Grey Water - IELTS Reading Answers & Explanations

You should spend about 20 minutes on Questions 1-14, which are based on Reading Passage 1 below.

Grey water

In Abu Dhabi, where fresh water sources are very limited, sustainable water management is a high priority. The region receives on average just 120 millimetres of rainfall every year but the country is seeing demand for water increase by almost 40% annually. In this situation, it is clear that Abu Dhabi needs to boost the efficiency of water use by increasing water recycling and using treated water safely. An example of how this can be done can be seen at one of Abu Dhabi's airport hotels, which is saving a quarter of its water by recycling grey water. The hotel's new grey water recycling system is reported to save 735,000 litres of water per month, or 60 litres for every one of its guests.

Every building where people live or work, such as homes, hotels, schools and offices uses two types of water, potable and non-potable. Put simply, potable water is drinking water and non-potable water is used for washing and cleaning. Non-potable water is further divided into grey and black water. Black water is waste water that has been contaminated because it has been used to carry away human waste from toilets, for example, and cannot be reused without extensive biological or chemical treatment. Grey water, however, requires less treatment prior to reuse, and although it cannot be used for drinking, as in the case of the hotel, it can help make significant savings to water usage by being used again for different purposes.

Grey water usually comes from showers, washing machines, sinks, dishwashers, and so on, and because of this it contains fats and oils from products such as soap and washing powder as well as flakes of skin and food waste. It typically makes up approximately 6% of a household's waste water but contains fewer harmful bacteria and less nitrogen than black or waste water. However, if it is recycled properly, around 70 litres of drinking water per person per day can be saved by domestic households. To separate grey water from black water, a separate water system is needed in the building because different pipes are required to keep grey water away from the contaminated water and flow it into the recycling process.

When the grey water undergoes biological cleaning, the water is collected from various sources around the house and fed into a course filter and surge tank. This serves two purposes: first of all, a valve regulates air and water flow into the surge tank so that an even flow and pressure is maintained. This is important for the filtering process to work properly and ensure an even flow of grey water through the filter. The water is then pumped into a second filtration tank, which contains natural materials. In this second tank, the water passes through different layers. The first filtration layer is made up of sand, which traps larger waste particles between the grains. Below this layer there are other layers made up of soil, finer sand and then coarse sand. Each layer traps more and more waste, thus purifying the water as it passes through. A lot of the purifying takes place in the soil layer, where microorganisms feed on the organic waste in the grey water and reproduce. Finally, the water flows through gravel or small stones to allow it to drain out of the system.

It is then pumped into a UV tank for a final mechanical treatment. In the UV tanks, ultraviolet light sterilises the water by killing any remaining microorganisms. Further filtration may also be applied after this. At this point, the water can be reused, not for drinking but for flushing toilets, or outdoor uses like washing the car or watering plants.

In contrast, the mechanical process for treating grey water is much simpler. It usually involves collecting the water from sinks, showers, etc. and passing it through a series of filters, each with a membrane that is finer than that of the previous filter. Chemicals, usually chlorine, can then be used to disinfect the water and it can also be treated with UV light.

In hot dry countries such as Abu Dhabi and in other environmentally sensitive areas, the benefits of recycling grey water are obvious. The main one is that it reduces the pressures on public water supplies. Moreover, the amount of polluted water entering rivers is reduced and the problem of poisonous blue-green algae in water reservoirs is also significantly reduced. Furthermore, grey water recycling can save on water bills in the long term.

However, grey water recycling can be expensive if it involves putting new or extra pipes into a building to move the grey water into the purification system. Furthermore, even after filtration and cleaning, the water still contains chemicals and microorganisms which limit its uses. There are few risks to human health when recycled grey water is used properly; one possible risk comes from eating fruit and vegetables that have been grown with it but the danger seems to be minimal. It is clear that commercial and domestic water use needs to be taken more seriously, and single-use water systems are hard to justify in a world of ever greater water scarcity.