John, thanks for your questions! We won't build until Spring, so I've only purchased the four used IBCs (2 extra) for $165 each, and the two 18in HDPE culverts 20ft long for $365 each. I probably have all the 55gal barrels I need for recirculation. The structure it'll be in will add cost, but we'll build it ourselves. I'm hoping to 3D print drainage cells to reduce cost and make them closer to a pipe in shape as I don't need the ~1ft width of standard drainage cells.
Yes, surface discharge as greywater is allowed (reasonable restrictions). When subsurface discharge is used, the goal is to subirrigate roots (reuse instead of disposal), so it would be nice if sub-irrigation pipes could be installed shallower than in a leach field. To help establish trees, shrubs, and other plants along the lateral (e.g. a hedgerow), pairing surface water (e.g. swale) with subsurface irrigation would help. Swales would increase how wet the soil can be around a subsurface pipe when freezing occurs, increasing the risk of cracking shallow pipes, a risk at least for PVC pipes. I think HDPE is more flexible than PVC, but I would assume drainage cells would be less prone to cracking than HDPE perforated pipes, but I don't know.
My understanding is that gravel mixed with dirt (either directly or from root dieback in the long run) creates an impermeable layer, hence French Drains put geo-fabric outside the gravel surrounding the pipe for more longevity. As far as clogging the geo-fabric wrapped around the drainage cell (called a Flo-Log), I'm guessing the risk factors would be similar to what causes leach fields to clog... BODs (biological oxygen demand), FOG (fats, oils and grease), suspended solids, and localized distribution. Recirculating vermifilters are great at reducing the first 3 risks, and I hope to use dosing to help with the fourth. I'm guessing that having something like 90% void space of a Flo-Log would be less likely to clog than the 20% void space of aggregate and perforated piping. Ideal would be to use open ended pipes, but I haven't found a cost effective way to distribute effluent over a large enough area to maximize reuse.
How big are the holes in HDPE perforated piping, and what are the hole positions across the circumference? I think you'd want holes at the 6 o'clock position to ensure complete drainage and the 12 o'clock position to reduce the risk of sweating, minimizing risk of root intrusion. I hear PVC becomes structurally weak with more than 3 holes across the circumference, and perhaps the closest you could buy commercially is the version with 3 evenly spaced circumference holes installed upside down (holes at 2, 6, and 10 o'clock), which might be good enough. I hear geo-fabric will dissuade roots as long as there is no standing water on the other side, but I suspect you could wrap perforated piping as well as you could a drainage cell. If you could get perforated piping to fully drain, prevent sweating, and not crack in freezing soils, then it may help the effluent spread farther, maximizing reuse, which would be really nice.
Too many users, undersizing the primary bins, media that breaks down slowly, colder temperatures, and use of toilet paper increase the risk that you'd need to pull off worm castings. Media that breaks down quickly (e.g. wood chips) create humus faster, increasing the risk of clogging and sending more suspended solids downstream. If I can get each IBC to at least take several years to fill before switching to the redundant one (to allow sufficient time for roundworm eggs to die off before removal of worm castings and humus), I'd prefer to use pine bark chips that break down very slowly.
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