Physicochemical properties of water treatment residuals generated from different water purification plants and the growth of vegetables using these residuals as a plant growing medium

In order to clarify whether the water treatment residual (WTR), i.e. an industrial waste, can be used as a plant growing medium or not, physicochemical properties of WTRs generated from two different water purification plants in Kitakyushu City in Japan were measured, and the growth of vegetables was experimented by using the WTRs to show the possible use of it as a plant growing medium. Komatsuna (Brassica rapa var. perviridis') was used as the vegetable. Bark compost (a soil conditioner) and/or phosphate (P) fertilizer were mixed to WTRs for improving its physicochemical properties. These mixture mediums were called cultivation soils. As a result, pH, and P absorption coefficient were not different between the WTRs and the cultivation soils. The pH, electric conductivity and effective cation exchange capacity of the cultivation soils were favorable, but the P absorption coefficient of the soils was too high for plant growth. Water-soluble and exchangeable manganese (Mn) concentrations of WTRs and cultivation soils differed largely between the two water purification plants. These Mn concentrations of the cultivation soils were high enough to cause Mn toxicity in plants. In the bark compost mixture soils, the water-soluble and exchangeable Mn concentrations were lower than those in the WTRs; the same characteristic of the Mn concentrations was also observed in the P fertilizer mixture soils. Concerning the vegetable growth experiment, almost no vegetables grew by using only the WTRs, and a positive effect of bark compost and P fertilizer mixtures on the vegetable growth was recognized. Mixtures of bark compost and/or P fertilizer to WTRs are, therefore, required for use WTRs as the plant growing medium.