GRP Septic Tank

  • 16 November, 2020
GRP Septic Tank Diagram drawing
GRP Septic tanks replaced brick built septic tanks about 40 years ago. GRP stands for Glass Reinforced Plastic and is another word for fiberglass. At Allerton we use this material in nearly all of our products. Pump stations, sewage treatment plants, sample chambers, and kiosks for example are all made with GRP. 
 
Fibreglass really is made of glass similar to that in windows or in kitchen drinking glasses. To manufacture Fibreglass, glass is heated until it is molten, then forced through super-fine holes. This creates glass filaments that are extremely thin – so thin, in face, that they’re best measured in microns.
 

Applications of GRP

 
These flexible filament threads can be used in several applications. For example, they can be woven into large pieces which look like rolls of coarse cloth. The final application is dependent on the length of the extruded strands (longer or shorter) and the quality of the fibreglass.
Form some applications, it’s important the glass fibres have fewer impurities, however, this involves some additional steps in the manufacturing process. Other materials can be used to give exceptional strength, and at the same time be much lighter. Kevlar and Caron Fibre are examples. They are expensive to manufacture and not needed when it comes to making septic tanks and containers. GRP is the economic material of choice.
 
We will come to these later but it is worth noting that glass fibre in the form of hundreds of bobbins in a factory combine, and are woven together to make high pressure pipes. It is also possible to force large diameter pipes under a busy road or railway line, without obstructing traffic.
 
The versatility of GRP means that it can be used not just for patching dents in a body of a car, but in the forming the bodywork of the car in the car plant. 
Fibreglass being put into a mould

Fibreglass: A Modern Material

Where brick, concrete and steel were the materials used before to build underground tanks, they are now GRP or fibreglass. After the second world war someone realised that by mixing threads of glass with resin a strong light material could be made. As glass is resistant to most corrosive materials the glass reinforced plastic (GRP) could be moulded to form any shape, and in the case of septic tanks, used to separate solid sewage from the liquor. 

60 years ago Klargester used this material to make GRP septic tanks and later cesspools and eventually many of the huge tanks we see transported on the roads today.

There are septic tanks functioning well enough after 50 years but it is the Gravel soakaway which fail. They simply fill up with black organic matter and 15 years is about the average life of the soakaway.

grp septic tank installation

The ConSept

Conversion units such as the Allerton ConSept use blown air to encourage aerobic bacteria , transforming the septic (anaerobic) regime into an aerobic one where the treated effluent can, with Environment Agency permission, be discharged into the river system. If no ditch is available, the ConSept utilises the air to lift, by siphonic action, the cleaned water to the surface. A shallow soakaway is far more effective than one deep in the ground.

Allerton ConSept unit ready for installation

It is worth noting that raw sewage has a value of about 300ppm. What comes out of a well maintained septic tank is 200ppm of horrible black water, but water which carries two thirds of what went into the tank!

Generally, sewage treatment plant (and the ConSept too) have to produce a final effluent of just 20ppm before it can go to the River system. Rainwater is about 5ppm.The ConSept works well in brick septic tanks but apart from the difficulty in removing the GRP Septic Tanks baffles, once inside the tank it is as if you have a brand new sewage treatment plant. Half the price and no machines on the lawn!

There is a downside for this of course: a capital cost of about £3000 and running costs of around £300 which includes servicing by Allerton, de-sludging every 12 to 24 months and electricity.

We hope you enjoyed reading this topic. If so, feel free to write about it in the comments below.

FAQ

Q: How often does a septic tank need emptying?

A: As a rule, once a year for the average household. It depends on the size of the septic tank and how many people are using it. Two years is possible, depending on the size of the septic tank. If the frequency is more than once a year, it is usually because of a failed soakaway, and that shouldn’t be happening. Check out this table about the pros and cons of septic tanks. Frequent emptying can be quite expensive. Read the section about septic tank emptying on our Septic Tank and Soakaway Problems page for more information.

Q: Are there septic tanks that don’t need emptying?

A. No. All septic tanks should be emptied to protect the soakaway! Read the section about septic tank emptying on our Septic Tank and Soakaway Problems page for more information.

Q: How do I know when my septic tank is full?

A: It’s difficult to gauge, so have it emptied every year. This will help lengthen the life of a soakaway. Learn more about soakaway problems. 

Q: But what if I use a drainage rod with a 3 inch rubber disc?
 
A: Carefully feel to see where the heavier sludge is. Deduct this from the depth of the tank and when there is a foot of sludge it is full. Any deeper and the settlement zone above the sludge level is compromised and the flow of water increases, carrying fine solids into the soakway. Not good.
 
Q: How do I clean my septic tank naturally?

A: It should emptied regularly by a registered waste contractor. The mess inside the walls of the septic tank is actually full of bacteria and should be left alone. Do not hose it down!

Q: Should I empty a Klargester septic tank?
 
A: Yes, every year to protect the soakaway.