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Question about my greywater system  RSS feed

 
Michael Kalbow
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Howdy,

   We are building a house, and I've already had the plumbing rough in done.  The greywater and the blackwater are plumbed seperately.  I'm planning on using a gravel filtration system to clean the greywater before it passes into some swales and eventually into a stock pond, assuming it doesn't get absorbed prior to reaching the pond.  My question is this, does the species of rocks used as gravel matter?  Would I see a benefit in using lava rocks for instance?  Or should I go with expanded shale?  Or pea gravel?  Does it matter?  I'll be filtering through a 275 gallon IBC tote with baffles, forcing the water to travel the height of the tote through the gravel twice before being discharged.
 
Peter VanDerWal
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Location: Southern Arizona
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For what it's worth, it's not the gravel that does the filtering, but the bacteria that grow on it.  The type of gravel isn't critical, I'd just go with whatever is cheapest.

You might look into a "constructed wetland" instead.  These are generally sized to be large enough so that it takes 5 days or so for the water to flow through it.  The main difference between a constructed wet land and you idea is that it's open on the top with plants in it to increase the bio filtering.  This wetland could be located right next to your pond with typical bog plants in it like cattails or bulrush, etc.
 
Michael Kalbow
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I like the idea of a constructed wetland, but how do you size one to be large enough so that it takes 5 days for the water to flow through it?  BTW, I was going to be putting reeds or cattails or the like into the ibc tote.
 
P Lyons
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Location: Ontario, Canada
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In order to calculate the Volume of the wetland:

Using the 5 day recommendation: Required Volume of wetland = 5 days X Volume of Greywater generated per day.

Assuming you will generate 100 L/day of greywater, the required volume would be 100L/day X 5 Days = 500 L.

Technically, in order to achieve 5 days of retention within the wetland you would need to factor in the volume occupied by the gravel, which would result in a design volume of 2 to 3 times larger, depending on the characteristics.  For your application with a final discharge is to a swale system, I think this would result in something unnecessarily large.

A few considerations if you choose to incorporate the constructed wetland as part of your system:

If kitchen wastewater will be part of your greywater, I would recommend using a woodchip filter prior to the wetland to remove any food particles, oils, greases etc. so you don't end up plugging the system.

protect the inlet and outlet piping and ideally allow for inspection.  Using milk crates or upside down buckets with holes drilled in them  surrounded by larger rocks than your gravel material will protect these critical areas from clogging and becoming matted with roots as well as facilitating inspection and maintenance.

For the gravel material, if you have pea gravel,  that is what is typically recommended, but if you have something else available you can use it, best to wash it well prior to use.

The typical recommended depth of a constructed wetland is usually around 0.5 m, so maybe you could use half of your IBC as the wetland reservoir, but any other water tight vessel or liner could also be used.
 
Michael Kalbow
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Thank you P.  We are planning on using a wood chip filter prior to the water entering the gravel filtration, and setting it up so that it will be easy to change the wood chips.  I like the idea of protecting in inlet and outlets of the water, and plan on doing so.  As for the gravel, I don't have any, so I will have to buy it and bring it in.  My thought was that the porous lava rock that is sometimes used in gas bbq grills has a tremendous surface area for bacteria to form on, so I was thinking that it might be better then the pea gravel. 
 
Peter VanDerWal
Posts: 120
Location: Southern Arizona
12
bee bike fish greening the desert solar woodworking
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Michael Kalbow wrote:My thought was that the porous lava rock that is sometimes used in gas bbq grills has a tremendous surface area for bacteria to form on, so I was thinking that it might be better then the pea gravel. 


If you were making a bio-filter for a pound water, etc. it might be helpful.  The problem is that more bacteria require more oxygen, in a wetland the plants help get a little oxygen down to their roots, but not a lot.  f you build it with a 4-5 day retention period, I don't think you'll be able to get enough oxygen into the water (short of running bubblers) to support all the additional bacteria so you wouldn't really benefit from the extra surface area.

If you can get the lava rock really cheap, then go for it, but otherwise just go with whatever's cheapest. Around here lava rock costs 5-10x as much as pea gravel, etc.
 
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