Estimating inorganic and organic nitrogen transformation rates in a model of a constructed wetland purification system for dilute farm effluents

This paper describes some of the concepts which are being built into a model of a subsurface flow constructed wetland system based on reedbeds. A nitrogen (N) cycling submodel has been adapted from a soil N model, with pools representing organic material with high biological oxygen demand (BOD), ammonium and nitrate. Microbiologically controlled transformations between pools are represented by first-order exponential kinetics, with N finally lost to the atmosphere either by ammonia volatilisation, or by denitrification to gaseous N 2 or nitrous oxide. Hydrology has been represented for both horizontal and vertical reedbeds, with contrasting partially anaerobic or fully aerobic conditions at each stage. The model has been set up to represent an experimental system with one horizontal and three vertical reedbeds, being tested for purification of dirty water from a dairy farm. Rate constant values have been selected so that simulated results are a reasonable approximation to measurements from the experimental system. The model has been developed to assist in optimising design parameters for new systems, including the numbers and sequence of reedbed types, dimensions and flowrates, for various incoming contaminant concentrations and target water purity standards.