A power plant that generates energy from left-over food scraps has started running in Singapore to process some 300 tonnes of food waste a day, starting from May 2008. At its maximum capacity, the facility can handle up to 800 tonnes of feed at maximum capacity and generate more than 6 MW of electrical energy per hour. That is enough power for over 10,000 households or equivalent industrial facilities. Currently most of the waste or garbage are sent to the incineration plants for disposal. The new Biomethanisation Plant will not only provides a cleaner energy alternative, but also contributes to long-term reduction of Green House Gas emissions and Global Warming. The highly sophisticated plant represents the first such venture in Asia for IUT Global, an environmental waste technology and management company headquartered in Singapore.So how does it work?
The whole process is called ADOS – Anaerobic Digestion of Organic Slurry: First Step: Garbage collectionWaste from households, restaurants and food centres, agriculture and industry are collected.
Second Step: Mechanical TreatmentThe organic waste is separated by a mechanical segregation system and the organic fraction is concentrated. Recyclables and undesirable/inorganic materials are removed to a large extent. A general rule that is used is: Anything that is larger than 15cm is removed (based on the logic that anything that is more than 15cm is likely to be non-organic).

Third Step: Milling and Anaerobic DigestionThe heart of the ADOS process is the ADOS mill, a patented wet mill performing a size reduction of organic solids while also effectively removing the reminent of inorganics and inerts (sand, metals, glass, plastics, etc.) prior to introduction into the Digestor.
The product discharged from the ADOS mill is an organic “slurry”. This slurry is pumped into a buffer silo to homogenize the stream of feedstock material before entering the Digestor and thus maximize bio-gas production during digestion.Within the Digestor, the decomposition and digestion process occurs. The carefully selected bacteria go into the Digestor filled with waste materials to grow and proliferate. They feed on the organic food and produce methane. The whole operation involves many factors, such as temperature, type of waste, combination of bacteria, and so on, to function at the optimum level. When the bacteria reach their equilibrium state, the system will continue to run indefinitely as long as it gets food waste. Fourth Step: Dewatering and CompostingThe finished product is pumped from the Digestor, dewatered before proceeding to the organic composting process. Final Step: The End ProductsCompost – In a downstream composting facility, high-quality compost is obtained after about 4 weeks, which owing to feedstock segregation by the ADOS mill, boast excellent levels of purity. The bulking material is sieved off and reused.
Bio-gas – The bio-gas produced is buffered, compressed and used as fuel in gas engines. The electrical power generated is fed into the power grid. Waste heat is recovered to heat the material being digested and fresh material being introduced into the digestor. Excess heat may be used for district heating, absorbent chillers, regeneration of dessicants for dehumidifying fresh air prior to chillers or processes that require 80ºC-90ºC heat.Because of the low energy requirements of the ADOS bio-methanisation process, it is very economical even in “small scale” facilities (e.g. 30 to 40 tons/day).
Last but not least, what is the biggest hurdle for such an operation? Segregation and changing people mindset. It requires a big change of mindset for people unaccustomed to thinking about what to do with their waste products, especially organic waste. The challenge lies in educating them about proper waste sorting at the source and making them aware of the contribution this makes to helping the environment. It takes only a little extra effort to segregate food from plates and containers.