Originally Posted by excav8ter
It's interesting how different parts of the country do things.... around here a septic and drainfield is basically 2 tanks totaling around 1500 -1800 gallons, and a drain bed that is a minimum of 900 square feet, depending on the amount of living space in the house....not the amount of bathrooms. I don't do to many, but we basically dig out the drain bed area, place 8"-10" of stone, then lay out the drainage pipe, and finish by placing more stone up to the top of the pipe, cover it with a layer of straw and your done. Sometimes the bed is square and somtimes it's rectangle.
The problems with just a bed is you get little or no aerobic bacteria colonies in the middle of the bed due too the lack of oxygen available for the soil through the trench sidewalls so they are a last choice for us here. The tank is a processing tank and the larger the tank is the cleaner the effluent is coming out of the tank. It is the first step of effluent cleaning. The tank relies on anaerobic bacteria for this. The water then goes in trenches, beds, etc and at the soil interface anaerobic bacteria forms a layer called a biomat that further cleans the effluent and slows the absorption process down. A small colony of aerobic bacteria setups there as well( if oxygen is available) and it actually feeds off the biomat and helps stop it from becoming a clogging mat( which is bad). Finally the remaining contaminants are remove as the effluent moves through the soil. Sizing a system is done different ways in different states, however waterflow is what each different method is establishing or how many gallons per day you will be running into the tank. Most systems are over designed by several 100% because waterflow can be so hard to establish and changes drastically from house too house, times of the year and same house after it is sold. Regardless of product used actual sq/ft of soil interface seems too be the most important design criteria other than GPD of flow and soil characteristics. Gravel and pipe is still one of the best solutions for conventional leach fields unfortunaetly cost deters the homeowner from using it most often times.