Empirical performance data provide a basis for the design of wetlands to specific performance limits. BOD and suspended solids are removed up to 95% from 10-250 mg l−1. Surface flow systems in Yorkshire achieved higher effluent standards but generally lower mass load removal than subsurface flow systems. All wetland systems fill up with solids at a predictable rate, and if they are not removed or degraded, they will overflow. Ammonia is not normally oxidised, except where relatively large land areas are used and/or BOD falls below 20 mg l−1. Phosphate is accumulated until saturation. Bacteria (faecal indicators) are removed, sometimes almost completely, but not reliably. The established theoretical knowledge of sewage treatment processes explains the performance of wetlands. Plants provide a renewable support medium for microbial growth, a matrix to retain solids and assist plug flow hydraulics, but do not appear to provide oxygen. BOD removal is mainly by the aerobic breakdown of organic matter. Bacteria growing on the soil and f-horizon (litter layer) use oxygen from the atmosphere and from nitrate in the sewage. Wetlands are oxygen limited and performance is enhanced if extra aeration is provided.
There were no obvious performance differences between surface and subsurface flow systems, unless extra aeration was involved, though strict comparative data were not found. Vertical flow wetlands appear to be inefficient percolating filters. The simplest designs, construction methods, maintenance and rejuvenation methods should be used. Near design performance of surface flow wetlands is achieved in about 14 days.