Piggery Wastewater Acidification by Completely-Stirred-Tank-Reactor and Plug-Flow Reactor

Abstract

The objective of this research was to investigate the acidification of piggery wastewater by completely-stirred-tank reactor (CSTR) and plug-flow (PF) reactor at hydraulic retention times (HRTs) of 36, 12, 6 and 4 h, consecutively. Two laboratory-scale reactors, with 15- L working volume, were comparatively operated. The reactors were seeded with anaerobically digested sludge at 23 gVSS/l. It was found that piggery wastewater consisted of faces which contained lots of anaerobic bacteria. The acidification had naturally occurred in wastewater, resulting in average volatile fatty acids (VFA) of 961-1,363 mg/L as acetic acid. Due to physical configuration, PF reactor could adjust to bacterial – substrate interactions better than CSTR with better succession of methanogenesis, resulting in lower VFA concentrations. The reactions along PF reactor’s length resulted in higher effluent pH than CSTR. To enhance acidification, HCl was added to adjust influent pH while effluent pH was maintained around 5.0 – 5.5. At HRTs 6 – 36 h, VFA reductions were observed in both reactors, indicating simultaneous methane production. At HRT 4 h, acidification in both reactors had prevailed, resulting in increase of VFA in the effluent with acidification efficiencies in CSTR and PF reactor at 3.03 and 2.42%, respectively. The performances of CSTR were inferior to PF reactor in terms of COD and SS removal while acidification in CSTR was better. At HRT 6 – 36 h, there were no distinct differences in biogas productions from both reactors while biogas from CSTR reactor had lower methane proportions. According to wastewater characteristics and results obtained, the application of two-phase digestion for piggery wastewater is not recommended.