Sources of Pollution of the Estuary of the River Nervion, Spain – A Case Study

Abstract

This paper describes part of a comprehensive multidisciplinary study of the pollution of the estuary of the River Nervion, Bilbao, Spain. The objectives of the study were to determine the sources and loads of pollution from domestic sewage and industrial wastewater; provide information on the extent of pollution; formulate hydrodynamic and predictive models capable of forecasting the effects on the estuary of the anticipated growth of population and industry; determine the amount of polluting matter that would have to be removed in order to reach defined estuary water quality standards; arrive at suitable technical methods able to attain these standards at minimum cost; determine the treatability of wastewater discharges and recommend methods for controlling industrial wastewater. These objectives necessitated a survey of the pollution loads from domestic sources and industry which were discharged to the fresh water and tidal reaches of four major tributaries in addition to the main river Nervion; the results of this part of the survey are described in this paper. The presence of iron in discharges from iron ore mining and metal processing was found to aid flocculation of organic matter and the consequent deposition of sludge in the estuary. This action and the rate of oxygen uptake during the decomposition of such deposits are also described. The domestic sewage of the study area receives treatment in septic tanks before discharging to rivers or the estuary. In spite of free hydrogen sulphide and high concentrations being present in the septic tank effluents, odour nuisance does not occur, because, as is shown in the paper, of the formation of ferrous sulphide. The hydrogen sulphide concentration in the atmosphere at a pumping station was found to be above the safety level. The industrial load of impurity was estimated to be approximately 15% higher than the domestic load, but it is extremly difficult to find a rational basis on which comparisons may be made between domestic sewage and a mixture of industrial effluents containing substances inimical to biological activity. The industrial effluent was contained in a very large volume of water, much of it taken from the estuary for cooling or process water. It is calculated that if the industrial pollutants had been contained only in the fresh water used by industry the COD of the mixture would have been 1317 mg/l; the suspended solids 2143 mg/l; the ammonia nitrogen 191 mg/l; iron 1037 mg/l; other heavy metals 118 mg/l; fluoride as F 14.5 mg/l; cyanide as CN 8.1 mg/l; oil 196 mg/l; phenols 8.1 mg/l; mercury 0.13 mg/l; and the mixture would have had an excess of acidity of 5188 mg/l. An estimate of the load of substances having an oxygen demand indicated that of the total oxygen demand discharged to the system of rivers and estuary approximately 50% entered the estuary from the main tributaries, the remainder being discharged directly to the estuary. An outline is given of the methods that would be used to determine the treatability of industrial effluents which were rendered acceptable for discharge into public sewers.