The actual effect of plants in SSF wetlands has been debated. <ref name = "Dan">Truong Hoang Dan, Le Nhat Quang, Nguyen Huu Chiem, Hans Brix, Treatment of high-strength wastewater in tropical constructed wetlands planted with Sesbania sesban: Horizontal subsurface flow versus vertical downflow, Ecological Engineering, Volume 37, Issue 5, May 2011, Pages 711-720, ISSN 0925-8574, 10.1016/j.ecoleng.2010.07.030.</ref> Generally, wetland plants provide improvements, although small, in BOD and pathogen removal. However, they enhance nutrient removal, although mostly through indirect means. Unless nutrient loadings are very low, net removal by direct plant uptake is generally a small proportion of total removal. Plants primarily affect treatment performance by enhancing nutrient processes such as nitrification and denitrification by transferring oxygen to soils and supplying of organic matter. <ref name = "Tanner">Tanner, C. C. (2001). Plants as ecosystem engineers in subsurface-flow treatment wetlands. Wetland Systems for Water Pollution Control 2000, 44(11), 9-17.</ref>
The inlet releases and distributes the influent wastewater at the wetland entrance. Inlet structures for FWS or HF SSF wetlands include perforated or slotted PVC pipe or open trenches perpendicular to the direction of the flow, and the influent is released onto the distribution medium for further dispersion and velocity reduction, creating uniform flow throughout the width of the wetland cell. In VF SSF wetlands, a grid of pipes or trenches is laid over the bed, and influent is released down into the substrate. The medium will assist in the spreading of the water throughout the bed, but it is important for the inlet grid to be as uniformly distributed as possible. Pipe sizes, orifice diameters, and spacing are determined by the design flow rate. <ref name = "UNhabitat"/>
The liner, at the base of the system, keeps the wastewater in and the groundwater out of the system. If the soil is clayey and impermeable, a liner may not be needed. However, if the intrinsic permeability of the soil is greater than 10-6 m/s, the wetlands must be lined. There are a few options for lining the system. A 30-mil PVC liner is the most common and the most reliable choice. 10-20 mil liners can be found in the developing world. <ref name ="Milhelcic"/> Geosynthetic clay liners are not recommended because they may crack. <ref name = "Gustafson"/> Another option is to decrease the soil permeability by mixing Portland cement or bentonite with the soil and compacting on-site <ref name = "UNhabitat"/>.