Characterization and Treatment of Emission Gas Condensate Generated from Biodrying of Sewage Sludge

Biodrying has been attracting much attention due to its capability of drying sewage sludge without using external energy. Although various aspects of biodrying have been studied, emission gas released from biodrying has rarely been characterized or treated. In this study, water vapors in emission gas were condensed by chilling and gaseous substances were further trapped in successive acid/alkali solutions. Mass balance analyses showed that water, C, and N removed from sludge by biodrying were completely recovered in the emission gas condensate (EGC), which contained 1,485.2mg/L of chemical oxygen demand (COD), 16,500mg/L of total nitrogen (TN, mostly ammonia nitrogen, NH3-N), and 662.7mg/L of total organic carbon (TOC). 22 volatile organic compounds (VOCs) were detected in EGC. Acid-trap solution contained a high concentration of NH3-N (16,472.0mg/L) and a moderate concentration of TOC (281.3mg/L). Alkali-trap solution contained 22,676.7mg/L of total inorganic carbon (TIC), but concentrations of other pollutants were low. Chlorination was demonstrated to be the best treatment method for EGC due to high removal ratios of TN, NH3-N, COD, and TOC at an optimized Cl:N mass ratio of 12.8:1. Since TIC was predominantly dissolved CO2, only the mixture of EGC and acid-trap solution was chlorinated at this ratio, and residual concentrations of NH3-N, TN, and TOC were 1.3, 163.3, and 62.0mg/L with removal ratios of almost 100%, 99.0%, and 85.8%, respectively. These results indicated that chlorination was effective for removing pollutants contained in EGC of biodrying.