My name is Steve Schneider. I have been in the excavation business for over 30 years. The last 10 years I have specialized in waste-water or septic design work. I do Design and Install work up to 60 miles from Spokane. I have put together information on my website that I hope will help with your understanding of your septic system.1st Gravity System. This is the least expensive system. The possible problem is that you need between 4 to 6 feet of good usable soils. 2nd Pressure Drain Field. This system uses a pump to equally distribute effluent over the drain field. This system needs 3 feet or more of usable soils. 3rd Drip System. This system uses a pump and drip line similar to drip line you would use for your landscaping. This system needs 30 inches or more of usable soil. 4th Pre-treated Drip System. This system requires a unit that cleans the effluent up before the effluent is introduced to the soils. This system needs 24 inches of usable soil or more. 5th Pre-treated Drip Mound System. This system uses pre-treatment Drip System and also, uses special imported soil made into a mound. This system needs as little as 12 inches of soil.