POWERSTEP Deliverable D5.5: Recommendations for ecoefficient new concepts of energy positive WWTP

This study analyses reference and innovative POWERSTEP schemes for municipal WWTP in their environmental and economic impacts using life-cycle tools of Life Cycle Assessment and Life Cycle Costing. Based on hypothetical scenarios at defined boundary conditions for WWTP size, influent quality, and effluent discharge limits, multiple process schemes have been modelled in a mass and energy flow model with a benchmarking software for WWTPs. This process data forms the basis to calculate operational efforts, and it is amended by infrastructure data for material demand and related investment costs. In addition, specific data has been added based on results of the POWERSTEP project (e.g. for N2O emissions) or information from literature. The results show that innovative schemes with advanced primary treatment operate with a superior electricity balance compared to current state-of-the-art schemes for municipal wastewater treatment as a reference, increasing electrical self-sufficiency from 27-82% to 80-170%. The POWERSTEP schemes reach this goal without compromising effluent quality targets of the schemes, i.e. reaching the same effluent quality than before. Concentrated influent with high COD levels supports the POWERSTEP approach and enables highly energy efficient schemes. However, nitrogen removal has to be realized with mainstream anammox after enhanced carbon extraction from concentrated influent. This process is still under development, and its performance and stability should be further validated in full-scale references. Sidestream N removal, advanced control of COD extraction and partial bypass of primary treatment are other options to guarantee nitrogen removal after enhanced carbon extraction with conventional denitrification. In the life-cycle perspective, POWERSTEP schemes significantly decrease primary energy demand of WWTP operation by 29-134% compared to the reference. In favourable conditions, their superior electricity balance can fully compensate life-cycle energy demand for chemical production, sludge disposal and infrastructure, resulting in real energy-positive WWTP schemes. Greenhouse gas emissions can also be substantially reduced with POWERSTEP (- 6 to 43%) due to savings in grid electricity production. GHG benefits of POWERSTEP are smaller than energy benefits on a relative scale, because direct emissions such as N2O from biological N removal and mono-incineration also deliver a major contribution to overall GHG emission profiles, and they are not reduced with POWERSTEP. In contrast, POWERSTEP schemes with mainstream anammox will most likely increase N2O emissions, compensating a large part of the electricity-related benefits in GHG emissions. Total annual costs are in a comparable range for both reference and POWERSTEP schemes. While the latter decrease operational costs by 3-16% due to lower purchase of grid electricity, they require higher investment for primary treatment, increasing capital costs by 4-17%. Overall, effects of POWERSTEP on operational and capital costs off-set each other and result in a net increase of total annual costs of 2-7%, which is within the uncertainty range of this cost calculation. Higher electricity prices (> 0.12 €/kWh) will increase the positive impact of POWERSTEP on operating costs, resulting in fully costcompetitive eco-efficient WWTP schemes at power prices of 0.25 €/kWh. Final recommendations are derived on the way to develop eco-efficient WWTP schemes of the future.

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