Abstract
The increasing concentration of livestock farms results in large amounts of waste production and the need for their management. The study of anaerobic digestion (AD) technology, under mesophilic conditions, applied to pig slurry is of the upmost importance for biogas recovery and sanitised digestate, contributing to a circular economy. The assessment of the effects of a feast/famine regime on biogas and biomethane (bio-CH4) yield with different feeding frequencies was performed. The evaluation was made in regards to three scenarios: the first is based on daily feeding (FR1); in the second, the feeding occurs once every two days (FR2); and in the third, the feeding happens once every three days (FR3). The results demonstrate that the biogas and methane yield increased by 34% and 37% between FR1 and FR3. The stability inside the reactor was maintained since specific loading energetic rate values did not exceed the recommended limit (0.4 d-1). It was also possible to conclude that the AD technology was efficient to sanitise the pig slurry, with the count of Escherichia coli going from 1 × 105 colony-forming units (CFU) g-1 to less than 100 CFU g-1, meeting the legal requirements for agricultural valorisation. The total anaerobic mesophile plate counts were significantly (p < 0.1) reduced from feeding to digestate, and the plate counts of Clostridia were significantly (p < 0.05) increased, reflecting the changes in the composition of the microbiota. The increasing yield in bio-CH4 in accordance with Clostridium counts suggests this genus as a positive microbiological key indicator of the AD performance.
Highlights
The continued use of fossil fuels and the impact of greenhouse gas (GHG) emissions has encouraged research into renewable energy production from biowastes and the implementation of economic models based on a circular economy – a system that is based on the recovery of materials, reuse, valorisation and recycling of natural cycles (Chojnacka et al, 2020)
A positive pattern in the production of bio-CH4 was evident in the regimes where starvation was induced (FR2 and FR3), which is an interesting fact as it suggests the adaptation of the microbiota to variations in the operational parameters of the Anaerobic digestion (AD) – namely, slurry storage time and reactor feeding frequency
With lower feeding frequencies, daily biogas and bio-CH4 flows were higher compared to feeds that were more frequent, which is supported by the fact that a higher famine period does not significantly impact biogas production
Summary
The continued use of fossil fuels and the impact of greenhouse gas (GHG) emissions has encouraged research into renewable energy production from biowastes and the implementation of economic models based on a circular economy – a system that is based on the recovery of materials, reuse, valorisation and recycling of natural cycles (Chojnacka et al, 2020).Biogas production plays a major role in waste management and valorisation contributing to mitigate climate change. AD has multiple environmental advantages: the production of renewable biogas, the elimination of odour, the sanitisation of digestate and the reduction of GHG emissions (Shi et al, 2018, Wainaina et al, 2020). This technique can be a favourable method for achieving a circular economy, closing the cycle of nutrients and promoting biofertiliser production (Chojnacka et al, 2020, Wainaina et al, 2020)
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