Treatment of biomass-gasification wastewaters by wet-air oxidation

Abstract

Production of synthetic natural gas from gasification of biomass results in the generation of a high-strength wastewater that is difficult to treat by conventional means. This study investigated the use of wet air oxidation (WAO) as a treatment method for these wastewaters. A literature review was conducted to identify the suitability of WAO for the treatment of high-strength industrial wastewaters and to determine typical operating conditions for such treatment. Data presented in the literature showed that WAO should be suitable for treatment. Data presented in the literature showed that WAO should be suitable for treatment of biomass gasification wastewaters (BGW), and a laboratory treatability study was designed. BGW, having an initial chemical oxygen demand (COD) of 30,800 mg/1 and initial color of 183,000 APHA units, was treated in a laboratory autoclave for 20, 40, 60, 120, and 180 min at temperatures and pressures of 150/sup 0/C, 5.1 MPa (750 psi); 200/sup 0/C, 6.9 MPa (1000 psi); 250/sup 0/C, 10.3 MPa (1500 psi); and 300/sup 0/C, 13.8 MPa (2000 psi). Maximum COD removals of 0% for the 150/sup 0/C, 5.2 MPa (750 psi) runs; 40% for the 200/sup 0/C, 6.9 MPa (1000 psi) runs, 55% for the 250/sup 0/C, 10.3 MPamore » (1500 psi) runs; and 85% for the 300/sup 0/C, 13.8 MPa (2000 psi) runs were measured. Maximum color removals for these respective runs were 56%, 82%, 97%, and 99%. Initial removal rates of COD and color were observed to increase with reaction temperature. The experimental results suggest that oxidation of BGW organics by WAO occurs in a stepwise fashion with large organic molecules first being hydrolyzed and then partially oxidized to low molecular weight intermediates. Complete oxidation of these intermediates is more difficult and most easily accomplished at high reaction temperatures. The best application of WAO to treatment of BGW appears to be as a pretreatment to biological treatment and it is recommended that this application be investigated.« less