5.1 Summary of Findings
The HRAP was investigated as a possible option for effluent polishing in South Africa through a laboratory model and the development of a deterministic design model. The thesis statement that was defined in section 1.3, was largely supported by the results obtained from the research. However, as explained below, effluent polishing with an HRAP may not always be practically attainable.
The nutrient removal that was measured from the HRAP during the laboratory experiments was modest. The algal growth was limited in these experiments due to a lack of high-intensity sunlight. This was an early indication of the importance of sufficient sunlight for an HRAP to be effective. The laboratory experiments also confirmed that algae have a strong preference for ammonia over nitrate/nitrite.
The calibrated computational model accurately predicted the ammonia and nitrate/nitrite concentrations. It was unsatisfactory in predicting the soluble reactive phosphorus (SRP) concentration, since it did not account for the phosphorus that was removed through precipitation. The model only gave an estimate of the SRP assimilated by algae. The COD and VSS estimations were also very inaccurate, presumably due to the model’s deficiency in accounting for the increase in soluble and particulate organics, caused by the algal respiration, excretion and mortality processes.
The calibrated HRAP model indicated that an HRAP could be very effective in Total Inorganic Nitrogen and ammonia removal in South Africa. The HRAP also has the prospect of effective SRP removal when SRP precipitation is considered in conjunction with assimilation. It was established that shallow ponds with long retention times are required to achieve these levels of nutrient removal. These conditions led to a large surface area requirement by the HRAP. It was estimated that an area of 60 m2 per cubic meter of daily flow is required for complete Total Inorganic Nitrogen removal. A smaller area requirement of approximately 20 m2/m3/day was estimated for complete ammonia removal.
The HRAP can theoretically serve as an effective method for eutrophication prevention. However, the large surface areas required to achieve satisfactory nutrient removals, may diminish the feasibility of these ponds. These area requirements are too large for the HRAP to be feasible in and around cities. It can potentially be an effective eutrophication prevention solution for small towns where land is inexpensive and widely available.
5.2 Contributions
The computational HRAP model developed in this thesis has its shortcomings and further development is required before it can be implemented in the HRAP design and operation
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phases. However, the potential improvement that a fully developed deterministic HRAP model can bring to the design and operation phases is evident. It can potentially enable an accurate prediction of the nutrient and organics removal, as well as the potential biomass production. A representative deterministic model can also greatly improve the design and operation of other types of waste stabilisation ponds.
This research revealed that an HRAP is theoretically very effective in nutrient removal, granted that the suspended solids can be removed effectively. However, due to the sunlight requirements of algae and the light absorption in deep ponds, very large surface areas are required for an HRAP to be effective.
5.3 Limitations and Future Research
This research had various limitations due to restrictions on the scope, time and funding of the project. This section explores these limitations and gives recommendations for the rectification of these limitations in future research projects.
The first major limitation of the research was that the data from only one laboratory experiment was used for the calibration of the computational HRAP model. It was therefore not possible to determine the accuracy of the model’s response to changing environmental conditions. It is recommended that batch experiments with different environmental conditions should be done to determine whether the calibrated model still applicable to varying environmental conditions.
All the laboratory experiments were conducted at the same pond depth. Supplementary research is required to determine if the model accurately responds to variations in the pond depth.
The computational model was limited by the kinetics of only two deterministic models. The HRAP model needs to be developed further to incorporate processes such as phosphate precipitation, and algal respiration, excretion, and mortality. The computational model can possibly be expanded to include these processes by incorporating some of the kinetics in existing surface water quality models such as the CE-QUAL-W2 model by Cole and Wells (2013).
The computational model did not include the carbon dioxide concentration as a possible limitation in the algal growth. Although the dissolved carbon dioxide concentrations were not limiting during the batch experiment, Craggs (2005b) suggested that carbon dioxide might become limiting at high pH. More research is required on the dynamics of carbon dioxide in an HRAP and its limitation of algal growth.
The research did not include an investigation into the removal of the suspended solids from the HRAP effluent. The implementation of an HRAP would generally require the removal of
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the solids before the HRAP effluent could be discharged. Algal settling ponds have successfully been used to remove the suspended solids from HRAP effluent (Rose, et al., 2002; Oswald, 1978). However, more research is required to determine the most effective and economical method of solids removal.
Other less significant research limitations include the indoors nature of the experiments and the synthetic wastewater used in the experiments. It is possible that the sunlight and actual wastewater effluent may have a different effect on the HRAP kinetics than the artificial lights and the synthetic wastewater. More research is required to determine the interchangeability between sunlight and the artificial lights, and between synthetic wastewater and the actual effluent from an underperforming WWTWs.
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