Developing energy and nutrient mass balances to inform value recovery options in municipal wastewater treatment systems

Abstract

Municipal wastewater contains valuable substances such as energy, organic carbon and nutrients. Recovery of these valuable substances may be a promising approach towards the ccircular economy. However, their fate in the treatment processes is rarely studied holistically and may have substantial variations between sites. The overall aim of this project is to reliably determine the chemical energy and mass balances of wastewater treatment plants in the UK to inform possible options for recovery of organic carbon (for energy), nitrogen and phosphorus. This study firstly developed a novel method for rapidly measuring wastewater energy content and determined the relationship between COD and chemical energy as 15.8 kJ/g COD. A similar relationship is estimated for the sludge. Subsequently, chemical energy balance is built on the COD balance. This study discovers only 20-30% of influent chemical energy can be potentially recovered as CH4 for energy recovery. With this limited amount of energy recovered, wastewater treatment is unable to be energy self-sustaining even that the influent energy is more than 5-14 folds of the electricity consumption. Potential energy recovery opportunities are found in the primary settlement, secondary treatment, sludge return liquor, and digested sludge. In terms of nutrient, 17-41% of influent TN and 33-52% of influent TP are accessible for recovery in the sludge return liquor and biosolid. The potential value recovery option is to increase the capturing of nutrient into the sludge. The impact of the implementation of energy and nutrient recovery measures on the overall energy balance and mass balance are studied via modelling. The nutrient recovery measure increases the nutrient loading in the sludge return liquor and biosolid and barely impacts the energy balance. However, whilst reducing the electricity deficit, the energy recovery measures tend to reduce the nutrient loading in the sludge return liquor but increase the proportion distributed in biosolid and the final effluent. The economic feasibility and sensitivity study of the implementation of the biosolid pyrolysis is part of this research. Although it appears not to be economically viable at the current time, it substantially improves the energy balance of the treatment work. Economically viability could be achieved in the future if the influential capital cost can be reduced or better incentive can be granted.