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Concept Understand your System Planning & Process Tools Implementation Tools Specific Topics Background

Home Implementation tools Wastewater Treatment Hardware Sludge Treatment Settling / Thickening Ponds

Implementation Tools

Introduction

Water Sources

Water Purification

Water Distribution

Water Use

Wastewater Collection

Wastewater

Treatment

Reuse and Recharge

Compiled by:

Overview of potential, modest-cost options for faecal sludge

treatment including pre-treatment of faecal sludge (solid-liquid

separation, left) and secondary treatment options (for thickened

sludge and liquid fraction respectively). Source: MONTANGERO &

STRAUSS (2004)

Settling/Thickening Ponds

Dorothee Spuhler (seecon international

gmbh)

Executive SummaryThickening ponds are designed to separate solid and liquid parts of faecal

sludge in order to process each other separately (e.g. in waste stabilisation

ponds, drying beds or by composting). They are designed as simple ponds in

which faecal sludge is filled and solids settle to the ground. Fresh sludge

(e.g. from latrines or unsewered public toilets) will require long retention

times during which settled sludge will undergo further stabilisation by

anaerobic digestion. Partially digested sludge with little organic matter and

easily settable solids (e.g. from septic tanks, anaerobic digesters etc.) will

require retention times of some hours only and settled sludge is desludged

more frequently. Short-retention-time thickening ponds are often called

settling tanks, while long-retention and stabilisation-thickening ponds are

called sedimentation or anaerobic ponds.

In Out

Faecal Sludge

Compost (for further off-site composting),

Soil, Treated Water, Fertigation Water or

Water requiering further treatment

All wet wastewater collection or treatment units produce sludge of variable consistency.

Generally, such sludge does not have a higher solids content than 2 to 10 % (SASSE & BORDA

1998). Thickening ponds or tanks are designed as a pre-treatment for faecal sludge to

separate the solid from the liquid part of the sludge in order to treat each one separately. The

supernatant liquid can be treated either in waste stabilisation ponds (optionally together with

other wastewater), mounds or just be infiltrated into the ground (e.g. greywater infiltration in

soak pits, or leach fields) depending on the degree of pollution. The thickened sludge occupies

much less volume and can be more easily transported. However, it can still contain pathogens

and needs to be further treated before reuse or disposal. Typically, sludge from thickening

ponds is transformed into biosolids on drying beds or co-composted together with organic solid

waste (co-composting small or large-scale). In some cases, it may also be spread out in thin

layers on the ground.

Sludge from on-site

collection systems such as

latrines or unsewered public

toilets (or activated sludge

systems) has not yet

undergone significant

degradation and still contains

a high amount of organics

(high-strength sludge,

HEINSS et al. 1998). This

“fresh” sludge is treated in

thickening ponds, which

provide a long liquid

retention time (hydraulic

retention time, HRT) of

several days or a few weeks

(so-called sedimentation

ponds), and hence, also allow for anaerobic degradation of organics (STRAUSS &

MONTANGERO 2002).

Related Topics

Sustainable Sanitation

Grasp what sustainablesanitation actually means andhow the water and thenutrient cycle are interlinked:

Sustainable SanitationThe Nutrient CycleThe Water Cycle

Sludge Drying

Alternative systems for sludgedrying are:

Drying BedsMechanical Dewatering

Precedent Steps

Almost every sludge can bedried in drying beds. Typicallythey are combined with:

Septic TanksBiogas SettlerAnaerobic Baffled ReactorAnaerobic Digestion (Small-scale)Anaerobic Digestion (Large-scale)Anaerobic Digestion (OrganicWaste)

Further Steps

The effluent from drying bedscan be used for fertigation,while the sludge may be usedas soil amendement(optionally after a furthercomposting step):

Fertigation(Co-)composting (Small-scale)(Co-)composting (Large-scale)Use of Compost

Search

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Non-mechanised settling thickening tanks and

sedimentation/anaerobic ponds for solid-liquid separation

(schematic). Source: STRAUSS & MONTANGERO (2002)

Different types of settling tanks. Source: KLINGEL et al. (2002)

Circular primary settling tank at Haran Al-Awamied treatment

plant. Source: MOHAMMED et al. (2009)

Sludge from septic tanks, or anaerobic digesters, biogas settlers, etc. has already been partly

mineralised and is much more easily settable (low-strength, HEINSS et al. 1998). Such “old”

sludge thickened in so-called settling tanks, which provide liquid retention times of a few

hours only, just enough to ensure quiescent settling of settable solids (STRAUSS &

MONTANGERO 2002).

Basic Design Principles

Both sedimentation ponds

and settling tanks are

designed based on the

storage volume required for

a desired depth and quantity

of accumulating solids

(STRAUSS & MONTANGERO

2002) in a given time

(depending on the frequency

of desludging). The specific

volume occupied by

separated solids may vary

from 0.02 (thin sludge) up to 0.15 (high-strength sludge) m3/m3 of raw sludge depending on

the type and composition and on the time allowed for solids consolidation and thickening

(HEINSS et al. 1998; STRAUSS & MONTANGERO 2002).

Sedimentation ponds are designed similar to anaerobic ponds in waste stabilisation ponds with

a sufficient volume for sludge accumulation over a long period (KLINGEL et al. 2002). The

anaerobic digestion process, which stabilises the fresh sludge, causes gas production

(MONTANGERO & STRAUSS 2004). When the gas bubbles rise, this can further limit the sludge

settling as solids are also pushed to the surface, but the produced biogas can be collected and

used for heating or cooking (i.e. by covering the pond with a gastight canvas). Sedimentation

ponds can be operated in batch and continuous mode and sludge is removed only once every 8

to 12 month (STRAUSS & MONTANGENRO 2002). Therefore they require generally more space

than simple settling ponds. The organic load of sedimentation or anaerobic ponds vary from

250 to 350 gBOD/m3 per day and the volume of accumulated sludge per incoming solids may

vary between 0.8 to 2 L/Kg TS (KLINGEL et al. 2002).

Settling tanks need not only

sufficient volume for sludge

accumulation, but also

sufficient depth of the liquid

column (> 1.5 m) in order to

allow a fast and good settling

(KLINGEL et al. 2002). The

loading/resting cycle in

settling tanks is much

shorter than in

sedimentation ponds and

sludge is removed about

every 2 to 4 months

(STRAUSS

& MONTANGERO 2002). The tanks should be equipped with baffles to maintain hydraulic

conditions favourable to good settling and to retain floating scum (KLINGEL et al. 2002).

For design, accumulating sludge volumes per incoming solids load of 5 to 9 L/kg TS can be

considered. However, the consistency and concentration of faecal sludge is highly variable

(depending on storage duration, tank emptying technology and pattern, performance of

treatment of collection unit, added matters, temperature, intrusion of ground- or surface

waters, etc. MONTANGERO & STRAUSS 2004) and treatment systems should be designed on a

case-to-case basis (EAWAG/SANDEC 2008).

Whether a sedimentation

pond or a settling tank is

used depends on the

consistency of the sludge

(e.g. fresh vs. old sludge),

the space available and the

means of sludge removal.

When only little space is

available, fresh toilet sludge

may be diluted with more

stabilised sludge in order to

be treatable in settling tanks

(KLINGEL et al. 2002). It is

also important to consider

that the quantities of solids,

which are produced and have

to be removed in settling

ponds, are much smaller than

the mass of solid to be

removed and handled from sedimentation ponds (EAWAG/SANDEC 2008). But frequent sludge

removal can also become expensive for operators.

Desludging (sludge emptying) can be done either after removal of the liquid column and a

period of drying manually by front loaders or mechanically through draw-off pipes without

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removing the supernatant. In-pond drying is more common for sedimentation ponds. In this

case, at least two parallel units need to be provided in order to assure continuous operation

(KLINGEL et al. 2002; STRAUSS & MONTANGERO 2002).

The removal rates, which may be expected in sedimentation ponds and settling tanks are

(STRAUSS & MONTANGERO 2002):

Sedimentation Pond Settling Tank

Suspended Solids 60 % > 95 %

BOD and COD (unfiltered) 30 to 50 % 70 to 95 %

BOD (filtered) 18 % 45 %

Costs Considerations

Pond systems are relatively low cost, as they do not require much maintenance and no energy.

However, large surface areas must be available and they need to be designed and operated by

skilled staff. Depending on the frequency of desludging, emptying may become expensive or

labour intensive.

Operation and Maintenance

The thickened sludge can be removed either mechanically or manually (after removing the

liquid column) every 8 to 12 or 2 to 4 months for sedimentation ponds or settling tanks

respectively (STRAUSS & MONTANGERO 2002).

Maintenance includes regular desludging and cleaning of the discharging areas (to reduce flies

and odours) and grit, sand and solid waste that are discharged along with the sludge must be

removed.

Trained staff for operation and maintenance is required to ensure proper functioning (TILLEY

et al. 2008).

Health Aspects

Faecal sludge is highly pathogenic and should be handled with care. Thickened sludge is also

infectious, although it is easier to handle and less prone to splashing and spraying (TILLEY et

al. 2008). The supernatant liquid can be polluted with microbes as well and needs to be

correctly treated or infiltrated. Open ponds may cause odour and should therefore be installed

away from densely populated areas.

At a Glance

Working Principle

Thickening ponds are simple ponds designed

for the solid-liquid separation of faecal

sludge. There are two types of thickening

ponds: Sedimentation ponds, which require

long retention times and hence also allow for

anaerobic degradation (similar to anaerobic

ponds in WSP); and settling ponds as a

primary treatment of already partly digested

sludge (e.g. from septic tanks or anaerobic

digesters) providing only short retention

times.

Capacity/Adequacy

Centralised; Requires large land-surfaces and

can cause odour (should be installed far from

communities); Is suited for almost any hot or

temperate climate.

Performance

Depends strongly on the local climate (rain,

runoff); Suspended solids can be removed up

to 60 % (sedimentation pond, fresh sludge)

and 95%, respectively (in settling tanks)

from the supernatant and concentrated in

the thickened sludge. Sludge and effluent

are generally still pathogenic and need

further treatment before being disposed or

reused in agriculture.

CostsModerate investment costs and low

operation costs.

Self-help Compatibility

Can be constructed with locally available

material, but requires expert design and

skilled operation.

O&M

Application of sludge, desludging, cleaning

of surrounding and incoming sludge from

solid wastes. Desludging every 2 to 4 or 8 to

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12 months for settling tanks or

sedimentation ponds respectively.

ReliabilityHigh, if the area is kept dry and hot or

temperate

Main strength Low-tech and no requirement of energy.

Main weaknessRequires space and desludging as well as

further treatment of both liquids and solids.

ApplicabilitySedimentation ponds can be used for partly stabilised faecal sludge such as sludge from septic

tanks, anaerobic baffled reactors, large or small scale anaerobic digesters, biogas

settlers, etc. They are not suited for fresh faecal sludge.

Settling tanks can be used as first step in fresh faecal sludge treatment (e.g. from latrines or

unsewered public toilets). Often, they are the first stage in a sequence of stabilisation ponds

(see also anaerobic ponds in waste stabilisation ponds).

The sludge is not hygienic and requires further treatment before reuse or disposal (drying,

composting) and the supernatant liquid needs to be infiltrated or further treated depending the

pollution/pathogen removal performance of the thickening ponds.

Thickening in ponds or tanks generally requires the availability of large areas of land, thus they

are appropriate where there is inexpensive, available space that is far from homes and

businesses.

Thickening ponds are low-cost and can be installed in most hot and temperate climates.

AdvantagesNo energy required

Relatively low cost

Potential for local job creation

Can be built with locally available material

DisadvantagesRequires large space

Requires expert design and skilled operation

Odours and flies can occur

In the case of frequent desludging, front-end loaders maybe required

References

EAWAG/SANDEC (Editor) (2008): Faecal Sludge Management. Lecture Notes. Duebendorf:

Swiss Federal Institute of Aquatic Science (EAWAG), Department of Water and Sanitation in

Developing Countries (SANDEC). URL [Accessed: 23.05.2012]. PDF

EAWAG (Editor); SANDEC (Editor) (2008): Fecal Sludge Management. Presentation.

Duebendorf: Swiss Federal Institute of Aquatic Science (EAWAG), Department of Water and

Sanitation in Developing Countries (SANDEC). URL [Accessed: 18.02.2011].

HEINSS, U.; LARMIE, S.A.; STRAUSS, M. (1998): Solids Separation and Pond Systems for the

Treatment of Faecal Sludges in the Tropics . Lessons Learnt and Recommendations for

Preliminary Design. . Duebendorf: Swiss Federal Institute of Aquatic Science (EAWAG),

Department of Water and Sanitation in Developing Countries (SANDEC). URL [Accessed:

12.04.2010]. PDF

KLINGEL, F. ; MONTANGERO, A. ; KONE, M.; STRAUSS, M. (2002): Fecal Sludge Management in

Developing Countries - A Planning Manual. Duebendorf: Swiss Federal Institute for

Environmental Science (EAWAG). URL [Accessed: 23.06.2010]. PDF

MOHAMED, A. ; KLINGEL, F.; BRACKEN, P.; WERNER, C. (2009): Effluent reuse from

constructed wetland system Haran Al-Awamied, Syria. Eschborn: Sustainable Sanitation

Alliance (SuSanA) . URL [Accessed: 26.01.2011]. PDF

MONTANGERO, A.; STRAUSS, M. (2004): Faecal Sludge Treatment. Duebendorf: Swiss Federal

Institute of Aquatic Science (EAWAG), Department of Water and Sanitation in Developing

Countries (SANDEC). URL [Accessed: 10.06.2010]. PDF

STRAUSS, M.; MONTANGERO, A. (2002): FS Management – Review of Practices, Problems and

Initiatives. London and Duebendorf: DFID Project R8056, Capacity Building for Effective

Decentralised Wastewater Management, Swiss Federal Institute of Aquatic Science (EAWAG),

Department of Water and Sanitation in Developing Countries (SANDEC). URL [Accessed:

24.05.2012]. PDF

STRAUSS, M.; DRESCHER, S.; ZURBRUEGG, C.; MONTANGERO, A.; OLUFUNKE, C.; DRECHSEL,

P. (2003): Co-composting of Faecal Sludge and Municipal Organic Waste. Swiss Federal

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Institute of Aquatic Science (EAWAG), Department of Water and Sanitation in Developing

Countries (SANDEC) and International Water Management Institute (IWMI). URL [Accessed:

23.06.2010]. PDF

STRAUSS, M.; BARREIRO, W.C.; STEINER, M.; MENSAH, A.; JEULAND, M.; BOLOMEY, S.;

MONTANGERO, A.; KONE, D. (2003): Urban Excreta Management - Situation, Challenges, and

Promising Solutions. In: IWA Asia-Pacific Regional Conference Bangkok, Thailand. URL

[Accessed: 23.06.2010]. PDF

TILLEY, E.; LUETHI, C.; MOREL, A.; ZURBRUEGG, C.; SCHERTENLEIB, R. (2008): Compendium

of Sanitation Systems and Technologies. Duebendorf, Switzerland: Swiss Federal Institute of

Aquatic Science and Technology (EAWAG) and. URL [Accessed: 15.02.2010]. PDF

See document in FRENCH

UNEP (Editor); Murdoch University (Editor) (2004): Environmentally sound technologies in

wastewater treatment for the implementation of the UNEP/GPA "Guidelines on Municipal

Wastewater Management". The Hague: United Nations Environment Programme Global

Programme of Action (UNEP/GPA), Coordination Office. PDF

SASSE, L. ; BORDA (Editor) (1998): DEWATS . Decentralised Wastewater Treatment in

Developing Countries. Bremen: Bremen Overseas Research and Development Association

(BORDA). PDF

Further Readings

ECOSAN CLUB (Editor) (2012): Faecal Sludge Management. Vienna:

Ecosan Club. URL [Accessed: 16.10.2012]. PDF

This issue presents studies from different regions (Bangladesh,

Cameroon, Burkina Faso) that mainly show the non-existence of

faecal sludge management. Additionally, the last paper describes a

new technological solution (LaDePa) for producing hygienically safe

organic fertiliser from sludge from ventilated improved pit toilets

(VIPs).

KONE, D.; STRAUSS, M. (2004): Low-cost Options for Treating

Faecal Sludges (FS) in Developing Countries - Challenges and

Performance. Duebendorf: Water and Sanitation in Developing

Countries (SANDEC), Swiss Federal Institute for Environmental

Science (EAWAG). URL [Accessed: 23.06.2010]. PDF

This article analyses and discusses the performances of low-cost

technology for treating faecal sludges in developing countries. It

shows that where septic tanks are the predominant type of on-site

sanitation installations, septage is the only or predominant type of

faecal sludge generated. It also shows that constructed wetlands,

settling tanks/ponds, or unplanted drying beds might prove

suitable as a pre-treatment.

MONTANGERO, A.; STRAUSS, M. (2004): Faecal Sludge Treatment.

Duebendorf: Swiss Federal Institute of Aquatic Science (EAWAG),

Department of Water and Sanitation in Developing Countries

(SANDEC). URL [Accessed: 10.06.2010]. PDF

This document reviews current practices of faecal sludge

management and treatment.

STRAUSS, M.; DRESCHER, S.; ZURBRUEGG, C.; MONTANGERO, A.;

OLUFUNKE, C.; DRECHSEL, P. (2003): Co-composting of Faecal

Sludge and Municipal Organic Waste. Swiss Federal Institute of

Aquatic Science (EAWAG), Department of Water and Sanitation in

Developing Countries (SANDEC) and International Water

Management Institute (IWMI). URL [Accessed: 23.06.2010]. PDF

The document gives an overview on the combined composting of

(faecal) sludges and organic solid waste based on a pilot project in

Kumasi, Ghana. Results of the investigation should help the city’s

waste management department to develop its biosolids

management strategy and enable the project team to develop

guidelines for planners and engineers on the option of co-

composting.

STRAUSS, M.; LARMIE, S.A.; HEINSS, U. (1997): Treatment of

sludges from on-site sanitation — Low-cost options. In: Water

Science and Technology 6, 129-136. URL [Accessed: 23.06.2010].

PDF

Published in 1997, this article gives an overview on current

literature-based knowledge regarding faecal sludge treatment

along with results and conclusions from field research.

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TILLEY, E.; LUETHI, C.; MOREL, A.; ZURBRUEGG, C.;

SCHERTENLEIB, R. (2008): Compendium of Sanitation Systems and

Technologies. Duebendorf, Switzerland: Swiss Federal Institute of

Aquatic Science and Technology (EAWAG) and. URL [Accessed:

15.02.2010]. PDF

This compendium gives a systematic overview on different

sanitation systems and technologies and describes a wide range of

available low-cost sanitation technologies.

See document in FRENCH

ANDREOLI, C.V. (Editor); SPERLING, M. von (Editor); FERNANDES,

F. (Editor) (2007): Sludge Treatment and Disposal. London:

International Water Association (IWA) Publishing. URL [Accessed:

01.11.2013]. PDF

Sludge Treatment and Disposal is the sixth volume in the series

Biological Wastewater Treatment. The book covers in a clear and

informative way the sludge characteristics, production, treatment

(thickening, dewatering, stabilisation, pathogens removal) and

disposal (land application for agricultural purposes, sanitary

landfills, landfarming and other methods). Environmental and

public health issues are also fully described.

Case Studies

HEINSS, U.; LARMIE, S.A.; STRAUSS, M. (1994): SOS - Management

of Sludges from On-Site Sanitation. Characteristics of Faecal

Sludges and their Solids-Liquid Separation. Duebendorf and Accra:

Swiss Federal Institute of Aquatic Science (EAWAG). PDF

This document gives an overview on the characteristics of different

sludges as well as monitoring results and recommendations for

design of solid-liquid separation. It is based on a field report.

MOHAMED, A. ; KLINGEL, F.; BRACKEN, P.; WERNER, C. (2009):

Effluent reuse from constructed wetland system Haran Al-Awamied,

Syria. Eschborn: Sustainable Sanitation Alliance (SuSanA) . URL

[Accessed: 26.01.2011]. PDF

In the village of Haran Al-Awamied a gravity sewer system already

existed and waste water was collected for irrigation without any

treatment. GTZ and MHC (Syrian Ministry of Housing and

Construction) initiated a project for a new ecological treatment

plant (settling tank and a vertical flow CW).

Awareness Raising Material

STRAUSS, M.; BARREIRO, W.C.; STEINER, M.; MENSAH, A.;

JEULAND, M.; BOLOMEY, S.; MONTANGERO, A.; KONE, D. (2003):

Urban Excreta Management - Situation, Challenges, and Promising

Solutions. In: IWA Asia-Pacific Regional Conference Bangkok,

Thailand. URL [Accessed: 23.06.2010]. PDF

The objective of this paper is to render planners, decision makers,

and consultants aware that faecal sludge management (FSM)

should form an integral part of the urban development planning

process. For this, three illustrative cases are presented, based on

which an array of measures or tools, as well as

institutional/regulatory, financial/economic, and technical aspects

are discussed.

Training Material

EAWAG/SANDEC (Editor) (2008): Faecal Sludge Management.

Lecture Notes. Duebendorf: Swiss Federal Institute of Aquatic

Science (EAWAG), Department of Water and Sanitation in

Developing Countries (SANDEC). URL [Accessed: 23.05.2012]. PDF

This module pays special attention to the haulage, treatment and

reuse or disposal of faecal sludge. It covers both technical and non-

technical (socio-cultural, economic, political etc.) aspects and

provides practical information on design, financing and planning of

faecal sludge treatment plants.

EAWAG (Editor); SANDEC (Editor) (2008): Fecal Sludge

Management. Presentation. Duebendorf: Swiss Federal Institute of

Aquatic Science (EAWAG), Department of Water and Sanitation in

Developing Countries (SANDEC). URL [Accessed: 18.02.2011].

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KLINGEL, F. ; MONTANGERO, A. ; KONE, M.; STRAUSS, M. (2002):

Fecal Sludge Management in Developing Countries - A Planning

Manual. Duebendorf: Swiss Federal Institute for Environmental

Science (EAWAG). URL [Accessed: 23.06.2010]. PDF

This manual is a first approach to provide guidance on strategic

planning of faecal sludge management. The study took place in the

City of Nam Dinh, in Vietnam. The main principles for strategic

sanitation planning have been adopted from the guide “Strategic

Planning for Municipal Planning” from GHK Research and Training

Ltd.

STRAUSS, M.; MONTANGERO, A. (2002): FS Management – Review

of Practices, Problems and Initiatives. London and Duebendorf:

DFID Project R8056, Capacity Building for Effective Decentralised

Wastewater Management, Swiss Federal Institute of Aquatic

Science (EAWAG), Department of Water and Sanitation in

Developing Countries (SANDEC). URL [Accessed: 24.05.2012]. PDF

A study on management and institutional aspects regarding the

challenges and possible improvements in managing faecal sludge.

Important Weblinkshttp://www.eawag.ch/ [Accessed: 05.05.2010]

http://www.unep.or.jp/ [Accessed: 23.06.2010]

Link to the online version of the “International Source Book On Environmentally Sound

Technologies for Wastewater and Stormwater Management” from the United Nations

Environmental Programme. This section is about drying beds.