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  • Writer's pictureSudheendra NK

Grey Water Treatment

Updated: Jan 29, 2023

Plants love organic matter in waste water and they can therefore treat grey water!

Bathroom -> Plants -> Well
Grey Water Treatment - from Bathrooms through Plants to Wells

Anyone from South Indian born in the previous century can relate to the above picture. This was an image of a typical house backyard, luckily even now, in some of the rural areas, this exists. This is a good example for a house level Grey Water Recharge system located in the large backyard of houses. Bathroom water and Kitchen Water is generally taken through culverts and let to circulate amongst Banana & Canna plants. There is also a Well located in close proximity of this setup.

Unknowingly or knowingly, we grew these specific plants which have an innate capacity to treat the greywater. They thrive in Grey Water! “Thrive” - because they absorb the organic nutrients from Greywater which is rich in either of these minerals - Nitrogen, Phosphorus, Potassium and of course Carbon, in various compositions. The result is that the Greywater gets largely treated and trickles through the sand layer below to become clean water which reaches the Aquifer below. The large green space along with the greywater is the reason why the wells never dried up even in summer.

Difficulties in an Urban Environment:

If this water cycle refilled our wells and kept a sustainable amount of water available, why did we abandon it slowly, at least in the cities? It is because of the combination of the following reasons :

  • On the one hand, land availability drastically reduced. Cementing of little available precious spaces made it impossible to achieve the proportion of plants and space required to treat the Grey Water discharge; which on the other hand became exponentially larger due to higher human density.

  • Wastewater let out rampantly started stagnating and became unmanageable

  • Mosquitoes bred in the stagnated water

  • Foul smell from the stagnant water became prevalent

  • During the rainy season, the condition even worsened when yet-to-be-treated water got mixed with rainwater and started moving into neighbourhoods

  • Water got recharged into the Aquifer before they were fully treated and hence the well water started getting contaminated with untreated organic waste

  • Therefore to overcome these problems, we replaced our traditional Greywater Treatment with Sewage Networks, and that sounded the death knell of our Aquifers - the cycle of water recharging the aquifer that feeds our wells with water is gone!

Resurrection of Greywater Treatment

Greywater Treatment is now a buzzword especially since rains have become extremely unpredictable. However, it is neither a fashion nor a fad. In a few countries, people have pioneered/demonstrated its success. So our traditional methods were good too!

If the Traditional system which worked for so many centuries had difficulties being incorporated in our cities, it is important to address them and find out methods to overcome the same. For someone who is interested to set up a greywater treatment system locally in their house, there are a few sets of things that would need to be taken care of:

  • Ensure Solid waste especially garbage doesn't enter the treatment area. Filter them at source - do separate out paper, plastic, glass etc. And if possible do compost of kitchen waste as well - can be really useful for our own garden.

  • Grease, Oil can be filtered by passing the greywater through a sand filter. This can also be done if the water is passed through a settling tank which would eliminate the grease and oil through floatation.

  • Have an anaerobic (absence of Air) treatment tank before the water enters the plant treatment area so that pre-treatment is done and most of the foul smell gets eliminated.

  • Household greywater flow does not get produced continuously throughout the day; on the contrary, morning bathing, cooking, washing clothes, add a shock load (more quantity in a very short interval) of Grey Water. We need to establish a buffer tank to moderate the water flowing through the plant treatment and avoid overloads.

  • Let Water not run on the surface - let the water flow through a sand medium into the plant roots a few centimetres below the ground level. Fill the sand top with Gravel, Stones to cover the plant bed. In this setup, no Mosquitoes can breed.

  • Ensure that the greywater stays in the treatment system for at least 24 hours before the water is taken out. This would give the plants time to ensure maximum treatment of Greywater before use.

  • Have at least 3 Sq.ft. of these plants for every 1 person regular Grey Water output (about 100 litres per day per person).

  • Ensure 20cm or higher bunds around the plant's bed so that rainwater overflow doesn't enter/exit the Greywater treatment area.

All the above list does make this look / sound complicated and hence easy to be given up on. But it need not be so; it can be done through the simple system explained below.

Easy Implementation at Homes

The following diagram shows a simple implementation that could be done at all our homes. The good news is that this can be implemented in our house setback areas with little plumbing and some supervision, and minimal budget.

Some Key Points:

  • Connect the greywater chamber output to a physical filter like a Gully/Trap which can easily separate out all the solid waste entering the system. Removing human hair from entering the system is very critical as it will choke the flow of water.

  • Let the water flow into the Settling/Sedimentation tank. The pipe is bent to let water below the scum layer where Grease, Oil and another organic matter float on surface forming a froth. Water entering this tank need to flow inside without disturbing the Scum layer.

  • Have an Anaerobic tank with Biomedia Enzymes - which is nothing but Bacteria; that can act on the organic waste in the water. A very simple implementation is to put a bundle of nets attached with cut waste bottles. The idea is to increase the surface area available for Bacteria to grow and come in contact with the greywater for effective treatment.

  • If you can build a Baffle tank (a tank with a wall built in the middle compartmentalizing it) which is in ~1:4 proportion then it can replace both the Sedimentation & Anaerobic Tanks.

  • The Anaerobic Tank and the plants supplement each other. Having the Anaerobic treatment for about 24 hours ensures breakdown of complex organic matters into simple nutrients which the plants can easily consume. Though the anaerobic tank is generally attached to a sewage treatment process, here it saves much space needed for the plants. Else we will need a much larger space for plants and a longer treatment which may not be practical in the house setback areas.

  • Plumbing lines run from the bottom of the tank to the top of the next one making sure that only water which has gone through the treatment enter the next treatment in line. Here water output from the Sedimentation Tank enters the Anaerobic Tank top which in turns lets another pipe from its bottom to the next tank.

  • As seen in the image this a continuous gravity-driven system. Once all the tanks get filled up, the rate of inflow is equal to the outflow of the system. So to avoid shock loading of the plants we need a Buffer tank before the treated water enters the plants.

  • Water from the Buffer Tank needs to be let out into the plant bed at a constant rate of about 40-50 litres per hour for our typical homes. Fix a regulator valve for this.

  • Build a shallow trench of about 30cm deep for the plants and have a bitumen/tarpaulin liner or any impervious material for the base of plant bed so that water does not go down to the soil before the treatment is completed.

  • Water output obtained at the end of the plants' bed can be recharged or even recycled if required.

  • Water output can be also tested occasionally because the entry and exit points of the treatment are controlled.

Concluding Remarks:

Let the complexity of the system not be a deterrent for Greywater treatment. One can start the treatment with the traditional method and then introduce additional checks & treatments over a period of time to keep improving or sustaining the system. Or in other words, one need not build the all-pucca-system right from day one. Continuous monitoring and improvement will make it much more robust over a period. The system may require further improvement, even beyond the above drawing, in the specific site condition and one needs to watch out for it. Neglect or no monitoring may even kill the system. That is perhaps the only disadvantage of this solution - it is NOT a fit-it-and-forget-it system. Like all good systems, this one needs a maintenance too - that of a human touch. On the other hand, with such continuous monitoring and improvements one can even go as far as treating black water with minor variations in the same system too!

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