Water Engineering: Membrane fouling for low energy advanced wastewater treatment

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.

This proposal seeks to identify the scientific mechanism of reversible foulant layer formation to enable significant energy reduction in membrane bioreactors (MBRs). Direct beneficiaries of this work include utilities that operate MBR at full-scale. The water industry generates 5 mn. tonnes of CO2/year; 2.5% of UK industrial emissions. A key driver for the water sector is in, 'Transforming wastewater treatment to reduce carbon emissions'. MBR presently use more energy than standard activated sludge processes due to the membrane which requires 0.4 kWh/m3 of product water. This is an acknowledged key process challenge (See Support Letter, Severn Trent Water) set out by the Department for Business Innovation and Skills Environmental Sustainability Knowledge Transfer Network. The proposed mechanism could reduce this energy demand by ten times enabling utilities to reduce directly attributable carbon. Direct beneficiaries also include the regulator (Environment Agency) and utilities currently concerned with meeting stricter consents set out in the Water Framework Directive, as the resultant process energy demand in MBR would be analogous to conventional systems, thereby lowering this critical barrier to implementation. Utilities can therefore also avoid alternative carbon intensive tertiary treatment process options which have been considered. Indirect beneficiaries include the general public through minimising the impact of effluent on receiving waters and enhancing security and sustainability of water resources.

The UK water sector is committed to maximising renewable energy to deliver carbon neutral wastewater treatment. Anaerobic MBR technology is critical to achieving this ambition. Research at Cranfield is currently funded by three UK utilities, with Yorkshire Water also committed to scale anaerobic MBR to 2000 m3/d within 5 years (See support letter, Yorkshire Water). Membrane energy demand remains the critical barrier using standard MBR design. Our research has indicated that hybrid anaerobic MBRs can enable highly reversible foulant layers similar to that developed through our patent. Importantly, if analogous foulant structure can be engineered and a ten-fold reduction in energy demand achieved, zero-energy sewage treatment could be realised in an industry which utilises >1% of electricity produced in England and Wales. Our close relationship with project partners means that findings could be widely and quickly (<5 years) deployed. Results of this research will be disseminated via quarterly project steering committee meetings to comprise of key representatives from the major UK utilities Anglian Water, Severn Trent Water and Yorkshire Water.

An innovative direct observation methodology will be developed for measuring the kinetics of reversible foulant layer formation. The research proposed will be of interest to a broad range of academic beneficiaries working on particle-surface interactions, including biofilm formation, adhesion, and membrane applications for water and wastewater treatment in the UK and globally. The proposed method will be a key academic impact enabling for the first time, a high resolution, comparatively low cost technique for exacting measurement of sub-micron particle transport combined with in-situ biofilm characterisation. The method will be mainly disseminated through publication in leading academic journals such as 'Journal of Membrane Science' and 'Water Research'. The PI has a proven track record, having published 7 papers in these journals which have been cited by researchers across the world (53 cites; China, USA and Europe), working on membranes, chromatography, polymer science and aquaculture engineering. The research will also generate significant new knowledge to enable better understanding of particle transport within biofilms, relevant to membrane scientists and others. Dissemination activities e.g. international conferences/trade journals will be used to reach wider audiences.