Optimized Nutrient Removal in Waste Water Treatment Plants

Transcription

1 Optimized Nutrient Removal in Waste Water Treatment Plants Activated Return Sludge Process Sille Bendix Larsen, Jeanne Agertved Madsen & Gert Petersen EnviDan East A/S Copenhagen, Denmark Jes la Cour Jansen Department of chemical engineering Technical University of Lund Lund, Sweden In the forthcoming years China has to face several obstacles in order to increase the treatment efficiency of the current waste water treatment plants. EnviDan and Aihua are now introducing the Activated Return sludge Process at Ma-an-Shan waste water treatment plant in China. The process can increase the capacity of existing plants with a demand for construction that is % of the volume demand when using conventional techniques for waste water treatment, thereby increasing the potential for nitrogen removal. Furthermore the process can be combined with biological phosphor removal, thereby saving cost for chemical precipitation. Envidan has implemented the process at several plants in Denmark and Europe with great succes. Thus it withholds a great potential for the Chinese waste water treatment sector. Nitrogen removal, phosphor removal, Activated Return Sludge Process, I. INTRODUCTION EnviDan, a Danish consulting company, have together with clients developed a process called Activated Return sludge Process (ARP). The process uses the all ready existing hydrolysis process in the WWTP in a new and controlled way. Furthermore the process uses the COD absorbed on the return sludge, thereby increasing the potential for full denitrification at WWTPs with low COD/T-N ratio. The process is operating in full scale at several WWTPs in Denmark and the Baltic countries in Europe. EnviDan has, together with Aihua Municipal Capital Engineering Company, Technical University of Lund and University of Tianjin obtained support from the Danish Ministry for Environment in order to develop the ARP process in full scale at the Ma-an-Shan WWTP in China. China is currently upgrading many of the existing waste water treatment plants (WWTP) from class III and II to class I, including effluent standard A and B. In order for the WWTP to oblige with standard A1 the plant will have to perform full nitrogen removal and phosphor removal. In Denmark, the standard of A1 has been enforced for the last years; hence the Danish consulting companies and WWTPs have a long experience in conducting full nitrogen removal as well as phosphor removal. II. THE ACTIVATED RETURN SLUDGE PROCESS The ARP process is built on two well known processes within waste water treatment: A: Sludge hydrolysis B: Adsorption of COD onto activated sludge A. Hydrolysis Sludge hydrolysis is a process where sludge is converted into soluble COD and ammonia (NH 3 ). The process takes places under anaerobic conditions. The process can be summarized to the chemical equation below: C 5 H 7 NO 2 + 3H 2 O 2.5 CH 3 COOH + NH 3 (1) The reaction is only showed as decomposition of biomass into acetic acid. The products of the reaction can also occur as Volatile Fatty Acids (VFA), for example acetate, propionate, isobutyrate, butyrate and many more. [1],[2] and [3] B. Adsorption of COD onto activated sludge Adsorption is a process that describes the adsorption of COD (soluble and suspended) on the activated sludge flocks. In the further waste water treatment process, COD is transported around in the WWTP, being decomposed during a full sludge age. Under normal operation conditions approximately 50 % of the COD will be removed from the WWTP by the surplus sludge; hence the surplus sludge holds a great potential for COD production. [4] The COD in the surplus sludge is for example used during anaerobic digestion, where it is converted into gas. It can also be used during the ARP process, where the COD is used for biological phosphor removal and denitrification. C. The Activated Return Sludge Process

2 The ARP process takes part in a volume separated from the main plant, where only return sludge is fed to the volume. The principal is illustrated in Figure 1. sufficient soluble COD for biological phosphor removal. 4. Minimizing surplus sludge: Since a larger amount of sludge is aerated and the total sludge age in the WWTP is increased, a larger amount of endogenous respiration will take place, thereby decreasing the surplus sludge. The process can be combined in order to obtain some of all of the above mentioned purposes or can be focused on one or two of the purposes. III. APPLICATION OF THE ARP PROCESS IN DENMARK The process is applied, among other plants, at Bjergmarken WWTP in Roskilde Municipality, Denmark. Figure 1. Principal diagram for the ARP process Since the ARP part of the plant is independent from the main plant, it can be applied for all plant types, hereunder alternating ditch plant, recirculation plants, and carrousel plants and so on. The ARP plant is equipped with mixers and aeration equipment and is therefore a concentrated version of the main plant (due to the higher sludge concentration from the return sludge). The ARP plant also performs both nitrification and denitrification in an operational cycle. The WWTP has a capacity of 125,000 PE, which makes it a middle size WWTP according to Danish standards. The plant is shown in Figure 2. ARP The ARP technology can be used for the following purposes: 1. Increasing the organic capacity: by establishing an ARP process as a supplement or in an already existing plant the total sludge mass in the plant will be increased. Hence the capacity of the plant will correspondent to the extra sludge mass in the plant. The main plant will still have to perform nitrification and denitrification, but the removal of COD will be moved to the ARP plant, thereby increasing the total capacity of the plant. 2. Increasing the hydraulic capacity of the secondary clarifiers: By implementing the ARP process in an existing WWTP the hydraulic capacity of the secondary clarifiers can be increased. This is due to the fact that 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. 3. Biological phosphor removal: The ARP tank can be used for biological phosphor removal because the tank produces easily degradable COD. 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 used for denitrification, hence the PAO will not be able to perform biological phosphor removal to an extend that is sufficient to reach the effluent standard. However in the ARP tank the conditions will change between aerobic, anoxic and anaerobic which produces in Figure 2. Bjergmarken WWTP The plant is an alternating ditch plant with secondary sedimentation and anaerobic sludge treatment. Previously the plant also had primary sedimentation. The primary sedimentation has been closed and the ARP process has been implemented in one of the previous primary sedimentation tanks. The tank has been equipped with brush aerators and mixers. Pictures from the ARP tank in operation are shown in Figure 3. Figure 3. ARP at Bjergmarken WWTP The main plant has a volume of 21,900 m3, whereas the ARP plant has a volume of 1,990 m3. Before implementation of

3 the ARP plant, the plant was operated with 5 kg MLSS/m 3, a QRAS flow of 71 % and a MLSS concentration of 12 kg MLSS/m 3 in the return sludge. The operation is illustrated in Figure 4. clarifiers can be increased with 35 %. The capacity increase of the secondary clarifiers is a consequence of a smaller return sludge flow together with a smaller sludge to the clarifier due to the lower MLSS concentration in the main plant. Normal Operation Winter Conditions 5 kg SS/m3 AS Volume m PE Extended Capacities using the ARP concept Extra Hydraulic Capacity (when needed) 4,15 kg SS/m m3 66% RAS ARP tank 33% RAS 1990 m3 SS to 71,4 % of inlet flow 8,57 kg SS/m3 of inlet flow SS to 4,8 % more SS in system 8,3 % more Volume 52,9 % of inlet flow 6,34 kg SS/m3 of inlet flow Figure 4. Normal operation of Bjergmarken WWTP without ARP A. Extra organic capacity At Bjergmarken WWTP 1,990 m 3 is converted into the ARP process. The ARP tank can be operated with a high sludge concentration of 12 kg MLSS/m 3. If the main plant is still operated with 5 kg MLSS/m 3, will the ARP process entail an increase of 23.9 t SS in the total sludge mass. The operational principal is shown in Figure 5. Extended Capacities using the ARP concept Extra Organic Capacity (when needed) Figure 5. 5 kg SS/m m3 Operation of Bjergmarken WWTP with extra organic capacity using the ARP Due to the extra sludge mass in the ARP tank the total sludge mass will be increased with 17 %, which corresponds to a capacity increase of 17 %. B. Extra hydraulic capacity 66% RAS The same ARP tank as described in the previous section can also be used for increasing the hydraulic capacity of the secondary clarifiers. This is done by decreasing the sludge concentration in the main plant, while the total sludge mass in the plant is sustained by the new sludge mass in the ARP tank. The principal for the operation is shown in Figure 6. The MLSS concentration in the main plant is decreased from 5 kg MLSS/m 3 to 4.2 kg MLSS/m 3 together with a decrease in the QRAS flow the capacity of the secondary ARP tank 33% RAS 1990 m3 SS efficiency, %: 80 21,8 % more SS in system 8,3 % more Volume SS to 71,4 % of inlet flow 8,57 kg SS/m3 of inlet flow Reduction in SS load to = 0,0 % Increased hydraulic capacity= 0,0 % Increased organic capacity= 17,4 % = PE Figure 6. Operation of Bjergmarken WWTP with extra hydraulic capacity using the ARP C. Actual operation at Bjergmarken WWTP The sections above are theoretical calculations, whereas the actual operation of the plant have had other concequences as well. The MLSS concentration in the return sludge can be as high as 25 kg MLSS/m 3. This gives an extra capacity of 31 %, which means that the ARP plant treats wastewater from PE in 8 % of the total volume. Due to the higher sludge mass in the total plant, the surplus sludge production has been decreased with 5-12 % before the anaerobic. After the the sludge production has been decreased with %. Despite the decrease in sludge production the biogas production has been suistained at a similar level to that before the implementation of the ARP. The mass balance is seen in Table 1. Table 1. Sludge mass balance for Bjergmarken WWTP before and after implementation of ARP (DS = Dry Solids) Sludge mass balance for Bjergmarken WWTP Period kg DS/week before kg DS/week after Biogas m 3 /week Spec. biogas m 3 /kg DS to ,787 24,433 10, st quarter of th quarter of th quarter of 2008 Reduction in SS load to = 26,0 % Increased hydraulic capacity= 35,1 % Increased organic capacity= 0 % 34,900 24,111 9, ,329 21,760 10, ,050 20,340 9, Besides the decrease in surplus sludge, the biological phosphor removal was optimized and the demand for chemical precipitation of phosphor was decreased. IV. APPLICATION IN CHINA With regards to the increasing demand for better effluent quality from the WWTPs in China, China stands across several obstacles. First of all most of the WWTPs in China are not

4 built to perform denitrification; hence a demand for extra volume is seen in order to oblige with the A1 standard. Secondly Chinese wastewater has a natural low COD/T-N ratio, which can cause problems for the denitrification due to lack of sufficient COD. The ARP process is thus a good alternative to conventional treatment plants because the demand for volume typically % of the volume demands of conventional wastewater treatment techniques. Furthermore the ARP process will also increase the potential for biological phosphor removal; hence eliminate the demand for chemicals for precipitation and thereby further eliminate the construction costs. with the A1 standard the capacity of the plant will decrease to 55,000 m 3 /d and the plant will have to conduct chemical phosphor removal. EnviDan and Aihua will reconstruct the plant in order to establish recirculation between the middle ring and the outer ring; furthermore two of the existing triple ditches will be converted into an ARP tank (in total 5,000 m 3 which corresponds to 15 % of the total volume of the plant). The plant will conduct biological phosphor removal as well as facilitating the necessary capacity increase. A principal sketch is seen in Figure 8. EnviDan and Aihua are supported by the Danish Ministry for Environment in order to set up a full scale combined process for ARP and biological phosphor removal at Ma-an- Shan WWTP. A. Ma-an-Shan WWTP Ma-an-Shan WWTP consists originally of a triple ditch. The ditch was later on closed when two new orbital ditches where constructed. A principal sketch of the plant is seen in Figure 7. Figure 8. Principal sketch of Ma-an-Shan WWTP with the ARP process Figure 7. Principal sketch of Ma-an-Shan WWTP (AN = Anaerobic, N = Nitrification, DN = denitrification and SC = Secondary Clarifier) The plant has a capacity of 60,000 m 3 /d and has to oblige with Chinese B1 standard, however the effluent standard has to be upgraded to A1. The load to the plant is presented in Table 2. Table 2. Load to Ma-an-Shan WWTP Load to Ma-an-Shan WWTP mg/l (average) kg/d (average) mg/l (max) kg/d (max) COD , ,441 BOD 150 7, ,730 SS , ,869 T-N , ,098 T-P The COD/BOD ratio is 1.9, the COD/T-N ratio is 8 and the COD/T-P ratio is 64. According to the experience by EnviDan a COD/T-N ratio of 8 should be sufficient in order to obtain full nitrogen removal (with COD with low inert COD content). Furthermore, a COD/T-P ratio of 63.7 should be sufficient to obtain biological phosphor removal. If the plant is to oblige By retrofitting the plant as described above, the plant will be able to reach an effluent quality of 5 mg T-N/l and the capacity will be increased to 65,000 m 3 /d. The plant will be operated with an online control system for both the main plant and the ARP plant. The control system will use online sensor for oxygen, ammonia and nitrate. The sensors in the main plant will control the aeration in the outer ring, whereas the sensors in the ARP will control the phases between aerobic, anoxic and anaerobic in order to also obtain biological phosphor removal. The testing period will operate for one year in order to include temperature variations. During the testing period rates for nitrification, denitrification and phosphor uptake will be established in the main plant as well as the ARP plant in order to compare the rate to the sludge content in each part of the plant. The results from the testing will have great influence on the waste water treatment in China in the forthcoming years, since a successful use of the ARP process on Chinese WWTPs will diminish a large part of the of the future construction need on Chinese WWTPs. V. REFERENCES [1] J. L. Bernard and K. Abraham, Key features of succesful BNR operation, Nutrient Management in waste water treatment processes and recycle streams, Krakow, 2005 [2] M. Henze, P. Harremoës, J. L. C. Jansen and E. Arvin, Waste water treatment, Biological and chemical processes, 3 rd ed. Springer, 2000, pp [3] J. Vollertsen, G. Petersen and V. R. Borregaard, Hydrolysis and fermentation of activated sludge to enhance biological phosphor removal, Nutrient Management in waste water treatment processes and recycle streams, Krakow, 2005 [4] M. Henze, C. P. L. Grady, W. Gujer, G. V. R. Marais and T. Matsuo, Activated sludge model No. 1 IAWPRC London, 1987

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