Practical approaches for retrofitting plug flow digester and process control to maximize hydrolysis and methane yield from piggery waste

Abstract

This study demonstrates and quantifies biomethanation improvements from practical schemes of retrofitting anaerobic plug flow (PF) digester for piggery wastewater (1% total solids concentration). First, batch assays were performed to seek the optimal scenario for substrate solubilization. By regulating micro-aeration at different levels, results showed that ORP −420 ± 10 mV promoted maximal hydrolysis efficiency (ηH), to as high as 49.5% above the control (without micro-aeration). However, organics were markedly lost by over oxidation beyond the optimal ORP. Sub-thermophilic (45 °C) was also proven as effective (p ≥ 0.05) as thermophilic (55 °C) temperature for hydrolysis with ηH 89% higher than that of the ambient (mesophilic) temperature. In continuous digestion studies, the two-stage operations, achieved by adding a pre-hydrolysis (pHY) reactor (HRT 1 day) to the main PF reactor (HRT 4 days), were assessed under four retrofitting schemes. Results showed incremental improvements of each strategy applied and suggested the best overall system configuration of pHY45C−420mV+PF45C with sludge re-hydrolysis. Methane yield was improved by 20.4% over the original PF digester operation. Next Generation Sequencing indicated significant changes in abundance and diversity of the major groups of organisms by the sub-thermophilic temperature that linked to the augmented biomethanation performance. Preliminary cost-benefit analysis is also presented. A full-scale implementation in a pilot farm in Thailand is under development.