Water Engineering: Membrane fouling for low energy advanced wastewater treatment

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The UK water industry treats over 3 billion m3 of sewage every day and so plays a major role in safe-guarding water sources for the protection of wildlife and human health through wastewater treatment. Within the EU, member states are required to meet a number of new, stricter sanitary determinant targets by 2015 which have been set out within the Water Framework Directive (WFD). Sewage treatment is currently predominantly facilitated by biological treatment systems, typically designed as activated sludge processes (ASPs). Whilst effective for treatment to existing standards, both operating utilities and the industry regulator, the Environment Agency, have raised concerns that the proposed WFD standards cannot be met by existing ASP assets. Membrane bioreactors (MBRs) are an advanced wastewater treatment process that couples membrane separation with the activated sludge process. The membrane units are typically comprised of pores with a nominal diameter in the range of 0.1 to 0.01 micrometres and thus enhance the separation of particles versus conventional ASP. Furthermore, the enhanced retention of active microbes enables more robust nitrification to be achieved and have therefore demonstrated a capability to exceed the proposed effluent compliance set out in the WFD.Consequently, MBRs represent the fastest growing advanced wastewater treatment technology with a global annual market value of around $1 Bn. However, the energy required to operate the membrane in the MBR process results in a markedly higher process energy demand than for conventional ASP technology. This constraint has therefore limited the uptake of this technology for municipal sewage treatment as it is in conflict with current regulatory and utility drivers which are seeking to, "Transform wastewater treatment to reduce carbon emissions" (Environment Agency Report, 2009), and in the long term move toward carbon neutral wastewater treatment. Nevertheless, the demand to meet stricter wastewater consents is imminent and is further exacerbated by the increased demand on scarce water resources. MBRs are an integral technology to fulfilling these challenges. This proposal therefore seeks to radically reduce the specific energy demand associated with membrane operation in MBR to enable uptake of this critical technology. During membrane filtration, particles accumulate at the membrane surface forming concentrated fouling layers at the membrane surface. This fouling layer gradually compresses with time, restricting flow further. The membrane energy demand arises from the air injection required to limit the accumulation of the concentrated particulate fouling layers. Recent studies at Cranfield have shown that by manipulating the hydrodynamics imposed by air injection, it is possible to restructure the particles within the foulant layer to make it more easy to remove, reducing the energy demand by up to ten times. Critical to understanding the scientific mechanism behind this relationship is in establishing the role of small particles (<1 micron) in these fouling layers as it is argued that small particles represent the critical fouling fraction. Whilst methodologies are available to measure foulant layers in such dynamic conditions, they are not sufficiently sensitive to detect particles in the sub-micron size range. Consequently, a novel Reflected Light Fluorescence Direct Observation method is proposed that will enable measurement of this critical group of particles. Once established, this method will provide quantitative evidence of particle distribution and particle transport within these complex fouling structures. The resultant evidence will be used to engineer highly reversible fouling layers within MBR, eliminating the critical energy barrier and enabling MBR utilisation as a reduced carbon technology option for advanced protection of the environment.