Abstract
High-rate anaerobic digestion (AD) is a reliable, efficient process to treat wastewaters and is often operated at temperatures exceeding 30°C, involving energy consumption of biogas in temperate regions, where wastewaters are often discharged at variable temperatures generally below 20°C. High-rate ambient temperature AD, without temperature control, is an economically attractive alternative that has been proven to be feasible at laboratory-scale. In this study, an ambient temperature pilot scale anaerobic reactor (2 m3) was employed to treat real dairy wastewater in situ at a milk processing plant, at organic loading rates of 1.3 ± 0.6 to 10.6 ± 3.7 kg COD/m3/day and hydraulic retention times (HRT) ranging from 36 to 6 h. Consistent high levels of COD removal efficiencies, ranging from 50 to 70% for total COD removal and 70 to 84% for soluble COD removal, were achieved during the trial. Within the reactor biomass, stable active archaeal populations were observed, consisting mainly of Methanothrix (previously Methanosaeta) species, which represented up to 47% of the relative abundant active species in the reactor. The decrease in HRT, combined with increases in the loading rate had a clear effect on shaping the structure and composition of the bacterial fraction of the microbial community, however, without affecting reactor performance. On the other hand, perturbances in influent pH had a strong impact, especially when pH went higher than 8.5, inducing shifts in the microbial community composition and, in some cases, affecting negatively the performance of the reactor in terms of COD removal and biogas methane content. For example, the main pH shock led to a drop in the methane content to 15%, COD removals decreased to 0%, while the archaeal population decreased to ~11% both at DNA and cDNA levels. Functional redundancy in the microbial community underpinned stable reactor performance and rapid reactor recovery after perturbations.
Highlights
The high demand milk and milk products has led to an increase in dairy production globally
The average total and soluble influent chemical oxygen demand (COD) during the trial fluctuated greatly (0.20 and 4.9 kg/m3 of total COD and between 0.05 and 3.1 kg/m3 for soluble COD), mainly due to changes in production processes of the factory (Table 1, Figures 1A,B). This corresponded to an organic loading rate of 1.3 ± 0.6 to 10.6 ± 3.7 kg COD/m3/day (Table 1)
Total COD removal was between 49 and 71%, while the average soluble COD removal was more stable over the trial, fluctuating between 71 and 84.3%
Summary
The high demand milk and milk products has led to an increase in dairy production globally. Dairy plants produce large volumes of wastewater; it is estimated that 1– 2 m3 of wastewater is produced per m3 of manufactured milk (Quaiser and Bitter, 2016; Slavov, 2017) These wastewaters are characterized by high organic load and nutrient composition (Demirel et al, 2005; Lateef et al, 2013; Gil-Pulido et al, 2018). Highrate AD technology relies on the retention of high levels of active microorganisms within the system This is achieved by the immobilization of the microbes on a support material or by the formation of granules (McKeown et al, 2012). These reactors tolerate short HRT (1–24 h) and high organic loading rates (up to 100 kg COD/m3/day; McKeown et al, 2012)
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