An evaluation of the eutrophication prevention potential of high rate Algae Ponds through the development of a deterministic design model

[摘要] ENGLISH ABSTRACT: Wastewater treatment is a major problem in South Africa. South Africa generally has thewastewater treatment infrastructure in place but often lacks the labour skills to operate theseplants efficiently. The increasing eutrophication in South African water bodies is an indicationthat this problem needs to be rectified. The characteristics of a High Rate Algal Pond (HRAP) make it an attractive option for effluentpolishing in South Africa. It has the potential of simultaneous nutrient removal and nutrientrecovery from the partially or poorly treated effluent of wastewater treatment works. Its simpleoperation would ensure that it is less susceptible to the poor operation practices in SouthAfrica. It is also relatively inexpensive to construct and operate, but the large footprint ofthese ponds makes its feasibility largely dependent on the availability and cost of land.A scale model HRAP was operated under laboratory conditions to investigate the nutrientremoval potential of these ponds. The nutrient removal measured during the laboratoryexperiments was believed to be modest, due to a lack of the high-intensity sunlight that thealgae require for photosynthesis. However, these were promising indications that the HRAP might be effective in the warm and sunny climate of South Africa despite the modest nutrient removals measured in the laboratory experiments. A deterministic design model for an HRAP was developed. The deterministic design wasprogrammed into Microsoft Excel with the use of Microsoft Visual Basics for Applications(VBA). The deterministic equations were solved numerically in the computational model. Theresults obtained from the laboratory experiments were used to calibrate the computationalHRAP model. The calibrated computational model accurately predicted the ammonia and nitrate/nitrite concentrations. It was unsatisfactory in predicting the soluble reactivephosphorus (SRP) concentration since it did not account for phosphate precipitation. Themodel only gave an estimation of the SRP assimilated by algae.The calibrated HRAP model was used to investigate the nutrient removal potential of anHRAP in South Africa. It was established that shallow ponds with long retention times, andconsequently large surface areas, are required to achieve satisfactory nutrient removal. Thearea requirement of an HRAP was estimated at approximately 60 square meters per cubicmeter of daily flow to achieve roughly 100% Total Inorganic Nitrogen removal. The estimatedarea requirement for roughly 100% ammonia removal was approximately 20 m2/m3/day. Itwas also established that HRAPs has the prospect of notable SRP removal.The theoretical calculations of the deterministic HRAP model indicated that an HRAP couldpotentially achieve sufficient nutrient removal for effective eutrophication prevention.However, the large surface area requirements might not make the HRAP practically feasiblefor effluent polishing in most cases.

[发布机构] Stellenbosch University