The first sample is effluent from the septic tank, the second is septic effluent after one pass through a recirculating sand filter, and the last one is the effluent produced after several passes through the recirculating sand filter. You can see that the septic effluent contains many impurities including a heavy organic load. What you can't see is that it also contains hundreds of millions of bacteria and viruses. If you could smell the septic effluent you would also know that it has a foul and disgusting odor. The middle sample shows that the organic load has been significantly reduced.
Wastewater treatment is the goal of all onsite systems.
Treatment and disposal of septic effluents has traditionally required fairly large areas of well-drained soils because the soils had to perform a large amount of the treatment required to turn septic effluent into clear water. When wastewater treatment is provided using sand filters or other similar devices, the soil requirements for treatment are reduced and the disposal areas may also be reduced. Evidence is also mounting that such treatment can vastly extend the life of a drainfield. Many in the onsite community believe that high levels of wastewater treatment offer benefits in increased levels of protection for the environment and public health over traditional septic systems, regardless of the soil type used for ultimate disposal. Historically, sand filters and other similar devices have added considerably to the cost of an onsite system.