Abstract
The impact of inoculum to substrate ratio (I:S) and the addition of water and percolate on stopping inhibition in dry batch anaerobic digestion of organic fraction municipal solid waste (OFMSW) was investigated. In particular, ratios of I:S from 1:2 to 1:16 and total solid contents from 40 to 25% with water and percolate addition were analysed. Tested I:S did not avoid acidification of the anaerobic digesters (ADs), and the highest biogas and methane production (16.2 and 1.7 L/kg VSadded, respectively) was achieved with the 1:4 ratio. Water addition was also insufficient to avoid acidification, and while biogas increased as TS decreased, 40.9 L/kg VSadded for 25% TS, methane yield remained low at 1.2 L/kg VSadded due to the inhibition of methanogenic archaea. Percolate addition proved a suitable strategy to increase pH buffering, with an increased methane production of 199.4 L/kg VSadded at similar TS ranges (27%). Impact on kinetics of methane formation was assessed by kinetic modelling with logistic model identified as the better fit for most of the ADs. Shorter lag phases were observed as TS were reduced, regardless of the acidification, as mass transfer limitations were reduced at the beginning of the batch, but an increase was observed when percolate was used instead of water. Increases of the maximum methane rate (Rmax) was also achieved with TS reduction, but only when acidification was avoided. This study has highlighted the need to profile percolate composition during batch digestion in order to balance recirculation of nutrients, microbial communities and toxic compounds.Graphic
Highlights
Dry Anaerobic Digestion (AD) is proven to be an appropriate technology to treat high solid organic wastes [1], producing biogas rich in methane that can be used as a renewable energy source and a digestate that can be used as fertilizer
This was consistent with values reported in literature, where the total biogas yield dropped for ratios smaller than 1:4 when food waste was used as a feedstock
This study focuses on the impact that changing I:S ratio and TS content has on preventing inhibition of OFMSW dry anaerobic digestion, where literature is scarce
Summary
Dry Anaerobic Digestion (AD) is proven to be an appropriate technology to treat high solid organic wastes [1], producing biogas rich in methane that can be used as a renewable energy source and a digestate that can be used as fertilizer. Batch operated reactors are the most common dry AD processes due to their simpler operation compared to continuous systems and the similarity to in-vessel composting processes [6,7,8]. In this process fresh material is usually inoculated with material digested from a previous batch, as a method to provide the necessary microbial population to the new substrate [7, 9], and usually loaded into the digesters using a front-end loader, to the composting processes. To compensate for the lack of mixing, the percolate collected from the reactor or water can be recirculated to the mixture, improving homogenisation of the system
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