Here is a overview of the different types of septic tanks and how the work:
Cylindrical and low profile septic tanks:Super Low Profile Septic Tank
These types of septic tanks work in the same way as a brick or concrete built septic tank works, using a baffle system the tank separates the solid waste from the liquid not allowing the sludge buildup to penetrate to to outlet area ensuring that the anaerobic breakdown works properly, this system is effective and simple and has no moving parts so the malfunction of the tank is unprovable.
Many cylindrical or low profile septic tanks like the Super Low Profile Septic Tank (pictured) can be installed without the use of large amounts of concrete, as long as the site is dry (no high water table) these tanks can be backfilled with gravel or sand.
Alpha or Onion Shape septic tanks:Alpha Septic Tank
These type of septic tank originally designed by Klargester is probably the most installed septic tank in the UK, manufactured using GRP glass fibre these thanks are cheap to purchase and quite simple in the way the work, using a floating baffle system to separate the solid waste from the liquid, the Alpha septic tank doesn't produce quite as good anaerobic breakdown as the cylindrical style septic tank but does work well in most cases, this tank does need to be installed using concrete as a backfill.