An Introduction to Septic Tank Systems

Some people have made elaborate preparations for a Level 2/3 event.  They have bought their retreat.  They have their food, their fuel, their guns, their water supply, and their generator.

They’ve planned the typical one gallon of water per person per day, and hopefully allowed more than that for all the other uses one needs water for too.  And their food stores are more than sufficient.

But ask them about what happens to the food and water after it is used, and they’ll look at you blankly.  Explain some more, and they’ll get into an elaborate discussion about toilet seats on 5 gallon plastic pails, about ‘Humanure’, maybe about cutting trenches and tossing handfuls of soil or lime on top of one’s outputs.

None of it sounds very pleasant, for sure.

Why aren’t more people simply having a septic system installed at their retreat?  Costs to install a system are low (in the overall scheme of setting up a retreat costs) and the impact on allowing for a much more comfortable life when living at the retreat is enormous.

Maybe some people look upon septic systems with underserved scorn.  They are an excellent, reliable, and low-tech way of handling human waste, and are good as a solution for people living away from a reliable city sewer system in normal times as well as in adverse times.  We’ve lived in septic served houses ourselves and have never had any problems – indeed, the most unreliable sewage system we ever had was where the line to the city sewer would get blocked, regular as clockwork, every two or so years, by roots from the neighbor’s nearby large tree.

Here’s a quick introduction to the subject of septics.

Septic systems vary in size depending on the ‘load’ that is expected to be placed on them (perhaps better to say the load that will be placed in them!).  The rule of thumb is to guess at the load based on the number of bedrooms in the house the system is servicing, and to assume two people per bedroom, and each person representing 60 – 75 gallons of liquid (and solid) a day.

If you are careful, and perhaps if you use 1.6 gallon per flush toilets rather than the earlier 3.5 – 5 gallon per flush toilets, and if you have modern water conserving washing machines and dishwashers, you’ll be able to get down below the 60 gallon per person guesstimate, but for designing your system to county code standards, you’ll need to use whatever their formula is.

Once you know the gallons per day that need to be managed, you double it and that tells you the size of the tank itself.  A three bedroom house would represent a load of about 360 gallons/day (at 60 gallons per person) which doubled represents 720 gallons of tank capacity.

A sweet spot in tank prices seems to be 1000 gallon capacity (which will probably cost you $500 – $800 or so).  This is also the usual capacity of a septic tank pumping truck, and it makes good sense to match your tank capacity with a pump truck’s capacity (prior to WTSHTF).  There is never any harm in ‘too large’ a tank capacity – this is a good thing as it extends the time between pump-outs.

The other thing you need to work out is more complicated.  This is the size of the drainfield, and it is determined not just by the gallonage per day, but also by the type of soil you’ll be draining the tank into.  That same three bedroom house might need as small as a 300 sq ft drainfield (best case scenario) or as large as an 1800 sq ft drainfield (a bad scenario).  You need to call in the experts to do soil analysis, perc tests, and drainfield design.

The all up cost of a septic system for a three bedroom house may be in the range of $2500 – $4500 to install, plus permit fees and initial consultation costs, assuming a reasonably positive situation and a gravity fed system.  If you need to have a pressure system, then the cost is more likely to be around $4000 – $6500 plus permits and consulting.  Worst case scenario, with a ‘mound’ type system, you could be spending $9,000 – $20,000.

A well designed septic system running at the level it was designed for (or less) will need to be pumped out or in some other way emptied every couple of years or so.  Larger tanks require emptying less frequently.  Other than that, they should reliably work for a couple of decades without needing much attention.

Gravity systems are, as their name implies, gravity fed.  Pressure and mound systems need pumps with infrequent duty cycles and low powered motors.  They should not overload your power resources at all.

If you are careful about what you put into your septic system – especially if you don’t use a sink garbage disposal unit to increase the amount of bio-mass going into the system (you shouldn’t waste food scraps that way, anyway – either use them for feeding animals or for composting), and limit the inputs to only the, ahem, human outputs, your system will largely take care of itself.

You should also try not to have sudden peaks of inputs.  If you have a system that anticipates 420 gallons a day, it is better to give it 400 gallons a day for three days in a row than to give it 100 gallons today, 1000 gallons the next day, and 100 gallons the third day, because you want everything to have at least two days in the tank before moving on to the drainfield.  This gives time for solids to settle to the bottom (and fats to the top) leaving just a relatively free of other stuff liquid to go into the drainfield.

Recommendations for Preppers

Clearly we are recommending you should build a septic system at your retreat.

There are two extra considerations to keep in mind.

The first is that, if at all possible, try to avoid the need to have a system that requires pumps.  This will reduce the number of things that might go wrong and/or need maintenance, and also reduce the energy cost of operating your septic system.  If the entire system is fully gravity powered, then you have a very resilient system – much better to spend more up front to get a robust system than to go a cheaper route only to find yourself with a system that becomes inoperative subsequently at a time when you can no longer easily access cheap energy and ready support.

The second consideration is to over-specify everything, and to massively increase the holding tank capacities too.  We’re not quite sure what you’d do at the point where the tank needs to be pumped, so best put that day off as long as possible.

Pumping is inevitable.  The septic system works by settling the solids to the bottom of the holding tank and having the liquid overflow feed out through the drainfield.  Sooner or later, the level of solids will reach the point where they need to be pumped out.  Better to make that time as far into the future as possible, by which time there will hopefully be some infrastructure available to respond to the need.

Here’s an excellent site with much more information on the topic.