Our Backyard Constructed Wetland for Greywater Treatment

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As you’ve probably guessed from the other articles on this site, we try and live as sustainable a life as we can manage. However there is one area where our performance sucks pretty comprehensively and that is treatment/re-use of greywater. One of the issues has been that while our banana circle (a greywater recipient) was in the front yard, all our greywater plumbing was at the back of the house. For years I fantasised about complicated tank/pump/plumbing set ups until I realised that it was not going to happen, so with the help of friends, we relocated the banana circle to the back yard. We also corrected a plumbing problem with our spa so now all shower/bath water was directed into said banana circle. We were on our way!

It is possible to put raw greywater onto lawns and fruit trees and when we had our original top loading washing machine I had a hose on the greywater outlet and did just that. The amount of water we used to wash our clothes was considerable so we upgraded to a front loader. That meant that we used less water (good!) but also meant that the concentration of pollutants in the greywater now produced by clothes washing was increased (bad!) so I didn’t feel right about running it directly out to the yard. I needed some way to treat it first. Hence this article!

My original thought was to put in a system similar to the 3 tier bathtub set up described by Scott Kellogg and Stacy Pettigrew in their book “Toolbox for Sustainable City Living”, to save horizontal space. I had problems working out where it would go even so and I would be back to needing a pump and surge tanks and the frame would need to be well engineered to suit the weight. Fortunately, with the removal of the lemonade tree to accommodate the banana circle it seemed to open the area up. There was room for two bathtubs horizontally, both below the level of the laundry sink which would act as a surge tank and I could still use gravity to move the water around.

The bathtubs

I have had two bathtubs sitting on top of the chook retirement village for some years waiting for this moment! Once I was able to manhandle them down onto the ground, it was a case of arranging them in the space that I had to see how they fit best. There is another dug-in bath acting as a water garden in that area and the other two seemed to fit best when lined up parallel with the water garden but the lower on a bit off set from the higher one. I used silicone and bath plugs to permanently block the drain holes before starting work.

Roughly positioning the tubs

The weight of the bathtub, plus gravel, plus water, plus plants means that I needed to have a stable base that could support the weight. I had some solid besser blocks, right angle ones with one long and one short side, left over from an old incinerator. So I laid one with the short side down at each end of the bath, and used a brick to support the other side. I dug them in a little bit to ensure the tub was level but also to give a more stable base. In the end it was not high enough to allow drainage into the next bathtub so I put in one besser block capping (40mm thick) under the drain end and two under the input end, when I realised the bath had a bit of fall the wrong way!

Supporting the first tub

The other bath I put in by turning two of the blocks upside down and placing one under each end of the bath. This ensured that the second bath was lower so that they would drain naturally by gravity. To improve stability of the baths and to make small adjustments to the fall, I put some offcuts of 6mm fibre cement board between the bath and the blocks to act as shims. In the end it all looked pretty good, or at least I thought so!

Supporting the second tub

The Plumbing

I needed to set the water flow up so that there was maximum contact with the bacteria which would (hopefully) grow on the gravel and the plants and plant roots so I didn’t want to just dump the greywater on the top of the gravel and hope for the best! I wanted to run it so that it went in at the bottom of the batch, rose up through the gravel, then drained into the next batch and did the same thing before draining out of the second tub onto the trees.

To do what I wanted to do I got hold of three one metre lengths of 50mm PVC pipe and a series of 50mm pipe fittings which included –

  • 3 x 50mm PVC end caps
  • 3 x 90⁰ elbows
  • 1 x Expandable Connector Waste Pvc Abey 50mm Trap Flxz22up
  • 2 x Holman PVC 50mm floor flange
  • 2 x 50mm mozzie proof vent cowl
  • 2 x 50mm to 25mm barbed reducer
  • 1 x 50mm two way valve

One of the reducers and the valve were for use on the inside part of the system (more on that later).

As it turned out a one metre length of pipe fits into the bottom of a bathtub really well so that became the basis for my horizontal part of the plumbing, which sits in the bottom of the bath. The first part of the work was to drill a whole stack of holes the length of the pipe, to let the water flow out slowly into the tub. A good size hole is 12mm or so but it can be difficult to drill into the pipe with this size twist drill and there is a tendency for the twist drill to snap out bits from the side as well as drilling the desired hole. To reduce this I used a 3mm twist drill to drill pilot holes about every 100mm down the pipe, with 4 rows at 90⁰ to eachother.

With the holes in place in two of the one metre lengths I glued an end cap on one end and a 90⁰ elbow onto the other of each one, using the blue PVC plumber’s glue. I then cut the third one metre length of 50mm pipe in half and slid half into the other side of the 90⁰ elbow giving two L-shaped sections. The idea was that the pipe with the holes would sit along the bottom of the tub horizontally and water would flow in through the vertical section of pipe.

I put them in place to try them out and found out that the vertical section on the one in the second tub was too tall to allow water to flow by gravity so my intention was to cut it off level with the edge of the bath. After some thought however I worked out that the drain from the upper tub could be directed down into the horizontal tube just by using the PVC expandable waste connector and ensuring it sealed by applying silicon both ends. That did work out to be the best way to do it and gave me some spare pipe.

The was one more job before they were complete, and that was to put some fly screen in place over the holes in the horizontal pipe to keep rocks and plant roots from blocking the pipes and preventing greywater from flowing into the system. I got hold of some aluminium fly screen because it is stronger than the polyester stuff and wrapped one layer around the horizontal section of pipe with the holes in it. To keep it in place I took some of the spare 50mm pipe and cut 12mm thick rings from it, then made a cut in the side of each ring so they could be opened out. These rings were then put in place over the fly screen and around the tube to hold the fly screen in place.

The next thing to organise was the drain holes in the bathtubs. Yes, I know I blocked the standard bath drain hole, but that is because I need the greywater to slowly move up through the gravel and overflow in a controlled way into the next tub then through it and out to the garden. I needed to drill a 55mm hole near the top of the tub at the opposite end to where the water flows in to fit the outside diameter of the standard 50mm (nominal) pipe. I didn’t have 55mm hole saw that would cut through ceramic and metal (I tried to sort it out but failed, loooooooong story!) but I did have a 70mm one.

Cutting through the side of the bath was a pretty noisy and hard job. It requires you to keep pressure on and some water going over the cut for lubrication for up to 10 minutes. I was using a battery drill and it was about as happy with me by the end as the neighbours were with the noise. I would use a 240v drill next time I think. The hole needed to be such that the bottom was 50mm to 75mm below the estimated level of the gravel. This was to ensure that there would be no standing water at the surface of the gravel and so no issues with mozzies.

50mm floor flange

The hole I had cut was about 15mm too big( because of the larger hole saw) so I inserted a 50mm floor flange in the hole and siliconed it in (silicon is my friend, again!). This took up the difference in hole size and allowed me to provide a seal to prevent leakage. The 50mm pipe I was using to make the drain from slid in with little moving about and was also siliconed in place. To stop the drain getting clogged with gravel or whatever I glued on a 50mm mozzie proof vent cowl on the inside end of the pipe. The cowl uses strong stainless steel mesh and it resisted any funny business by the gravel quite well.

Attachment of the 50mm mozzie proof vent

With that in place I siliconed the outside end of the drain in the first bathtub to the expandable waste connector referenced above, expanded it down so it fitted into the top of the elbow bend in the second tub and also applied a bit of silicon. Now the two bathtubs were connected I needed to fit the drain to the second tub.

Expandable waste connector in place

That followed mostly the same process: cut out the hole, insert the floor flange, apply silicon, insert 100mm or so of 50mm pipe and glue on the 50mm mozzie proof vent cowl. To allow me to connect a 25mm hose I put on a 90⁰ elbow after inserting and gluing in the 50mm to 25mm reducer into it. The reducer has a barbed fitting so that the hose slips on and won’t fall off again.

System for the rear drain

In place

With all of the plumbing in place all that was needed was to fill both baths to the top with 20mm blue metal gravel. This is heavy stuff! I ordered half a cubic metre which filled both baths fully with enough left over to make a bathtub wicking bed. I had to shovel the gravel into a barrow then barrow it from the front yard to the back yard then, due to the position of the bathtubs, shovel it out of the barrow, into each tub. During this process two things occurred to me –

  1. It was hard work! (yes I know I am a genius!), and
  2. If I had set up the baths as I originally wanted to (3 of them, one above the other) I have no idea how I would have gotten the gravel into the middle bath, let alone the top one!)

With the gravel in place I ran some tank water into the system to check flow and, believe it or not, it worked perfectly!

Gravel in place

To finish this part of the system off I needed to put in plants, preferably from a functioning constructed wetland. Fortunately two of my friends have such a beast and I was able to score irises, taro and papyrus which were then dug into the gravel far enough for any part of the plant bearing roots to be submerged. Plants in a greywater system should not be used for food but they can be periodically trimmed and cut up to make mulch or compost. Where out baths are located also means that the foliage with shade some of the back wall of the house in summer, to reduce the solar heat gain of that part of the house.

Plants in Place

The Inside Bit

The last bit of engineering required was to put something in place to get the greywater from inside house to outside the house and into the system. The easiest way to do this was to just run a flexible 25mm hose from the discharge hose of either the washing machine or the dish washer, whichever was in use directly to the input pipe of the first bathtub. I didn’t want to do that for two reasons –

  1. It would be effectively increasing the length of the discharge hose which would have meant the waterpump in the appliance would have to work much harder, resulting in possible early failure of the part, and
  2. If the discharge from either of the appliance pumps was at a greater rate than the system could accept, it would overflow everywhere, defeating the purpose of the plumbing and reducing the effectiveness of the greywater system.

What I needed was an intermediate or surge tank, which would accept the discharge from the appliances, but then allow it to drain into the system via gravity. I worked out that I could do this using the laundry sink. I bought a 50mm two-way valve so that if the valve was in one position the greywater would go straight to waste, in the other position I could run it outside through a 25mm flexible tube into the greywater system.

Pre-valve

Valve set up

Post Valve

To fit the valve I cut the drain pipe leading out of the bottom of the sink about 220mm up from the floor using a crosscut wood saw (which made short work of it!). I unscrewed the S-bend and upper part of the pipe from the bottom of the sink and then cut off about 100mm from the free end of the pipe I had taken out. I then reassembled everything with the valve in place, just to make sure everything fitted where it should. It did! So I used the blue plumbers glue and put everything back in place and re-screwed the s-bend back onto the bottom of the sink. I also glued the other 50mm to 25mm barbed reducer into the horizontal outlet of the valve. I then left everything in place and did not use the sink for 24 hours to allow the glue to set.

The Testing

The next morning I attached some 25mm flexible tube onto the 25mm barbed fitting and ran it out the back door and into the input pipe of the first bathtub. Unfortunately the 25mm flexible tube is flexible (funny that!) so it goes well around corners but is not self-supporting, any greywater in it would cause it to sag dramatically. To get around this I ran it through a spare one metres section of 50mm pipe supported on a couple of buckets and it did the job fairly well. It is only temporary for testing purposes and I will need to develop something a little more robust for final installation. Anyway the test went perfectly, no leaks anywhere. I am somewhat impatient (alright I am very impatient!) so I put the washing machine on and allowed one load of wash water to go through the system and again it performed flawlessly.

Once the constructed wetland is put together you are supposed to leave it for 3 months to allow the plants and beneficial bacteria to establish themselves before running any greywater. After the test I diluted the greywater in the system with more tank water and it seems to be doing OK. Just after completion we had two freakishly (for spring at least) hot days, both over 37⁰C but the system seems to have handled it OK and we haven’t lost any plants. The secret is to keep an eye on them and top up with clean water periodically.

Another fun project is completed and once the establishment period has elapsed we will be using it full time.


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