For improved exploitation of the energy content present in the organic matter of raw sewage, an innovative concept for treatment of municipal wastewater is tested in pilot trials and assessed in energy balance and operational costs. The concept is based on a maximum extraction of organic matter into the sludge via coagulation, flocculation and microsieving (100 µm mesh size) to increase the energy recovery in anaerobic sludge digestion and decrease aeration demand for carbon mineralisation. Pilot trials with real wastewater yield an extraction of 70–80% of total chemical oxygen demand into the sludge while dosing 15–20 mg/L Al and 5–7 mg/L polymer with stable operation of the microsieve and effluent limits below 2–3 mg/L total phosphorus. Anaerobic digestion of the microsieve sludge results in high biogas yields of 600 NL/kg organic dry matter input (oDMin) compared to 430 NL/kg oDMin for mixed sludge from a conventional activated sludge process. The overall energy balance for a 100,000 population equivalent (PE) treatment plant (including biofilter for post-treatment with full nitrification and denitrification with external carbon source) shows that the new concept is an energy-positive treatment process with comparable effluent quality than conventional processes, even when including energy demand for chemicals production. Estimated operating costs for electricity and chemicals are in the same range for conventional activated sludge processes and the new concept.

Due to intensive aeration in the activated sludge basins, a significant part of the organic matter in the wastewater often expressed as chemical oxygen demand (COD) is mineralized to the greenhouse gas CO2. Therefore the organic content in municipal wastewater is yet a widely untapped source of renewable energy. The Carismo project vision is to reduce the specific energy demand with a new treatment scheme based on a low energy microsieve separation process and at the same time, increase the specific energy recovery with an advanced separation of the organic fraction which is valorized in a digester. Therefore two treatment schemes were evaluated at lab scale and pilot scale with real wastewater. The raw wastewater contained a high COD concentration of 1000 mg/l. The first scheme treated the raw wastewater with a coagulation and flocculation step before a microsieve separation with a drum filter at 100 µm. The second scheme was similar to the first one with an additional MBBR (Moving Bed Biofilm Reactor) installed upstream the coagulation tank. The specific goal of the microsieve process was to increase the organic carbon extraction rate in scheme 1 to 60–80 %. The Pilot trial results showed an average COD extraction of 73–81 %. The average suspended solids (SS) removal was > 95 %. The soluble phosphorus removal was between 15 % and 70 % depending on the coagulant type and dose. With 20 mg Al/l, the effluent phosphorous concentration was around 2 mg/l. The MBBR upstream increased the COD transfer in the sludge by 3–8 %, but simultaneously the mineralization decreased the yield for the biogas process. This and the additional energy consumption of the aeration speaks against the separation process with an upstream MBBR.

This paper describes an innovative concept for treatment of municipal wastewater, targeting the improved exploitation of the energy content present in the organic matter of raw sewage. The concept is based on a maximum extraction of organic matter into the sludge via coagulation and micro-sieving (100 µm mesh size) to increase the energy recovery in anaerobic sludge digestion and decrease aeration demand for carbon mineralisation. Pilot trials with real wastewater yield a COD extraction of 70-80% of total COD into the sludge while dosing 15-20 mg/L Al and 5-7 mg/L polymer with stable operation of the microsieve and effluent limits below 2-3 mg/L total phosphorus. Anaerobic digestion of the sludge results in high biogas yields of 600 NL/kg organic dry matter input (oDMin) compared to 430 NL/kg oDMin for mixed sludge from a conventional activated sludge process. The overall energy balance of the new concept for a 100 000 pe treatment plant (including biofilter for post-treatment with full nitrification and denitrification with external carbon source) shows that the new concept is an energy-positive treatment process with comparable effluent quality than conventional processes, even when including energy demand for chemicals production. Estimated operating costs for electricity and chemicals are in the same range for conventional activated sludge processes and the new concept

Unbehandeltes Abwasser ist ein wertvoller Energieträger. Die hier enthaltenen organischen Stoffe haben so viel chemische Energie, dass sich damit die bisher in der Abwasserbehandlung benötigte Energiemenge komplett kompensieren und sogar noch ein Energieübersch uss erzeugen ließe. Wissenschaftler vom Kompetenzzentrum Wasser Berlin (KWB) haben einen neuen Prozess der Abwasserbehandlung entwickelt und im Pilotmaßstab getestet, um das erhebliche Energiepotenzial im Abwasser besser auszuschöpfen. Das Forschungsprojekt CARISMO ("CARbon IS MOney") hat eine Expertenjury für den Deutschen Nachhaltigkeitspreis nominiert.

A MBBR before an advanced sedimentation step was operated as new wastewater process scheme for maximum COD extraction. The objective of this biological reactor was to modify the soluble COD ratio in primary wastewater. At high loads, the MBBR is able to consume the soluble COD for bacteria activity with very little oxidation. This process changes the soluble COD into particulate COD which is better separate from the wastewater during the following step with coagulation, flocculation and micro sieve filtration. Goals were 95% removal of suspended solids and 80% of COD extracted through separation. To check these new scheme performances, a pilot plant (0.5 to 3 m³/h) was operated at the Stahnsdorf WWTP in the south of Berlin. First results showed that a HRT of 20-30 min and a load 40-60 g CODf /(m2*d) can be recommended for maximum accumulation and minimum oxidation and that the 80% of COD extraction can be achieved (at low oxygen concentration below 1 mg/L). However the performance difference between the scheme with or without MBBR did not exceed 8 %