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Application of Sonar for the
Measurement of Sludge Heights in
Wastewater Stabilisation Ponds
David Morgan
Supervisors: Dr Anas Ghadouani & Dr Marco Ghisalberti
1 November 2010
This dissertation is submitted as partial fulfilment of the requirements for the degree of Bachelor of
Engineering (Environmental Engineering) with Honours at the University of Western Australia
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ABSTRACT
Wastewater Stabilisation ponds are an established method for the treatment of wastewaters. They are
a sustainable and low maintenance technology that provides robust treatment. Ponds promote
ecological processes that decompose organic matter, remove contaminants and inactivate pathogens.
Construction costs are low and ponds are the most effective method for treatment where land costs are
not prohibitive. For this reason, wastewater stabilisation ponds are used for wastewater treatment
throughout regional Western Australia by the Water Corporation. During the course of wastewater
treatment, a sludge layer is formed at the base of the pond. Over time this sludge will accumulate,
reduce the working volume of the pond and affect pond performance. Sludge removal and disposal is
a necessary part of pond maintenance. Despite the wide scale use of treatment ponds, the process of
sludge accumulation is not well understood.
Assessment of pond performance and budgeting of sludge removal both require the effective
measurement of pond sludge volume in-situ. Despite this, current methods of sludge measurement are
time-consuming and inconsistent. The use of sonar for the purpose of measuring sludge levels has a
number of potential advantages over conventional methods. These include the possibility for
continuous measurement and GPS co-ordination, associated efficiency and accuracy gains and the
potential for unmanned operation.
This study has investigated the potential for a low-cost, GPS equipped sonar unit (model HDS-5,
Lowrance Electronics) to effectively measure sludge levels. Comparisons between sonar
measurements and sludge readings and pond sludge distributions are also discussed. There were three
major phases to this investigation; literature review, field work and data analysis. The primary aim of
the literature review was to identify the requirements of a sludge measuring device including possible
sources of error and useful performance criteria. The shallowness and turbidity of Western Australian
WSP systems was of concern. Major performance criteria included resolution, usability, efficiency,
replication of results obtained from current measurement techniques and robust operation in a range
of pond environments. The relationship between sludge management and pond operation and
hydrodynamics was also investigated.
Initial field work was carried out at the Sunset Hospital Jetty site in Nedlands to ascertain the
effectiveness of the pond in shallow water. In this testing the sonar was capable of reading down to
the minimum water depth of approximately 300mm. Field work was then conducted at Harvey and
Brunswick wastewater ponds to ascertain the effectiveness of sonar replicating current measurement
techniques. Spot readings by sonar and sludge judge were highly correlated at both ponds with r-
squared values of 0.9761 and 0.8701 respectively.
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Full-scale sonar sludge surveys were conducted at both Harvey and Brunswick ponds to provide
highly detailed sludge distribution profiles. The sonar survey took 50% less time than traditional
methods whilst producing data that was an order of magnitude more detailed. Sludge volume analysis
demonstrated that volume calculations for sonar were 7.5% lower than those obtained by sludge judge
for both ponds. Both ponds possessed an uneven sludge distribution that resulted in channel depths
fluctuating by as much as a factor of three.
The work conducted in this study demonstrates that the Lowrance HDS-5 depth sounder provides
effective measurement of sludge heights and sludge volumes in Wastewater Stabilisation Ponds. It is
anticipated that the method investigated will be applicable to Wastewater Stabilisation Ponds in
general, and not limited to Western Australia. Sludge distribution has been shown to be highly uneven
in the two ponds investigated. It is anticipated that such variability will have an effect on pond
hydrodynamics and performance. Recommendations include assessing the relationship between
sludge distribution and pond hydrodynamics and design of an autonomous or remote controlled
vehicle equipped with sonar.
ACKNOWLEDGEMENTS
Dr Anas Ghadouani
Marco Ghisalberti
Brett Kerenyi, Water Corporation
Dean Italiano, Water Corporation
Christopher Evans, Water Corporation
Dr Jimmy Seow, Department of Environment and Conservation
Dean Puzey, Water Corporation
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TABLE OF CONTENTS Abstract .................................................................................................................................................................. 2
Acknowledgements ................................................................................................................................................ 3
List of Figures .......................................................................................................................................................... 6
List of Tables ........................................................................................................................................................... 6
Acronyms ................................................................................................................................................................ 6
1 Introduction .................................................................................................................................................... 7
2 Literature Review............................................................................................................................................ 9
2.1 Wastewater Stabilisation Ponds ........................................................................................................... 9
2.1.1 Pond Types ........................................................................................................................................ 9
2.1.2 Removal Mechanisms ..................................................................................................................... 12
2.1.3 Design Procedures .......................................................................................................................... 14
2.2 Sludge Accumulation & Management ................................................................................................ 16
2.2.1 Sludge Composition ........................................................................................................................ 16
2.2.2 Sludge Removal and Disposal ......................................................................................................... 18
2.2.3 Sludge Accumulation ...................................................................................................................... 20
2.2.4 Sludge Distribution ......................................................................................................................... 20
2.2.5 Sludge Monitoring .......................................................................................................................... 21
2.3 Pond Hydrodynamics .......................................................................................................................... 23
2.4 Sonar ................................................................................................................................................... 24
3 Approach and Methodology ......................................................................................................................... 25
3.1 Equipment ........................................................................................................................................... 25
3.2 Study Sites ........................................................................................................................................... 27
3.3 Methodology ....................................................................................................................................... 30
3.3.1 Sludge Judge and Sonar Comparison .............................................................................................. 30
3.3.2 Pond Sludge Surveys ....................................................................................................................... 30
3.4 Data Analysis ....................................................................................................................................... 31
4 Results .......................................................................................................................................................... 32
4.1 Spot Comparisons ............................................................................................................................... 32
4.1.1 Harvey pond .................................................................................................................................... 32
4.1.2 Brunswick pond .............................................................................................................................. 34
4.2 Sonar Logging ...................................................................................................................................... 35
4.3 Sludge Volumes ................................................................................................................................... 36
5 Discussion ..................................................................................................................................................... 38
5.1 Spot Tests ............................................................................................................................................ 38
5.2 Sludge Surveys .................................................................................................................................... 38
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5.3 Volume Analysis .................................................................................................................................. 40
5.4 Pond Analysis ...................................................................................................................................... 41
6 Conclusions ................................................................................................................................................... 42
7 Recommendations ........................................................................................................................................ 43
7.1.1 Autonomous vehicle ....................................................................................................................... 43
7.1.2 Pond Hydrodynamics ...................................................................................................................... 43
7.1.3 Further use of Sonar ....................................................................................................................... 43
7.1.4 Detailed Volume Analysis ............................................................................................................... 43
8 Reference List ............................................................................................................................................... 44
9 Appendix a – Sample of sonar log output ..................................................................................................... 47
10 Appendix B – Lowrance HDS-5 specifications.......................................................................................... 48
11 Appendix C – Hawk sonar sludge level monitoring system ..................................................................... 50
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LIST OF FIGURES
Figure 8-1: Aerated pond, Harvey WA .................................................................................................................... 9
Figure 8-2: Typical pond Train for Temperate Climates ....................................................................................... 10
Figure 8-3: Typical factors affecting pond performance (Water Corporation 2010 b) ......................................... 11
Figure 8-4: Pond series in Spain featuring anaerobic facultative and maturation ponds (Moreno 1990) ........... 12
Figure 8-5: Overloaded facultative pond (Water Corporation 2010 b) ................................................................ 14
Figure 8-6: Surface loading method (Mara 1987) ................................................................................................ 15
Figure 8-7: Profile of facultative pond illustrating algal lifecycle (Water Corporation 2010 b) ............................ 17
Figure 8-8: Dry desludging (Water Corporation 2010 b) ...................................................................................... 18
Figure 8-9: Large scale sludge incineration plant, Netherlands (EEA 1997) ......................................................... 19
Figure 8-10: Sludge distribution in Mexican Facultative pond. Inlets and outlets identified by arrows (Nelson
2004) ..................................................................................................................................................................... 21
Figure 8-11: Sludge judge, Harvey ........................................................................................................................ 22
Figure 9-1: Sonar set-up, transducer is mounted on a float ................................................................................. 25
Figure 9-2: Sludge measurement using a sludge judge ........................................................................................ 26
Figure 9-3: Initial testing on Swan river, Nedlands ............................................................................................... 27
Figure 9-4: Pond layout, Harvey WWTP ............................................................................................................... 28
Figure 9-5: Pond layout, Brunswick WWTP .......................................................................................................... 29
Figure 9-6: Maturation pond, Brunswick .............................................................................................................. 30
Figure 9-7: Transducer mounting and sludge judge, Brunswick pond ................................................................. 31
Figure 10-1: Sludge judge height comparison, sludge judge and sonar, Harvey .................................................. 33
Figure 10-2: Scatter plot of spot test data, regression line shown, Harvey ......................................................... 33
Figure 10-3: Sludge judge height comparison, sludge judge and sonar, Brunswick ............................................. 34
Figure 10-4: Scatter plot of spot test data, regression line shown, Brunswick .................................................... 35
Figure 10-5: Depth feed on sonar display, identified by white line ...................................................................... 36
Figure 11-1 – Sonar profiling, Harvey Pond .......................................................................................................... 39
Figure 11-2 – Sludge distribution, measured by sonar survey, Harvey ................................................................ 40
Figure 11-3 – Sludge distribution, measured by sonar survey, Brunswick ........................................................... 41
LIST OF TABLES
Table 1: Comparisons of Spot Test Data, Harvey.................................................................................................. 32
Table 2 – Comparison of Spot Test Data, Brunswick ............................................................................................ 34
Table 3 – Comparison of sonar and sludge data, both integrated and averaged depth volume, Harvey pond ... 37
Table 4 - Comparison of sonar and sludge data, both integrated and averaged depth volume, Brunswick pond
.............................................................................................................................................................................. 37
Table 5 – Survey comparison, sludge judge and sonar ......................................................................................... 39
ACRONYMS WSP – Wastewater Stabilisation Pond
WWTP – Wastewater Treatment Pond
CSTR – Continuously mixed stir-tank reactor
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1 INTRODUCTION
Wastewater Stabilisation Ponds are a simple means of wastewater treatment and their use is
widespread. The systems use earthen ponds and long residence times to allow natural mechanisms to
decompose organic material, degrade contaminants and inactivate pathogens. The systems are low
cost, require little maintenance and provide a robust and effective treatment of wastewaters.
Despite their simple design and appearance, the treatment processes involved in Wastewater
Treatment Pond‟s are decidedly complex and poorly understood. The ponds contain an intricate
ecosystem including algae, protozoa and aerobic and anaerobic bacteria (Kehl et al. 2009).
Decomposition and removal mechanisms are complex; empirical studies have demonstrated that site
conditions are influential. Factors such as temperature, pond geometry, residence times, pond
hydrodynamics and solar radiation are important (GHD 2007; Kehl et al. 2009).
Over the lifetime of a WSP, influent suspended solids and products of decomposition combine to
form a sludge layer at the base of the pond. This sludge will accumulate and increase in volume as the
pond continues to operate. The sludge accumulation rate is highly variable and will change widely
from pond to pond. Factors of influence include temperature, pond design and treatment procedures.
The distribution of sludge is also highly variable; surveys have shown sludge heights to vary in the
horizontal, whilst sludge density is highly variable in the vertical (Picot et al. 2005; Nelson et al.
2004). The sludge-water interface is difficult to define. As the sludge volume increases, the effective
pond volume is reduced and the hydrodynamics of the pond are altered. For this reason, WSP‟s
require occasional desludging.
Achieving a better knowledge of pond sludge distribution is important for two reasons; simplifying
effective pond management and permitting a more thorough investigation of pond hydrodynamics.
Current methods of sludge measurement are cumbersome, time consuming, inaccurate and labour
intensive, usually requiring the use of a boat with the ensuing health and safety requirements.
The use of sonar offers a range of possible advantages over conventional methods of sludge
measurement. The opportunities for increased accuracy, efficiency and autonomous operation are
apparent. Sonar sludge level detectors have been utilised to some extent in the petrochemical industry
and some wastewater applications. Additionally, some attempts have been made at testing sonar use
in agricultural sludge ponds (Singh et al. 2007). To date, no comprehensive studies have been carried
out on the use of sonar for measuring sludge levels in Wastewater Stabilisation Ponds. There is a clear
need to quantify the effectiveness and usability of sonar in measuring sludge heights and the influence
of sludge distribution on hydrodynamics.
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This study will assess the effectiveness of sonar in measuring sludge levels of Waste Stabilisation
Ponds in Western Australia. Major criteria will be accuracy, resolution, usability, efficiency and
replication of results obtained by current measurement techniques. Sludge Distribution will be
discussed with regard to pond hydrodynamics and performance.
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2 LITERATURE REVIEW
2.1 WASTEWATER STABILISATION PONDS
2.1.1 POND TYPES
Wastewater Stabilisation Ponds are used around the world for the effective treatment of domestic
wastewaters. They are cheap to build and operate, with minimal maintenance requirements. Such
ponds are resilient to variable flows and produce effluent of a quality comparable to that of
conventional mechanical systems (Racault et al. 1995; Esen & Al-Shayji 1999). The significant land
requirements of WSP systems are their major disadvantage, for this reason they are primarily used in
rural and regional areas where land costs are not prohibitive (Xian-wen 1995).
WSPs utilise the purification processes of natural systems such as rivers and lakes. Thus despite their
simple design the systems contain complex ecosystems which include viruses, algae, bacteria,
protozoa, insects, parasites, crustaceans and fungi (Kehl et al. 2009). Through the action of
microorganisms, complex organic cells are broken down into simple non-organic substances. A major
goal of WSP systems is thus to provide optimum growth conditions for these organisms that promote
FIGURE 2-1: AERATED POND, HARVEY WA
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decomposition (Water Corporation 2010 b). The treatment processes cannot be fully controlled and
are largely guided by physical and environmental constraints; these include solar radiation,
temperature, wind, pH, pond geometry, organic loading and pond hydraulics (GHD 2007; Kehl et al.
2009). Wastewater characteristics, particularly the presence of toxins also affects pond performance
(Thirumurthi 1974). For this reason pond design is a complex task requiring consideration of many
different and often uncontrollable variables. There are four major pond types that are used for
wastewater treatment; these ponds are largely differentiated by the availability of dissolved oxygen.
FIGURE 2-2: TYPICAL POND TRAIN FOR TEMPERATE CLIMATES
Anaerobic ponds promote breakdown of organic wastes through the action of anaerobic bacteria.
They treat wastes with a high organic load, typically with Biological Oxygen Demands of above
2000mg/L in the absence of Oxygen (GHD 2007). Minimising oxygen intrusion and minimising
temperature fluctuations are constraints considered in the construction of such facilities. For this
reason, ponds are deep, typically 3-5m in order to minimise temperature fluctuations and oxygen
diffusion at the surface (WHO 1971). Ponds support no widespread algae populations, have long
detention times and are highly turbid (Saqqar & Pescod 1995). Solid deposition and fermentation are
the major modes of treatment. When anaerobic ponds are used, it is typically for pre-treatment of
highly concentrated wastes, such as those from feedlots, abattoirs and wineries, before further
treatment. Anaerobic ponds are rarely used in Western Australian Wastewater Treatment systems.
Facultative Ponds are utilised in almost all pond systems. They contain a complex ecology and allow
for the robust removal of contaminants through settling, biodegradation and disinfection (Bryant
1995; WHO 1971). These processes create a layer of sludge at the base of the pond. The structure of a
facultative pond is guided by the presence of dissolved oxygen. Due to the turbidity of the pond,
sunlight cannot penetrate through the entire water column and a thermocline can develop (Water
Corporation 2010 b). Such ponds can become stratified, influencing the flow conditions (Pearson et
al. 1995). The top layer of the pond is thus rich in Carbon Dioxide, nutrients and sunlight, promoting
the growth of algae and aerobic bacteria (Tadesse et al. 2004). This is a highly aerobic environment.
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Levels of dissolved oxygen decrease throughout the water column, forming an anaerobic layer at the
base of the pond (Boran & Kargi 2005). The sludge layer is an anaerobic cell that is responsible for a
significant degree of decomposition that takes place (WHO 1971; Water Corporation 2010 b). Ponds
are typically 1.5 to 2m deep allowing the penetration of sunlight and maintain some mixing.
Facultative ponds are often the first pond in a series and will be expected to contain almost all
suspended solids and reduce the Biological Oxygen Demand by approximately 80% and above (GHD
2007; Racault et al. 1995). For this reason their correct design and operation is critical. Figure 8-3
below demonstrates some interactions associated with Facultative ponds.
FIGURE 2-3: TYPICAL FACTORS AFFECTING POND PERFORMANCE (WATER CORPORATION 2010 B)
Maturation ponds, also known as oxidation or polishing ponds are designed for the purposes of
disinfection. They can serve to remove residual biological oxygen demand and achieve some solids
deposition (Maynard et al. 1999). The main objective of design is the disabling of pathogens,
particularly viruses, harmful bacteria and parasites. The goal is to maximize sunlight radiation which
is known to inactivate pathogens (Water Corporation 2010 a; Maynard et al. 1999). Ideally the pond
should be as shallow as possible, however in order to prohibit the growth of pond plants, depths of
approximately 1m are often optimal. These ponds will typically be the last in a series.
Aerated ponds utilise mechanical aerators to enhance the concentration of dissolved oxygen in the
pond. They are useful when influent loads are increased and space is limited, or new pond
construction postponed. The aerators increase oxygen diffusion at the surface whilst also providing
mixing throughout the water column. The dissolved oxygen is thus spread throughout the pond
allowing enhanced action of aerobic bacteria (WHO 1971). Aerators are often retrofitted to facultative
ponds to increase treatment capacity.
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FIGURE 2-4: POND SERIES IN SPAIN FEATURING ANAEROBIC FACULTATIVE AND MATURATION PONDS (MORENO 1990)
2.1.2 REMOVAL MECHANISMS
Wastewater Stabilisation ponds are complex systems, that include a range of treatment processes in
one system (Bryant 1995). The major aims of a pond system are the settling of solids, reduction of
biological oxygen demand through decomposition of organic material and pathogen inactivation
(WHO 1971). A functioning facultative pond is expected to accomplish these objectives whilst
minimising odours. The large residence times and low flow rates of a facultative pond promote the
deposition of solids. Removal of suspended solids in an effective facultative pond should approach
100% (Maynard et al. 1999, Water Corporation 2010 b). The settlement of solids has a significant role
in removing organic matter from the water column and creating an anaerobic cell at the base of the
pond. The removal of heavy metals and parasites is largely achieved through settlement. Pond
systems typically achieve poor removal of nutrients, in the range of 30-40% removal (Maynard et al.
1999). For this reason, aluminium dosing is sometimes used to precipitate phosphorous and achieve
higher rates of nutrient reduction.
The decomposition of organic matter and the associated reduction in Biological Oxygen Demand is a
crucial part of effective treatment. Removal is largely achieved through bacterial decomposition; both
anaerobic and aerobic (Tadesse et al. 2004; WHO 1971). Anaerobic Respiration takes place in the
sediments of facultative ponds, where oxygen is limited. The mechanisms involved are highly
complex and the range of products far greater than in aerobic environments (WHO 1971). The
decomposition is a multi-step process, and largely results in the emission of methane. Many of the
compounds produced in an anaerobic environment are odiferous and this can be a significant issue in
design and management (DOW 2009). Aerobic respiration takes place in the oxygenated parts of a
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facultative pond and is undertaken by Carbon Bacteria, fungi and protozoa. Much of the Carbon
Dioxide is captured by algae in the water column (Water Corporation 2010 a; WHO 1971).
The presence and concentration of Oxygen is the driver of aerobic decomposition in facultative ponds.
The primary source of Oxygen is algal photosynthesis, whilst mass exchange at the surface can also
be significant (Boran & Kargi 2005). For this reason, algal growth is pivotal to encouraging aerobic
decomposition and strong pond performance. Temperature, solar radiation, nutrient availability,
ammonia/sulphide toxicity and predation all affect algal growth (Lawty et al. 1996; WHO 1971).
There is a complex inter-relationship between Biological Oxygen Demand, bacterial growth and algal
growth. The decomposition of organic matter produces Carbon Dioxide. Much of this Carbon is then
transformed by algae into cell mass, which upon death of the algae becomes an additional oxygen
demand (Boran & Kargi 2005; Water Corporation 2010 b). For this reason algal blooms can increase
the level of organic matter considerably, affecting pond performance.
Pathogens of importance in WSP systems are bacteria, viruses and parasites. Wastewater Stabilisation
Ponds provide very effective removal of pathogens, typically above 99% (Water Corporation 2010 b).
Despite this, methods of removal and inactivation are poorly understood (Maynard et al. 1999).
Removal of pathogens is usually measured by the presence of indicator bacteria, primarily E. coli.
Such indicators are easy to measure and identify, though there are significant shortfalls with this
method. Little work has been done to determine if removal rates are transferable between E. coli,
other bacteria, viruses and parasites (Maynard et al. 1999). Experiments comparing E. coli and Vibrio
Cholerae have shown that these bacteria species react differently to changes in conditions such as pH
and solar radiation (Maynard et al. 1999). Studies focusing on mechanisms of bacteria removal have
proved inconclusive, despite significant research. Factors affecting removal are thought to be solar
radiation, pH, predation, competition, temperature, residence time and dissolved oxygen (Maynard et
al. 1999; Water Corporation 2010 b). It is thought that absorption of viruses to solids and subsequent
sedimentation is important (Water Corporation 2010 b). WSPs are thought to provide good conditions
for virus removal. They have long detention times and expose viruses to solar radiation, adsorptive
solids and other micro-organisms. Removal of intestinal parasites in WSP systems is excellent;
commonly exceeding 99.999% (Water Corporation 2010 a). Sedimentation of cyst and eggs is thought
to be the primary method of removal. Cysts can exist for long periods, exceeding a year in sediment
(Nelson et al. 2004). For this reason, sediment disturbances including high flows and sludge
extraction can affect removal rates. The hydraulic residence time is an important element of all
pathogen removal mechanisms. Short circuiting induced by stratification or preferential flows can
reduce the residence time and allow the transport of viable pathogens through the system (Maynard et
al. 1999). For this reason, any reduction in hydraulic performance is of critical importance.
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2.1.3 DESIGN PROCEDURES
Design Procedures for Wastewater Stabilisation Ponds have undergone considerable study since their
widespread inception in the 1970s, and continue to evolve. Wastewater ponds are simple in their
construction, but contain a complex ecology and intricate processes that make characterisation
difficult. For this reason creating a comprehensive method for the dimensioning of ponds is
challenging. Factors such as solar radiation, temperature, wind and hydrodynamic conditions are
usually relevant (Kehl et al. 2009; Esen & Al-Shayji 1999). Hydrodynamic conditions are often
ignored and approaches typically assume some sort of ideal flow, assuming either plug-flow or mixed
conditions (Wood et al. 1995). In practice ponds exhibit dispersed flow that is difficult to model or
characterise. There are three major approaches to pond dimensioning that have been applied to pond
design. These are empirical approaches, kinetic methods and surface loading.
FIGURE 2-5: OVERLOADED FACULTATIVE POND (WATER CORPORATION 2010 B)
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Equations for empirical design, such as those of Aloyna (1971), attempt to account for numerous
variables that influence pond performance without considering the actual pond processes. They are
usually based on past experience, pond field observations and lab testing. One of the major issues
with this method is the extrapolation of correlations from simple laboratory experiments with few
variables to a vast pond environment with many more variables and relationships (Kehl et al. 2009).
These early approaches determined residence time to be an important variable.
Reactor Methods such as the kinetic model of Marais (1961) model a Wastewater Stabilisation Pond
as a biochemical reactor, producing treated wastewater (Prats & LLavador 1994). They use reactor
theory from the process engineering field. Typically such an approach assumes first-rate decay of
organic material and ideal mixed flow, both of which are known to be erroneous (Pearson et al. 1995;
Kehl et al. 2009). This approach is again susceptible to the extrapolation of laboratory data to a pond
environment. In addition, characterisation of flow regime is critical to the design of these ponds, and
is often not well known (Prats & LLavador 1994).
The surface loading method as defined by Mara (1987) is the predominant contemporary method for
Wastewater Stabilisation Pond design (Pearson et al. 1995). Past methods have largely emphasised
residence time as the major influence on pond treatment. Unfortunately, maximising residence time
has often resulted in deeper ponds, which suffer from poor mixing and reduced potential for solar
radiance and inactivation of pathogens (Water Corporation 2010 a). This is particularly relevant to
maturation ponds. The Surface Loading Equation is shown in Figure 8-6. Despite the simplicity of
this equation, this model has been used very successfully and is based on extensive research of pond
systems around the world. The equation relates the surface loading rate λ, in kg of BOD per hectare
per day to the average winter temperature.
FIGURE 2-6: SURFACE LOADING METHOD (MARA 1987)
There are several common issues with the use of pond design procedures, particularly in regard to past
construction using obsolete design procedures. These include construction of ponds that are either too
deep or shallow, application of methods with little respect for local conditions and a lack of
appreciation for the role hydrodynamics plays in pond performance. Wastewater Stabilisation Ponds
can be as simple as a pond with an inlet at one end and an outlet at the other. In Western Australia
some systems are no more complex than this (Water Corporation 2010 a). Most systems were
16
constructed in the 1970s and 1980s when wastewater stabilisation ponds became popular around the
world. The design regime utilised for many of these ponds was an interpretation of the kinetic method
of Marais (1961).
The majority of Western Australian facultative and maturation ponds are of depths that do not agree
with current design procedures and methodology (Water Corporation 2010 b). Facultative ponds are
often shallow at 1-1.25m whilst maturation ponds are too deep at 1.5 – 1.8m. Historically maturation
ponds have been designed to achieve higher residence times through increases in depth. Research
identifying light inactivation of bacteria and viruses as the primary method of disinfection has meant
shallow ponds of around 1m are advised (GHD 2007). Whilst maturation ponds are often too deep,
facultative ponds in Western Australia are frequently too shallow. Deeper ponds provide a more
hospitable depth range for green algae to thrive, maximising oxygen production and minimising the
chance of algal collapse (Water Corporation 2010 b). Deeper facultative ponds produce less sludge,
which reduces sludge removal costs. Deeper ponds promote the development of an aerobic zone at the
base of the pond, and thus anaerobic decomposition. Anaerobic decomposition produces methane that
is largely released from the pond; in contrast aerobic decomposition produces Carbon Dioxide, much
of which is captured by algae in the pond, processed into cell mass and eventually incorporated back
into sludge (WHO 1971). In addition deeper ponds promote the compaction of sludge.
Wastewater Stabilisation Ponds are hydrological features with complex flow regimes and
hydrodynamics are known to have a significant impact on pond performance. Despite this, hydraulic
influence is largely ignored in pond design and management (Persson & Wittgren 2003). The
characterisation of pond hydrodynamics is expensive, time consuming, and often unsuccessful,
requiring tracer tests (Moreno, MD 1990). Design procedures typically assume ideal flows, characterised
by either fully mixed or plug flow conditions. In reality flows are far more complex, and dead zones
and short circuiting are known to occur both in the horizontal and vertical directions (Esen & Al-
Shayji 1999; Torres et al. 1996). The hydrodynamics of pond systems will be discussed later in this
study.
2.2 SLUDGE ACCUMULATION & MANAGEMENT
2.2.1 SLUDGE COMPOSITION
The operation of Wastewater Stabilisation Ponds results in the creation of a sludge layer at the base of
the pond. This sludge layer increases in volume throughout the life of the pond, due to the slow
deposition of sediments. Deposition occurs due to both sedimentation of influent solids and the death
and deposition of pond algae and bacteria (Picot et al. 2005). The compaction of this sludge layer by
the weight of sludge above it means that sludge density increases with sludge age.
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FIGURE 2-7: PROFILE OF FACULTATIVE POND ILLUSTRATING ALGAL LIFECYCLE (WATER CORPORATION 2010 B)
The process of anaerobic decomposition takes place in this sludge layer. In temperate regions, this
anaerobic decomposition only takes place to any appreciable extent in summer and is known as
benthic feedback, as the sludge adds to the organic loading of the pond (Bryant 1995). Studies by
Banks et al. (2005) have shown that benthic feedback can add considerably to the organic loading of
the pond. In some cases, the level of benthic feedback in summer months has been equivalent in
magnitude to the incoming load. This has significant implications for pond performance and design
criteria.
The rate of sludge deposition is highly variable and influenced by a range of factors. These include
temperature, wastewater characteristics and treatment processes such as aluminium dosing which are
designed to precipitate nutrients and add to sludge deposition (Picot et al. 2005). Sludge deposition is
most evident in primary ponds where sedimentation is predominant and organic loads are at their
peak. As the sludge volume increases with pond operation, the effective pond volume decreases,
residence time is reduced and pond performance is adversely affected (Nelson et al. 2004; Picot et al.
2005). To retain design performance the regular removal of sludge is required to maintain pond
volume and residence time. The successful long-term operation of a Wastewater Stabilisation Pond is
reliant on the effective management of sludge levels. Desludging intervals of 10 years or so are
typical.
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2.2.2 SLUDGE REMOVAL AND DISPOSAL
Sludge removal and disposal incurs significant costs, both in extraction, storage, transport and
eventual disposal. Analysis of data in Austria has demonstrated that sludge treatment and disposal
typically amount to more than 50% of total operational costs of small Wastewater Treatment Plants
(Nowak, O 2006). The extraction of sludge from the pond must be considered in conjunction with
disposing of the sludge. There are two major methods of sludge removal, dry desludging which
requires the emptying of the pond and under-water desludging which does not require bypassing of
the pond (Picot et al. 2005; NC State 2008). Both of these techniques have their advantages. Dry
Desludging has traditionally been used because of its lower cost. The pond is bypassed and allowed to
dry, typically over the warmest months of the year; sludge can then be removed using conventional
earth moving equipment. Implicit in the use of this technique is the loss of water treatment capacity
over a significant part of the year. In systems without surplus capacity this will usually result in
overloading of the system.
Desludging without emptying the pond is typically utilised when the receiving water body is sensitive
to pollution and bypassing of the pond is not an option (Picot et al. 2005; NC State 2008). Extraction
is usually carried out using a sludge pump which may be mounted on a raft or other flotation device
for a period of many days. This sludge is then dewatered using a centrifuge to reduce transport costs.
This approach requires the continuous use of equipment over many days and the associated increase
in costs. A French study analysing sludge removal techniques has established that under-water
desludging costs approximately 50% more than conventional desludging (Picot et al. 2005).
FIGURE 2-8: DRY DESLUDGING (WATER CORPORATION 2010 B)
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Wastewater sludge exhibits a range of properties depending on the wastewater characteristics and
pond treatment procedures. Substances of particular consideration include volatile solids, nutrients,
heavy metals and organic pollutants (Abis & Mara 2005). The concentrations of these contaminants
determine methods and cost of disposal. There are three major methods of sludge disposal; land
spreading, land fill and incineration (Nowak, O 2006). Land spreading is considered the most useful
method as it utilises the high nutrient content of wastewater sludge for agricultural use. There a
number of regulations associated with land application of sludge. These include restrictions on the
application of heavy metals and nutrients, hydraulic conditions of the site and timing of application to
maximise nutrient uptake (Nowak, O 2006; EEA 1997). Sludge storage facilities are often required as
sludge can only applied at certain times throughout the year. In addition spreading is typically
conducted using regular farming equipment, which may not be suited to the purpose. The composition
of sludge and effects of land spreading on food chains is not well known (EEA 1997).
Land filling is probably the predominant means of sludge disposal in current use. It has traditionally
been used because it is simple and low-cost (Nowak, O 2006). The increasing regulation of land-fill is
likely to make it a more expensive and less suitable method for the disposal of pond sludge (EEA
1997). Incineration is increasingly being used for the disposal of pond sludge. Despite the high costs
of operation and large capital investment required, incineration is currently used for 15% of sludge
disposal in Western Europe (EEA 1997). The major advantages are the significant reduction in sludge
volume after incineration and the minimisation of odours. The increasingly costly nature of sludge
disposal necessitates the more accurate estimation of sludge volumes for the purposes of budgeting
and planning.
FIGURE 2-9: LARGE SCALE SLUDGE INCINERATION PLANT, NETHERLANDS (EEA 1997)
20
2.2.3 SLUDGE ACCUMULATION
Despite the inevitable formation of sludge, management of sludge and its disposal has typically been
ignored in design. Sludge disposal in all its forms is becoming more expensive and more highly
regulated. For this reason, estimating in-situ sludge volumes is becoming a more important part of
pond management. Attempts have been made to estimate per annum sludge accumulation rates in
order to allow the more simple management of sludge volumes. Sludge deposition is known to be
influenced by temperature, influent loads, site conditions and methods of treatment such as aluminium
dosing (Picot et al. 2005). The approach of Nelson et al. is typical; relating the accumulation rate to
the time the pond has been in operation. Accumulation rates of 0.021-0.036m3/person.yr were
estimated, whilst a similar French study calculated rates in the range of 0.04-0.148m3/person.yr
(Nelson et al. 2004; Picot et al. 2005). It is well known that sludge deposition per capita is higher in
cold climates, design guidelines often recommend a value of 0.04m3/person.yr for climates with
average temperatures above 20C whilst values of 0.1m3/yr are recommended for areas experiencing
average winter temperatures of below 10C (Mara & Pearson, cited by Picot et al. 2005).
There a number of problems associated with the application of accumulation rates to pond planning
and sludge disposal plans. Firstly, experiments have demonstrated that sludge deposition is not a
linear process as sludge depths stabilise with time due to compaction and anaerobic decomposition
(Abis & Mara 2005; Picot et al. 2005). After two years of pond operation, the annual accumulation
rate decreased as sludge volumes became denser. For this reason, application of a simple linear per
annum rate is a significant approximation. The influence of local pond conditions and influent
loadings on sludge accumulation rates is significant. Communications with Dean Puzey of the Water
Corporation have established that aluminium dosing has a very significant effect on sludge
accumulation, to the point of doubling accumulation rates. For these reasons practical sludge
measurement will likely be required as a means of pond management.
2.2.4 SLUDGE DISTRIBUTION
Sludge distribution in Wastewater Stabilisation Ponds is highly variable in both the horizontal and
vertical, and is influenced by a variety of factors (Picot et al. 2005; Nelson et al. 2004). Studies have
demonstrated sludge is composed of two layers. A stabilised, older and denser layer is situated
beneath a less dense and more volatile layer above it (Abis & Mara 2005). The relative depth of
sludge and sludge age are closely related (Picot et al. 2005).
Numerous studies have shown sludge distribution to be highly uneven (Nelson et al. 2004; Picot et al.
2005). Sludge levels are often found to be higher at the inlet, outlet and in the corners (Abis & Mara
2005). Higher sludge levels in the corners are attributed to wind action, as gaseous products of
anaerobic distribution force sludge to the surface, where it is blown into the corners. Patterns of
sludge distribution are often attributed to pond geometry (Nelson et al. 2004). Local conditions are
21
important and sludge distributions are known to change throughout the year. There are obvious
interactions between sludge distribution and pond hydrodynamics with regard to differences in
channel depth and differing flow velocities throughout the pond. The processes involved are poorly
understood and have not been well studied.
FIGURE 2-10: SLUDGE DISTRIBUTION IN MEXICAN FACULTATIVE POND. INLETS AND OUTLETS IDENTIFIED BY ARROWS (NELSON 2004)
2.2.5 SLUDGE MONITORING
Due to the difficulties associated with standardising accumulation rates, active sludge level
monitoring is an important part of sludge management and pond operation in general. In addition,
attempts to characterise hydrodynamics of such systems are helped by a detailed knowledge of pond
sludge distribution. There are two components of sludge monitoring; estimating sludge volume and
taking sludge samples for density analysis (NC State 2008). Numerous techniques are utilised for
measuring sludge, all involving the use of a boat. Standard practice entails splitting the pond into a
grid using flags or other markers on the bank. Somewhere between 8 and 40 depth measurements are
then taken using the measuring apparatus (Nelson et al. 2004). An average sludge height is calculated
for the pond and using the pond area, the sludge volume can be calculated. Some studies have used
this data to construct three-dimensional plots of pond sludge distribution (Picot et al. 2005; Nelson et
al. 2004).
Due to the difficulties associated with standardising accumulation rates, active sludge level
monitoring is an important part of sludge management and pond operation in general. In addition,
attempts to characterise hydrodynamics of such systems are helped by a detailed knowledge of pond
sludge distribution. There are two components of sludge monitoring; estimating sludge volume and
22
taking sludge samples for density analysis (NC State 2008). Numerous techniques are utilised for
measuring sludge, all involving the use of a boat. Standard practice entails splitting the pond into a
grid using flags or other markers on the bank. Somewhere between 8 and 40 depth measurements are
then taken using the measuring apparatus (Nelson et al. 2004). An average sludge height is calculated
for the pond and using the pond area, the sludge volume can be calculated. Some studies have used
this data to construct three-dimensional plots of pond sludge distribution (Picot et al. 2005; Nelson et
al. 2004).
Typical measuring devices include light meters/infrared meters which include an emitter and detector
at the end of a length of chord. These devices emit a beep when density of sludge is such that the
sludge layer is reached. Disk-on-rope devices are made of a PVC disk attached to some chord. This
disk is designed to be lowered slowly through the water column and will settle on the sludge layer. In
many locations a pole is used, passed through the water column until the sludge layer is identified by
feel. Sludge judges are devices that use a clear plastic pipe to obtain measurements. Standard practice
in Western Australia is to use a 2m cylinder with a closing switch at one end. These measuring
apparatus‟ share a number of common disadvantages. All require the use of a boat, with the ensuing
safety issues (Water Corporation 2010 b). All are labour-intensive, requiring two people on the boat
and one or two onshore. Measurements can only be taken when stationary, making the exercise time
consuming. In addition, many methods are subjective, possessing an element of “feel,” and requiring
the opinion of the operator.
FIGURE 2-11: SLUDGE JUDGE, HARVEY
23
2.3 POND HYDRODYNAMICS
The hydrodynamic behaviour of Wastewater Stabilisation Pond‟s is complex and poorly understood
(Polprasert & Bhattarai 1985). The residence time is determined by hydraulic performance, which is
thus critical to the effective operation of such ponds. Despite this, many of the pond design methods
are empirical in nature and do not consider fully hydraulic conditions, which are highly variable
between sites (Esen & Al-Shayji 1999). Investigators have usually assumed completely mixed flow
conditions or the other extreme, plug-flow conditions. Both are ideal models, use of completely mixed
flow assumes CSTR conditions and homogeneity throughout the pond. Use of plug-flow conditions
assumes perfect hydraulic performance with no mixing. Plug-flow conditions typically overestimate
the residence time and performance of a pond. A more realistic approach to flow conditions is the use
of a non-ideal dispersed flow model (Nameche & Vasel 1998). There are a number of variations, the
common theme is that the dispersion coefficient is crucial and is usually calculated using a tracer test.
A dispersion coefficient of 0 indicates plug-flow conditions, whilst a dispersion coefficient
approaching infinity is representative of mixed flow conditions.
Wastewater Stabilisation ponds are largely uncontrolled systems and are affected by a range of
variables, these include design variables as well as environmental conditions. There are a number of
factors that contribute to pond hydrodynamics these include pond geometry, inlet and outlet
placement, wind, temperature, evaporation and rainfall (Agunwamba JC 1992). Pond Geometry is
particularly important as it is a design variable and can often be altered in existing systems (Persson, J
2000). Length/Width ratio has been demonstrated to be the single most influential factor for
determination of hydraulic conditions, though there is disagreement on the required ratio. Ponds with
L/W ratios of four and above will possess some elements of plug-flow, whilst flow condition of ponds
squarer than this will most likely be able to be approximated by mixed-flow conditions (Persson &
Wittgren 2003). Wind has an important effect on hydraulic conditions and is a usually a strong mixing
force. In ponds where inlet and outlets are aligned along the prevailing wind direction, winds facilitate
the bulk movement of water from inlet to outlet, causing a short circuit and negatively affecting pond
performance (Moreno, MD 1990). Wind can also act to disrupt stratification induced short-circuiting
created by temperature differences in the water column (Aldana et al. 2005; Meneses 2005).
Tracer tests are time-consuming, labour intensive and are often poorly validated. Despite these
shortfalls they are the only current method of characterising hydraulic conditions and identifying short
circuiting and zones of no mixing. Research has demonstrated that in practice, actual residence time is
always lower than the nominal or design residence time. Results range broadly, Australian studies
have demonstrated facultative ponds possessing an actual residence time of 10 hours in comparison to
24
a design residence time of 15-20 days; a Spanish study has identified short circuiting ranging from 10-
42% in the ponds studied (Sweeney et al. 2005; Moreno, MD 1990). Thermally induced stratification
is known to occur in WSP‟s and is an often cited as a cause of zones of no mixing and decreased
hydraulic performance (Torres et al. 1996; Gun & Stefan 1995; Pearson et al. 1995). Other studies
have shown short-circuiting to occur in ponds without thermal stratification (Moreno, MD 1990).
Wastewater Stabilisation Ponds are low velocity environments and the small but consistent jet at the
inlet of such systems has been shown to influence flow patterns (Fyfe 2007). The creation of a large
circulating cell at the perimeter of the pond causes a large dead zone in the centre of the pond and a
direct short circuit from inlet to outlet along one side. This effect has been linked to short-circuiting of
90% (Frederick & Lloyd 1996).
2.4 SONAR
The use of sonar for measuring water depths is well developed for marine applications. The ability to
continuously measure at high resolution, whilst travelling at speed allows the quick and detailed
mapping of the seafloor. Sonar has also been utilised for the measurement of sludge levels in static
applications, such as clarifiers and commercial devices such as those manufactured by Hawk (see
Appendix C) are available. The use of sonar for measuring sludge levels has been identified as
possessing a number of potential advantages over conventional measuring apparatus‟. These include
the potential for continuous measurement, co-ordination with GPS co-ordinate systems and associated
savings in time and labour.
A paper by Singh has identified the benefits of sonar for measurement of sludge levels in agricultural
sludge ponds, which are similar in many ways to Waste Stabilisation Ponds (Singh et al. 2007). Both
are reasonably shallow and are highly turbid (Banks et al. 2005). This study utilised a Lowrance depth
sounder equipped with GPS mounted on a remote control airboat. The study determined the co-
ordination of sonar and GPS data to be an effective means of obtaining sludge readings, though the
results were not compared to conventional methods of sludge measurement. Use of the sonar was
identified as having significant efficiency gains, being 15 to 20 times faster and 70% less labour
consuming than general surveys (Singh et al. 2007).
25
3 APPROACH AND METHODOLOGY
3.1 EQUIPMENT
Sonar sludge depth measurements were taken using a Lowrance HDS-5 Fish Finding Sonar and GPS
combination. It is a relatively typical fish finder/depth sounder produced for the recreational fishing
industry. There were a number of reasons for the selection of a fish-finder type device. Such products
are relatively inexpensive and user-friendly. The inclusion of built-in GPS allows for the co-
ordination of depth and co-ordinate data, and continuous data logging. Depth measurements are taken
multiple times per second and data is easily exported to excel. Output is composed of depth
measurements and associated GPS co-ordinates. The sonar and transducer combo was calibrated for
shallow water operation. Ping speed was set to maximum and all shallow water settings were selected.
The 200kHz transducer cone was utilised. The sonar transducer was attached to a float that allowed
the transducer to be towed or pulled along as shown in Figure 9-1. There were two main functions of
the sonar that were utilised in this study.
FIGURE 3-1: SONAR SET-UP, TRANSDUCER IS MOUNTED ON A FLOAT
26
Spot testing was conducted using the displayed depth on the sonar head-unit. The sonar provides a
constant measurement of water depth that on the display that is continuously refreshed. The resolution
is low, only differing in increments of 100mm and readings must be transcribed. Spot readings were
largely used for comparison with sludge judge readings. The sonar unit also allowed the continuous
recording of coupled GPS and depth data. Temperature and boat speed were also recorded, whilst
water depth resolution was 1cm, higher than of spot readings. The GPS receiver possessed an
accuracy of approximately 1-3m. This function was utilised for the continuous collection of data for
the purposes of profiling Wastewater Stabilisation Ponds. Recordings were taken in the LS2 file
format with a maximum resolution of 3200 Bytes a ping.
The Water Corporation predominantly uses a sludge judge for the purpose of sludge monitoring and
management. It is composed of a 2m rigid clear plastic cylinder of approximately 5-10cm in diameter.
A switch at the top can be turned to block air exchange at the top of the pipe. The sludge judge is
pressed into the sludge until it reaches the pond bottom, at this point the switch is closed forming a
vacuum and the device is raised from the water column. A sludge depth can be read off the cylinder
using the attached measuring scale. The actual position of the sludge interface is subjective; the point
where light no longer passes horizontally through the cylinder is typically used. Resolution is in the
range of 50mm. Past sludge judge surveys of the ponds in question were available from the Water
Corporation. In these surveys the ponds were split into a grid with grid points spaced 10m apart in
both axes. For a typical pond there would thus be approximately 20-100 points depending on pond
size.
FIGURE 3-2: SLUDGE MEASUREMENT USING A SLUDGE JUDGE
27
3.2 STUDY SITES
The main aims of this study have been to test the capability of sonar in profiling sludge levels in
Wastewater Stabilisation Ponds. Major aims are to assess usability, accuracy and determine sludge
distributions in one or more ponds. Depth sounding was conducted at three study sites; these were
Sunset Hospital Jetty in Dalkeith, Harvey WWTP and Brunswick WWTP. Major points of interest
were accuracy, usability, reliability and pond sludge distribution. Preliminary sonar testing was
completed at the Sunset Hospital Jetty site in Nedlands to determine the capability of the sonar device
in shallow water. The site was chosen because of its shallow depth and location near UWA. Sonar
measurements were taken and compared to actual values. In addition sonar depth data was logged and
analysed for cohesiveness. Results from this exercise demonstrated that the sonar was capable of
reading water depths as small as 300mm and that the sonar measured depths progressively in
increments of 10mm.
FIGURE 3-3: INITIAL TESTING ON SWAN RIVER, NEDLANDS
28
Sonar profiling was conducted at the Harvey WWTP on the 22nd
September 2010. The pond train is
composed of two aerated facultative ponds followed by two maturation ponds. Profiling was
conducted on the 2nd
pond, as shown in figure 9-4. This facultative pond has an average depth of 1.6m
and is one of the largest ponds by area in Western Australia. Desludging was carried out in 2004;
despite this the pond contains 56% sludge by volume. The pond is aluminium dosed and mechanically
aerated. The Aluminium dosing creates a denser, more noticeable sludge layer. As such the pond was
chosen due its depth, high sludge depth variability in the horizontal and well-defined sludge layer.
FIGURE 3-4: POND LAYOUT, HARVEY WWTP
The Brunswick WWTP was also chosen as a suitable site for sonar profiling. Surveying was carried
out on the 6th of October 2010. The system was composed of a single facultative pond followed by
two maturation ponds. Sampling was conducted in pond number 2 as demonstrated in figure 9-5; the
maturation pond was approximately 1.1m deep and contained 50% sludge. The pond received no
aluminium dosing or mechanical aeration. It was predicted that this pond would have a “fluffier”
sludge that would be more difficult to measure.
29
FIGURE 3-5: POND LAYOUT, BRUNSWICK WWTP
30
3.3 METHODOLOGY
Sludge Surveying was conducted using a small inflatable dinghy equipped with an electric motor. The
sonar device was towed near to the boat whilst the display, containing the GPS receiver was located
onboard, approximately 1m away. There were two main elements to the testing; these were
comparisons of sludge judge and sonar results and full sludge surveys.
3.3.1 SLUDGE JUDGE AND SONAR COMPARISON
Sludge surveys are currently carried out by the Water Corporation using a sludge judge. These
devices are subjective and somewhat inaccurate. However, it was determined that their current use
made them the most useful proxy for actual sludge judge depth. A comparison between sludge judge
and sonar depth would provide useful information on sonar accuracy in a Wastewater Stabilisation
Pond environment. A number of points were selected randomly to take both sludge judge and sonar
depth readings. The sludge judge has an effective resolution of 50mm whilst the sonar spot readings
have a resolution of 100mm.
3.3.2 POND SLUDGE SURVEYS
Sonar Sludge Surveys were carried out at both Harvey and Brunswick Wastewater Treatment Plants.
The large size of the chosen Harvey pond meant only one section, approximately a third could be
surveyed due to time constraints. At Brunswick the entire chosen pond was surveyed. Surveying was
conducted using an inflatable dinghy with sonar device mounted and an electric motor. Data was
collected using the logging function of the sonar which compiles both GPS and depth data
continuously. Transects were taken approximately 10m apart, though due to the continuous nature of
the sonar logging, turns and cross-transect paths were also included in the data. Past sludge survey
data compiled by the Water Corporation was available for comparison.
FIGURE 3-6: MATURATION POND, BRUNSWICK
31
3.4 DATA ANALYSIS
Spot depth readings for were recorded for both the sludge judge and the sonar at specific locations.
Due to the low resolution of the data, simple comparisons were deemed to be the most useful. Results
were plotted in both bar charts and scatter plots and linear regression line analysis conducted. Sonar
Logging was conducted over both Harvey and Brunswick Wastewater Treatment Ponds. Output was
composed of Depth readings and coupled GPS co-ordinates. By subtracting water depth
measurements from average pond depth, sludge heights were calculated. Surfer software was utilised
to create three dimensional plots of the sludge surface for sonar log data. Using these plots, a sludge
volume estimate could be calculated and compared to Water Corporation sludge volume estimates
determined by sludge judge surveys. These plots were also compared and discussed in relation to
known pond information such as the presence of aerators and pond geometry.
FIGURE 3-7: TRANSDUCER MOUNTING AND SLUDGE JUDGE, BRUNSWICK POND
32
4 RESULTS
4.1 SPOT COMPARISONS
4.1.1 HARVEY POND
Results from spot comparisons between the sludge judge and the sonar device at the Harvey pond are
indicated in table 1 below. Sludge Heights range from zero to 1.14m. The measurements taken for
both techniques are reasonably similar in magnitude. It is noticeable that for each sample point, sludge
heights measured by sonar are higher than sludge judge measurements in all cases. On average sonar
sludge height is 14% higher than the sludge judge height. This data is compared side by side in Figure
10-1.
TABLE 1: COMPARISONS OF SPOT TEST DATA, HARVEY
Sample
Point
Pond Depth
(m)
Water Depth -
sonar
(m)
Sludge Height -
sonar
(m)
Sludge Height - Sludge
judge
(m)
Error (%)
1 1.6 1.5 0.1 0
2 1.6 0.8 0.8 0.66 21.212121
3 1.6 0.4 1.2 1 20
4 1.6 0.8 0.8 0.68 17.647059
5 1.6 0.6 1 0.85 17.647059
6 1.6 0.5 1.1 0.97 13.402062
7 1.6 0.4 1.2 1.14 5.2631579
8 1.7 0.7 1 0.95 5.2631579
9 1.6 0.4 1.2 1 20
10 1.7 0.7 1 0.9 11.111111
11 1.6 0.6 1 0.9 11.111111
average: 1.618181818 average: 14.265684
33
FIGURE 4-1: SLUDGE JUDGE HEIGHT COMPARISON, SLUDGE JUDGE AND SONAR, HARVEY
A scatter plot of the same data is shown in Figure 10-2. As demonstrated by the r-squared value of
0.9761, sonar and sludge judge measurements at the Harvey site are highly correlated.
FIGURE 4-2: SCATTER PLOT OF SPOT TEST DATA, REGRESSION LINE SHOWN, HARVEY
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1 2 3 4 5 6 7 8 9 10 11
Slu
dge
He
igh
t
Sample Point
Sludge Height
Sonar
Sludge Judge
R² = 0.9761
0
0.2
0.4
0.6
0.8
1
1.2
1.4
0 0.2 0.4 0.6 0.8 1 1.2
Son
ar R
ead
ing
Sludge Judge Reading
Sludge Height
Harvey
Linear (Harvey)
34
4.1.2 BRUNSWICK POND
Results from spot comparisons for Brunswick pond are shown in table 2 below. Sludge Heights vary
from 0.15m to 0.56m with a range of 0.41m. With the exception of Point 10, sonar sludge heights
were greater than sludge judge heights. The average difference was 19%. A side by side comparison
of the data is shown in figure 10-3.
TABLE 2 – COMPARISON OF SPOT TEST DATA, BRUNSWICK
Sample
Point
Pond Depth
(m)
Water Depth - sonar
(m)
Sludge Height -
sonar
(m)
Sludge Height - Sludge
judge
(m)
Error
(%)
1 1.2 0.7 0.5 0.45 11.11111
2 1.15 0.7 0.45 0.4 12.5
3 1.13 0.7 0.43 0.36 19.44444
4 1.19 0.6 0.59 0.51 15.68627
5 1.23 0.5 0.73 0.56 30.35714
6 1.17 0.6 0.57 0.42 35.71429
7 1.12 0.8 0.32 0.27 18.51852
8 1.15 0.8 0.35 0.34 2.941176
9 1.16 0.8 0.36 0.3 20
10 1.14 0.9 0.24 0.26 -7.69230
11 1.05 0.8 0.25 0.15 66.66667
12 1.01 0.7 0.31 0.22 40.90909
13 1.18 0.7 0.48 0.47 2.12766
14 1.1 0.7 0.4 0.36 11.11111
15 1.16 0.8 0.36 0.32 12.5
average: 1.142666667 average: 19.45968
FIGURE 4-3: SLUDGE JUDGE HEIGHT COMPARISON, SLUDGE JUDGE AND SONAR, BRUNSWICK
00.10.20.30.40.50.60.70.8
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Slu
dge
He
igh
t
Sample Point
Sludge Height
Sonar
Sludge Judge
35
A scatter plot of the same data is shown below in Figure 10-4. The r-squared value of 0.8709 indicates
that sonar and sludge judge measurements are highly correlated at the Brunswick site.
FIGURE 4-4: SCATTER PLOT OF SPOT TEST DATA, REGRESSION LINE SHOWN, BRUNSWICK
4.2 SONAR LOGGING
Sonar logging data was compiled for both Harvey and Brunswick ponds. At Harvey, the middle half
of the pond was profiled whilst the entirety of the pond at Brunswick was profiled. Many thousands of
readings were recorded at both Harvey and Brunswick ponds. Results were initially screened for false
readings, including water depths equal to zero, and water depths greater than the maximum pond
depth. All water depths equal to zero where identified as false readings by the sonar software. Both
surveys used an inflatable dinghy equipped with small electric motor, taking 20 minutes at Harvey
and 15 minutes at Brunswick. In the Brunswick pond readings were taken in conditions containing
algae and areas of noticeably low density sludge. Winds, wave action and boat speed had a noticeable
effect on the transducer mounting stability.
At Harvey there were approximately 28,500 data points. Of these, 3501 were labelled false readings
and identified by a zero. There were an additional 73 water depth readings that were greater than the
maximum pond depth of 1.7m, ranging from 2.2m to 3.5m and occurring at only three co-ordinates.
Of the remaining 25,000 data points, there were many possessing duplicate GPS co-ordinates, due to
the low boat speeds and low resolution of the GPS. Approximately 25,000 data points were reduced to
532 independent points by dismissing duplicate data. Brunswick sonar data possessed no readings of
zero or readings exceeding the maximum pond depth of 1.2m. Of the 20,700 data points, 416
independent readings were obtained.
Figure 10-5 shows a screenshot of the sonar display taken whist sonar logging was taking place. As
can be seen there is a clear and progressive response to changes in sludge depth as the boat is in
R² = 0.8709
00.10.20.30.40.50.60.70.8
0 0.1 0.2 0.3 0.4 0.5 0.6
Son
ar R
ead
ing
Sludge Judge Reading
Sludge Height
Brunswick
Linear (Brunswick)
36
motion. The Figure contains data from approximately the last 5m of transect. Data shown in appendix
A also demonstrates the clear response to incremental changes in sludge height whilst in motion.
FIGURE 4-5: DEPTH FEED ON SONAR DISPLAY, IDENTIFIED BY WHITE LINE
4.3 SLUDGE VOLUMES
Past sludge survey data measured by the Water Corporation were also available, though of a lower
resolution. The sludge survey of the Harvey Pond contained 116 data points, was conducted in April
2010 and took 120 minutes. As discussed earlier the lack of perimeter data and the difficulties
involved in grid sampling using line of sight of markers on the bank make this data difficult to
compare. However, a section of data was selected that most closely corresponded to the GPS
boundaries of the area sampled by sonar. This section contained 78 grid points and covered a
rectangular area of 5040 m2.
One method of comparing sludge height results for both sonar and sludge judge surveys is to calculate
the total volume of sludge contained in the ponds. Surfer 8.0 was used for this purpose, integrating
using Simpson‟s 3/8 rule. Sludge volume estimates recorded by the Water Corporation by averaging
sludge judge survey measurements are also shown. Sonar estimated volume is 5.6% lower than
integrated sludge judge data and 7.4% lower than the averaged sludge judge volume.
37
TABLE 3 – COMPARISON OF SONAR AND SLUDGE DATA, BOTH INTEGRATED AND AVERAGED DEPTH VOLUME, HARVEY POND
Harvey Area (m2) Volume (m
3) Volume, equated to
sonar survey area (m3)
Sonar Survey 5928 4193 4193
Sludge Judge
Survey
5040 3775 4440
Official Estimate 10920 8346 4530
Sonar sludge surveys of the Brunswick pond encompassed the entire pond and obtained 416
independent data points. The three dimensional sludge profile as measured by sonar was plotted using
Surfer 8.0 and is shown below. Sludge Surveys of the pond using a sludge judge were conducted in
April 2010 took 30 minutes and contained only 20 data points, as is the case with the Harvey pond,
sludge levels were not recorded within 10m of any pond bank. For this reason the bounded area was
only 1200m2. Sludge Volumes were calculated as for the Harvey pond. The sonar survey volume was
thus 24% lower than the integrated sludge judge volume and 7.5% less than the averaged depth
estimate of volume.
TABLE 4 - COMPARISON OF SONAR AND SLUDGE DATA, BOTH INTEGRATED AND AVERAGED DEPTH VOLUME, BRUNSWICK POND
Brunswick Area (m2) Volume (m
3) Volume, equated to
sonar survey area (m3)
Sonar Survey 3180 1433 1433
Sludge Judge
Survey
1200 713 1890
Official Estimate 3068 1489 1543
38
5 DISCUSSION
5.1 SPOT TESTS
It is evident from this study that sonar provides effective measurement of sludge heights in the
Wastewater Treatments ponds tested. Spot testing demonstrated that sludge judge and sonar readings
were highly correlated. The r-squared value of correlation between sludge judge and sonar data was
0.9761 at Harvey and 0.8701 at Brunswick. To conserve time, display readings of sonar depth were
taken, which possesses a much lower resolution of approximately 100mm compared to that of sonar
log data of approximately 10mm, sludge judge measurements possess a resolution of approximately
100mm. The range of sludge heights at Brunswick was 400mm versus 1140mm at Harvey. The
smaller scale of sludge heights at Brunswick is thus expected to have magnified the scale of
inaccuracies involved in measurement compared to the Harvey pond.
At both Harvey and Brunswick ponds there was a tendency for sonar sludge height readings to be
higher than that of the corresponding sludge judge reading. This difference was similar at both Harvey
and Brunswick ponds, averaging 14% and 19% respectively. Discrepancies at the Harvey pond ranged
from 5% to 21%. At Brunswick the differences ranged from -7% to 67%. The larger variation may
again be due to the magnification of inaccuracies inherent in readings gained from Brunswick due to
the low range of sludge heights present. The presence of a more complex sludge-water interface may
also have influenced measurement in comparison to the more distinct sludge layer at Harvey pond.
5.2 SLUDGE SURVEYS
Sonar surveys achieved significant efficiency gains compared to sludge judge survey techniques.
Profiling of Harvey and Brunswick Ponds took 66.66% and 50% less time respectively than that taken
for sludge judge surveys. Sonar surveying was far more extensive than that of the historical sludge
judge surveying data. At Harvey 700% more data points were taken, whilst at Brunswick 2000% more
data points were recorded. The resolution of logged sonar data was significantly higher than that of
historical sludge survey data. Sonar survey data was highly sensitive to changes in sludge height;
possessing a resolution of below 10mm whilst a sludge judge has a resolution of approximately
50mm. Transect data from the sonar display demonstrates that the sonar device is capable of reading
small changes in sludge depth progressively and in high detail. Data contained in Appendix A reveals
that almost all readings are within 10mm and typically 3mm of consecutive readings. This shows that
the sonar is effectively identifying all sludge height differences above 3mm.
39
TABLE 5 – SURVEY COMPARISON, SLUDGE JUDGE AND SONAR
Time taken Data Points Resolution
Sludge Judge Survey – Harvey (Half Pond)
60 minutes 78 100mm
Sonar Survey – Harvey (Half Pond)
20 minutes 532 10mm
Sludge Judge Survey – Brunswick
30 minutes 20 100mm
Sonar Survey – Brunswick
15 minutes 416 10mm
Pond conditions had a significant effect on the stability of the transducer mounting. At both ponds
there was a reasonably strong wind and wave action was evident. At Brunswick a thicker foam pad
was used for flotation to increase stability. At both ponds the float was rocked heavily by wave action.
Due to float stability issues, boat speeds were kept quite low, typically below 3km/hr. Speeds above
this had the effect of generating wash over the float which caused submergence. The maintenance of
such a low speed was the major impedance to more efficient sampling. It is likely that a more stable
transducer mounting would allow higher speeds and more efficient sampling.
FIGURE 5-1 – SONAR PROFILING, HARVEY POND
40
Screening of Harvey Sonar data identified 3500 zero readings and 73 water depths exceeding the
maximum pond depth. No false readings were recorded for the Brunswick pond. This can largely be
attributed to the use of a more stable float and a more vigilant approach to boat speeds. All 3500 false
zero readings were self-identified by the sonar software. Of the 73 depth readings that exceeded water
depth, all were above 2.2m versus a pond depth of 1.6m, occurred at only 3 locations within the pond
and occurred in two approximately 5 second periods. For these reason it is likely that they occurred
due to float stability issues.
5.3 VOLUME ANALYSIS
Sonar sludge survey data was plotted in three dimensions to give an indication of sludge distribution
at both ponds studied. Surfer 8.0 software was utilised for the purpose as it also allowed the
calculation of sludge volumes by integration using Simpson‟s 3/8 rule. Results demonstrated that
sonar calculated sludge volume was 7.5% and 7.4% lower than official estimates calculated using
depth average sludge judge survey data for Harvey and Brunswick ponds respectively. It is interesting
to note that despite generally overestimating sludge heights in spot readings, sonar measurements
have slightly underestimated sludge volumes. Nevertheless, the relatively similar readings obtained by
sludge judge and sonar survey volume analysis would indicate that sonar provided an effective
measurement of sludge volume at the two ponds investigated.
FIGURE 5-2 – SLUDGE DISTRIBUTION, MEASURED BY SONAR SURVEY, HARVEY
41
5.4 POND ANALYSIS
There are a number of factors that are known to affect sludge distribution. An analysis of sludge
distributions at both Harvey and Brunswick demonstrates a number of factors identified in the
literature that are involved. The section of pond profiled at Harvey contained one of the pond‟s two
aerators, located at approximately (0,40). It is expected that the velocity generated by the aerator
would have an effect on sludge distribution. As can be seen in Figure 11-2 sludge heights are low near
this feature as the aerator has mobilised sludge and formed a highly variable sludge distribution. It is
also apparent that there is a deeper channel throughout the middle of the pond, travelling parallel to
the x-axis, from the inlet at the right to the outlet. The Brunswick pond profile shown in Figure 11-3
also exhibits a high level of variability. Sludge heights are higher in the corners and there is an
obvious spike in sludge levels at the inlet. The lowest sludge levels are located near the outlet. In this
pond there is a clear sludge gradient increasing from left to right.
FIGURE 5-3 – SLUDGE DISTRIBUTION, MEASURED BY SONAR SURVEY, BRUNSWICK
Sludge distribution is highly variable in both ponds profiled. In the Harvey pond water heights vary
from approximately 1.5m to 0.5m, a three-fold change in channel depth. At the Brunswick pond water
depth varies from 0.4m to 0.8m indicating a twofold difference in channel depth. This large scale
variation in depth will have a significant effect on the hydrodynamics of the pond. The occurrence of
a channel at Harvey and a sludge gradient at Brunswick is of a particular note. It is likely tracer test
analysis would be required to ascertain the existence of short circuiting.
42
6 CONCLUSIONS
The field work conducted in this study has demonstrated that the Lowrance HDS-5 sonar provided
accurate and reliable measurement of sludge heights in the two ponds tested. Correlation of sonar and
sludge judge measurements was high; r-squared values of 0.9761 and 0.8709 were recorded for
Harvey and Brunswick ponds respectively. There is a clear tendency for sonar to overestimate sludge
heights by a factor of approximately 15%. The use of an integration based approach to calculate
sludge volume, based on sonar readings gave results comparable to those obtained by conventional
volume measurements. For the two ponds profiled, volume estimates were 7.4% and 7.5% lower than
those on file by the water corporation. Sonar surveys were 50% quicker and approximately ten times
more detailed than conventional sludge judge surveys.
The use of a floating mount for the transducer was largely satisfactory. False readings that occurred at
Harvey Pond were largely attributed to float instability, caused by wind and wave action. For this
reason, it is advisable the future studies attempt to use a more stable flotation platform. The overall
success of this system would suggest there is significant scope for development of an autonomous or
remote control device using such a platform.
The successful replication of both sludge heights and pond sludge volumes demonstrates that sonar is
a valid method of calculating sludge height, volume and distribution. The device was user-friendly
and offered robust measurement of sludge levels in the ponds tested. There is significant scope for the
application of such methods to the sludge pond management and research.
Profiling of ponds using sonar has demonstrated that horizontal sludge distribution is highly variable
in Western Australian Water Stabilisation Ponds. Effective channel height was found to differ
dramatically throughout both ponds due to sludge distribution, ranging by a factor of 2 in the
Brunswick pond and a factor of 3 in the Harvey pond. It is likely that such a change would have an
effect on pond hydrodynamics.
43
7 RECOMMENDATIONS
The research contained in this study forms an introduction to the use of sonar for measuring sludge
levels of Wastewater Stabilisation Ponds in Western Australia. As such, there is a range of
opportunities for further study. The following recommendations identify further use of the Lowrance
HDS-5 or other depth sounder based system.
7.1.1 AUTONOMOUS VEHICLE
The ability of the sonar device to continuously measure without human interaction makes it a prime
candidate for an autonomous or remote controlled sludge measuring device. There is significant
potential for such systems to provide significant efficiency gains, reduced labour requirements and
eliminate the need for manned craft and related safety issues. Craft design, stability, propulsion,
manoeuvrability and the ability to profile the pond effectively whilst sustaining wind and wave action
are the main problems to be solved.
7.1.2 POND HYDRODYNAMICS
There is a complex inter-relationship between pond hydrodynamics and sludge distribution. This
study has established that Western Australian ponds have a highly uneven sludge distribution and
demonstrated an effective technique for the characterisation of such a distribution. Tracer tests have
not been carried out in Western Australian Wastewater Stabilisation Ponds and are necessary to
characterise pond hydrodynamics, performance and dead zones and short circuits. Development and
application of tracer test methods is an essential part of further research.
7.1.3 FURTHER USE OF SONAR
Sonar is a robust technology that is not limited to discrete water depth measurements. More complex
systems may be able to provide a more thorough penetration of the sludge layer. Possible output
includes active measurement of both top and bottom of sludge measurements and identification of
sludge densities based on signal response. It is likely such approaches will require more powerful
sonar systems that can be more broadly programmed.
7.1.4 DETAILED VOLUME ANALYSIS
Approximate estimates of volume in this study demonstrated integrated sonar volumes to be
approximately 7.5% lower than sludge estimates calculated using averaged sludge judge depths. In
contrast, sonar spot readings were found to be approximately 15% greater than sludge judge readings.
This could be due to difficulties in defining boundaries for both sludge judge and sonar data. A more
thorough investigation of these discrepancies is required.
44
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9 APPENDIX A – SAMPLE OF SONAR LOG OUTPUT
48
10 APPENDIX B – LOWRANCE HDS-5 SPECIFICATIONS
49
50
11 APPENDIX C – HAWK SONAR SLUDGE LEVEL MONITORING SYSTEM