Conversion of biological treatment plant sludge to organic fertilizer for applications in organic farming

Abstract

Conversion of the sludge generated in the biological treatment plants of glove dipping industries of Sri Lanka to a valuable organic fertilizer after removing toxic metal ions such as heavy metals and excess Zn and Al present in the sludge to allowable limits is described. In order to do so, the raw materials used were analysed for these species and for their nutritional values. Removal of metal ions by different acids such as HNO3 and acetic acid digestion processes are revealed and the results are compared. Dilution of the metal ion-removed sludge with other raw materials used in organic fertilizer production to enable maintain right C:N ratio and the use of these materials in the fertilizer production process used are presented. Application of the fertilizer to soils of fruit and vegetable plantations and measurement of Zn, Al and heavy metals in the soil and plant parts and their crops as a function of time is also described. The quality of vegetables and fruits produced by applying this new fertilizer is compared with those obtained in the open market. It has been shown that the former contains no or much less than maximum allowable limits of heavy metals or toxic species when compared to those grown using other organic fertilizers. This study is useful for industrial biological treatment plant sludge management by con-verting it to a useful product.