As effluent fills the first compartment, clarified liquids are forced to leave the second compartment and flow out to the leach field. You don't need to hire an engineer to design your septic system in most areas. Some local codes require at least a 1250 gallon septic tank. Slower percolation test results require larger leach fields. This is generally an eight foot deep pit which is excavated in the area of the leach field to provide a visual confirmation that there are no problematic soil conditions, groundwater, or bedrock where you intend to put the leach field.
Codes allow field size reductions with dosing.
This is a very common mistake that could result in your leach field failing and having to be replaced. Septic filters keep larger particles from getting into your leach field and eventually causing it to fail. Without water on the inside, any type of septic tank just doesn't have the strength to completely resist the weight of the soil around it. If the tank cracks, it will leak and have to be replaced. The number one reason that your septic system will fail is because the leach field becomes clogged with sediment.