Application of high-rate activated sludge processes in municipal wastewater treatment

Abstract

Along with the circular economy and sustainability concepts, energy efficiency and resource recovery have become vital in wastewater treatment plants (WWTPs). Wastewater sludge is no longer considered as a waste to be disposed of but an energy source due to its organic matter. In anaerobic digesters, biogas can be produced from the process (or excess) sludge of WWTPs, which can be used to produce energy. By producing energy from the organic content of the wastewater, achieving energy neutral and/or positive WWTPs gained popularity throughout the world. Activated sludge systems, which biologically remove organic matter, nitrogen, and phosphorus from wastewater, have been widely applied worldwide in the last century. High aeration energy and large area requirements of the conventional activated sludge (CAS) processes led to the development of low-cost wastewater treatment technologies. One of these technologies is two-stage adsorption-bio-oxidation (AB) process. The purposes of A-stage and B-stage are removal of organic matter and nitrogen, respectively. One of the processes applied in A-stage of the AB process is the high-rate activated sludge (HRAS) process. HRAS process is a modificition of CAS process and has around 20 times higher organic loading rate compared to CAS process. Sludge retention time (SRT), hydraulic retention time (HRT), and dissolved oxygen (DO) concentration in the HRAS process are considerably lower than those in CAS process. The HRAS process aims to mineralize the organic matter in wastewater as minimum as possible and to increase carbon capture in the sludge with the biosorption mechanism. By this way, the amount of biogas produced from the excess sludge in anaerobic digesters can be increased. AB process has drawn considerable attention after the 1980s. The first AB process was established in Germany (Krefeld WWTP), which was taken into operation in 1981. Later, several AB processes have been established in Europe. Strass WWTP in Austria is one of the best examples of AB process since it achieves energy positivity. Most of the literature studies on HRAS process and full-scale HRAS process applications have conventional clarifiar. With lamella clarifiers, which have higher settling performance in a smaller footprint than conventional clarifiers, the footprint of the treatment plant can be further reduced. Nitrogen removal efficiency in HRAS process is not as high as the organic matter removal efficiency. For this reason, nitrogen concentration in the effluent of the HRAS process is higher than the effluent of CAS systems. In order to improve the nitrogen removal efficiency, low-cost nitrogen removal processes such as nitritation-denitritation or nitritation-anammox can be used in B-stage after HRAS process. The purpose of this thesis was to investigate the application of HRAS process including a lamella clarifier in municipal wastewater treatment and determine optimum operational conditions. In this study, the effluent of a grit chamber in a full-scale preliminary wastewater treatment plant was fed to a pilot-scale HRAS plant. Since HRT and DO concentrations are two important operational conditions, two HRTs (75 and 50 min) and three DO concentrations (0.2, 0.5, and 0.8 mg/L) were tested. The optimum conditions were determined based on the effluent quality and carbon capture. During the study, the system was operated with a SRT of 0.5 days. In the first part of the study, the optimum HRT was determined, while in the second part of the study the optimum DO concentration of the system was determined. Firstly, the pilot-scale HRAS system was operated with 75 and 50 min HRTs with the same DO concentration of 0.5 mg/L in Stage-1 and Stage-2, respectively. The optimum HRT was determined in terms of higher carbon capture and better effluent quality. Then, at the selected HRT, HRAS system was operated with DO concentrations of 0.2 and 0.8 mg/L in Stage-3 and Stage-4, respectively. It has been observed that Stage-1 (HRT: 75 min; DO: 0.5 mg/L) was the optimum operation condition. Total suspended solids (TSS), chemical oxygen demand (COD), total nitrogen (TN), ammonia nitrogen (NH4-N), and total phosphorus (TP) removal efficiencies of Stage-1 were 80±5%, 58±3%, 32±6%, 27±7%, and 58±10%, respectively. Based on a mass balance of the HRAS system, it was determined that 41.7% of the influent COD, 34% of the influent TN, and 60% of the influent TP were captured in the sludge in Stage-1. Comparison of different HRT and DO concentrations showed that low HRT and DO concentration increased carbon redirection. The reduction of HRT from 75 min to 50 min decreased COD lost via oxidation, which resulted in an increased biosorption. However, decrease in HRT worsened particulate matter caprute of the clarifier and caused a decrease in effluent quality. Increase in DO concentration caused further oxidation and hydrolysis of particulate matter. As a result, more soluble COD (SCOD) and colloidal COD (CCOD) were observed in the effluent. The extracellular polymeric substances (EPS) in sludge makes flocs come together and increase solid/liquid separation. While the highest EPS production was observed in Stage-1, the lowest EPS production was observed in Stage-3. Low EPS production in Stage-3 was the reason for weak flocs and particulate COD (PCOD) loss through effluent increase. Sludge volume index (SVI) was between 20-30 mL/g throughout the study, showing that HRAS process sludge had good settleability characteristics. Compared to the literature, TSS removal efficiency was quite high in the pilot-scale HRAS process, which may the effect of lamella clarifier. With HRAS process, capturing organic matter to the sludge would positively affect the biogas production in anaerobic digesters enabling energy efficient WWTP operation. Since nitrogen and phosphours are resources for agriculture, the presence of nitrogen and phosphorus in the effluent of HRAS process offers the possibility for use of the effluent for irrigation purposes. However, effluent quality should be improved due to particulate matter in the effluent and micropollutants that are likely to be found in domestic wastewater. Therefore, reuse of the effluent of HRAS process for irrigation purposes should be investigated.