Optimising Tertiary Treatment Within Integrated Algal Ponding Systems

Inadequate sanitation is one of the leading causes of water pollution and consequently illness in many underdeveloped countries, including South Africa and, specifically, the Eastern Cape Province, where cholera has become endemic. As modern wastewater treatment processes are often energy intensive and expensive, they are not suitable for use in these areas. There is thus a need to develop more sustainable wastewater treatment technologies for application in smaller communities. The integrated algal ponding system (IAPS) was identified as a possible solution to this wastewater management problem and was investigated for adaptation to local conditions, at the Rhodes University Environmental Experimental Field Station in Grahamstown, South Africa. The system was monitored over a period of nine years, with various configuration adjustments of the high rate algal pond (HRAP) unit operation investigated. Under standard operating conditions, the system was able to achieve levels of nutrient and organic removal comparable with conventional wastewater treatment works. The mean nitrate level achieved in the effluent was below the 15mg.l-1 South African discharge standard, however, nitrate removal in the IAPS was found to be inconsistent. Although the system was unable to sustain chemical oxygen demand (COD) removal to below the 75mg.l-1 South African discharge standard, a removal rate of 87% was recorded, with the residual COD remaining in the form of algal biomass. Previous studies in the Eastern Cape Province have shown that few small wastewater treatment works produce effluent that meets the microbial count specification. Therefore, in addition to the collation of IAPS data from the entire nine year monitoring period, this study also investigated the use of the HRAP as an independent unit operation for disinfection of effluent from small sewage plants. It was demonstrated that the independent high rate algal pond (IHRAP) as a free standing unit operation could consistently produce water with Escherichia coli counts of 0cfu.100ml-1. The observed effect was related to a number of possible conditions prevailing in the system, including elevated pH, sunlight and dissolved oxygen. It was also found that the IHRAP greatly enhanced the nutrient removal capabilities of the conventional IAPS, making it possible to reliably and consistently maintain phosphate and ammonium levels in the final effluent to below 5mg.l-1 and 2mg.l-1 respectively (South African discharge standards are 10mg.l-1 and 3mg.l-1 in each case). The quality of the final effluent produced by the optimisation of the IAPS would allow it to be used for irrigation, thereby providing an alternative water source in water stressed areas. The system also proved to be exceptionally robust and data collected during periods of intensive and low management regimes were broadly comparable. Results of the 9 year study have demonstrated reliable performance of the IAPS and its use an appropriate, sustainable wastewater treatment option for small communities