Septic Tank lab




Most houses outside of cities use septic tank for deposal of house hold waste. The design is controlled by state law. In New Mexico the state regulations  are controlled by the New Mexico Environmental Improvement Board Title 20: Environmental Protection Chapter 7: Wastewater and Water Supply Facilities Part 3: Liquid Waste Disposal.

Before starting lab go through the septic tank tutorial. Look at picture of leach line installation 




Design a septic tank for the first house on Tellbrook Rd in Las Cruces NM. Use the terra server to locate and measure the size of the lot. Roughly  150 m by 150 m . Calculate the size of the lot in ac.


Look up the soil type and topography in the soil survey. Place the location of the septic tank downstream from the house. Design the size of the tank and the leach line.


Determine the size of the house from the aerial photo graph and that will determine the number of people living in it and the number of rooms. House size is roughly 4000 ft sq. and should support a family of 5.


Look at the topographic maps to determine the slope of the land. See the terra server. Capture the topographic map and draw in the house and location of septic tank parts. Use a graphic program to accomplish this. 



Design criteria  

The typical flow is 75 gallons per person per day and design for 2 people per bedroom. The leach field design equation:


A= (Q*T^0.5)/5   multiple by 1.5 for some states

A=  ft^2

Q= gpd/person

T= soil percolation rate, minutes/inch    


Minimum size leach field



LTAR = long term acceptable rate gpd/FT^2

Soil type

LTAR Design gpd/f^2


Medium sand



Sandy loam to loam






Silt loam



Clay loam



Silty clay loam




The outlet pipe needs to flow downhill at least 1/8 inch per foot downhill to the leach field where the septic tank effluent enters a manifold or distribution ("D") box.


Assume a 4000 ft sq house has six rooms. Max number of people 12. Max flow of wastewater= 900 gpd.