Design of a small scale iron and manganese removal system for Copperbelt University’s borehole water

Abstract

F acilities like deep wells where oxygen content is low, the iron and manganese-bearing water is colourless. This is because the iron and manganese are dissolved. When the water is exposed to air, the iron and manganese are oxidized and change from colourless, dissolved forms to coloured, solid forms. Oxidation of dissolved iron particles in water changes the iron to white, then to yellow and finally to red-brown solid particles that settle out of the water. Iron that does not form particles large enough to settle remains suspended [colloidal iron] and leaves the water with a red tint (1). Manganese is usually dissolved in water, although some shallow wells contain colloidal manganese [black tint]. These sediments are responsible for the staining properties of water with high concentrations of iron and manganese (2). These precipitates or sediments may be severe enough to plug water pipes (1). Additionally, iron and manganese can affect the flavour and colour of food and water. According to Skimpton D (1) manganese is objectionable in water even when present in smaller concentrations than iron. There is therefore need to treat water for iron and manganese. Iron and manganese is naturally present in many aquifers throughout the world. While iron can start causing aesthetically undesirable taste and odour problems at concentrations above 0.3 ppm, concentrations of up to 3.0 ppm are often acceptable to local people; higher levels could cause people to revert to traditional unprotected sources (2) which are most times contaminated. Furthermore, according to (3,4), a big problem that frequently results from iron or manganese in water is iron or manganese bacteria. These bacteria occur in soil, shallow aquifers and some surface waters. The bacteria feed on iron and manganese in water and form red-brown [iron] or black-brown [manganese] slime in toilet tanks and can clog water systems. Technically the main problems associated with iron in water can be summed up as follows (3-6) (i) It can cause an unpleasant taste in the water; (ii) when iron precipitates out of solution it can clog up valves, small bores, pipes and other water accessories; (iii) the "brown water" is ineffective for washing; (iii) the iron can give rise to "iron bacteria [organisms that prey on iron compounds, for example frenothrix, gallionella and leptothrix]". There are a number of Iron and Manganese removal methods. However, these methods are dependent on the form and concentration of the iron and manganese in the water. WHO (7) mentions that, the approach to dealing with naturally occurring chemicals will vary according to the nature and source of the chemical. Key water treatment methods for iron and manganese are briefly explained below: (i) Water Softener Iron and Manganese Removal: this method is suited for removing low concentrations of iron and manganese. It relies on the process of cat-ion exchange to remove minerals that cause hard water such as calcium, magnesium and other constituents such as iron and manganese (8); (ii) Manganese Greensand [Iron removal filter]: this is a purple-black filter medium coated with manganese oxide and is capable of reducing iron, manganese and hydrogen sulphide from water through oxidation and filtration. Its actual removal capacities vary and depend on the characteristics of each compound (8); (iii) Removal of iron and manganese by oxidation and microfiltration [MF]: this is especially suitable when the combination of these metals are high and variable. The results from experimentation on MF show that the oxide particles, with sizes ranging from 1.5 to 50 micro-meters, can be efficiently micro filtered (9); (iv) Aeration and Filtration Iron and Manganese removal: This method is used for removing iron in cases of concentration levels higher than 0.3 ppm but not more than 32 ppm. Aeration methods can be of two types which include a single and double tank variety. At the Copperbelt University in Zambia, high levels of iron and manganese are suspected to be present in the borehole water. This has led to the decommissioning of two existing boreholes because of the observed damage and rusting caused to the tanks, pumps, pipe systems and related elements. This is also noticed from the slightly brown tint, when the water is collected. This has caused the institution to limit the domestic usage of water from the boreholes, and the water is now used mostly for construction purposes. Additionally, a study by WaterAid (10) shows that iron and manganese concentrations of above 1 mg/l and 0.5 mg/L respectively have been recorded on the Copperbelt province. According to WaterAid (10), increased iron and manganese concentrations are also likely in areas affected by mine drainage such as the Copperbelt province in which region the university campus lies. To improve the quality of underground drinking water with high contents of iron and manganese the use of iron and manganese filtration systems must be considered. As highlighted above, a number of iron and manganese removal systems exist, therefore implementing the most suitable and affordable design would solve the problem by lowering iron and manganese concentrations to acceptable levels. Selecting the most appropriate design will depend on the economical availability of materials and ease of construction all this in relation to its efficiency. This study was majorly aimed at designing and performance testing of a suitable small scale Iron and Manganese removal system for Copperbelt University's Borehole water. Materials for the filtration system were Siwila S, Chota C, Yambani K, et al. Design of a small scale iron and manganese removal system for Copperbelt University's borehole water. J Environ Geol 2017;1(1): 24-30. The study aimed at designing and performance testing of a suitable small scale Iron and Manganese removal system for Copperbelt University's Borehole water. Materials for the filtration system were locally sourced within the Copperbelt province of Zambia. Tests were carried out on borehole water and system filtered water. The results show that the system performed relatively well on reducing Iron and Manganese from groundwater. The model constructed was a small scale version of an Up-flow filtration system. Evaluation showed performance efficiencies of 81.67% and 32% on iron and manganese removal respectively. The Up-flow design is better because water takes relatively more time to pass through the filter increasing contaminant removal capacity. The aerator tray was good but retention time for air was not sufficient and could be made better. Based on the results from the lab scale model a full scale Prototype was proposed and designed. Selecting the most applicable design largely depends on economical availability of materials, ease of construction and operation all this in relation to system efficiency. The Up-flow design in this study can be improved with respect to (i) the sedimentation tank for the settlement of oxidized iron and manganese by improving retention time of water (ii) spray nozzle to increase the surface area for aeration of water. Additionally, a substitute aerator tray with cascades to increase retention time for aeration is proposed. Furthermore, based on the model and literature review of similar designs, the sand layer depth should be at least 20 cm.