Contaminant loads of CSOs at the wastewater treatment plant of a city in NW Spain

The city of Santiago de Compostela in north-west Spain (population about 100,000) has a combined sewer system that feeds to a grossly under-sized wastewater treatment plant. This problem is compounded by the very high rainfall (1600 mm per annum) and the age and poor repair of the sewer system (so that estimated groundwater input is about 13.4 Hm³ per annum). Combined sewer overflow (CSO) events are thus frequent, and have severe pollutant effects on the receiving watercourse, the River Sar. CSOs were monitored daily over a 40-month period, and determined the hydrographs and pollutographs for seven representative moderate-magnitude CSO events occurring during this period. Event total contaminant loads, often discharged to the river over a very short period, were in many cases very high: up to 9500 kg of suspended solids, 900 kg of total Kjeldahl N, and 3.62 kg of Zn. In most events, most contaminants showed a “first flush”, though variability was high and in many cases the pollutograph did not meet the stricter of the various criteria that have been proposed for definition of the first flush. That these stricter criteria were not met may be attributable to the dilution effect of the large volumes of groundwater that are continually entering the system.