Environmental study of the alternatives for the sewer system of a small coastal community in the bay of Biscay

Abstract

Zumaia is a small tourist community on the northern coast of Spain, situated along the estuary of the Urola river. This ecosystem has been affected by urban discharges coming from the villages surrounding the estuary. According to the 91/271 European Community Directive, regarding wastewater treatment, this situation should be addressed in order to achieve the quality objectives established for the restoration of the natural values of the area. The local Water Authorities of the Basque Country (Spain) have adopted an integrated model for the design of the sewer system of this community. The model was developed specifically for the study of coastal sewerage under dominant wet weather conditions and includes: 1) Establishment of the site specific statutory requirements and environmental objectives; 2) Design of sewer system components with specific reference to influences on marine water quality; 3) Development of monitoring programs. The construction of a sewer pipe alongside the river to collect the foul and the stormwater flow will prevent a large proportion of the discharges of contaminants into the receiving waters. However, the presence of overflows in the system during storms leads to the release of a great deal of pollutants into these waters. The main problems related to this type of discharge in the littoral zone arise from the bacteriological contamination of the bathing waters and from the oxygen depletion over the estuary. These two topics have been the predominant issues conditioning the consideration of different schemes of management of the sewer system. The aim of this paper is to analyze, from an environmental point of view, the structural and functional scheme considered for the sewage management of this community.