Plastic Sewage Chambers Using Rotational Molding Technology

by Miki Burmil, Rotational Molding Consultant

Twenty two years ago I founded a plastics production plant using Rotational Molding. I decided to focus on underground chamber products including:

• Manholes and inspection chambers for sewage
• Chambers for electricity and telecommunications
• Pump stations

These products have traditionally been produced using concrete. Their transition to plastic has brought about many benefits. After wide market research and a comprehensive study of this family of products, I have found additional reasons that support my decision, one of them being the high added value.

The Correct usage of RM technology offers the ability to provide solutions resulting in improved products. In this article, I shall concentrate on manholes and inspection chambers for sewage, being the largest segment of the underground product family.

The manholes and the inspection chambers are an integral part of the sewage system, which is used for transporting wastewater from buildings to purification facilities. This is a gravitational system, and the flow is achieved by installing the pipes at an incline of 1-2%. The main components of the sewage system are:

• Pipes for transporting waste water
• Fittings for connecting pipes
• Chambers for Cleaning, inspection, and 
changing the direction of sewage flows.

The benefits of using plastic instead of concrete 
for sewage chambers are:

• Excellent resistance to chemicals
• Smooth flow preventing blockages in the system
• Fast and easy installation
• Low weight (for example: if we take the very popular chamber of 1,000 mm diameter and height of 1,750 mm, the plastic one weighs 100 kg as opposed to the concrete one that weighs 2,500 kg, which is 25 times more!)

Protecting the environment:

• The chambers are totally sealed. This prevents 
leakage from the sewage system and contamination 
of ground water.
• Using plastic manholes significantly reduces costs 
of energy, transportation, and installation. No cranes and heavy equipment are required.
• Prevention of root penetration into the chambers.
• The products are recyclable.

Designing Manholes Based on a Modular System
When designing the product it is essential to specify it and understand the function it takes in the sewage system in order to give an appropriate response to market requirements.

On the one hand, Rotational Molding technology enables the production of volumetric structures with complicated shapes. On the other hand, the design must offer an adequate solution to the main disadvantage of the technology – the difficulty in keeping the accurate dimensions of the product.

Product Specification
The product is an underground technological product, which should meet stringent standards and be capable of withstanding heavy loads. Since the system carries wastewater, it is essential that it is properly sealed in order to prevent ground water pollution. It is also necessary to take into account that the range of products should be wide with many variables.

The height of the chamber depends how deep down it is placed. This height changes according to the topography of the terrain where the project is to be carried out and on the length of the pipes. Since the pipes are installed at an incline, the longer the pipes are the installation is deeper, resulting in a deeper chamber. The chamber height can vary between 0.5 – 6 meters and more. The chamber diameter also changes and depends on the depth into which the chamber is placed. The bigger the depth, the wider the chamber diameter. The maximum height for low chambers is 1.25 meters with accepted diameters of 400, 500, and 600 mm. These are called inspection chambers.

Chambers, which are higher than 1.25 meters, have accepted diameters of 800, 1,000, and 1,250 mm. These are called manhole chambers. On top of these chambers there is an opening of 600 mm to allow entry of a man, and steps are required in order to descend to the bottom of the chamber.

The shape of the chamber should be designed in such a way as to withstand heavy loads and should include reinforcing ribs and appropriate wall thickness. A suitable design should provide solutions for all these requirements.

The Modular System
The idea is to use three basic elements as a modular system. 
The three elements are:

• The Base Unit is the lower part of the chamber to which the pipes are connected. There is one outlet and a variable number of inlets (from 1-5) at the base of the chamber manhole. The inlet diameters into the chamber for the pipes vary and have to correspond to the pipe diameter which varies from 100 – 1000 mm. The inlets have to be compatible to the different types of pipes used including smooth and corrugated pipes.

There is a wide range of bases in different diameters, number of inlets, and pipes diameter required. It is necessary to use special inlet seals, which have been specifically developed. In case of PE or PP piping, it can be joined to the base by welding instead of using seals. There are a number of methods for welding.

• The Elevation Unit is the part that allows setting the desired height of the manhole.
• The Upper Unit (cone) allows the reduction of the 
manhole diameter to the uniform diameter of the cover – 60 cm (man-hole).

The number of units required for installing a chamber varies according to the height required. One or two elements are required for installing inspection chambers.

Manholes – chambers with large diameters, higher than 1.25m, require a larger number of units. For example, a 6 meter high requires 6 units: one base unit + 4 elevation units + one upper unit.

The units are joined in two different ways – by using special rubber seals or welding by hand extruder.

Covers for the Chambers
Covers are required to complete the product. Standard covers are usually used for concrete chambers. These covers are made of concrete or cast iron.

Manholes chambers installed under roads require the capability of withstanding heavy loads. Therefore, concrete rings are used in order to spread the vertical load. In this case, the chambers meet the D Class category standard, which means withstanding the weight of a 40-ton truck.

Chambers installed in pedestrian areas require the capability of withstanding lower loads. In this case, plastic covers can be used. They are produced either by rotational molding, injection molding, or reinforced polyester.

Standards and Quality Control
In 2009, after 6 years of discussions, the European Committee has confirmed the Standard for plastic manholes – EN13598. ISO Standard based on EN Standard was confirmed on October 2011– ISO 13272. The EN13598 Standard includes two parts:

• Standard EN -13598-1 is the Standard for shallow inspection chambers of less than 800 mm diameter.
• Standard EN-13598-2 1 is the Standard for manholes in traffic areas and deep underground installations – above 1.25 meter height and 800 mm diameter and bigger.

The main standard tests are:

• Vacuum test
• Stiffness
• Water tightness
• Vertical load test
• Load test for steps
• Impact test

Maintaining Quality Control and quality assurance is essential. Setting up a laboratory with suitable equipment for testing materials and products is of the very utmost importance. Manufacturers are advised to build their own testing facilities for regular quality testing and product improvement.

A high level of molds is needed in order to obtain high-level products.

European Market Situation
The pipes and fittings are usually manufactured from PVC, PE and PP by extrusion and injection molding. These technologies are very common and the transition to plastic pipes and fittings was relatively quick even though chambers were still being manufactured from concrete. So, for many years, plastic pipes were used together with concrete chambers requiring the use of special connectors, which were very costly.

The transition to the production of plastic chambers started at a later stage, with the solutions offered by RM technology. Today’s sewage systems use over 90% plastic pipes and fittings. However, over 90% of the chambers are still produced from concrete. There is every reason to believe that the market for plastic chambers will grow and equal that of the pipes. This market segment has great potential.

There is a very strong lobby of concrete manufacturers trying to prevent the entry of plastic chambers into the market. The completion of the Standard for chambers will promote the marketing of plastic chambers. Increased awareness the world over of the use of advanced products, which contribute to protecting the environment, also helps towards the marketing of these products.

The chambers are usually sold for infrastructure projects with a scope of up to several thousand products per one project. For example, for one specific project we were asked to supply around 2,000 chambers. As there is an average of four elements per chamber, this meant that we had to produce around 8,000 units for this project.

Efficient Production of Chambers
It should be pointed out that, despite RM technology being somewhat slow, chamber production is faster.

The wall thickness of the products is 10-12 mm and they weigh on average about 30kg per unit. The volume of the product is low compared to its weight, thereby enabling more efficient production – approximately 500 kg/hr.

It is possible to assemble two spiders on a straight arm using a suitable machine with a 4.5m diameter oven. Four molds can be connected to each spider giving a total of eight different molds on one arm. Oven time per arm is about 25 minutes. In this way, it is possible to achieve an average cycle time of about 3 minutes only per single product (8 products at 25 minutes).

Conclusion
Today, I can say with great satisfaction that my decision to embark on this topic 22 years ago was successful, very varied, and fascinating. Eight years after having founded the plant we sold it to our competitors. I continued as a consultant to clients interested in developing production lines for these products. I have gained a great deal of experience with production of underground products including:

• Layout of the plant
• Acquisition of machinery and suitable auxiliary equipment
• Designing and producing molds
• Determining production/recipe data to assure the production 
of quality products
• Choosing suitable raw material
• Erecting laboratories and testing facilities
• Writing Procedures for Quality Control and Quality 
Assurance Systems
• Developing improved products and finding solutions for 
market demands

I have no doubt that the next generation of plastic sewage chambers will replace the traditional concrete products.

from an article at RotoWorld®
https://rotoworldmag.com/plastic-sewage-chambers-using-rotational-molding-technology/