Approaches to maximize energy recovery in sludge management

Abstract

Wastewater treatment plants (WWTPs) are typically designed with the aim of meeting effluent criteria. The recovery of energy is frequently regarded as a secondary concern. In recent years, there has been a shift in terminology, with WWTPs being rebranded as water resource recovery facilities (WRRFs). This reflects a shift in focus towards the recovery of resources present in wastewater streams, including energy and nutrients. The biological treatment of wastewater relies on bacterial activity to facilitate the conversion of organic matter into carbon dioxide (CO₂). As a consequence of this process, a significant amount of sludge is produced as a byproduct. The recovery of energy from this sludge is achieved through anaerobic digestion, whereby the organic matter is converted into methane (CH₄) gas. In today's world, novel configurations for wastewater treatment systems have been designed with the aim of enhancing the capture of organic compounds from wastewater and contributing to the development of energy-neutral or energy-generating WWTPs. The selection of an appropriate configuration may influence the digestibility of the sludge, and therefore affect the overall energy balance and the total costs incurred. This thesis explores the characterization of the excess sludge from different novel compact municipal wastewater treatment processes including the aerobic granular sludge (AGS) process, and integration of high rate activated sludge (HRAS) with membrane bioreactor (HRAS+MBR) process with a focus on energy recovery and feasibility. The findings were comparatively evaluated with the conventional anaerobic/anoxic/aerobic (A2O) process so as to identify the optimal process considering energy recovery and feasibility. Then, this thesis focused on developing strategies to maximize energy recovery for the optimal process in sludge management. In order to gain insight into the rationale behind the thesis, Chapter 1 provides a concise overview of the innovative wastewater treatment processes and anaerobic digestion. Chapter 2 provides comprehensive information regarding the wastewater treatment process, energy and resource recovery from excess sludge, and the economic viability of the novel wastewater treatment processes. Chapter 3 involves a comprehensive description of the materials and methods utilized in the studies presented in this thesis. It includes detailed information on the characteristics of wastewater and sludge; the experimental setups; analytical methods; operational conditions; and experimental plan. Additionally, it explains the techno-economic analysis and mass balances. Chapter 4 presents the findings obtained from four distinct studies. Chapter 5 outlines the key conclusions and offers insights into potential future research directions and perspectives. The present thesis addresses the following research topics, which are discussed in Chapter 4: Study 1, benchmarking of the digestibility of the excess sludge obtained from three different wastewater treatment processes including A2O process, AGS process and HRAS+MBR process, Study 2, techno-economic feasibility analysis for these wastewater treatment configurations, Study 3, evaluation of co-treatment of wastewater and food waste in the AGS process, and methane potential of mono- and co-digestion of the excess sludge from the AGS process, Study 4, exploration of the digestibility of low-temperature thermal-alkali pretreated excess sludge from the AGS system and the phosphorous recovery potential. These topics were investigated through laboratory scale studies using real municipal wastewater. Study 1 characterized the excess sludge generated from the A2O process, HRAS+MBR process, and AGS process. For this purpose, the biomethane potential (BMP) test was conducted to investigate the digestibility of these excess sludges under mesophilic conditions. The physicochemical characteristics and digestibility of each sludge were investigated. Degradation of organic fractions, and change in sludge characteristics during anaerobic digestion were determined. The sludge from the A2O process consists of proteins, hemicellulose, cellulose, lignin, lipid, volatile fatty acids (VFA), and carbohydrates in 46%, 30%, 8%, 7%, 6%, 2% and 1% in order. Organic compounds in sludge from the AGS process were mainly 41% protein, 33% hemicellulose, 9% lignin, 7% cellulose, 7% lipids, 2% VFA, and 1% carbohydrates. Sludge from the HRAS+MBR process was mostly composed of a high amount of proteins (53%), hemicellulose (14%), lipids (14%), celulose (11%), lignin (5%), VFA (2%), and carbohydrates (1%) . Results showed that anaerobic digestion of sludge from the HRAS+MBR process yielded the highest methane with a value of 212 ± 18 mL CH4/g VS. Methane yield of sludge from the AGS process (173 ± 11 mL CH4/g VS) was slightly lower than that from the A2O process (180 ± 1 mL CH4/g VS). Based on the chemical oxygen demand (COD) mass balance, only 18.8% of COD could be converted into methane gas if the sludge from the A2O process was digested. Compared to the A2O process, the organic matter in wastewater converted into methane gas was around 21.0%, and 23.3% for the AGS process, and HRAS+MBR process, respectively. Study 2 revealed a comprehensive techno-economic feasibility analysis of three systems including A2O process, AGS process, and HRAS+MBR process. The distributions of different components in capital and O&M costs were evaluated comparatively for each system in the study. Moreover, energy balance and environmental benefit analysis were conducted to have a better understanding of the economic performance of all systems. 21% lower energy consumption was observed in the AGS system in comparison to the A2O system. This finding might be associated with the absence of mixers, return sludge pumping and recirculation of wastewater for nitrogen removal in the AGS system. The unit total cost was found to be 0.113 €/m3, 0.091 €/m3, and 0.195 €/m3 for A2O process, AGS process, and HRAS+MBR process in order. Based on the energy balance and economic analysis, AGS system was found to have lower net energy consumption and total cost compared to A2O process and HRAS+MBR process. Study 3 focused on the effect of co-treatment of municipal wastewater and food waste on the treatment performance of the AGS process. The impact of co-treatment of municipal wastewater with food waste on the morphology of granules and treatment performance of the AGS process was investigated. At stage 1, raw wastewater from a full-scale municipal WWTP was fed to a laboratory-scale AGS reactor, whereas a mixture of waste- water and food waste was fed to the AGS reactor at stage 2. Besides, the BMP test was performed to determine the methane yields of the digestion of the excess sludge from the AGS system and co-digestion of the excess sludge from the AGS system with food waste. The addition of food waste into wastewater enhanced the nutrient treatment efficiency in the AGS process. BMP of the excess sludge from the AGS process fed with the mixture of wastewater and food waste (195 ± 17 mL CH4/g VS) was slightly higher than BMP of excess sludge from the AGS process fed with solely wastewater (173 ± 16 mL CH4/g VS). The highest methane yield was observed for co-digestion of excess sludge from the AGS process and food waste, which was 312 ± 8 mL CH4/g VS. Integration of food waste as a co-substrate in the AGS process would potentially enhance energy recovery and the quality of effluent in municipal wastewater treatment. Study 4 investigated the digestibility of low-temperature thermal-alkali pretreated excess sludge from the AGS process and phosphorus recovery potential as struvite from digested sludge. The highest methane yield was obtained for the pretreated sludge sample at 100 C and pH 10 (S3), which was 216 ± 22 mL CH4/g VS. The low temperature thermal-alkaline pretreatment before the anaerobic digestion can be proposed as a good option to enhance the BMP with an enhancement of 27% for S3 concerning the sludge sample without any pretreatment (S0). On the other hand, pretreated sludge sample at 100 C and pH 11 (S4) and pretreated sludge sample at 100 C and pH 12 (S5) had lower improvement in methane production compared to S3, which might be explained by the inhibition effect of sodium hydroxide (NaOH) concentration. The phosphorus recovery rate increased with the increment of pH values for each Mg/P ratio. The optimum phosphorus recovery rate with a value of 91.9 ± 3.7% was obtained at the Mg/P molar ratio of 1.5:1.0 and pH 10. Besides, the unit total cost and benefit for the struvite production were calculated to be 3.40 $/ton sludge, and 16.23 $/ton sludge, respectively. Overall, low-temperature thermal-alkali pretreated followed by struvite crystallization provided an economic, and sustainable approach with environmental benefits for enhancing energy and phosphorus recovery from excess sludge. This thesis elucidates the potential of the AGS process to enhance energy and phosphorus recovery within the framework of a circular economy. Based on the energy balance and economic analysis results, it was found that the AGS system had the most energy efficient treatment system and the lowest net total cost compared to the A2O process and HRAS+MBR process. The findings of this thesis indicate that the integration of food waste as a co-substrate in the AGS process has the potential to enhance energy recovery and the quality of effluent in municipal wastewater treatment.