Abstract
Anaerobic pig manure digestion holds potential to contribute to a bio-based economy. This work assesses the stability of the thermophilic mono-digestion process. Thermophilic mono-digestion experiments with (i) fresh liquid pig manure and (ii) the fresh fecal fraction from source separation by a pig housing construction were conducted in semi pilot-scale continuous stirred tank reactors. The effect of separation on the digestion stability was studied by comparing thermophilic mono-digestion of fresh liquid (unseparated) and fresh source separated pig manure. Influencing factors and inhibitors were identified during the experiments. An unstable thermophilic mono-digestion process was observed for fresh liquid pig manure at a digester retention time of 60 days, due to high levels of ammonia and sulfur-containing components. Thermophilic mono-digestion of the fresh fecal fraction was more promising in terms of stability, provided enough time for digestion. In addition, the effect of low temperature (70 degrees C) thermal pre-treatment of manure on the digestion stability was investigated. In the case of liquid pig manure, no improvement in the digestion stability was noted upon thermal pre-treatment. For the fecal fraction, the stability of the thermophilic mono-digestion process did improve. Moreover, thermal treatment and subsequent thermophilic mono-digestion of the fresh fecal fractions from two different farms with a similar pig housing construction suggested an effect of the (organic) dry matter content on the process stability.
Highlights
Pig production in Flanders, the Northern part of Belgium, has a very high density of animals per surface area
Both fresh liquid pig manure and the fresh fecal fraction from source separation by a specific pig housing construction were mono-digested under thermophilic conditions
Methane production values and relative efficiencies obtained for Liquid pig manure (LPM) Ap were similar to somewhat higher than those for LPM A (Figure 1A). This observation was in agreement with the results found by Appels et al (2010), who investigated the influence of thermal pre-treatment at for example 70◦C on mesophilic anaerobic digestion of sludge from a waste water treatment plant
Summary
Pig production in Flanders, the Northern part of Belgium, has a very high density of animals per surface area. The more than 5.8 million animals in 2018 accounted for 90% of the total pig population in Belgium and were divided over approximately 3,750 farms Given the actual annual manure production per pig category in terms of nitrogen (VLM, 2019b) and considering the fixed composition of liquid manure from these pig categories (VLM, 2019c), the estimated annual mass of pig manure produced is close to 10 million tons. The produced biogas can be burned in a combined heat and power unit to supply electricity and heat, while digestate can serve as an organic fertilizer. Scarlat et al (2018) estimated the electricity production potential by digestion of manure from livestock and poultry in Europe to be close to 58 TWh per year, only taking into account collectable manure. For the specific case of Belgium, it was nearly 1.5 TWh
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