Presentation on theme: "The Drain Field Function What goes into the drain field? How the drain field functions? What causes the drain field to fail?"— Presentation transcript:
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) ~ gallons of wastewater per individual per day (average)
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 + CO 2 ) 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 NO 3 then to N 2 (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.
Aerobic soil needed for treatment Well Groundwater Aerobic zone = unsaturated flow Drain field size, trench construction, tank maintenance, appropriate site selection – the soil properties are suitable for and can accommodate processed domestic wastewater Anaerobic zone Anaerobic micro-sites