Disappearance of Faecal Coliforms after Marine Discharge of Urban Wastewater by a Sea Outfall off the Island of Mauritius
Mauritius is widely known for its sandy beaches and as a tourist destination. The tourist industry has played an important role in the economic development of the island and is currently the main engine of economic growth. This sector is very dependent upon marine recreational facilities and as such the bacteriological water quality of the coastal lagoons is of crucial importance for the sustainable development of the tourist industry. Disposal of wastewater through sea outfalls will always remain a feasible option in an island-state like Mauritius. T90, the time necessary for the concentration of the microbial indicator selected to be reduced by 90%, is the most important parameter when designing an outfall. In the literature, the design T90 value ranges from 2 to 6 hours. There is presently no code of practice for the T90 value in Mauritius and it is usual for consultants to take values derived in temperate climates. However, in order to get the best results from an outfall, it is essential to carry out specific studies on T90 in the local context. The aim of this study was to assess the bacteriological quality of a popular public beach – Pointe aux Sables – which is under the direct influence of a sea outfall discharging a combination of trade and domestic effluents. T90 values were also determined along the plume, in situ and in the laboratory. Despite the presence of an outfall in the vicinity of the bathing zone of Pointe aux Sables, the level of faecal coliforms was low when compared to the guidelines for coastal water quality. The decay constant, K (base e), along the plume was in the range of 0.56 to 1.01 per hour, which gave a T90 value ranging from 2.27 to 4.05 hours. During the sampling exercise, the distance at which the concentration of faecal coliforms was reduced by a factor of 10 was within 3 km, with an average of 1.6 km when considering the tidal phases. An average T90 value of 23.65 hours was obtained in a confined environment in situ, while the laboratory T90 value was about 60 hours. It is reasonable to use a value of 4 hours as a code of practice for sea outfalls in Mauritius. It should be mentioned however that, to achieve an environmentally safe design for outfalls, models with variable T90 must be used to take into account the diurnal variation. If designs are done with T90 values determined at night, they will be on the safer side. Further studies are needed to determine the variation of T90 on an hourly basis.
Keywords: outfall, bacteriological quality, faecal coliforms, T90, coastal environment, Mauritius