Levels of sewage treatment accomplished by private septic systems:

This septic system operations article explains the Septic bacteria contamination levels that occur in residential septic system wastewater and soil treatment systems.

The soil environment is a hostile one for septic bacteria and for many viruses, so with adequate time and space for effluent treatment a drainfield can be successful. But some drainfields may not work this well (sandy soils, shallow bedrock, nearby waterways), in which case owners may elect for a disinfectant or other system to improve the treatment level.

Septic bacteria contamination levels in wastewater and soil treatment systems

Jantrania/Gross point out that "Microorganisms in raw wastewater can be present in millions of counts per 100 ml, thus reducing them by 90% [presumably typical onsite treatment levels] will still leave a large quantity in the effluent before discharge." [- Jantrania. p.22]

Those authors conclude that if the target is a 10-fecal coli count in finished wastewater we need a 99.9999% reduction from the starting level of bacteria. Disinfection (such as a chlorine injection system, ozone, or a UV-light system) is used to reduce the levels of bacteria and viruses.

A Pollution Scale proposes a range from 0 (water) to 10 (sewage). (Other literature typically refers to simply primary, secondary, or tertiary levels of sewage treatment.)

A typical septic tank achieves just 45% of the desired total level of effluent treatment, or 5.5 on the 0-10 scale, and a properly working drainfield (see above for our doubts about this), achieves additional treatment from between 65% and about 91%, or treatment levels 3.5 down to about 0.9 on the 0-10 wastewater pollution scale.

[We would not want to drink effluent that was 90% treated towards drinking water quality, but we might discharge this into groundwater.] [Op.cit. p48].

Advanced and alternative septic system designs change the ratio of wastewater treatment between in-tank and in-soil, or we should say "before soil" and in "in soil" since advanced treatment can take any of a variety of forms (aerobic systems, aboveground foam cube trickle-down systems, etc.).

It should be apparent that improving wastewater treatment before it is discharged into the soil reduces the treatment load on the soil - that is the soil can either treat to a higher level or can achieve the prior level with less area.

The rate of disposal of effluent by discharge into soil is probably unchanged by improving its treatment prior to disposal.

In other words, you may still need a drainfield of the same size to adequately dispose of the volume of water being discharged even though a smaller area might have been capable of achieving the level of treatment needed. It depends ... on the local soil properties and local surface and subsurface groundwater levels.

This article series is a supplement to the introduction (SEPTIC SYSTEM BASICS) to our online book SEPTIC INSPECTION & TEST GUIDE

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Continue reading  at WASTEWATER TREATMENT BASICS or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

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Recommended Articles

• SEPTIC SYSTEM BASICS
• WASTEWATER TREATMENT BASICS
  • FIVE WASTEWATER TREATMENT TASKS
  • FIVE WASTESATER TREATMENT PROCESSES
  • PERCENTAGES of WASTEWATER TREATMENT ACHIEVED
  • SEPTIC TANK TREATMENT of WASTEWATER
  • DRAINFIELD TREATMENT of WASTEWATER
  • DRAINFIELD TREATMENT EFFECTIVENESS
  • SEPTIC BACTERIA CONTAMINATION LEVEL

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