Efficient use of water means using only the amount of water required for a given task without any water going to waste. Using water efficiently results in water being conserved. It also results in savings in energy and chemical use, since less treated water has to be put into the water distribution system. In this study session you will look at efficiency in water use, and also consider alternative sources of water (such as rainwater and treated sewage) for non-drinking purposes such as irrigation.
When you have studied this session, you should be able to:
11.1 Define and use correctly all of the key words printed in bold. (SAQ 11.1)
11.2 Describe the technical options for minimising the use of drinking water. (SAQ 11.2)
11.3 Outline how rainwater can be collected and used. (SAQ 11.3)
11.4 Describe how sewage can be treated and used safely so that water is conserved. (SAQ 11.4)
Water is a precious resource and has to be conserved in order that future, increased water demands due to population and industrial growth (as is happening in Ethiopia) can be satisfied. Increasing economic prosperity also results in higher water use per person. As people become more affluent and have a piped water connection in their home, they are likely to purchase and use more domestic appliances that require significant quantities of water, such as washing machines and dishwashers. Personal bathing also tends to increase and people shower more frequently. Increasing demand means that more water will need to be supplied, bringing with it the costs and challenges of developing new water sources.
There are other costs to consider. Producing drinking water in water treatment plants requires significant inputs of energy and chemicals, so saving on water use will also save energy and chemicals.
How are energy and chemicals used in drinking water treatment?
You may recall from Study Session 5 that energy is used to pump raw (untreated) water to the treatment plant. Energy is used in the various treatment units at the plant, and finally energy is used to pump the treated water into the distribution system.
Chemicals such as aluminium sulphate and ferric chloride are used as coagulants, and chlorine is used as a disinfectant.
The water produced by the plant is treated so that it is clean and safe for drinking and cooking with but, as you know, it is also used for many other purposes, some of which do not need water to be of potable quality. It makes sense to avoid using fully treated drinking water for purposes that do not need it. It is also important to note that using less water has economic benefits for consumers, since their water bills will be lower!
Publicity campaigns to encourage people to use water sparingly are undertaken by water utilities. Flyers are distributed to customers, and radio and television are used to disseminate the message of water conservation. In Tigray, the motto is ‘Value every drop of water!’ Ways of using less water are constantly being found and Table 11.1 lists some that apply to the home.
|Fix leaking taps by replacing the washers.||A dripping trap can waste about 13 litres of water a day.|
|Never let the water run while brushing teeth, face washing, shaving, etc.||Leaving the tap running can waste up to 9 litres of water a minute.|
Install aerator nozzles on taps (Figure 11.1).
Figure 11.1 An aerator nozzle for a tap.
|These draw air into the water flow and reduce water consumption by 10–20%.|
Install aerated showerheads (Figure 11.2).
Figure 11.2 A water-saving aerated shower-head.
|These pull air into the water flow and discharge 8 litres of water a minute (because the air replaces some of the water), compared to 12 litres a minute in a standard showerhead.|
Use water-economical washing machines and use them only with a full load.
Figure 11.3 Label indicating water use by the appliance.
|Labels indicate the water usage efficiency of washing machines (Figure 11.3).|
Install water-efficient dual-flush toilet cisterns where possible.
Figure 11.4 The buttons on a dual-flush toilet cistern.
|Dual-flush cisterns have two buttons (Figure 11.4). If there is faecal matter to be flushed away, the right-hand button is pressed; this will use 6 litres of water to take away the faecal matter. If urine only is to be disposed of, the left-hand button is pressed; this uses 4 litres of water to take the waste away.|
|In older toilet cisterns, put one or two 1-litre plastic bottles filled with sand inside.||This reduces the flush volume by displacing the volume of bottles from the cistern, thereby reducing its effective capacity.|
|Use rainwater for toilet flushing, and for watering plants.||Rainwater can be collected from the roof, filtered and stored in a tank.|
|Use greywater (used water from showers and sinks that is not faecally contaminated) for toilet flushing or watering the garden.||The water should be filtered and disinfected before use.|
|Water garden plants and vegetable plots early in the morning or late in the evening.||This reduces the amount of water lost through evaporation.|
Many water-saving measures are simple and inexpensive to carry out (for example, fixing a leaking tap). Others (such as buying and fitting an aerated showerhead) cost money, but the savings made by using less water make these worthwhile.
Rainwater offers a relatively clean source of water for numerous uses. Normally rainwater can be considered clean but if the air is polluted with chemicals or particles (for example, in an industrial area), it can become contaminated. It can also pick up contaminants from roofs and gutters.
Where water is very scarce and there is no safe alternative source, rainwater harvesting provides households with the water they use for all their domestic purposes. If the water is used for drinking and cooking, it should be treated using one or more of the methods described in Study Session 10. Some families may use rainwater directly without treatment because they do not have treatment facilities – but this is not recommended, as the water could be unsafe.
What are some of the ways in which rainwater is harvested?
From Study Session 3 you know that rainwater can be collected by roof catchments, ground catchments and sand dams.
People in urban areas are less likely to have to use rainwater for their basic needs but it can still provide a useful source for numerous purposes in the home. Devi et al. (2012), in research undertaken in rural and urban areas (Guma, Gambe, Suntu, Jimma and Daraba) in Oromia Region, found that rainwater was harvested and used for house cleaning, utensil cleaning, vehicle cleaning, washing clothes, bathing, giving to animals and watering plants. A total of 2050 people were interviewed and about 30% of those from rural areas used rainwater for drinking. The rainwater was collected from tin roofs in urban areas and thatched roofs in rural areas, and led into a collection tank. Rainwater was also harvested from surface run-off on the ground and stored in small storage reservoirs.
The researchers found that rainwater harvested from tin roofs, if disinfected, would be within the WHO guidelines for drinking water, while the rainwater from thatched roofs was not suitable for drinking. They calculated that a house with a tin roof area of 100 m2 could collect 126,000 litres in the rainy season, which would be twice the water requirement for a family of five, for a year.
If 126,000 litres of rainwater is enough for twice the water requirements of a family of five for a year, what is the daily water usage of each person used in the calculation?
It would be litres
In urban areas, where space is more limited, roof collection is probably the only feasible method of rainwater harvesting. The recommended practice for roof collection is to let the first 15 to 20 minutes of the rain drain away, and then collect the water.
Why is it advisable to avoid using the first few litres of rain, especially after a dry period?
The first few litres of rainwater from a roof may contain contaminants from the roofing material, or from substances (such as dust, leaves and bird droppings) that have accumulated on the roof or in the gutter.
The diversion can be done automatically using proprietary devices like the one in Figure 11.5.
In the floating ball first flush system, when the rain starts to fall it accumulates together with any debris in a chamber with a conical top. As the chamber fills, a ball floats on the surface of the collected water and eventually becomes stuck in the conical chamber entrance, blocking the bottom chamber and therefore redirecting subsequent collected rainwater into the main clean rainwater storage tank. This storage tank should have a tight-fitting lid that prevents sunlight from entering and encouraging algal growth, and also prevents entry of mosquitoes.
A small-bore pipe is used to slowly drain the water from the lower chamber of the floating ball first flush system, to automatically reset the device. A tap can be used if water is at a premium, since water dribbles out continuously when it is raining.
A lot of water is used for purposes where high-quality drinking water is not necessary.
Can you think of water uses where the quality does not have to be of drinking water standard?
You may have thought of the washing of outdoor areas (like the house yard), washing of vehicles and gardening, but you could have mentioned anything that does not involve ingestion of the water.
One of the biggest uses of water is in irrigation, not only for food crops but also for landscaping schemes in cities like Addis Ababa (Figure 11.6). For uses such as these, it is possible to use treated sewage effluent to water plants, if this is available. Sewage treatment can be an economical process using a simple system of ponds called waste stabilisation ponds, which are described below. (Note that reuse of the water is only feasible for fully treated sewage; septic tank discharge should not be used in this way.) In Ethiopia the opportunities for reusing treated sewage effluent may be limited at the moment, but future changes could see more sewage treatment systems, which would increase the potential.
Waste stabilisation ponds are natural or constructed ponds used for treating sewage or other wastewaters biologically by harnessing the power of sunlight and wind. They are therefore ideal for tropical countries, and in Ethiopia there is such a system at Kality, treating the sewage from Addis Ababa.
In a typical waste stabilisation pond system, effluent that has passed through a screen is sent through a series of ponds with a total retention time of 10–50 days. (The retention time, in this context, is the length of time the effluent stays in the ponds.) No mechanical equipment is used in the ponds, so operation and maintenance costs are very low. Figure 11.7 shows a typical layout for a waste stabilisation pond system treating domestic sewage.
Bacteria in the ponds oxidise the pollutants and work symbiotically with algae, which provide oxygen through photosynthesis. (A symbiotic relationship means two types of living organisms live together for their mutual benefit. In this case, the algae produce the oxygen that the bacteria need, and the bacteria produce carbon dioxide and release ammonia and phosphate that the algae consume.)
Oxygenation also occurs through the action of wind, and by diffusion. Diffusion is the movement of a substance from a region of high concentration to one of low concentration. In the present context it means the movement of oxygen from the air, where it makes up 21% of the composition, to the water, where it is in low concentration.)
The major part of the biodegradation of the sewage (the breaking down of complex substances in sewage into simpler compounds, by micro-organisms) takes place in the facultative ponds (Figure 11.7). These are ponds in which the upper portion is aerobic and the lower portion is anaerobic. Facultative ponds are 1–1.5 m deep, with a retention time of 5 to 30 days. Solids settle to the bottom and are anaerobically digested by bacteria, so that sludge removal is rarely needed.
Maturation ponds are ponds placed after facultative ponds, for the purpose of pathogen reduction. They are usually 0.5–1.5 m deep, with a retention time of 15 to 20 days. The ponds serve to inactivate pathogenic bacteria and viruses through the action of UV radiation from sunlight and by the greater algal activity in these shallow ponds, which raises the pH to above 8.5 (when pathogens are rapidly killed off). The long retention time in each of the ponds also enhances the sedimentation of eggs of intestinal nematodes (parasitic worms).
To prevent sewage from leaching away, and to preserve the effluent for reuse later, the ponds should have a liner. This can be made of clay, asphalt, compacted earth, or any other impervious material (material that does not let anything through). To prevent run-off from entering the ponds, and to prevent erosion, a protective, raised earth barrier can be constructed around the ponds using the excavated material from their construction. A fence is needed to keep people and animals out (Tilley et al., 2014).
Any scum that builds up on the surface of the facultative and maturation ponds should be removed to allow sunlight to reach all the algae, and also to increase surface aeration. Large plants that are present in the water should be removed.
A further benefit of waste stabilisation ponds is that at the same time as treating wastewater, they can be used to increase protein production through the rearing of fish (such as Tilapia) and ducks in the maturation ponds.
Figure 11.8 shows part of waste stabilisation pond system.
Waste stabilisation ponds are especially efficient in hot climates. Although they require large areas of land, this need can be satisfied by locating the ponds at the outer perimeter of cities or on disused land. At the Kality waste stabilisation ponds on the outskirts of Addis Ababa, the system consists of one facultative pond and three maturation ponds, with a total retention time of 28 days. In an evaluation of Addis Ababa’s sewage treatment system in 2010, Dagne found that an average of 83.6% of the organic pollutants was removed by the process.
The critical parameter for assessing the suitability of using the effluent from waste stabilisation ponds for irrigation is microbiological analysis for the presence of pathogens. Treated effluent can be reused in crop irrigation if safe limits of faecal coliforms and intestinal parasites are achieved in the treatment process. The World Health Organization (2006) addresses this in Wastewater Use in Agriculture, Volume 2 of its Guidelines for the Safe Use of Wastewater, Excreta and Greywater. An important consideration is the type of crop that is being grown. If it meets specified standards, wastewater can be used to irrigate crops that are not eaten raw. This is called ‘restricted irrigation’ and includes non-food crops (such as cotton or oilseed), food crops that are processed before consumption (such as coffee or wheat), and crops that have to be cooked (such as rice or potatoes). For this type of crop, the WHO has specified that the level of faecal coliforms should not exceed 105 per 100 ml of treated effluent, and there should be no more than one intestinal nematode egg in 1 litre of treated effluent (WHO, 2006). (If children under the age of 15 are exposed to the treated effluent, for example by working or playing in fields irrigated with treated effluent, the limit for intestinal nematode eggs becomes stricter at one egg per 10 litres of treated effluent).
Another important consideration is the type of irrigation. If spray irrigation is used, there should be a buffer zone of 50–100 m between the irrigated fields and any nearby houses and roads, so that local people are not affected by airborne wastewater. In Ethiopia, flood irrigation or furrow irrigation is common; this is where farmers flow water down small trenches running through their crops (Figure 11.9). In this situation, the possibility of human contact with treated effluent is high.
In all situations, anyone who comes into contact with untreated or treated effluent should wear appropriate protective clothing, including gloves and boots. After working at the ponds, or after working on the irrigation of crops, people should wash themselves thoroughly. While the ponds may be designed for the production of water safe for irrigation, it is best to be safe and avoid all direct contact with the effluent.
In Study Session 11, you have learned that:
Now that you have completed this study session, you can assess how well you have achieved its Learning Outcomes by answering these questions.
Match the following words to their correct definitions.
Using the following two lists, match each numbered item with the correct letter.
waste stabilisation ponds
a.the process by which something moves naturally from a region of high concentration to one of low concentration
b.material that does not let anything pass through it
c.a system of ponds to treat wastewater using sunlight and wind
d.ponds that have oxygen at the top and are without oxygen at the bottom
e.ponds in which the pathogenic bacteria and viruses in an effluent are eliminated
f.the time that the effluent spends in an environment
g.a relationship where two parties live together for mutual benefit
h.the breaking down of complex materials by micro-organisms
Select the false statement from those below, and give the reason why it is false.
D is false. After sand filtration there may still be pathogens in the rainwater, and these have to be eliminated to make the water safe for humans to drink.
The five statements below are on waste stabilisation ponds. Select the statements that are false and give the reasons why they are so.
B is false. The bacteria and algae help each other to survive through a symbiotic relationship.
D is false. Other safety measures, such as wearing gloves and boots, having a buffer zone if spray irrigation is used, washing thoroughly after being in its vicinity, and avoiding all contact with the effluent, are all necessary.
E is false. The maturation ponds in the treatment system can be used for farming fish and ducks.