Screening removes large debris that may interfere with other treatment steps. Storage of water obtained from rivers and other sources in reservoirs for some time (days to months) may allow natural biological purification of the water to occur. This is important when using slow sand filters.
Best Methods of Improving Drinking Water Quality
1. Adjustment of pH
Pure water has a pH close to 7 but fresh water can vary in pH depending on the geology of the drainage basin or aquifer and the presence of contaminants.
If water is acidic, lime, soda ash, or sodium hydroxide can be added to increase the pH during water purification. The addition of lime increases the calcium ion concentration thereby raising the water hardness. For very acidic waters, degasifiers can be used to raise the pH of the water by removing dissolved carbon dioxide from the water.
Making the water alkaline helps coagulation and flocculation processes, and minimizes the risk of lead dissolving from pipes into drinking water. The acid may be added to alkaline waters to reduce alkalinity or pH.
2. Coagulation and flocculation
Coagulation entails the addition of chemicals to reduce suspended particles in water (e.g clay, silt, algae, bacteria, viruses, protozoa, and organic matter). Organic and inorganic particles contribute to the turbidity and color of the water.
Addition of inorganic coagulants (e.g aluminum sulfate -alum or iron III salts e.g. iron III chloride cause many reactions among particles in water.
Inorganic coagulants neutralize negative charges and precipitate metal hydroxides (iron and aluminum).
Flocculation refers to the formation of large amorphous metal hydroxides e.g. iron III and aluminum which adsorb and enmesh particles in suspension and allow their removal by sedimentation and filtration.
3. Sedimentation or clarification
Water from the flocculation process enters the sedimentation basins where there is low water velocity which allows the floc to settle to the bottom. The sedimentation basin depth must be of sufficient depth so that water currents do not disturb the sludge and settled particle interactions are promoted.
The settled particles form a sludge at the bottom of the sedimentation basin which must be removed and treated.
After removing the sludge, the water is filtered to remove the remaining suspended particles and unsettled floc. Types of filters include rapid sand filters, pressure filters, slow sand filters, and membrane filtration.
• Rapid sand filters
This is the most common type of filter and contains activated carbon or anthracite coal above the sand. It removes organic compounds that produce taste and color but cannot remove smaller particles. Some particles block the pores’ spaces or adhere to sand particles. This requires back flushing, backwashing, or air scouring to clear the pores.
• Pressure filters
The pressure filters work on the same principle as sand filters but differ in that their medium is enclosed in a steel vessel and water is forced through it under pressure. They can filter smaller particles than paper and sand filters; they are thin and liquids flow through them rapidly; they can withstand high pressures and they can be cleaned and reused.
• Slow sand filters
These filters are useful where there is sufficient space and land since water must pass through the pores very slowly. They rely on biological treatment processes rather than physical filtration.
They are constructed with graded layers of sand with gravel at the bottom and fine sand at the top. Another type of slow filtration is bank filtration in which natural sediments on the river bank filter out contaminants from incoming water.
• Membrane Filtration
This method is widely used for drinking water and sewage. For drinking water, membrane filters can remove all particles larger than 0.2µm.
They are an effective part of tertiary treatment when the water is to be reused in industry, for some domestic purposes, or before being discharged into streams. No filtration can effectively remove dissolved substances like phosphorus, nitrates, and heavy metal ions.
• Removal of ions and dissolved metals
Ultrafiltration membranes use polymer membranes with chemically formed microscopic pores that filter out dissolved substances without the use of coagulants. Ion exchange systems use ion exchange ion resin or zeolite-packed columns to replace unwanted ions e.g. the removal of Ca2+ and Mg2+ ions replacing them with Na+ and K+ ( removal of water hardness). This method is also used to remove toxic ions like nitrite, lead, mercury, arsenic, etc.
• Precipitative softening
Water rich in hardness (calcium and magnesium ions) is treated with lime (calcium oxide) and/ or calcium carbonate to precipitate calcium carbonate out of the solution using the common ion effect.
Water is passed between a positive and a negative electrode. Ion exchange membranes allow positive ions to migrate from the treated water towards the negative electrode and negative ions towards the positive electrodes. The complete removal of ions from water is called electrodialysis.
This is accomplished by filtering out micro-organisms and by adding disinfectants to kill pathogens that remain after filtration. Disinfected water is allowed to stand for some time before use.
• Chlorine disinfection
Chlorine or its compounds such as chloramine or chlorine dioxide are used. Chlorine is a strong oxidant and rapidly kills many harmful microbes. But chlorine is a toxic gas that should be used with care. Sodium hypochlorite when used does not release chlorine gas. Chlorine is ineffective against protozoa that form cysts in water such as Giardia spp and Cryptosporidium spp.
• Ozone disinfection
Ozone is an unstable molecule that readily gives up one atom of oxygen providing a powerful oxidizing agent which is toxic to most water-borne organisms. It is a very strong broad-spectrum disinfectant that can inactivate cyst-forming protozoa. It also works against many other pathogens but leaves no residual in the water.
• Ultraviolet disinfection
This is very effective at inactivating cysts in low turbidity water but its power decreases with increasing turbidity of the water. It leaves no residual in the water. Residual disinfectant can be provided by adding chloramines
• Solar water disinfection can be used in remote areas and has a lower impact on the environment.
6. Re-using Wastewater or Reducing its Volume
• Reusing sewage indirectly for drinking water
In places where clean water is scarce, sewage can be treated and reused for drinking water. For example, in the space station urine is purified for drinking, food preparation, and washing. Recycled wastewater is also used in Israel, Singapore, and parts of Europe.
• Reducing the volume of wastewater
Grey water describes all wastewater except that flushed down toilets. Grey water usually goes down the drains to the sewage line but some water-scarce areas recover it and are used to flush toilets, wash cars, or irrigate plants.
Wastewater reclamation is important in arid areas with large populations. Treated wastewater is used as cooling and process water for commercial washing, ornamental fountains, firefighting, golf course irrigation, creation of artificial wetlands, and groundwater recharge. The major concern when using reclaimed water is surviving pathogens.
6. Construction of Wetlands
Wetlands can be constructed to treat wastewater. The wetlands remove nutrients and reduce suspended solids and BOD. The effluent can be used for plant irrigation, flushing toilets, and groundwater recharge. Mature plants grown in the greenhouse can be sold.
7. Reducing Nonpoint Sources of Water Pollution
Agriculture is a major cause of non-point source runoff of soil, pesticides, fertilizers, and animal wastes into rivers, lakes, and other water bodies.
Steps to reduce runoff from non-point sources include:
1. Plant grass or trees as buffer strips next to water bodies to absorb the runoff.
2. When possible, use zero-tillage in which crop residues are not tilled into the ground, but left on the soil to limit soil erosion and runoff.
3. Fertilizers should only be applied when and where necessary. The excess application runs off into surface water or infiltrates down into groundwater as surplus.
4. Contour strip-cropping (planting rows of different crops) helps to lower both chemical runoff and soil erosion.
5. Use integrated pest management to reduce pesticide use.
6. Choose chemicals that are needed in small amounts to kill weeds or use herbicides that are less water soluble and bind more tightly to soil.
7. In Livestock feed lots, methods exist to minimize runoff, but large operations may allow animal waste solids to settle out in detention ponds.
8. Practice crop rotation e.g. between maize and legumes, apply less fertilizer and apply fertilizer with less runoff.
9. Reducing Nonpoint Source Runoff from other Activities:
• Provide buffer zones of grass or trees near water bodies onto which runoff can flow.
• Build detention ponds or constructed wetlands into which runoff can flow for natural treatment.
• Seal off any open mine that is the source of runoff.
• For large construction sites, reduce runoff by laying out the sites to make use of or modify the land’s natural contours.