Sridhar D. Iyer
In our daily use, we encounter three types of water such as ‘white,’ ‘grey’ and ‘black,’ which are respectively, potable, untreated faecal-free waste water generated by routine activities, and toilet water. Greywater constitutes at least 60% of consumed water and because there is a concern over water scarcity and sustainable living, it is an important source to conserve and reuse. Let us examine what is greywater and how it can be recycled and repurposed.
Greywater is household wastewater from sinks, showers, and washing machines that does not contain faeces and can be treated and reused for non-drinking purposes.
All greywater is wastewater, but not all wastewater is greywater. The untreated and stored wastewater often becomes cloudy, which gives it a grey colour and is also called ‘sullage’ or ‘foul water.’ Though greywater looks dirty, it can be used prior to discharging into the sewage system.
Greywater has less pathogens, in contrast to blackwater, and hence is suitable for non-potable uses. But it needs to be treated and reused soon, since long-time storage causes deterioration by organic matter and heat, that create microbial growth and
foul odour.
Quality and quantity
Considering the quality of greywater, it can be effectively used. Greywater from washing machines has lint and dirt, organic material, and chemicals from detergents. Greywater from washbasins, kitchen sinks and dishwashers has organic materials, food particles, bacteria, detergents, oils, grease; hairs, soaps, shampoos, toothpastes, among others.
The quantity of greywater produced would depend on climate and weather, number and age of persons in a house, office or industrial estate, style of living, water usage patterns, etc. Estimations have shown that on average a person uses about 150 litres per day of water and from this about 100 litres of greywater is generated. Assuming a house with four members, annually about 1,46,000 litres of greywater would be produced. Just imagine the mind boggling numbers that will ensue if we include corporate offices, industries, factories, educational institutions and others.
Greywater uses
Treated greywater is an excellent source for several purposes such as for gardening, irrigation, toilet, car washing (especially on a commercial scale and in housing societies), firefighting equipment and hydrants, landscaping, artificial waterfalls and fountains. It can be used in cooling towers and manufacturing processes. Instead of potable and tanker/well waters, greywater could be sprayed to settle the dust at construction sites. Importantly, local laws must be followed to reuse greywater in terms of storage, treatment and permitted levels of contamination.
How greywater system works
Treatment of greywater depends on the purpose and amount of usage. On-site treatment and recycling reduce the centralised treatment processes and saves time, energy, efforts and money. Greywater contains fewer contaminants that require simple and cost-effective remedies:
Collection: The volume of greywater generated needs to be calculated and collected through several ways from various sources by dedicated plumbing pipelines.
Storage: Proper storage of the greywater is important.
Filtration: The stored water is filtered for a longer duration usage. Screen and mesh filters help to remove dirt, hairs, soap scum and other waste. Multi-stage filtration systems of sand, gravel, and charcoal could separate finer particles and organic material. In settling tanks, heavier particles sink and cleaner water that rises can be drained and used. Biofilters such as plants, microbes and bacteria breakdown the contaminants. Constructed wetlands catalyse the decomposition of organic matter because soils and plant roots are natural filters.
Disinfection: Greywater needs to be disinfected within 24 hours using chlorine and/or ultraviolet light to kill germs, viruses and bacteria. Ozone generators would decompose organic matter and destroy the pathogens.
Properly stored and treated greywater is an asset for day-to-day uses, during water shortage and drought. Greywater lessens the demand for water supplied by the government and drawn from wells, especially in the areas or states where monsoon is erratic. Water bills could be lower and money saved by housing societies, resorts, hotels and corporate offices. Stored greywater would minimise its flow and contamination of nearby waterbodies.The groundwater gets recharged and soil gets enriched with nutrients. Usage of greywater is an example of sustainability and profitability especially in tourist-centric regions.
Greywater implementation in Goa
In India, generally in housing and office complexes, greywater and blackwater are combined and discharged into a public sewer or into a sewage treatment plant (STP) if there are no public sewers. This is done because of space constraints and to reduce cost of having separately treating greywater and blackwater.
The Goa Land Development and Buildings Regulations, 2010, states that complexes with 50 or more flats must have an STP but not for those that are linked to existing sewerage lines. The Goa State Pollution Control Board (GSPCB) studied the quantum of waste generated by housing complexes and has reduced the number of units to 24. It is mandated that the builder constructs and maintains the STP for three years or till the complex is handed to the society. But builders may circumvent the rules because of space to be allotted, and expertise needed to install, operate and maintain an STP. Even societies would find it difficult to run an STP.
Therefore, the government could have a two-pronged strategy: increase the number of sewerage lines for blackwater, and compel the complexes (housing, offices, industries, institutions) to set-up greywater treatment plants. This would also be safe since STP fails often, clog pipelines and create bad odour and health hazards.
To conclude, there are major differences between sewage and greywater treatment plants in terms of contents, managementsand applications. An STP is complex, costly and any failure would impact the environment. In contrast, collecting, treating and repurposing greywater are simpler, less pricey and have insignificant environmental impacts.
(The writer is Chief Scientist
(retired) at CSIR-NIO, Dona Paula)
