Two-stage anaerobic digestion of biodegradable municipal solid waste using a rotating drum mesh filter bioreactor and anaerobic filter

doi:10.1016/j.biortech.2009.03.066

Bioresource Technology

Volume 100, Issue 18, September 2009, Pages 4121-4126

https://doi.org/10.1016/j.biortech.2009.03.066 Get rights and content

Abstract

A rotating drum mesh filter bioreactor (RDMFBR) with a 100 μm mesh coupled to an anaerobic filter was used for the anaerobic digestion of biodegradable municipal solid waste (BMW). Duplicate systems were operated for 72 days at an organic loading rate (OLR) of 7.5 gVS l⁻¹ d⁻¹. Early in the experiment most of the methane was produced in the 2nd stage. This situation gradually reversed as methanogenesis became established in the 1st stage digester, which eventually produced 86–87% of the total system methane. The total methane production was 0.2 l g⁻¹ VS_added with 60–62% volatile solids destruction. No fouling was experienced during the experiment at a transmembrane flux rate of 3.5 l m⁻² h⁻¹. The system proved to be robust and stably adjusted to a shock loading increase to 15 gVS l⁻¹ d⁻¹, although this reduced the overall methane production to 0.15 l g⁻¹ VS_added.

Introduction

There is increasing interest in anaerobic digestion (AD) as a means of stabilising biodegradable municipal waste (BMW) with the co-benefit of producing biogas as a renewable energy source (De Baere, 2006). In this work a novel high rate 1st stage rotating drum reactor was developed which incorporated a mesh filter allowing the solids and liquids retention time to be uncoupled. The purpose of this work was to explore the possibility of using a rotating drum mesh filter bioreactor (RDMFBR) in series with an anaerobic filter (AF) in a two stage configuration to provide a high rate, compact and robust AD process.

The use of advanced membrane materials in membrane bioreactors (MBR) with pore sizes around 0.05–0.2 μm is growing rapidly in full-scale applications, particularly in the wastewater treatment industry where they can show a number of advantages. These include the elimination of secondary sedimentation for biomass retention, and reduced energy consumption (Yang et al., 2006); and in some situations a decrease in sludge production (Bohdziewicz et al., 2008). Most commercial applications of MBR employ micro-porous membranes and are primarily used in the treatment of liquids or suspensions with very low solids such as domestic (Saddoud et al., 2007) and industrial wastewaters (Choo and Lee, 1996) and landfill leachate (Bohdziewicz et al., 2008).

The use of membranes with larger pore sizes of 10–100 μm has also been reported for wastewater/sludge treatment, in what are described as mesh filter bioreactors (MFBRs). These units retain the flocculent biomass that provides the biological treatment capacity and produce an effluent similar to that of as a conventional activated sludge plant. This is not as high a quality as may be achieved by an MBR but the advantages are the high membrane fluxes that can be achieved, of up to 150 l m⁻² h⁻¹ at transmembrane pressures (TMP) of 30–100 Pa, leading to reduced reactor footprint and cost (Kiso et al., 2005).

High flux rates at low TMPs open up the potential for using MFBRs in the anaerobic digestion of solid materials, as a means of reducing the hydraulic retention time (HRT) relative to the solid retention time (SRT). This concept of uncoupling the solids and liquids retention time in the 1st stage of a two-stage system has been developed using high rate methanogenic reactors for recovery of biogas from the liquid phase. It has been used to provide greater process stability and improved substrate degradation rates for treating problematic feed materials, such as mixed abattoir wastes (Banks and Wang, 1999). Uncoupling of retention times also allowed increased process loadings compared to single stage reactors when degrading a mixture of paper and wood (Banks and Humphreys, 1998). Little work has been carried out on the potential application of mesh filtration as the solids liquid separation system in the uncoupling process, except in the field of rumen inoculated anaerobic digestion where promising results were found in terms of biological performance (Gijzen et al., 1987, Dalhoff et al., 2003). In previous work (Walker et al., 2008) a 1st stage digester with an integral nylon mesh filter cartridge was used as part of a two-stage AD process, and it was found that meshes with pore sizes in the range 30–100 μm gave good digestion performance when used with BMW as feedstock. The maximum organic loading rate (OLR) that could be achieved in these stirred digesters was 3.75 g VS l⁻¹ d⁻¹, due to difficulty in mechanically stirring the digestate rather than flux limitation of the nylon mesh filter. The rotating drum mesh filter bioreactor (RDMFBR) described in the current paper was designed to overcome this limitation.

Section snippets

Methods

A pair of two-stage digestion systems (S1 and S2) were operated in parallel. Each system consisted of a 1st stage RDMFBR (volume 1.5 l) and a 2nd stage AF reactor (volume 4 l). Each system was operated at mesophilic temperature and used a primary feedstock of simulated BMW which had the properties shown in Table 1. This was prepared based on compositional analysis of waste collected as part of a waste audit performed by the Resource Recovery Forum (RRF, 2001). The simulated BMW had a biochemical

Results

The experimental run lasted for a total of 87 days during which time the RDMFBRs operated continuously without fouling. The rotating drum was able to mix the high solids input material applied at a high loading with little mechanical effort. A summary of the main results is given in Table 2, which shows averages of measurements taken between days 56 and 72. This period was chosen since at a SRT of 15–20 days most of the inoculum solids would have been removed, and the data are likely to represent

Discussion

During the first 30 days of the trial, the 1st stage reactors went through an acidic phase and produced an effluent with a COD of 4000–5000 mg l⁻¹. During this time each AF reactor produced up to 1.5 l d⁻¹ of methane. Gradually methanogenic conditions were established in the 1st stage reactors and these began to contribute a greater and greater proportion of the system methane and the AF reactor methane production decreased to bellow 0.5 l d⁻¹. At the same time the effluents from the 1st stage were

Conclusions

The RDMFBR was able to maintain continuous filtration over an 86-day period and provide an effective means of uncoupling the solids and liquid retention time in the 1st stage reactor, allowing the system to operate at an OLR of 7.5 g VS l⁻¹ d⁻¹ when using SBMW. The rotating drum provided a means of mixing the waste at this high OLR which was not possible in a previously used mechanically stirred digester design (Walker et al., 2008). The mixing energy in the drum system was also considerably

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