Potability analysis of raw water from Bospoort dam, South Africa

The South African constitution states that everyone has a right to have access to an environment that is not harmful to their health or well-being. This includes a constant supply of clean and safe drinking water. In order to fulfil this constitutional right, it is imperative to carry out continuous monitoring of water at different stages of the water supply system so that real data can be obtained about the state of water quality in the environment. Management interventions can then be made to maintain or improve water quality.

Magalies Water is a state-owned entity providing a wide range of related water and sanitation services in South Africa, and operates in four provinces that include Gauteng, Limpopo, North West and Mpumalanga. It obtains a significant fraction of its raw water from the Bospoort Dam, a gravity and earth-fill type dam on the Hex River, which is a tributary of the Elands River, a part of the Crocodile River (Limpopo) basin. It is located near Rustenburg, North West, South Africa. The reservoir mainly provides water for irrigation purposes, municipal water supply and industrial uses.

Magalies Water recognises the critical role played by the monitoring of water quality and therefore carries out continuous measurement and collection of various water quality data. This data was handed over to the authors for analysis, which has culminated into the production of this paper.

The major water quality parameters that have been analysed include pH, conductivity and dissolved solids, turbidity and colour, total coliforms, Escherichia coli (E. coli), hardness, alkalinity, precipitation potential, chlorophyll and organic carbon.

The term pH (Jonnalagadda & Mgere 2001) describes the extent to which water is acidic or basic (alkaline). The pH scale ranges from 0 to 14. Water that is neutral has a pH of 7; water that is acidic has a pH less than 7 and water that is basic has a pH higher than 7. Very acidic or very basic water can be corrosive and can also interfere with water treatment process efficiency. pH is an important treated water quality parameter mainly due to the fact that (among other reasons), potable water should be non-corrosive.

Conductivity is a measure of the ease with which water transmits an electric current. Conductivity is due to, and is therefore an indicator of the presence of inorganic dissolved solids in the water such as chlorides, nitrates, sulphates and phosphate ions (ions that carry a negative charge), or sodium, magnesium, calcium, iron and aluminium ions (ions that carry a positive charge). Potable water should possess these substances in controlled amounts, as excess proportions can affect the taste and colour of water, and may be responsible for more adverse health effects such as reducing the oxygen carrying capacity of blood, or lowering blood sugar levels.

Turbidity is a measure of water clarity or the extent to which colloidal substances in water decrease the passage of light through the water (Juntunen et al. 2012). Colloidal substances are chemically reactive suspended substances that cannot be removed by filtration and may include clay, silt, algae, plankton, microbes, and other substances. Colour in water may be caused by the presence of minerals such as iron and manganese or by substances of vegetable origin such as algae and weeds. Colour tests indicate the efficiency of the water treatment system. Potable water should be colourless and non-turbid.

It is difficult, time-consuming, expensive and impractical to monitor drinking water for every possible microbial pathogen (disease-causing) bacteria, viruses, or protozoans that might occur with contamination (Reddy & Lee 2012; Onda et al. 2012; Bartram et al. 2014). A more viable approach is the detection of organisms normally present in the faeces of man and other warm-blooded animals, as indicators of faecal pollution, as well as of the efficiency of water treatment and disinfection. The most commonly tested faecal bacteria indicators are total coliforms, faecal coliforms, E. coli, faecal streptococci, and enterococci. All members of the total coliform group can occur in human faeces, but some can also be present in animal manure, soil, and in other places outside the human body. E. coli is a single species of faecal coliform bacteria that is specific to faecal material from humans and other warm-blooded animals. Its identification implies the presence of hazardous disease causing organisms.

Hardness is a measure of the amount of divalent salts, or positively charged ions, particularly calcium (Ca²⁺) and magnesium (Mg²⁺), in water. Total hardness is the sum of the concentrations of Ca²⁺ and Mg²⁺, expressed in ppm (mg/l) calcium carbonate. Calcium carbonate hardness is a general term that indicates the total amount of divalent salts present, although it does not specify which salts are causing water hardness. Hard water is generally not harmful to human health but wastes soap and forms scale precipitates upon heating (Kumar & Puri 2012; Sengupta 2013).

Alkalinity, on the other hand, is a measure of the capacity of water to neutralize acids. Alkaline compounds in the water such as bicarbonates, carbonates, hydroxides and phosphates lower the acidity of the water. Alkalinity, like hardness, is reported as milligrams per litre of calcium carbonate (mg/l CaCO₃), and owing to this fact, hardness and alkalinity are often confused. However, alkalinity measures negative ions (carbonates and bicarbonates) while hardness measures positive ions (calcium and magnesium), and sometimes these values can differ greatly. If limestone (calcium carbonate) is the cause of hardness and alkalinity, these values will be similar or identical. However, if sodium bicarbonate (NaHCO₃) is responsible for high alkalinity, it is possible for water to have high alkalinity and low hardness.

Measured in milligrams per litre of calcium carbonate, this parameter indicates the level of corrosivity of the water (Randtke 2011). A negative potential indicates that the water is corrosive i.e. can dissolve the metal of the pipeline. A positive potential indicates that calcium carbonate can be precipitated from the water and deposited into the pipeline. This action can have the advantage of protecting the pipeline through lining, but can, if in excess, reduce the hydraulic capacity of the pipeline, hence the establishment of a limit.

A pigment in plants that enables photosynthesis to take place. The concentration of Chlorophyll is a convenient indicator of the amount of algae in the water. Too much algae can cause undesirable tastes and odours, clog filters, consume chlorine and can lead to the production of undesirable disinfection by-products. All of this can increase the cost of treating water for domestic use.

This parameter measures the amount of organic matter in the water (Toming et al. 2016). Too much organic matter will attract undesirable microorganisms that will grow in pipelines.

Temperature can affect parameters such as pH, precipitation potential, as well as the concentration of various dissolved solids.

Metals such as potassium and sodium exist in low concentrations in raw water. Zinc, chromium, aluminium, iron, copper, cobalt and manganese, were found to exist in trace concentrations. These concentrations are desirable for human health. Health hazards attributed to these elements are minimal and moreover, the body can get rid of them easily. However, high concentrations of these metals are known to be deleterious to human health.

Other parameters, whose health hazards can only be realised when consumed in high concentrations (which concentrations rarely occur in drinking water) include: sulphates, nitrates, ammonium, fluorides and phosphates (Thi Minh Hanh et al. 2011; Yan et al. 2015; Liu & Chan 2016). Their concentrations were found to be sufficiently and consistently low for the period under consideration. Unfortunately, for most of these parameters, a thorough analysis could not be carried out, owing to little and/or missing data.

To facilitate the understanding of the results, each graph has been drawn with a linear trend line in order to demonstrate the general trend in the variation of data for the period under consideration. An alarm level (the maximum recommended concentration of a parameter) has also been included in the graphs (in a red colour), to show the deviation of the data values from the limit.

• (i) In general terms, the pH, colour, turbidity, dissolved solids, alkalinity, precipitation potential, chlorophyll and organic carbon were found to be within acceptable limits.

• (ii) The hardness of the water was found excessive.

• (iii) The conductivity of the water is high, surpassing the alarm level for approximately 65% of the period under consideration, even though the causative dissolved solids are appreciably below their alarm levels.

• (iv) The total coliform and E. coli counts were significantly lower than for Vaalkop dam, with a few isolated incidents of sharp increases in the counts.

• (i) A further investigation, requiring more data, is required to explain the contradiction between high conductivity and low dissolved solid concentration.

• (ii) It is important to investigate the events that led to the E. coli count increase at the beginning of 2009, 2011 and 2012.