Enhanced Primary Sludge Fermentation and Anaerobic Digestion for Integrated Carbon Upgrade and Resource Recovery
Abstract
The emerging paradigm shift towards renewable resource recovery, and energy minimization in municipal wastewater treatment plants (WWTPs) coupled with increased concern over nutrients-related eutrophication accelerated the development of biosolids treatment technologies for simultaneous waste minimization, resource recovery, and carbon upgrade. Biological nutrient removal (BNR) processes, often need excess carbon source to meet stringent quality standards. Despite successful use of sludge fermentation liquid to enhance the BNR, different techniques, are applied to improve the low conversion yield of fermentation and optimize resource recovery. In this context, insights on the impact of two commonly used primary treatment techniques (i.e. conventional primary clarifier and the emerging rotating belt filtration (RBF) technology) on the single and integrated anaerobic fermentation and digestion of wastewater biosolids was investigated in this study. Techno-economic assessment and optimization to simultaneously maximize volatile fatty acids (VFAs), and biomethane recoveries, and further application of internal carbon source to enhance the BNR process using a plant-wide approach, were also among the main objectives of this project.
The fate of cellulose study revealed that roughly 80% of the raw wastewater cellulose was removed in either of the primary treatment options, while represented 35%, and 17% of the total suspended solids (TSS) in the RBF and primary clarifier sludges, respectively. Cellulose was biodegradable irrespective of the biological process configuration and tested Solids retention times (SRTs), with effluent concentrations of about 2-3 mg/L.
pH-controlled fermentation was effective in improving the VFA yields by up to 93% and 72% at pH 9, for RBF and primary sludges, respectively. Furthermore, pH 6 was proposed as optimum considering significant enhancement in VFA production, while also lowering the amount of consumed chemicals. Interrelated impact of enzyme, temperature, and SRT on the enhancement of primary and RBF sludges fermentation showed a positive impact of enzyme dose as well as temperature and SRT on the VFA and soluble COD production. Cellulase increased the VFA yields by up to 36% and 86% for primary and RBF sludges, respectively. Response surface methodology (RSM) model depicted the existence of an optimum in the high-enzyme (1%-1.5%), long-SRT (3d-4d) range. The economic viability of fermentation at full scale was confirmed by proving that VFA recovery could save up to 7.2±2.0% (RBF), and 7.6±2.7% (PS) of the overall sludge disposal costs. Integration of fermentation and anaerobic digestion negatively impacted the biogas production of the residual fermented solids by 8.4% and 12.7%, compared to fresh primary and RBF sludges due to the VFA recovery, respectively; but still economically outperformed the single stage digestion under all tested scenarios.
Both primary and RBF sludge fermented liquid (SFL) were effective in enhancing BNR. Removal efficiencies in the rectors were reached up to 57% (total nitrogen) and 92% (total phosphorus), upon supplementation with the SFL. Effluent nitrogen and phosphorus of the reactors were closely matched for the two trains in the range of 15± 6 mg N/L, and 0.5 ± 0.3 mg P/L, respectively. A case study incorporating experimental results into a plant-wide model showed a moderate (3.4%-8.5%) improvement in the effective COD:N and COD:P ratios (compared to the original feed); but a significant increase in readily biodegradable (rbCOD) and VFAs (2.5-6.1 times) in the combined feed could be achieved by utilization of fermeners.