Biogas production from high volume industrial effluents at ambient temperatures (AmbiGAS)

The project is formulated under the ERA-net programme and brings together an interdisciplinary team of academics, industrial process design engineers and end users from 4 countries. The research aims to develop in a systematic manner the concept of using anaerobic digestion (AD) at ambient temperatures for large volume, low-strength effluents produced in biomass-based process industries. Experience to date shows that low temperature AD is feasible, and the work will attempt to improve rates of reaction to match those of more conventional mesophilic systems. The research will establish the fundamental mechanisms underlying the acclimatisation of anaerobic microbial populations to low temperature conditions, and attempt to identify control parameters that will ensure rapid adaptation and stable operation. Reaction kinetics will be determined for different types of anaerobic biomass (granular, biofilm, dispersed or in flocs), and for industrial wastewaters from both food processing and non-food sectors. Studies will be carried out using laboratory-scale digesters to test different methods of retaining the anaerobic biomass by altering the reactor configuration, with a particular focus on the use of innovative membrane systems. The digesters will also be tested in conjunction with novel in-situ and side-stream biogas upgrading units, again using advanced membrane technologies. Our existing knowledge combined with the information gained in these targeted laboratory studies will be used to design a pilot-scale digester that will be trialled by one of the industrial end-user partners. The goal is to demonstrate the concept successfully at a pilot scale, while gaining sufficient knowledge and data to be able to show net energy production, and other savings calculated by using process models. The research outputs will provide process industries with the necessary information to make decisions on adopting these new technologies to take advantage of the potential energy savings.

The research will also develop customised membrane-based gas upgrading systems to refine biogas to give methane that is pure enough for use as a vehicle fuel or direct injection into the gas network. Membrane systems will also be used to recover dissolved methane from treated effluent, which will not only increase energy yield but will also offer a solution to a major environmental issue, as methane is a powerful greenhouse gas that can contribute to climate change.

The research is industry-linked, and the benefits and penalties of the new approach will be fully assessed using industry-standard process optimisation tools, which can provide the basis for further economic and environmental assessment. The research will have significant outputs in a range of areas: it will increase our scientific knowledge of methanogenic microorganisms, and improve process control in anaerobic systems; it will introduce new concepts in biogas upgrading and methane recovery, with added environmental benefits; it will create a database of effluent and process parameters to allow modelling and optimisation of biomass-based industries; and it will provide a practical demonstration of ambient temperature AD. The research and its implementation will thus provide a roadmap for the rapid uptake of this concept, which offers second generation biofuel production from a previously untapped source.

The research combines fundamental scientific and engineering approaches to treatment of high volume, low-strength industrial effluents from bio-mass based industries. The technology uses anaerobic conversion of organic carbon in the effluent streams to CH4 and CO2. The novel aspect is to adapt this process, which normally operates at mesophilic or thermophilic temperatures, to ambient temperature. This will provide significant energy savings compared to conventional aerobic treatments. Microbial populations will be assessed to elucidate which factors, both metabolic and genetic, allow low temperature adaptation. These changes will be mapped using techniques such as gene pyrosequencing, Fluorescent in situ Hybridisation and radio-labelled tracer experiments. The process kinetics of the reactions will be determined in lab-scale continuously-operated trials and engineering innovations will be introduced, including selective membranes to retain biomass allowing use of different reactive beds and hydraulic regimes. Experiments will assess the effect of in situ gas removal on reaction kinetics and overall rates of biogas production. The results will be used to adapt Anaerobic Digestion Model 1 (ADM1) for low-temperature process simulation.

Low temperature operation results in a higher proportion of CH4 remaining in solution and this represents not only a loss of energy but also an environmental risk from GHG emissions. Devices will be developed based on membrane cartridges in which dissolved methane is either extracted through a membrane into a solvent, or desorbed into a gaseous stream. In both cases dense membranes will be used to prevent evaporation or leakage of the extraction fluid.

The results will be used to design a pilot-scale demonstration of low-temperature AD. Process data will be modelled using both ADM1 and Aspen Plus as an optimisation tool to estimate overall energy and utility savings when the plant is fully integrated into an industrial process.

The research opens up new opportunities for the use of anaerobic technology in place of conventional effluent treatment, offering not only energy generation but also energy savings. The transition to more energy-efficient systems will provide increased economic competitiveness, particularly in the food and non-food biomass sectors. This will be realised through: a move from net energy consumption for effluent management to net gains; efficiency in energy utilisation by adopting holistic management approaches in which energy from a waste resource is integrated into the production process; improved environmental performance by offsetting GHG emissions through fuel substitution; and savings in natural resources through optimisation of utility services.

For a typical low-strength industrial effluent with a COD of 3 g l-1 the gross energy potential from complete conversion to CH4 is ~11 kWh/m3. To treat this in conventional aerobic systems typically consumes 3 kg O2/m3 at an energy demand of ~7.5 kWh. The total potential gain in converting from aerobic to anaerobic treatment is thus ~19 kWh/m3, ignoring energy costs for pumping, mixing etc in both aerobic and anaerobic systems. It is not realistic to scale this up across every industry sector producing low-strength effluents, but in 2010 for example the EU27 produced around 0.276 Mtonnes of COD in low-strength slaughterhouse wastewaters, equivalent to a gross production of 122 MW continuous or to the energy yield from digestion of around 900,000 tonnes of maize silage. This is from one sector: other large potential sources include dairies; fish and seafood processing; fats and oils; vegetables including potatoes; soft drinks and alcoholic beverages; pharmaceuticals; textiles; and paper and pulp industries. In practice industrial effluents are produced at a wide range of temperatures and strengths but any decrease in operating temperature represents a potential energy saving: particularly where process integration and optimisation techniques are applied. A high-profile example of an integrated design with an enhanced energy balance compared to existing technologies will have a positive impact on the sustainability of the sectors concerned, and offer new market opportunities for technology providers.

A well as supporting the application of new technology, the research will underpin the development of process control and optimisation. This will allow the establishment of reliable and competitive ambient-temperature AD plants by exploiting micro-scale mechanisms, and linking these to macro-scale system performance. To facilitate this, digester designs are considered in relation to the needs of psychroactive communities through concepts such as enhanced biomass retention, selective control of growth rate, and manipulation of metabolic pathways.

A further strength of the work is the collation of AD plant performance data in replicated studies, to provide both microbiological datasets and detailed process information. Use of these to upgrade Anaerobic Digestion Model 1 will create a powerful simulation tool that can be further developed for enhanced process control. Another exciting concept is the potential to enhance performance by shifting the equilibrium of biochemical reactions through selective removal of intermediate metabolites and product gases: this cutting-edge approach may create new opportunities in the biorefinery field, in terms of process control and selective product recovery. The work will also accelerate the development of novel membrane contactors for gas upgrading, which can be used across the industry sector.