By Martin Harris

Issue #63 • May/June, 2000

Q. We have just purchased 15 acres, and our question is how to install our septic system, how far away from the on-site stream and can we use cinder blocks?

A. The short, common-sense answer is that, with any sort of normal soils, a properly-functioning system with cinder (actually, these days, they’re concrete) blocks or even zero-cost fieldstone could be installed as close to the stream as, say 100 feet.

The longer answer is that common sense may not be enough. That’s because most states have very detailed requirements for septic system installation, ranging all the way from professional-engineers-only design to minimum septic tank size and offsets from streams and property lines, including everything from fairly elaborate soil tests to bureaucratic on-site installation inspection.

It happens that Vermont law still includes our so-called “10-acre exemption,” which allows any landowner with at least that lot size to design and install any arrangement he/she wants, as long as no effluent can be observed crossing the property line. This exemption has been under severe pressure by anti-rural-development advocates in recent years and won’t survive much longer, but while it does it allows a rural homesteader substantial design freedom at mimimal installation cost. There may be similar acreage-based exemptions in other states.

Most states now have regulations prohibiting the use of site-built septic tanks using concrete blocks. They require instead factory-manufactured pre-cast concrete units, with minimum size ranging from 500 to 1000 gallons in which primary (anaerobic) bacterial treatment occurs. The effluent then flows by gravity to the typical perforated-plastic-pipe field, where runs of pipe are set into gravel trenches for secondary (aerobic) bacterial treatment to take place. From the holes in the pipe, the treated effluent then flows harmlessly through the gravel into the ground. Could you build a tank out of block, laid up in waterproof manner with mortar joints and surface parging, setting it on a fairly solid concrete slab so it won’t settle and allow cracks to open up? Sure, if the authorities will let you.

The authorities might even let you build a tank out of dry-laid block, no mortar, with the core holes set horizontally, allowing the effluent to seep immediately into the ground. Such construction used to be made out of fieldstone and was called a cesspool; the design basis was that both anaerobic and aerobic digestion could take place in the same container, and the final effluent would be just as clean as in the more modern two-step system. If you go this route, you’ll most likely want to emulate the old-timers and build two or three cesspools, directing the sewage flow into one for a few years, while the others are allowed to “rest” and the sludge build-up in the surrounding soil to biodegrade.

Whether it’s to be cesspool or septic tank, the top should be a couple of feet below grade in northern climates so that bacterial action isn’t slowed by low soil temperatures. There should also be a removable lid so that the indigestible sludge which builds up at the bottom of the tank and the organic mat at the top don’t fill up too much of the tank volume. Pumping every five years is usually recommended. If your design calls for the septic tank flow to go to a tile field, remember that the field pipes should be less than a foot from ground surface, so that the aerobic bacteria can get oxygen. You’ll need either a sloping site or an electric pump to meet this design essential.

If you don’t fancy digging every five years to get to the tank lid, install an access route which is exposed at the soil surface. My own tank set up is properly buried, but over the lid opening there’s a three-foot length of three-foot diameter concrete pipe, with the lid on top of that. To pump the tank we just step through the surrounding flowerbed, lift the lid, and drop the pump hose down the hole into the tank. Slick.

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