Performance of a Horizontal Subsurface Flow Constructed Wetland for the Removal of Nutrients, Organic Matter and Selected Pesticides from Wastewater of a Flower Farm, in Bishoftu, Ethiopia
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Date
2020-02-02
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Addis Ababa University
Abstract
In Ethiopia, surface water pollution from streaming of wastewater of agro-industries is a major environmental and health concern that requires efficient treatment systems before they are discharged into the environment. One of the low cost, eco-friendly treatment systems is construction of different configuration of artificial wetlands to treat the waste water. The purpose of this study was to optimize treatment performance of horizontal subsurface flow constructed wetland (HSSFCW) to treat waste water laden with residual agro-chemicals including nutrients and pesticides from a flowers farm, near Bishoftu Town, Ethiopia. For this purpose, five pilot-scale (HSSFCW) units were constructed to evaluate the effect of wetland plants, hydraulic retention time, seasonal temperature, supplementing of additional substrate (bio-stimulation) and addition of effective microorganisms (EM) (bio-augmentation) and optimize the performance of the system. The CW units were single planted with Typha latifola (Tp), Cyperus papyrus (Cp) and Pennisetum purpureum (Pp) and mixed plant with equal proportions of the three plants (Mp), operated under selected days of retention time, with addition of different volumes of molasses and EM. Water, plant tissues and sediment samples were periodically taken for analysis of physico-chemical parameters and microbial profiles following standard methods. The result showed that mean removals for BOD5, COD, NH4+ and NO3- were remarkably higher (P < 0.05) in planted CW units than unplanted control. Among the single planted units, Typha latifola (Tp) was the most efficient. CW unit with mixed plantation (Mp) showed the best performance with mean removal of 75.9%, 75.7%, 91.6%, 72.9%, and 60.3% for BOD5, COD, NH4+, NO3- and TP respectively. Extending the system operational retention time from 1 to 4 days significantly reduced (P < 0.05) the levels of BOD5, COD, NH4+, NO3- and TP in out flow samples by > 42%, >33%, >48%, >36% and >29% respectively that were below the discharging limits set by iv
Environmental Protection Authority (EPA) except for TP. Based on RISA profiles, higher diversity indexes of bacterial communities’ were detected from Tp and Mp CW units which was positively linked with their higher treatment performance. Removal efficiency of the CWs for BOD5, COD, NH4+, NO3- and TP showed direct dependency with intermittent water temperature where the average removal values obtained from temperature ≥ 200C were significant higher (t > 0.177, p < 0.05) compared to below it for planted CW units. Moreover, data of rDNA Illumina sequencing of the sediments bacterial community revealed a higher relative sequence abundance at all taxonomic levels (phylum, class, genus, OTUs etc’) in a higher performing planted CW units compared to the unplanted control. Specifically, measures of species richness (OTUs) and Shannon diversity index (H) as well as some of the dominant bacterial taxa from the CW units showed a direct correlation (r > 0.50) with removal of BOD5, COD, NH4+, NO3- and TP. Alternatively, Typha planted CW units bio-augmented with EM at 1:1800 dilutions enhanced removal of BOD5, COD, NH4+, NO3- and TP than its corresponding control by 7.8%, 5.7%, 8.5%, 16.1% and 15.6% respectively while both operating at 3 day of retention times. A serial increase of EM doses from 1:1800 to 1:600 also accompanied with an increase of sediment bacterial counts and remarkable reduction (p < 0.05) of the levels of BOD5, COD, NO3- and TP in out flow samples. This corresponded to 4.4, 19.4, 0.1 and 2.5 mg/L of BOD5, COD, NH4 + and NO3- respectively in treated effluent samples which were far below the stringent environmental tolerable standards. Moreover, the additions of sugar cane molasses (MS) and EM to the system improved the removal of 10 μg/L inflow endosulfan used as a trial pesticide by > 15 % and > 19.0% respectively with reference to the average removal efficiency of 77% obtained in untreated. Nevertheless, it was only the EM treated CW units reduced this level of endosulfan close to the permitted discharge limit of 0.1 μg/L. The high removal rate of endosulfan (≥ 77%) v
without any accumulation in plants tissue and low sediment sorption may suggest biodegradation as removal mechanism for endosulfan in the CW units. In general, this study showed HSSFCWs planted with Typha latifola or mixed plantation with equal proportion of Typha latifola, Cyperus papyrus and Pennisetum purpureum effective for reducing the levels of BOD5, COD, NH4+, NO3-, TP and endosulfan emanating from floriculture waste water below the EPA discharging limit, particularly when integrated with EM bio-augmentation. Thus, the system can be recommended for management of wastewater from point and non-point sources of floriculture industries before they are discharged into the environment.
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Keywords
Bio-Augmentation, Bio-Stimulation, Cyperus Papyrus, Typha Latifola, Removal Efficiency, Retention Time