Abstract
A study was conducted to assess the performance of an integrated system consisting of an anaerobic digester (AD), an aerobic sequence batch reactor (ASBR), and three horizontal subsurface flow constructed wetlands (HSSFCWs) vegetated with the perennial grass Phragmites karka for removal of nitrogen in tannery wastewater. Nine grab wastewater samples were collected weekly between September and December 2016 from the influent and effluent at each unit of operation of the plant and analyzed for physicochemical parameters. The AD removed 67% of oxidized nitrogen (NOX–N); the removal was improved to 76% in the ASBR with a further aeration period in the HSSFCWs, and the NOX–N removal efficiency improved progressively to 96%. The removal efficiency for nitrate (NO3–N) was 82% and for nitrite (NO2–N) 88%; for ammonium (NH4–N) removal efficiency was 77% higher in the HSSFCWs compared to the AD and the ASBR. Despite the considerable percentage of NO3–N removed, in the effluent was above the country’s permissible limit (39.3 mg/l). The high concentration might be due to a weaker denitrification process in HSSFCWs caused by lack of carbon and external organic sources in the wastewater sufficient to carry out the process. The results suggest that the addition of supplementing carbon sources (methanol, sugars, or volatile fatty acids) to the effluent would achieve better performance. Based on these findings, the application of combined anaerobic/aerobic system connected with constructed wetlands process allows the achievement of higher efficiency in removing nitrogen compared to the use of an anaerobic, aerobic, or constructed wetland system along.
Highlights
In the face of the emerging concern for environmental preservation and governmental regulations that are becoming more stringent, the development of innovative, sustainable, environmentally friendly, and low-cost processes for efficient treatment of tannery wastewater has gained the interest of researchers in clean technologies
This study found considerable amounts of solids and high levels of B OD5, COD, and nitrogenous compounds in tannery wastewater, and the effluent showed a high B OD5/ COD ratio explained by the presence of organic matter in the wastewater
Higher removal efficiency for TSS, turbidity, BOD5, COD, NO3–N, NO2–N, NH4–N, S2−, and PO43−were observed in CW than in AD and aerobic sequence batch reactor (ASBR), whereas aerobic SBR showed higher removal efficiency for total dissolved solids (TDS) and total nitrogen (TN) and anaerobic digesters achieved the highest removal of sulfate (SO24)
Summary
In the face of the emerging concern for environmental preservation and governmental regulations that are becoming more stringent, the development of innovative, sustainable, environmentally friendly, and low-cost processes for efficient treatment of tannery wastewater has gained the interest of researchers in clean technologies. Tannery effluent contains large amounts of ammonia nitrogen (NH3–N) and total nitrogen (TN) due to the addition of ammonium salts during deliming and batting and the removal of non-collagenous proteins from raw hide in beam house processes (UNIDO 2000, 2011). The discharge of high nitrogen concentrations into receiving waters without proper treatment can lead to an imbalance in the natural ecological system that boosts eutrophication; depletion of dissolved oxygen in surface waters, killing fish and creating septic conditions; and odor problems.
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