Evaluating the Effects of Operations to Increase Nitrogen Discharge from Sewage Treatment Plants on Concentrations of Organic Matter and Nutrients in Surface Water at Harima-nada in the Seto Inland Sea

The effects of operations to increase nitrogen discharge from sewage treatment plants to prevent oligotrophication at Harima-nada in the Seto Inland Sea on concentrations of organic matter, nitrogen, and phosphorus in surface water were evaluated. First, the sea waters around the Futami treatment center, which has been conducting the operation, were analyzed. The results show that no significant changes in water quality were observed during the operation. Next, the effects of the operations of 20 sewage treatment plants at Harima-nada on the water quality were simulated using a numerical simulation model. As a result, it was estimated that dissolved inorganic nitrogen and phosphorus were taken up by phytoplankton, and organic matter and nitrogen concentrations in the entire area of Harima-nada were increased by phytoplankton advection and diffusion. The observed values could not confirm the effect of the operation to increase nitrogen discharge at the Futami treatment center owing to dilution by seawater and the drainages of other nitrogen emission sources. However, it was shown that when the number of operations to increase nitrogen discharge from sewage treatment plants increased, the water quality in a wide area of Harima-nada may be affected.