Septic Design (Surrey)

1.introduction

Septic fields must be designed by eligible geotechnical engineers, and every project has its own challenges and specifications. in this very case, Evertek engineers dealt with peat and very soft clay in Surrey. the main problem with clayey soils especially in septic design is their low permeability and low percolation rate accordingly (about 100 min/in). high level of groundwater exacerbates the situation as water cannot be drained through a soil which is already saturated.  

2.Septic fields

 A conventional septic system works by collecting wastewater from your toilets and drains. It retains solids and scum in a septic tank and pipes liquids to your septic drain field. In your drain field, bacteria break down wastewater pollutants and treated effluent returns to your property’s soil and groundwater.

That is the high-level answer. For a fuller understanding of how a conventional septic system works, we can break the wastewater treatment process into 10 steps:

  1. Wastewater from your toilet (called blackwater) and your shower, bathtub, sink, washing machine, and dishwasher (called greywater) runs into a main drainage pipe that leads to your septic tank.
     
  2. Solid materials sink to the bottom of your septic tank and form sludge.
     
  3. Fats, oils, and greases float to the top of your septic tank and form scum.
     
  4. Anaerobic bacteria (bacteria that do not need oxygen) in your septic tank feed on organic wastewater pollutants.
     
  5. Liquids pass through an effluent filter near the outlet of your septic tank.
     
  6. Filtered liquids flow through a pipe that leads to your septic drain field.
     
  7. Perforated pipes in your drain field allow wastewater to seep into a layer of gravel.
     
  8. Aerobic bacteria (bacteria that need oxygen) break down contaminants as wastewater percolates through the layer of gravel and into your native soil.
     
  9. In your native soil, any remaining impurities are removed. These impurities may include harmful bacteria, parasites, viruses, and nutrients.
     
  10. Treated wastewater reaches groundwater, which eventually drains into nearby streams, rivers, and lakes.

An advanced septic system works by receiving wastewater from your septic tank and lowering concentrations of organic matter, suspended solids, pathogens, and, in some cases, nutrients like phosphorus or nitrogen. After treatment, the system discharges effluent downstream, where it eventually returns to your soil, a ditch, or a watercourse.

Advanced septic systems are most often used on properties with:

  • limited or no space for a conventional septic drain field
  • shallow soil that is unsuitable for wastewater percolation
  • impermeable soil or bedrock that prevents treated wastewater from infiltrating into the ground
  • sensitive environments that require extra wastewater treatment processes
  • strict local environmental regulations

 

 

3.Percolation Test

The soil percolation rate indicates how quickly water moves through soil and helps evaluate the ability of the soil to absorb and treat effluent — wastewater that has received preliminary treatment in a septic tank. The percolation rate is measured in minutes per inch (mpi). Soils with slower percolation rates, through which it takes longer for water to travel, need larger drainfields to handle a given amount of wastewater than those with faster percolation rates. Soils with very slow percolation rates may be unsuitable for drainfields. The percolation rate is determined by conducting a percolation (perc) test. The percolation test measures the amount of time it takes for water in a test hole to drop 1 inch. As it was previously mentioned, due to clayey underlying layers the percolation rate in this project was quite low (100 min/in). 

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Refs:

[1] https://www.premiertechaqua.com/en-ca/wastewater/what-is-septic-system#:~:text=A%20septic%20drain%20field%2C%20also,treatment%20process%20that%20removes%20pollutants.

[2] https://extensionpublications.unl.edu/assets/html/g1472/build/g1472.htm