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Grey Water Clean-up - Mulch Filter

Updated on September 18, 2016

Revision of Current Experiment

In my other Hubs, concerning the use of Azolla filiculoides for the final "polishing" of grey water effluent, the process of initial filtration was described. (Further information about Azolla sp. may be obtained from the website of "The Azolla Foundation.")

This involves passing the raw grey water vertically downward, first through a layer of mulch, then through a deeper bed of gravel.

Within the mulch there is a diverse biological population of earth worms (predominantly Eisenia foetida), plus innumerable microscopic organisms. This population works its way through the mulch and any organic solids arriving within the raw grey water. It is able to deal with food waste, fats and grease (NOT those derived from fossil fuels), skin and hair particles, bacterial pathogens.

The gravel used below the mulch must be sufficiently aerobic. The particles must be large enough and irregular in shape, so that air pockets exist between the particles. If sand is used, this can exclude air to a large extent and water can ultimately not pass through easily. The result is anaerobic conditions, which we do not want. Blockages and offensive odours will result.

Mulch filter in upper drum..  Gravel in lower drum.
Mulch filter in upper drum.. Gravel in lower drum.

What happens in these two filter layers?


The organisms working away within the mulch are simply doing what they do in the natural environment. If the mulch is obtained from the local forest floor, or under a hedge, or from a hay field, the micro-organisms present in those natural habitats will be quite happy in the mulch filter. Thus this technology can be adapted for any locality in the world.

(I put this to anyone who is interested, please experiment in your area. If what I have said turns out not to be true, please write and tell us. Such information will be invaluable.)

The materials used as mulch will gradually be broken down by the diverse biota into humus. "Humus" is considered, as I understand it, to be basically the dead bodies of billions of bacteria, which in their lives took in nutrients broken down from the food waste, etc. which arrived with the grey water. Those dead bodies conglomerate into a Gel. This Gel is the Humus. It has the beautiful property of being a natural slow-release fertilizer.

(18 Nov 2013 - I have a new theory regarding this decomposition of the organic material. I suggest that some of the nutrients/elements become soluble in water and pass on to the Azolla pond, providing better nutrition for the Azolla. I have no way of establishing this, but welcome any feed back from the scientific community. )

Ultimately, (I have found over 9-12 months), the mulch is almost entirely broken down into humus, and does not allow fast-enough passage of water through. Passage is so slow that I get a back-up of grey water onto the surface of the mulch. Overflow can occur.

(One might be fearful of the worms being drowned by such a situation. However, I have found the worms multiplying enormously in that environment, provided the water does ultimately drain through.)

Therefore one needs to remove a quantity of the composted material to leave room for addition of new mulch. I have just done this after having the filter in operation for approximately one year. A small quantity of the old compost was left in the drum, complete with the attendant worms. The worms will rapidly colonize the new mulch.


This part of the filter is constructed so as to be aerobic, in other words, the water pulls air through the gravel as it falls. A tiny layer of water clings to each gravel particle and in this layer there forms another population of bacteria and microbes, which continue to remove pathogens from the water.

The mulch above is effectively an ecosystem. The gravel is another ecosystem. Each area of the complete filter performs a natural process, without any other assistance from us, other than providing the ideal conditions.

I have no expertise in microbiology, and remain open to others wishing to contribute here. It is my guess that potential pathogens will be destroyed within these two filter layers. Further research would be most valuable, to determine what depth of filtration, how slowly the water must traverse and the mortality rate of pathogens in such an environment.

Please check back to my other two Hubs regarding the use of Azolla. I have included a lab report, which shows E.coli are still present in the final effluent. Therefore the final effluent must be regarded as potentially unsafe (if the grey water is derived from the bathroom, laundry or shower) and should be either sanitized (with UV light or some other means) OR discharged into the topsoil, according to local recommendations. As a purely kitchen waste water disposal, i.e., simply as a grease trap, such sophistication would not be necessary, in my opinion.

What Materials Do You Have Available?

For the Mulch Filter

Which food crops are grown in your area?

Rice? Do you have a big stack of rice straw, close to your house, for feeding your goats? And around the rice stack, there is straw that gets trodden into the ground, it gets very dirty, the goats will not eat it? This has begun to partly decompose on the ground...... you could collect it up and use it as your Mulch Filter. Over time it will become good compost for your garden.

Bananas? Those dried, dead leaves on the ground below the banana plants could be broken up small, mixed with any other weeds or leaves you can find. It needs to be brown and dry when you first use it. Collect up enough to provide you with the Mulch Filter.

Pine trees? Collect up the pine needles from under the tree. This will make a good filter medium.

Sweet Corn (Maize) Sorghum or Millet? Find out which part of the plant is usually discarded as waste, and try using that as mulch.

In the desert? Maybe in Rajasthan? No Trees!!! No Leaves!! I suspect you already have an efficient way of dealing with your washing water, because it is very precious and you are very careful with it. If you still need a way to deal with dirty water, please look around you and find a solution.... I would most interested in hearing how you get on.

For the Gravel
The size of the particles is important. Too small and it will block up. Too big and the water will rush through too fast and not give the bacteria time to do their work. 7mm average size of particle is ideal.

Practical Considerations

It is desirable that as the raw grey water reaches the surface of the mulch filter, it is spread all over the surface instead of pooling in one spot.

Simplicity is the key

I imagine that in a family, or small community situation, where the women folk will simply pour their bowl of dirty water onto the top of the mulch filter, they can just spread the water over the filter by hand. No fancy technology is involved. They will soon get the hang of it.

You could obtain a domed plate of some kind, place it on the surface of the mulch so that water coming from the pipe splashes outwards and is quite simply distributed. Please see photograph.

This old hub-cap (nothing to do with HubPages!) is used to distribute water over the surface of mulch.
This old hub-cap (nothing to do with HubPages!) is used to distribute water over the surface of mulch.

Hoping for Feed Back

It would be much more informative to have some laboratory tests done on my filter system.

1. After the grey water has passed through the mulch, then down through the gravel, I would like to know the bacterial/viral load, the dissolved oxygen, suspended solids, nitrogen level, etc. at this point.

2. Presuming that a slime layer builds up on each particle of gravel, I would like to know if this is true. Does that slime layer really reduce pathogen levels?

3. The water then passes upwards through a 300mm layer of crushed glass. This is designed to be anaerobic for de-nitrification to take place. Does it? Is the water passing through this area slowly enough?

4. What are the dissolved nutrient levels in the final effluent? Is this water valuable for watering plants?

5. Is the final effluent safe? What about viral die-back? Are there any viruses that could get past the relatively short path through this system.

6. Research is also going on in other parts of the world, in the use of Azolla filiculoides. (Please see my other Hub, for details.) How effective is this plant in "cleaning up" the water?

If anyone reading through my Hub is able to give useful feedback, I would be most grateful.

© 2013 Alan


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    • jonnycomelately profile imageAUTHOR


      3 years ago from Tasmania

      Thank you Shawn, for that suggestion.

      As I understand it, such a slow sand filter would have to be of a sufficient depth, and experimented with, providing several laboratory analyses over a few months, in order to be sure it was performing as intended.

      Currently I discharge it into top soil as described in my hub. Also, being in the cool temperate climate of Tasmania, I don't expect there to be any pathogens in my effluent. If this were set up in a warmer climate, where enteric and other pathogens are more common, then one would need to be more careful.

      Do you have any first-hand technical or professional background? Again, thank you for your interest.

    • profile image

      Shawn Richardson 

      3 years ago

      If you want to get rid of those coliforms, run the output of the gravel filter through a slow sand filter.

    • jonnycomelately profile imageAUTHOR


      4 years ago from Tasmania

      Greetings, SaveTheSoil. I will look into that.

      My intentions are along the lines of Permacultue. In other words, not building up another set of rules to be ridgedly followed, but encouraging an attitude of thinking, a "culture" that guides our thinking. A rule constricts, whereas a guide opens us up to lateral - and deeper - ideas.

      Please let us know how you get on.

    • profile image


      4 years ago


      I just came across your hub after searching for a greywater mulch filter. I really like the simplicity of your system. I may implement it in my greywater system. I just wanted to let you know that there is a new book out called" the wastewater gardener" that was written by the person who was in charge of the Biosphere 2 wastewater treatment. He speaks a lot about the issues and concerns that you have brought up here.

    • Nadine May profile image

      Nadine May 

      5 years ago from Cape Town, Western Cape, South Africa

      Thank you Jonny, I will have a look at that website.

    • jonnycomelately profile imageAUTHOR


      5 years ago from Tasmania

      Thanks for visiting, Nadine. I hope it proves useful for you in practice.

      I am currently in South India learning lots more about Azolla. There is scientific data becoming available about it's chemical and biological make-up and uses in livestock supplimentary feed. Much of this is beyond my background knowledge but can be further researched via if you are interested.

      My experiments are just that, although with considerable success so far. With some innovative "tweeking" it should be possible for others to take it beyond the one-person household set-up to one that will serve one or several families.

      Please let us know of your efforts - successful or otherwise, especially the latter which we can all learn from.

    • Nadine May profile image

      Nadine May 

      5 years ago from Cape Town, Western Cape, South Africa

      Wow this is a very informative hub on a grey water system. I published a page on Wikinut on the same subject titled: Our dream is to be totally 'off the grid' Many thanks for all your valuable information.

    • pramodgokhale profile image


      5 years ago from Pune( India)


      i want to know your other activities regarding recycling. If you can send on my email


    • jonnycomelately profile imageAUTHOR


      5 years ago from Tasmania

      Cooperation and mutual understanding I think are the key points. Learning to listen and share each others points of view is sometimes difficult and needs practice to become reality.

      My personal vision, which does change from day to day but has a consistency about it, is that we need all manner of individuals, both professionals, scientists, technologists, and "hands-on" people doing the work, to act in unison for better outcomes.

      All "waste" streams, which might now be seen as problematic, need to be researched and worked in together, producing new materials and resources. These processes will employ the minds of many, many people, inventively, intelligently. There might be many failures and mistakes made along the way, but making these known and the results shared is a major part of learning. In that way we can advance and provide better choices for later generations.

      I believe that India, in particular, will be very much in the forefront of this progress. It has the talents and the needs.

    • pramodgokhale profile image


      5 years ago from Pune( India)


      well if you visit India then it will boost recycling processes and rural India certainly welcome you.

      This is real international cooperation and understanding.


    • jonnycomelately profile imageAUTHOR


      5 years ago from Tasmania

      I am planning to visit South India very shortly, to study Azolla applications further.

    • pramodgokhale profile image


      5 years ago from Pune( India)


      great ,wonderful. We Indians always try to make frugal methods and achieved and you also innovated such processes.

      I am retired blue collar worker-technician and liked such innovation , want to know more. Blue collar worker is a tribe of the past and knowledge based technocrats are emerging and i think it is necessary to make room for them.

      We need to cope with new trends and cost effective solutions . All these efforts will make atmosphere clean and more and more Eco-friendly

      ideas comes out.

      Thank you.


    • jonnycomelately profile imageAUTHOR


      5 years ago from Tasmania

      Thank you Silva. That is most encouraging. I wish you well and would love to hear how you get on with it.

      Please check back into this hub... I have added a paragraph about the gravel part of the filter. A lab test showed some interesting results.

    • Silva Hayes profile image

      Silva Hayes 

      5 years ago from Spicewood, Texas

      I don't have anything to add to this but I have to say, I LOVE this hub. It makes the process so easy to understand. I am going to save this and pass it on. Thank you. Perhaps I may attempt to rig up something similar.

    • jonnycomelately profile imageAUTHOR


      5 years ago from Tasmania

      Thanks for visiting and your interest, Moonlake.

      Just a brief explanation if you like: Grey water is the waste water that comes from the shower, laundry and kitchen. It contains particles of food waste, lint, skin, hair, and some bacteria, etc. If you were to store that grey water for more than 24 hours, it would become smelly and septic. Some people pass water from the kitchen sink through a "grease trap," but the conventional type can become extremely smelly after a few weeks and does not filter out the particles very well anyway.

      I have developed the "mulch" filter that you have read about in my Hub, which uses natural biota (worms, etc.) to devour most of those particles and thereby remove them from the grey water.

      It's really mimicking what happens on the surface of the soil in a forest.... copying nature to good effect.

      I hope that gives you a better idea.

    • moonlake profile image


      5 years ago from America

      I wish I understood all of this but I don't. I do understand worms and mulch. We put worms in compost here. I do understand we need a reservoir of alternative methods.

      Your hub was interesting. I voted up and shared.

    • jonnycomelately profile imageAUTHOR


      5 years ago from Tasmania


      Each and every one of those worms has a name..... but they do not infect one's!

      Thanks for your interest and input.

    • tobusiness profile image

      Jo Alexis-Hagues 

      5 years ago from Lincolnshire, U.K

      Fascinating, but way beyond me, maybe I can get my old man to read this. Afraid you got me running for cover at the first mention of worms :). Brilliant and informative.

    • jonnycomelately profile imageAUTHOR


      5 years ago from Tasmania

      Thank you!!! Green but not Left, lol.

      I love to tinker like this, especially when it suits my lifestyle living in a bush setting.

      Without wishing to be "doomsday-ish," I feel we need to explore "other" ways of doing things so that, where we depend upon advanced technology now, if circumstances become very difficult sometime in the future, we have a reservoir of alternative methods.

    • Brisbanelocksmith profile image

      John Magee 

      5 years ago from Brisbane, Qld, Australia

      Great info. The bike wheel turbine just makes a grey topic a bit more fun! I am starting to think you are more green then the greenies in Tasie :)

    • profile image

      Akhilesh kumar 

      5 years ago

      really very innovative and informative experiment , i feel this is very useful for gray water treatment, i am sure this type of low cost technology bring revolution water filtration...


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