Treatment of agricultural wastewater in downflow reed beds : Experimental trials and mathematical model

This paper presents an examination of the performance of a full-scale downflow reed-bed system for the treatment of high-strength agricultural wastewater with BOD5 in the range 400-1500 mg/l. The effects of organic loading, hydraulic loading and the frequency of intermittent feeding on the treatment results were identified. The removal of organic matter in terms of BOD5 and COD averaged 74.3 and 53.0%, respectively. These removal rates appeared to increase with organic loading and feeding frequency but decrease with hydraulic loading. Percentage removals of suspended solids, NH4-N and PO4-P were 39.6, 23.1 and 34.7%, respectively. Organic pollutants, expressed as BOD5, were postulated to be retained in the downflow reed beds when the influent flow flushed through the beds; they were then aerobically decomposed by micro-organisms during the intervals between intermittent feeding when oxygen can diffuse into the beds more readily. A mathematical model was developed to describe this treatment mechanism based on an assumption that the pollutants are removed from the wastewater by the consecutive processes of adsorption and aerobic decomposition. The values of the constants for the model were derived from the experimental data. Comparison between the experimental and calculated results demonstrates that the mathematical model can be used for design purposes.