Bio-methane production from urban organic matter

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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.