Bio-methane production from urban organic matter

Anaerobic biological treatments can be used for stabilisation of the sludge produced in water and wastewater purification, in solid waste degradation and in the treatment of high-strength industrial effluents. The degradation of organic matter in anaerobic conditions has the following advantages over aerobic degradation:
- It does not require an oxygen supply and consequently has a lower energy demand.
- Sludge production is reduced by >90%, with lower in operational costs and environmental impacts.
- It produces biogas (composed mainly of CH4 and CO2) which is a valuable renewable biofuel.

In the design of these systems consideration must be given to the fact that slower-growing microorganisms may be washed out during high-rate treatment of liquid effluents. New techniques based on the utilisation of membranes to keep microorganisms in the system can overcome this limitation. This configuration is referred to as a membrane bioreactor (MBR) and combines two functions: biological degradation by the retained microorganisms, and solids separation in which the treated effluent is separated from the suspended solids and microorganisms responsible for degradation. The MBR ensures the production of high-quality effluent as a high concentration of microorganisms can be maintained; this has the further potential advantage that it may allow wastewater treatment even at lower operating temperatures. Use of membranes also allows the retention of species that have become adapted to particular wastewater types, including those that contain persistent pollutants that would otherwise not be easily degraded.

It is clear that membrane systems are one of the most promising technologies in wastewater treatment. The application of MBRs for aerobic treatment is increasing, due to the development of membranes that are able to work at high permeate flow rates, and the production of more compact, cheaper and exchangeable membrane modules. Despite these advances, however, membrane technologies also have several important drawbacks which hold back their wider application. The main issues are investment and operating costs: both of which are closely linked to operational problems such as membrane fouling, which limits the maximum flow rate that can be achieved. Methods to reduce membrane fouling include gas recirculation and back-flushing, both of which consume considerable amounts of energy, reducing the potential energy gains from an anaerobic system. Gas scouring is also not completely effective, and it may be necessary periodically to remove the membrane from the reactor for chemical cleaning. This has implications for operating costs, and continual use of chemical agents may affect the membrane lifespan and separation efficiency, making it highly desirable to reduce the frequency of this type of cleaning. A key aspect of the current research is therefore to develop and test alternative methods of membrane cleaning. The work carried out by the University of Southampton will specifically investigate the use of purpose-designed support particles which encourage the growth of microbial biomass while also providing a mildly abrasive cleaning action. This will be coupled with the application of low-intensity ultrasound, based on adaptation of the StarStream technology developed at the University, which has already won a series of major awards for innovation. StarStream uses low-intensity ultrasound and micro-bubbles in a stream of low-pressure water, and is effective at cleaning a variety of surface and fouling types. The combination of these two approaches may have synergetic effects on reactor performance allowing higher flux rates to be achieved with lower energy usage.

The research will develop new methods of biomass immobilisation and membrane cleaning that allow higher flux rates to be maintained over longer durations. Purpose-designed support matrices that can be colonised by the microbial community will allow separation of biofilm from the membrane surface via a mild abrasive action. Examples include activated carbons of different pore and granule sizes, plastic media of different buoyancies and densities, and reticulated polyurethane foam with extremely high specific surface areas.

Mobilisation of particles allows the use of novel low-intensity ultrasound technology developed at Southampton. This has found widespread application as an effective cleaning system for a variety of surface and fouling types, and will be adapted to the present use. The system is potentially low energy in relation to performance, and could be configured to provide either continuous or intermittent cleaning as needed. For external cartridge systems, ultrasound offers large potential advantages. Cleaning is currently based on the application of back pressures to clear the membrane pores: this is less effective with surface biofilms, however, due to the formation of preferential flow pathways once an area is cleared.

The interaction of ultrasound with the microbial community may also benefit the process in several ways: by causing biomass dispersion and rapid colonisation of support particles; by promoting surface interactions that enhance mass transfer; and by directly affecting cell viability, to give better control over the system growth rate and metabolic activity. Growth rates are also linked to the production of extracellular polymeric substances, a key factor in membrane biofouling.

A further potential impact of low-power ultrasound may be the release of gaseous products from the bulk solution, thus improving recovery: a particularly important aspect when operating at low temperatures due to the increased solubility of gases.

The development of effective cleaning systems for membranes will open up a very wide market for their use in both treatment and biorefinery applications, where there is a requirement to maintain cells in active culture whilst selectively removing product streams. A major advantage of in situ acoustic cleaning in this respect is that it would facilitate the maintenance of stable reactor conditions, including opportunities for aseptic operation in non-wastewater applications. The development of membrane technologies thus offers major opportunities in technology transfer to other process industries, as well as direct application in a very large potential market in the treatment of effluents from a variety of industries and from municipal sources.

The use of anaerobic technology in place of aerobic systems in wastewater treatment applications presents a considerable opportunity for energy savings, as well as the potential to generate a valuable biofuel as a by-product. Typically, to treat one tonne of domestic wastewater using a conventional process requires at least 3.6 MJ of energy input: the anaerobic system can yield 6.3 MJ, giving possible net energy gain of around 10 MJ per tonne. Moving from an energy-negative to an energy-neutral or energy-positive technology for wastewater treatment could also contribute significantly to reduction of greenhouse gas (GHG) emissions. Previous studies have suggested that an average decrease of around 80% in GHG emissions could be expected in converting a typical wastewater treatment plant to anaerobic treatment. Because of the size of the industry, which treats around 27000 million m3 of municipal wastewater per year, this would be equivalent to avoided GHG emissions of around 50 million tonnes CO2 equivalent per year in Europe alone. These savings could make a useful contribution to the targets stated in the Kyoto protocol and Council decisions (280/2004//EC and 2005/166/EC), according to which GHG gas emissions in 2012 must be reduced by 8% compared to 1990 standards. In further communique on 'Limiting global climate change to 2 oC - The way ahead for 2020 and beyond' (COM/2007/0002 Final) the EU makes a firm commitment to achieve at least a 20% GHG emissions reduction by 2020. The energy production from the anaerobic treatment process will also make a contribution to meeting renewable energy targets both for power generation and for liquid and gaseous biofuels.

It would be challenging to change the existing UK and EU wastewater infrastructure overnight: but the development of small-footprint, high efficiency anaerobic MBRs that could be pre-fabricated in the case of smaller installations would facilitate this, and help to address major issues of replacement of ageing infrastructure in the UK, as well as creating business opportunities and new jobs in the engineering and construction sectors through expansion of treatment systems across the world. The global market for MBR technology is already predicted to reach US$888 million by 2017, and contributions to resolving the key issues of membrane fouling and in situ cleaning are likely to accelerate uptake of these technologies.

Anaerobic digestion is recognised by BBSRC as an enabling biotechnology, with the output available for heat and power generation, for blending into gas distribution systems, and for compression to power alternative-fuel vehicles. The development of anaerobic membrane bioreactors for wastewater treatment also represents a halfway house to technologies for the extraction of other intermediate fermentation products as building blocks to hydrocarbon-based bulk chemicals. To use wastewater as a resource in this way would be have major impact in demonstrating the potential for provision of raw materials for a future bio-based economy.