Phosphorus Recovery and Reuse Using Calcium-Silicate Hydrate Made from Rice Husk

Abstract

Calcium silicate hydrate was synthesized from rice husk and used to recover phosphorus from model wastewaters. The phosphorus removal capacity of calcium silicate hydrate was substantially lower in a high alkalinity model animal wastewater (5,000 mg/L as CaCO3 at pH 9), compared to deionized water, due to the formation of calcite, which reduced the concentration of free calcium available to react with phosphorus. By gradually decreasing the alkalinity, a proportionate increase in phosphorus uptake was observed, and model wastewaters with alkalinity levels of up to 307 mg/L as CaCO3 showed a phosphorus uptake (≥99%) similar to deionized water with no alkalinity. Phosphorus was extracted from spent calcium silicate hydrate by modified soil phosphorus tests. For calcium silicate hydrate exposed to phosphorus in deionized water, the best recovery was under acidic and alkaline conditions, and for calcium silicate hydrate exposed to phosphorus in high alkalinity model wastewaters, the best phosphorus release was under acid and neutral conditions. The results indicate that calcium silicate hydrate synthesized from rice husk shows promise for phosphorus recovery from wastewaters and subsequent reuse as fertilizer.