The generation and disposal of biodegradable waste without adequate treatment result in significant environmental pollution. Besides health concerns for the people in the vicinity of disposal sites, degradation of waste leads to uncontrolled release of greenhouse gases, such as methane and carbon dioxide, into the atmosphere. Conventional means, like aeration, is energy intensive, expensive and also generates a significant quantity of biological sludge. In this context, anaerobic digestion offers potential energy savings and is a more stable process for medium and high strength organic effluents.
In the waste water plant of Bromma, Stockholm, about 10000 tons of sewage sludge (dry matter) is produced annually. In such a plant the sludge is biologically treated under anaerobic conditions. The product is biogas and a solid organic rest product. The biogas was earlier for internal heating at the WWTP, and also sold for external use. In 1996-1997 a pilot biogas upgrading plant was installed, and in 2001 a full-scale upgrading plant was constructed. The upgrading plant separate methane from carbon dioxide and other contaminants in order to produce a vehicle fuel of natural gas quality. 1.5 million m3 gas is produced annually, which corresponds to 1.5 million litres of petrol.
There are six fuel stations in Stockholm providing gas as fuel. Biogas is one of the least environmentally harmful fuels economically available today. Because it is based on biologically produced substances it is part of recirculation, and does not contribute to the CO2 content in the atmosphere. Emissions of hydrocarbons, CO and nitric oxides from production and use are small. The rest product has to a large extent been used as soil improvement and fertilizer thanks to the decreasing amounts of pollutants e.g. heavy metals. However, the use in agriculture is nowadays limited. In the future, with still better control of the quality of the sewage water input, the rest product can be utilized. This is important from the recycling point-of-view, because the rest product contains phosphorus, which is a limited natural resource.
The anaerobic digestions technology for sewage sludge and organic water treatment is well established. The by-product is sanitised. If the sewage mainly comes from controlled sources as food industries and manure from farms, the rest substance is well suited as soil improvement for agriculture, and the phosphorus is recycled. The rest product is better as fertiliser than ordinary manure because of better nitrogen availability and less odour. If farm manure is used in the digester, it is transported to the plant with trucks. When the trucks go back, they bring the sanitised fertilisers back to the farms. Problems occur when more than 10-20% of the sludge is household waste and/or waste water sewage sludge. Then there is a risk that it will contain too much pollutants to be accepted in agriculture. The quality of the sewage water must be controlled for recirculation of plant nutrients.
The process is controlled, and can be used anywhere. In areas with a large production of e.g. waste from food industries, the technology is well suited for combination of solid waste and sewage sludge, and the gas as well as the rest product can be utilized. If the origin of the main part of the water treated is urban sewage sludge, methods must be developed to control, limit and decrease pollutants. However, in any case the gas produced can be utilized.