Examination of the OSA process for the purpose of excess sludge minimization at activated sludge process: Modelling of a full-scale plant

Activated sludge process, which is used for domestic and industrial wastewater treatment, is consisted of an aeration basin (biological processes occur) and a final clarifier respectively. Certain amount of the sludge is recycled back to the to the aeration tank, in order to achieve a desired sludge age at whole of the system. Thus, a part of the organic matter is converted to biomass while nitrification is carried out. Excess sludge production emerges as a serious problem in activated sludge processes even if it is implied as an advantageous system with biomass formation and the capability to do nitrification. Control of the excess sludge is possible with digestion, stabilization, chemical treatment and thickening; though still burdensome in terms of cost. Some alternatives are investigated and the one, whose applicability's easy and that can approximately provide % 60 reduction in sludge minimization in comparison with the conventional activated sludge system, called as "OSA (Oxic-settling-anoxic) process", has been chosen of other systems due to high costs. In this research "conventional activated sludge" and "OSA process" were compared in terms of excess sludge reduction capacity via using a mathematical modelling program which is called GPS-X®. Study was conducted following the three steps; experimental set-up (daily analysis were conducted), respirometric analysis and modelling. In the first phase, reactors were set up to exhibit Activated sludge process and OSA process. Sequencing batch reactor was used to simulate conventional activated sludge process.An anaerobic bioreactor integrated with the sequencing batch reactor was used to simulate the OSA process. Sequencing batch reactors were chosen in both of the systems due to it's performance on both carbon removal and nitrification/denitrification. Both systems were operated at the same time, having a total of 4 cycles each including 6 hours during the day. The sludge used at the beginning of the installation of the reactors and the wastewater that's used during the operation were obtained from the same treatment plant. Domestic and industrial wastewater's are treated in the wastewater treatment plant. Around % 40 of the wastewater is coming from industrial acitivities, which are almost textile industries. The rest of the wastewater is coming from domestic activities. Though it's known that wastewater entering to the plant was intensive in terms of suspended solids; for this reason wastewater was added to the lab-scale systems, after having settled about 30 minutes. Sequencing batch reactor's volume used in the experimental set-up were 2 liters, both were fed with 500 ml of wastewater during feeding phase. Timed pumps were used for the addition of the wastewater to the systems, for the exchange (1/10th of the aerobic sludge was exchanged with the anaerobic sludge on a daily basis) and for the discharge from the systems. Some analysis were conducted to the samples taken from sequencing batch reactors and to the discharged wastewater from the systems. Samples were taken from each reactors twice a week to perform suspended solids and volatile suspended solids analysis. Daily samples were taken from discharges to do SS and VSS analysis after having a stable condition in both of the systems. In addition to these experiments COD analysis was also performed to interrogate the COD removal rate of the systems. Samples were taken once a week from the anaerobic reactor to conduct suspended solids, volatile suspended solids and soluble COD analysis. All of the experiments performed in the first phase were to ensure of the steady-state conditions in the systems. In the second phase, respirometric tests were conducted to measure the oxygen uptake rate of the systems after having steady-state conditions. Experiments were done to have a comparable result of the systems by means of the oxygen consumption rate of the microbial population during the operation of the systems. System's differ from each other in terms of anaerobic reactor integrated to the sequencing batch reactor. Three sets of respirometric test were performed to differentiate the endoogenous decay levels. In the first two set-up, sludge was taken from the SBR's and then used with the addition of nitrification inhibitor, wastewater and tap water for the oxygen uptake rate test. For the last set-up, sludge was taken from OSA system's SBR and also from the anaerobic reactor; while nitrification inhibitor, wastewater and tap water was added. According to the respirometric experiments some of the COD fractions were estimated by using the OUR values in the calculations. In the third phase, a model was created in order to represent the wastewater treatment plant. While doing it a mathematical model approach was adopted to perform the optimization of the treatment plant by using a commercial software called GPS-X® program. In modelling study, every unit in the plant configuration was entered to the model. Calibration was carried out just after entering the processes occurring in every unit including its dimensions. Plant influent wastewater's characterization was demonstrated in the model with the data obtained from laboratory experiments. Beneath these data, COD and flow values of 1 year relevant to the influent wastewater has been entered for the calibration step of this phase. Some kinetic and stoicihometric coefficients were tried to calibrate the system. Shortly after the calibration step, verification was done by using 1 year COD and flow data of the plant. After having a steady-state in the model, scenarios were created. 4 scenarios were simulated additionally to the current situation of the treatment plant. These scenarios were build up in accordance with the literature review and also on the basis of treatent plant's configuration. Model data was compared in terms of sludge production at the system in the absence of Cannibal tank and belt filter. Sludge amount is increased about 27 tonnes on a daily basis and yield was decreased to 0.506 gVSS/gBOD5 from 0.729 gVSS/g BOD5 within this scenario. Sludge age in the anerobic tank was changed as 10 days and 15 days. In both of the sludge ages little difference was observed. Interchange rate value was assessed to interrogate the sludge production. IR value was % 10 in the current condition; for the modelling this was changed as % 5 and % 15. Regarding the yield and sludge production it's found that increase in IR value is feasible via reduction of excess sludge. Sludge was reduced 12 tonnes on a daily basis and yield was also decreased when compared to the current situation. In the OSA process, some approaches were followed to explain the mechanisms of the sludge reduction. While none of them are verified. According to the literature review sludge is reduced by one of the following hypothesis: (1) microorganisms in the system is related to the slow growers, then due to this assumption they have low yield. (2) energy yielded in the metabolic activities of the microbial population is not equally shared between anabolic and catabolic acitivities. (3) sludge decay rate in the anaerobic reactor is above the normal values. Further research should be done to clarify the uncertainties of the sytem caused by inadequate information on the microbial population of the OSA system. For a better understanding of the excess sludge reduction mechanism molecular methods could be performed to observe microbial population for having an idea on the metabolic activities of the organisms.