Five Residential Septic Tank and Drainfield Wastewater Treatment Tasks

This article explains the basic tasks of a residential septic tank and drainfield - the onsite wastewater treatment processes.

Burks and Minnis (on whose work this chapter is based) list five wastewater treatment processes [which I call "tasks" as these authors confuse the reader by also listing five functions of bacteria in wastewater treatment and five types of wastewater treatment processes], one or more of which may occur together simultaneously in a particular treatment system, and each of which can be accomplished by quite a variety of methods.

Page top photo: during a septic loading and dye test conducted by the author this water seepage down rock confirmed that the septic system was built in shallow soil atop solid rock - it was not a functional septic system.

Five Wastewater Treatment Tasks

1. Mechanical filtration of solids:
   
   such as filtration that occurs in soil below a drainfield or sand bed. Because soil particles may also have a small electrical charge, some small (previously suspended) solids will also adhere to some charged soil particles.
   
   The biomat that forms below a conventional drainfield or on media in advanced systems has a key role in additional filtering as well as oxidation and disinfection discussed next.
   Filtration in advanced design septic systems occurs in the textile, foam, or other media. Some septic system install textile or media filters to further clean effluent before it is sent to the soil absorption system.
   
   Filtration also occurs, in a sense, in the septic tank by the settling of solids to the tank bottom (sludge) or the coagulation of some light solids and greases at the tank top (scum layer). The net free working volume of a septic tank and the sewage inflow rate determine the tank's retention time - a period necessary to allow sludge and scum formation.
2. Biological oxidation
   
   of organic material: such as may occur to a limited degree in a septic tank, or to an advanced degree by the introduction of additional oxygen in aerobic septic systems. This process is also referred to as microbial oxidation since other microbes besides bacteria are at work. Microbial oxidation is an "aerobic" process, meaning that oxygen is required for the process to complete. Biological oxidation is a two-step process.
   • Aerobic Processing of septic waste: First organic material in the wastewater is consumed by bacteria or other microbes (carbonaceous biochemical oxygen demand). Second the bodies of older dead bacterial cells are used to make new cells (endogenous respiration).
     
     Burke/Minnis explain that after these two steps, except for some un digestible bacteria hulls, all of the other products have been assimilated into new bacterial bodies.
   • Anaerobic Processing of septic waste: other bacteria can "digest" wastewater contents without requiring oxygen, breaking down organic matter ultimately into methane (which is explosive, explaining some of the exciting anecdotes I warn about in my septic safety web pages).
     
     Anaerobic processes break down organic waste first by acid fermentation, second by acid regression, and third by alkaline fermentation, in case you see these terms floating around.
   
   Keep in mind, however, that in a conventional septic system this process is quite incomplete in the septic tank (perhaps 45%) and that the remaining oxidation has to occur in the soil absorption system. Aerobic and other advanced systems achieve different levels of oxidation at different stages.
   
   Burks & Minnis ascribe five functions (these authors like fives) to bacterial processing of wastewater: removal of carbonaceous organic matter, nitrification, denitrification, phosphorous removal, and conversion of organic waste into new cell mass, CO₂ and water.
3. Disinfection:
   
   which may occur by the processing of pathogens by microorganisms in the septic system or by actual disinfection in some advanced systems which require the insertion of a disinfectant
4. Water disposal:
   
   such as by absorption into soils below a septic drainfield or soil absorption system, or by evaporation into the air in an evaporation/transpiration system
5. Byproduct disposal:
   
   such as the pump out of settled sludge and floating scum from a septic tank - (the septic pumper has to haul this septage to an approved dumping facility for ultimate disposal).
   

...

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

Or see these

Recommended Articles

• 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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