Effect Of The Designed And Operational Parameters On The Performance Of The Subsurface Constructed Wetlands

The domestic Wastewater from the small town and village is becoming the main source of pollution to water column after the completion of the Three Gorges Project. Constructed wetland is gaining importance as an effective and low-cost alternative Wastewater treatment technology for domestic Wastewater in small villages. The constructed wetland treatment system has advantages over the conventional Wastewater treatment unit as it can be constructed in the same place as where the Wastewater is produced and maintained by relatively untrained personnel. Due to its relatively low-cost for construction, the constructed wetland is usually utilized as secondary and even as primary treatment unit in rural area.Experiments were carried out in a pilot-scale subsurface flow constructed wetlands (SSCW) system comprising four pairs of lined beds of almost equal surface area (15m² each bed), with the following aspect ratios: 1:1, 1.5:1,2:1 and 2.5:1. The size of the granular medium of each pair varied from coarse granitic gravel (D₆₀=20 mm) to small granitic gravel (D₆₀=12 mm). The beds of the pair with longest aspect ratio were made shallower (0.5 m) than the rest (0.7 m). The SSCW system was sampled weekly from September 2005 to September 2006.The Wastewater samples during one year operation of SSCW are analysised for COD, BOD₅, TN, AN, TP and TSS. Also, the ORP and the electron acceptors such as SO₄^2-, NO₃^-, NO₂^- and DO are analysed for gaining insight into the mechanism of pollutants removal possiblely occurred in the constructed wetlands system. The main conclusions from one year investigation are summarized as follows:1,The operational parameters including flow (Q), hydraulic retention time (HRT), hydraulic loading rate (HLR) and season are four major factors controlling the performance of the SSCW. Better performance is found for pollutants removal under low flow (14 t d^-1), low HLR (0.1 m d^-1), long HRT (2.4 d) and high temperature (summer).2,No significant differences are observed between two water depths, the similar result is observed between the granular medium size. This may be due to the no enough difference of the water depth and relatively narrower granular medium size ranges for substrate selection. The difference of the water depth does not cause strong difference of the redox status in the SSCW, leading to unobvious changes of biochemical reactions responsible for the degradation of organic matter.3,The strong anaerobic condition is observed in SSCW system (ORP<-300m V). The SO₄^2- become electrons acceptors instead of dissolve oxygen in SSCW for organic matter removal. A high SO₄^2- removal efficiency is observed in this study (58%～79%). The result for the estimated organic matter removed by sulphate reduction is only about 30% of the total organic carbon (TOC), NO₃^-and NO₂^- are detected occasionally in some vertical and spatial samples, although NO₃^- and NO₂^- are hardly detected in influent and effluent samples, suggestion that nitrification-denitrification reaction possibly occurs in the SSCW which is possibly attributed to the oxygen transportation by the plants roots. Some part of the rhizosphere inside of SSCW is aerobic condition which promotes nitrification-denitrification. So it is highly possible that the organic matter removal is a combined process that merged the sulphate reduction, nitrification- denitrification, and other biological process such as methanogenesis.