Today, tanks are made of precast concrete, plastic and fiberglass.  State regulations require progressively larger tanks as predicted loading goes up, as in a large number of bedrooms.  A septic tank's primary function is settling and separation of solids from the wastewater.  The tank must be big enough to allow settling, hence the larger tanks for the larger flows.  It is important to provide a baffle on the  outlet to prevent floating solids from leaving the tank and fouling the dispersal field or pump station.   Today, we fit the outlet baffles with easily removable cartridge filters capable of removing solids down to 1/16".   Proper use of a septic tank requires that certain items and materials are not send down the drain.  They include large amounts of paper, grease, excessive quantities of water, plastic non-biodegradeable items,  chemicals, industrial waste, medicine.  They should be pumped and cleaned every 3 years or when solids fill the bottom 40% of the tank, which ever comes first.  

1000 gallon precast septic tank with 2 access covers

Effluent filter - septic tank outlet

Polylok model PL-122