Transcript 3 - Basics of Wastewater Treatment Presentation

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 – 200-600 mg/l
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’
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.
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.
What we have is
‘failure to treat’
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.
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
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
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 microenvironment 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.
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)
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.
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.
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