The Potential of Greywater Reuse
By Jennifer Lu and Mujtaba Jebran
With rising urbanization rates, cities face a common problem of water usage, encompassing issues such as drought and water scarcity amidst rapidly growing populations. Water is an input in several different facets of life, from agricultural irrigation to drinking water. As an output, water waste can be collected either in the form of sewage, also known as blackwater, or greywater, which contains less concentration of organic materials compared to sewage. Greywater reuse is a promising way to reduce consumption of water resources.1
Greywater Uses
Water recycling is typically applied to two types of water: blackwater and greywater. Blackwater is dirty water from plumbing and sewage waste, while greywater is water from sources like sinks, baths, shower drains, and laundry pipes. Reuse purposes of greywater include forest, agriculture, and garden irrigation, livestock watering, cooling, industrial cleaning, industrial processes, fisheries, recreational water, and finally, public water supply, or potable (drinkable) water.2
Greywater Treatment
Greywater reuse processes are divided into primary, secondary, and advanced treatment (Figure 1). The primary treatment is sedimentation, the removal of suspended matter from water. The secondary treatment consists of disinfection, which can be done with chemical disinfection or biological oxidation. Chemical disinfection typically involves chlorine or bromine dosed in a liquid solution, while biological oxidation uses hydroxyl (-OH) groups in chemicals to remove organic material. Membranes can aid in the process of biological treatment. For example, microfiltration removes protozoan parasites, while ultrafiltration involves the removal of viruses.3 After this level of treatment, the reused water can be used for surface and landscape irrigation. Biological treatment is necessary to remove biodegradable material from water.4 Membrane filtration is one method of carrying out biological treatment. Additionally, the recycled water can replenish certain non-potable aquifers, ecosystems, and industrial cooling processes.
There also exists a tertiary level of advanced treatment of greywater that includes chemical coagulation in addition to repeated processes of filtration and disinfection. There are two different types of coagulation: colloidal suspension and flocculation. Colloidal suspension is used to remove smaller water soluble solids with some positive or negative charge. Metallic salts can be added to neutralize the charge, leaving neutral solids that will naturally coagulate together. Flocculation is used to remove small hydrophobic solids. Larger polymers are added to water, causing all of the solids to clump together, thus making the solids easier to remove. This water can be used for landscape irrigation, toilet flushing, vehicle washing, food crop irrigation, and recreational water.5
| Treatment Level | Primary Treatment | Secondary Treatment | Tertiary Treatment |
|---|---|---|---|
| Process | Sedimentation |
Chemical filtration Biological treatment |
Chemical coagulation Additional filtration and disinfection |
| Uses after level of treatment | None |
Surface Irrigation Landscape Irrigation |
Landscape irrigation Toilet flushing Vehicle washing Food crop irrigation Recreational Water |
| Table 1. Greywater Treatments | |||
Cost and Effectiveness
Water reuse can be applied easily at the individual home level; families can install greywater reuse systems.6 A typical greywater reuse station installed in a home can cost as little as $1,000 and saves about 50-80% of the water used.7 The reuse of greywater is inherently more energy efficient than bringing in new water from external sources as a result of reduced transport and filtration costs. Additionally, specialized management depending on the destinations of reused water can also reduce energy in certain treatments. For instance, water that will end up as toilet water requires a significantly fewer number of treatment steps than future potable water.
On a large-scale, city-wide scope, investing in water reuse in homes allows cities use less energy and money as costs go less towards transport and filtration of new water from outside sources and disposal and storage of wastewater.8 When individual homes use less water, the city saves money overall. For example, Morocco enjoyed about $2.035 Billion net benefit from implementing greywater reuse for agriculture in 2008.9 Learning from Morocco, cities are urged to implement methods of recycling greywater to save both financial and water resources.
As the growth rate of urbanization and population increases, the demand for scarce water resources increases. To satisfy this growing demand, fresh water sources need to be accompanied by recycled water. Cities are encouraged to reduce the amount of water wasted and costs of transporting and filtering water from outside sources through greywater reuse, getting closer to the goal of zero water waste than ever before.