Gray-water refers to household wastewater other than water from the toilet, known as black-water; gray-water accounts for 60% of all wastewater from a household and includes water from sinks, showers, baths and laundry machines (Christonova-Boal et al., 1996). Although the water contains suspended pathogenic material and bacteria, the amounts are significantly less than those found in toilet water; therefore, the amount of treatment required as part of recycling is much lower than that of toilet water or sewerage.
Recycling of gray-water involves separation of the plumbing system handling toilet water from that carrying the rest of the household wastewater; the gray-water is used to carry out functions other than drinking including watering of lawns and as water for flushing the toilet (Christonova-Boal et al., 1996).
In my opinion, recycling of wastewater is a legitimate way of conserving water; however, the need to conserve through recycling of gray-water should be tampered with deliberate caution when handling it
The issues surrounding the reuse of gray-water stem from the dwindling reserves of groundwater as an alternative for surface-water as a source of potable water to households. The need to conserve potable water spurred the use of gray-water for activities that do not require potable water such as flushing the toilet (Christonova-Boal et al., 1996).
The use of this water has also been supported by countries where water is scarce through out the year or where the drought period are very long; countries in the middle-east, Mediterranean and Australia have policies encouraging the recycling of gray-water. In the United States, the greatest proponents for recycling include New Mexico, Arizona and California, where shortage of fresh-water is a perennial problem.
Benefit of recycling gray-water
Most significant among this is the reduction of household use of fresh-water; this is by substitution with gray-water in activities that do not require potable water; especially in areas where shortages of fresh-water occur during the dry season.
Another benefit cited is the reduction of the volume of wastewater going into the sewer system thus putting it under less strain; since gray-water makes up about 60% of all the wastewater from the household (Christonova-Boal et al, 1996), diverting this water would reduce the flow to the system by a similar amount. Recycling of the water through irrigation or other method that involve application of the water onto soil also helps in recharge of ground-water; this way, the exceptional abilities of soil to filter water to purity-levels which conventional systems cannot achieve at the same cost are utilized to increase the amount of potable water available to the region.
The use of gray-water for irrigation also improves the fertility of the soil; the food particles and the nitrogen in the water is broken down by bacteria on the topsoil and used up by the plants as nutrients; this mechanism reduces the cost of fertilizer used for farming (Godfrey et al., 2009).
The recycling of gray-water has been cited as one of the ways of solving growing water needs in arid countries (Godfrey et al., 2009); this is especially so in the middle-east and the Mediterranean regions where rapidly growing economies and populations are increasing the demand for water past the traditional sources capacity; in this case, the benefit-cost ratio of recycling water has been shown to be better than that of desalinizing the same volume of sea water (Prathapar et al., 2005).
Finally, the reuse of household gray-water as a method of conserving the environment invokes a feeling of satisfaction and contentment as one feels that s/he is playing a part in making the environment cope better.
Risks of recycling of gray-water
Gray-water does contain bacteria and suspended organic matter similar to that contained in black-water albeit at lower concentration. Therefore it has a potential of causing health problems as sewerage. There has been no documented case of infection arising from use of gray-water; however, several concerns regarding its safety have been raised.
As we have seen, gray-water is derived from activities involving hygiene; as such the water contains not only organic material and bacteria, it also has varying concentrations of inorganic chemicals such as soaps and detergents, bleaches and others (Nolde, 2000). These components may not be satisfactorily decomposed in the environment; for example, some detergents have high concentrations of salts used as bulking agents (Prathapar et al, 2005); these can mimic drought situations causing the plant to die.
In addition to this, the level of ‘grayness’ is no ascertained by any standard method; for example, water used for washing soiled diapers may be labeled as gray while containing elements at the same level as toilet water labeled as black; the innocent use of this water as gray has a potential for public-health crisis.
The nature of gray-water also makes it unsuitable for use in multiple-residential-units; in a single-dwelling-unit, the bacteria in the water originate from the members of the unit and are thus therefore less at risk from these pathogens (Nolde, 2000). However, a shared system would give the pathogen the opportunity to spread to other households from a single one.
Recycling of gray-water has the potential of contaminating both surface and ground-water. In cases where the water is used on fields or lawns, pooling of the water would offer an opportunity for run-off to wash the water into the nearest river. Thus, the water cannot be recycled in this manner in the wet seasons. Additionally, this type of use is dependent on the topsoil bacteria quickly breaking down the organic material; this is not possible during winter and all attempts at recycling by irrigation should be avoided. Depending on the soil type, water applied on the ground may not be adequately filtered before it reaches the aquifer; thus contaminating ground-water.
The traditional source of clean water may not be as sufficient in providing adequate supplies to a growing population. Additionally, in the face of an increasing erratic climate stemming from the effects of global-warming, severe drought has become more common and lasting longer in many parts of the world causing severe water shortages.
It is therefore necessary to put in place measures that minimize the usage of fresh-water in households to avoid completely depleting the limited source of water; in my view, recycling of gray-water is one of the legitimate ways of conservation.
The health concerns raised of the potential of this water being a source of disease are legitimate; strict measures are needed to ensure safety. The use of gray-water should be avoided when the soil or the climate is unsuitable for such use. Recycling systems should be installed strictly by certified engineers to avoid contamination of clean water. The user should additionally asses the level of material in the water before use; excessively contaminated water should be routed to the sewer; avoiding detergents harmful to plants.
Application of the water on the soil should never be through sprinkler systems to avoid inhalation of contaminated droplets; discharge should be below the ground to facilitate percolation into the soil.
In the face of growing populations, reduction in fresh-water supplies and the global climate change, it is prudent to put gray-water to some use to avoid unnecessary wastage; the benefits achieved are much greater than the potential losses from recycling.
The need to conserve through recycling of gray-water should be tampered with deliberate caution when handling it. Systems and policies that encourage recycling will in the long run standardize the methods and place the practice firmly on the stage with other conservation methods.
Christova-Boal Diana, Robert E. Eden, Scott McFarlane ”An investigation into greywater reuse for urban residential properties” Desalination, Volume 106, Issues 1-3, (1996) Pages 391-397.
Godfrey Sam, Pawan Labhasetwar, Satish Wate “Greywater reuse in residential schools in Madhya Pradesh, India—A case study of cost–benefit analysis” Resources, Conservation and Recycling, Volume 53, Issue 5, (2009) Pages 287-293.
Nolde Erwin. “Greywater reuse systems for toilet flushing in multi-storey buildings – over ten years experience in Berlin” Urban Water, Volume 1, Issue 4, (2000) Pages 275-284.
Prathapar, S.A. A. Jamrah, M. Ahmed, S. Al Adawi, S. Al Sidairi, A. Al Harassi “Overcoming constraints in treated greywater reuse in Oman” Desalination Volume 186, Issues 1-3, (2005) Pages 177-186.