Towards an integrated performance model for subsurface flow constructed wetlands

Detailed investigations have been conducted on a set of four pilot scale su bsurface flow (SSF) constructed wetlands in order to characterise heat tran sfer, mass dispersion and biological performance mechanisms. These studies have followed the beds from post construction through unplanted hydraulic b ase line studies to the current status of mature stands of Phragmites austr alis. Experimental observations indicate that in unplanted beds, daily ther mal fluctuations are depth dependent and range from 1 to 9 degrees Celsius. These fluctuations result in daily thermal inversions, and enhanced mixing and oxygen transport. For planted beds, thermal fluctuations are depth ind ependent, and have a constant amplitude of 2 degrees Celsius. Planted beds may be thermally stratified. Lithium tracer studies corroborate these resul ts for the planted bed. In addition, performance studies indicate that orga nic pollutant removal is probably limited to organic suspended solids remov al, with subsequent biological breakdown. Current first-order plug flow mod els can not account for these operational issues. A combined model is neces sary to account for lateral dispersion, temperature gradients and settling of suspended solids to accurately reflect real biological removal mechanism s.