Good morning, Hugh,
Many homes are equipped with septic tanks to perform as a holding tank for waste, allowing waste decomposition to occur. Reduction of solid waste through bacterial action works, but is a slow process and often incomplete. Additionally, a large number of chemicals we regularly introduce into our septic tanks, such as common soap, dishwashingj and clothes detergents, bleach, commercial toilet cleaning solutions, et cetera, are toxic to the bacteria performing the job of decomposition.
Septic tanks are one part of the equation; the other being the leach field. Leach fields are the fluid distribution pipes running from the septic tank into the ground and are intended to operate with “clear” liquids only; “clear” liquids does not refer to their color but means “no solid materials”. Solids will fill the spaces between dirt particles and eventually form a sufficient barrier to liquid absorption to cause the leach field to fail. The only fix for this is dig a new leach field in dirt that has not been contaminated by particles, or replace the dirt in the existing field. Either solution will be expensive and quite inconvenient.
Other things that wind up in the septic tank can render the leach field useless, specifically, greases and oils, including those created during cooking. Washing a grease-encrusted broiler pan sends thick animal fats down the drain; these fats will float on the liquid in the septic tank and be the first to be carried out of the tank into the leach field. It’s better to wipe out greasy cookware with paper towels to be disposed of as trash before sink washing, to minimize the amount of grease and oils entering the septic tank.
Harsh chemicals should be avoided in houses with septic systems. Soap and detergent use cannot be avoided, but things such as commercially-available toilet cleaning chemicals can be replaced with scrubbing with small quantities of isopropyl alcohol , which can be carefully burned off in the toilet bowl.
Since waste decomposition is slow and usually incomplete, all septic tanks will need to be pumped out periodically to remove the solids. Depending on how many people are in the family and the size of the tank, that period may be as short as three years or as long as seven. Most residential tanks are 1,000 gallons, but they are commonly available in sizes from about 500 to 3,000 gallon. I’ve seen a series of three 55-gallon blue plastic barrels  used successfully as a septic tank where the quantity of waste is small or the need is temporary. (I doubt those can be pumped out with regular equipment, so they must be replaced periodically.)
Most septic tank pumping trucks have 2,000 gallon tanks, so there’s a disadvantage to having a septic tank larger than that; one pumping session won’t completely empty the tank, necessitating a second trip, always at additional expense to the homeowner.
Regardless of how often it’s necessary, more frequent septic tank pumping is recommended. I have my tank pumped every three years, even though I could probably go another two years, or perhaps three, without a problem. Should economic and social conditions deteriorate sufficiently, it may be impossible to get a tank pumped at all. When it’s completely full, solids will start entering the leach field, eventually killing it, and could back up into the supply pipe coming from the house preventing use of any of the house’s plumbing fixtures. Knowing that I have a recently pumped tank that could go as long as six years without attention provides a reasonable operational cushion.
If one has the land and money resources, a dual septic system can be a good investment. One tank is connected only to the toilets and becomes the sanitary system; the other handles all other waste water, from sinks, showers, washing machines, et cetera. This keeps chemicals that are toxic to bacteria out of the sanitary tank, allowing it to perform its decomposition job unaffected. This will extend the required pumping intervals, and the non-sanitary tank should probably never need pumping. Leach field killers, such as grease, oils et cetera, should still be avoided in the non-sanitary system.
Equipping toilets, or at least one toilet, with its own independent water supply (most easily accomplished during construction or major remodel) provides an advantage. In SHTF times, family use shifts to the single toilet equipped with an independent water supply; disconnecting that toilet’s water piping from the whole house system and attaching it to an independent water source allows use of that toilet without using the entire house’s water system. Additionally, non-potable but uncontaminated water can be used for toilet flushing since that independent water delivery system for the one toilet gets disconnected and is separate from the house’s potable water system. Rainwater collected from roofs, after the first 10 minutes of rain cleans the roof, is one good source of toilet-flushing water, and will not require any expensive purification to use in a toilet. “He who flushes also pumps” becomes the order of the day.
If usage is confined to one toilet, the water traps in the others will eventually dry out from evaporation allowing sewer gases and insects into the house. (The same applies to sinks, tubs, and showers not in regular use.) Evaporation losses can be reduced by covering the toilet bowl with plastic wrap, but since the DWV (drain-waste-vent) system is connected to vents, those are the pipes extending above your house’s roof that allow the plumbing system to drain properly, the water in a toilet bowl is still exposed to atmosphere and will eventually evaporate from that. Periodic inspection of unused toilets is necessary, as will be the occasional refilling of the toilet bowls to maintain water seal in the trap. – N.K.