Phosphorus Removed from Wastewater as Fertilizer

Abstract

This research is a study concerns in using the complex compound called hydroxyapatite Ca₅(PO₄)₃OH which was obtained by precipitating phosphorus from simulated wastewater using Ca(OH)₂ as fertilizer. The concentration of Ca(OH)₂ was varied (180-380 ppm) to obtain the best for a fixed phosphorus concentration (5 ppm), stirring speed (400 rpm) and temperature (20 °C). The important thing for a fertilizer is its dissolution in the soil. Therefore; this study investigates the dissolution of the insoluble precipitate (hydroxyapatite) in oxalic acid which is an organic acid not harmful for plants. The dissolution was reversible because oxalic acid is a weak acid. The factors studied were the concentration of oxalic acid (100 -180 ppm), the stirring speed (100 – 500 rpm) and the temperature (20 – 60 °C). The best conditions were taken as that gave the maximum recovery of phosphorus, i.e. the maximum concentration of phosphorus in solution (1.8 ppm). The best conditions were: oxalic acid concentration (160 ppm), stirring speed (200 rpm) and temperature (30°C). Acidity was recorded during the experiments. It was shown that the acidity (as general trend) increased with increasing mixing speed and decreased with increasing temperature. The dissolved phosphorus increased with increasing mixing speed and decreased with increasing temperature. The study reached to enhance the dissolution of the precipitate by using a strong mineral acid like sulfuric acid in addition to oxalic acid with such little amount as two drops (0.5M) in (34ml) solution. The dissolution enhanced and became irreversible. The concentration of dissolved phosphorus was (2 ppm) at the best conditions. The best conditions were applied on a real precipitate obtained by phosphorus recovery from industrial wastewater.