Chemical vs. Biological Treatment of Grey Water

Abstract

The major difficulty in treating grey water is the large variation in quality observed over short timescales. The use of cleaning agents has also been reported as having a major impact on treatment. Many treatment schemes proposed use mainly physical and biological processes and have problems adjusting to the shock loading of organic matter and chemicals.

A series of experiments were undertaken to investigate the use of chemical processes to treat grey water. The processes investigated were coagulation / flocculation to remove solids and TOC, and advanced oxidation using titanium dioxide and UV to remove TOC. Results were compared with results from biological processes such as biological activated filters (BAF) and membrane bioreactors.

Jar tests using ferric sulphate on both real and synthetic (weak and strong) grey water showed that at a dose of 30 mg/l ferric chloride, 90% of the solution’s turbidity and 80% of the TOC could consistently be removed. This compares well with the 85% COD removal reported for grey water treated by biological activated filters. The effects of pH and alkalinity on removal were also investigated.

The use of titanium dioxide and UV as a treatment for removing organic carbon from real grey water was also investigated. The results show that the combined TiO₂/UV process could reduce organic carbon concentrations from 160 mg/l to 30 mg/l within 3 minutes, an 82% reduction. The rate of the oxidation reaction has been shown to be dependent on the concentration of TOC and the concentration of TiO₂.

Whilst most operational grey water recycling schemes in the UK use physical or biological processes, the present study has shown that both traditional and novel chemical processes can treat grey water to the required level whilst dealing with changes in influent quality.