AbstractDecreasing water quality is a major environmental concern. Phosphorus (P) discharges from wastewater treatment plants (WWTPs) contribute to eutrophication, posing a threat to aquatic life. Stricter discharge regulations are expected in the near future to limit phosphorus entering the aquatic environment, including consents on small WWTPs. Existing P removal technologies such as chemical dosing or biological treatment, which are commonly used in larger
treatment plant operations, face challenges for the implementation on small-scale. Hence, a need for a low-maintenance, low-cost and efficient technology for P removal arises.
Reactive media filters are a technology that could treat P on small WWTPs as a tertiary treatment system. The filter retains P via adsorption and precipitation processes. This research study aims to investigate the performance of several reactive media under various conditions to gain a better understanding of their performance and mechanisms in order to optimise the design of such filter systems. Many materials have been tested for their P uptake, but only few studies were concluded on real wastewater, real world conditions and at full-scale over a long time period to establish the longevity of the media filters. In addition, the exact removal mechanisms and their interaction remain unresolved, where this research intends to improve this understanding by creating a comprehensive data set.
Large variability in experimental design between different studies makes directly comparing different materials difficult. Therefore, this research project compared several media under the same conditions to evaluate their performance.
The efficiency of the media in terms of P removal were examined via long-term bench-scale column testing and a pilot-scale trial using real wastewater. The mechanisms involved in the P uptake by the materials were investigated using scanning electron microscope (SEM), x-ray fluorescence (XRF) and x-ray diffraction (XRD) analysis. The different media were compared in their chemical make-up, particle size and properties to gain knowledge of the working principle of P removal and pH increase. Several parameters including the influent wastewater characteristics such as P concentration and alkalinity, and operational factors such as the hydraulic retention time (HRT) and flow conditions were examined for their impact on the performance of the three studied media. Water quality parameters in terms of organic content, solids, pH and metal concentrations were also monitored.
Results showed that the tested media examined all decreased the P content efficiently to levels below 0.1 mg/L. A correlation between HRT, P removal and pH was established. In addition, the media showed a decrease in solid content and biochemical oxygen demand (BOD) as well as metal uptake (e.g. Al, Fe, Mn and Zn). High effluent pH and initial metal leaching of Cr and V were of concern.
Bonding of media particles by precipitates combined with accumulation of organic matter and solids during operation led to a decrease of surface area available for P retention and a reduction in HRT, further deteriorating P removal by the media.
A pH adjustment step was tested for treating the highly alkaline effluents and wood bark filters were a promising solution. However, an initial increase in BOD, chemical oxygen demand (COD), P and colour will need to be addressed for full-scale applications. Insights were given to the impact of acid release and microbial activity within the bark filters on the pH of the treated water. In addition, the effect of the pH correction step on the different media effluents was evaluated, showing the largest pH reduction for the Phosclean effluent due to its lowest buffer capacity.
Recommendation based on the results obtained from this research were made in terms of size and lifetime of the filter bed and factors to be considered for efficient P removal performance by the media. A whole life cost analysis for a case study was carried out using data obtained from the long-term pilot-scale trial.
The findings of this study were utilised to optimise the design of a full-scale filter application for the implementation on a small WWTP in the South East of England.
|Date of Award||Nov 2021|
|Supervisor||Fay Couceiro (Supervisor), John Williams (Supervisor) & Stephanie Barnett (Supervisor)|