Domestic wastewater (WW) contains thermal energy in the form of organic matter (close to 4 kWh/kg Chemical Oxygen Demand (COD) for 500 mg COD/l – or about 2 kWh/m3), but in conventional WW treatment plants (WWTP) the organic matter is oxidised with an electricity consumption of at least 0.5 kWh/m3, and producing sludge.
In larger WWTP with capacities above 50,000 population equivalent (PE), this biomass can be converted to biogas in anaerobic digestion, but on average only up to 50% of the energy can be recovered by co-generation.
Other valuable resources in WW are nutrients, essential ingredients for plant growth in agriculture, as each European produces yearly about 4.5 kg N, close to 1 kg P and 2 kg K. However, centralised WWTP can only recover a small fraction of diluted nutrients by precipitating up to 50% of the phosphorus in sidestreams, while two thirds of the nitrogen is converted to gas and lost to the atmosphere. In the production of artificial fertilizer, this nitrogen is gained back from the atmosphere with an energy input of 10 to 15 kWh/kg.
One approach to increase recovery rates and optimizing the energy balance is by treating concentrated streams in a decentralized concept, before the waste is diluted by the freshwater use of 150 l/PE/d. The project H2020 Run4Life (Recovery and Utilization of Nutrients 4 Low Impact Fertiliser) demonstrates the options of decentralized WWTP, based on the source-separated collection of greywater (from showers, basins…), blackwater, from toilets), and kitchen waste, with each flow receiving optimal treatment for resource recovery.
As part of Run4Life, blackwater from an office building in Nigran, near Vigo in Galicia, is treated with the AnMBR process, combining Anaerobic Digestion (AD) with membrane filtration to separate biosolids and clean water. As ultrafiltration with pore sizes between 0.04-0.1 μm retains solids, bacteria and even some viruses, permeate is suitable for reuse while providing nutrients for agriculture (fertigation). This in turn reduces CO2 emissions by recovering organic matter and avoiding the need of mineral fertilizer.
Blackwater treatment at room temperature removes above 90% of the organic matter, while biogas with >75% of methane concentration is obtained and biosolids production is cut by half compared to conventional WWTP. The higher organic concentration of blackwater allows for direct AD treatment, and incorporating the organic kitchen waste increases biogas production and nutrient recovery, while avoiding biowaste collection and transport.
This concept will also be applied by Aqualia in the new Life Zero Waste Water project, in collaboration with the University of Valencia, to exploit their joint patent (EP3225596). AnMBR has been demonstrated at different WWTPs, achieving low power requirements (0.15 kWh per m3 of treated water in Life Memory) whilst producing a nutrient-rich, pathogen-free permeate.
AnMBR has also been used to retrofit of an old septic tank of a small village (20 m3/d WW), as OPEX was 20% less compared to conventional WWTP. A larger AnMBR to treat domestic WW from an industrial park (400 m3/d), to generate bioenergy and reuse water in the factories and green spaces is under design as a sewer mining concept in H2020 Rewaise.
The Run4Life project receives funding from the EU Horizon 2020 Research and Innovation programme, GA no 730285. This article reflects only Aqualia's view. The European Commission is not responsible for any use that may be made of the information it contains.