Septic Design = Onsite Wastewater Treatment Design ​

The term septic is short for onsite wastewater treatment systems which make up all our societal wastewater infrastructure outside metropolitan areas.

The main concepts in septic design are:

Wastewater Flow Rates ​

Residential Flow Rates

Residential flow rates are handled slightly differently depending on local regulations. Typically, the method to calculate this is a flow per day per bedroom

Residential Flow Methods used across the industry:

1. Flow Per Bedroom Per Day
2. Flow Per Fixture Count
3. Flow Per Resident Count

In some areas, deviation is allowed from the standard flow may be allowed with the use of low flow fixtures and appliances like washing machines, toilets and faucets. A great resource for determining which fixtures meet these criteria are the US EPA WaterSense Program.

Commercial Flow Rates

Commercial flow rates are typically the quantification of a type of unit of business and an associated flow per day with each unit.

For example: a church with a per seat of 2 gallons per day

landplan handles flow rates with our Building with Flow Tool

Soil Data Analysis ​

In the practice of onsite wastewater treatment system planning, the soil where the system will be installed is considered the soil absorption area, or the location where the soil will absorb all the wastewater generated at the property.

There are a few different types of soil analysis for onsite wastewater treatment:

1. Profile/Horizon Classification
2. Percolation Test

Different regulatory agencies have different requirements for soil analysis. Some use one of the above or both to classify soil permeability. Soil permeability is the quantification of a soil's ability to move water through its pores/void space. In general, soil permeability corresponds to a soil loading rate or gallons per day per area.

There is another classification used in a few areas (Ohio, British Columbia) called a linear loading rate. This is a value of gallons per day per linear measurement. It is used to determine how much wastewater can be applied per foot/meter.

Elevation ​

NewSeptic tools generate elevation at specific vertices of the drawn components. These are collected, and the user can replace them with more accurate elevation data. The end goal of this is to provide a hydraulic profile showing elevations of all components. The hydraulic profile shows important details including the anticipated elevations of all components, including important information can be learned from the hydraulic profile such as tank dig depths and pump elevation required.

Soil Absorption Sizing ​

Soil absorption area is typically determined by dividing flow by your selected soil loading rate. Some regulatory jurisdictions have limits on how long or wide the area can be, but many offer a variety of options for planning your required absorption area.

Wastewater Treatment Levels ​

Wastewater is water that contains human and possibly industrial wastes. Incoming residential waste streams are quantified in terms of Biological Oxygen Demand (BOD), Total Suspended Solids (TSS). These are measurements of how much organic matter is in the wastewater and how much treatment is required.

Many different manufactured treatment systems are available for use in residential treatment systems. All treatment systems use the same methods of treatment. The main methods are:

1. Aeration. Introducing air into the wastewater mixes the wastewater and also provides oxygen to bacteria naturally present in the wastewater. The oxygen allows the bacteria to be many times more efficient at breaking down organic wastes into their building block molecules, such as methane gas and ammonia, etc.
2. Filtration. Wastewater filtration of varying levels from gravel/large pores, to reverse osmosis/small pores. Varying media is available for filtration products, from synthetic fabric and plastic media, to natural bi-products such as coconut husks, and peat, to mined aggregate, such as gravel/sand.
3. Disinfection. Wastewater disinfection is typically done with a chlorine/dechlorination process or with the use of ultraviolet (UV) light.

Distribution Media ​

A common form of absorption area are trenches or beds where wastewater is dispersed into the ground. The media in these trenches or beds can be natural occurring aggregate like gravel or synthetic plastic such as chambers or large diameter pipe.

Distribution Methods ​

The wastewater must be pumped or delivered by gravity pipe to the soil absorption area. For pumping applications there are many options for distributing the wastewater including:

1. Solenoid valves
2. Automatic Distributing valves

Gravity applications typically use a distribution box to distribute the wastewater evenly to all beds/trenches. There are a few methods for distribution through distribution box(s):

1. Serial. The wastewater flows from one trench to the next, as if there is one long connected trench.
2. Sequential. Wastewater delivered in sequential method means it fills up one trench, then moves to the next
3. Parallel. Wastewater delivered in parallel should all exit the distribution box at the same time.

Hydraulics ​

Onsite wastewater treatment requires moving water, in many cases up hill. The study of hydraulics is the study of movement of liquid, in this case wastewater, and accounting for friction losses, changes in elevation and flow in piped systems.

Isolation Distances ​

Different regulatory jurisdictions require different isolation distances from septic components. Each generally have the same functionality...to protect existing surface water, private/public drinking water and utility infrastructure.