What size tank will I need?

Aerobic units should be large enough to allow enough time for the solids to settle and for the wastewater to be treated. The size of most units range from 300 to 1,500 gallons per day, but local regulations often require that the unit be at least large enough to handle 500 gallons of wastewater per day.
The needed size of an aerobic unit is often estimated the same way the size of a septic tank is estimated, by the number of bedrooms (not bathrooms) in the house. It is assumed that each person will use approximately 50 to 100 gallons of water per day, and that each bedroom can accommodate two people. When calculated this way, a three-bedroom house will require a unit with a capacity of 300 to 600 gallons per day. Some health departments require that aerobic units be sized at least as large as a septic tank in case the aerobic unit malfunctions and oxygen doesn’t mix with the wastewater. In such cases, the aerobic unit will work as a septic tank—which will, at least, provide partial treatment for the wastewater.

Temperature
Lower temperatures tend to slow down most biological processes, and higher temperatures tend to speed them up. The aerobic process itself creates heat, which, along with the heat from the electrical components, may help to keep the treatment process active. However, cold weather can have adverse effects on the
performance of aerobic units. In one study of aerobic units, there were problems when the temperature of the wastewater inside some of the units fell below 15 degrees Celsius (59 degrees Farenheit). Problems can sometimes be avoided by insulating around the units. Your health department should know whether aerobic systems are suitable for your area.
PIPELINE - Winter 1996; Vol. 7, No. 1