Water testing is almost always used to see what the problems are. When testing the water, it is important to think about the range of water conditions that may occur. What land use, activities or situations can affect the water quality? Some examples of activities that can have an intermittent effect on water quality are rainfall, land slips, excavation, tree felling and cows crossing streams. The should be tested at the times when the quality is poor (and needs more treatment), and also under normal conditions. The most common treatment is disinfection. Most water suppliers add chlorine or another disinfectant to kill bacteria and viruses. Other treatment might also be needed, according to the quality of the source water. Table 3 indicates the treatment that may be needed for various contaminants. Only common contaminants are shown. The need for treatment will depend on their concentration. Table 3: Treatment needed for various water contaminants Contaminant Description Sources Likely treatment methods Micro-organisms Bacteria Bacteria are a type of contaminant, some of which are capable of causing waterborne disease. Bacteria are very small. They cannot be seen with the naked eye. They are capable of multiplying quickly and are responsible for such waterborne diseases as cholera, typhoid and campylobacteriosis or symptoms of gastroenteritis. Disease-causing bacteria from farm animals, birds, possums, sewage discharges, treated effluent. Also backflow and recontamination in the network. Bacteria are usually killed by dosing with chlorine. This has the advantage of remaining in the water as protection against recontamination. Some other chemicals are effective but are less common. Sometimes UV light is used. Other methods are boiling, and physical removal (eg, reverse osmosis). Viruses A virus is an extremely small particle capable of causing waterborne disease. Disease-causing viruses that are capable of being transmitted through water include hepatitis A and poliomyelitis. Human sewage treated Also other the Viruses are usually controlled by dosing with chlorine. This has the advantage of remaining in the as protection against recontamination. Some chemicals are effective but less common. Sometimes UV light is used, but it is not very effective against some viruses.
Fluoride At low concentrations, fluoride is beneficial to but at higher levels it can be damaging to health. Dissolved from rocks and soil, particularly in geothermal areas. In most New Zealand source waters, fluoride concentrations are too low to be beneficial and some communities adjust the fluoride level. Hardness Hard water is water that contains high levels of calcium and manganese ions. It becomes difficult to lather with soap, and scale can form on the inside of kettles and hot water cylinders. It is not a health concern but can make the water less palatable. New Zealand waters are not usually hard. This is why they are often aggressive. Dissolved (usually limestone) and soil. If any treatment were to be undertaken, a small water supply would probably use the ion exchange process. Iron Iron is an element whose compounds can cause reddish-brown colouration of the water and can give it a metallic taste. Iron can be present in the source water and it can also dissolve out of steel pipes when the water is aggressive. Often water coming from a bore source is low in oxygen and, when it comes into contact with air, the iron oxidises and forms reddish brown particles in the water. Dissolved rocks, especially in bore water. Aeration followed by filtration is usually effective in removing iron. Sometimes increasing the pH, chemical oxidation followed by filtration, greensand filters or ion exchange is used. Manganese Manganese is an element whose compounds can cause brown/black stains on laundry and can give water an unpleasant taste. High levels of manganese can be harmful to health. Dissolved rocks and soil. Normally, chemical oxidation followed by filtration, greensand filters or ion exchange is used. Nitrate At high levels nitrate can methemoglobinemia, a disease that is particularly toxic to babies. Surface waters are usually low in nitrate but high levels can occur in groundwater. Fertilisers, animal run-off, raw sewage or treated effluent. The ion exchange process is normally used but this is very expensive and difficult. Usually a new source is
Pre-settlement If water is left in a tank for long enough, particles will settle to the bottom (or, in some cases, float to the top). In the pre-settlement process, water is passed through a tank at a slow rate and suspended solids fall out of suspension to the bottom of the tank (Figure 4). The outlet of the settling tank should be near the top of the structure so that the clear water is removed. Figure 4: Pre-settling basin The solids are removed by either draining the tank or by suction from the bottom. In a small system, a swimming pool vacuum system could be used. A common problem with settling in tanks and ponds is ‘short-circuiting’, where water effectively takes a short cut. To avoid this problem, consider: placing the inlet and outlet as far apart as possible introducing the water to the tank as gently as possible to avoid turbulence installing baffles to direct the flow. Simple settling will not remove very fine-grained particles (including micro-organisms) because they settle extremely slowly. Therefore the water can still be cloudy after Nevertheless, settling reduces the load on downstream treatment processes. Pre-filtration Pre-filters can be worthwhile because they do not require chemicals, have few working parts and are robust. Unfortunately they need frequent cleaning and do not remove fine particles such as silt and clay. It is common to install coarse steel screen and plastic disc filters to remove large items such as small animals, leaves and sand (Figures 5 and 6). Gravel or coarse sand filters are sometimes used to reduce turbidity enough for the downstream processes to work effectively. Infiltration galleries (or bankside filtration) can work in this way (see Figure 7). When the media becomes blocked, the filter is backflushed or drained, depending on the design. Figure 5: Infiltration gallery Figure 6: Disc filter Figure 7: Infiltration gallery
3. inactivation – the use of chlorine, ultraviolet light or other methods to disinfect the water. All principles help to make drinking-water free of pathogens. Examples The following worked examples indicate some of the solutions that may be available in making decisions about treatment options. They do not constitute a complete list, nor will they be appropriate in every situation. In most cases, water testing and a pilot are needed to make sure that local conditions are suitable for a particular treatment method. Part of the process of considering treatment options is to calculate the costs of the alternatives. Develop the costings for your particular plant using up-to-date and local costs. An important element to include is the effect of the long-term operating costs. Sometimes an option that looks cheap initially can cost a lot more in the long term. 10.1 Scenario 1: Shorthop Community Hall Community: A community hall uses up to 2000 litres per day. There may be little or no water use for many days at a time. Water source: Rainwater is taken from a roof catchment and stored in a 20,000-litre tank (Figure 15). Existing treatment: None Water quality issues: There is no chemical contamination, but water quality testing is finding faecal coliforms in the water. The roof is the source of the contamination. Figure 15: Existing water supply system at Shorthop Community Hall Option 1: Add a second storage tank Trimming overhanging branches around the building reduce the tendency for birds to rest above the rooftop and would prevent access by possums. This measure therefore minimises source contamination. A screen on the gutter outlet is useful to divert debris such as leaves (Figure 16). Self cleaning designs are available. When it rains, the first flush of water from the roof may contain most of the pathogens from bird and animal droppings and other debris. It may also contain sediments, waterborne heavy metals and chemical residues that have collected on the