Investigating the effective removal of fat, oil and grease (FOG) in water treatment plants of the Mangaung Metropolitan Municipality

Abstract

The most common type of wastewater under low-strength waste streams is municipal wastewater. It is characterised by low organic strength and high particulate organic matter constituents. Common pollutants in municipal wastewater include viruses, bacteria, protozoa, helminths, suspended solids, total dissolved solids, a few chemicals, nitrates, phosphates and fats, oil and grease (FOG). The presence of high concentrations of nitrates and phosphates in wastewater effluent can cause eutrophication of receiving water bodies. On the other hand, FOG in the wastewater treatment plants leads to inefficiencies of the wastewater system, resulting in severe environmental degradation and contamination of water resources. As a result, optimal wastewater treatment technology is one of the most used criteria for the treatment of wastewater. The activated sludge and trickling filter systems are both known to remove FOG; however, the degree of efficiency of each method against the other in the removal of FOG is not known. The effectiveness and degree of the activated sludge compared to the biological trickling filter system in the removal of FOG showed no difference, since both treatment plants release FOG in their effluents. This study presented findings from investigating the effectiveness of the wastewater treatment technologies in two treatment plants in the Mangaung Metropolitan Municipality (MMM) in South Africa. The study was conducted to compare the efficiency of the trickling filter and activated sludge systems in the treatment of FOG, nitrogen and phosphorus and the possible correlation of nitrates and phosphates to FOG. The effects of other parameters such as pH, atmospheric pressure, temperature, dissolved oxygen, salinity, conductivity, resistivity and total dissolved solids were also investigated. Twenty grab samples of both the influent and effluent of the NEWTP and the BWTP were collected in total for the months; June, July, August, September and October. Samples were analysed at the laboratory of the MMM situated at the BWTW. The sampling and laboratory analysis of FOG followed the description of the Environmental Protection Agency (EPA 1664B). The hexane extraction and gravimetric method was used in the analysis. Nitrates were sampled and analysed using a SOP Chem 005 Spectroquant nitrate test, while phosphates were sampled and analysed using a Spectroquant Prove spectrophotometer in the laboratory. The Hanna HI98192 and Hanna HI98193 multimeters were used for field measurements of pH, dissolved oxygen, temperature, atmospheric pressure, conductivity, resistivity, salinity and total dissolved solids. The results showed that the trickling filter system from BWTP removed FOG by 61.36%, while the activated sludge system from NEWTP removed FOG by 52.81%, which showed that the trickling filter is more effective than the activated sludge in the removal of FOG. Furthermore, the removal of nitrates and phosphates was found to be within the South African effluent discharge standards of ≤ 2.5 mg/ℓ. 0.99 was found as the regression for BWTW and 1.00 for the NEWTW meaning there is a strong relationship between the presence of nitrates and FOG. Furthermore, In the NEWTW, it was noted that for the duration of the study, phosphate levels were lower in the effluent indicating that the plant is highly effective in eliminating phosphates. 0.00 was found to be the correlation coefficient of phosphates and FOG indicating no correlation. While, correlation was 0.999 for the NEWTW, showing a strong correlation. In conclusion, the volume of FOG found in the effluents of these plants was influenced by the volume of nitrates and FOG. In addition, the comparison of the trickling filter and activated sludge systems indicated that the trickling filter system is an efficient treatment method for the treatment of FOG, compared to the activated sludge system.