Abstract
Phosphorus occurs in natural water and wastewater mainly as orthophosphates and polyphosphates. The major sources of phosphorus arising in municipal wastewater originate from both domestic and industrial waste flows. Approximately 50 to 70% of the phosphorus in domestic wastewater comes from human wastes and the remaining 30 to 50% comes from synthetic detergents containing phosphate components that are utilised for washing clothes. The fertiliser industry and commercial laundry systems comprise the bulk of industrial sources of phosphorus. The presence of excess phosphorus in the effluent discharged to natural waters has long been viewed as the cause of algae blooms and eutrophication. The average molar ratio of nitrogen, phosphorus and carbon in algae protoplasm is approximately 15:1:105, and therefore represents an optimum nutrient requirement ratio. The constituent that is present in the lowest concentration, taking into account this ratio requirement, will effectively limit algal growth. It can be deduced, therefore, that a minimal amount of phosphorus can still support substantial algae growth and its removal is more effective than nitrogen removal for preventing eutrophication in surface water. To this end, feasible methods for the removal and recovery of phosphorus from wastewater need to be studied. This study aimed to investigate the feasibility of using liquid-liquid extraction and enhanced coagulation methods to remove and recover phosphates from wastewater and water resources. The results revealed that to achieve the maximum phosphates removal by a liquid-liquid extraction method, the best extractant was a mixture of kerosene and benzyldimethylamine at a volume ratio of 2:1. The optimum volume ratio of the extractant and wastewater sample is 1:1, while the optimum extraction period was 6 hours with a shaking speed of 250 rpm. A phosphate extraction efficiency of greater than 80% was achieved across the three categories of water samples tested; a model water, lake water in the UNIS campus and real wastewater. A high stripping efficiency of greater than 90% was achieved from stripping the extractant used in treating each of the wastewater samples, using 6M sulphuric acid at a volume ratio of 1:1 with an agitation speed of 250 rpm. It was possible to re-use the resulting extractant from the stripping process nine times, when the overall phosphate removal efficiency was maintained by mixing the recycled to fresh extractant (kerosene and benzyldimethylamine at a volume ratio of 2:1) at volume ratios of 4:1 and 2:1 for the lake water and wastewater samples respectively. Aluminium sulphate, aluminium chloride and anhydrous iron chloride were used as chemical coagulants in the enhanced coagulation study. The doses applied for the two aluminium salts used were 4, 8, 12, 16 and 20 mg/L as while that of anhydrous iron chloride was 8, 16, 24, 32, and 40 mg/L as Fe3+. Turbidity removal efficiency of >80% was achieved when aluminium salts were used. The iron salt produced an efficiency of >80% for the sample pH 6 and 8. In addition, the removal efficiency increased with increase in the coagulant dose for all the coagulant salts used. The phosphates removal efficiency increased with increase in the coagulant dose and showed dependency on the pH of the wastewater samples. The major drawback of coagulation/precipitation is the excess sludge production in the process. This research revealed that a liquid-liquid extraction method is superior in respect of phosphate removal and recovery and has potential for use as an alternative method for industrial applications.