The Sjölunda wastewater treatment plant in Malmö, Sweden, was upgraded for extended nutrient removal in 1998-1999. The design was based on future effluent standards of 10 mg BOD7/l, 0.3 mg total-P/l and 8 mg total-N/l. The upgrading concept took into consideration existing processes and structures, resulting in a cost-effective and compact upgrading. To introduce nitrification, the existing trickling filters for BOD-removal were converted to a nitrifying mode. A sequencing batch reactor for nitrification of supernatant was necessary to control the ammonia load. Denitrification was accomplished in a moving bed biofilm reactor with addition of external carbon source. The future effluent standards could be met by the upgraded plant. The trickling filters were stable despite varying loading conditions. High rates and low effluent ammonia concentrations were achieved. Essential features for stable post denitrification were control strategies for carbon source dosage and avoiding phosphorus limitation.

Anaerobic digestion of sludge has been part of the treatment plant in Malmö for many years and several projects on optimisation of the digestion process have been undertaken in full scale as well as in pilot scale. In order to facilitate a more sustainable solution in the future for waste management, solid waste organic waste is sorted out from households for anaerobic treatment in a newly built city district. The system for treatment of the waste is integrated in a centralised solution located at the existing wastewater treatment plant. A new extension of the digester capacity enables separate as well as co-digestion of sludge together with urban organic waste from households, industry, restaurants, big kitchens, food stores, supermarkets, green markets etc. for biogas production and production of fertiliser. Collection and pre-treatment of different types of waste are in progress together with examination of biogas potential for different types of organic waste. Collection of household waste as well as anaerobic digestion in laboratory and pilot scale has been performed during the last year. It is demonstrated that organic household waste can be digested separately or in combination with sludge. In the latter case a higher biogas yield is found than should be expected from digestion of the two materials separately. Household waste from a system based on collection of organic waste from grinders could be digested at mesophilic conditions whereas digestion failed at thermophilic conditions.

Problems with eutrophication and algae blooms initiated the need for enhanced nutrient removal at European wastewater treatment plants in the end of the eighties. When upgrading the two WWTPs in Malmö, site specific upgrading concepts had to be considered because of the existing plant structures. As phosphorus removal already was implemented, the upgrading was focused on enhanced nitrogen removal. The key element in the upgrading procedure was the retrofit with an appropriate post-denitrification process. A newly developed concept with a moving bed biological reactor process was found to be cost-effective. Pilot plant tests with an MBBR process for post-denitrification were performed for demonstration purpose and to get some ideas for proper design criteria. The later experiences from the full-scale plant operation showed good agreement with the selected design criteria from the pilot plant data. The future effluent standards could be met by the upgrading of the plants. The pilot plant tests as well as the experiences from full-scale operation pointed out the need for a control system for external carbon dosage. The control action was based on the instantaneous nitrate load calculated from on-line measurements of nitrate and wastewater flow. The need for an active control of the phosphorus concentration in the MBBR process was also stressed by the experiences.

At many large wastewater treatment plants (WWTPs) the increased hydraulic load, caused by combined sewer systems during storm events, results in primary effluent overflow when the capacity of further treatment is exceeded. Due to stringent effluent standards, regulating the total discharge from the WWTPs, the Rya WWTP in Göteborg and the Sjölunda WWTP in Malmö will have to reduce the impact of primary effluent overflow. Separate, high rate, precipitation processes operated only during high flow conditions have been investigated in pilot units at the two WWTPs. Precipitation in existing primary settlers operated at a surface loading of 3.75 m/h removed phosphorus to 0.35 mg/l. The Actiflo process was also shown to remove suspended solids and phosphorus well. BOD was reduced by 50-60%. With such processes the overall effluent concentrations from the plants can be reduced significantly. Key upgrading features are small footprints, short start up time and high efficiency.