Wet air oxidation of waste water sludge

Working with Wetox Ltd, a lab scale plant for the continuous oxidation of wastes from municipal water treatment and industrial processes has been developed at IRL using wet air oxidation (WAO). The WAO process can be used to significantly reduce the chemical oxygen demand (COD) of waste sludge and to recover components such as aluminium and phosphate. WAO requires high temperature (240 - 300 degreesC) and high enough pressure (typically 5 - 7 MPa) to maintain a liquid aqueous phase and excess oxygen content, therefore a stirred autoclave vessel was employed. The plant was designed to allow the continuous throughput of a range of waste materials with air as oxidant; and continuous recovery of reaction products (vapour, liquid aqueous and solids) in separate streams. Significant reduction in the COD of sludge from a waste water treatment plant was achieved, based on measurement of the collected product streams. Furthermore, a corresponding increase in CO2 was detected in the product gas vapour stream. Metal salts and phosphates were detected only in the blow-down (solid phase) as desired. This plant has provided process data that will assist with configuration of a 200 L pilot plant to be commissioned by Wetox Ltd, together with Palmerston North City Council and the Ministry of the Environment. Based upon recovery of the chemicals and the destruction of the organic sludge, the process has been shown to be commercially viable.