Optimized Nutrient Removal using the Activated Return sludge Process

Optimized Nutrient Removal using the Activated Return sludge Process Larsen, S., B*, Petersen, G.*, Liu Zhixiao** and Dines Thornberg*** * EnviDan A/S, Fuglebækvej 1A, 2770 Kastrup, Denmark ** Capital China Aihua Municipial & Environmental Engineering Co., LTD RM 4F, New Gold Dragon Building, No 21 Weijinnan Road, Hexi destrict, Tianjin, China *** Lynettefælleskabet A/S, refshale 265, Copenhagen Abstract EnviDan has, together with clients, developed the Activated Return sludge Process (ARP). In order to increase the hydraulic and/or organic capacity of existing and new waste water treatment plants. The process has been implemented on several WWTP s in Denmark, Sweden, the Dominican Republic and in China. In 2010 the Danish Environmental Protection Agency (EPA) supported a developed project, where the ARP process is documented in full scale as well as pilot plant scale. Attentive paper includes the results from full scale and pilot scale experiments as well as results from implementation in full scale on a Chinese WWTP. The results showed that the capacity of a WWTP s is clearly connected to the sludge amount in the plant and that not only the conversion of COD can be increased by using the ARP process, but also the nitrification rate increases. Thus the process withholds great potential for upgrade of existing WWTP in order to decrease the necessary volume for biological processes, and thereby minimizing the overall carbon footprint at WWTP. Keywords Organic capacity increase, Activated Return sludge Process, waste water, sludge hydrolysis, oxygen consumption, OUR, AUR, NUR, INTRODUCTION In Denmark continuous focus and effort on optimizing the biological processes used in connection with wastewater treatment takes place. The ARP technology has been developed by EnviDan A/S during the last 10 years. The ARP process uses the sludge hydrolysis in a new and controlled way. The process is operating in full scale at several wastewater treatment plants (WWTPs) in Denmark and the Baltic countries in Europe as well as in the Dominican Republic and China. In the ARP process a part stream of the return sludge is led to a separate tank (ARP tank) having a hydraulic retention time of app. 20-30 hours. In this tank the process conditions are controlled and sludge hydrolysis takes place. Typically, the return sludge flow is decreased to 30-50% of the influent flow when introducing the ARP process resulting in a high return sludge concentration. As the sludge hydrolysis rate is proportional to sludge concentration a very compact design is used when expanding WWTP s capacity by introducing the ARP technology. The ARP technology has been implemented with the primary purpose of increasing the hydraulic and/or the organic capacity of WWTPs. In addition, the ARP process has been used for minimization of the amount of excess sludge at plants where either the sludge handling facilities had limited capacity or the operation costs related to final sludge handling where high. This paper focuses on the results from full scale and pilot scale experiments that have been conducted in order

to clarify the rates for nitrification and denitrification as well as the potential for COD conversion when implementing the ARP process at existing waste water treatment plants. THE ACTIVATED RETURN SLUDGE PROCESS (ARP) The ARP process is based on two well-known processes within wastewater treatment: A: Adsorption of COD components to activated sludge flocks B: Sludge hydrolysis of slowly degradable COD into easily degradable COD Adsorption The majority of both the soluble and the particulate organic compounds (COD) in the waste water are adsorbed on the activated sludge flocks when waste water and return sludge is mixed, see Figure 1. Hence, the sludge fraction holds a great potential for soluble COD production which can be utilized and released using the hydrolysis process. Under normal operation conditions approximately 45 % of the incoming COD will be removed from WWTP s with the excess sludge withdrawn from the activated sludge stage (Gujer et al., 1999). The COD content in the excess sludge fraction can be converted into biogas by anaerobic digestion or hydrolysed to produce easily degradable organics. Figure 1: COD adsorption and hydrolysis in activated sludge in the activated sludge process Hydrolysis The main fraction of organic compounds in wastewater cannot be used directly by the microorganisms in the activated sludge. Initially, the organic compounds are hydrolyzed and the slowly degradable organic compounds in wastewater are transferred into easily biodegradable organic compounds. The hydrolysis process is slower than processes for biological growth, thus the hydrolysis rates are regularly the limiting factor in regards to wastewater treatment processes (Henze et al., 2002). The hydrolysis process under anaerobic conditions is often expressed as the decomposition of biomass into acetic acid as below: C 5 H 7 NO 2 + 3H 2 O 2.5 CH 3 COOH + NH 3 (1) It is well known that sludge hydrolysis is facilitated by microorganisms which excrete extracellular enzymes capable of decomposing larger particulate organic compounds thus generating easily degradable soluble organic compounds like Volatile Fatty Acids (VFAs). The main part of VFA (60-80%) is acetate but also propionate, isobutyrate, butyrate and many more short chain organic compounds can be produced (Vollertsen et al., 2006, Bernard and Abraham, 2005). Recent studies have suggested that that propionate may be a more favorable substrate than acetate for EBPR performance. On numerous occasions, superior P removal performance has been reported with propionate as compared with acetate during long-term enrichment studies at laboratory scale. (Oehmen, et al, 2007)

The sludge hydrolysis rate is proportional to the sludge concentration and the temperature. As a consequence, the efficiency of the hydrolysis process expressed as transformed amount of COD pr. m 3 increases proportionally to the sludge concentration. The sludge hydrolysis processes can be used to support biological phosphorous removal, improve the nitrogen removal or sludge minimization depending on the configuration and process conditions. ARP configuration The ARP process takes place in a volume separated from the main plant, where only return sludge is fed to the volume. The principal is illustrated in Figure 2. Figure 2: Principal diagram for the ARP plant Since the ARP tank is operated independently from the main plant, it can be applied for all plant types. A part of return sludge is led directly to the ARP plant resulting in a higher sludge concentration compared to the main plant. The ARP volume is designed using a hydraulic retention time of 20-40 hours. The ARP plant is equipped with mixers and aeration equipment and can be considered as a high MLSS version of the main plant. In the ARP plant COD conversion using oxygen or nitrate as well as both nitrification and denitrification is performed by an operational cycle controlling oxic, anoxic and anaerobic phases. The ARP technology can be used for the following purposes: 1. Increasing the organic capacity: By establishing an ARP process as a supplement to the main treatment process the total sludge mass in the plant will be increased. Hence, the plant capacity will increase correspondently to the extra sludge mass in the plant. The main plant will still have to perform nitrification and denitrification, but part of the COD removal will be moved to the ARP plant, thereby increasing the total capacity of the plant. 2. Increasing the hydraulic capacity: By implementing the ARP process in an existing WWTP the hydraulic capacity of the secondary clarifiers can be increased. This is achieved as a part of the sludge mass in the plant is moved to the ARP tank (thereby sustaining the total sludge amount in the plant). By moving a part of the sludge to the ARP tank, the MLSS concentration in the main plant can be decreased, thereby decreasing the sludge load to the secondary clarifiers. In this way, the ARP configuration accounts for a permanent storm water control system which among the benefits does not require a prior notice for conversion of the plant operation. 3. Biological phosphorus removal: The ARP process can be used for improving the biological phosphor removal as readily bio-degradable COD is produced by hydrolysis in the ARP tank. The Phosphor Accumulating Organisms (PAO) needs soluble COD and anaerobic conditions followed by aerobic conditions in order to perform biological phosphor removal. In the main plant the soluble COD is also used for denitrification, hence the activity of the

PAO can be limited resulting in a poor or instable biological phosphor removal. However, in the ARP tank the conditions changes between aerobic, anoxic and anaerobic ensuring sufficient soluble COD for biological phosphor removal. 4. Minimizing excess sludge production: the ARP tank can be used for increasing the overall total sludge age in the WWTP. In this way, the amount of excess sludge will decrease by endogenous respiration taking place in the tank. The processes can be combined in order to obtain some or all of the above mentioned purposes or can be focused on one or two of the purposes depending on the applications needed (Petersen et al. 2009). RESULTS FROM DOCUMENTING THE ARP PROCESS Attentive section includes the results obtained through the following tests: 1. Analysis of full scale ARP plant, at Bjergmarken waste water treatment plant, Denmark 2. Analysis of pilot scale ARP plant, at Lynetten waste water treatment plant, Denmark 3. Implementation of full scale ARP at Wangjiashan waste water treatment plant, China Full scale experiments at Bjergmarken waste water treatment plant Bjergmarken WWTP is located in Roskilde, Denmark and is dimensioned for capacity of 125,000 person equivalents (PE). The plant has had an ARP tank in a previous primary clarifier for more than 5 years. The biological treatment is performed using the BIODENIPHO principle. The main plant has a volume of 21,900 m 3, whereas the ARP plant has a volume of 1,990 m 3 (equal to 8% of the total process volume). The ARP tank has been equipped with brush aerators and mixers. The main part of the return sludge from the secondary clarifiers is led to an anaerobic tank prior to the alternating ditches and approximately 15 % of the return sludge is led to the ARP tank ensuring a retention time of 20-30 hours. A more detailed description of the plant and operating data from implementation of the ARP process can be found in Jakobsen and Petersen, (2008). The plant has a capacity of 125.000 PE and is loaded with approximately 86.000 PE. On average 24 % of the total sludge mass in the plant is placed in the ARP tank. The amount of excess sludge is 96 m 3 /d containing 5.2 % TS. The total sludge age is 24 or 32 days based on total sludge mass in the aeration tanks excluding or including ARP tank, respectively. The overall goal of the full-scale tests was to quantify the removal potential in the ARP tank. Hence the following was tested: 1: Rates for nitrification and denitrification, both in laboratory test and by using the data from the online sensors 2: Conversion of COD by using the oxygen amount supplied to the ARP tank The standard oxygen requirement (SOR) based on 2 x 6 m rotors in operation 8,4 h/d is calculated to 800 kg O 2 /d equal to an actual oxygen requirement (AOR) of 620 kg O 2 /d. In the ARP tank, the oxygen consumption is used for removal of COD and ammonium-n generated from the hydrolysis in the tank. Initially, this COD is hydrolyzed to easy-degradable organic compounds and ammonium-n according to equation (1) previously presented. The hydrolyzed COD and ammonia- N is then oxidized to carbon dioxid and free nitrogen according to equation (2): 2.5 CH 3 COOH + NH 3 + 5.75 O 2 5 CO 2 + 0.5 N 2 + 6.5 H 2 O (2) Thus, 150 kg COD and 14 kg N is oxidized using 184 kg O 2, which corresponds to 0.82 kg COD is oxidized per kg O 2 used and 0.0761 kg N is oxidized per kg O 2 used in the tank. Based on these values the amount of COD and N oxidized in the ARP tank can be calculated from the AORs:

COD removal: 622 kg O 2 /d x 0.82 kg COD/kg O 2 = 510 kg COD/d N removal: 622 kg O 2 /d x 0.0761 kg N/kg O 2 = 47 kg N/d Thus the ARP convert 510 kg COD/d, which corresponds to 0.614 kg TS/kg O 2 according equation 2. This corresponds to approximately of 400 kg TS/d, or 10 % of the total sludge production before anaerobic digestion. Looking at the sludge production before anaerobic digestion, from several years reveals that the excess sludge production has decreased after implementation of the ARP process. While the sludge production has decreased the gas production has been stable, which can be explained by a slightly higher sludge retention time in the digester due to the lower sludge production. Based on the on-line data from the equipment placed both in the main plant and ARP plant biological uptakes like Ammonium uptake rate (AUR), oxygen uptake rate (OUR) as well as nitrate uptake rates (NUR) have been calculated. In general, using on-line data a large quantity of data is available. Calculating the biological uptake rates is subjected to some uncertainties eq. selecting the day and time has a significant influence on the results. In addition, temperature as well as the waste water composition affects the uptake rates. Thus, the on-line data material has been evaluated and three maximum rates have all been calculated at different temperatures in the main plant. An example of an on-line data curve indicating where the rate has been calculated is presented in Figure 3. Selected results are presented in Table 2. Figure 3: On-line data curve showing where the rate has been calculated marked with red. In Situ AUR NUR Date Temperature Main plant Main plant ARP C [gn/kg VSS/h] [gn/kg VSS/h] [gn/kg VSS/h] 11/2 2011 8 1.03 3.15-1.29 2.44-2.04 6.45-22/4 2011 13 0.93 2.43 1.09 1.77 5.04 0.86 1.57 4.27 0.63 10/6 2011 19 0.77 1.28 3.21 0.82 4.72 3.40 0.69 1.89 2.66 Table 1: Ammonium uptake rates (AUR) and Nitrate uptake rates (NUR) for Phase 1 based on on-line data. The ammonium uptake rates achieved in the main plant is between 0,8 and 2 gn/gvss*h, which is in accordance with the theory (Henze et al., 2010). The nitrate uptake rates in the main plant are relatively high between 2.5 and 6,5 gn/gvss*h even at low temperatures compared to Henze et al., 2010. This is most likely obtained by the higher content of easily degradable organic compounds

compared to normal raw waste water. The easily degradable organic compounds are produced during hydrolysis in the ARP tank. The nitrate uptake rates in the ARP tank are in general lower compared to main plant as the nitrate concentration in the ARP tank is low resulting in a low uptake rate. Pilot plant experiments at Lynetten waste water treatment plant Lynetten WWTP is the biggest waste water treatment plant in Denmark and has a capacity of 1,100,000 PE. At the plant, a pilot plant is installed in a scale of 1:14700. The pilot plant is fed with primary clarified waste water and is fully equipped with aeration, mixing and online sensors. The pilot plant has three hydrolysis tanks, where only return sludge is fed to and two classic anaerobic selector tanks where the primary clarified waste water is mixed with the return sludge from the hydrolysis tanks. The anaerobic selector tank was rebuild, so a part of the tank was used for the ARP process and a part was continuously used for biological phosphor removal. The flow diagrams can be seen in Figure 4. Figure 4: Flow diagrams for the pilot plant in Lynetten, left; reference operation without ARP operation and right; operation with ARP configuration During ARP operation, the selector tanks was equipped with intermitted aeration in order to increase the aerobic sludge amount in the plant, furthermore the return sludge flow was decreased in order to increase the sludge concentration in the return sludge. This entailed that 33 % of the total sludge mass was saturated in the ARP tank during ARP operation. The operation of the pilot plant was divided into 3 phases: 1. Phase 1: Reference operation, where the plant was loaded accordingly to the capacity using the original plant configuration (left flow diagram in Figure 4) 2. Phase 2: ARP operation with the same load as in reference period 2 using ARP configuration (right flow diagram in Figure 4) 3. Phase 3: Reference operation with 30 % overload, where the plant was loaded with 30 % overload accordingly to the capacity using the original plant configuration (left flow diagram in Figure 4) The effluent values from the pilot plant in the three phases are depicted in Table 2. Effluent Parameter Phase 1 Phase 2 Phase 3 Effluent demand for Lynetten WWTP COD [mg/l] 30.8 34.7 39.8 75 T-N [mg/l] 7 12.2 19.7 8 NH 4-N [mg/l] 2.26 1.8 13.6 - T-P 0.9 2.6 2 1.5 Table 2: Comparison of effluent data from the pilot plant in all three phases Phase 1 represented the original BIODENIPHO operation at the plant, whereas phase 3 represented BIODENIPHO operation including 30 % overload of the plant. As can be seen from the table the plant is not able to fulfil the effluent demand in phase 3, which was expected. In phase 2, where the ARP configuration is introduced, the plant is not fully capable of fulfilling the effluent demand, however the T-N concentration in the effluent is still 38.7 % lower than in phase 3, which has a similar load to the plant.

In order to verify the operation OUR, AUR, NUR and hydrolysis rate was measured at the plant in all phases of the pilot experiment. The result and comparison of these parameters are shown in Table 3. Parameter Phase 1 Phase 2 Phase 3 Phase 2/Phase 1 Phase 3/Phase 1 Phase 2/Phase 3 Organic load [Kg COD/kg SS/d] Aerobic sludge age excluding ARP [d] Aerobic sludge age including ARP [d] Nitrogen load [KgN/Kg SS/d] Nitrogen conversion [Kg N/d[ OUR [g O 2/Kg VSS/h] AUR [g NH 4-N/Kg VSS/h] NUR [g NO 3-N/Kg VSS/h] Hydrolysis rate [g COD(S)./Kg VSS/h] Table 3: Comparison of data from phase 1-3 0,26 0,38 0,38 +46% +46% 1 4,3 4,3 4,1 1-5% +5% 4,3 5,1 4,1 +19% -5% +24% 0,027 0,044 0,037 +63% +37% +19% 0,49 0,81 0,48 +65% -0,02% +69% 15,6 15,3 15,0-2% -4% +2% 3,0 4,3 1,9 +43% -37% +126% 1,2 4,6 3,7 +283% +208% +24% 1,3 1,4 1,1 +8% -15% +27% The aerobic sludge age, excluding the sludge in the ARP, bas been more or less the same in all of the phases. If the aerated sludge in the ARP is included, the aerobic sludge age in phase 2 has been 24 % higher compared to the other phases. According to the theoretical part, the capacity should be connected to the aerobic sludge age, hence the capacity for both COD removal and nitrogen removal should be increased with 24 % in phase 2 compared to the other phases. The organic load to the plant in phase 2 and 3 has been 46 % higher than the load in phase 1. This higher load entails that the effluent concentration for COD is increased in phase 3 and compared to phase 1. Despite the 46 % higher load, the effluent concentration in phase 2 is increased with 11 %, which indicates that the ARP dies increase the capacity fir COD removal. The nitrogen removal in phase 2 has also been significant higher than in phase 1. When the two phases is capered it is seen that the nitrogen removal has increased with 63 % in phase 2 compared to phase 1. The higher removal rate is due to the fact that the pilot plant has received higher nitrogen loads in phase 2 than in phase 1 and 3. The plant has received 38 % more nitrogen in kg N/kg SS/d in phase 2 compared to phase 1 and 16 % higher nitrogen load than compared to phase 3. Despite the higher nitrogen load, the plant with the ARP configuration still shows better effluent results than phase 3. The much higher nitrogen load to the plant during phase 2 has resulted in a T- N concentration that exceeds the effluent demand in phase 2. The biological rates measured in the three phases showed that average OUR correlated to 20 º C is similar in all three phases. The AUR correlated to 20 º C was 4.3 and 1.9 g NH 4 -N/kg VSS/h in phase 2 and 3 respectively. The higher rate for ammonia removal is in good coherence with the higher nitrogen conversion in phase 2. The average NUR correlated to 20 º C was 4.6 and 3.7 g NO 3 -N/kg VSS/h in phase 2 and respectively. This is within the theoretical area when using waste water a source of carbon (Henze et al., 2010). The denitrification is approximately 20 % higher in phase 2 compared to phase 3, which could be due to the higher hydrolysis in the ARP tank that produces VFA for the denitrification, resulting in higher rate for denitrification (Henze et al., 2010). The hydrolysis rate is very similar in all three phases, which is due to the fact that the rate is dependent on the specific redox conditions (Jensen, T.R. (2009)). However, since the hydrolysis rate is expressed in g COD(S)/kg VSS/h the total amount of COD(S) produced will be higher in the

plant with ARP configuration because the total sludge mass in the pilot plant is higher. The potential for production of soluble COD in the plant with ARP configuration can be estimated to 1,226 kg/d from the hydrolysis rate and the sludge amount in the plant, whereas the potential production of soluble COD in the plant with BIODENIPHO configuration is 948 kg/d and 926 kg/d in phase 1 and 3 respectively. The extra production of soluble COD in the plant with ARP configuration can be used for biological processes such as denitrification or biological phosphor removal. The pilot plant has been operating without any addition of chemicals in order to precipitate phosphor; hence the phosphor concentration in the effluent is a measurement for the activity of the biological phosphor removal. The average concentration in phase 1 was 0.9 mg/l, whereas the average concentration in phase 2 and 3 was 2.6 and 2 mg/l respectively. The higher concentration in the effluent in phase 2 and 3 indicates a higher BIO-P activity in the reference phase, phase 1. In the reference operation the anaerobic sludge age was 30 % higher than in phase 2 with ARP configuration, where only 3 BIO-P tanks were in operation. It was not possible to optimize the biological phosphor removal in the reference period; however this could have been done by operating the BIO-P tanks and the ARP tank in parallel instead of in series. By operating in parallel the readily bio-degradable COD, produced by the hydrolysis in the ARP tank, could have been used more efficiently and thereby increased the biological phosphor removal. Full scale implementation at Wangjiashan waste water treatment plant EnviDan has several references in Denmark, where the process has been implemented for many years. Chinese regulation will in the forthcoming years start to enforce more stringent effluent standards. With regards to this, China stands against several obstacles. First, a lot of Chinese sewage plant is based on the earlier years old discharge standards of construction, according to the new standards, the existing biological tank volume is insufficient to meet the strict discharge requirements.. Secondly Chinese wastewaters often have naturally low COD/T-N, COD/TP ratios, which might limit the nitrogen and phorphorus removal. The ARP is a good alternative for plant upgrades compared to conventional treatment processes as the volume demand is typically 20-40 % compared to conventional designs for nitrogen removal, thus diminishing construction costs. Furthermore, production of soluble COD by the hydrolysis will increase the potential for both nitrogen and Bio-P removal. EnviDan has together with our Chinese partner Aihua, implemented the ARP process at Wangjiashan waste water treatment plant in Maanshan, China. The plant consists of an old triple ditch plant, which is no longer used for wastewater treatment. The plant has instead been upgraded with two Orbal multichannel oxidation ditches. Prior to the ditches is an anaerobic selector tank. The plant is designed for a capacity of 60,000 m 3 /d and is currently loaded with 48,710 m 3 /d, or 109,000 PE. AN N/DN SET. AN/N/DN ARP/BIO-P Figure 5: Process configuration proposed by EnviDan and Aihua for Achieving both nitrogen and phosphor removal based on A1 standard Analysis of the influent data shows a COD/BOD ratio of 1.4, which indicates that the organic

material is suitable for biological processes. However the COD/T-N ratio of 4.7 is low and can cause problems for the denitrification (Henze et al., 2002). In the BPR process, the PAOs take up VFAs under anaerobic conditions and store them as polyhydroxyalkanoic acids (PHAs). Therefore, a successful BPR process depends on having sufficient VFAs available in the anaerobic zone. But the influent wastewater at Maanshan WWTP was not well-suited for BPR because of its low VFA content. Fermentation of a portion of the RAS stream was proposed as a potential method for making additional VFAs available. A high COD/T-P ratio of 58.7, should favour BIO-P removal (Barnard, J. L. and Abraham, K., 2006). However, the limited nitrogen removal in the main tank renders the RAS with a high nitrate concentration, where the average T-N concentration in the effluent in 2012 was 18 mg/l. This has limited the BIO-P removal when the pre-treatment tank is rendered anoxic (Janssen et al., 2002) as PAOs will be outcompeted by heterotrophic organisms. The inhibition of the BIO-P removal was also seen in the T-P concentrations in the effluent where the average concentration in 2012 was 1.2 mg/l. In October to December 2011 the rates for nitrification and denitrification was measured at Wangjiashan waste water treatment plant. Date AUR [g NH 4-N/kg VSS/h] NUR [g NO 3-N/kg VSS/h] P release [g PO 4-P/kg VSS/h] 22/11-2011 2.18 0.78 0.70-0.75 24/11-2011 - 0.75-6/12-2011 2.51 0.51 0.49 7/12-2011 2.67-0.22 Table 4: Obtained rates for nitrification and denitrification at Wangjiashan waste water treatment plant at 20 º C (Grossman, in press) The nitrification rates presented in Table 4 are similar to those found in Swedish WWTPs with similar influent BOD/T-N ratios (Jansen et al., 1991). The denitrification rates are however very low when compared to Swedish WWTPs, (Hagman, 2007). The low denitrification rate could be due to the plant configuration, which is based on simultaneous denitrification. The configuration proscribes a continuous aeration in all rings, though with low oxygen setpoints. This entails that oxygen is present most of the time during operation, which causes the denitrification rate to decrease (Henze et al., 2002). The observed P-release rates can also be regarded as low as Janssen et al (2002) describes that release rates lower than 3 g P/kg VSS h can be regarded as moderate. This corresponds well with the fact that the BIO-P activity is limited by the high nitrate concentration in the return sludge, which entails that only very little phosphor is incorporated into the cells of the PAO bacteria, which again entails that only very little phosphor can be released from the cells. In May 2012 the plant was retrofitted to include an ARP process for both capacity increase and biological phosphor removal in the previous triple ditch. Furthermore an online control system was implemented in order to control the aeration equipment and thereby the nitrogen removal as well as the biological phosphor removal. After implementation of the ARP process at Wangjiashan waste water treatment plant the T-N concentration in the effluent has been decreased to 12 mg/l and the T-P concentration has been decreased to 0.7 mg/l. The decrease in T-N in the effluent is due to a lesser amount of nitrate. The nitrate reduction is a consequence of the control system for the main plant as well as the soluble COD production in the ARP tank. The soluble COD concentration in the ARP tank reaches an average concentration of 860 mg/l excluding the amount phosphor incorporated in the cells of the PAO bacteria. This entails in a total COD production of approximately 4.1 t soluble COD/d, which can be used for denitrification in the main plant.

The BIO-P activity has also increased. This is seen by the decrease of the T-P concentration in the effluent but also on the soluble PO 4 -P concentration in the ARP tank. Here the PO 4 -P concentration reaches an average of 9 mg/l in the end of an anaerobic cycle. The phosphate concentration shows a moderate BIO-P activity. The activity will not be higher at Wangjiashan waste water treatment plant, due to the low inlet concentrations of phosphor, which inhibit the growth of the PAO bacteria. CONCLUSION In December 2010, full scale experiments at Bjergmarken WWTP were initiated. The experiments were to clarify the potential for COD conversion as well rates for nitrification and denitrification. A number of process parameters e.g. biological uptake rates have been calculated using both on-line data from the main plant and from the ARP tank. The AUR rates obtained are between 0,8 to 2 gvss*h N/ whereas the NUR rates are between 2,5 and 6,5 gn/gvss*h in the main plant. The NUR rates are relatively high compared to rates typically obtained using raw waste water as a carbon source and is obtained by leading the easily degradable organic compound produced in the ARP tank to the main plant. A pilot plant experiment was conducted at the Lynetten waste water treatment plant in order to verify and elaborate the results seen in the full scale experiments. Here it was seen that introducing the ARP resulted in an increased capacity of 63 % with regards to nitrogen removal and approximately 40 % with regards to COD removal, this despite the fact that the total sludge age was increased with only 24 %. The BIO-P activity was decreased in the period with ARP operation, due to smaller volume used for the sludge hydrolysis compared to the reference operational scheme. The process has been implemented in full scale at Wangjiashan waste water treatment plant in China. This has entailed a decrease in the T-N concentration in the effluent as well as decrease in the T-P concentration in the effluent. ACKNOWLEDGEMENTS The project has been carried through with the support of Lynetten A/S, who has put their pilot plant available for use in the project period and has maintained the pilot plant as well. Furthermore the full scale project at Wangjiashan WWTP has been carried out with support from the Danish Environmental Protection Agency. The results at the plant have been obtained by master student Alexander Grossmann and professor Jes la Cour Jansen at Lund University. REFERENCES Barnard, J.L. and Abraham, K. (2005) Key features of successful BNR operation. Proceedings of the International Water Association Specialty Conference on Nutrient Management in Wastewater Treatment Processes and Recycle Streams, Krakow, Poland, September 19 21, 2005. Barnard, J. L.; Abraham, K. (2006) Key features of successful BNR operation. Wat. Sci. Tech., 53 (12), 1. Grossman, A. (in press). Assessment of the activated return sludge process at Wangjiashan wastewater treatment plant. Master Thesis, Water and Environmental Engineering, Department of Chemical Engineering, Lund institute of Technology, Lund, Sweden Gujer, W., Henze, M., Mino, T. and van Loosdrecht, M. (1999) Activated sludge model no. 3. Wat. Sci.Tech. 39 (1), 183 193.

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