Septic tank for grey water

[Bassosa]

Hi guys,

My builder is proposing a plumbing system that seems strange to me in its setup.

Basically, grey water and black water are split.

One 4000 liter septic tank handles the waste of 21 toilets

A separate 4000 liter septic tank (same make & model) handles all the grey water, but grey water passes through a 1200 liter grease trap before it enters the septic tank
 

So, is it common in Thailand to use a septic tank to treat grey water?

Is 4000 liter enough for the waste of 21 toilets? The building is a hotel.

Also, I've spent hours looking online for a calculator which tells you how big your septic tank should be, but they all assume both black and grey water pass through it. 

 

Any input greatly appreciated.

[JimShortz]

In my opinion that septic tank size should be manageable for your 21 toilets - at worst you will need fairly frequent pump outs - but cheap and available. All of the modern tanks have an outflow of liquid from the surface that I assume will go to a French Drain of some basic sort (leaky pipe into the ground), so you only really need to pump out build ups of solid waste. My concern would rather be the connecting of so many toilet pipes to one tank sounds like blockage heaven to me - but I am no expert, only homebuilding experience. I chose two smaller tanks for my four toilets to improve the straightness/ease/length of connections.
Seperating greywater here is perfectly normal, and if you don't you are risking toilet gas smells backing up into your hotel bedrooms via crappy shower water traps that are in common use here. Most actually either go straight out into street drains or directly into French Drain soak aways, as mine does, with just the kitchen sink getting its own grease trap to try and reduce the underground fouling.
Good luck!

 

[Langsuan Man]

In my opinion and after having a Thai septic system with the large black tank as described installed,  I think the grey water thing is a myth.  I have never seen grey water in over 7 years in my drains and since my entire development has the same system feeding into the main mooban drainage system, no grey water there either

 

So far spent 1,200 THB to have the tank emptied after 4 years, and 700 THB recently so I am not worried whether or not the system works 

 

There certainly is no Thai agency that is going to come check to insure I am not polluting,  unlike the US,  where the local health department checks every year    

[Fruit Trader]

1 hour ago, Langsuan Man said:

In my opinion and after having a Thai septic system with the large black tank as described installed,  I think the grey water thing is a myth.  I have never seen grey water in over 7 years in my drains and since my entire development has the same system feeding into the main mooban drainage system, no grey water there either

 

So far spent 1,200 THB to have the tank emptied after 4 years, and 700 THB recently so I am not worried whether or not the system works 

 

There certainly is no Thai agency that is going to come check to insure I am not polluting,  unlike the US,  where the local health department checks every year    

Grey water a myth?

 

Our rent rooms separate the grey water and septic run off with some of the grey water used for garden irrigation. Grey water from the laundry area and rooms would most likely kill or reduce the septic system efficiency if allowed to enter the septic line.

 

The local authority has checked how we handle waste water before allowing it to enter city drains.

Below is a typical home system separating grey water from septic. Its a good idea because the grey water lines in this example often contain bleach and detergent from laundry machines.

 

[Bassosa]

On 1/17/2020 at 1:06 PM, JimShortz said:

In my opinion that septic tank size should be manageable for your 21 toilets - at worst you will need fairly frequent pump outs - but cheap and available. All of the modern tanks have an outflow of liquid from the surface that I assume will go to a French Drain of some basic sort (leaky pipe into the ground), so you only really need to pump out build ups of solid waste. My concern would rather be the connecting of so many toilet pipes to one tank sounds like blockage heaven to me - but I am no expert, only homebuilding experience. I chose two smaller tanks for my four toilets to improve the straightness/ease/length of connections.
Seperating greywater here is perfectly normal, and if you don't you are risking toilet gas smells backing up into your hotel bedrooms via crappy shower water traps that are in common use here. Most actually either go straight out into street drains or directly into French Drain soak aways, as mine does, with just the kitchen sink getting its own grease trap to try and reduce the underground fouling.
Good luck!

 

Thanks for your comment.

I definitely like the idea of separating black and grey water, for the reasons you mention.

I guess we're going to find out whether a 4k liter tank is enough for our use!

[Bassosa]

Just now, Bassosa said:

Thanks for your comment.

I definitely like the idea of separating black and grey water, for the reasons you mention.

 

And yes, our effluent does get released into the road drain.

I guess we're going to find out whether a 4k liter tank is enough for our use!

 

[MikeN]

4 hours ago, Bassosa said:

Thanks for your comment.

I definitely like the idea of separating black and grey water, for the reasons you mention.

I guess we're going to find out whether a 4k liter tank is enough for our use!

According to the DOS website a 2000L tank is enough for 30 people at a business premises.