Detailed source water monitoring showed large variations in the total concentrations of aluminium and iron in the Vaal Dam, South Africa, which were highlighted as a specific concern for one of the largest drinking water treatment plants in South Africa. This study aimed to better understand the presence of these metals in the source water, removal of these metals through the conventional treatment process, and final water quality trends, for the period 2008 to 2014. Aluminium and iron concentrations were highly variable and showed significant influence on colour and turbidity in source water. Sedimentation performed well, and removed over 70% of the metal concentration from the raw water. Filtration removed 15% of the remaining aluminium and iron concentrations. The pH and turbidity of the final water had minor effects on the metal concentration in the final water. The conventional treatment process was shown to be capable of removing aluminium and iron from the source water as both were within water quality limits in the final water. This study highlighted the importance of source water quality monitoring and treatment plant efficacy in evaluating whether the current treatment technology is appropriate for current and future challenges.
In drinking water treatment, there is a direct relationship between an optimised water treatment plant and maximum public health protection in the form of safe, reliable drinking water (US EPA 1998). Drinking water treatment processes generally employ multiple treatment barriers. The presence of a number of treatment barriers ensures supreme water quality, as the failure of one barrier is compensated by effective operation of the remaining barriers, thus minimising the likelihood of contaminants passing through the treatment system (LeChevallier & Au 2004). The consideration of water treatment plant efficacy to produce drinking water quality that is acceptable for human consumption must continuously be evaluated to ascertain if the current treatment technology employed is appropriate for current and future challenges.
Detailed raw water quality trending is integral for water treatment plants, as it is important to be conscious of parameters which are of specific concern to a water treatment plant. The success of a treatment plant in dealing with source water quality changes can help to guide whether the process is capable enough to produce safe drinking water, both at present and in the future, or whether a change in process technology is necessary. The implementation of new treatment technology will likely come with an increase in water treatment costs, but drinking water treatment plants should above all be primarily focused on the production of safe drinking water (Ewerts et al. 2014).
Yearly water quality monitoring of source water from the Vaal Dam, which supplies a large water utility in South Africa, showed large variations in the total concentrations of aluminium and iron in the source water, which were deemed as problematic parameters in terms of treatment. This study aimed to better understand the nature, variation and presence of these metals in the source water. This investigation also assessed whether the performance of the conventional water treatment process was consistently capable of removing these parameters to acceptable limits in the final potable water. To assess the treatment efficiency, it is essential to understand each element in terms of occurrence, uses, environmental fate, guideline limits and possible public health aspects, which will be described below.
Aluminium is the most abundant metal element in the earth's crust and occurs naturally in the environment as silicates, oxides and hydroxides, combined with other elements, such as sodium and fluoride, and as complexes with organic matter (Symons et al. 2000; WHO 2011). Aluminium-based salts are frequently used in drinking water treatment during coagulation for the removal of particulate, colloidal and dissolved constituents (Srinivasan et al. 1999).
Aluminium mobility and transport in the source water is dependent on a number of factors including: chemical speciation, hydrological flow, soil–water interactions, underlying geological composition, and pH (WHO 2011). Increasingly acidic conditions, such as acid mine drainage and acid rain, could lead to elevated levels of dissolved aluminium in the water bodies (Schecher & Driscoll 1988; Cook et al. 2000). Aluminium concentration in treated water is affected by pH, temperature, and turbidity (Srinivasan et al. 1999).
The World Health Organization (WHO 2011) does not have a guideline limit for aluminium, however it suggests that a practical limit of 0.1 mg/L for large water treatment plants and 0.2 mg/L for small water treatment plants should be adhered to, while the SANS 241 (2006) South African National Drinking Water Standard indicates an operational guideline limit of 0.3 mg/L.
Effective removal of aluminium during treatment is crucial, as elevated levels of aluminium in the final water could cause a number of problems within the distribution network. Aluminium precipitates can increase the final water turbidity (Costello 1984) and may enmesh micro-organisms to obstruct the disinfection process (Hoff 1978). Pipeline capacity and pressure within distribution pipelines was found to be affected by aluminium hydrolysis products that coat pipeline walls (Hudson 1966; Costello 1984). Zhang et al. (2016) showed that certain aluminium species attached loosely to pipeline walls, hence it is likely that these species may release back into the water if hydraulic or chemical conditions change, resulting in high aluminium concentrations in tap water.
Aside from the operational hindrances during distribution, elevated levels of aluminium in the treated water could affect public health. Various epidemiological studies have shown an associated relationship between aluminium and neuro-degenerative disorders such as Alzheimer's disease (Klatzo et al. 1965; Martyn et al. 1989; Jekel 1991; Neri & Hewitt 1991; Gauthier et al. 2000).
Iron is the second most abundant metal on earth, and is rarely found as elemental iron, due to ferrous iron Fe(II) and ferric iron Fe(III) readily combining with oxygen- and sulphur-containing compounds, to form oxides, hydroxides, carbonates and sulphides (WHO 2011). Iron is mainly used as a construction material in metal industries, and iron-based salts are used as coagulants in drinking water treatment (WHO 2011; Chaturvedi & Dave 2012).
Iron occurs mainly as a precipitated, complexed ferric iron form in surface water, and the most commonly found compounds are the iron oxides (Dimitrakos-Michalakos et al. 1997). The state of iron in a water body depends mainly on the pH and the redox potential; the soluble ferrous iron form Fe(II) can be converted to the precipitated ferric iron form Fe(III) by increasing the pH and/or the oxidation potential.
The WHO (2011) states that the operational guideline limit for iron is 0.3 mg/L, which is the level at which water has a noticeable metallic taste. However, there is no health-based guideline value stated for iron. The SANS 241 (2006) South African National Drinking Water Standard indicated an aesthetic limit of 0.3 mg/L and chronic health limit of 2 mg/L. It has been reported (Kontari 1988; Dimitrakos-Michalakos 1997; Sharma et al. 2001; Chaturvedi & Dave 2012) that if the iron concentration in the treated water surpasses 0.3 mg/L it could lead to the following unfavourable conditions within the distribution network: colour problems, increased turbidity, metallic taste, and precipitation on pipeline walls, and it could act as a substrate for bacterial growth.
In spite of the unfavourable conditions, effective iron removal from source water during treatment is merely an aesthetic and operational concern, as iron in moderate doses is considered as an essential nutrient for human health (Chaturvedi & Dave 2012).
MATERIALS AND METHODS
The drinking water treatment plant
The Vaal Dam lies on the Vaal River within South Africa. The Vaal Dam barrier was constructed in 1938 and stores water for irrigation, industrial use and potable use and is approximately 30 kilometres upstream from the water treatment plant. The Vaal Dam has a catchment area of 38,500 square kilometres and a storage capacity of 2,603,400 megalitres. The Vaal Dam drains a rural agricultural area; hence the water is relatively unpolluted, but is considerably turbid. Coagulation involved the use of hydrated lime as primary coagulant and activated silica as coagulant aid. The water treatment chemicals were dosed into the source water and then underwent flash mixing to achieve instantaneous and complete homogenisation of the coagulants. The coagulant dosages depended on the incoming raw water quality and the optimum dosages were determined using weekly jar tests performed at the water treatment plant. The hydrated lime was preferred to ensure sufficient removal of the colloidal, suspended solids which were prominent in the source water.
Coagulation was followed by flocculation in spiral flocculators, and sedimentation through horizontal sedimentation tanks to produce settled water turbidity of less than 5 Nephelometric Turbidity Units (NTU). Due to the use of lime, the pH after sedimentation was above 10, which had the advantages of limiting algal and bacterial growth; however, this water was highly unstable and possibly scale forming. Carbon dioxide was employed for water stabilisation to lower the pH to below 8.5. Following stabilisation, ferric chloride was added to aid the removal of particles during filtration.
The drinking water treatment plant utilised rapid gravity sand filters as the final physical barrier for particulate removal. The filters were covered to prevent algal growth and silica sand was used as the filter media, with gravel as the supporting layer. The operational limit for the final water turbidity after filtration was set at less than 0.5 NTU. The final step before distribution was primary disinfection using chlorine gas.
Samples were collected from 2008 to 2014 to determine the raw water quality trends and the plant efficiency. Samples were taken fortnightly at each unit process as follows: (a) source water, (b) after sedimentation (settled), and (c) after filtration (final). Samples were collected in 1 litre high density polyethylene bottles with tamper-evident caps, and were transported and stored at 4 °C before analysis.
All the chemicals and standards used for this research work were of RG grade. All standard solutions were prepared in deionised water (Milli-Q system, Millipore).
The total aluminium and iron concentrations were measured using a Spectro Spectroblue inductively coupled plasma optical emission spectrometer (ICP-OES) (radial configuration). Aluminium was measured at an analytical wavelength of 167.078 nanometres, with a resolution of ±7 picometres. The typical accuracy of quality control standards was 103.05% and the relative expanded uncertainty was 5.43%. Iron was measured at an analytical wavelength of 259.941 nanometres, with a resolution of ±7 picometres. The typical accuracy of quality control standards was 101.90% and the relative expanded uncertainty was 6.98%. The instrument was calibrated and the background profiles were checked before each run. The total aluminium and total iron were reported in milligrams per litre (mg/L).
Colour and turbidity were determined using a HACH 2100AN photometer. Colour was measured in milligrams per litre Pt-Co (mg/L), and turbidity was measured in NTU. The pH measurements were done using a Mettler Toledo pH meter (DHL-53). Both these instruments were calibrated on a daily basis.
Samples were filtered through 0.45 micrometre membrane filters, and then acidified using 1% nitric acid before the samples were run through the instrument.
RESULTS AND DISCUSSION
Raw water quality
Metal removal through the treatment process
The removal of the total metal concentrations was higher through the sedimentation process as compared to filtration, with the majority of the total aluminium and iron concentration being removed through settling. A minimum of 70% of the incoming total aluminium and iron was removed by sedimentation, in conjunction with the preceding processes of coagulation and flocculation. Iron removal was likely enhanced by the use of hydrated lime as coagulant, as at high pH values, insoluble iron hydroxide complexes are formed, which precipitate out during sedimentation. It is interesting to observe that aluminium is also removed quite efficiently, even though it is generally found to be insoluble at such high pH values. Filtration removed a maximum of 15% of the remaining total aluminium and iron after sedimentation; hence filtration acted as a final polishing step. It is clear that the unit processes at this treatment plant were effective in dealing with low and high levels of total aluminium and iron in the source water, as the removal of these metals was good.
Final water quality
Detailed source water monitoring of the Vaal Dam, South Africa, showed large variations in the total concentrations of aluminium and iron in the source water, which were highlighted as a specific concern for the water treatment plant. This study aimed to better understand the nature, variation and presence of these metals in the source water, and to assess whether the performance of a conventional water treatment process was consistently capable of removing these parameters to acceptable limits in the final potable water.
It was found that the two metals showed large variations over the 7-year study, and that their presence in the Vaal Dam was due to similar inputs. Maximum concentrations for both metals were seen during 2011 when there was heavy rainfall in the area (total aluminium = 1.34 mg/L; total iron concentration = 0.84 mg/L). It was found that 71% of aluminium and 43% of iron concentrations were higher than 0.3 mg/L.
In an attempt to understand the two metals and their influence on other water quality parameters in the source water, it was found that the metal concentrations in the source water showed significant positive correlations with colour (aluminium R2 = 0.7539; iron R2 = 0.7837) and turbidity (aluminium R2 = 0.908; iron R2 = 0.9266), hence aluminium and iron have a great impact on these parameters in the raw water in the Vaal Dam, with iron having a greater effect.
More than 70% of the total metal concentrations were removed during sedimentation and the coagulation/flocculation processes that preceded it, which was likely aided by the use of hydrated lime, which increased the pH during coagulation. Filtration acted as a final polishing step, with a maximum of 15% of the remaining total aluminium and iron after sedimentation being removed here.
The total aluminium and iron concentrations in the final water were consistently compliant with the SANS 241 (2006) South African Drinking Water Standard and WHO guidelines (2011). It is clear that the unit processes at this treatment plant were effective in dealing with low and high levels of total aluminium and iron in the source water, as the removal of these metals was good. Both pH (aluminium R2 = 0.1605; iron R2 = 0.161) and turbidity (aluminium R2 = 0.3683; iron R2 = 0.1987) showed weak positive relationships with the metal concentrations in the final water, hence aluminium and iron were not affected by these parameters in the finished water.
The conventional treatment process, which was employed at the large water treatment plant in South Africa investigated in this study, was optimally run and highly effective in removing aluminium and iron from the source water. Hence the current treatment technology is still appropriate in providing good drinking water quality, based on the current highlighted problematic parameters.
This study highlighted that in-depth source water quality trending is essential for water treatment plants, as it is important to be conscious of parameters which are of particular concern to a water treatment plant. The success of a treatment plant in dealing with source water quality changes can help to guide whether the process is capable enough to produce safe drinking water, both at present and in the future, or whether a change in process is necessary, as drinking water treatment plants should above all be primarily focused on the production of safe drinking water.
The authors would like to thank the Analytical Services Department at Rand Water, especially Mr Henry Foden, for analytical assistance during this study.