The quality of drinking water in Cape Town is of a high standard. This is due to the fact that most of the water consumed in Cape Town is abstracted largely from unpolluted mountain catchments. The Water Laboratory at the Scientific Services Branch, located within the Water and Sanitation Department of the City of Cape Town, is largely responsible for the chemical monitoring of potable water in the Cape Metropolitan Area (CMA). The laboratory is also responsible for the chemical monitoring of the raw (untreated) ground and surface water, and also water from the public swimming baths and distribution network.
Drinking Water Quality
Access to safe drinking water is essential to hetitleh and considered to be a basic human right. The City of Cape Town and its Scientific Services Branch is committed to providing quality tap water to its consumers by ensuring that the drinking water supplied by its various water treatment plants does comply with the recommended guidelines as stipulated in the South African National Standards (SANS 241:2006) for Drinking Water. Every effort is made to meet the highest national and international standards for drinking water.
Water Laboratory (Scientific Services)
The Water Laboratory is not only equipped with some of the most sophisticated water testing instrumentation in the water industry, but also consists of a group of highly skilled, qualified and experienced scientists and technicians who ensure that the consumer is supplied with drinking water of acceptable quality. Most of the laboratory’s instrumentation has been interfaced with the Laboratory Information Management System (LIMS) which provides for the automatic and instantaneous transfer of analytical data from the instrument to the LIMS in preparation for the production of the final laboratory report.
- Discreet Analyzer/Aquakem 250 – This selective chemistry analyzer is an integrated system solution for convenient and automatic testing of routine photometric tests. It is capable of
performing up to 200 tests per hour. This instrument is used by the Water Laboratory to test for nitrates, nitrites, chlorides, sulphates, fluorides and colour in the water.
- Ultraviolet Visible (UV/Vis) Spectrophotometer – This instrument determines the absorbance of the water by measuring the intensity of light passing through the sample. The absorbance is measured at 300 nm in a 4 cm quartz cuvette and is a measure of the organic content of the water. The absorbance of the raw water is used to determine the dosage concentration of the treatment chemicals at the water treatment plant.
- Gas Chromatograph Mass Spectrophotometer (GCMS) – One of the analyses performed on the GCMS is the measure of Geosmin in the water. Geosmin is a naturally occurring compound found in surface waters (rivers and dams) as an organic molecule produced by blue-green algae. This compound is responsible for the earthy, musty taste and odour sometimes found in the water and is removed with powdered activated carbon in the water treatment process.
- Inductively Coupled Plasma Optical Emission Spectrophotometer (ICP-OES) – The Water Laboratory uses the ICP-OES to perform weekly analyses on various water samples to
determine their metal content. Some examples of these metals are sodium, calcium, magnesium, potassium, aluminium, iron, manganese, lead, copper, zinc, chromium, vanadium, etc.
- TIAMO Analyser – The Metrohm automated titration system is a computer controlled and fully
automated titration system with a sample tray capable of analyzing 59 samples. This analyzer is used to automatically determine the pH, conductivity, alkalinity and turbidity of a water sample.
- Wet chemistry analysis – Various wet chemistry analyses are performed in the laboratory which includes analysis of ad-hoc samples (e.g. identification of seepage samples) and water treatment chemicals (e.g. analysis of lime, ferric
sulphate, aluminium sulphate, etc.). These wet chemistry analyses also include pH, conductivity, alkalinity, chloride, residual chlorine, ammonia, etc.
Water Quality Monitoring
The Water Laboratory performs numerous tests on water samples which are taken from the water treatment plants, service reservoirs, various points within the distribution system and from swimming baths. Various chemical analyses are also performed on ‘ad hoc’ water samples which include seepages and complaint samples from the consumer. Over 320 000 chemical and biological tests are performed each year on the raw water entering the water treatment plants and also on the treated water leaving the plants into the reservoirs and distribution network.
Frequency of monitoring
- Water Treatment Plants (WTP) – raw, treated and intermediate composite water samples are taken weekly at 11 water treatment plants and submitted to the Water Laboratory for chemical
testing. Unlike the other water treatment plants which use conventional chemical water treatment on its raw surface water, the Atlantis WTP treats groundwater by first passing it through a softening plant after which it is blended with the treated water from the Voelvlei pipeline via the Melkbos Reservoir.
- Reservoirs – Treated water samples are taken weekly at 14 reservoirs of various capacities and submitted to the Water Laboratory for chemical testing.
- Distribution – Treated water samples are taken weekly at approximately 120 sampling points situated across the entire distribution network within the Cape Metropolitan Area and submitted to the Water Laboratory for chemical testing. These sampling points include those situated in the various informal settlements spread across the city.
- Swimming Baths – Samples are taken at approximately 40 public swimming baths during the
summer season and submitted to the Water Laboratory for chemical testing and monitoring.
- Non-Routine (Ad-hoc) samples – These non-routine (adhoc) samples, which include seepage samples for source identification and complaint samples from the consumer, are submitted daily to the Water Laboratory for chemical testing and monitoring.
- WTP Treatment Chemicals – Samples of water treatment chemicals (e.g. lime), which are used in the treatment processes at the various water treatment plants, are submitted weekly to the Water Laboratory for chemical testing and quality monitoring.
- Water Treatment Plants – All raw, treated and intermediate samples are sampled every 2
hours at the water treatment plants and tested for various chemical parameters e.g. pH, Alkalinity, Turbidity, UV Absorbance, Residual Chlorine, Hardness, etc.
These results are used to optimise plant operations and to monitor the quality of the water throughout the treatment process. The Bulk Water Branch recently acquired 9 ProcessLab (ATLab) analysers for each of the water treatment plants to perform the above-mentioned analyses automatically.
The methods used feature fully automated electrode calibration as well as record keeping of calibration data. The ProcessLab analyses samples within minutes with a much higher repeatability and accuracy compared to the previous manual determination methods.
All the analytical results which are stored in the system’s database are immediately transferred, over distances greater than 80 km, to a central server where it can be viewed by the Bulk Water Management and the Water Laboratory staff at Scientific Services. These automatic ProcessLab analysers play an important part in maintaining the high quality of drinking water leaving the water treatment plants.
- Distribution network – Approximately 20 on-line chlorine analysers have been strategically
placed within the distribution network where it is expected that the residual chlorine concentration will be low (e.g. the furthest point from the reservoir, the outer perimeter of the distribution network, etc.). These on-line analyzers constantly monitor the free chlorine in the drinking water and send the results to a central server and website where it can be accessed by the Reticulation Branch management who will then authorise the necessary operational changes accordingly. These on-line chlorine analysers play an important part in maintaining the desired residual chlorine concentration in the drinking water in all areas of the distribution network.
Water Treatment Plant Optimisation
The Water Laboratory plays an important role in assisting the water treatment plants with the optimisation of its various water treatment processes. The Water Laboratory’s Analytical Chemists perform regular water treatment plant inspections to monitor the quality of water throughout the water treatment process and to give scientific guidance where needed. Research projects are also conducted at the various water treatment plants to monitor the quality of the water as it passes through the various water treatment processes, to monitor the efficiency of each process and to further optimize each process and the treatment as a whole.