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Question DetailsAsked on 11/10/2016

Need a small bath in detached garage. Are there any self contained septic systems I could use and what is involved?

I have septic, but know cost of adding another septic or the ability to add to mine is probably not allowed or cost prohibited. Had once heard of self contained units you can have pumped. Can't seem to get good info and regulations. I live on two acres and have woods behind this building. It would not have regular use, but want it as overflow for company. Most use would be gray water. Any suggestions helpful.

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2 Answers


All the self-contained systems I have seen have been a major maintenance headache, many have odor issues, and a lot of the "Green" solutions will not work at all with a highly irregular and sporadic inflow like you will have.

In most septic-legal areas (which yours may or may not now be for new construction even if you have a current septic system) uyou can go with a tank-only septic system (no leach field) but requires overfill alarm and level gauge, then emptying by a truck. Will fill quickly with graywater because that will not be going out to a leach field as normal - and you would have to check it and get it pumped if needed before having a party or houseguests - meaning advance planning. Basically same concept as a porta-potty but with running water.

Probably simplest if your septic tank is downhill from the garage (accounting for the fact the sewage line will have to be typically 3-10 feet underground depending on your local winter frost penetration depth, so "downhill" at least a couple of feet below that depth to the tank inlet or household line), would be to tie into the tank or the household drain line and use existing septic system.

Generally, the sizing of a septic system is based on the number of bedrooms (as a proxy for the number of residents), not the number of bathrooms - so in many areas adding a bathroom does NOT mean necessarily having to upsize the septic system, so you might just be looking at the plumbing in the garage for the bathroom, trenching to the septic tank or house (whichever is closer), and tying into the household septic system.

A note - if this will be infrequently used (not at least weekly or preferably more frequent toilet flushing or shower/tub water flow) make sure the inlet of the garage line comes in a foot or more ABOVE the line from the house if tying into that, or as close to a foot above the outlet level from the septic tank as possible if going straight to the septic tank. The reason for this is a seldom used line will back up and clog with solids and paper waste from the main line if not used enough to keep flushing it out before it goes to sludge, so a higher entry point (coming in from above to the connection rather than at same level) avoids that backup risk. Of course, you still need your proper slope on the drainage line to the connection, so sometimes that does not work out.

IF the septic tank/house line is uphill from the garage, then for a detached house one would normally use a septic lift pump in the house to pump it to the tank. However, with just a seldom used bathroom that is not going to produce enough volume to clean the line - so you would have to run to the tank or house (probably the house for close power and warmth if in cold area) going downhill (meaning maybe deep pipeline ditch) to a lift station at the house or tank, and have the pump located there (at the low point) to lift the sewage up to the household line at a convenient connection point. You would still have to be sure to run water out there every week or two to prevent it from sludging up and jamming the pump - not an ideal situation, and of course lift pumps are another maintenance item, and bathroom would be unusable in a power outage.

One other possibility would be, if allowed in your area (generally only rural areas, and with no well within 100-300 feet depending on area), would be a cesspool - which is a hole in the ground like a shallow well, which the sewage dumps into - acting as both septic tank and leach pit. Still is pumped periodically to remove solids, and when pumped out is normallhy pressure-jetted to restore the flow capacity for the water to the surrounding soil. Requires a relatively permeable soil condition, and does not last as long as a normal septic system with interceptor tank and leach lines.

The regulations you should be able to get easy from local building department, health department, or water district (different agencies control septic systems in different areas depending on politics and on whether there is a special agency for wells and water use because of water shortages or water quality issues). Commonly on their website - google for septic system permits, adding on your town name (or county if not in a legal town or city).

For professional help on planning/costing this, a Civil Engineering firm that does site development and septic sytem design (not an Angies List category) would be the right one to go to, because in virtually all areas septic system changes or construction has to be designed by and inspected during construction and approved by a licensed civil engineer.

Answered 3 years ago by LCD



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Answered 3 years ago by Member Services

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