Water Journal : Water Journal May 2015
MAY 2015 water 39 Feature article With this scheme the pot ale and draff are separated from the spent lees and wash waters, mixed with dust and culms, and dewatered, dried and burned in a specially designed dual fuel (biomass and biogas) fired boiler to produce steam and electrical energy that is used on site. The spent lees, as commonly occurs in distilleries, is contaminated with a low level of copper picked up from the still. In order to ensure that the copper consent is met, this spent lees is treated by an ion exchange plant to remove the copper prior to mixing with the liquors from the dewatering process. The combined liquors are treated in an expanded granular suspended bed (EGSB) high rate anaerobic reactor. This uses a granular anaerobic biomass to convert a high proportion of carbonaceous COD into methane-rich biogas, which is used as an additional fuel for the boiler. Following the anaerobic treatment stage, the aqueous stream is passed to a membrane bioreactor (MBR). This is an aerobic process in which aerobic bacteria remove most of the residual COD and also convert ammonia into nitrate (nitrification). It also incorporates an anoxic denitrification stage to reduce total nitrogen. Meeting the SEPA discharge consent necessitates membrane filtration as a final process stage, and this produces a high-quality effluent that can be fed directly to a reverse osmosis unit to be reused as boiler make-up water. Excess biomass generated in the biological treatment processes is mixed with the draff prior to dewatering to provide additional biofuel for the boiler. The system came on-line in 2009 and provides 98 per cent of the steam and 80 per cent of electrical power used at the distillery, reducing annual carbon dioxide emissions at the site by approximately 56,000 tonnes. A similar plant at Leven in Scotland is currently treating a flow of 6,100m3/day containing 118 tonnes of COD. Water recovery is 60 per cent and the recovered energy meets 80 per cent of the distillery’s needs while reducing carbon dioxide emissions by 95 per cent. a new approach While this total bio-energy approach is both economically attractive and sustainable, Veolia has been working to improve efficiency and reduce costs. Memthane® combines the advantages of anaerobic digestion and membrane bioreactor (MBR) technologies in a compact anaerobic membrane bioreactor (AnMBR). The system consists of a high-efficiency anaerobic bioreactor followed by a membrane ultrafiltration system. Wastewater is fed to the anaerobic bioreactor where the organic components are converted into biogas, which usually more than satisfies the process energy requirements, with surplus being available as a renewable energy fuel for boilers or CHP plants. The effluent from the anaerobic reactor is pumped to the ultrafiltration modules, where permeate is separated and biomass is returned to the bioreactor. The permeate has negligible suspended solids concentration and can potentially be fed directly to a reverse osmosis plant, allowing around 70 per cent or more of the wastewater to be recovered at a quality similar to that of softened mains water. This water can be used for applications where potable water is not mandatory, such as boiler or cooling tower make-up and CIP (clean in place). The ultrafiltration membrane retains sludge in the system, minimising surplus sludge production and disposal costs. It also retains high molecular weight COD (which is often difficult to biodegrade in conventional anaerobic reactors) for a sufficiently long period that most of it is broken down. This not only reduces the COD, but also enhances biogas production. Combining two steps in one, the Memthane process typically reduces the influent COD by more than 99% and this usually means that no additional expensive aerobic post-treatment is necessary. Further, if the wastewater contains significant levels of nitrogen and phosphorus it may also be possible to recovery valuable nutrients by struvite precipitation. Treating high-strength wastewater by anaerobic membrane bioreactors produces a treated effluent that is usually low enough in COD to be discharged directly to sewer and with low enough turbidity to be recycled via reverse osmosis. Biogas production provides a sustainable source of renewable energy and the possibility of nutrient recovery offers a potentially saleable by-product. There are now 10 Memthane plants in operation around the world and these have demonstrated that the combination of reduced effluent discharge costs, low operating costs, energy recovery and water recycling makes the process not only technically and economically attractive, but also sustainable, reducing both carbon and water footprints. Currently, pilot plant trials are being carried out at three distilleries and it is estimated that a typical 8MLa distillery would benefit by about £800,000 pa in energy, as Figure 3 shows. Figure 3. Memthane® in a distillery application. Adding water recycling via reverse osmosis and the benefits in reduced discharge cost and mains water savings could easily add a further £100,000 pa [approximately $AU195]. This type of project, converting by-products into valuable energy and recoverable clean water, can make a major contribution to the sustainability of distilleries and will set a new standard in the industry. wJ the aUthors Craig Menouhos (email: craig.menouhos@ veolia.com) is an industrial chemist with 21 years of experience in water treatment. He has a comprehensive understanding of business and technical solutions in the food and beverage, and municipal sectors. He is currently Client Manager – Food & Beverage for Veolia in Australia. Ian Hart (email: email@example.com) is Industrial Business Development Director, Veolia Water Technologies UK. He is responsible for developing the business in the food, beverage, pharmaceutical and energy sectors. Ian has more than 25 years of experience developing project solutions for water treatment and other process industries. Before joining Veolia, he was a Director of Irish-based consultant engineers, PM Group.
Water and CSG
Water Journal June 2015