Removal of Residual Phosphorus and Nitrates from Sewage Treatment Plants by a Fine Sand Filter

Studies on eutrophication affirm that phosphorus is the limiting factor on which action must be taken to avoid the production of algal biomass and the appearance of green tides in waterways. A concentration of 1 mg/L of phosphorus in a body of water is sufficient to trigger eutrophication. However, according to the wastewater discharge standards in force in Côte d'Ivoire, the secondary water of a WWTP must have a phosphorus concentration of less than 2 mg/L. With the exception of lagooning, conventional biological purification processes, such as activated sludge, eliminate in the best case 50 % of the initial phosphorus contained in the raw wastewater. As a result, refining processes, called tertiary treatment, are implemented at these stations. Since the WWTP subject to our study does not have a refining system, it often turns out that the water leaving this station does not meet the standards. In addition, in some cases, when work is carried out on this station, it is shut down. In this case, the water is collected and discharged directly. Since eutrophication has harmful impacts on the receiving environment, it is important to conduct studies to minimize it. Thus, we were asked to set up a refining process for the elimination of phosphates. To refine this water, we opted for direct filtration on a sand bed. Having four types of sand, we carried out filtration tests, according to an experimental plan, in the laboratory by playing on the parameters: the type of treatment, the height of the sand, the volume of water poured and the grain size of the sand and measured the phosphorus and nitrate contained in the filtrates in order to determine optimal refining conditions. Thus, this study has shown that it is possible to refine the secondary waters of this WWTP by carrying out direct filtration on a bed of sand with a grain size of 0.5 mm < ∅ < 1 mm treated with acid, which can then serve as a basis for other future additional studies to determine the sizing parameters of this sand filter. We have also been able to see that the parameters that are: the type, grain size and height of the sand, and the volume of effluent passing over the filter are parameters that influence the efficiency of said filter on the retention of phosphorus and nitrate in discharge water.