Gravity fed or pumped chamber to drainage field?

[flanagaj]

I wanted to ask whether there are any merits in having a collection chamber after the sewage treatment plant, which collects and then periodically pumps the contents into the drainage field pipes quickly so that the waste water in dispersed around all of the perforated pipe.   Our initial plan was to simply allow the waste water from the STP to drain as and when it is required.  I know some STP systems do exactly this and others Graf do not.  The logic apparently, is that by pumping a large volume you will utilise all of the perforated pipe, where as, allowing it to trickle out, means that only the early section of the field is used.  Obviously, as time moves on the early section of the field will probably slowly start to silt up and the water will naturally travel further into the field.   So my reasoning on this one, is that overtime the net result is the same and the field would have the same lifespan.

Logic correct or flawed?

[Bonner]

Keep it simple, use gravity to do the job. The drainage field will balance itself over time. I would only use a pump if absolutely necessary.

[ProDave]

Definitely.  Our past house had a pump out of necessity as the drainage field was higher than the treatment plant.  The pump would last somewhere between 5 and 7 years before it burned out and needed replacing.  I was SO glad when our present house was given permission to discharge to the burn, no pump, no drainage field.

[saveasteading]

What comes out is liquid and looks almost clean. If you have a test chamber you will have the comfort of seeing, even testing,  this. A chamber is a good idea anyway.

What muck remains in the effluent will continue to break down in the soakaway.

A pump should be unnecessary and is another thing to go wrong. I wonder why a manufacturer has one included apart from draining uphill.

 

[Kelvin]

The Graf system discharges the water twice a day using the compressor ‘pump’ the water out of the tank into the drainage field which makes quite a loud bubbling noise so I wouldn’t want it right near the house. It also has a built in sampling chamber. 

Edited August 11 by Kelvin

[crooksey]

Your logic is correct, but flawed in execution.

 

A correctly designed drainage field that matches the percolation rate of the ground (during the winter) and sized accordingly to building regs will have no issues dealing with an output from the graf style outlets (twice a day volumes).

[Kelvin]

Here’s video that explains how their air lift system works. As @crooksey says the Graf system would work perfectly well with no extra pumps in the tank. It’s all done from the compressor. 
 

 

[flanagaj]

2 hours ago, Kelvin said:

The Graf system discharges the water twice a day using the compressor ‘pump’ the water out of the tank into the drainage field which makes quite a loud bubbling noise so I wouldn’t want it right near the house. It also has a built in sampling chamber. 

Thanks.  I was unaware of this and thought it just tricked out as new effluent entered the tank.

[Kelvin]

3 minutes ago, flanagaj said:

Thanks.  I was unaware of this and thought it just tricked out as new effluent entered the tank.

It shouldn’t ever be that full that it could flow out. It’s quite a pronounced bubbling sound as it lifts the water up the pipe into the sampling chamber. I have an SVP after the tank (best practice Graf install) and that’s where the sound is loudest. 

[saveasteading]

2 hours ago, flanagaj said:

thought it just tricked out as new effluent entered the tank.

Anybody know a good reason for pumping it out. Gravity seldom fails so I like it.

 

2 hours ago, Kelvin said:

shouldn’t ever be that full that it could flow out.

Ditto. Always full for the chambers that work that way.

A big plus for me is that they will continue to work 90% ( well enough) if there is no power or the pump breaks

 

[Russell griffiths]

The water from our graff tank is clear and completely odourless, it’s not likely to bung up your drainage field 

unlike my other septic tank which is horrible. 

[Kelvin]

2 hours ago, Russell griffiths said:

The water from our graff tank is clear and completely odourless, it’s not likely to bung up your drainage field 

unlike my other septic tank which is horrible. 

It surprised me how clear it is. pH of 7 too. It’s likely better quality than the rainwater going into our other drainage field. 

[saveasteading]

46 minutes ago, Kelvin said:

better quality than the rainwater

For fertiliser yes.

[Kelvin]

I have a full water testing kit. I might run a sample through it. 

[flanagaj]

I was just looking at invert levels again today and as the soil pipe run is 28 metres from the first inspection chamber.  The invert levels are as follows.

 

1:40 - 1340mm

1:60 - 1110mm

1:80 - 990mm

 

This is because the first IC has an invert of 640mm.  I don't think I can do anything about that as the slab floor make up of concrete/insulation/sand blinding/harcore is 500mm, add 110mm and you are at 610mm.  I assume that the soil pipe has to go under the type 1?

 

The reason for my post was that I was considering going with the Graf pumped option as it would mean the 8m * 4m field that I need to excavate will be 500mm higher than if I went with the gravity outlet option.  Just wondering whether the pump will be loud.

 

 

Edited Tuesday at 21:04 by flanagaj

[Kelvin]

If you mean the Graf One2Clean with the blower unit then it can be sited remotely from the treatment plant by up to 20m (from memory) You could also build it into a small shed of some sort to deaden the sound. They do a green plastic housing for it which is what we have. It’s not noisy but you would hear it if it was close to your garden sitting area or an open window. It’s a gentle hum that comes on and off. Like I said before what makes more noise is when the air ram pump is emptying the water assuming you fit an SVP at the tank (which is their standard install) You can hear the water bubbling out of it. 

[flanagaj]

23 minutes ago, Kelvin said:

If you mean the Graf One2Clean with the blower unit then it can be sited remotely from the treatment plant by up to 20m (from memory) You could also build it into a small shed of some sort to deaden the sound. They do a green plastic housing for it which is what we have. It’s not noisy but you would hear it if it was close to your garden sitting area or an open window. It’s a gentle hum that comes on and off. Like I said before what makes more noise is when the air ram pump is emptying the water assuming you fit an SVP at the tank (which is their standard install) You can hear the water bubbling out of it. 

No. It's the unit where the invert level of the outlet is higher than the inlet.  It has a pump that then pumps the treated water to another tank higher up.

 

[Kelvin]

Ah right the  thing they call the +O module. Ask them for the technical specs it will possibly have some details on how quiet it is. Presumably it also switches on/off to pump the water. To some extent something switching like that can be more annoying than a constant hum you might get used to. Depending on where the tank is you could screen around it with something. 

[Russell griffiths]

You want your first ic as high in the ground as possible, if your bathrooms are on an outside wall this is easier, if a bathroom is in the centre of the house it will get deeper as you cross the house. 

[flanagaj]

1 hour ago, Russell griffiths said:

You want your first ic as high in the ground as possible, if your bathrooms are on an outside wall this is easier, if a bathroom is in the centre of the house it will get deeper as you cross the house. 

I agree, but if the sub floor structure is as follows, then the soil pipe needs to be buried below the layers in the diagram below?   This will then dictate that the invert of the first IC 28 metres from where the tank will be located?

[Russell griffiths]

Depends what distance this pipe is traveling across the house

if the bathroom is on an outside wall you can lift the pipe up in the layers, it doesn’t have to be under the type 1. 
if the bathroom is in the middle of the house you need to consider the structure of the floor above the pipe. 
 

lift the house higher 

[flanagaj]

The problem is now resolved.   Graf are pretty useless when it comes to their online documentation and the various configurations of their tanks.   It was only when I came across a web site selling them, did I realise that there are actually 4 outlet configurations.  Every other site just lists the tanks as 'Gravity' or 'Pumped' outlets.

 

So you can raise the outlet by either 300mm or 515mm with the standard lift system that comes with the tank.  I was just about to hit buy on a pumped tank.

Having a raised outlet helps as it means it puts my drainage field above the fast draining chalk layer and into the soil layer that gives me a > 12 Vp reading.   This not only saves a huge amount of excavation, but also having to put a 700mm sand layer underneath the drainage field to increase the Vp so that it meets the criteria.

[Kelvin]

Good stuff. I found Graf more helpful when I called them. Their website is quite poor. 

[crooksey]

The key to remember here is all to do with the winter water table level.

 

If your tank is deep, there is a chance your drainage field could be below the level of the winter water table (therefore useless). If you have a pumped outlet, you can have the field 300mm below the finished ground level and remove any doubts about winter water table level.

 

Or go gravity, and have an NRV on the drain-field and have an overflow tank, so that in the event your drainage field is under-water, the outlet of the PTP goes into an ancillary tank, that can then be pumped out whenever back to the drain-field when the water table drops.

 

I have been on sites where one winter the water table was -3m, the next winter it was minus 200mm. Sandy soils are worse for high water table as there is no resistance to it (although offer low vp).