One of the most important functions of a septic tank is to hold wastewater for a period of time so that solids within the incoming wastewater can settle at the bottom of the tank in the sludge layer. This settling process, along with the consumption of organic waste by the bacterial environment within the septic tank, are crucial processes to maintaining drainfield permeability.
Many factors affect the organic matter settling rates in a septic tank:
- Suspended solids in the incoming wastewater range in sizes and buoyancy. Obviously, heavier particles will require little time to travel to the bottom of the tank, regardless of the conditions within the tank. Some particles, however, have specific gravities (the density of a substance relative to the density of water) very close to water. In other words, these particles can stay suspended in water for long periods of time.
- Particle size and surface area affect settling rates. For example, two particles may contain the same volume but one may have more surface area than the other. The particle with larger surface areas will, in general, stay suspended longer than the particle with the smaller surface area. Also, small or fine particles tend to stay suspended longer than larger particles.
- The concentration of particles can affect settling rates. Higher concentrations of particles will result in more particle collisions. It has been well documented that particle collisions will slow down settling rates.
- An unpredictable and unquantifiable variable in particle settling rates is the fluid velocity and subsequent turbulence in the septic tank. These can have a huge affect on the particle movement and overwhelm all other particle settling factors. Obviously, zero fluid velocity and turbulence is optimum, but is unrealistic. It is best to try and keep it to minimum levels. For example, when septic tanks continuously receive a high percentage of incoming wastewater as compared to their holding capacity, the resulting fluid velocity and turbulence can easily offset any progress the lighter and finer particles have made towards the bottom of the tank. As a result, these fine particles stay suspended and are very likely to be pushed out into the drainfield.
Fine organic waste particles in a drainfield are a nightmare scenario. They readily and naturally settle and fill cracks, holes, seams and pores in the drainfield, becoming very effective at preventing water from passing. This mass of fine particles will eventually seal the drainfield. Once sealed, the septic system is shut down.
So, septic tank hold time is a critical to keeping a septic system healthy. It is very important for septic system owners to understand this and prevent overloading their system with gray water. Laundry loads and showers should be spaced apart and any water leaks should be immediately fixed. Giving a septic tank time to allow solids to settle reduces the incidences of fine organic waste particles passing into and settling in the drainfield.