Evaluation Of Biogas Production From Anaerobic Digestion Of Pig Manure And Grass Silage

Abstract

The research project evaluated biogas production from anaerobic co-digestion of pig manure (PM) and grass silage (GS) in laboratory- and pilot- scale studies. In addition, improvement of the biogas yield from GS via thermo-chemical pretreatment and leaching bed reactors was studied.

Using the laboratory-scale batch experiment, anaerobic co-digestion of concentrated PM with GS at five PM to GS volatile solid (VS) ratios was evaluated. The highest specific CH4 yields obtained were 304.2 ml CH4/g VS at a PM to GS ratio of 3:1. The results show that it is feasible to co-digestate PM and GS when PM/GS VS ratios are not less than 1:1. Three laboratory-scale 3 litre continuously stirred tank reactors were set up for further examination of biogas production with different organic loading rates (OLRs, 1.0, 1.5, 2.0 and 3.0 kg VS/m3/d) at various separated PM to dried GS VS ratios. It was found that the OLR affected the digester performance more than the dried GS proportion in the feedstock. Tripling the OLR increased the volumetric methane yields by 88% but decreased the specific methane yields by 38%.

Thermo-chemical pretreatment of dried GS was examined at different NaOH loading rates and temperatures to determine effects of pretreatment on its bio-degradability in terms of the hydrolysis yield and degradation of ligno-cellulosic materials for biogas production. 100 oC and the NaOH loading rate of 5% is recommended as a proper GS pre-treatment condition, under which biological methane production potentials of GS pretreated were improved by 38% in comparison with untreated GS. Six identical column-type stainless steel leaching bed reactors (LBRs) with a working volume of 2-litre were set up to examine the hydrolysis and acidification of GS under three OLRs. The highest GS hydrolysis yields of 51.5-58.2%, acidification yields of 57.2-60.3% and VS removals of 62.1-66.3% were obtained in the conditions of addition of inoculum, leachate dilution with tap water (1:1 - v/v) every 6 days and pH adjustment to 6.5. The cellulase activity can be used as an indicator to the hydrolysis process.

A pilot-scale CSTR (480 litres) with a working volume of 360 l was fabricated to further examine biogas production from PM and GS in the Teagasc pig research unit in Fermoy, Co. Cork. The experiments consisted of two phases: Phase I at an OLR of 0.87 kg VS /m3/d with the feedstock of PM only and Phase II at an OLR of 1.74 kg VS / m3/d with the mixture of PM and GS at 1:1 of VS basis as the feedstock. The specific methane yields were 154 ml CH4/g VS added when mono-digestion of PM and 251 ml CH4/g VS added when co-digestion of PM and GS.

This PhD study shows that anaerobic co-digestion of pig manure and grass silage should be adopted for sustainable animal manure management. It can produce methane-rich biogas and nutrient-rich fertilizer, and can reduce the greenhouse gas emissions in the pig industry. This study also provides experience and operation data for this co-digestion technology.