Evaluation of the operation of continuously stirred tank reactors (CSTR) for the anaerobic co-digestion of broiler litter and laying heng manure

Abstract

Poultry production in Uruguay is concentrated in the southern region of the country, where egg and broiler production generate 26% of agro-industrial waste. The main residues produced in poultry farms are broiler litter (BL) and laying hen manure (LHM), reaching 81,000 and 3,300 tonTS/year, respectively (Benzano et al., 2016). Although the solids wastes have been used as fertilizer in the region, the excessive application and lack of prior stabilization generated the overload of nutrients in the water basin where the poultry production is concentrated. In this context, the valorization of BL and LHM through anaerobic digestion (AD) is proposed to produce biogas and digestate. The research strategy consists of evaluating the technological, economic, and environmental feasibility of implementing centralized anaerobic co-digestion (CoAD) projects using these residues. In order to assess maximum energy production, the Biochemical Methane Potential (BMP) of BL, LHM, and their mixture at the nationally generated ratio (24:1 in TS) was evaluated. To evaluate the technology in full-scale centralized plants, the performance of AD was studied in reactors operated under continuous flow conditions. Specifically, six continuous stirred-tank reactors with 2 L of effective volume (BRS Bioprocess Control) were fed over 176 days with BL, LHM, and the BL:LHM mixture (24:1). The aim was to determine the operating conditions that yield the highest methane (CH₄) production with minimal reactor volume. Reactors were operated at a constant Hydraulic Retention Time (HRT) of 20 days with a progressive increase in Organic Loading Rate (OLR). Process monitoring included biogas volume measurement and digestate analysis of pH, TS, VS, FOS/TAC, and NH₄⁺. Three steady states were established corresponding to OLRs of 1, 2, and 3 kgVS/m³·d, where the CH₄ concentrations, the digestate characterization (COD, TP, TN, volatile fatty acids, macro- and micronutrients), pathogen, antibiotic resistance, and microbial community composition were also determined. Throughout the assay, pH (6.94 - 7.94), FOS/TAC (< 0.3), NH₄⁺ (200 - 1151 mgN-NH₄⁺/L), and NH₃ (52 - 74 mgN-NH₃/L), were in the reported ranges for these type of processes (Mezes et al., 2011; Dai et al., 2016; Guebitz et al., 2015; Yenigün and Demirel, 2013). Maximum methane yields (YCH₄) were obtained at OLR = 1 kgVS/m³·d (BL: 106 NLCH₄/kgVS, LHM: 229 NLCH₄/kgVS, BL:LHM: 103 NLCH₄/kgVS), which decreased as the load increased to 2 kgVS/m³·d (BL and LHM: 23%, BL:LHM: 17%). When the OLR was raised to 3 kgVS/m³·d, RCH₄ remained similar in the LHM reactors (184 NLCH₄/kgVS), while those containing BL and BL:LHM progressively ceased CH₄ production after day 100. It is presumed that the poor degradation of BL, evidenced by the accumulation of solids in the reactors at the end of the assay, led to a 40% reduction in effective reaction volume, thereby reducing HRT and increasing OLR. As a result, the maximum RCH₄ for LHM was 88% of its BMP, while for BL and BL:LHM it was considerably lower than these BMP results (28% and 37%, respectively).