Environmental and energy efficiency of different sewage treatment processes

This paper summarises the Scandinavian experience that chemical wastewater treatment gives “value for money” in the sense that such treatment gives a low construction volume per removed unit of Oxygen Consumption Potential (OCP) in the receiving water and a low energy consumption per unit of Oxygen Consumption Potential (OCP) removed. The OCP evaluation takes into consideration both the primary oxygen consumption caused by organic matter and ammonium as well as the secondary oxygen consumption from degradation of algae resulting from the discharge of nutrient. The paper demonstrates that in comparison to conventional biological activated sludge treatment (with pre-settling), chemical treatment plants (primary precipitation) are cheaper in terms of cost per unit of OCP removed and that they require less energy per unit of OCP removed. Chemical treatment is considered to be the most suitable method when discharging municipal sewage to sensitive marine receiving waters and should be categorized as secondary treatment, like biological treatment. It is important when ecological aspects are considered, for instance in terms of energy consumption, that all elements that contribute to the total consumption of energy must be included. For instance, a compact treatment plant, like a chemical one, may require less energy for concrete, while it requires more energy for chemicals. The problem is evaluated in two different ways: (i) an evaluation of energy consumption based on the major energy-associated elements: chemicals, air and biogas; and (ii) an analysis of all ecological aspects based on a model for evaluating the ecological value of the products and processes.