UPGRADE OF CONVENTIONAL ACTIVATED SLUDGE SYSTEM TO A BIOFILM BASED TREATMENT SYSTEM

Imre Tóth 1, Jack Ryl 1, Robert Freudenberg 1

1. Organica Water, Budapest, Hungary

ABSTRACT

One of the three main wastewater treatment plants (WWTP) in Budapest, South Pest WWTP (80 ML/d), was upgraded to a biofilm based treatment technology. Tertiary treatment units for nitrogen and phosphorus removal (BIOFOR® NP and BIOFOR® DN) were part of the existing process. Prior to the upgrade these treatment units could not be operated efficiently due to substandard performance of the secondary treatment process units. The upgrade successfully addressed these issues. The aim of this paper is to describe the main concepts behind the South Pest upgrade and to evaluate treatment performance of the retrofitted units including the most recent operational.

INTRODUCTION

One of the three main wastewater treatment plants (WWTP) in Budapest, South Pest WWTP (80 ML/d), was upgraded to a type of fixed bed biofilm activated sludge (FBAS) technology. The upgrade was based on retrofitting biological reactors with a novel attached growth system where a natural food chain phenomenon takes place. The plant received almost 60 ML/d of wastewater at the time the upgrade was commissioned. The main drivers for this upgrade were as follows:

1. influent to the WWTP became more and more concentrated while hydraulic load also increased considerably,

2. the population within the catchment area was increasing rapidly.,

3. more stringent effluent quality requirements were imposed on the plant,

4. odour started to become a serious problem as the residential areas came closer to the plant.

In order to address these issues the Budapest Sewage Works Ltd. (BSW Ltd.) – the operator of the WWTP - undertook a feasibility study with the aim of identifying suitable available technologies on the market to find the best possible solution. The study

also identified the plant capacity “bottle necks”. One of the major capacity limitations was poor performance of the existing clarifiers because of the high solids loading at the time. Therefore the only suitable solution for the upgrade was to increase the biomass in the reactors and reduce solids loading on the clarifiers by adopting membrane-based or biofilm-based technologies. The advantages and disadvantages of the membrane bioreactor (MBR), moving bed biofilm reactor (MBBR) and fixed-bed biofilm activated sludge (FBAS) technologies were listed and compared. The upgrade was designed to meet the following objectives:

• increase the organic removal efficiency of the secondary treatment,

• reduce suspended solids loading on the secondary clarifiers in order to decrease TSS and BOD loads on the tertiary BIOFOR® N and BIOFOR® post-DN units for nitrogen and phosphorus removal,

• minimize odour.

Table 1 shows the final effluent quality after advanced treatment.

Table 1: Final effluent requirements

Effluent limits Value unit

TCOD 50 mg/L

TCBOD5 25 mg/L

TSS 35 mg/L

NH4+-N* 2 mgN/L

TN** 10 mgN/L

TP 1.8 mgP/L

*temporary wintertime limit is 4 mgN/L **temporary wintertime limit is 20 mgN/L Prior to the upgrade, the tertiary treatment units could not be operated efficiently due to substandard performance of the secondary treatment process. In addition, as residential areas developed closer to the plant, odour started to become a serious problem.

The upgrade successfully addressed these issues at the South Pest WWTP. The Environmental Department of Guangdong Province, China, was also looking for a solution to upgrade an existing WWTP (oxidation ditches) of HeYuan city to an increased capacity of 30 ML/d. Since the treatment plant discharges its effluent to HeYuan River – a major source of Hong-Kong’s and Shezhen’s drinking water reservoirs - the local government wished to meet Surface Water Category III effluent discharge limits (Table 2.) for the treated effluent.

Table 2: Surface Water Category III. limits

Effluent limits Value unit

TCOD 20 mg/L

TCBOD5 4 mg/L

TSS 5 mg/L

NH4+-N 1 mgN/L

TN 10 mgN/L

TP 0.2 mgP/L

The oxidation ditches were retrofitted with the same biofilm system and the plant was successfully commissioned. Since there are no applications of this technology in Australia (despite being successfully operated around the world), Melbourne Water decided to demonstrate this technology at a pilot scale for an upgrade of the Western Treatment Plant. METHODOLOGY General Technology Description The Food Chain Reactor® (FCR) system has two different types of biofilm carriers (see Figure.1). The first one is a natural biofilm carrier namely plant roots. Plants are located on the top of the reactors, with their roots immersed in the content of the reactors. The plant roots serve as biofilm carriers in the top ~0.5m below the water level. The second biofilm carrier is artificial, installed below the plant roots in order to provide biofilm carrier surface in the deeper parts of the reactor. Due to the special structure of the biofilm carriers, a unique biofilm develops, which achieves higher biomass amount in fixed form in the reactors (no sludge recirculation is used, thus no significant suspended biomass can be found in the system). This leads to higher sludge retention times (SRT) within the same reactor volume and more stable operation.

Figure 1. General structure of one reactor (1 –

reactor, 2 – plants, 3 – artificial carrier, 4 – fine

bubble diffusers

Influent and effluent wastewater characterization During the investigation phase, daily samples were taken from the influent wastewater, secondary clarifier effluent and final effluent at South-Pest WWTP. COD, BOD, TKN, TN and NH4

+-N concentrations were measured by certificated laboratory in accordance with the standards of MSZ ISO 6060:1991, Labor-4:1998 and MSZ 260-3:1973. Loading rates were calculated and statistical analysis was carried out in order to compare the performance of the system during the cold seasons from November till April. Process Control Philosophy In order to reduce aeration costs a number of process control philosophies were assessed to identify the most efficient. Dynamic process simulations are excellent and cost effective tools to evaluate the effectiveness of process control strategies before their implementation. They also facilitate testing and developing a customised design of a preferred control strategy for a specific plant. SIMBA# software was used to dynamically simulate a number of control strategies for various temperature and effluent quality requirement scenarios. Apart from the benefit of reducing aeration costs, which results in energy savings, certain process control strategies also reduce external carbon addition, improve denitrification potential and bio-P performance.

The following aeration control strategies were evaluated:

- Level 1: DO-based aeration control employing DO probe in one reactor

- Level 2: DO-based aeration control employing DO probes in two reactors

- Level 3: Ammonia-based aeration control employing DO probes in two reactors and ammonia probe in the train effluent.

In addition a nitrate probe was employed to control internal recycle rates for TN targets. Figure 2 shows a comparison of air supply required to meet the same effluent quality targets at three different temperatures using the following process control approaches:

- Base Case – DO control with design DO set-points without adjusting them with temperature

- Case 1 – DO control with seasonally adjusted DO set-points (good operational skills required)

- Case 2 - NHx control sets DO set-points based on the difference between measured ammonia effluent and ammonia set-point (good operational skills not essential).

The figure shows that similar level of energy savings can be achieved by either skilful operation using DO control or employing more sophisticated process control in the regions where there is shortage of good operational skills. Figure 2. Airflow requirement

Upgrade description

The South Pest WWTP was built in 1966 away from the city. Since then several upgrades were carried out. Before the last retrofit upgrade with the presented biofilm technology, the system consisted of the following units (see Figure 5): mechanical pre-treatment (including primary clarifiers), activated sludge reactor for carbon removal with secondary clarifiers, BIOFOR NP and BIOFOR DN for nitrification and denitrification steps, anaerobic digesters for sludge treatment and biogas

production. Apart from treating primary and secondary sludge, the anaerobic digesters also treat organic food waste. Supernatant from the digesters returns back to the biological reactors. The existing WWTP has three reactor trains (see Figure 6). The effluent from three primary clarifiers flows into the distribution chambers before entering the activated sludge reactors. Each reactor train has two activated sludge reactor lines. The first train (Line 1-2) was constructed in 1966, while the second and third train (Line 3-6) were put into operation in 1983 with a larger reactor volumes compared to the first train. The reactor lines consist of two anoxic and six aerobic reactors. Mixing in the anoxic tanks is provided by mixers. The air is introduced into the aerobic reactors by fine bubble diffusers. Each line is followed by a secondary clarifier. One of the main capacity limitation before the upgrade was clarifiers’ inability to accept higher solids loading caused by the increased growth related loading on the plant. With these limitations and challenges, the activated sludge reactors and the secondary clarifiers were not able to provide adequate water quality for the BIOFORs. Moreover, intensive foaming often formed on the top of the overloaded reactors. The aim of the upgrade was to increase the treatment capacity of the reactors and decrease the clarifier overload in order to be able to produce the required water quality for the BIOFORs, so that the plant would be capable of meeting the effluent requirements (see Table 1). During the construction phase biomodules (Fig. 7) were installed into the aerobic reactors with a special matrix structure. Aeration panels were installed below the artificial biomodules in order to provide sufficient oxygen and proper mixing within the biofilm carriers. Plant racks were installed on top of the reactors. The project demonstrated that replacing the existing activated sludge system with a biofilm system provided two following benefits critical to the upgrade:

• significant increase in the treatment capacity of the reactors since the biomass amount increased 3 to 5 times within the existing reactor volume

and

• reduced solids loading on the clarifiers as the biomass is attached to the carriers.

DISCUSSION AND RESULT ANALYSIS

Averages of influent concentrations, daily loading rates and volumetric loading rates were calculated for the investigated periods (Table 4). It should be noted that the values in Table 4 are based on daily grab samples taken from raw influent wastewater. Reliable measurement results from primary clarifier effluent are not available, therefore the influent

wastewater results are used here to calculate volumetric loading rates on the biological reactors. It is important to note that they are conservative since the return flow from the anaerobic digester is not included and the actual volumetric loading rates on the biological system are different due to primary sedimentation tanks’ performance.

An increasing trend in the calculated daily loading rates of COD and BOD is evident, whereas the TSS load remains in the range of 9.4-13.1 tons/d. The loadings during winter of 2014-2015 were lower. After presenting the influent flows and loads, the secondary clarifier effluent is evaluated.

Figure 3: Carbonaceous BOD5 in secondary clarifier effluent

Figure 3 shows that carbonaceous BOD5 concentrations in the secondary clarifier effluent decreased after the upgrade. An average BOD5 of 25.3 mg/L was produced after the retrofit, compared to 48.4 mg/L BOD concentration prior to the upgrade. In order to confirm this statement, statistical test was carrier out. Student’s t-test shows (Table 5) significantly lower BOD5 concentration after the upgrade, therefore the organic loading onto the tertiary filtration unit is also lower. . The same analysis was performed to investigate total solid concentration in the effluent of the secondary clarifiers.

Figure 4 shows that TSS concentration in the secondary clarifier effluent is lower after the upgrade. An average secondary effluent TSS concentration of 22.5 mg/L is produced compared to 34.6 mg/L before the upgrade. Student’s t-test was used to confirm the statement. Based on the statistical analyses’ results (Table 6), the solids concentration in the secondary clarifier effluent is significantly lower after the upgrade, exerting a lower solids loading onto the tertiary treatment.

Figure 4: TSS concentration in secondary clarifier effluent

Based on the analysis of secondary clarifier effluent quality it can be concluded that the biological treatment capacity significantly increased. Since the biomass is now in attached form and MLSS concentration in the reactors decreased to 800-1600 mg/L, the solids loading on the secondary clarifiers decreased substantially. Analysis shows that the organic and solids loading on the BIOFOR® units was reduced. Final effluent was also evaluated and as Table 3. shows, the effluent quality improved significantly across all parameters.

Table 3: Percent reduction in final effluent

Final effluent Reduction

TCOD 32%

TCBOD5 54%

TSS 52%

TKN 55%

Ammonia-N 59%

TN 22%

CONCLUSION:

The conversion of a conventional activated sludge system to a biofilm based system was successfully implemented at South-Pest WWTP in Hungary. The analysis shows that the performance of the secondary treatment significantly improved and the solids loading rate on the secondary clarifiers decreased, exerting lower solids loading on the tertiary treatment units. Final effluent quality significantly improved. Oxidation ditches in China were also converted to biofilm based systems and the system was successfully commissioned. Nitrification-denitrification is part of the process providing high quality effluent to meet the Surface Water Category III requirements. (Table 2.).

Figure 5: Process units of the South-Pest WWTP

Figure 6: South-Pest WWTP

Figure 7: Biomodules installed in the basins

Figure 8 Biological reactors covered with a

glasshouse

Table 4: Influent flows and loadings during the investigated period

Table 5: Student’s t-test, secondary effluent BOD concentration

BOD-secondary effluent

Before upgrade

After upgrade

Mean (mg/L) 48.76294821 26.04

Variance 1864.208582 329.55

Observations 251 279

Hypothesized Mean Difference 0

df 328

t Stat 7.75

P(T<=t) one-tail 5.96331E-14

t Critical one-tail 1.649512493

P(T<=t) two-tail 1.19266E-13

t Critical two-tail 1.967222827

Table 6: Student’s t-test, secondary effluent TSS concentration

TSS secondary

effluent Before

upgrade After

upgrade

Mean (mg/L) 34.6 24.5

Variance 685.9 270.5

Observations 253 282

Hypothesized Mean Difference 0

df 415

t Stat 5.46

P(T<=t) one-tail 4.05X10-8

t Critical one-tail 1.65

P(T<=t) two-tail 8.1X10-8

t Critical two-tail 1.97

Average influent flows and loads

influent flow (MGD) 63 ± 7 49 ± 5 56 ± 12 49 ± 5 58 ± 6

COD concentration (mg/L) 430 ± 170 584 ± 147 519 ± 175 630 ± 234 387 ± 99

COD load (t/d) 26.9 ± 10.4 28.1 ± 7.5 28.6 ± 9.0 30.7 ± 11.2 22.3 ± 5.1

COD volumetric loading rate (kg/m3·d) 2.98 ± 1.15 3.11 ± 0.83 2.56 ± 0.81 2.75 ± 1.00 1.99 ± 0.45

BOD concentration (mg/L) 243 ± 83 372 ± 88 338 ± 110 393 ± 175 212 ± 60

BOD load (t/d) 15.1 ± 5.1 17.9 ± 4.4 18.3 ± 5.6 19.3 ± 8.7 12.2 ± 3.0

BOD volumetric loading rate (kg/m3·d) 1.68 ± 0.56 1.98 ± 0.49 1.63 ± 0.50 1.72 ± 0.78 1.09 ± 0.27

TSS concentration (mg/L) 204 ± 96 194 ± 106 182 ± 73 266 ± 182 169 ± 39

TSS load (t/d) 12.7 ± 5.9 9.4 ± 5.4 10.2 ± 4.5 13.1 ± 9.0 9.8 ± 2.4

TSS volumetric loading rate (kg/m3·d) 1.40 ± 0.65 1.04 ± 0.60 0.91 ± 0.40 1.17 ± 0.81 0.88 ± 0.22

2010-2011 2011-2012 2012-2013 2013-2014 2014-2015