High-Rate Activated Sludge Systems: A Promising Technology to Harvest Organic Carbon from Municipal Wastewater Open Access
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The high-rate activated sludge (HRAS) systems are shifting the energy intensive processes to low-energy and sustainable technologies for wastewater treatment. In order to recover energy-rich organic carbon from wastewater, the HRAS system is a key technology for moving towards energy neutral (or positive) treatment processes. This study investigates the various HRAS technologies and their best selection, by understanding their operational and kinetic parameters and mechanistic relationship with process performance, carbon redirection and carbon harvesting for high and low-strength wastewater treatment.A series of pilot-scale studies were performed to compare conventional high-rate activated sludge systems (HRAS) (continuous stirred tank reactor (CSTR) and plug flow (PF) reactor configurations) with high-rate contact-stabilization (CS) technology in terms of carbon recovery potential from chemically enhanced primary effluent (low-strength) at a municipal wastewater treatment plant (WWTP). This study showed that carbon redirection and recovery could be achieved at short solids retention time (SRT). However, bioflocculation became a limiting factor in the conventional HRAS configurations (total SRT ≤ 1.2 days). At a total SRT ≤ 1.1 day, the high-rate CS configuration allowed better carbon removal (52-59%), carbon redirection to sludge (0.46-0.55 g COD/g CODadded) and carbon recovery potential (0.33-0.34 gCOD/gCODadded) than the CSTR and PF configurations (28 - 37% carbon removal, carbon redirection of 0.32-0.45 g COD/g CODadded and no carbon harvesting). The presence of aerobic stabilization phase (famine), achieved by aerating the return activated sludge (RAS), followed by aerobic contact with the influent (feast) wastewater was identified as the main reason for improved biosorption capacity, bioflocculation and settleabilty in the CS configuration. The fundamental mechanisms behind feast-famine condition in CS were further studied to understand the bioflocculation improvement. The pilot reactor operations at SRT range of 0.2-1.7 d SRT showed that the carbon harvesting efficiency in high-rate CS systems increased with the increase of extracellular polymeric substance (EPS) production in the aerobic contactor. In addition, the aerobic contactor had comparatively better carbon removal, redirection, capture efficiency and most importantly better effluent quality compared to anaerobic contactor. This was due to the increase of EPS in aerobic contactor than stabilizer, whereas no increase of EPS was observed in the anaerobic contactor. This confirmed that dissolved oxygen is necessary for fast EPS response to improve bioflocculation and carbon capture in high-rate CS systems. This study also suggested that food to microorganism (F/M) ratio can play a vital role for high-strength wastewater systems for maximizing carbon recovery linked with EPS response, whereas optimum stabilization time to induce starvation phase is necessary for low-strength wastewater systems. A unique approach to determine stabilization time in CS was proposed in this study using sludge oxygen uptake rate measurement. This followed the Monod kinetic model in which half-saturation coefficient (Ks) indicated the optimum stabilization time needed to induce starvation phase for microorganisms. The higher the SRT of a system, the longer the stabilization time required for enough starvation, and vice-versa for shorter SRT system.The fundamental mechanism behind biosorption to recover carbon from wastewater through HRAS system is insufficiently studied in the literature. The most accepted method of quantifying biosorption capacity (mixing wastewater to sludge and then settling) of activated sludge in the batch system depends on good settleability (sludge retention time, SRT ≥ 3 d), which is difficult to apply on HRAS where settleability characteristics are different. However, results showed that the interference of settleable particulate matter (approximately 55%) of wastewater in conventional biosorption quantification approach are among the key factors that invalidate its application on any SRT conditions and sludge characteristics. A unique approach of quantifying biosorption called “biosorption yield” by measuring ex-situ oxygen uptake rate (OUR) on different SRT sludge was proposed in this study, which was able to predict biosorption. In addition, an in-situ based biosorption quantification approach was established in this study which incorporated wastewater characterization, operational parameters and EPS of a high-rate continuous system. Both “biosorption yield” and “in-situ biosorption” approaches correlated well with SRT, carbon redirection, oxidation and observed yield of the system, and had the potential to quantify biosorption. Besides low-strength wastewater systems study, the plug-flow type A-stage (adsorption) and high-rate CS systems were investigated at very short SRT range of 0.15-0.3 d to evaluate their carbon recovery potential from high-strength raw wastewater treatment. The study observed that decreasing SRT from 0.3 d to 0.15 d in A-stage, the carbon oxidation decreased and effluent quality deteriorated but carbon capture did not improve. In CS, the carbon oxidation and carbon capture increased and effluent quality improved, which was opposite to A-stage performance. This suggested that A-stage was limited by biomass and thus flocculation at 0.15 d SRT, while CS improved its performance due to better bioflocculation caused by the feast-famine condition and higher EPS productions. A-stage and CS had similar carbon capture (42-43%) but A-stage had better effluent quality (67 mgVSS/L) and higher oxidation (22%) than CS (85 mgVSS/L and 18%). This suggested that optimum carbon oxidation is necessary for achieving better effluent quality. In addition, particulate and soluble carbon removal efficiency in both systems increased with increased EPS production. Both A-stage and CS was able to capture significant fractions of nitrogen (19-26%) and phosphorous (30-36%) through waste activated sludge. This confirmed that HRAS systems not only redirect higher carbon, but also nitrogen which can be treated through energy efficient sidestream treatment processes, and at the same time move towards energy neutral treatment.