Optimisation of energy and nutrient recovery in wastewater treatment schemes (Executive Summary)
The research project CoDiGreen (2010-2012) targets the optimisation of energy and nutrient recovery in the wastewater treatment schemes of Braunschweig and Berlin. Therefore, pilot experiments are conducted to test the effect of addition of co-substrates (grass silage, topinambur) and the thermal hydrolysis of excess sludge on the biogas yield of anaerobic digestion. In addition, co-digestion of grass silage is also tested in a full-scale digestor of the wastewater treatment plant (WWTP) Braunschweig-Steinhof. Beside the experimental part, the environmental footprint of the wastewater treatment scheme in Braunschweig and the sludge treatment line in WWTP Berlin-Waßmannsdorf is analysed with Life Cycle Assessment (LCA) to identify potentials for optimisation and assess selected technical options in their effects on the environmental profile. Finally, a market review of the concept of agricultural reuse of effluent and sludge in Braunschweig is conducted to get an overview of the market situation, and a risk assessment is initiated to identify potential risks associated with this practice. The results of the pilot experiments show that both the addition of co-substrates and thermal hydrolysis can substantially increase the biogas yield and quality (CH4 content) during mesophilic digestion (HRT = 20d). Methane yields can be increased by 10%, 9% and 13% for thermal hydrolysis of excess sludge, addition of grass silage (+10% TS), and the combination of both (if the methane yield is only related to the VS of the sludge, the increase was 10%, 31% and 38%). A two-step digestion with intermediate hydrolysis (“DLD”) yields +19% CH4. No exceedance of legal requirements for inorganic and organic pollutants can be detected, whereas lab-analysis indicate positive impacts on sludge dewaterability and polymer demand for dewatering. For a full scale realisation of co-digestion it can be estimated that a 100.000 PE WWTP would require approximately 30 ha of extensively cultivated area to add +10% VS of grass substrate. However, the promising results of co-digestion with grass cannot be confirmed in full-scale trials, where only -8% of biogas yield can be measured (+2% if related to the VS of the sludge only). Even though the technical feasibility of grass addition can be shown, operational difficulties (fibre size, hydraulic mixing, low HRT) seem to prevent the realisation of the maximum potential of grass addition in full-scale. The environmental assessment of the systems in Berlin and Braunschweig reveals a high degree of energy production in both systems, lowering associated impacts of carbon footprint and other environmental impacts. However, potentials for optimisation are identified in terms of energy production and nutrient recovery, and recommendations for the future testing of technical options are given based on the scenario analysis within the LCA. Environmental benefits of the reuse approach in Braunschweig are quantified and relate mostly to the lower discharge of nutrients and other pollutants into surface waters. The normalised environmental profile underlines the primary functions of wastewater treatment (= protection of surface waters), which should not be compromised while optimising energy demand and carbon footprint.