Is a drainage field required for a STP?

[flanagaj]

So I am reading a lot of conflicting information regarding whether a STP needs a drainage field.  If I am correct drainage fields are historically used for septic tanks, but if you read section 5.3 of NHBC-Standards-2025-Chapter-5-3-Drainage-below-ground.pdf you see that they mention discharging to a soakaway.  Interestingly, they make no mention of a the percolation test draining away too quickly (like one of our trial holes).  

So what is the actual requirements for discharging from a STP if you cannot discharge to a water course?   Ideally, I'd like to be able to use soakaway crates or the Graf infiltration tunnels.  Our site is quite tight and excavating and installing a conventional drainage field will be tricky.

[Kelvin]

Soakaways and drainage fields get used interchangeably incorrectly with a soakaway being a catch all term. Our planning approval specifically said a drainage field. Even the SEPA document I read on their website talking about soakaways to discharge into from a treatment plant. It really confused me at the time. 

 

The rules are also slightly different between Scotland and England regards using existing water courses. In Scotland you’re generally not allowed unless there is no alternative and you need approval from SEPA. 
 

 

Edited Sunday at 15:33 by Kelvin

[Gone West]

1 hour ago, flanagaj said:

So what is the actual requirements for discharging from a STP if you cannot discharge to a water course?

I don't know what the actual requirements are, but when we installed our STP in 2010 we discharged to a Klargester reed bed system. Several years later the supplier said the effluent could be discharged into a soakaway. As the reed bed system was quite high maintenance we installed a soakaway.

[saveasteading]

2 hours ago, flanagaj said:

whether a STP needs a drainage field.  

No it doesn't. It is a fancy option when a soakaway won't work.

 

So you have good percolation figures. Too fast at one location which in theory means your treated water runs away too fast.

1. The stp manufacturers have long stopped claiming you could drink it, but it shouldn't be nasty so a soakaway will allow bugs to treat it further then it dribbles slowly to the aquifer, among the natural water containing worm and rabbit remnants etc. No harm.

2. If it ran away too fast it might open a stream and stop filtering it. So at that area of fast percolation, rather take drainage elsewhere or  close the French drain off...use umperforated  pipes to bypass it. 

3. A drainage field would be pointless as your French drains are underground and self emptying. I think some people say drainage field wrongly, meaning a soakaway system.

 

4. As another current discussion, I like French drains rather than crates. They spread the water and are easy to lay. The gravel surround continues the treatment as any stuff sticks to it and is eaten by bugs.  The formulae tend to be verrrry conservative. I can't suggest putting in half as i don't know your site circumstances.  Perhaps build it in phases for programming reasons and look out for any problems.

[IanR]

2 hours ago, flanagaj said:

So what is the actual requirements for discharging from a STP if you cannot discharge to a water course? 

If your site is in England you must meet the General Binding Rules, otherwise you require a permit from the Environmental Agency. The GBR is the far easier route.

For a new discharge you can not use a soakaway, for discharges to ground it must be via a drainage field. Both STP and Drainage filed must be in line with the relevant British Standard. See Rule 9 for the specifics
 

Quote

Rule 9: make sure your treatment system meets the right British Standard

Your system must meet the relevant British Standard that was in place at the time it was installed.

If different parts of your treatment system were installed at different times, each part must meet the relevant British Standard in place at the time of installation.

The current standards for new systems are:

• BS EN 12566 for septic tanks and small sewage treatment plants
• BS 6297:2007 for drainage fields

Ref. https://www.gov.uk/guidance/general-binding-rules-small-sewage-discharge-to-the-ground

 

Lots of rules, make sure you meet all of them.

Under the GBR you can discharge to a Water Course in some circumstances, so when you say "cannot discharge to a water course" I assume you mean you do not have access to a suitable water course.

Edited Sunday at 17:44 by IanR

[flanagaj]

1 hour ago, IanR said:

If your site is in England you must meet the General Binding Rules, otherwise you require a permit from the Environmental Agency. The GBR is the far easier route.

For a new discharge you can not use a soakaway, for discharges to ground it must be via a drainage field. Both STP and Drainage filed must be in line with the relevant British Standard. See Rule 9 for the specifics
 

Ref. https://www.gov.uk/guidance/general-binding-rules-small-sewage-discharge-to-the-ground

 

Lots of rules, make sure you meet all of them.

Under the GBR you can discharge to a Water Course in some circumstances, so when you say "cannot discharge to a water course" I assume you mean you do not have access to a suitable water course.

Correct.  There are no water courses within the vicinity of the tank.  Hence, discharging to ground.

[saveasteading]

Ok so I'm getting mixed up by the terminology now. 

 

Drainage field in the English regs is just french drains.  So use French drains but call it a drainage field.

 

They call it a drainage field for the small amount of liquid from an stp, but soakaways if it is rainwater.

 

What I say still applies.

[IanR]

50 minutes ago, flanagaj said:

Correct.  There are no water courses within the vicinity of the tank.  Hence, discharging to ground.

Then to meet the General Bing Rules you'll need a Drainage Field that meets BS 6297:2007

I'm not sure if that document is available free of Charge, but if you ask Gemini, it gives you a good idea of what is required:
 

Quote

Designing a drainage field for a small treatment plant that meets BS 6297:2007 requires a systematic approach, starting with site assessment and ending with detailed construction. Here's a comprehensive guide:

Key Principles from BS 6297:2007 for Drainage Fields

• Further Treatment: The drainage field provides additional biological treatment of the effluent from the treatment plant as it infiltrates the soil. This requires an aerobic environment, which means the pipes should be laid at a relatively shallow depth.
• Controlled Infiltration: The ground must be permeable enough to allow the effluent to seep away but not so permeable that it drains too quickly, leading to insufficient treatment or pollution of groundwater.
• Protection of Water Sources: The drainage field must be sited safely away from buildings, watercourses, and water abstraction points (wells, boreholes).
• Site Suitability: Not all sites are suitable for a drainage field. A thorough site investigation, including percolation tests and a trial site assessment hole (TSAH), is essential.

Steps to Design a Drainage Field

1. Preliminary Planning and Site Investigation

• Identify Suitable Location:
  • Distance from Buildings: A minimum of 15m from any habitable building.
  • Distance from Watercourses: A minimum of 10m from any watercourse or ditch.
  • Distance from Water Abstraction Points: A minimum of 50m from any water supply abstraction point (e.g., well, borehole).
  • Groundwater Protection Zones: Ensure the site is not within a Groundwater Source Protection Zone 1.
  • Underground Services: Keep the area clear of any underground services or water supply pipes.
  • Access Roads/Paved Areas: No access roads, driveways, or paved areas should be within the disposal area.
  • Topography: The field should generally be downslope of any groundwater source. Avoid floodplains.
• Trial Site Assessment Hole (TSAH):
  • Excavate a hole at least 1m square and 2m deep, or a minimum of 1.5m below the proposed drainage field pipework.
  • The primary purpose is to determine the position of the seasonally highest groundwater table.
  • The groundwater table must never rise to within 1.2m of the bottom of the drainage field pipework (i.e., if your pipes are at 700mm deep, the water table needs to be at least 1.9m deep). Look for "mottles" (brown/grey discoloration) in the soil, which indicate seasonal waterlogging.
  • Leave the hole open for at least 48 hours to observe any rise in the water table. If the water table is too high, the site is unsuitable.
• Percolation Test (Vp Test):
  • Purpose: To determine the soil's permeability (how quickly water drains away).
  • Procedure (as per BS 6297:2007):
    1. Excavate Holes: Dig at least two holes, 300mm square, to a depth at least 300mm below the proposed invert level (bottom of the infiltration pipe) of the drainage pipes. Space them along the proposed line of the system. Note and record changes in soil characteristics at measured depths.
    2. Saturate the Soil: Fill each hole with water to a depth of at least 300mm and allow it to seep away completely.
    3. Repeat Saturation: If water drains rapidly (within 10 minutes), refill the hole up to a maximum of 10 times to ensure saturation. If it continues to drain rapidly, the ground may be unsuitable.
    4. Unsuitable Ground: If the water has not soaked away within 6 hours, the area is not suitable.
    5. Measure Percolation Rate: Refill each hole with water to a depth of at least 300mm. Observe and record the time in seconds for the water to seep away from 75% full (225mm depth) to 25% full (75mm depth) – a drop of 150mm.
    6. Calculate Vp: Divide this time (in seconds) by 150 (mm). This gives the average time in seconds for the water to drop 1mm.
    7. Repeat Tests: Repeat the test at least three times in each hole.
    8. Average Vp: Calculate the average Vp value by summing all the test results and dividing by the number of tests.
    9. Acceptable Vp Range: Drainage field disposal is only suitable if the average Vp value is between 15 and 100 seconds/mm.
       • Vp < 15: Water drains too quickly, insufficient treatment, risk of groundwater pollution.
       • Vp > 100: Water drains too slowly, leading to ponding and system failure.
    10. Further Tests: If Vp results vary widely (more than 50% above or below the average), conduct more tests in different locations.

2. Detailed System Design

• Calculate Required Drainage Field Area (A):

  • For a package wastewater treatment plant (which provides additional treatment compared to a septic tank), the formula is: A=p×Vp×0.20 Where:
    • A = required drainage field floor area in square metres (m2)
    • p = number of people served by the treatment plant (for domestic properties, this should be the maximum number of people that could live in the house). Consult BS EN 12566 for population equivalent guidelines if unsure. For a typical small domestic plant, this might be based on the number of bedrooms (e.g., 5 persons for a 3-bedroom house, plus 1 for each additional bedroom).
    • Vp = average percolation value (in seconds/mm) obtained from the percolation test.

• Trench Dimensions and Layout:

  • Trench Width: Between 300mm and 900mm wide.
  • Pipe Depth: Perforated pipes should be laid at a minimum depth of 200mm and a maximum depth of 700-800mm to ensure aerobic contact with the soil.
  • Pipe Type: Use 110mm downward-facing perforated pipes. "Flexicoil" type land drainage pipe is NOT allowed.
  • Gradient: Lay pipes with a uniform gradient, not steeper than 1:200.
  • Gravel/Granular Fill: Pipes should be laid on a 200-300mm layer of clean gravel or granular fill material (graded 16-32mm or 20-50mm).
  • Geotextile Membrane: Trenches should be filled to a level 50mm above the pipe and then covered with a layer of geotextile membrane to prevent silt entry. The remainder can be backfilled with topsoil.
  • Undisturbed Ground: Maintain a minimum of 1m wide strips of undisturbed ground between parallel trenches.
  • Maximum Trench Length: The length of a single trench should not exceed 30m.
  • Inspection Chamber: An inspection/distribution chamber should be installed between the treatment plant and the drainage field to distribute effluent evenly to the trenches and for monitoring.
  • Loop System: The drainage field should ideally be set out in a continuous loop fed from an inspection chamber.
  • Venting: Upstream ends of distribution pipes may be extended vertically above ground level and capped with a cowl or grille for venting.

Example Calculation (Illustrative)

Let's assume:

• Small treatment plant serving a 4-bedroom house: p=6 persons (typical for a 4-bed house, 5 for 3-bed +1 per extra bed)
• Average Percolation Value (Vp) : 30 seconds/mm (within the acceptable range of 15-100)

1. Calculate Area (A): A=p×Vp×0.20 A=6×30×0.20 A=36 m2

2. Determine Trench Length: If you decide on a trench width of, say, 0.6m (600mm): Total Length of Trenches = A/Trench Width Total Length of Trenches = 36 m2/0.6 m=60 m

   You would then design a layout of parallel trenches, each no longer than 30m, to achieve a total length of 60m. For example, two trenches each 30m long, with 1m of undisturbed ground between them.

Important Considerations and Next Steps

• Professional Advice: It is highly recommended to engage a qualified wastewater engineer or a British Water Wastewater Accredited Service Technician for site assessment, percolation testing, and detailed design. They can ensure full compliance with BS 6297:2007 and local regulations.
• Building Regulations: The installation must comply with Part H of the Building Regulations 2010 (Drainage and waste disposal) and be signed off by a building inspector.
• General Binding Rules: Ensure compliance with the General Binding Rules for small sewage discharges to the ground in your specific UK nation (England, Scotland, Wales, Northern Ireland), as these may have additional requirements or nuances.
• Maintenance: A drainage field requires proper maintenance of the treatment plant to ensure the quality of effluent entering the field, which prolongs its lifespan. An inspection chamber before the drainage field allows for monitoring.
• Record Keeping: Keep all test results and design documents for regulatory purposes and future reference.

By following these guidelines and engaging appropriate professionals, you can design a drainage field that effectively treats wastewater and complies with BS 6297:2007.

Edited to add what looks like a good reference to clear up any confusion on what a Drainage Field is:

https://www.homeseptic.co.uk/drainage-field-size-calculation-and-design/
 

Edited Sunday at 19:49 by IanR

[flanagaj]

4 minutes ago, IanR said:

Then to meet the General Bing Rules you'll need a Drainage Field that meets BS 6297:2007

I'm not sure if that document is available free of Charge, but if you ask Gemini, it gives you a good idea of what is required:
 

 

This is where it gets confusing.  My understanding is that a sewage treatment plant does primary and secondary treatment.  That therefore negates the requirement for the aerobic treatment provided by the soil.  

[IanR]

2 minutes ago, flanagaj said:

That therefore negates the requirement for the aerobic treatment provided by the soil.  

That's a misunderstanding. While the discharge has low toxicity, if it is allowed to build up it can get to a hazardous level, so the GBR's require their discharge to be controlled either into a drainage field or a water course. 

[flanagaj]

10 hours ago, IanR said:

https://www.homeseptic.co.uk/drainage-field-size-calculation-and-design/
 

This link was very helpful and clearly details my question regarding drainage fields / infiltration tunnels for a STP.  

"Currently, written legislation has fallen behind the technology available, notably in Sewage Treatment Plants. Sewage Treatment Plants produce near clean water, thus the need for secondary treatment is redundant.  A Septic Tank with a Drainage field is nearly always cheaper than a Sewage Treatment Plant with a Drainage Field.  Written legislation allows you to build a slightly smaller drainage field but the impact is minimal. Essentially a rational economic person would not install a Sewage Treatment Plant with a discharge to ground and a Drainage Field.

While explicit written standards have not caught up, building control and local authorities have. With consent, it is possible to install a series of Infiltration tunnels to discharge the treated effluent from a Sewage Treatment Plant. Again there are calculations to perform based on porosity etc to ensure an element of secondary treatment and adequate dispersal. A Sewage Treatment plant with an infiltration system is generally cheaper than a Septic Tank with Drainage Field.

Further, an infiltration system can be installed in a far smaller area increasing flexibility and applicability. For instance, the majority of Septic Tanks currently do not have a Drainage field that meets the general binding rules. The Environment Agency is unlikely to check on your Drainage Field. However, when you come to sell your house or replace a failed Soakaway a Drainage Field will need to be installed. Drainage fields cover a large area, in most cases four to five times the surface area of a traditional Soakaway. Many properties with Septic Tanks do not have sufficient space to install a Drainage Field constructed in line with legislation. A Sewage Treatment plant with an infiltration tunnel system could be the only viable option."

[Kelvin]

11 hours ago, IanR said:

That's a misunderstanding. While the discharge has low toxicity, if it is allowed to build up it can get to a hazardous level, so the GBR's require their discharge to be controlled either into a drainage field or a water course. 

Yes exactly the conversation we had with building control. I had in my mind at the outset it would be a soakaway largely because that’s what the SEPA website said and what they told me when I called them. When I had the land survey done the guy that came out to do it said he it would likely be a drainage field or discharge to the burn but that would need permission so we submitted the planning application with that in mind. When we applied for the warrant we had a more detailed design done for a drainage field. The original design in terms of size was 24m x 1m but this was changed to 9m x 2m for the warrant. We were fortunate that the Goldilocks position for it happened to be the only place it could go and the place where the percolation test was perfect. Despite having 1.5 acres of space there was nowhere else it could have gone due to our neighbour’s borehole location,  the burn and the nature of our land. 
 

The guy I spoke with said the reason it needs to be a drainage field is for your reason above, build up of toxicity over time or if the treatment plant has reduced efficiency over time for some reason but he also pointed the need for the homeowner to correctly maintain the treatment plant and have it checked annually (or do it yourself) 
 

 

Edited Monday at 07:23 by Kelvin

[Gone West]

14 hours ago, saveasteading said:

I like French drains rather than crates

For a soakaway, have you come across the idea of crates with 'bio filter media' in them to further reduce the toxicity of the effluent. IIRC it was something that I discussed with my supplier many years ago.

[saveasteading]

49 minutes ago, Gone West said:

you come across the idea of crates with 'bio filter media' in them

Yes. But gravel does that and,  I think, better . It's the same principle as gravel in trays for car parking....no oil interceptor required.  The oil, or the stp discharge runs on and into the gravel and coats the very large surface area. Micro-organisms consume it.

So gravel around perforated pipes is my choice...Best and cheapest.

I've done this for very testing facilities. 1000m2 office, 4 team sports pavilion plus others. No problems ( I would have heard).

The crate with media may be viable where there is no land for French drains.

But the flow out of an stp is nearly clean...when I've taken a non -technical sample it has looked clean and had no smell.

I vaguely recall one installation (by others) needing maintenance but it was the wheel-going-round type that had stopped going round.

 

In summary. The crate ticks a box.

[SteamyTea]

14 hours ago, IanR said:

Drainage Field that meets BS 6297:2007

Is that also known as a reed bed system, or is that something else again.

 

I quite like the idea of my waste going through a treatment plant (having seen inside one, they don't seem very complicated, a bucket with an air bubbler in it) and then into a hole in the ground with some damp living plants growing out the top.

Must be more to it than that.

[IanR]

28 minutes ago, SteamyTea said:

Is that also known as a reed bed system, or is that something else again.

It's not unfortunately, in codifying the requirement natural options have disappeared for perforated tubes with their fixed cross-section and calculable flow-rates, so something more like


 

28 minutes ago, SteamyTea said:

a bucket with an air bubbler in it) and then into a hole in the ground

Still relatively simple, but a second chamber is required for an STP, the first being a settlement chamber to seperate the solids (that needs emptying every 1 - 2 years), then the aerated liquid chamber with a filter media.

Edited Monday at 10:53 by IanR

[Kelvin]

1 hour ago, SteamyTea said:

Is that also known as a reed bed system, or is that something else again.

 

I quite like the idea of my waste going through a treatment plant (having seen inside one, they don't seem very complicated, a bucket with an air bubbler in it) and then into a hole in the ground with some damp living plants growing out the top.

Must be more to it than that.

A reed bed system is different and not allowed nowadays.
 

They aren’t complicated at all. Our field just has grass on top of it and it all looks like the rest of the field so there’s no change in the soil. Our Graf system bubbles the air on an on/off cycle so is slightly cheaper to run. After a year ours has 30% sludge depth so will likely need emptied before year 3 is up. pH level is 7, water clear and ammonia levels within spec. 

Edited Monday at 12:05 by Kelvin

[dpmiller]

5 hours ago, IanR said:

but a second chamber is required for an STP

not always. SBRs do an aeration/ settle/ decant cycle, within a single chamber.

[saveasteading]

8 hours ago, SteamyTea said:

Must be more to it than that.

Nope. Except sizing the chambers.

Bacteria wouldn't understand a complex system.

[crooksey]

You can use infiltration tunnels with BCO and EA approval, its very quick to get it approved.

 

Graf make the infiltration tunnels so BCO usually state they want a Graf plant to go with it.

[saveasteading]

What is inside the tunnel? I'm guessing filled with plastic shapes / nodules as found in pond filters. Ie maximum surface area for crud to stick to for bugs to consume.