The effect of seasonal variation on nutrient removal from municipal wastewater using a constructed wetland microcosm
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Date
2018
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University of Zululand
Abstract
South Africa is facing a severe water scarcity due to the exponential deterioration of natural water bodies such as dams and lakes. The deterioration of natural waterways is fuelled by anthropogenic activities that produce bulk amount of wastewater. The haphazard disposal of nutrient-rich wastewater from household, industries and institutions may lead to the occurrence of eutrophication which impair the integrity and quality of water in natural water bodies. This has resulted in the urgent need for the development and implementation of new innovative green technology for wastewater treatment.
Constructed wetlands have proven to be an ideal alternative technology for wastewater treatment. This is because they are environmentally friendly and economically sustainable treatment systems. In addition, these systems have a potential of reducing contaminants to acceptable levels that pose no threat to human and environmental health. Despite these advantages, their application is still challenging in some parts of the world. This is due to the limited information about the seasonal performance of these systems and poor understanding of the influence of environmental parameters in pollutant assimilation. This study delineates the effect of seasonal variation in microbial community structures in wetland microcosm. In addition, this study also investigated the seasonal effect of physiochemical parameters on nutrient assimilation in these systems.
The constructed wetland microcosms were setup at the Empangeni (University of Zululand), and was divided into planted (planted with Amaranthus hybridus and Bidens pilosa) and unplanted (reference) section. These systems were operated in warm and cold seasons for one month. The physiochemical parameters (dissolved oxygen, pH and temperature) were monitored. The removal efficiency of chemical oxygen demand, ammonia, nitrite, nitrate and
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phosphorus were measured pre- and post-treatment using spectrophotometric methods. The spectrophotometer was used with commercial kits (Merck) following manufacturers protocol.
Nutrient removal was seasonal and varying degree of nutrient removals were observed in planted and reference section of the wetland microcosms. The highest reduction efficiencies were obtained in warm than cold seasons. In warm season, the highest removals were 97%, 95%, 90%, 70% and 74% for ammonia, nitrite, nitrate, phosphorus and COD in planted section, while in reference section the removals were 69%, 69%, 82%, 57% and 59% for ammonia, nitrite, nitrate, phosphorus and COD respectively. In cold season, the removals were 60%, 73%, 65%, 68% and 64% for ammonia, nitrite, nitrate, phosphorus and COD in planted section, while in reference section the removals were 42%, 64%, 50%, 46% and 50% ammonia, nitrite, nitrate, phosphorus and COD respectively. The increase in physiochemical parameters was directly proportional to nutrient reduction in the microcosms. The correlation of physiochemical parameters with the nutrients removal ranged from very poor (temperature (0.11≤r≤0.95), moderate negative (COD (-0.44≤r≤0.94) (pH (-0.45≤r≤0.89) and to a very strong positive correlation (DO (-0.72≤r≤0.89). Based on the discharge limits of nutrients, the effluent for nitrite, nitrate and phosphorus were within the discharge limit while ammonia did not meet the discharge standards in both seasons as per South Africa’s Department of Water and Sanitation.
Microbial community structure and diversity occurred in the microcosms. However, their occurrence was seasonal with warm season showing high abundance than the cold season. Furthermore, the planted sections showed high microbial abundance and diversity than the reference sections. This indicated that macrophytes supported the growth, diversity and activity of microorganisms within these systems. This was supported by the high removal of nutrients in the planted sections than in the reference sections. Nitrosomonas and Nitrobacter were the most dominant nitrifiers in the microcosms in both seasons while Thauera, Pseudomonas and
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Acidovorax were the dominant denitrifiers in the both warm and cold seasons. Phosphate accumulating microorganisms were dominated by Accumulibacter in warm season and Bacillus in cold season.
Description
A Dissertation Submitted for the Requirement for the Degree of Masters of Science (Microbiology) in the Department of Biochemistry and Microbiology Faculty of Science and Agriculture at the University of Zululand, 2018
Keywords
Seasonal Variation, Wastewater