Methane management in sewage treatment

Poly-di-methyl-siloxane (PDMS) hollow fibre membrane modules were designed and built for the specific de-gassing of real and synthetic process liquids to understand: (i) the feasibility of operation; and (ii) classify the mass transfer characteristics to aid design at full scale. Liquid saturated with pure methane or a binary methane and carbon dioxide mixture was introduced into the shell side of the extraction unit, whilst sweep gas or vacuum was employed counter-currently as a stripping medium. From data analysis of operation in both anaerobic effluents obtained from Expanded Granular Sludge Blanket (EGSB) reactor and synthetic liquids, when operating under optimum conditions 93% of methane and 88% of carbon dioxide was recovered. The obtained data indicate that the extraction process is controlled by diffusivity of gases through the PDMS membrane and is proportional to the thickness of membrane wall. When applying vacuum to promote methane mass transfer, the process was highly sensitive to vacuum pressure; the highest de-gassing efficiency was recorded under the lowest absolute vacuum pressure. However, when vacuum was replaced by sweep gas, the process was insensitive to changes in gas velocity. When utilising PDMS membrane contactor for de-gassing of EGSB effluent, the net electrical output achieved by the EGSB increased by c. 24% and indicates that by integrating methane recovery, treatment of domestic wastewater using low temperature EGSB processes can become carbon positive. The potential of directing recovered methane to porous hollow fibre membrane absorbers and upgrading to national gas (NG) standards to use in national gas grid or as a vehicle fuel has been demonstrated

Dissolved methane recovery from anaerobic effluents using hollow fibre membrane contactors

Hollow fibre membrane contactor (HFMC) systems have been studied for the desorption of dissolved methane from both analogue and real anaerobic effluents to ascertain process boundary conditions for separation. When using analogue effluents to establish baseline conditions, up to 98.9% methane removal was demonstrated. Elevated organic concentrations have been previously shown to promote micropore wetting. Consequently, for anaerobic effluent from an upflow anaerobic sludge blanket reactor, which was characterised by a high organic concentration, a nonporous HFMC was selected. Interestingly, mass transfer data from real effluent exceeded that produced with the analogue effluent and was ostensibly due to methane supersaturation of the anaerobic effluent which increased the concentration gradient yielding enhanced mass transfer. However, at high liquid velocities a palpable decline in removal efficiency was noted for the nonporous HFMC which was ascribed to the low permeability of the nonporous polymer provoking membrane controlled mass transfer. For anaerobic effluent from an anaerobic membrane bioreactor (MBR), a microporous HFMC was used as the permeate comprised only a low organic solute concentration. Mass transfer data compared similarly to that of an analogue which suggests that the low organic concentration in anaerobic MBR permeate does not promote pore wetting in microporous HFMC. Importantly, scale-up modelling of the mass transfer data evidenced that whilst dissolved methane is in dilute form, the revenue generated from the recovered methane is sufficient to offset operational and investment costs of a single stage recovery process, however, the economic return is diminished if discharge is to a closed conduit as this requires a multi-stage array to achieve the required dissolved methane consent of 0.14 mg l−1.Yorkshire Water; Severn Trent Water; Anglian Water; Northumbrian Water; EPSR