Danes lead the way in biowaste to energy

Jon McAteer, Technical Manager at Veolia Water Solutions & Technologies, talks about the success of a Danish plant that co-digests household waste, sewage sludge, food waste and organic industrial wastewater to produce biogas for electricity generation. by Jon McAteer Disposal of sewage sludge is probably the biggest problem currently facing the water industry. The most widely used technology is anaerobic digestion. In the UK, although the same process is used to generate landfill gas from organic household waste, it is unusual for both types of waste to be treated together. This is mainly due to a lack of commonality between the organizations involved. In contrast, Denmark has demonstrated ‘joined up thinking’ by developing a major programme for biogas production using anaerobic digestion technology. The Danish government’s ‘Energi 21’ plan sets out integrated solutions for energy, waste management and nutrient redistribution, and provides support for biogas development as part of its policy target to meet 35% of the country’s energy needs from renewable sources. With Germany now the leading EU user of biogas, it is perhaps time for the UK to learn from Denmark’s experience and adopt a joined-up approach to maximize energy from waste and sewage sludge. Veolia Environnment’s four business divisions (water, waste management, energy and transport) make it well-placed to provide complete solutions. Krüger A/S is part of Veolia Water Solutions & Technology (VWS) (a subsidiary of Veolia Environnment’s water division) and was responsible for developing much of the anaerobic digestion technology currently being used in Denmark. Anaerobic digestion requires a digestible feed and this often involves source segregation. The availability of suitable industrial waste streams can often be an advantage, adding to the capacity for biogas generation and providing a cost-effective means for factories to treat their waste. There also needs to be a viable disposal route for sludge and for treated liquid digestate and, of course, the necessary infrastructure for energy distribution. A typical anaerobic digestion plant provides collection, storage and blending of the waste to provide a consistent feed which is then pasteurized prior to mesophilic digestion at 35°C. The dewatered sludge is spread on land while the biogas produced by anaerobic digestion is collected and burned in a combined heat and power (CHP) plant to generate electricity. The anaerobic digestion plant at Grindsted Kommune is one such scheme. Designed by Krüger A/S, it produces almost 7000 Nm3/day of biogas which is used to generate electricity and provide district heating. Grindsted Kommune is a mainly agricultural area with local food processing industries. The anaerobic digestion plant was designed in 1996 to co-digest organic household waste, sewage sludge, food wastes from supermarkets and restaurants, and food industry wastewater. Table 1 summarizes the composition of the waste received and the amounts treated. The greatest challenge to the engineers responsible for the scheme was not a technical one, but having to arrange for household waste to be sorted prior to collection. They considered the power consumption required for mechanical sorting of the waste to be too high in the context of plant sustainability, and undertook a major programme to raise public awareness about the scheme. The result was the introduction of a waste collection regime which allows the organic fraction of household, restaurant and canteen waste to be collected with minimum inconvenience to the public. Each household is provided with a rack to hold a special degradable paper bag for digestible organic material (but not garden waste) and a plastic bag for the remainder. The bags are collected on alternate weeks. The refuse collection team is specially trained and samples of the separated waste are taken twice a year for quality testing. Industrial waste is delivered into a 25-tonne capacity underground blending tank before being comminuted and mixed with liquid wastes in a 20 m3 slurry hopper. The bagged household waste is added and the mixed solids conditioned in a strain press before being transferred to the anaerobic digestion plant. Pre-treatment consists of pasteurization of all the feed to ensure that the final product can be used as a high quality soil improver. The technology used is Veolia Water Solutions & Technology’s BioPasteur®, which consists of two 20 m3 tanks operating on a staggered fill–pasteurize–empty cycle so as to provide a semi-continuous feed to the digester. Once filled, the tank’s contents are heated to 70°C and held there for an hour. At the heart of the BioPasteur process is the SWS (sludge–water–sludge) heat exchanger, which is designed to recover as much heat as possible. The pasteurized waste slurry is pumped through the SWS heat exchanger where it is cooled to 35°C before entering the digester, which has a hydraulic residence time of 14 days. Most of the heat from the pasteurized waste slurry is recovered through its use to pre-heat the raw waste slurry prior to pasteurization and to pre-heat boiler make-up water. Above: Grindsted Kommune – mainly an agricultural area. Below: Anaerobic digestion plant located near the sewage treatment works. The digester currently produces about 2.5 million m3 per year of biogas. This is delivered, via a 500 m3 gas buffer storage bag, to a gas engine that generates 250 kW of electricity and 340 kW of heat. A gas boiler produces a further 700 kW of heat, with potential to supply a district heating system. The odourless digested sludge is dewatered to 22% dry solids (ds) on a belt press and then spread on agricultural land. In the winter, when the ground is frozen or too wet for spreading, it is held in store for use in the spring. Overall, the plant achieves 60% degradation of the waste and reduces its mass by a similar percentage. The anaerobic digestion plant, together with suitable refuse bins and collection vehicles, cost about €8.5 million in 1996. Table 2 summarizes the annual income and operating costs. Figure 1. Anaerobic digester Stakeholders The success of the Grindsted Kommune anaerobic digestion plant is in no small way due to the co-operation of the local public and the policy of good communications which has provided motivation to sort their waste at source – supported by a fines system for households that fail to sort. Future plans include increasing the throughput to over 6400 tonnes ds per year with biogas production in excess of 26 million m3 per year, engine efficiency more than 30% to electric power, and the export of heat to district heating as a benefit to the public. The Danish model clearly works and has demonstrated the ability of current anaerobic digester technology to co-digest household waste, industrial waste and both primary and secondary sewage sludge. There is a valuable lesson to be learned by countries such as the UK. The UK already has a culture of sorting and recycling waste, so the Danish approach could be easily adopted here – with the political will. The reduction in the quantity of waste going to landfill is significant and there are clear environmental benefits to be gained. Anaerobic digestion technology is continuing to develop and, along with the latest high efficiency CHP systems, it is possible to maximize the economic benefits of waste to energy. And, with double Renewables Obligation Certificates (ROCs) currently available for biogas-produced electricity to help support this development, the future is definitely bright – the future is biogas. Jon McAteer is Technical Manager at Veolia Water Solutions & Technologies, Birmingham, UK.e-mail: enquiries@veoliawaterst.co.uk More Waste Management World Articles Waste Management World Issue Archives