Power generation and heat recovery Case study: a synergy project between a waste water treatment plant and a green waste composting platform

A common valorization of digester gas and composting gas increases the efficiency of the co-generator installation by 10% to 15%. In this case study, a green waste fermentation and composting platform and its neighboring waste water treatment plant (WWTP) opted for a common co-generator: about 600,000 m³ of digester gas and 1,900,000 m³/year of fermentation gas (biogas) transformed into electricity and heat. The energy content of this combined gas source is about 13,800 MW/a, out of which about 38% is transformed into electricity, about 42% is converted into heat, and 20% is lost. The electrical energy produced (600 kW) is sold to the Swiss electrical grid (Swissgrid Program) as Ökostrom, or “green power,” at a higher price than that of normal power. The heat produced (660 kWh) is used to heat the composter (60 kWh), the digester (125 kWh), and the buildings (25 kW). The excess heat (450 kWh) could also be used for a future low-temperature biosolids drying project, whose life-cycle costs would be counterbalanced by the reduction in disposal costs. This project allows for an optimal use of the energy content of biogas and digester gas. Once drying is implemented, the environmental impact will be even more beneficial with a reduction in transport and the facilitation of phosphorous recovery from dried biosolids. In this case study, the large amount of biogas produced would enable the implementation of low-temperature biosolids drying using the excess heat of the co-gen facilities. The ratio of the biogas to the digester gas production has to be at least 2.5 to 3.0 in order to produce sufficient excess heat for a low-temperature dryer. Low-temperature drying is the most ecological and sensible way of using locally produced waste-heat all year long.