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Minimization of Sludge Generation Using Anaerobic Biomass Reactor

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The activated sludge process (ASP) is a dominant method for the biological treatment of municipal and industrial wastewater. Despite its high organic matter removal efficiency, ASP generates large amount of excess sludge as a byproduct. The majority of total operational costs of the facility have been spent for the treatment of excess sludge, associated with conditioning, dewatering, disposal and treatment of odor generated during the solids handling processes as well. One of efforts for sludge reduction in ASP is to increase solids retention time (SRT), in other word, reduce sludge wastage in the system (e.g., extended SRT). The SRT is a key operational and modeling parameter of the ASP, the primary method of treating wastewater. It is believed that SRT determines the growth rate of microorganisms, effluent quality of the process, and yield of sludge. From the current understanding of SRT-based activated sludge system, long SRT leads to deflocculation of sludge via formation of pin-point floc, causing poor sludge settling and eventually washout of sludge. These are attributed to an old and over-oxidized sludge due to the accumulation of inert organic substances or traditionally referred to as cell debris. Recently, the activated sludge process that incorporates a high rate anaerobic side-stream reactor has shown more effective sludge reduction along with sound operational performance in effluent total suspended solids and sludge settling. Extremely long SRT (such as over 100 days) is involved in this process, yet did not show to cause negative effects on operational performances for the activated sludge system. These results have cause considerable amount of controversy as it could not be simply be explained by traditional SRT-activated sludge concept.

Based on our preliminary research on the anaerobic side-stream reactor process and examination of literature reviews, the hypothesis for sludge reduction with sound operational performance in extremely long SRT has been proposed. In the anaerobic side-stream reactor (ASSR), anaerobically digestible fraction of sludge is degraded along with the release of subfraction of extracellular polymeric substances (EPS) and key floc cations. The addition of anaerobically treated sludge into the activated sludge basin enhances the expression and activities of exoenzymes (such as protease and glucosidase) under aerobic conditions, leading to the degradation of sets of EPS which cannot be degraded in single aerobic or anaerobic mode. This degradation process reduces the accumulation of excessive EPS pools, allowing for balanced EPS pools even under extremely long SRT. Overall, the process with ASSR enables sludge to be kept refreshed in spite of extremely long SRT (sludge refreshment).

In order to examine the hypothesis, anaerobic digestion batch test and three laboratory scale systems with different ASSRs conducted and operated. Anaerobic digestion batch reactor was conducted in room temperature (21°C). Total solids, soluble chemical oxygen demand (sCOD), protein, polysaccharide, cations, and anions were measured for each anaerobic digestion batch sample. Three laboratory systems were: system #1 (ASP with 10-day SRT ASSR), system #2 (ASP with 2.5-day SRT ASSR, high rate ASSR), and system #3 (control ASP without ASSR). The systems were operated for more than 100 days. The primary effluent from a full-scale wastewater treatment plant was used as a feed to all three systems. Sequencing batch reactor (SBR) was used to operate a main activated sludge reaction basin. For systems #1 and #2, 10 % volume of sludge in the SBR was wasted, thickened, and fed into their own ASSRs. Prior to feeding, the same volume of anaerobic sludge was taken from each ASSR and most of them were recycled back to a main reactor while 10% of anaerobic sludge was removed for sampling and a regular wastage purpose. System #3 served as control activated sludge for this study. This system, therefore, did not include an ASSR and 10% volume (or mass) of sludge in the main reactor was permanently wasted to maintain SRT of activated sludge at 10 days.

As results of anaerobic batch, the concentration of released soluble protein was higher than polysaccharide, while the concentration of sCOD was much higher than soluble protein throughout the anaerobic digestion batch test. These results are consistent with the data seen by Novak et al. (2003). It was important to observe that protease activity reached a maximum level within 2 days of SRT and then declined. It indicates that the enzyme to degrade protein is more released during early period of anaerobic digestion although actual VSR was small at this point. This result suggests that AS with a high rate ASSR which contains higher enzyme content could be more effective for sludge reduction. In addition, the concentration of divalent cations (Ca2+ and Mg2+) and potassium reached maximum levels within 2 days of SRT while the concentration of sodium was stable during entire anaerobic batch digestion. These results indicate that significantly different biochemical reactions occur during early period of anaerobic digestion.

In order to verify the anaerobic batch test results, three systems were operated: system #1 (AS with 10-day SRT ASSR), system #2 (AS with 2.5-day SRT ASSR), and system #3 (control AS without ASSR). The overall observed yields for systems #1, #2, and #3 were found to be 0.25, 0.18, and 0.44 mg VSS/mg COD, respectively. System #2 (AS with a short-SRT ASSR) led to the lowest sludge yield among the three systems. Based on these yield values, systems #1 and #2 achieved 43%, and 59% more sludge reduction compared to a control AS (system #3), respectively. Furthermore, systems #2 showed about 30% more solids reduction than system #1 which had a long-SRT ASSR (AS with 10-day SRT ASSR). These results clearly indicate that the system that employs a high rate ASSR is very effective in reducing sludge generation and is even more effective than sludge reduction conditions used in System #1. In addition, these observed sludge yield results suggest that sludge recirculation between a high rate ASSR and the aeration basin provides more favorable condition for sludge reduction.

It was found that the amount and composition of EPS were similar for the activated sludge collected from system #2 and conventional activated sludge system. This indicates that EPS which are not usually degraded in a single aerobic or anaerobic mode are effectively degraded through the repeating conditions of aerobic basin and a high rate anaerobic side-stream reactor. This degradation mechanism avoids the accumulation of excessive EPS pools, allowing sound operational performances for activated sludge even under extremely long SRT. The interaction between the aeration basin and the high rate anaerobic side-stream reactor leads to very effective sludge reduction and sound operation performances in wastewater treatment.

Currently, to further identify the main sludge reduction mechanism of ASSR treatment, 300 gallon sized pilot-scale system is operating. Pilot scale system consists of ASP and high rate ASSR and built in Amherst Wastewater Treatment Plant, MA USA. Further research will be conducted through pilot-scale system such as inorganic recovery, microbial community and its role, and biogas production.
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Keywords: Activated sludge; aerobic digestion; anaerobic digestion; biological sludge reduction; yield

Document Type: Research Article

Publication date: 01 January 2011

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