Re-engineering of existing wastewater stabilisation ponds for improved contaminant removal

Abstract

Wastewater treatment plants (WWTPs) are critical in resource recovery, recharging surface waters, and recycling essential nutrients. WWTPs also significantly reduce pollutant loads, which are sometimes marred by operational inefficiencies leading to inadequately treated wastewater effluent. Therefore, this research attempts to solve some of the above-mentioned problems by means of the re-engineering of wastewater stabilisation ponds (WSPs) for improved contaminant removal. Firstly, the wastewater influent and effluent samples from the Bainsvlei, Botshabelo, North-East, and Sterkwater WWTPs located in the Mangaung Metropolitan Municipality, Free State province, South Africa were obtained and analysed for a 3-year period. Thereafter, in-situ sampling at effluent discharge points, i.e., Bloem Spruit, Klein Modder River, and Renoster Spruit, was conducted to establish the associated impact on the environment and public health. Finally, the re-engineered WSPs were modelled and simulated for four distinctive design configurations using the Hydromantis extended simulation software tool (GPS-X 8.5). The monthly wastewater influent and effluent samples at all the WWTPs failed to comply with the South African National Standards (SANS) pertained to the Chemical Oxygen Demand (COD ≤ 75 mg/ℓ). For the period under consideration, the average COD concentrations at the four WWTPs varied between 107.6 mg/ℓ and 141.8 mg/ℓ. Further investigations revealed that the ammonia (NH3-N) levels for all the WTTPs are also higher than the specified national standard which resulted in lower levels of Dissolved Oxygen (DO) at the three in-situ sampling/ effluent discharge points. A DO deficiency typically impacts negatively on the survival of aquatic species, while aggravated (lower) levels will eventually lead to eutrophic waters and increasing treatment costs. Subsequently, the four re-engineered WSP design configurations were evaluated in terms of the Volatile Suspended Solids (VSS), Total Suspended Solids (TSS), and organics and nutrient (ammonia, nitrogen, and phosphate) removal efficiencies. The results demonstrated that Scenario 3 produced higher TSS values compared to the other three design scenarios to subsequently result in the highest VSS/TSS ratio ≥ 0.8, which reflects an efficient removal rate of organics and nutrients. Hence, WSPs can be successfully re-engineered by changing design parameters (e.g., flow rate, number of baffles, the arrangement of ponds, and the type of ponds) to meet the SANS criteria for wastewater effluent.