Tank should be about 8x12 feet.
Note that the inlet for black water and gray water should be higher than the "T" pipes in the other two overflows. The "T: should have several inches of pipe top and bottom to raise the top above the water level and to keep floating scum out of the outlet tubes.
Septic systems can be heavily regulated in much of the country. You may end up doing a system of your own and later being required to remove or replace it with a different system. One way of avoiding this is to make it according to proscribed designs for your area and soils test.
This particular one could equally easily be done with two huge plastic containers. There are similar self contained plastic systems that are easy to install yourself.
Septic systems are no great mystery. There is nothing stopping you from building or installing your own. In a regulated area, you just need to meet the basic requirements set out in the codes in your town or state. Pay someone to come to your house to do a design for you. Then do it yourself.
There will be plenty of digging, but most people can handle that either by hand or by using rented equipment. Most of the parts are available in home centers, and the only problem is transportation.
Always do more than is required. The point of doing these things for yourself is not to Beat "THE MAN", but to save money and be self reliant.
With self reliance, comes responsibility. Codes are usually in place for a reason...So...meet them, or exceed them, even if you use a different approach.
Protect your land, your community, the greater environment and your health.
There are commercially available systems available that look for all the world like the Yellow Submarine in the Beatles' film. Just carry it in and bury it. There are usually big plastic Dry Wells to distribute the treated water into the soil.Alternatives are:
1. French Drains.:
Long trenches are dug. Filled in partially with rocks and coarse gravel. Perforated PVC pipes are laid into the top of the gravel. An impermeable sheet of some sort is laid over the pipe so that soil and other particles do not sift into the perforated pipe and block them. Then they are attached to your final overflow pipe and buried.
2. Huge Dry wells:
are buried after attaching to the final overflow from the tanks. When they are buried, they are often filled with rocks.
3. Reed beds:
These can be wonderful, and are installed all over the world, but they come with risks. As the water is released onto the surface of the soil into beds of water loving plants, maintaining the beds puts you in direct contact with the plants and the water. Infections of all sorts are possible. How do your keep small children away from them, or pets for that matter.
The system above:
Build this yourself in areas where it would be impossible to carry in huge tanks etc. This could even be put directly under an outhouse that has water and a true toilet. Just put the outhouse right on top of the first tank, install a concrete floor with a hole and mounting hardware in it, and install a toilet over the hole just like in a house. Either pipe water into the house for summer, or in warm weather areas of the country, or carry a pitcher of flushing water to the toilet in the winter. A small amount of bio safe antifreeze would be needed in the water that is sitting there when unused. Also, heat from the decomposition may keep it warm enough. Trial and error will have to lead you. Flush twice, once to get rid of the antifreeze(environmentally safe only, for trailers), and the second to get rid of the waste. There are also units made commercially that only have a well with a trap door to drop the waste out the bottom or simple open bottom toilets like you see in government parks etc.. You still need water for the septic system to work, so dump in a pitcher of water, washing the inside of the chute as you do.
Gray water is usually introduced into the second chamber. Remember to use only environmentally safe shampoo, soap, detergents etc. for all your washing and cleaning. Whatever you use, is being dumped into your land!
Whatever you use for a system. try planting nitrogen and other fertilizer craving plants all around the area where you are draining your water. Fruit trees(as long as there is no direct contact with the water on the surface) Peonies, ground covers...etc...just be certain that they do not have invasive roots. You just need Heavy Feeders. No root crops please! Plants with big, dark green leaves love nitrogen! Roses also come to mind. Phosphorus may be absorbed by the soil.
Note that French drains or leach fields are typically laid out in a trident pattern. The pipe from the septic usually enters a distribution box. This is usually a concrete cube with four holes in the four sides. One, higher than the rest, receives the fluid. The other three holes lead to the right, left and forward. Pipes from the right and left usually turn at right angles after a few feet, to run parallel with the middle one. The ends of the perforated pipes are capped, and the water filters into the drain trenches. If the soil is clay, it may not drain properly. Always test the soil. Dig a few test pits all over the area to be used. Pour in several gallons of water and observe the drainage of the water. Doesn't drain?...not a good place for a drain field.
Heavy metals are always a concern in any septic system. Recycle anything that may contain these, and watch out for foods that may contain them. Never flush anything that contains heavy metals, and recycle batteries etc., instead of discarding them on your land. Tuna and other ocean fish comes to mind. It is possible that these heavy metals may be taken up by food crops. Be especially careful with children in the area as they are affected more than adults by such things as mercury.
Above all, be mindful of local wells, not just yours. Putting systems like this down hill from wells, especially dug wells, is a good plan, and keep them as far away as possible. Consult local experts about well contamination and the flow of water into local wells. Very deep wells may be using water that is centuries old in layers of deep rock, but they may also be fed directly by surface water through cracks in the bedrock.
Remember that animals have been crapping on your land for eons. The land is still OK. The big difference is that we are concentrating our waste in one area as we do not move around, and we use chemicals in our waste water. Take reasonable precautions, and you may do no more harm to the land than those animals.
Composting waste is a good alternative if you have no other choice, but this does not remove chemicals....
Pump out septic tanks regularly, and keep them well supplied with bacterial cultures that will digest the solids. There is usually a greasy scum layer on the top of the water in the septic tank. This is good to be pumped away from your land regularly.