9.4.E - Water Quality


Water for human consumption must be regularly tested for its quality. Such water is stored in dams such as Warragamba Dam that supplies Sydney's water and is treated so that it meets the Australian drinking water guidelines.

To ensure that drinking water is of good quality and that guidelines are met, water quality testing and monitoring takes place at every stage of the supply system. Samples are taken in the catchments, after water is treated, in distribution pipes and in several hundred homes in Sydney.

Water must also be treated before it is suitable drinking. Purification process remove non-living components from the water such as dissolved ions and leaf litter. Sanitation involves killing micro-organisms so that the water does not cause disease.

In this unit you will:

  • Identify and describe different tests that are used to monitor and test for water quality
  • Describe your local water catchment area
  • Identify factors that affect the concentration of ions in the catchment
  • Identify sources of pollution in the catchment
  • Describe the processes used to purify and sanitise water supplies


Water Catchment Area

A catchment is an area of land where, when it rains, water can run from creeks, streams and rivers into one of the major water supply dams. There are over 16,500 square kilometres of catchment area in Sydney's water system including seven major dams, the largest being Warragamba dam.

On leaving dams, the water passes to one of the ten water filtration plants around the Sydney Water network. The Prospect water filtration plant supplies water to around 80% of Sydney. These plants remove contaminants so that water meets quality health guidelines. Water then enters a complex series of pipes and reservoirs for delivery to homes and businesses.

Pollution

The catchment must be protected from possible sources of contamination. The contamination may be from farming communities upstream that release fertilisers and pesticides into the waterways. It could be in the form of chemicals leached into ground water from dumped rubbish or even from air pollutants settling in the dams. It could also be from dangerous chemicals released into waterways from nearby factories or even from dead and decaying wildlife in the water system.

Concentration of Ions

A number of factors can affect the concentration of ions in the rivers and creeks that flow into dams. Some of these factors include:

  • The flow rate of the stream - The faster the flow rate, the more sediment is picked up and dissolved in the stream.

  • The terrain through which ground water moves before reaching a dam - The more ground it moves through the more ions will dissolve.

  • The temperature of the water - The higher the temperature the more solids will dissolve.

  • The surface area of the dams and rivers - The more evaporation, the more concentrated the solutes.

  • Proximity to farms - Rivers pick up phosphate and nitrate from fertilisers used on farms.

  • Proximity to factories - Factories may release heavy metal ions or acids or bases into water increasing the concentration of these ions.

 

Testing for Water Quality

Water quality can be determined by considering some or all of the following tests.

Concentrations of Common Ions

Fluoride

Fluoride ions are added to drinking water to promote and enhance dental health. Fluoridated water has fluoride at a level that is effective for preventing tooth decay. Fluoridated water operates on tooth surfaces by creating low levels of fluoride in saliva, which reduces the rate at which tooth enamel decays and increases the rate at which it reforms in the early stages of tooth decay. A 1994 World Health Organization expert committee suggested a level of fluoride from 0.5 to 1.0 mg/L.

The concentration of fluoride ions can be determined in a water sample by using fluoride ion specific electrodes.

Chlorine

Chlorine in the form of hypochlorous acid (HOCl) or sodium hypochlorite (NaOCl) is usually added to water supplies to kill bacteria and other microbes in drinking water. Chlorine in water is more than three times as effective as a disinfectant against Escherichia coli than an equivalent concentration of bromine, and is more than six times more effective than an equivalent concentration of iodine. A free chlorine concetration of 0.5 - 1.0 mg/L in water is enough to maintain the quality of the water throughout the distribution network, but most likely not adequate to maintain santitation of the water when stored for extended periods.

The concentration of free chlorine can be determined in a water sample by using chlorine specific electrodes.

Iron

Iron concentration in rivers is most often around 0.7 mg/L. Concentrations of iron in drinking water should normally less than 0.3 mg/L but can be higher when various iron salts are used as coagulating agents in water treatment plants and where cast iron, steel, and galvanised iron pipes are used for water distribution.

Too much iron in drinking water can be a problem because it can give a rusty color to laundered clothes'leave brown stains in sinks and basins and can also affect the taste of the water.

The concentration of iron ions in a water sample can be determined using AAS.

Nitrates

Nitrogen is an essential nutrient in aquatic biological systems for healthy plant growth. Natural sources of nitrates in rivers and lakes include rain water and the products of the decay of plant and animal material. Another notable source of nitrates in waterways is farming involving the use of fertilisers high in nitrogen compounds. Excess fertiliser is swept into rivers by run off after rain and by soil erosion. industrial wastes also contain nitrogen compounds and can be released into rivers. Sewage released into rivers is another source of compounds which contain nitrogen.

Bacteria generally break down compounds which contain nitrogen, often from plants, animals and their wastes, to form nitrate ions. Normal levels of nitrates in rivers and creeks are normally fairly low (less than about 0.05mg/L). Pollution as a result of human activities can lead to higher levels of nitrate ions. High levels of nitrate ions (in conjunction with higher than normal levels of other nutrients) can lead to the growth of algal blooms, excessive growth of aquatic weeds and the loss of some species of organisms.

The concentration of nitrogen ions can be determined in a water sample by using nitride ion specific electrodes.

Phosphates

Phosphorus is another essential nutrient in aquatic biological systems for healthy plant growth. It occurs naturally in Australian waterways at fairly low concentrations. Some native plants are sensitive to high levels of phosphorus. Phosphorus enters waterways from the weathering of rocks and from the decomposition of organic matter such as leaf litter and animal wastes. Human activities can affect the levels of phosphorus in a body of water. Sewage, industrial wastes and fertilisers which wash off soils can add phosphorus to rivers and lakes. The use of detergents containing polyphosphates also increases the phosphorus levels in sewage. Removing this phosphorus from the sewage is an expensive process which may leave other undesirable materials in the water. High levels of phosphorus (along with other nutrients) can lead to the growth of algal blooms.

Analytical chemists measure phosphorus concentrations using phosphate ion specific electrodes.

Eutrophication is when nutrient levels in waterways get too high and aquatic plants and algae grow abundantly. When phosphates and nitrates enter waterways in high concentrations, algal blooms can result. These nutrients are absorbed by aquatic plants and photosynthetic cyanobacteria which grow rapidly and can form an algal bloom which covers the surface of the water. These then use up the available oxygen in the waterway, block sunlight from reaching into the water to deeper aquatic plants and the cyanobacteria excrete toxic substances. Eventually aquatic organisms cannot survive and they die and are decomposed by bacteria which consume more oxygen. The waterway can now become unlivable for aquatic organisms and dangerous for humans to use as well.

The Environment Protection Agency, EPA, recommends a nitrogen to phosphorous ratio of 10:1, nitrogen concentrations less than 1 ppm and phosphorous concentrations less than 0.1 ppm.

Heavy Metal Ions

Heavy metals are metals with high atomic masses such as mercury, lead and cadmium. They are toxic to living things. Heavy metals are a problem in water because they:

  • interfere with enzyme systems and metabolism in the body
  • cannot be easily eliminated from the body
  • bioaccumulate (that is, they build up to high levels in the tissues of living organisms)
  • bioconcentrate (concentration gets greater as it moves up the food chain)

Heavy metals mostly settle in the sediments of waterways. Tests suggest a considerable amount of contamination of sediments occurs, not only heavy metals, but also trace metals and insecticides. Disturbing sediments by activities such as dredging causes them to become suspended again and allows them to accumulate in fish and shellfish.

The EPA sets guidelines for the maximum allowable concentration of heavy metals in waste water discharge into waterways. The levels are:

  • 10 ppm for lead
  • 2 ppm for mercury
  • 10 ppm for cadmium

Analytical chemists working for the EPA monitor heavy metal concentrations using AAS. Gravimetric or volumetric methods are more difficult due to low ion concentrations. The analytical chemists monitor concentrations of heavy metals in bodies of water as well as in the tissues of aquatic organisms. In food, the maximum allowed level of lead and cadmium is 2 ppm.

Total Dissolved Solids

Total Dissolved Solids (TDS) is a measure of the total concentration of dissolved solids in the water. The ability of the water in a river or creek to conduct electricity can be used to estimate TDS in the water. This assumes that most of the dissolved solids are ionic. The greater the ability of the water sample to conduct electricity, the greater the amount of dissolved solids. Conductivity is measured in millisiemens per metre or mS/m. The conductivity can be used to estimate the total dissolved solids in a sample of water by multiplying by a factors of 0.68. When the conductivity is measured in mS/m, if it is multiplied by 0.68, an estimate of the amount of dissolved solids in mg/L (ppm) is obtained. Some TDS meters carry out this conversion automatically.

Some typical values for TDS in different types of water samples include:

distilled water - 1 mg/L
rain water - 7 mg/L
river water - 120 mg/L
sea water - 34 000 mg/L or 34 g/L

In the school laboratory, a known volume of water can be evaporated to dryness and the remaining solid's mass determined using an electronic balance. A calculation of concentration can then be determined.

Hardness

Water is said to be hard if it does not lather well when soap is added to it, but instead forms an insoluble scum. Soaps are the sodium salts of long carbon chain fatty acids. These will dissolve in pure water. However, often the calcium and magnesium salts of these fatty acids are insoluble. So if the soap is added to water which contains calcium and magnesium ions, the soap precipitates with these metal ions and forms an insoluble scum. Eventually, when enough soap has been added to remove all of the calcium and magnesium ions from the water, extra soap will form a lather (bubbles in the water). Therefore, hard water contains high concentrations of calcium and magnesium ions.

Many rivers and lakes contain water that is naturally hard. Hardness of water does not pose ecological problems. It can pose problems, however, for people in their use of the water. Hard water forms scums with soaps and leaves precipitates inside hot water systems, kettles, boilers and pipes.

The hardness of a sample of water can be accurately determined by titrating with ethylenediaminetetraacetic acid (or EDTA for short). The EDTA forms soluble complexes with the calcium and magnesium ions. A simpler method of determining the hardness of a sample of water is to add measured amounts of a soap solution to the sample until a stable froth forms on shaking. The amount of soap which needs to be added for this to happen is an indication of the relative hardness of the water.

Total hardness is a measure of the concentration of calcium and magnesium ions in water expressed as the equivalent amount of dissolved calcium carbonate. Soft water has a calcium carbonate concentration less than 60mg/L while for hard water it is greater than 60mg/L. In Sydney the average equivalent concentration of calcium carboante is 46mg/L.

Turbidity

The turbidity of a sample of water is a measure of how clear it is. The clearer the water, the lower its turbidity. An increase in suspended solids in the water increases its turbidity. These solids could be clay, silt, algae, industrial wastes or sewage. It can be a problem if water is too cloudy, since this can reduce the amount of sunlight which penetrates the water which, in turn, can reduce the plants ability to carry out photosynthesis. This reduces the amount of dissolved oxygen in the water. Suspended solids may also clog fish gills, reduce growth rates and prevent egg and larval development.

At high levels of turbidity the water loses its ability to support a large number of different organisms. High turbidity can be the result of soil erosion, industrial and sewage wastes and the presence of excess nutrients leading to the growth of algal blooms.

Turbidity can be measured by pouring a sample of the water down a turbidity tube. At the bottom of the tube is drawn a black cross. As the tube fills up with water, it becomes harder to see the cross. The depth at which the cross can no longer be seen can be used as a measure of the water's turbidity. Turbidity tubes can be calibrated so that the level of the water in the tube when the cross can no longer be seen gives the amount of suspended solids in the water in mg/L.

Turbidity can also be measured by determining the amount of light scattered by the water sample using a nephelometric turbidity meter. The more suspended solids there are, the more the sample will scatter light. Trubidity is measured in nephelometric turbidity units (NTU) and the maximum turbidity for treated drinking water in Australia in 5 NTU.

Acidity

The pH of water in natural streams in Australia is usually between about 5.5 and 8.5 pH units. Changes in the pH of water in rivers and lakes are important because most aquatic organisms cannot survive in water which has either a very low or a very high pH. Low pH levels can be caused by natural factors such as high concentrations of naturally occurring acids in soils and rocks. High pH levels can be caused by the water flowing through areas containing alkaline rocks which lead to high concentrations of ions such as carbonate, hydrogen carbonate and hydroxide.

The pH of water can be measured using indicator paper or using a pH meter.

Dissolved Oxygen

The amount of oxygen dissolved in the water of a river or creek is a very important indicator of the water quality. Aquatic life forms need dissolved oxygen to survive. Waters with consistently high levels of dissolved oxygen (between 80 and 100% saturation) are able to support a large variety of aquatic organisms. Dissolved oxygen comes mainly from the atmosphere, so processes which increase the contact between the water and the air generally increase the amount of oxygen which is dissolved. Such processes include tumbling of the water and the formation of waves. Rain falling through the air and running into the waterway also increases levels of dissolved oxygen. Plants carrying out photosynthesis can increase the oxygen in the water.

Dissolved oxygen can be reduced by the addition of organic material to the water (such as sewage and some mineral processing effluents). When such material is added to the water, bacteria start to break it down and consume oxygen in the process. Algal blooms also reduce oxygen levels. The ability of water to dissolve oxygen is reduced with increasing temperature.

Dissolved oxygen can be measured using an oxygen sensitive electrode and a data logger and should be greater than 6 mg/L for healthy water.

Biochemical Oxygen Demand

Oxygen is required by all the organisms which live in a river or lake. It is used not only by larger organisms such as fish and water weeds, but also by microscopic organisms such as bacteria. Sometimes, the oxygen in a body of water can fall to very low levels because there is such a high demand for it. This can happen if there is a lot of decaying material in the water. The bacteria carrying out the decay processes can use up so much of the oxygen that other organisms don't have enough. Therefore, it is a useful indication of water quality to determine how fast the oxygen in the water is being used up by microscopic organisms. This is called the Biochemical Oxygen Demand or BOD. Often the BOD ( the amount of oxygen used up) is measured over a period of five days. This is then called the BOD5.

Human activities can affect the BOD of a body of water. Adding sewage or industrial wastes to a river can greatly increase the BOD of the water as bacteria populations increase and feed off the organic wastes. Some inorganic wastes from industrial and mining activities can also consume oxygen in the water by chemically combining with it.

The BOD5 can be determined in the following way. The dissolved oxygen content of the water is first determined. Then a sample of the water is collected, wrapped to exclude light (so photosynthesis does not occur) and left for five days. After this time the dissolved oxygen content of the water sample is determined. The reduction in the dissolved oxygen content of the water is the BOD5.

A BOD5 of greater than 3mg/L may indicate that the water is polluted.


Water quality test results from Sydney Water in 2003.

 

Water Treatment

Most water from Warragamba and Sydney's other dams travels to the Prospect Water Filtration Plant which treats about 3000 megalitres of water every day. The treatment process includes two main stages: purification and sanitation.

Purification



  1. A cutaway diagram of a typical sand/anthracite filter designed to rapidly
    filter suspended solids after flocculation.

    Screening
    Screening involves water passing through a sieve-like device that removes solid objects like twigs, plant matter and deceased animals.

  2. Flocculation
    Flocculation involves adding chemicals like iron(III) chloride, aluminium hydroxide and cationic polymers to form precipitates with some unwanted ions and make fine suspended solids clump together so that they can then be filtered.

  3. Filtration
    Filtration involves the water passing through a sand and anthracite filter which catches most solids.

Sanitation

Sanitation involves the addition of chlorine, often as sodium hypochlorite. The hypochlorite ion, OCl-, is what kills bacteria. Chloramine, a mixture of chlorine and ammonia, is added later to keep the water disinfected until it reaches households and businesses. Purification and sanitation of Sydney water is very effective. We have no need to boil our water before drinking it. It is clear and clean looking and rarely contains coloured or cloudy impurities.

Microscopic Membrane Filters

Microscopic membrane filters are thin membranes that act like a sieve with pores of a uniform size. The type of filter is classified according to the size of the pores. Water is forced through the pores under pressure to accelerate the filtration process.



A typical membrane filter used to purify and sanitise water. These filters can be cleaned by backflushing
clean water back through the filter in the opposite direction.

Ultrafiltration uses a pore size of 10 - 100 nm while nanofiltration uses a pore size of between 1 - 100 nm. All particles larger than these pores are trapped. These filters are used in the last stage of water treatment but aren't used by Sydney Water because they are too expensive and not really needed anyway.

Each square centimetre of membrane contains millions of pores allowing the flow rate to remain high. The membranes can be made of many different materials including polymers such as polypropylene, polyvinylchloride and nylon and glass fibre. These filters can remove organic and biological particles including suspended solids, algae, faecal bacteria and protozoans such as cryptosporidium and giardia. Ultrafiltration can remove viruses as well. To use these filters, a vacuum pump draws water through the filtering surface. No purification system can remove all contaminants; each system has its advantages and disadvantages. Most filters cannot remove particles smaller than 1 nm, so atoms and dissolved ions can not be removed. However nanofiltration membranes have very small pores and special surface coatings and can remove some heavy metal ions and very small molecules. Most microscopic membrane filters consist of polymer sheets wound around a central, rigid core to form a cartridge that can be replaced. Some consist of fine, hollow capillaries that are housed inside a filtering unit. The filtered particles are trapped on the outside of the capillaries and the filtrate passes through the centre of the capillary. Water that is to be filtered is made to flow across the surface of these membranes to prevent clogging of the pores.