Abstract Description

Mounting global pollution, particularly from nitrate (NO3⁻) and ammonia (NH4⁺) accumulation, poses a serious threat to aquatic and terrestrial ecosystems. Elevated NO3⁻ levels promote harmful algal blooms and eutrophication, while also increasing human health risks such as colorectal cancer through the formation of carcinogenic N-nitroso compounds when contaminated water is consumed. Human activities, including agriculture and management of wastewater treatment plants (WWTPs), contribute further to nitrogen pollution by releasing nitrous oxide (N2O), a potent greenhouse gas and ozone-degrading gas. Incomplete removal of NO3⁻ in wastewater often results in secondary nitrogenous pollution in effluents. Conventional WWTPs, especially those constrained by limited resources and outdated infrastructure, struggle to efficiently remove nitrogen compounds. Thus, biological denitrification using microbial consortia offers a cost-effective and sustainable approach by reducing NO3⁻ to inert nitrogen gas (N2), avoiding reliance on chemical treatments. Physicochemical analyses were performed to measure NO3⁻ concentrations in WWTP effluents. Indigenous denitrifying bacteria were enriched through batch culture experiments, followed by optimization using trace metals to enhance denitrification efficiency. Results: The current study investigates the potential of indigenous bacterial consortia enriched from four KwaZulu-Natal WWTPs to achieve complete in-situ denitrification. Physicochemical analysis confirmed that NO3⁻ levels in effluents exceeded the SANS drinking and discharge water guideline of 11 mg/L with a range of 22.8-121.7 mg/L of NO3- as N (NO3-N). Batch experiments were conducted using glucose as the sole carbon source at concentrations of 1000, 500, 250, and 0 mg/L where complete NO3⁻ removal was achieved within 24 hours with 500 and 250 mg/L of glucose attaining 87.7% and 92.74% NO3⁻ removal respectively. At 1000 mg/L, only 68.91% of NO3⁻ was removed, while the 0 mg/L treatment showed removal however accumulated 28.5mg/L of NO2⁻ by the end of the 24 hours. This highlights the importance of maintaining an optimal C:N ratio. In quest to attain complete denitrification within the WWTPS, optimization targeting production of terminal inert N2 gas was the ultimate task. Trace metals known to be metal co-factors to functional enzymes facilitating denitrification were explored: Copper (Cu2+), Molybdenum (Mo2+) and Iron (Fe2+) with biogenic gasses analyzed using GC-MS. It was proved that concentrations between 20 and 50 mg/L of Cu2+ and Mo2+ promoted complete denitrification with over 80% N2 as biogenic gas. Iron supplementation promoted NO3- and NO2- reduction rates which corroborated with reported recent literature but novel in its application within WWTPs. Indigenous microbial consortia show promise reducing NO3⁻ pollution and mitigating N2O emissions from KwaZulu-Natal WWTPs, supporting environmental and public health.