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FAQs on Septic System Design - Basics
Q&A Choosing Septic Tank Size, Drain Field Size, Basic Septic Design

Questions & Answers abouit ABCs of septic tank and drainfield design:

This septic system design article series provides basic septic system design parameters such as finding the recommended septic tank volume and conventional recommended onsite wastewater soil absorption system (leach field or drainfield) size, along with some notes on how to calculate these from simple water usage and site conditions.

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Basic Septic System Design FAQs

Photograph of a conventional septic tank during installation.These questions & answers

On 2019-01-15 17 by (mod) - who is permitted to design a septic system?

Eric,

Septic systems are usually designed by a septic engineer who has special training and expertise in septic system design and who knows local site conditions, conducts perc tests, knows local regulations, and prepares a plan to be submitted to the local building or health department for approval.

However an architect is permitted to conduct those same tasks in many jurisdictions - assuming that she has also the appropriate expertise.

Your local building and health departments have the final legal say so for your question.

On 2019-01-15 by Eric

Who design the septic system is it the architect or engineer.

On 2018-12-28 by (mod) - what keeps a septic tank from freezing?

Septic

Please see you question repeated and a very detailed answer now found at

SEPTIC TANK FREEZE PROTECTION https://inspectapedia.com/septic/Septic-Tank-Freeze-Protection.php

On 2018-12-27 by septic tank freeze

what keeps a septic tank from freezing, we had to open ours yesterday and the soil was is now loose above. it has been in use for 40 years and needs to be cleaned and will be as as soon as the weather in iowa permits. we covered the dirt with leaves and a tarp. thanks, bob

On 2018-09-30 by (mod) - do septic systems require root killers like Rid-X?

No it does not.

Thanks for asking, Carl.

The short answer is NO, not for conventional septic tank and drainfield designs - please take a look at SEPTIC TREATMENTS & CHEMICALS

Buy some aerobic septic system designs and some disinfection systems require treatments or additives. You can read about those in the ARTICLE INDEX

And let me know if you have other questions.

On 2018-09-30 by Carlzbad

Does a septic system need a chemical like rid x to properly dispose of sewage?

On 2018-08-14 by Robert

I am looking for any literature or past BQ of any septic tank for 2 flats each with 64 2 bedroom houses and a future expansion plan of about 160 more houses each 2 bedroom?

On 2018-03-28 by Al

I would like to know what size is my septictank they are for adults living in the house

On 2017-10-16 by Anonymous

Tank ke upar
ghar

On 2017-08-23 by (mod) - OSSF system for a Kid's Summer Camp

Right. Yet there has to be an engineering assumption underneath the "number of bedrooms" that translates to "number of occupants" that in turn translates to average daily wastewater flow. That's what we both should look for and report back here.

See for starters WATER QUANTITY USAGE GUIDE

and be sure to see SEPTIC TANK SIZE

In my OPINION the rule of thumb of 50 to 100 gallons of water per person per day may not be the right match for a kids' summer camp
but it would be really interesting to calculate:

- toilet flushes

- showers

- use of laundry facilities

- dishwashing and cleaning by camp staff

You may need a mini onsite wastewater treatment system

see COMMERCIAL SEPTIC DESIGN

and SEWAGE TREATMENT PLANTS, RESIDENTIAL

On 2017-08-23 by Robby

I am working on a project that will utilize septic tanks (several in parallel and series). The WW flows are 20,000 GPD, and influent WW BOD strength is about 450 mg/L and it will be permitted with an effluent strength of 100 mg/L. The effluent will be disposed of via surface irrigation. I have not been able to find any literature on how to size a septic tank using influent and effluent BOD loadings. All literature I have found is based on number of bedrooms. This OSSF system will serve a Kid's Summer Camp.

Thanks and I look forward to your response. Robby

On 2017-08-23 by (mod) - size a septic tank using influent and effluent BOD loadings

Robby, Please see your question and our detailed reply now found at SEPTIC TANK SIZE

On 2017-08-23 by Robby

I am working on a project that will utilize septic tanks (several in parallel and series). The WW flows are 20,000 GPD, and influent WW BOD strength is about 450 mg/L and it will be permitted with an effluent strength of 100 mg/L.

The effluent will be disposed of via surface irrigation. I have not been able to find any literature on how to size a septic tank using influent and effluent BOD loadings. All literature I have found is based on number of bedrooms. This OSSF system will serve a Kid's Summer Camp.

Thanks and I look forward to your response. Robby

On 2017-04-18 by (mod) - how to seal septic tank concrete lid or lid on the D-box

Mikie

If your concrete septic tank lid-to-tank mating surfaces have a pre-cast tongue and groove shape then you'll want to use a gasket such as the products sold by ConSeal (http://conseal.com/ Tip City Ohio USA, 800 332 7325 ) (InspectApedia has no relationship with companies / products discussed here and we do not sell anything - Ed.)

If you are sealing a concrete septic tank lid that does not use an integral gasket, you'll want to use butyl sealant/caulk as it has good adhesion properties with concrete. But take care: the concrete surfaces must be clean and dry, otherwise the sealant will not bond nor seal properly.

On 2017-04-18 1 by mikie

what type of sealant do i use when reinstalling concrete lid on my distrabution box
on my septic system

On 2016-11-13 by (mod) - definition of "step" in drainfield or soakbed piping

Charlie:

A for-sure answer would depend on the context in which the word "step" or "stepped" draining piping is used, but typically stepped piping refers to how the wastewater distribution pipes are placed in soil along a slope: runs parallel to the fall line have to step down at the end of each run, across a required separation space, then begin the next run across the fall line going back in the opposite direction.

On 2017-01-22 20:41:59.112863 by charlie

What doe's the word "step" mean when speaking of septic drainage pipe to drain field?

On 2016-11-13 by (mod) - who is allowed to design septic tanks?

Abdul:

A septic engineer or perhaps a civil engineer trained in onsite waste disposal can design septic tanks in most countries. Beware that home-made "septic tanks" can be dangerous, risking fatal collapse.

On 2016-11-13 by Abdul Sayed

who can we design septic tanks i want information by email saeed.ghafoori@yahoo.com tanks

who can we design a septic tank by formula

On 2016-08-01 by josh

Thanks for the response

Will discuss floating material position, scum and sludge depths and visual inspection results in depth with a septic guy next time I pump.

On 2016-07-31 by (mod) - does it matter if a septic is turned the wrong way with incoming sewage entering at the tank outlet end?

Well indeed backwards piping of a septic tank can cause trouble, as the relative elevation of inlet is usually intended to be higher than outlet - we are avoiding flooding and backflow and clogging.

The normal level of sewage in a tank is just at the bottom of the outlet pipe. Search InspectApedia for SEPTIC TANK LEVELS to see details.

Search InspectApedia for SEPTIC TANK PUMPING FREQUENCY to see a guide on how often to pump your tank.

On 2016-07-31 by Josh

My septic service pump man says the top half of my 1000 gallon concrete tank was installed 180 degrees wrong. In other words, the inlet opening is where the outlet opening should be. It is also not level - it is pitched a bit toward the outlet. This system has been working fine for 46 years, however.

In it's first eleven years there were six people in the house, then two for a few years when my wife and I moved in, then four as my family grew, and now two again. I used to have it pumped every five years. My septic guy said something about toilet paper a few years ago after pumping it and suggested it should be pumped every two years.

The effluent level is just at the bottom of the outlet. I can't see how the sludge level can be higher than it used to be. What questions should I ask to understand this?

 

On 2015-07-28 by (mod) - how to find out what kind of septic system we have

Lauren it sounds as if you are describing either a mound system or a pre-cast concrete seepage pit or cesspool system. Because there are life safety hazards (falling into a septic tank is fatal) you would be best served by having an on-site inspection and if necessary excavation to be sure that you not only know the type, but the condition of the equipment and to be sure that there are no missing or unsafe covers.

Beware: don't let a madman with a backhoe tear up the whole property: doing so is unnecessarily expensive and destructive. An experienced contractor knows how to spot probably equipment locations and to dig by hand or with a gentle touch.

On 2015-07-27 by Lauren

Would you be able to tell me how to find out what kind of septic system we have and exactly where leaching field is? We live in MA and were told it is a beehive. There are 2 tanks and they are made of concrete.
Many thanks!

On 2015-04-13 by (mod) - assumptions when debating acceptable septic design with the code official?

Dep

With all due respect, things are getting a bit out of hand here. We are piling assumptions atop of assumptions and waving our arms wildly. This is just too much speculation for me to have confidence in any conclusions we might try to draw. Having seen septic systems installed, done a few, made lots of measurements, dug lots of holes, I am not likely to bet the farm (or your farm) on so much speculation.

If there is a problem with system function it needs to be explored and diagnosed.

If there is a compelling need to know more accurately how the drainfield trenches were constructed, let's take a post hole digger or a spade and dig some representative cross-sections up to see what's there.

On 2015-04-13 by Deputy B

Thanks for your response regarding my question on the measurements. I have a follow-up septic system/math question. :-) The inspector indicated to me that he did use the as-built plan to determine the 5 feet 8 inch measurement for the bottom of the soil absorption system.

For argument's sake, let's assume the as-built plan is accurate. Based on my "knows enough to be dangerous" research, it seems like the inspector used the top of foundation as a benchmark and determined the level of the lowest distribution box by subtracting 95.3 from 100.0. This gives you 4.7 feet or 4 feet 8 inches.

If you then add 12 inches of stone, you get 5.8 inches, which is the amount the inspector indicated the soil absorption system was below grade. However, my house is a split-level with windows in the basement so the top of my foundation is about 2 feet 8 inches above the ground. I'm assuming that the measurement should have taken that into account.

If my assumption is accurate, the distribution box with the 95.3 measurement would be about 2 feet below ground (which seems to be a reasonable number compared to 4 feet 8 inches) and the stones a foot below that would put lowest point of the soil absorption system at 3 feet. In addition, the last inspection of the system in 2002 indicated the ground water level was at 5 feet and the system passed.

If the lowest part of the soil absorption system was 5 feet 8 inches, it should have failed. I've owned the house since then and there have been no changes to the septic system. Let me know your thoughts.

Question: hole in a septic piping elbow?

(Feb 19, 2014) Anonymous said:

I have a separate tank for my waste water (grey water). my solids run into my first tank and then into the waste water tank. The waste water is pumped uphill from my waste water tank. the pipe ,which is 2" , makes an upside down 'u'. where the 2" pipe turns horizontally and heads up hill, there is a 3/8" hole in the bottom of the elbow.

Is this hole supposed to be there? when I turned on the pump after unthawing the tank I noticed the hole. I replaced the elbow and hooked everything back up. I guess I am just looking to make sure that hole is supposed to be there or not. I didn't put a hole in the new elbow. I am hoping that this is correct.

Reply:

Anon I'm not sure I have a clear understanding of your system and piping but I'm guessing that the "upside down U or elbow" you describe may have included an air inlet to allow air back into the line so that when the pump stops and wastewater wants to drain back down into the downhill-loated wastewater tank we don't back-siphon water out of the drainfield.

If that's the case, the opening is needed. But why it would be on the underside instead of on top is a point I don't get either. On the underside I'd think that wastewater would leak out of the vent opening during pumping.

Question: how could the septic inspector have measured distance to ground water?

Deputy B said:

We had a septic system inspection done and the company measured the ground water level at 48 - 50 inches based on the information from an abutting property. They indicated that the soil absorption system was at the proper level but the stone in the trenches was 5 feet 8 inches below grade.

There is no engineering plan on file. The as-built document says that there are 12 inches of stone under the pipe in the trenches, but I'm having a hard time figuring out how they came up with the 5 feet 8 inches measurement.

The only related information in the as-built plan indicates the elevation for the top of the foundation at 100.0 and the various "d" boxes at 96.2, 96.0, 95.6, and 95.3. I'm not sure how that information is used to determine the level of the stone in the trenches.

Can you explain how that measurement was derived with the information in the as-built plan? Assuming the measurements by the inspection company are accurate, is it possible to rectify the situation without putting in an entirely new system? Thanks in advance

Reply:

Deputy

Like you I'm impressed that it sounds as if someone used X-ray vision or some other technique to determine the distance between the bottom of the septic system drainfield trenches and the seasonal high level of local ground water: at least that is what one should be measuring.

It is possible that your inspector is reporting data that was on the original or as-filed septic system plans for your home, in which case Kudos to the inspector for taking the trouble to go dig up local septic system records.

At the time that a septic system was originally designed and approved an engineer had test holes dug, percolation rates assessed, and the septic soakbed or drainfield plan would have (or should have) included an assertion that the design will meet your local requiements for the separation distance between bottom of the trenches and top of seasonal groundwater. Typically that's around 24". More in some areas.

Stone levels in trenches might also have been specified by the septic engineer.

Really? However in my OPINIOIN it is misleading (to put it politely) to suggest that those data were actually visually or physically inspected at the time of your septic system inspection and test unless the inspector actually did so.

And it's worth keeping in mind that "as filed" or "as planned" is often not exactly the same as "as built" since the plan can't necessariliy know that the excavator is going to hit a boulder, find higher ground water than records indicate, or that she or he took a short cut and left out most of the gravel (which after all costs something).


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