Osmotic Membrane Technologies for Energy Neutral Wastewater Treatment: Process Performance and Optimization

The United Nations estimates that the world produces around 1,500 cubic kilometers of wastewater annually, of which more than 80% is untreated. On average, treating 1 cubic meter of sewage consumes ~0.5-0.6 kWh of energy, ranking the fourth most energy intensive sector in the UK. With the increasing concern over declining quality of natural water bodies, greenhouse gas emission and the escalating price of fossil fuels, the conventional paradigm of sewage treatment needs a step-change. There is a rapidly expanding global water market in creating and delivering low energy and environmentally sustainable sewage treatment technologies which are required not only for enhanced treatment efficiency but also resource exploitation.

Compared to energy-intensive aerobic counterparts, anaerobic sewage treatment processes are more attractive due to their low energy consumption and sludge production and their production of bioenergy. However, it is crucial to pre-concentrate dilute sewage before anaerobic digestion in order to achieve improved treatment efficiency and energy recovery.

Forward osmosis (FO) is a natural process by which clean water passes from dirty feed water towards a salt 'draw' solution with higher osmotic pressure when the two solutions are separated by a semipermeable membrane. It is an emerging water treatment technology and has been identified as an ideal candidate for concentrating liquid due to its superior retention of organic matter as well as its low energy requirement. However, as yet, the full potential of FO has not been realised in full-scale treatment technologies. The greatest gains in deploying FO may be achieved by eschewing the more traditional reactor designs and exploring radically new technologies that integrate biotechnology and membrane science to deliver a step change in the efficiency and sustainability of water supply, treatment and reuse.

In this study, we propose to develop a novel osmotic membrane bioreactor for energy-neutral anaerobic wastewater treatment. Our conceptual design comprises two stages: (1) osmotic pre-concentration and (2) high retention anaerobic membrane bioreactor (HRAnMBR). In Stage-1, clean water passes from sewage to ocean water that is separated by a semi-permeable membrane at nearly zero energy input. It is even possible to gain energy when a small amount of pressure is added on the ocean water side, a process called pressure retarded osmosis (PRO). Nearly all organics are retained in the concentrated sewage due to the high retention nature of membranes. This pre-concentration stage facilitates the efficient anaerobic digestion in Stage-2. The water that has permeated through membrane has high quality and can be discharged back to the ocean with no environmental impact. In Stage-2, the organics in pre-concentrated sewage are degraded and converted into renewable energy by anaerobic digestion. In the HRAnMBR, high retention membrane plays two roles: (1) further clean the anaerobically treated sewage and (2) significantly enhance biogas recovery and thus the overall energy balance during sewage treatment. This 18-month project will focus on simplified systems where we can build models of water flux, pollutant removal and biogas production in response to key variables (e.g., membrane materials, wastewater/draw solutions composition, temperature, reactor configuration). Design and operation guidelines of the system will also be established.

The whole system is highly attractive in terms of treating wastewater to meet the further stringent water quality standards, reduced footprint and reduced energy costs. More importantly, it has the potential to make sewage treatment a net energy producer. We already have a relationship with Scottish Water who is interested in the proposed technology to provide water and wastewater services to small rural communities. The technology can be sold in the huge global market for decentralised water supply and treatment.

The proposed research will ultimately deliver a sustainable sewage treatment technology in coastal regions. Most major cities and rural area in the UK are located on the coasts and nearly all coastal areas discharge wastewater offshore. In some cases, the nutrients in the wastewater cause harmful algae blooms in coastal zones. This project concept is based on "converting waste to resource". Our vision is to ultimately combine osmotically driven membrane process with anaerobic digestion to (1) eliminate the release of eutrophication-causing nutrients and ecologically harmful contaminants through wastewater, (2) convert wastewater to potential energy sources in the form of biogas, (3) significantly reduce overall operating cost, energy consumption and carbon footprint, (4) improve environmental sustainability, and (5) reduce the desalination costs and energy consumption by co-locating wastewater treatment with desalination plants. Hence, the proposed technology will have important impacts on the water security and sustainability. The success of this technology will benefit the UK in terms of technology development and commercialization, research competiveness, economic growth and environmental sustainability.

The European Commission through Horizon 2020 cites the need for Europe to become a leading developer and supplier of high-end technologies for the rapidly expanding global water market (US$ 770 bn/year by 2016). In this project, novel osmotic pre-concentration technologies will be designed and implemented to allow sewage pre-concentration at little energy input. Meanwhile, the waste organic energy contained in the pre-concentrated sewage can be efficiently harvested using a high retention anaerobic membrane bioreactor to enhance the overall energy balance in sewage treatment. Upon the successful implementation of the proposed treatment scheme, sewage treatment plants will potentially generate a significant amount of net energy output. Thus, the proposed research will likely lead to a breakthrough in energy-efficient sewage treatment. The proposed technology has great commercialisation potential which will help take strides towards meeting the ever more stringent environmental regulations. Once successfully implemented, the technology will bring the UK to the forefront in membrane technology, wastewater treatment and seawater desalination; and will have significant positive impacts on the economic performance of the water sector, related consultancy firms, the supply chain, spin-outs and SMEs, and hence job creation and security in the sector.

The proposed technology can be used in both developing and developed worlds. It will provide knock-on benefits to other sectors including food, medical, pharmaceutical, health, and military by supplying water that is sustainable and high quality. Furthermore, it is envisioned that the integrated membrane system can be applied on-site to recover high quality water and valuable solutes not only from sewage but also other industrial wastewater with complex compositions. For example, the refineries and petrochemical industries which use large amounts of fresh water for production processes and thus generates large amounts of wastewater, will be able to utilise the proposed membrane process to reduce their clean water consumption and negative environmental impact.

Scottish Water is particularly interested in the technology to provide water and wastewater services to small rural communities (many of which are coastal). Furthermore, the proposed technology can be sold in the huge global market for decentralised water supply and treatment. Through the collaboration with our industrial collaborators, the novel technology can be more readily tested in pilot scale. This may allow an accelerated adoption of the technology. In the long term, we will engage with industry partners who are able to manufacture the proposed water treatment system and take it to market.