High solids anaerobic digestion of the putrescible fraction of municipal solid waste

Abstract

Anaerobic digestion is increasingly becoming essential for treating the organic fraction of municipal solid waste (MSW) according to the growing 'energy from waste' recycle concept. Over the last decade, efforts have been made to control the anaerobic digestion of the putrescible fraction of municipal solid waste in engineered reactors, rather than continuing with 'free' digestion in landfill sites. This way of increasing treatment efficiency can be achieved with significantly reduced retention time (days rather than years) and enables the recovery of valuable energy from waste without biogas emissions into the atmosphere, which cause adverse greenhouse effects. Three basic approaches have been explored for the anaerobic treatment of MSW: (a) conventional 'low solids' slurry digestion; (b) two-phase digestion; and (c) high solids 'dry and semi-dry' digestion. The advantages of high solids digestion processes include high loading rates, and high yields of biogas and methane. Anaerobic digestion at high solids content is also reported to reduce capital and operating costs through smaller reactor volume. Different reactor design configurations have been applied at full-scale and their performance evaluated. Commercial plants which use a high solids digestion process, recycle a portion of the digested solids residue in addition to the liquid from the digested residue. However, the exact recycling ratio and data concerning the recycling impact on the stability and performance of the digestion process, are not well documented in the literature. Also, it is recognised that mixing is important to enhance anaerobic digestion but little research has concentrated on the impact of mixing on continuous or semi-continuous high solids anaerobic digestion systems. The main aim of this research was to evaluate the potential methane production in the stabilisation of the putrescible fraction of MSW using two novel reactor designs. A vertical sequential reactor (VSR) was used to assess the effects of digested recycle ratio (DRR) and leachate recycling (LR) and a horizontal rotary reactor (I-IRR) was used to evaluate the impact of mixing on the digestion of the PFMSW. Experiments were conducted in both batch and semi-continuous mode using three 70 1 reactors, under mesophilic conditions (3 5°C) and high total solids content. Process performance was assessed using various waste parameters for solid waste and leachate treatment. Regarding solid waste treatment, VS removal, biogas and methane production and yields were the main parameters used for assessment, among several others. Accordingly, COD, VFA, pH, alkalinity, N1H3-N, TKN etc. have been some of the parameters used to monitor leachate quality. Furthermore, internal biological activity in the VSR was assessed using the observed VFA concentrations and degradation under different operational conditions (DRR and LR). Initially, batch experiments were carried out in order to evaluate the first start-up of high solids digestion. Furthermore, they provided seed digested residues for the subsequent semi-continuous studies, in addition to providing essential information regarding ultimate process efficiency in terms of methane yield and VS removal. Also, rapid start-up batch experiments were used to determine the required RT for semi-continuous studies. Optimum DRR and LR were determined with the use of the VSR semicontinuous process. A DRR of 0.5 was found to be optimum in terms of volumetric methane production, while LR2 (3.5 £ leachate recycle) provided a desirable economical option, because of its lower liquid requirements. I-IRR gave limited indicatory results expressing high treatment potential for high solids anaerobic digestion of the putrescible fraction of MSW. Finally, a mathematical model was developed for optimising the VSR operational conditions, utilising data produced under semi-continuous conditions.