High-rate activated sludge process for energy efficient wastewater treatment

Abstract

The conventional activated sludge (CAS) process used in wastewater treatment is inefficient in terms of energy and requires a large space. CAS plants can only recover around 33% of the energy in wastewater, which is a significant drawback. The growing urban population and wastewater volume pose a challenge in land-constrained areas like Istanbul. Therefore, it is important to adopt innovative treatment processes that minimize land requirements, maximize organic matter capture, and reduce carbon dioxide (CO2) losses. This approach, known as carbon capture, redirection, or harvesting, focuses on diverting carbon from wastewater to the sludge line for biogas production in anaerobic digesters. High-rate activated sludge (HRAS) systems, developed in the 1980s, aim to redirect carbon from wastewater to anaerobic digesters for energy production, offering an alternative to primary sedimentation in wastewater treatment plants (WWTPs). They are operated at high organic loading rates and have short sludge retention times (SRTs) and hydraulic retention times (HRTs). In the B-stage, pollutant removal primarily occurs through biological oxidation at lower organic loading rates. The HRAS process has proven to be superior to CAS and primary sedimentation units with regard to carbon redirection and methane generation during anaerobic digestion. However, several areas for improvement have been identified based on existing literature. Firstly, there is a lack of information regarding the performance of HRAS systems when equipped with lamella clarifiers, which can enhance the performance of solid-liquid separation in WWTPs. Secondly, existing mathematical modeling studies of the HRAS process are either overly complex or overly simplistic, often overlooking important processes like adsorption and focusing primarily on carbon removal. Furthermore, sensitivity and uncertainty analyses, commonly employed to assess the robustness of models, have not been extensively applied to HRAS system models. Thirdly, despite its potential, there is a scarcity of literature on integrating the HRAS process into water reclamation practices, which could benefit from its smaller spatial footprint, lower energy consumption, and nutrient-rich effluent. Lastly, it is essential to address the fact that the use of coagulants in the HRAS process improves phosphorus removal efficiency while increasing operational costs. Limited research has been conducted on the impact of using water treatment plant (WTP) sludge on the anaerobic digestion of waste sludge in WWTPs and its influence on HRAS process performance and energy recovery. This thesis explores the potential of the HRAS process in sustainable wastewater treatment and investigates the following research topics: Study 1, optimum operational conditions for the HRAS process with a lamella clarifier, Study 2, mathematical modelling of the HRAS process to provide a practical tool for future implementations of HRAS plants, Study 3, integration of membrane filtration for reclaimed water production for industries, Study 4, post-treatment alternatives for HRAS process effluent for irrigation purposes, and Study 5, the reuse of WTP sludge to enhance treatment performance and resource circularity. These topics were investigated through pilot and laboratory scale studies using real municipal wastewater. A pilot-scale HRAS plant with a lamella clarifier was constructed in a full-scale preliminary WWTP (PWWTP) in Istanbul. The plant was operated under various conditions for two years to address the research topics. Laboratory-scale experiments involving membrane filtration and chemical precipitation were conducted using real municipal wastewater collected from the same PWWTP. Study 1 focused on determining the optimal operational conditions for the pilot-scale HRAS system coupled with a lamella clarifier. The study found that using a lamella clarifier resulted in lower total suspended solids (TSS) concentrations in the effluent and required a smaller footprint compared to a conventional clarifier. The optimum operational condition was identified as Stage 1, with an HRT of 75 minutes and a dissolved oxygen (DO) concentration of 0.5 mg/L. This condition demonstrated the best effluent quality, highest carbon capture, and highest production of extracellular polymeric substance (EPS). The study also found that reducing the HRT increased biosorption but led to increased chemical oxygen demand (COD) loss through the effluent. Lower DO concentrations promoted carbon redirection but resulted in weak floc formation and increased particulate COD (xCOD) loss. Meanwhile, higher DO concentrations enhanced COD oxidation but allowed more particles to escape through the effluent. Overall, the HRAS process with a lamella clarifier showed promising particulate matter removal efficiency and the potential for reclaimed water production. Studies 3 and 4 further investigated the inclusion of the HRAS process in reclaimed water production systems. Study 2 developed and calibrated a mathematical model for HRAS systems by integrating Activated Sludge Models No. 1 and 3 (ASM1 and ASM3), accounting for substrate adsorption and storage. The calibration utilized dynamic data from the pilot-scale HRAS plant and identified influential parameters like maximum specific growth rate (µ), growth yield (YH), storage yield (YSTO), storage rate (kSTO), decay rate (b), and readily biodegradable substrate half-saturation coefficient (KS1). The calibrated model demonstrated satisfactory efficiencies for mixed liquor suspended solids (MLSS), total COD (tCOD), soluble COD (sCOD), xCOD, total nitrogen (TN), ammonia nitrogen (SNH), total phosphorus (TP), soluble TP (sTP), and particulate TP (xTP), all above 70%. However, an uncertainty analysis exposed discrepancies in sCOD. The study also highlighted the potential to enhance sTP dynamic behavior estimation. The low model efficiency is likely due to variations in wastewater characteristics, especially the phosphorus (P) fractions, which were not dynamically considered in the model. Another reason could be the precipitation of phosphate salts, which was not included in the model. Overall, the study offers valuable insights into influential parameters and opportunities for refining the HRAS process modeling. Study 3 encompassed both experimental and cost analyses to evaluate different treatment configurations for water reclamation. Six configurations were examined, incorporating pre-treatment options like direct membrane filtration (DMF) via microfiltration (MF) and ultrafiltration (UF) membranes, and HRAS, and final treatment alternatives such as nanofiltration (NF) and reverse osmosis (RO). The performance of NF and RO membranes ensured that the reclaimed water from each scenario met the required quality standards for cooling tower makeup water. Despite HRAS producing effluent with higher turbidity compared to MF and UF membranes, the cost analysis revealed that the HRAS+NF configuration (C3) offered the most cost-effective treatment, with a cost of 0.38 €/m3 of wastewater. This cost advantage was due to the lower expenses associated with the HRAS process compared to MF and UF membranes.