Sizing a Community Septic System

Wastewater systems in the US are sized based on the maximum number of gallons per day they can treat.

A 300-room hotel, for instance, might require a 50,000 gallon-per-day system. Depending on soil loading rate*, that system might need a 2 acre drip field for effluent disposal.

Here are some factors that determine how many gallons per day your community septic or other wastewater system must be able to handle:

• Capacity in gallons per day is determined by state and local design specifications.
• These regulatory agencies calculate required treatment capacity in terms of maximum gallons used per person per day or maximum flow per bedroom per day, etc.
• Commercial wastewater systems use more complex formulas that take their specific usage into account. The hotel mentioned above might need to account for 75 gallons per bed per day but might also have a restaurant and a bar attached for which another 12 gallons per seat per meal would have to be added.
• Design criteria must also assume the level of pollution present within wastewater from different sources. Very dirty wastewater takes longer to treat which means systems must have higher capacity than what is released to give the system the time needed.

Here is an example of a design criteria matrix from an actual state regulatory agency:

• Design criteria tables such as the one above provide a starting point to determine size, but in most cases, regulatory agencies grant variances based on actual flow and treatment level.
• We at Aqua Tech will research the design criteria required for your project and budget around them. As the build gets closer, we reevaluate your treatment needs and work with civil engineers and regulatory authorities to ensure regulatory compliance without excess expense.

Bottom line: Use this table to get a rough estimate. When you’re ready, let’s talk and get more specific.

*Soils differ in how much moisture they can absorb per hour. Very dense soil might only be able to absorb one tenth of a gallon per square foot every hour while porous soil can absorb almost a full gallon per square foot. Soil absorption per hour is called its “loading rate.” The higher the loading rate the smaller the drip field needed.