Drain Field Repair

Septic Drain Field Repair/Replacement/Maintenance

If you live in a house or property connected to a septic tank, it’s important you know how to maintain the drain field. Many homeowners seem to forget about this responsibility until it backs up into the yard and house, causing extensive damage and requiring professional help. Here, we’ll go over the steps to properly maintaining your septic tank drain field, as well as how to repair and replace it when necessary.

Anyone using a septic tank instead of a sewer system should familiarize themselves with how a drain field works. Basically, when you flush organic material down the toilet, it travels down your bathroom pipes until reaches one large sewer pipe which eventually leads to the septic tank. Once inside the septic tank, the solid materials sink and the liquids float to the top. A baffle then works to carry the liquids down another pipe and into a gravel-filled area of the drain field.

When everything with your septic tank and drain field works properly, you shouldn’t be able to tell you even have a drain field. However, if problems arise, the yard around your drain field can become a disastrous area filled with sludge and anything else you’ve flushed down the toilet, which is why it’s important to maintain and repair it promptly.

The number one rule to maintaining a proper drain field is watching what you flush down the toilet. It’s easy to forget about even having a septic tank when you can’t see it. Trust me, though, it’s there and you need to be aware of it. Ideally, you should only be flushing water, waste and soap down the toilets and drains. Never pour cooking oil or grease down into your septic tank, as these are some of the primary culprits for damaging a drain field. Other things you should avoid flushing are paper towels, cigarettes and diapers.

To keep your drain field working as it should, you’ll need to have your septic tank professionally pumped and drain every so often. How often really depends on the number of people living in your home and the size of the tank. If it’s just you living there, you can probably get away with only having it pumped once every 5 years. Although, if you have a large family, you’ll need to have it pumped about once every 2 or 3 years. If you don’t take the time to have your septic tank pumped, the solids can back up and eventually overflow into the drain field. Once this happens, extensive and costly repairs will be needed to get it back to working condition.

While this isn’t necessary, it’s not a bad idea to hire a professional to come and inspect your septic tank and drain field once a year. They will be able to come out and make sure nothing is clogged or getting backed up in the tank or drain field. It’s a small price to pay that could potentially save you thousands of dollars on big repairs down the road.

What is a Septic Drain Field?

From Wikipedia,the Free Encyclopedia

Septic drain fields, also called leach fields or leach drains are used to remove contaminants and impurities from the liquid that emerges from the septic tank. A septic tank, the septic drain field, and the associated piping compose a complete septic system. The septic drain field is effective for disposal of organic materials readily catabolized by a microbial ecosystem. The drain field typically consists of an arrangement of trenches containing perforated pipes and porous material (often gravel) covered by a layer of soil to prevent animals and surface runoff from reaching the wastewater distributed within those trenches.[1] Primary design considerations are hydraulic for the volume of wastewater requiring disposal and catabolic for the long-term biochemical oxygen demand of that wastewater.

Hydraulic design:
Many health departments require a percolation test (“perc” test) to establish suitability of drain field soil to receive septic tank effluent. An engineer or licensed designer may be required to work with the local governing agency to design a system that conforms to these criteria.

Wastewater from toilets is assumed to contain bacteria and viruses capable of causing disease. Disinfection methods used prior to surface disposal of municipal sewage cannot be used with septic tanks because disinfection would prevent wastewater treatment by killing the septic tank and soil ecosystems catabolizing the putrescible contents of the wastewater. A properly functioning drain field holds and deactivates pathogens before they leave the drain field soil.

The goal of percolation testing is to ensure the soil is permeable enough for septic tank effluent to percolate away from the drain field, but fine grained enough to filter out pathogenic bacteria and viruses before they travel far enough to reach a water well or surface water supply. Coarse soils – sand and gravel – can transmit wastewater away from the drain field before pathogens are destroyed. Silt and clay effectively filter out pathogens but allow very limited wastewater flow rates.[2] Percolation tests measure the rate at which clean water disperses through a disposal trench into the soil. Several factors may reduce observed percolation rates when the drain field receives anoxic septic tank effluent:[3]

Microbial colonies catabolizing soluble organic compounds from the septic tank effluent will adhere to soil particles and reduce the interstitial area available for water flow between soil particles. These colonies tend to form a low-permeability biofilm of gelatinous slime at the soil interface of the disposal trench.[4]
Insoluble particles small enough to be carried through the septic tank will accumulate at the soil interface of the disposal trench; non-biodegradable particles like mineral soil from laundry or vegetable washing, or bone and eggshell fragments from garbage disposals will remain to fill interstitial areas formerly available for water flow out of the trench.[5]

  • Cooking fats or petroleum products emulsified by detergents or dissolved by solvents can flow through prior to anaerobic liquifaction when septic tank volume is too small to offer adequate residence time, and may congeal as a hydrophobic layer on the soil interface of the disposal trench.[6]
  • Rising groundwater levels may reduce the available hydraulic head (or vertical distance) causing gravitational water flow away from the disposal trench. Effluent initially flowing downward from the disposal trench ultimately encounters groundwater or impermeable rock or clay requiring a directional shift to horizontal movement away from the drain field. A certain vertical distance is required between the effluent level in the disposal trench and the water level where the effluent is leaving the drain field for gravitational force to overcome viscous frictional forces resisting flow through porous soil. Effluent levels in the vicinity of the drain field will appear to rise toward the ground surface to preserve that vertical distance difference if groundwater levels surrounding the drain field approach the level of effluent in the disposal trench.[6]
  • Frozen ground may seasonally reduce the cross-sectional area available for flow or evaporation.


Dosing schedules or resting periods:
A drain field may be designed to offer several separate disposal areas for effluent from a single septic tank. One area may be “rested” while effluent is routed to a different area. The nematode community in the resting drain field continues feeding on the accumulated biofilm and fats when the anaerobic septic tank effluent is no longer available. This natural cleansing process may improve hydraulic capacity of the field by increasing available interstitial area of the soil as accumulated organic material is oxidized. The percolation rate after resting may approach, but is unlikely to match, the original clean water percolation rate of the site.

Catabolic design:
Just as the septic tank is sized to support a community of anaerobic organisms capable of liquifying anticipated amounts of putresible materials in wastewater, the drain field should be sized to support a community of aerobic soil microorganisms capable of decomposing the anaerobic septic tank’s effluent into aerobic water. Hydrogen sulfide odors or iron bacteria may be observed in nearby wells or surface waters when effluent has not been completely oxidized prior to reaching those areas.[6] The biofilm on the walls of the drain field trenches will use atmospheric oxygen in the trenches to catabolize organic compounds in septic tank effluent. Groundwater flow is laminar in the aquifer soils surrounding the drain field.[7] Septic tank effluent with soluble organic compounds passing through the biofilm forms a mounded lens atop groundwater underlying the drain field. Molecular diffusion controls mixing of soluble organic compounds into groundwater and transport of oxygen from underlying groundwater or the capillary fringe of the groundwater surface to micro-organisms capable of catabolizing dissolved organic compounds remaining in the effluent plume.[8]

When a septic tank is used in combination with a biofilter, the height and catabolic area of the drain field may be reduced. This technology may allow higher density residential construction, minimal site disturbance, more usable land for trees, swimming pools, or gardens. With adequate routine maintenance it may reduce the chances of the drain field plugging up. The biofilter will not reduce the volume of liquid that must percolate into soil, but it may reduce the oxygen demand of organic materials in that liquid.

Inappropriate wastes:
Septic tank and drain field microorganisms have very limited capability for catabolizing petroleum products and chlorinated solvents, and cannot remove dissolved metals; although some may sorb onto septic tank sludge or drain field soils, and concentrations may be diluted by other groundwater in the vicinity of the drain field. Cleaning formulations may reduce drain field efficiency. Laundry bleach may slow or stop microbial activity in the drain field, and sanitizing or deodorizing chemicals may have similar effects. Detergents, solvents and drain cleaners may transport emulsified, saponified or dissolved fats into the drain field before they can be catabolized to short-chain organic acids in the septic tank scum layer.[6]


1.^ Steel, E.W. & McGhee,Terence J. “Water Supply and Sewerage”McGraw-Hill Book Company (1979) isbn=0-07-060929-2 pp.576-577
2.^ Alth, Max & Charlotte “Constructing and Maintaining your Well & Septic System” Tab Books (1984) isbn=0-8306-0654-8 pp.166-167
3.^ Alth, Max & Charlotte “Constructing and Maintaining your Well & Septic System” Tab Books (1984) isbn=0-8306-0654-8 p.217
4.^ Alth, Max & Charlotte “Constructing and Maintaining your Well & Septic System” Tab Books (1984) isbn=0-8306-0654-8 pp.164-165&219
5.^ Alth, Max & Charlotte “Constructing and Maintaining your Well & Septic System” Tab Books (1984) isbn=0-8306-0654-8 p.219
6.^ a b c d Hammer, Mark J. “Water and Waste-water Technology” John Wiley & Sons (1975) isbn=0-471-34726-4 pp.407-408
7.^ Linsley, Ray K. & Franzini, Joseph B. “Water-Resources Engineering (2nd Ed.)” McGraw-Hill Book Company (1972) isbn=07-037959-9 p.88
8.^ Perry, Robert H., Chilton, Cecil H. & Kirkpatrick, Sidney D. “Chemical Engineers’ Handbook (4th Ed.)” McGraw-Hill Book Company (1963) p.14-13