Influence of operation time, hydraulic load and drying on phosphate retention capacity of mineral filters treating natural swimming pool water

Abstract

Three parallel mineral filters, each with a volume of 2550 cm3, were used for phosphate removal from natural swimming pool (NSP) water. Filters worked continuously in saturated conditions. Hydraulic loading rates (HLRs) were 15 m3/m2h, 15 m3/m2d and 5 m3/m2d for Filter 1 (F1), Filter 2 (F2) and Filter 3 (F3), respectively. In the middle of the length of the filter, an inspection hole was located for sampling the filtration material. Samples were collected on the 55th, 87th and 120th day of filter operation. The phosphate sorption capacity of materials sampled from the filters was significantly lower than the sorption capacity of the fresh material, with their values varying between filters (different HLRs) and sampling events. In the case of F3 (slow filter), a decreasing phosphate sorption capacity was observed along with the time of operation. However, filtration material sampled from filters F1 and F2 had a higher P sorption capacity at the end of the monitoring period (day 120th day) than at the time of second sampling (day 87th day). This may suggest that the biofilm which developed on the grains of the filter blocked the contact between reactive material (RM) and treated water. Breaking the biofilm due to a faster flow (F1 and F2) resulted in increased filter sorption capacity. The remaining material sampled from F1, F2 and F3 was dried and also tested for P sorption capacity. Results showed that air drying increased the P sorption capacity of material from below 50% up to 100% of initial value, which suggests that periodic drying of filters may be a simple option for the regeneration of filtration material. P-bounded forms in filters were also analysed and discussed.