Treatment of high-strength wastewater in tropical constructed wetlands planted with Sesbania sesban: Horizontal subsurface flow versus vertical downflow

Abstract

a b s t r a c tTreatment of various types of wastewaters is an urgent problem in densely populated areas of many tropical countries. We studied the potential of using Sesbania sesban, an N 2 -fixing shrub, in constructed wetland systems for the treatment of high-strength wastewater. A replicated horizontal subsurface flow system and a saturated vertical downflow system was established with planted and unplanted beds to assess the effects of system design and presence of plants on treatment performance. The systems were loaded with a mixture of domestic and pig farm wastewater at three hydraulic loading rates of 80, 160 and 320 mm d −1 . The S. sesban plants grew very well in the constructed wetland systems and produced 17.2-20.2 kg dry matter m −2 year −1 with a high nitrogen content. Mass removal rates and removal rate constants increased with loading rate, but at 320 mm d −1 the effluent quality was unacceptable and hydraulic problems appeared. Mass removal rates and removal rate constants were much higher than reported in other studies probably because of the high-strength wastewater, the high loading rates and the tropical conditions. Planted systems removed pollutants much more efficiently than the unplanted controls. Direct plant uptake constituted only up to 8% of the total-N removal and 2% of the P removal at the lowest loading rate, and was quantitatively of low importance compared to other removal processes. The significant effects of plants were therefore related more to their indirect effects on the removal processes. This study for the first time documents that S. sesban can be used in constructed wetland systems for the treatment of polluted water while at the same time producing a valuable N rich biomass that can be used for animal fodder or soil amendment.

Effectiveness and Temporal Variation of a Full-Scale Horizontal Constructed Wetland in Reducing Nitrogen and Phosphorus from Domestic Wastewater

“…Pan et al [13] also observed that, when HLR is increased, the removal efficiencies of NH 4 + -N and TN decreased, which is similar to what was found by others authors [43,44]. Although lower HLR generally resulted in better removal efficiencies of N compounds, RE did not significantly (p > 0.05) correlate with HLR and the linear relationships between HLR as independent variable and removal percentage were rather weak for all nitrogen compounds and for all monitoring periods, with coefficient of determination (R 2 ) always less than 0.4356.…”

“…The negative average removal in NH 4 + -N in autumn-winter may also be related to high flushing due to the high flow rate of water through the wetland bed during the rainy months. Several authors reported that increase in N loads within certain limits tends to contribute to grater removal rates in HSSF-CWs [16,18,43,46]. The results obtained at the present study have shown that the concentrations of TN and NH 4 + -N in the effluent tend to significantly (p < 0.05) decrease with increasing respective incoming mass load rates, although the linear relationship between the two variables for the whole period (12 August-13 June) is moderate, as shown by the values found for the coefficient of determination (TN: R 2 = 0.3738; NH 4 + -N: R 2 = 0.3521).…”

Comparative evaluations of organic matters and nitrogen removal capacities of integrated vertical-flow constructed wetlands: Domestic and nitrified wastewater treatment

“…[3,5] However, anaerobic decomposition might account for a considerable proportion in the IVCWs, especially at high COD LR due to the low DO levels in the wetland beds. [18,19] COD mass removal rate of the IVCWs increased with the increasing LRs, which was consistent with the results obtained in other CWs, [20][21][22][23][24] and was probably due to the enhanced microbial production and metabolism under higher LR. [20] Nevertheless, a tendency of leveling off for mass removal rate at high LR was observed by Dan et al, [23] suggesting that the organics removal capacity of CW was not infinite.…”

“…In addition, the k values of COD removal in the IVCWs were in the high range of some references, [18,23,24] and higher than some others. [5,10,21] Very low effluent COD concentration (7.2 mg L…”

“…Moreover, the TN removal efficiencies decreased significantly at the high LR, showing a much more remarkable influence of LR on TN elimination than that on COD, which agreed with other references. [10,23,31] Taking the low and fluctuated nitrogen removal efficiencies at the high LR into consideration, a loading rate of around 1.0 and 1.5 g N m ¡2 d ¡1 were recommended for DW and NW to be dosed into the IVCW, with substantial reduction in winter, to achieve satisfactory nitrogen removal if no approach for improvement of N removal was applied. Further investigation on saturation kinetics of pollutant removal in IVCW is considerably necessary, which is significant and profitable for its better design and operation.…”