The effect of water treatment plant sludge addition to the performance of a pilot-scale high-rate activated sludge system

Abstract

The rising energy demand driven by rapid population growth has encouraged the adoption of energy-efficient technologies in wastewater treatment, as in many other sectors. While the primary goal of wastewater treatment is removing pollutants, certain processes also enable greater recovery of energy from the organic matter present in wastewater, therby contributing to sustainability. Among the range of biological wastewater treatment methods, the activated sludge process is highly favored. In the conventional activated sludge (CAS) process, most of the organic material in wastewater is converted into carbon dioxide under intensive aeration. This process serves as an energy source for microorganisms and also contributes to their growth. Excess microorganisms are removed from the system as waste sludge based on the predetermined sludge age. The organic material in this excess sludge can then be converted into biogas containing approxiamtely 60-70% methane through anaerobic digestion. Biogas is subsequently utilized for power generation in gas motors, enabling energy recovery from wastewater. From an energy management perspective in wastewater treatment, the conversion of organic matter to carbon dioxide by aerobic microorganisms in CAS systems represents an energy loss, as this carbon can no longer be converted to methane in anaerobic digestion. To maximize energy recovery from wastewater, organic matter should be concentrated in excess sludge as rapidly as possible while minimizing its oxidation to carbon dioxide. The high rate activated sludge (HRAS) process can provide these conditions, as the design of the HRAS process relies on adsorbing and storing organic matter from wastewater while minimizing mineralization. Unlike CAS systems, which typically operate with a sludge age exceeding 5 days, dissolved oxygen (DO) levels above 2 mg/L, and a hydraulic retention time (HRT) of around 8 hours, HRAS systems keep organic matter mineralization to a minimum. In HRAS systems, the sludge age is kept below 2 days, the DO concentration below 1 mg/L, and the HRT under 2 hours, all to minimize the conversion of organic matter to carbon dioxide. Due to the presence of coagulants in it, water treatment plant (WTP) sludge holds potential as a valuable resource. Reduced access to landfill sites due to rising landfill disposal costs, including taxes, together with the growing emphasis on zero-waste targets, are driving the adoption of more sustainable and resource-oriented approaches to sludge management, such as the reuse and recycling of WTP sludge. Due to its low combustibility and organic content, incineration and anaerobic digestion are generally considered as unsuitable methods for WTP sludge management. Consequently, the exploration of options for recycling and reusing WTP sludge has gained substantial attention. For instance, one viable approach involves transforming it into cement, ceramics, or lightweight aggregates. Additionally, alternative approaches include using WTP sludge as a bed layer material in constructed wetlands or as a coagulant in wastewater treatment plants (WWTPs) to enhance solids removal, improve sludge settling, and facilitate dewatering processes. The primary aim of this thesis was to comprehensively investigate the feasibility of reusing WTP sludge as a low-dose coagulant in the HRAS process. To achieve this goal, a pilot-scale HRAS system was continuously operated with and without WTP sludge addition to treat preliminary treated municipal wastewater received from a full-scale WWTP in Istanbul, Türkiye. Samples of alum and iron sludges were collected from the dewatering units of full-scale water treatment plants, and subsequent jar tests were conducted in the laboratory to determine the appropriate WTP sludge dose for addition to the bioreactor of the HRAS plant. Based on the jar tests conducted using seed sludge mixed with iron and alum WTP sludges diluted to 3% solids and applied at a dosage of 0.3 mL/L HRAS sludge, the amount of WTP sludge added to the HRAS system was limited to a level that would increase excess sludge generation by no more than approximately 10%. Iron sludge increased total suspended solids (TSS), chemical oxygen demand (COD), and total phosphorus (TP) removal by 1.3±0.1%, 2.6±1.4%, and 3.0±1.7%, respectively, compared to the without WTP sludge addition case. In contrast, alum sludge resulted in smaller improvements of 1.0±0.1% for TSS, 0.4±1.0% for COD, and 0.6±0.9% for TP. Iron sludge was selected because it demonstrated overall higher removal efficiencies compared to alum sludge. Accordingly, the iron sludge dose was restricted to 20.1±1.6 mg dry matter per liter of wastewater to avoid increasing the inorganic matter content of HRAS sludge and not to deteriorate its methane potential. Since the average solids content of the WTP sludge was around 30%, it was diluted with tap water to approximately 3% solids to facilitate pumping. This diluted mixture was then fed to the bioreactor of the HRAS system via a peristaltic pump. Following the addition of WTP sludge, modest improvements were observed in the removal efficiencies of TSS, particulate chemical oxygen demand (pCOD), and TP, increasing from 78.0±0.4%, 75.2±2.3%, and 60.1±4.1% to 80.0±3.2%, 81.3±0.9%, and 63.8±2.3%, respectively. Moreover, the addition of WTP sludge significantly influenced some sludge characteristics, particularly by increasing the median particle size (D50) in the aeration tank from 96±3 µm to 163 ±14 µm while reducing sludge volume index (SVI30) from 38±6 mL/g to 32±4 mL/g. These changes enhanced sludge settleability and dewaterability, potentially reducing sludge treatment costs. However, biomethane potential (BMP) tests showed that WTP sludge addition reduced BMP by 8.9%, decreasing the average BMP from 330 NmL CH₄/g volatile solids (VS) to 301 NmL CH₄/g VS. The environmental impact of the investigated system, assessed using the effluent quality index (EQI) calculated from the average effluent concentrations of TSS, COD, total nitrogen (TN), and TP from both stages with literature-based weighting factors, showed a 2% improvement in case of WTP addition. When different sludge management scenarios, namely adding and not adding WTP sludge to the HRAS pilot-scale system, were analyzed to compare the costs of using WTP sludge in a hypothetical HRAS plant considering sludge transportation, sludge landfilling, and treatment costs, along with energy recovery from biogas production, the results indicated a total cost reduction of approximately 11% compared to the not adding case. This analysis included the transportation and landfilling of both WTP and HRAS sludge, as well as the energy and chemical use in the HRAS plant. This result underscores the benefits of reusing WTP sludge as a coagulant in wastewater treatment. This approach promotes a symbiotic relationship between WTPs and WWTPs, contributing to the development of a circular economy and enhancing resource circularity. This thesis found that integrating WTP sludge into the HRAS process resulted in limited improvements in treatment performance; however, future research could explore ways to investigate the impact of WTP sludge addition on treatment efficiency and biogas production. Moreover, advanced options for coagulant recovery, such as acidification, could also be investigated in this context. Then, long-term performance assessments would be beneficial for evaluating the operational stability, economic feasibility, and environmental sustainability of WWTPs with different configurations utilizing WTP sludge. Key areas for future research also include detailed process optimization, investigation of the effects of WTP sludge characteristics (e.g., iron vs. alum sludge) on anaerobic digestibility, and comprehensive cost-benefit analyses that consider both expenses and energy recovery potential. Additionally, examining relevant regulatory frameworks for wastewater management and aligning sludge reuse practices with broader waste management policies could help identify institutional barriers and opportunities. Collectively, these future directions provide valuable insights into the circular integration of water, wastewater and waste management systems and support the transition toward more sustainable and resource-efficient practices.