Septic Tank Solutions and Cautions

ok, 60 year old septic design, It is useful to view the conventional septic system as a fluid compost (composting organic and inorganic materials) and water dispersal system that may contain human pathogens, is hidden from view, and difficult to service(other than pumping (empting) the tank).

It starts with 1.5 to 2 inch drainage pipe from sinks, 3 inch from toilets all with 2% slope to septic tank and ideally every p-trap(especially the toilet) with a nearby vent.

the septic tank is ideally two or more compartments typically constructed of acid resistant concrete, poly ethylene plastic or fiberglass, with at least 2 horizontal meters between drop inlet and outlet pipes, with a depth of over 1.4 meters (for storage of sludge and scum) and volume that yields a couple days of retention time (minimum, more is better) (typical for calculation is about 180 liters per person per day). lids at both the inlet and outlet.

discharge from tank in at least 2 inch pipe to a distribution box with equal flows going to each leach or weeping trench (note all trenches about the same length). clean gravel(no Fines)in trench of about 60 cm wide and 75 cm deep, about 6 to 12 meters long per person, based on perk tests. 30 cm gravel then 7.5cm leach pipe surrounded by gravel, topped with a soil barrier of small grain straw or uncoated paper and 30 cm of backfill soil(mounded to allow for settling, do not compact). each trench level bottomed and on slope contour (note each trench level bottomed but there is no need to make the bottom of all the trenches the same elevation).

So, can you violate some of the above and still get adequate performance? of course, just like Russian roulette. but similarly you get a mess when it doesn’t work.

When I noticed that buildings in cr had no roof vents, I asked how they flash roof penetrations and got either the doe in the headlight look or ‘use silicone seal caulk’, so I vented the disposal pipe after the last fixture which was about 3 meters before the inlet of the tank, topped it with an activated charcoal filter and tucked it under the eave, near the beginning of the run i installed a vacuum relief valve, the kitchen sink drainage is before the either the vac relief or the vent and has slow drainage, otherwise the system functions as it should.

so what are some common avoidable mistakes here?

drain lines not strong enough, use only sch 40 or sdr 17 or heavier pipe, clean inside pipe burrs, use pvc primer or sand joints before applying pvc glue. if you need to compromise 2% rule to 1%, use one size larger pipe. use long sweeps, 45′s and ‘y’ junctions not hard 90′s. flow wise plastic pipe is more forgiving than the case uron, clay , cement or steel pipe that was in use when many of the codes and practices were developed.

Septic tanks, there was a Canadian building and delivering precast concrete septic tanks in 2007, I don’t know if his operation still exists. other wise build a poured in place tank or block tank, or use a series of amanco’s little hort poly(?) tanks. avoid vertical cylinder tanks, their inlet/outlet separations are terrible. jet sand or pea gravel backfill and incrementally fill poly tanks at installation, it is a good idea to anchor lightweight tanks, even some concrete tanks will float when empty. immediately refill poly tanks after pumping, poly is tough but not rigid. use a defector between compartments. install an effluent filter at the outlet of the tank. triangulate from known and durable points to septic tank lids or attach a colorful nylon rope to the lids, so you can find the lids when needed.

distribution box needs to be installed fairly level then each outlet pipe absolutely level with the others. ‘speed levelers’ quickly level the pipes. install a tee fitting vertically if there is more then a few meters elevation difference between the tank and the box. omitting the box on a passive gravity system is a fatal flaw, it leaves the system without a brain.

the leach trenches typically are gravel filled with river rock of varying size, instead use quarter stone free of fines, it is the space that is created between the rock that is the point, to handle the surges. don’t use the corrogated flexible pipe with slits, too week and the slits are prone to plugging with slimes, rather drill 75mm pipe with two rows of 15mm holes on 15cm spacing with 120 degrees between the rows. again this is the conventional way there are other ways that are better. install a vertical perf pipe at both ends of each trench for observation if there are problems, also marks the extent of each trench.

leach trench suitability and sizing will have to be another day.

operation, use aerators on faucets, space loading, don’t have wash days, do a load a day, 2 if you must, use front load washing machines as they use less water and laundry supplies. minimize chlorine use.

web resources: orenco, tuf tite, delta whitewater, geoflow

question: what is the general design of your biofilter system? if open air is it sized for maximum effluent load as well as maximum precip?


About Trevor

I have lived in Costa Rica for 14 years and have been active in real estate development over that period and construction in general. I am a qualified Journeyman carpenter and cabinetmaker from Canada. My current focus project is building custom green homes and housing packages all built out of Magnesium Oxide SIPs.

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