초록
<P><B>Abstract</B></P> <P>Wheat straw is an abundant, cheap substrate that can be used for methane production. However, the nutrient content in straw is inadequate for methane fermentation. In this study, recycling digestate liquor was implemented in single-stage continuous stirred tank processes for enrichment of the nutrient content of straw with the aim of improving the methane production. The VS-based organic loading rate was set at 2 g/(L d) and the solid retention time at 40 days. When wheat straw alone was used as the substrate, the methane yields achieved with digestate liquor recycling was on average 240 ml CH<SUB>4</SUB>/g VS giving a 21% improvement over the processes without recycling. However, over time, the processes suffered from declining methane yields and poor stability evidenced by low pH. To maintain process stability, wheat straw was co-digested with sewage sludge or supplemented with macronutrients (nitrogen and phosphorous). As a result, the processes with digestate liquor recycling could be operated stably, achieving methane yields ranging from 288 to 296 ml CH<SUB>4</SUB>/g VS. Besides, the processes could not be operated sturdily with supplementation of macronutrients without digestate liquor recycling. The highest methane yield (296 ± 16 ml CH<SUB>4</SUB>/g VS) was achieved by co-digestion with sewage sludge plus recycling of digestate liquor after filtration (retention of nutrients and microorganisms). This was comparable to the maximum expected methane yield of 293 ± 13 ml CH<SUB>4</SUB>/g VS achieved in batch test. The present study therefore demonstrated that digestate liquor recycling could lead to a decreased dilution of vital nutrients from the reactors thereby rendering high process performance and stability.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Poor nutrient content of wheat straw limits its use as feed for methane production. </LI> <LI> Effluent recycle, co-digestion and macronutrients addition were studied in lab scale. </LI> <LI> Effluent recycling could lead to high methane yield and process stability. </LI> <LI> Effluent recycling plus co-digestion was best for long term process performance. </LI> <LI> Long hydraulic retention time is a not precondition for high methane yield from straw. </LI> </UL> </P>