Water quality comparison of secondary effluent and reclaimed water to ambient river water of southern Okinawa Island via biological evaluation.

The objective of this work was to evaluate the biological effect of the secondary effluent (SE) of a wastewater treatment plant and reclaimed water treated via ultrafiltration (UF) followed by either reverse osmosis (RO) membrane filtration or nanofiltration (NF) to be used for environmental use by comparing the results of algal growth inhibition tests of concentrated samples of the SE and permeates of RO and NF with those of six rivers in southern Okinawa Island. Although the SE water had no adverse effects on the growth of the algae Pseudokirchneriella subcapitata, it could lead to water quality degradation of rivers in terms of its toxic unit value, whereas the use of RO and NF permeates would not lead to such degradation. The recharge of rivers, into which domestic wastewater and livestock effluents might be discharged in southern Okinawa Island, with reclaimed water subjected to advanced treatment could dilute the concentrations of chemicals that cause biological effects and improve the water quality of the rivers, based on the results of the bioassay using P. subcapitata. Comparing the results of bioassays of reclaimed water with those of the ambient water at a site might be effective in assessing the water quality of reclaimed water for environmental use at the site.