Assessment of the possibility of recycling backwashing water from the swimming pool water treatment system

The paper presents the physicochemical analysis and toxicological assessment of backwashing samples taken after the process of washing filter beds in a raw condition after the process of their aeration and dechlorination. The backwash water under investigation originated from circulation systems existing in two indoor swimming pool facilities. The backwash water, as used at the preliminary and the main stages, was characterized by different physicochemical properties. For toxicological assessment, the Mictorox bioluminescence inhibition test, the Chaoborus sp. insect larva survival test and the phyto test using Lemna minor fine cilium were involved. The investigation presented in the paper included a preliminary phase focusing on the ecotoxic characterization of backwash water subjected to aeration and dechlorination processes. In turn, at the main stage, the effect of aeration duration on the quality of backwash water in terms of its physicochemical parameters was analyzed. The results of the preliminary stage investigation indicate that backwash water, both in a raw condition and after 30 minutes’ aeration, could not be discharged directly to the environment due to the threat to living organisms caused by its high toxicity. Whereas, using 160 minutes’ aeration duration contributed to a significant improvement in the quality of the backwash water and elimination of its toxic properties with respect to the indicator organisms used. The chemical dechlorination process brought about varying effects. In the case of the Microtox test, a stimulation of bacterial bioluminescence was noted, but, at the same time, the death of individual insect larva specimens was observed. In spite of the high biomass increase in the Lemna minor test, a gradual discolouration of fronds under the influence of backwash water was observed. Because of the presence of numerous compounds being disinfection by-products, as well as coagulant residues in backwash water deriving from swimming pool systems, it is necessary to seek further solutions that will allow them to be recycled, which will result in a reduction of water consumption and effluent discharges.