Improving Water Quality

To improve the physical water quality factors (PWQF) of huge volumes of water used for irrigation is quite easy and cheap compared to the processes required to improve the chemical quality factors (CWQF).

A high salt concentration or total dissolved salts (TDS) of irrigation water is the most important chemical quality factor. The TDS in water has a direct effect on most of the other water quality factors. Reducing the concentration of salts in irrigation water involves huge inputs of energy and is not an economical option for agriculture in South Africa. Processes that result in desalting (desalinization) of the water involve reverse osmosis (RO), distillation, and de-ionisation or demineralisation.

Reverse Osmosis: A water treatment process whereby dissolved salts, such as sodium, chloride, calcium carbonate, and calcium sulphate may be separated from water by forcing the water through a semi-permeable membrane under high pressure.

Distillation: The purification of salt or brackish water by removing the dissolved salts by allowing the water to evaporate and then condensate against a smooth surface, such as glass, from where it is collected again.

Deionisation: Removal of ions from water by exchange with other ions associated with fixed charges on a resin.

Click here to view a video that explains desalination.

The quality of irrigation water can however be improved by focusing on specific factors that can be improved economically.

Improving Physical Water Quality Factors

Improving the physical quality factors of irrigation water involves processes such as:

Sedimentation: Sedimentation is a process whereby the flow of water is reduced, and the suspended material is allowed to settle to the bottom. This is done by letting the water into a storage dam from where it is pumped to the orchards. During the time the water is in the storage dam, the flow is reduced to zero and solid particles settle. This process is effective at removing solids such as clay, silt, sand, cuttings and some organic debris. However, when the water has a high SAR, the clay settles slowly and in severe cases, the clay remains in suspension.

Filtration: Filtration is the process where water is passed through a structure with very small openings which allow the water to pass through but trap insoluble suspended particles. Filtration does not remove dissolved salts. The small openings are created by a container filled with sand (a sand filter), with a series of plastic discs containing narrow spleens on their flat surfaces that are fitted together (a disc filter), or with a series of steel plates with small holes in or mesh wire (screen filter).

Improving Chemical Water Quality Factors

Chemical water quality factors can either be improved or dealt with under certain conditions, as follows:

Correcting the pH – It is not always economically viable to correct the pH, because large volumes of water are required for irrigation. However, when applying pesticides, adjusting the pH is always justifiable due to the cost of the pesticide, the small volumes of water required, and the potential loss in exportable yield if the pesticide is less effective. The volume of acid needed for the correction of the pH of water can be determined by titration, using a pilot plant (see practical below) or based on the carbonate plus bicarbonate content of the water.

The pH of water can also be reduced by adding commercially available buffers. These buffers are usually mixtures of inorganic and/or organic acids. Application directions are supplied with each product.

Some pesticides are more active at pH values higher than 6.50. Although this is not required often, special formulations of buffer solutions are available to increase the pH to the desired level. Increasing the pH of irrigation water is seldom required, but if high water acidity causes too much damage to the irrigation system, in the form of corrosion, a specialised investigation is called for.

Example: Correcting the pH of Water

Information supplied:

7.5ml acid is required to decrease the pH of 20 litres of water to 5.5

The spray machine has a capacity of 2,000 litres

Calculation:

Requirement per litre: 7.5 / 20 = 0.375ml per litre

Requirement for 2,000 litres: 0.375 x 2,000 = 750ml per 2,000 litres required to reduce pH to 5.5

Oxidation – When water gets in contact with air, dissolved metals like iron and manganese are oxidised to their insoluble form and settles to the bottom of the water. Oxidation can be accelerated by spraying the water through the air into a storage dam.

Dealing with Sodium Absorption Ratio (SAR) – The SAR of water influences the soil. A SAR value of more than one indicates a possible sodium hazard. It is dealt with as part of the soil maintenance program.

Dealing with Chlorides – Too high concentrations of chlorides can be dealt with, to some extent, by supplying to the tree more or all nitrogen in the form of nitrates and supplying the tree with more calcium. This is best done with fertigation and a drip irrigation system.

Fertigation - Fertigation refers to the practice of applying fertilisers through the irrigation system. Drip irrigation systems are most suited to fertigation.

Improving Quality Of Water Leaving The Farm

A very important aspect of water quality management is to improve the quality of the water leaving the farm, thereby improving the downstream quality. If the upstream users prevent or reduce contamination, downstream users will have fewer problems. Prevention involves the following processes:

Preventing Nitrogen Leaching – Nitrate nitrogen is easily leached from the soil and will end up in underground or surface waters. Optimising the application of nitrogen and water will reduce the leaching of nitrates.

Preventing Excessive Application of Chlorides – Like nitrates, chloride also leaches easily and will end up in the subsoil, underground- or surface water. The application of chlorides should therefore also be controlled carefully.

Preventing Excessive Runoff – When water is applied at rates in excess of the infiltration rate of the soil, runoff occurs. Runoff water carries nutrients and soil particles, and adds to the problem of suspended solids, unwanted nitrates and phosphates, and increasing the electrical conductivity of water. Runoff water also causes soil erosion.

Preventing Dumping – Dumping of any chemicals or materials that can cause pollution or contamination of water sources is strictly prohibited. However, even approved dumping sites can add to the deterioration of the quality of water. Even French drains and other approved methods of getting rid of refuse can cause pollution problems. Regulations regarding the safe disposal of chemicals and other harmful materials must be strictly adhered to at all times.