Achieving Stable Performance by the Co-digestion of Livestock Manure and Food Waste in an Anaerobic Sequencing Batch Reactor (AnSBR)

Introduction Anaerobic digestion (AD) can be briefly described as a series of processes involving microorganisms degrading organic matter in the absence of oxygen with final by-products of methane, carbon dioxide, hydrogen sulfide, ammonia, and new biomass (Veeken et al., 2000; Kelleher et al., 2002). Although AD is one of the most cost-effective technologies due to the plausible high energy and resource recovery from organic matters (Ding et al., 2017), there are challenges and the need for innovative methods to tap into the total resource recovery and utilization of the wastewater treatment process. In the AD process, various types of microorganisms sequentially degrade organic matters in multi-steps and parallel reaction. The AD process of complex organic matters generally involves three interrelated steps: hydrolysis, fermentation (acidogenesis and acetogenesis) and methanogenesis by methane producing bacteria (MPB). The major challenges of AD are, therefore, the slow hydrolysis rate; where unhydrolyzed organic matter is discharged and directly impact the growth of MPB thereby lowering the methanogenic rate. An innovative approach was tested and applied in this study; including, increasing the solid retention time (SRT) using an anaerobic sequencing batch reactor (AnSBR); a patented highly effective mixers and a new settled sludge discharge system at the bottom of the reactor. An enhanced performance and stability of the AAD is achieved in the evaluation of a full-scale (100 ton/d) plant in Korea. Methods The full-scale Advanced Anaerobic Digestion (AAD) process was operated at the Miryang livestock treatment plant in Korea for 300 days, processing 80 m3/d of livestock manure and about 18~39 m3/d of food waste (household, restaurant). The AAD was operated with the AnSBR with 8 hours cycle mode (thus, 80 mins filling — 280 mins reaction — 100 mins settling — 20 mins decanting). The AAD distinguishes from the conventional AD in that, the SRT is significantly increased to improve biogas production, and the HRT on the other hand shortened. The patented novel discharging system at bottom of digesters allows removal of settled non-biodegradable sludge (sands and others) without affecting the operation of the AD and with the high-performance agitators (Fig. 1.1), settled biodegradable solids quickly mixed with other solids in each cycle. The scum layers are not formed in this system and this new design thus utilizes the full digester volume without non-usable volumes found in conventional digesters for scum layer and accumulated non-biodegradable solids settled at the bottom. Table 1.1 summarizes the operational parameters of the AAD. Results Process performance: Over the operational period, influent volatile solids (VS) concentration fluctuated averaging 30,091.9 +/- 5,788.8 (16,381.6-43,373.9) mg/L. This fluctuation of influent VS was mainly due to the change of food waste input leading to changes of the OLR. Effluent VS concentration however remained stable and average VS removal rate was 70.6+/-3.1 (Fig. 1.2). In Fig. 1.3, the fluctuating biogas and methane production resulting from varying influent VS is shown. However, the AAD's unique operational procedures allowed for a stable methane content and the performance yield. (Fig. 3 (b) and (c)). In the conventional AD process, while higher OLR improve the processing efficiency by dropping operational pH, it causes accumulation of volatile fatty acids (VFAs) and excessive production of carbon dioxide, this inhibits the MPB and in effect accelerate the process failure. On the other hand, lower OLR generally decline efficiency. Thus, the AAD process uniquely withstands these highs and lows in OLR with its SBR operational procedures. Process stability: Process stability was attained early in the first 60 days of operation of the full-scale AAD. The AnSBR retains high biomass level due to bioflocculation followed by biogranulation. The advantages of employing AnSBR including biomass flocculation, solids separation, high SRT etc. hastened the stability of the AAD process at Miryang. The AAD's innovative discharge system equipped at the bottom of the digester for the discharge of non-biodegradable matters (usually heavier and settle faster during the setting period), such as fixed solids (FS) that accumulate at the bottom of the digester also enhanced stability. The communicative nature of the radar level sensor (installed at the top), pressure level sensor (install at the bottom) and discharge system improve the overall process efficiency and maintains a uniform water level pressure on the sensors. (Table 1.2 and Fig. 1.3). Khanal, 2008, emphasized the essence of maintaining a uniform temperature during settling in digesters due to the sensitive nature of MPB to temperature. Table 1.3 shows the temperature changes observed at the top, mid and bottom of digester during settling. Although total solids (TS) concentration at the top of digester dropped to 1.58% from 2.46%, there were no significant changes in temperature along the different digester levels over the settling period a good indication of the AAD stable performance. Conclusion This study evaluated the stability and performance of a full-scale AAD process with AnSBR to produce biogas using livestock manure and domestic food wastes. The advantages of employing an AnSBR with patented mixing system and settled non-biodegradable sludge discharge system, was seen in the stable perfo