Transcript The Drain Field Function

The Drain Field Function
•What goes
into the drain
field?
•How the
drain field
functions?
•What causes
the drain field
to fail?
What goes into the drain field?
•Septic ‘tanks’ function
to remove settleable
solids, greases and to
facilitate breakdown or
decomposition of
inorganic and organic
materials
•Wastewater – dissolved organic and inorganic
materials (salts, caffeine, detergents, cleaners,
chemicals, OTC and pharmaceutical drugs),
suspended solids, microorganisms (bacteria,
viruses)
•~ 50-75 gallons of wastewater per individual per day
(average)
http://www.co.el-dorado.ca.us/EMD/envhealth/homeowner_septic.html
How does the drain field function?
Effluent from tank to drain field
Effluent absorbed by or infiltrates into soil pores
Suspended solids trapped in soil pores –good/bad
Dissolved organics and some solids digested by
microorganisms – this is good (water + CO2)
Organic nitrogen converted to ammonia nitrogen
Most bacteria and viruses destroyed in soil with time
Soluble inorganics may be degraded, broken down
Ammonia converted to NO3 then to N2 (anaerobic
process)
Soluble inorganics and some organics may
continue to leach downward in soil – to local
aquifers – this is not good
Some wastewater moves upward from drain field
by capillary action and is removed by evaporation
or plant water use (ET)
What causes the drain field to fail?
• The drain field can fail:
– Hydraulic failure – water no longer infiltrates
– Treatment failure – water infiltrates at such a rapid
rate that there in not sufficient time or opportunity for
treatment of waste water
– Biological (or treatment) failure – the drain field and
soil above and below becomes anaerobic (without
oxygen) and biological breakdown of organics and
destruction of bacteria and viruses does not occur
– Chemical failure – either the soil fails, becomes
plugged, sealed due to dispersion or chemicals are
introduced which either kill functionally beneficial
microbes or are not absorbed and/or degraded in the
soil: example: OTC, solvents, sodium-rich water,
nitrate, chloride salts, sulfates.
– Some anaerobic condition is good!
What causes hydraulic failure?
Basically – the percolation rate decreases or is
inadequate for the field size and waste water load
In short – a sizing or soil suitability issue
• due to clay particles being flushed or leached into or
below the drain trench
• due to abundance of clay in the soil and excessive
dosing resulting in persistent swelling and pore plugging
• due to clay aggregate and particle dispersion due to
sodium-rich water dosing
• due to build-up/accumulation of excessive bacterial slim
mat below the trench line
• due to pore plugging by solids which flow from septic
tanks that have not been pumped
• due to flooding due to high groundwater or sewage
flowing from neighboring leach fields.
http://www.co.el-dorado.ca.us/EMD/envhealth/homeowner_septic.html
Aerobic zone = unsaturated flow
Well
Anaerobic zone
Anaerobic micro-sites
Aerobic soil needed
for treatment
Groundwater
Drain field size, trench construction, tank maintenance,
appropriate site selection – the soil properties are suitable
for and can accommodate processed domestic wastewater
Questions?