The development of a mesh bioreactor for the anaerobic digestion of biodegradable municipal waste

Abstract

A laboratory scale prototype mesh bioreactor (MeBR) for the two-stage anaerobic digestion (AD) of biodegradable municipal waste (BMW) was successfully designed and tested. The development involved a number of preliminary stages; creation and characterization of a synthetic BMW (SBMW), exploration of its single-stage AD characteristics under both methanogenic and hydrolytic conditions, and AD trials of a two-stage reactor system where SBMW was fed to a 1st stage hydraulic flush (HF)reactor and centrifuging was used as a method to produce liquid effluent which was fed to a 2nd stage anaerobic filter (AF) reactor. The single stage digestion of SBMW suffered from process instability at very low organic loading rates (OLR) of 2-2.5 gVSl-1d-1 whilst the two-stage HF/AF system was robust up to a maximum OLR of 7.5gVS/ld. The HF reactors became methanogenic due to the effect of effluent recycling. After this, two different prototypes designs of MeBR were built and tested in continuous two-stage AD trials (AF 2nd stage). The aim was to replace the centrifuging of the HF reactors with continuous mesh filtration whilst maintaining the stable and robust digestion process. The first design confirmed the ability to filter SBMW digestate through nylon meshes of pore size 30-140 >m at an OLR of 3.75 gVSl-1d-1. The mesh system operated similarly to the HF/AF system and efficient two-stage AD of the SBMW was shown. Problems with stirring thick digestate limited the OLR on both the mesh and HF systems. To address this limitation on OLR, a 2nd MeBR was designed which employed a rotating drum for low effort mixing and 100 >m nylon mesh sections on the drum surface for filtration. This reactor system operated stably at an OLR of up to 15 gVSl-1d-1 albeit with reduced specific methane production. Application of this type of system will be dependant on requirements for high plant throughput, system robustness and a compact process to make up for slightly lower methane production and waste stabilisation compared to single stage digestion.

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Identifiers

ORCID for C.J. Banks: orcid.org/0000-0001-6795-814X

ORCID for S. Heaven: orcid.org/0000-0001-7798-4683

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