Applicability of thermophilic aerobic process for efficient treatment of sewage sludge and food wastewater

Abstract

With significant increase of municipal solid waste (MSW), efficient treatment and disposal of MSW has attracted more attention recently. Among the various options, anaerobic digestion (AD) is a practical and widely used technology for the stabilization of organic fraction of MSW such as waste activated sludge (WAS), animal manure and food waste (FW), since it produces methane, the main component of biogas, which can be a renewable energy source. In addition, residues from AD can be used as fertilizer and soil amendment. An effective two-stage sewage sludge digestion process, consisting of thermophilic aerobic digestion (TAD) followed by mesophilic anaerobic digestion (MAD), was developed for efficient sludge reduction and methane production. Using TAD as a biological pretreatment, the total volatile suspended solid reduction and methane production rate (MPR) in the MAD reactor were significantly improved. According to denaturing gradient gel electrophoresis (DGGE) analysis, the results indicated that the dominant bacteria species such as Ureibacillus thermophiles and Bacterium thermos in TAD were major routes for enhancing soluble organic matter. TAD pretreatment using a relatively short solid retention time (SRT) of 1 day showed highly increased soluble organic products and positively affected an increment of bacteria populations which performed interrelated microbial metabolisms with methanogenic species in the MAD; consequently, a quantitative real-time PCR (qPCR) indicated greatly increased Methanosarcinales (acetate-utilizing methanogens) in the MAD, resulting in enhanced methane production. To elucidate the effects of the TAD process as a biological pre-treatment step on the methane production and sludge digestion in MAD under different SRTs, a process involving two-stage TAD-MAD were investigated. The TAD reactor (RTAD) was operated with a 1 day SRT and the MAD reactor (RMAD) was operated at three different SRTs: 39, 19 and 9 days. For a comparison, control MAD (RCONTROL) was operated at three different SRTs of 40, 20 and 10 days. Our results reveal that the sequential TAD-MAD process has about 42% higher MPR and 15% higher TCOD removal than those of RCONTROL when the SRT decreased from 40 to 20 days. DGGE and qPCR results indicate that RMAD maintained a more diverse bacteria and archaea population compared to RCONTROL, due to the application of the biological TAD pre-treatment process. In RTAD, Ureibacillus thermophiles and Bacterium thermus were the major contributors to the increase in soluble organic matter. In contrast, Methanosaeta concilii, a strictly aceticlastic methanogen, showed the highest population during the operation of overall SRTs in RMAD. Interestingly, as the SRT decreased to 20 day, syntrophic VFA oxidizing bacteria, Clostridium ultunense sp., and a hydrogenotrophic methanogen, Methanobacterium beijingense were detected in RMAD and RCONTROL. Meanwhile, the proportion of archaea to total microbe in RMAD and RCONTROL shows highest value 10.5 and 6.5 % at 20 day SRT operation, respectively. Collectively, these results demonstrate that the increased COD removal and methane production at different SRTs in RMAD might be attributed to the increased synergism among microbial species by improving the hydrolysis of the rate limiting step in sludge with the help of the biological TAD pre-treatment. To evaluate the applicability of single-stage TAD process treating high-strength food wastewater (FWW), TAD process was operated at four organic loading rates (OLRs) from 9.2 to 37.2 kg COD/m3 d. The effects of OLRs on microbial community changes were also examined. The highest volumetric removal rate (13.3 kg COD/m3 d) and the highest thermo-stable protease activity (0.95 Unit/mL) were detected at OLR = 18.6 kg COD/m3 d. DGGE profiles and qPCR results showed significant microbial community shifts in response to changes in OLR. In particular, DGGE and phylogenetic analysis demonstrate that the presence of Bacillus sp. (phylum of Firmicutes) was strongly correlated with efficient removal of organic particulates from high-strength FWW. Given this information, we developed a novel combined biological process which consists of MAD combined with TAD post-treatment solid and separation unit for treating high-strength FWW. During the experimental period, most of solid residue from the mesophilic anaerobic reactor (R1) was separated by centrifugation and introduced into the thermophilic aerobic reactor (R2) for further digestion. Then, thermophilic aerobically-digested sludge was reintroduced into R1 to enhance reactor performance. The combined process was operated with two different Runs: Run I with hydraulic retention time (HRT) = 40 d (corresponding OLR= 3.5 kg COD/ m3 d) and Run II with HRT = 20 d (corresponding OLR= 7 kg COD/m3). For a comparison, a single-stage mesophilic anaerobic reactor (R3) was operated concurrently with same OLRs and HRTs as the combined process. During the overall digestion, all reactors showed high stability without pH control. The combined process demonstrated significantly higher organic matter removal efficiencies (over 90 %) of TS, VS and COD and methane production than did R3. qPCR results indicated that higher populations of both bacteria and archaea were maintained in R1 than in R3. Pyrosequencing analysis revealed relatively high abundance of phylum Actinobacteria in both R1 and R2, and a predominance of phyla Synergistetes and Firmicutes in R3 during Run II. Furthermore, R1 and R2 shared genera (Prevotella, Aminobacterium, Geobacillus and Unclassified Actinobacteria), which suggests synergy between mesophilic anaerobic digestion and thermophilic aerobic digestion. For archaea, in R1 methanogenic archaea shifted from genus Methanosaeta to Methanosarcina, whereas genera Methanosaeta, Methanobacterium and Methanoculleus were predominant in R3. The results demonstrated dynamics of key microbial populations that were highly consistent with an enhanced reactor performance of the combined process. Collectively, experimental results show that TAD is a promising strategy for efficient treatment of WAS and FWW, especially for methane production. Further studies regarding cost and energy balance should be conducted to confirm the economic benefits of using TAD pre- or post-treatment.