A comparison of novel and conventional sewage treatment plants in terms of energy requirements, operational costs and organic micropollutants removal

Abstract

Conventional sewage treatment plants (STPs) are expected to be replaced by a new generation which is aimed to be more energetically efficient. These novel STPs consist on a first stage for maximum organic matter capture followed by a partial nitritation-anammox unit. The organic matter recovered as sludge is further transformed into biogas during anaerobic digestion, whose production is enhanced by applying a sludge pretreatment technology such as thermal hydrolysis. However, the different technologies included in novel configurations have not been sufficiently tested, and moreover, they have not been integrated to holistically determine the energy requirements, the operational costs and the fate of organic micropollutants (OMPs). Therefore, the goal of this Thesis is to evaluate the energy requirements, the operational costs and the OMPs removal in different novel STP configurations based on rotating belt filters, chemically enhanced primary treatment and high-rate activated sludge or combinations for organic matter recovery and to compare the results with those of a conventional STP configuration. This thesis demonstrates that the different novel STP configurations evaluated significantly decrease the energy requirements in comparison with conventional STPs due to two reasons, the lower aeration demand and the higher self-produced electricity obtained from biogas combustion. Thermal hydrolysis before anaerobic digestion also contributes to decrease the STP energy demand, although a higher degree of thickening is required than in conventional configurations. Yet, the energy self-sufficiency is not reached in any of the studied configurations. This thesis also highlights that the energetic analysis must be accompanied by an economical evaluation since lower energy requirements do not necessarily mean a reduction of the operational costs. For example, novel STPs based on chemically enhanced primary treatment for organic matter recovery lead to the lowest energy demand but to the highest operational costs, even higher than those of a conventional configuration. In contrast, the alternative based on high-rate activated sludge reaches very comparable energy demand and also considerably lower operational costs than the other evaluated configurations. In terms of OMPs, all the studied novel STP configurations reach similar removal efficiencies from wastewater for most them, indicating that the technology used for organic matter recovery does not affect OMPs elimination in the water line. However, it strongly affects the presence of hydrophobic OMPs in the sludge line, and subsequently, in digested sludge. Among the studied alternatives, the novel STP configurations based on the high-rate activated sludge reactor achieve the lowest presence of OMPs in digested sludge. In conclusion, the STP configurations based on high-rate activated sludge for organic matter recovery become the preferable option from a holistic point of view because, although it presents similar OMPs removal efficiency from wastewater and slightly a higher energy demand than the other novel configurations, it achieves the lowest operational costs and presence of OMPs in digested sludge.