1. Septic Tank x Drain Field Biological and Chemical Functions
BOD5 = the biological oxygen demand, i.e., the quantity of oxygen consumed by microorganisms during a 4-day period, in the process of organic substrate decomposition.
BOD is a surrogate measure or gauge (somewhat like a scale) of the amount of biodegradable organic material in sewage or wastewater. Thus, a high BOD means that the wastewater contains a high concentration of biodegradable organic material.
Ideal - < 1 mg/l
Impaired water – 2-8 mg/l
Treated effluent discharge - < 20 mg/l
Untreated sewage – mg/l

2. BOD = reflection of oxygen demand in ‘aerobic’ processes
Sewage treatment within septic tank is ‘anaerobic’
Anaerobic because sewage entering the tank is so high in BOD that any oxygen present in the sewage is rapidly consumed.
Anaerobic digestion does reduce some of the BOD in the septic tank
Settling of solids also reduces the BOD of the sewage
Residual BOD (biologically oxidizable organic material within the wastewater) flows into the leaching or drain field.
Thus, it is essential that the drain field remain ‘aerobic’

3. BOD (this biologically oxidizable organic material in the wastewater) serves as a food (energy) source for ‘digesting’ microbes.
Thus the BOD actually serves a beneficial purpose in supporting the microbial biomat which forms under the drain field. Caveat – the drain field remains ‘aerobic’.
The good – a healthy biomat will
contribute to the physical breakdown and reduction of oxidizable organic material,
will serve as an inhospitable environment for bacteria and viruses,
will facilitate the biological conversion of ammonia and nitrate to nitrogen gas
will facilitate the sequestration and/or precipitation of phosphorus compounds
and will facilitate the biological and physical breakdown of some OTC pharmaceuticals.

4. If either the BOD is so high that all available oxygen in the waste water and drain field is consumed, Or
The drain field is poorly or inadequately aerated – because of submergence, compaction, flooding, poor drainage, deep burial, lack of aeration and/or venting,
The biomat can (and likely will) go anaerobic and discontinue properly functioning.
Desirable bacteria and protozoans in the biomat die, resulting in diminished treatment of the sewage. Anaerobic bacteria proliferate, producing a mucilaginous biofilm which further clogs the drain field.
What we have is
‘failure to treat’

5. In short – BOD (within limits) is a good and integral part of properly functioning drain field
Aerobic conditions within the drain field are essential to a properly functioning drain field
Excess BOD in sewage can cause a leaching field to function poorly or improperly and can even result in system failure – hydraulic and/or treatment failure
Solutions – properly functioning drain field – site selection (caution about drain field location in ‘tight’ soils, i.e., poorly drained or collapsible silts and clays)
- drain field properly sized to accommodate the anticipated BOD
- waste water pretreatment to reduce BOD – actively oxygenate the sewage before it enters the drain field, to facilitate the reduction of BOD of waste water

6. Soil-facilitated processes essential to effective septic drain field waste water treatment
Humans excrete nitrogen in organic form – dead cell material, proteins, amino acids, urea, residue of food digestion (feces)
Organic nitrogen is broken down fairly rapidly and completely to ammonia (NH3) by microorganisms in the septic tank.
In the presence of oxygen (in the drain field), ammonia is converted to nitrate by bacteria (an oxidation process).
In the absence of oxygen, nitrate is converted to nitrogen gas.
Step 1 = ammonia to nitrite to nitrate = nitrification
Step 2 = nitrate to nitrogen gas = denitrification
Both steps mediated by bacteria

7. Any waste water treatment system that is intended to remove nitrogen by the nitrification/denitrification processes (traditional septic systems) must be designed to provide both aerobic and anaerobic environments so that both nitrification and denitrification can proceed.
A combination of an aerobic environment in the immediate vicinity of the discharge point and an anaerobic micro-environment within, at the bottom of, or immediately below the biomat.
Neither a poorly drained nor excessively well drained leaching environment will provide the desired treatment.

8. Phosphorus
Principal forms in human waste are organically bound phosphorus, polyphosphates, and orthophosphates.
Organically bound phosphorus – human and food wastes
Converted to orthophosphates during decomposition
Polyphosphates – sourced from synthetic detergents
Polyphosphates – automatic dishwasher detergent
Polyphosphates hydrolyzed to orthophosphates
Because of these conversions, the principal form of phosphorus in wastewater entering the drain field is orthophosphate.
Orthophosphates are negative ion (anionic) forms of phosphorus
PO43- (phosphate anion)
HPO42- (hydrogen phosphate)
H2PO4- (dihydrogen phosphate)

9. Anionic charge (negative), as in the case of phosphates, is the net charge of most fine-textured soil, i.e., soil is negative in electrical charge.
Thus, in the absence of a complexing, binding, or precipitating environment, phosphate would readily leach in soil. This is the reason for concern about phosphorus impairment in coastal and shoreline environments. Coarse-textured, sandy, very-well drained soils predominated by sand and silt are likely to have limited phosphorus attenuation capacity.
Fortunately – most phosphorus which is not removed in the septic system (due to complexing and precipitation in solid form) is likely removed under the drain field by chemical precipitation.

10. In slightly acid, aerobic environments (pH < 7.0):
Orthophosphates (anions) combine with tri-valent iron or aluminum cations (both generally abundant in soil) to form insoluble precipitates FePO4 and AlPO4.
FePO4 – ferric orthophosphate
AlPO4 – aluminum orthophosphate (Berlinite)
pH < 7.0 common in highly organic soils, forested areas, high rainfall x well drained areas, very sandy soils.
In alkaline, aerobic environments (pH > 7.0):
Orthophosphates (anions) combine with calcium to form insoluble apatite, i.e., rock phosphate).
Ca5(PO4)3 – apatite (rock phosphate)
pH > 7.0 common in marine sediments, low-rainfall areas, arid and semi-arid environments, prairie landscapes, calcium-rich soils.

11. Just like nitrogen, there is a caveat that applies to phosphorus!
If the soil below the drain field becomes anaerobic (devoid of oxygen), iron may become chemically reduced (changed to the Fe2+ form), which is soluble and can be leached within the soil.
What leads to the anaerobic conditions – as previously mentioned:
lack of adequate aeration or ventilation in the drain field
soil collapse, hydraulic failure
limited vertical separation to groundwater
excessively high BOD in the sewage
– settling tank full
- drain field under-sized