Effects of Biogas Substrate Recirculation on Methane Yield and Efficiency of a Liquid-Manure-Based Biogas Plant

Abstract

Biogas plants are the most complex systems and are heavily studied in the field of renewable energy. A biogas system is mainly influenced by biological and technical parameters that strongly interact with each other. One recommended practice when operating a biogas plant is the recirculation of the substrate from the second fermenter into the first fermenter, which extends the recirculation amount (RA) and, in turn, the recirculation rate (RR). This technique should be applied to support and secure the biogas process. In this investigation, the RA was varied, starting with the recommended "best practice" of 10.0 m 3 /d (RR 40%). Every ten days, the RA was reduced in steps of 1.5 m 3 /d, with 5.5 m 3 /d (RR 27%) being the final value. The basic question to be addressed concerns to what extent the RR influences the methane yield and thereby influence the efficiency of a manure-based biogas plant in practice. Diverting the "best practice" to a RR of 27% stabilised the fermentation process and lead to significantly higher methane yields with smaller standard deviations. In addition, with a reduced RR, the standard optimal acid concentration within the biogas substrate was approximately reached.

Energy and Waste Management

“…Around half of the studies are based on experiments on a laboratory scale [12][13][14] or pilot scale [19]. Some non-experimental studies use models [16][17][18] while two studies [15,19] are based on calculations which cannot be considered complex mathematical models.…”

“…The waste type in most studies is municipal solid waste (MSW), but two studies [16,19] deal with manure. Wet anaerobic digestion of liquid manure from livestock [19] is a fairly well established technology in contrast to anaerobic (wet or dry) digestion of horse manure.…”

“…Two studies have electricity generation in focus [15,19] while one study recovers oil as fuel which can later be used for different purposes. Depending on local climate conditions, district cooling may be more interesting than district heating in many parts of the world.…”

“…Wet anaerobic digestion of liquid manure from livestock [19] is a fairly well established technology in contrast to anaerobic (wet or dry) digestion of horse manure. Biogas from manure also presents two environmental benefits: avoiding methane emissions from traditional manure treatment and avoiding emissions from conventional fuels in vehicles or in heat and power supply.…”

“…Combining the experimental approach used in [12,13] with anaerobic digestion and manure [16] set the scene for the study performed by Müller et al [19], where the effect on methane yield and efficiency from substrate recirculation is investigated. Recirculation of substrate between fermenters or from raw digestate to fresh material is a recommended practice to support and secure the biogas process, but to what extent parameters-such as recirculation amount (RA) and recirculation rate (RR)-affect the methane yield is not known.…”