Project #2 - Subsurface Flow Wetland for Household Greywater Treatment

       I've begun designing a constructed wetland to treat kitchen sink and laundry greywater from the house I'm currently living at.  The system will reduce BOD, nitrogen and phosphorous levels from the influent greywater before it's released to the environment, where it will be used for garden irrigation.  It should be a fairly straight-forward project, as lots of documentation and guidelines for similar projects are available online, and the construction looks fairly simple.


      A popular source of diy greywater information is Art Ludwig's book Create an Oasis with Greywater, and his website, Oasis Design.  If you're looking for background information on Greywater, his website looks like a good starting point.  Another is the well-named Greywater Guerrillas (Because greywater treatment is clearly pretty subversive stuff. I find the anti-establishment side of DIY culture kind of funny).  Both seem to advocate simple designs that route the greywater through drainage pipe laid in mulch basins adjacent to plants with high water uptake (mostly fruit trees).  While this looks to work quite well in mild climates, I expect the winter performance would suffer in areas where plants are dormant for much of the year.  I think a wetland system would be better suited to colder climates.


      Two types of constructed wetlands exist: surface flow and sub-surface flow.  By keeping the water surface below the soil/gravel surface, the subsurface design avoids odour and mosquito breeding issues that may arise with surface flow systems, while also minimizing potential for human contact with pathogens in the untreated water.  For these reasons, I'll be building a sub-surface flow (SF) wetland.  


The US EPA has made available a lot of information about wastewater treatment with constructed wetlands, including specific design and construction guidelines: 


Handbook of Constructed Wetlands [pdf]
Subsurface Flow Constructed Wetlands for Wastewater Treatment [pdf] 

The EPA SF design guideline models the system as a first-order plug flow reaction, and provides an empirically-determined rate constant.  Sizing a wetland  using these guidelines is a simple matter of entering the influent flow rate and BOD, desired effluent BOD, bed depth, media (gravel) porosity and water temperature into the supplied equation, and solving for the wetland's plan area.  The maximum length-width ratio is limited by the hydraulic capacity of the bed, and must be considered to prevent the water surface from rising above the media surface.

I've roughly calculated the wetland size for my application using published domestic greywater flow, BOD, N, P, K and TSS characteristics  listed in a few different studies I found online. To achieve 97% BOD reduction from kitchen sink greywater in winter (design temperature = 4 degrees C), it looks like my system will need to be about 2.5m long x 1.5m wide x .6m deep.  During summer (design temperature = 15 degrees C), the reaction rate is much higher and the same wetland should be able to handle the additional flow from a washing machine.

   I intend to build a rectangular wetland, as pictured below:

• Inlet and outlet structures will each be constructed of PVC or ABS drain pipe, connected to respective sumps. 
•  A height adjustable standpipe in the outlet sump will allow control of water depth, which is useful for conditioning root growth of wetland plants
• Liner will likely be PVC or EPDM
• Basin will be partly dug, and partly built from retaining wall 

    I'm presently doing some foundation waterproofing work on the house I'm living in and will soon be ordering drain rock.  When I do, I'll assess the porosity of the available material, come up with a final design for the wetland and order enough extra for the wetland project.


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