A comparison of municipal wastewater treatment plants for big centres of population in Galicia (Spain)

Background, Aims and Scope. It is clear that a wastewater treatment plant brings about an enhanced quality of wastewater; however, it also implies such environmental side effects as material and energy consumption as well as involving the generation of waste. This study is maintained within the boundaries of a research project that aims at the evaluation, from an environmental perspective, of the most common technical options focused on the removal of the organic matter present in urban wastewater. In particular, the paper presents the results for four centres of population with more than 50,000 inhabitants. The differences present among the facilities on their configurations will allow their comparison and the definition of the less environmentally damaging scheme for the treatment of this type of wastewater. Methods. As done before, LCA was the tool used for the evaluation of the environmental performance of the systems under study. In particular, the Centre of Environmental Science (CML) of Leiden University methodology was considered. The collection and transportation of wastewater across the pipeline was considered to be unaffected by the operation of the WTP and therefore, these stages were excluded. Within the boundaries of the treatment plants, the analysis was limited to the operation stage and no considerations were given to the building phase. Average annual data from several years were provided to obtain the inventory data, avoiding extraordinary conditions such as flooding or prolonged stop of units. CML factors (updated in 2002) were chosen for the impact assessment stage. SimaPro 5.1 software was used to make the calculations easier and to provide the data for the background systems. Results and Discussion. The comparison performed showed that the different configurations entail variations on the impact categories under study. Although higher electricity consumption was reported for those facilities with secondary treatment, its implementation is recommended as better results are obtained for eutrophication, undoubtedly an important criteria when wastewater treatment systems are analysed. In particular, the discharge of ammonium and phosphorous was identified as the main contributor within this impact category. The digestion of the sludge entails several benefits as the sludge is partially stabilised, its volume significantly reduced and the impacts associated to its application to soil are minor. Concerning the several dewatering systems compared, the use of different chemicals to facilitate the water removal turned to have an influence on the environmental performance so attention to this should be paid in addition to the energy consumption associated. Conclusions and Recommendations. Four WWTPs of capacity ranging from 75,000 to 125,000 inhabitants were evaluated in order to get more knowledge of their environmental performance. Data on material and energy consumption as well as characterisation of the water and the sludge entering and leaving the facilities were collected from the facilities in order to build the inventory required to perform the environmental assessment. The comparison performed among the four facilities made possible the definition of a less environmentally damaging WWTP, where secondary treatment at the water line as well as filter band and anaerobic digestion at the sludge line should be included. As mentioned, this study is part of a research project in which twenty treatment plants (divided in groups according to their capacity of treatment) are being evaluating. At the present time only results from the study of those from the highest populated areas were considered. On-going research is focused on the analysis of the plants representative for less populated areas.