Water treatment

We saw in the last step that the source of our water can create challenges to sustainable development. The next element that needs to be explored is how and why our water is treated.

Does water need to be treated?

Most water sources will require some form of treatment, but the amount is dependent on the quality of the water source (see Step 3.4). As highlighted in Step 3.3, some microorganisms in water can cause acute illnesses, such as cholera or typhoid from bacteria. Other compounds may cause more chronic illnesses if ingested over long periods of time at relatively low doses. Lead, for example, can cause disorders of the nervous system while long-term exposure to arsenic can promote various types of cancer (see Step 3.4).

Hence, the aim of water treatment should be to attain a quality suitable for human consumption, as defined by the health-based World Health Organisation (WHO) standards.

However, it needs to be highlighted that not all water that comes into our homes is necessarily taken into our bodies. Water used to fill toilet cisterns, for example, doesn’t need to be of drinking water standard as it is just used as a transport mechanism to move urine and faeces out of the house. Rainfall harvested water can be used to fill cisterns, for example, in certain climates. From a sustainability perspective, the aim should be to treat water to an appropriate level for its intended use.

How can water be treated?

In general water can be treated using a combination of physical and chemical treatment processes. The original form of treatment, which is still used very effectively, is to pass water through a bed of sand where many contaminants are filtered out. This can be done at any scale, from centralised plants, using either slow or rapid gravity sand filters, down to household scale biosand filters (see figure below).

In larger scale systems additional treatment processes are normally included up-front of the filters, and aim to reduce the solid load by adding chemicals called coagulants and flocculants. These act to clump the particles together so that they can be removed by settling to the bottom of large tanks (clarifiers). This prolongs the life of the filters before they become blocked and need to be cleaned.

However, very small molecules and microorganisms (particularly bacteria and viruses) can still get through the sand filters and so a final stage of disinfection is needed to ensure that the water is safe to consume. This targets the complete removal of any viable microorganisms by either adding a powerful oxidising chemical (chlorine or ozone), or using ultra-violet light or a barrier called a membrane which allows water molecules to pass through but not larger microorganisms.

^{Slow sand filters at large-scale centralised treatment plant in Ireland (left) © Laurence Gill and schematic of biosand filter for single household use (right) Click to expand.}

Sustainability of water treatment

So, how can this knowledge be used to achieve the goals of SDG 6?

First of all, it is clear that the water from most sources does need to be treated to some extent before being safe for human consumption. We may take this for granted in a lot of places as we open taps in our houses, but a safe water supply is still not the reality for over 660 million people around the world.

Then, with respect to the nature of water treatment itself, the following aspects need to be considered in relation to achieving sustainable development.

• What is the water quality of the source as this will dictate the level of treatment required? Taking water from a surface water source might seem more attractive from an energy perspective compared to pumping groundwater from a borehole, but will probably need more extensive treatment with ongoing requirements of energy and chemicals compared to the cleaner groundwater source.
• At what scale should the appropriate treatment be targeted – centralised/decentralised/household? This will be linked to factors such as: the size of the resource versus the size of the demand; the proximity of the water source to the population; who is financing the infrastructure; and who will own and operate the water supply.
• How sustainable is any ongoing requirement for chemicals in the treatment process from an economic, availability, and security as well as environmental perspective? For example: will the chemicals end up as part of a byproduct from the process; how will that be dealt with responsibly without long-term environmental impacts?
• How sustainable is any ongoing requirement for energy to run the treatment processes? Can this be minimised by careful location of the plant in order to benefit from any natural fall of the water from the source to drive the water through the system (if taking water from a reservoir or springs in the nearby hills for example)? How will the energy be generated and how reliable is it? An unreliable energy supply could mean that water gets cut off several times per week, as happens in many cities at present in poorer countries, which is a major risk to public health. Can the energy be generated by more sustainable local renewable sources?
• What is the financial model used to set up and run the water supply? Who is supplying the investment and does it need to be recouped? How will the water be paid for? Will it be affordable for the community? Who will own and maintain the system? Is the structure socially acceptable to the community?