Recycle Greywater to Reduce Urban Water Needs

Some weeks ago, there was a buzz in our residential complex about having to set up “grey water” processing, because it may be made mandatory by the municipality. I got curious and started investigating the topic.

At the end of my study I was left wondering why it was not yet mandatory – grey water processing, when fully implemented, could reduce the water requirements in cities by 30-35%!!

What is Greywater?

Greywater is used to refer to relatively clean wastewater. Taking residences as an example (refer the schematic below), wastewater from the kitchen sink, water filter, shower/ bathtub, washbasin, washing machine/ laundry is called greywater, whereas wastewater from the toilet flush, or water after cleaning floors and vehicles can be called blackwater.

Without greywater recycling, clean water is supplied for all uses. Wastewater from all uses is collected and channeled to the same sewage system.

Normal Water UsageGreywater Recycling/ Reuse

We need relatively clean water in our kitchens, bathrooms, wash basins and for washing our clothes. However, we do not need water of the same level of purity for flushing our toilets or watering our gardens.

Greywater recycling attempts to recycle relatively cleaner wastewater (usually after some basic treatment) for toilets, car cleaning and gardening, as shown in the schematic below.

Greywater Usage

How is Greywater Recycling Implemented?

  1. Input supply to taps needing relatively clean water (kitchen, water purifier, showers, wash basins, washing machines) is separated from the input water supply to taps that do not require very clean water (toilets).
  2. Drainage that collects ‘greywater’ is separated from the drainage that collects ‘blackwater’.
  3. The greywater is collected, treated, and stored. It is used supply water for rougher uses like toilets, etc.
  4. Blackwater is sent to the sewage system.

Impacts of Recycling Greywater

  1. Reduction of clean water requirement by 30-35% – reduction of water bill for the consumers as well as reduction of load on the city’s total water needs.
  2. Reduction of electricity, pipes, pumps, and storage requirements in clean water supply system.
  3. Reduction of load in the transportation, treatment and disposal of the sewage (blackwater).

Some Details of Greywater Treatment

There are many technologies and methods available to treat greywater; the choice depends on multiple factors like open space available, possible uses of greywater, estimated amount of greywater to be handled, etc. Listed below is a high-level set of steps that are involved in handling and recycling greywater.

  1. Collect greywater in a surge drum.
  2. Do basic filtration to remove large particulate matter.
  3. Reduce contaminants using biological treatment (like using water loving plants in Constructed Wetlands) or chemical/ mechanical processes.
  4. Store and distribute the treated greywater.

Our Cities Must Implement Greywater Processing

Every year we hear and read of water shortages in cities. Cities also use water from rivers and lakes, thereby reducing the availability of water for agriculture and residential uses in rural areas. Delivering water to homes in cities also involves large infrastructure and consumption of electricity.

Reducing the water requirement of cities by 30-35% is indeed worth pursuing.

Successful, institutionalized implementation will take many years – and we need research, guidance, equipment, awareness, expertise, laws, verification and compliance in this field – so another opportunity for start-ups (alas, this one is not based on ‘mobile apps’ :-)).

Here is a youtube video of a residence with greywater collection, treatment, and use on the rooftop in Bangalore:

Use the link https://youtu.be/3-VmVC0K8v8 if the video does not load.

Your comments are welcome!

More such posts are available here.


I'm Rajesh Naik, and this is my personal website If you are interested in contacting me professionally, I am available on LinkedIn and I will be glad to accept your invite. You may also check out my professional website, Alignmentor.com.

One thought on “Recycle Greywater to Reduce Urban Water Needs”

  1. Very nice, elegant and crisp diagrams. Reminds me of the books at school:)

    Water is a precious commodity and its scarcity is already being felt across cities and villages. The culture of saving water is yet to take root in our societal consciousness.

    We do some bit of grey water re-circulation. We reuse water from washing utensils for the garden. And from washing machines for flushing the toilets.

    Along with rain water harvesting, if grey water reuse is enabled in new buildings and townships( like Nikoo homes:), that would enable people to switch to water conservation practices. I feel the building codes need to be revised to make these practices and underlying plumbing and drainage systems mandatory for all new constructions. School education should involve site visits to inculcate thinking and consciousness building at an early age.

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