Water and Effluent Treatment Magazine
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| ANAerobic DigesTioN Taking AAD to the next Biomethane Gas-to-Grid injection plants may be widely used in mainland europe, but they are a new concept in the UK. Imtech and Severn trent Water are currently building one of these at Minworth, which already produces 3,400m³/h of biogas. David O'Malley explains why the plants are an attractive prospect. Over the last decade, the adoption of Advanced Anaerobic Digestion(AAD) by the UK water industry has been significant. This technology has enabled many water companies to move away from highcost and energy-intensive sludge treatment processes such as thermal drying and potentially odorous processes, such as lime disinfection and dewatering. AAD has also helped maximise solids destruction, using either biological or thermal hydrolysis pre-treatment to produce typically 50% more biogas than conventional digestion. When integrated with high-efficiency CHP units, renewable power generation is maximised. Imtech is one of the leading engineering and delivery partners for AAD and has successfully delivered projects for Anglian, Dŵr Cymru Welsh Water and Northumbrian Water, with further projects nearing completion for Thames Water at Crossness and Beckton. We are working closely with Severn Trent Water to further enhance the economic benefit of Anaerobic Digestion (AD) with the introduction of a relatively new process in the UK, Biomethane Gasto-Grid injection. Digester Capital investment into this Biomethane Gas-to-Grid plant is economically attractive, and a threeyear payback period is anticipated gas is cleaned and injected into the National Grid as a sustainable replacement for natural gas in addition to it being used in Combined Heat and Power (CHP) units for renewable power and heat generation. With the largest commercial Biomethane Gas-to-Grid plant in the UK now under construction at Minworth, the next stage of sustainable wastewater service will be completed in 2014. New concept The key driver of Biomethane Gas-to-Grid is increased renewable energy production and improved incentives for biomethane production when compared to renewable power generation using CHP units. Biomethane gas is produced by the extensive cleaning of the biogas produced from anaerobic digestion of sewage sludge. The biogas is cleaned, scrubbed, dried and compressed to produce a high-grade biomethane gas. Although widely adopted at many plants across Europe, the use of Gas-to-Grid injection plants is still a comparatively new concept in the UK. Minworth is Severn Trent Water's largest sewage treatment works. It already has 16 existing anaerobic digesters, which produce the £6.3M project is scheduled for final completion in 2015 14 | WET News | January 2014 about 3,400m³/h of biogas. Sewage sludge is biologically converted to biogas, which is approximately 60-65% methane with the balance being mainly carbon dioxide and trace quantities of hydrogen sulphide and other contaminants. Once complete, the new Biomethane Gas-to-Grid plant will take approximately 1,200m³/h of the biogas and remove carbon dioxide and trace contaminants to produce biomethane (97-98% methane). The remaining biogas will still be processed through the existing CHP units to generate electricity and heat for use within the wastewater treatment process with excesses exported off site to the national electricity grid. Capital investment into this Biomethane Gas-toGrid plant is economically attractive following the recent introduction of the government's Renewable Heat Incentive scheme. It has an anticipated threeyear payback period. When complete and combined with the existing CHP plant, it will ensure that the Minworth treatment site is energy selfsufficient, with increased income. Imtech, as part of the LORImtech joint venture between Laing O'Rourke and Imtech, worked closely with Severn Trent Water in developing this project by firstly identifying, assessing and reviewing the most suitable upgrade technology applicable to this specific application. A technology package from Malmberg has been selected, which utilises a simple but effective high pressure water wash process. Laing O'Rourke is carrying out the civil work, while Imtech is the EPC contractor. Consultants CNG Services and GL Noble have also worked closely with the development team and have facilitated a successful interface between the team and National Grid. The process Malmberg's high pressure wash water system has been selected as the most suitable solution. It uses absorption, which provides a high pressure water scrubbing process to clean and biogas and increase the methane concentration. Throughout the process the biogas is dried, cleaned, scrubbed, cleaned and compressed before the resulting methane enriched biomethane product is injected into the local high pressure National Grid pipeline. The biogas is initially dried and cleaned upfront of being introduced into the upgrade plant by using activated carbon adsorption. The cleaned gas is then pressurised before being efficiently water scrubbed through a randomly packed column of high performance packing material. Under these pressurised conditions, the process water absorbs the carbon dioxide and other contaminants from the biogas and thus enriches the methane content within the gas stream. The gas exiting this wash column is now high grade biomethane gas (97-98% methane). The biomethane is then dried and cleaned before being compressed (up to 20bar) and chemically analysed to ensure it meets the stringent gas quality