Molecular Size Distributions of Dissolved Organic Matter in Wastewater Transformed by Treatment in a Full‐Scale Trickling Filter

Abstract

The performance of biofilm reactors is primarily limited by the rate of diffusion of biodegradable molecules into the biofilm. Wastewater entering and leaving a trickling filter was therefore separated into three molecular weight size fractions of <1000 daltons (<1K), 1 to 10K, and >10K to <0.45 μm, and the concentration of 5‐day soluble biochemical oxygen demand (sBOD5) and dissolved organic carbon (DOC) were measured for each size fraction. On average, a large percentage of both the sBOD5 (82%) and DOC (78%) was contained in the smallest size fraction (<1K). Removal rates of sBOD5 were high. Influent sBOD5 values were reduced from 27.3 to 33.3 mg/L to 4 mg/L in the effluent. A comparison of a trickling filter model (TRIFIL2) indicated that removal rates of sBOD5 were much greater at this site than was typical of other trickling filters treating domestic wastewater. The large concentration of this small molecular weight fraction was believed to account for greater than normal removal rates. To demonstrate this, the molecular size distributions assumed in the trickling filter model TRIFIL2 were adjusted to fit the measured size distributions. The measured average removal of 92% compared well to the 93% removal predicted by the TRIFIL2 model with these measured molecular size distributions. These measurements and model simulations demonstrate that the performance of trickling filters can be linked directly to physical characteristics of the wastewater such as the molecular size distribution of biodegradable organic matter.