The development of an approach for the production and use of algae to treat urban wastewater

Abstract

The presence of huge quantities of nutrients in water bodies heavily promotes the growth of algae. Excess nitrogen and phosphorus in discharged wastewaters can lead to downstream eutrophication thereby damaging the ecosystem. Algae bioremediation involves the use of live algae in the removal of excess dissolved nutrients from wastewater and subsequently diminish the pollution load. It is an alternative economical and sustainable way of treating sewage wastewater compared to conventional treatment processes. With dwindling water resources, it is imperative to find ways to minimise pollution of many streams. The main objectives of this investigation were to determine whether there is a difference in the optimum conditions in pools and raceways that are required to cultivate Chlorella vulgaris and Chlorella protothecoides, and for using microalgae to reduce nutrient loads in urban wastewater treatment works. Several physicochemical variables that includes pH, dissolved oxygen (DO), temperature, conductivity, total dissolved solids (TDS), chlorophyll a and ammonium (NH4+) were studied. These variables were measured to ascertain how they affect the production of these two selected Chlorella spp. Chlorella vulgaris was cultured in round pool 1 and C. protothecoides in round pool 2. These two Chlorella spp. were then mixed together and cultured in the raceway pond. Chlorella vulgaris showed that they grow faster than C. protothecoides. This was indicated by the high concentration levels of chlorophyll a averaging 179.03 ?g/? in round pool 1. Round pool 2 showed different results as compared to round pool 1 with low levels of chlorophyll a averaging 41.32 ?g/? and high levels of ammonium averaging 104.91 mg/?. The algae cultures were introduced to stabilization ponds at Motetema wastewater treatment works with a depth of 2 m and a capacity of 20 M? but without mechanical aeration, and the effluent quality was monitored. The concentration of nitrates decreased from 0.88 mg/? to 0.81 mg/? and chlorophyll a showed a 25.94 % decrease from 45.38 mg/? to 33.61 mg/? after the introduction of microalgae. The study showed that these two Chlorella spp. responded differently to the prevailing ranges of physicochemical variables during culturing, and they have the potential to be utilised for wastewater treatment on a large scale.