We pretty much finished building our worm septic system last Friday! It took us about two years to get a conditional permit to build this experimental system, detailed in our previous post. We started digging last April, and have been working on it (on and off, interspersed with other jobs) ever since. It won't go into service until we've plumbed the house, so then we will add worms. That's still months away, so I think it's time to share our build process.
I think it'll make most sense if I share our work by elements of the system rather than chronologically, since we were back and forth over the entire layout day by day and week by week.. so you may notice, for example, a photo of the unplastered tank after the section where I show us plastering...
The waste plumbing in our house comes to the outside through the front stem wall. You can see the white PVC pipe stubs in this photo, coming out at ground level between the two right porch posts, between the Home Depot bucket and the generator.
Installing the line from the house to the septic tank
Below is the stub coming out of the house looking out toward the eventual tank location, and then the septic trench with the stubbed out pipes coming through the wall. The larger lower pipe is for sewage (blackwater) that will dive down and out to our septic tank. The smaller higher pipe is our greywater pipe from the shower and bath that we will send to an infiltration bed in front of the porch. (We can also switch it inside so that shower and bath water also goes to the septic tank through the main sewer line.)
Here is the line heading out from the house toward the worm bin septic tank. We had Randy and Isaac out to do the earth moving for us, and then Chip plumbed this run. This sequence shows the line marked, dug, laid out, glued, and installed at 1/8" to 1/4"+ drop per foot. Randy dialed in the digging slopes, and Chip kept the pipes spot-on.
Placing the vermifilter septic tank
The septic tank is a recycled IBC tote with a 6" opening at the top (sewage in from the house) and a 2" bulkhead fitting at the bottom (effluent out toward the drainfield). Randy calculated the depth needed to get our lines in and out of the tank, all with the appropriate slopes, including the ~3-foot drop within the tank itself. Then he dug the pit to accommodate the tank and housing with a level floor, and the top edge following the contours of the terrain.
The last two pictures above show from the tank back toward the house, and then from the opposite direction the line out toward the drainfield.
Plumbing the vermifilter septic tank
Our septic tank is constructed pretty much according to Wendy Howard's fantastic open-source plans found at vermicompostingtoilets.net. We made a perforated drain pipe to fit into the outlet hole of the tank. We cut an access hatch into the top of the IBC tote, being careful to leave the intake cap intact. Once we had the pipe installed, we realized that it needed more drainage openings, so Chip drilled those in place.
We did make a few adjustments to Wendy Howard's plans for our situation. We needed to plumb in a test port for our quarterly water quality tests, and we modified the vertical drain pipe. While the open-source plans favor maximum drainage, we adjusted to give us a good margin for best water quality. Our final approval will depend on having good test results. We think it will be easy to meet the conventional septic requirements. We would love it, thought, if we knock it out of the ballpark!
To that end, most of our drain holes in the vertical pipe are lower to capture the more thoroughly processed water, and our higher holes are placed at the side of the pipe opposite from where the sewage enters the tank to minimize contamination. We did also drill holes at the very top all the way around, to handle an emergency inundation. Those holes are well above the filter, and higher than we ever expect the water to be.
Once the pipe was in, we covered it all with fiberglass screen to prevent larger pieces from going into the perforation holes.
Filling the tank
At the bottom of the tank is a layer of gravel for free-flowing drainage, which Chip covered with a layer of fiberglass screen to keep sediment out of the gravel. Then we filled the tank to the 2/3 mark with wood chips (larger chunks toward the bottom, and finer toward the top). When the tank is active, the worms will consume the sewage solids and will also chew through the wood chips creating a network of pores that retain water. The worms also filter water through their skin, and they come with a whole biome of microbes that eat pathogens, cleaning the water that is held within the matrix.
Making the water quality test port
For our experimental permit we were required to add a test port for quarterly testing of effluent quality. We are required to meet the same level of effluent cleanliness expected from a conventional septic system ("primary treatment"). Research on this type of vermifilter shows that these systems can also meet secondary treatment standards, meaning biological treatment and cleaner water. Research also shows that almost all pathogens are removed ("tertiary treatment" or "disinfection"). We only need primary treatment, but if we could get to secondary or tertiary treatment, it would be a great argument for pushing this technology toward wider regulatory acceptance.
We plumbed our outlet with a three-way Jandy diverter valve. It will normally be set to flow straight out to the drainfield. We can switch it, though, to divert the effluent at a right angle to an outlet that will let us collect effluent for testing. Chip built a housing around the outlet and test port. The last picture in this set is taken looking straight down into the housing. the Jandy valve is the black handle at the bottom right of the photo, and the test port points up in the picture with the blue handle, and a plastic box to catch any overflow from the test bottle. The circle at the bottom center is for a cleanout at the start of the line toward the drainfield (before we added the actual cleanout fitting and cap). Chip later covered the test port housing with a trap door with a piano hinge.
Insulating and housing the tank
Chip built a small housing out of OSB sheets to enclose the tank before we built a shed around it. Before encasing the tank we put a layer of recycled insulation around the whole thing -- foam sheets that had protected our weekly veggies from Misfits Market during the pandemic.
With the initial layer of insulation and protection in place, I started building our earthbag shed. More than two feet below grade we used solid packed tubes filled with earth, stepping back like a retaining wall.
The photos below show the steps for laying earth bags. We originally bought our big roll of tube netting from Volm Companies in Antigo, Wisconsin for our house walls (on the sides above the tire bales), described in an earlier blog post. I sewed the ends of the bags on a machine, gathered a section of tube onto a bottomless five-gallon bucket, and shoveled dirt into the bucket, feeding the tube out bit by bit for one row of the wall. Then I used a tie wrap to cinch the end shut, and tucked the loose end under the bag. I staggered the row ends like you do laying bricks, so the ends of the rows interlock with each other.
In the back the hole was three or four feet deep, so I used hard packed dirt in the bags as a retaining wall. I pounded these down with a steel tamper. The hole was excavated perfectly with just enough room for the bag width at the bottom, and tapering away from the tank as it went up. I laid the bags in tight against the dirt so at the top there is a gap between the bags and the OSB housing. Once the bags were within two feet of grade I switched from dirt fill to scoria fill, for better insulation. Toward the top I built the bags around the septic line that entered from the back.
Chip and I decided that we need one trap door to service the main tank access port (to check on the worms and drainage, and to add straw or wood chips as needed), and a second trap door to access the test port. We chose to angle these two roofs in different directions to follow the grade contours. To step down the earthbags, I sewed a vertical gusset toward the end before sewing the end shut. Earthbags have to be compressed as much as possible. If you put in less filling to a full width tube, it will just compress to a wider width. The gusset created a smaller volume which let me create tapered bag ends to get the roof angle right.
For vents, I used 4" PVC pipe covered with screen (to discourage insects) and hardware cloth (to discourage rodents and provide friction with the earthbags to hold them in place). I put one low vent in front and one high vent in back.
Chip anchored the earthbag shed by sharpening rebar to a point and pounding it down through the bags into the earth, after drilling a hole in the wood bond beams.
Chip and I got to try out our three-jet wall gun from mortarsprayer.com to cover the earthbags with lime plaster. The earthbag tubing material is not UV-stable and needs to be covered wherever it will be exposed to sunlight. We sprayed as far down as we could inside and out.
Fitting the two-directions roof to our wonky earthbags sunk into the natural contours of the surrounding hill involved cobbling together scraps of wood from our lumber pile to build a bond beam that was level side to side and sloped down in the desired direction. Once the frame was complete I attached the fixed portion of each roof, and built the two trap-door frames for the test port and tank access.
The worms prefer temperatures between 55 and 80 degrees. Our outdoor temperatures range from below zero to the high 90s. Worm farmer Sam McCarthy raises worms in a similar climate in Santa Fe, and wrote this article about keeping worms alive and thriving through the winter. According to Sam, our bin itself is big enough to provide appropriate temperatures within the worm bed as long as we keep it fed and watered, and topped off with bedding (straw or wood chips). Since we will also have water running through our bin regularly, that should help keep it cooler in midsummer and warmer in winter.
We went the extra step of building the earth sheltered insulated earthbag shed, as Wendy Howard recommends. To top it all off, we filled the inner cavities with insulation. Here we are peeking into the tank access trap door. In the back where the tank is sealed, I put plastic bags filled with recycled Misfits Market insulation batts, tucked in and overlapping to fill the whole area, while leaving the upper vent clear. In the front, covering the tank opening, I sewed a big burlap bag and stuffed it with straw. My intention was to provide some insulation while still allowing air circulation into the tank. I also lined the roof and trap doors with rigid insulation.
With the roof on before adding any insulation, our temperature monitors showed the temperatures inside the housing as even more extreme than our outdoor diurnal swings, as you can see in the above graph. Once we insulated, the blue line on the graph shows the temperature within the bin in a much narrower range than the yellow outdoor temperature. Isn't it beautiful what some thermal mass and insulation can do? We look forward to seeing how the winter temperatures hold up. Also, our temperature monitor is on top of the tank, under the insulation. The temperature inside the worm bed should be even more moderate.
Building the drainfield
We were required to build a drainfield for our experimental septic system, but were allowed to set this up with some experimental features as well. It is sort of a hybrid between the vermicompostingtoilets.net design and a conventional septic trench. It is sized according to New Mexico requirements for a one-bedroom house: 44 feet long, 3 feet wide, and 3 feet deep, with a minimum of 6" of gravel at the base. I laid out an outline on contour. Randy and Isaac did the earth moving for us.
Once the gravel was in, we were allowed to use organic materials as the rest of the required "aggregate." We put larger sticks and branches from our land at the bottom. Then Chip and I hauled a few truckloads of chipped wood from the landfill for the bulk of the bed. We ran some of that wood through our chipper shredder to have a finer layer toward the top. This deep bed of organic materials should become populated with the beneficial microorganisms from the septic effluent, and we will plant wildlife shrubs and pollinator flowers in this area.
Randy dug the pipe trenches from the worm bin to the drainfield, again sloping them perfectly dropping 1/4" per foot for the long runs. Since the effluent should be clear, we were allowed to use 2" pipe which is another departure from the 4" code requirement. The drainfield plumbing was another melding of the vermicompostingtoilets.net design; this time Art Ludwig's Create an Oasis with Greywater book series was my other main reference. I used a binary branched drain system where every split is level to divide the flow in half.
I split the main flow into halves, and the pipes therefore enter the trench in two places. Once the pipes enter the drainfield there is another split with one fourth of the flow going in each direction.
The photo below shows what happens to one fourth of the total septic effluent, entering from the right in the lower pipe. This then splits with half of this water (or 1/8 of the total effluent) dropping into a greywater Oasis-style mulch outlet within a mulch shield (a perforated five-gallon bucket with lid). Since our water should be clearer than greywater, I routed the other half of the water through Vermicomposting Toilet-style perforated pipe at the top of the picture. There is another split so that 1/16 of the total effluent goes into each of the top pipes, which are perforated along the bottom (not visible in this picture). The setup in the photo is repeated four times along the trench.
The pipes will all be buried, but the mulch shield will be accessible from above so we can see how it works. We did get a small septic scope that we can run through the pipe to see how the perforated pipe is performing.
NM code requires 2" of filtering material over the pipes. We used finely chipped wood and straw. We got large stainless steel dog bowls from Tractor Supply to cover each mulch shield, and topped those with octagonal saltillo tiles. This photo shows the trench ready for its final 6" of dirt:
The finished system
So last Friday we wrapped up this job by having Steve come out to finish filling everything back in with his tractor. Here's the tour out, from the house cleanout looking out toward the tank, the finished tank, looking from the tank toward the drainfield, and the finished drainfield. Oh, we do still have to cut the cleanouts to ground level, add the cleanout plugs, and cover them (likely with more dog dishes and saltillo tiles).
And here's looking back from the drainfield, to the tank and toward the house.
...Now the worms are waiting for us
to plumb the house...