Horizontal Subsurface Flow Gravel Bed Constructed Wetland with Phragmites Karka in Central India

Abstract

Treatment performance of a field-scale horizontal subsurface (SF) constructed wetland (CW) was evaluated for removal efficiency of BOD, TSS, NH4-N, NO3-N, TKN and P from municipal wastewater emanating from a small community of residential areas in Ujjain, Central India. The SF wetland had a rectangular size and covered an effective surface area of 41.82 m2 with a water retention capacity of 18 m3. The SF medium was composed of a gravel bed supported below on a layer of puddled local clay and overlaid by a thin synthetic liner. CW was planted initially with locally grown grass, Phragmites karka. Plants placed in this rectangular design at the rate of 3 to 4 plants per m2 increased to 6157 plants within ten months producing a biomass of 121 tonnes ha−1. The influent was pretreated before entering the SF system, through two baffles; a grass covered small ditch, followed by a narrow tunnel of packed biofilmed boulders. Removal rates of TSS (48%), TKN (36%), NH4-N (22%) and NO3-N as zero percent were realised. An earthen channel provided initial pretreatment by a land treatment system. Average treatment performance after five months from this SF system recorded removal efficiencies of 78% for NH4-N, TSS; 58-65% for P, BOD and TKN. Effluent dissolved oxygen levels increased to 34% indicating existence of aerobic conditions in the rooted-gravel bed. The SF system overall results established: (a) very cost-effective treatment technology, (b) SF removal efficiency above 50% for BOD, NH4-N, TKN, and P. This SF system presents a unique design consideration compared with the land-intensive Kickuth standard system design.