Central biodigester for fresh market waste in Dhaka

Fresh markets in Dhaka (vegetables, fruits, meat, fish) produce large amounts of waste which are currently transported to landfill sites. However, landfilling requires land area and causes an atmospheric emission of methane, a potent greenhouse gas. Therefore, the policy of the city corporations (Dhaka North City Corporation (DNCC) and Dhaka South City Corporation (DSCC)) is to stop landfilling and process fresh market waste in a different way. As part of the FAO – WUR project “Support for Modelling, Planning and Improving Dhaka’s Food System (DFS)” alternatives for valorising fresh market waste were studied: biogas (a renewable fuel) production from the organic fraction and black soldier fly larvae production (a feed for chicken and fish). In this report decentral and central digestion (biogas production) are evaluated. Decentral digestion (11.5 tonnes organic waste per day) appeared to be more cost effective than the current method (landfilling) in which high costs for transport and landfilling are made. However, the land area requirement for decentral biogas production plants (250 m2) is the most important drawback. Generally it is not available near current markets. The alternative is central digestion. That processes the waste from a number of markets (up to 10) on a site outside the residential areas, e.g., on the current landfill sites, benefits from the economy of scale (less costly per tonne waste processed) and better availability of skilled operators and engineers at a large waste processing plant at which more activities take place (e.g., eco-town with composting plant, recycling/sorting facilities, and waste-to-energy plant). Larger plants can hire more operators and engineers which have different specializations and skills. A design was made for processing 100 tonnes of the organic fraction of fresh market waste per day. It is estimated that this plant produces net 444 kW renewable electricity, which can be supplied to the local electricity grid. The solid fraction of the digestate can be converted into compost. The investment costs for such a central digestion plant, including engineering and installation are estimated at 103 million BDT, an order of magnitude lower than the investment costs previously estimated by DSCC (2019). The revenues from the sales of the electricity almost cover the costs for the transportation of the waste, the interest for the investments, the maintenance, and labour costs. This is a considerable cost saving compared to landfilling. The payback time for the investments is estimated at 3.4 years. If the investments would be 50% higher, the payback time may still be acceptable (6 years). It can be concluded that central digestion has an interesting business case, it saves land area (landfilling) and it prevents emission of large amounts of methane (a potent greenhouse gas) into the atmosphere.