In a nutshell: the issue

The process

Organic Sulphates and Inorganic Sulphides are present in the slimes of sewerage flows and form the Total Sulphides present in the sewerage.

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• A fraction of the dissolved Hydrogen Sulphide is released into the sewer atmosphere as a gas.

• A process of bacterial reduction occurs when the Hydrogen Sulphide, in combination with the other gases, such as Carbon Dioxide, is progressively oxidised, reducing the Ph to the point where it splits off with water molecules to form Sulphuric Acid.

• The powerful inorganic Sulphuric Acid reacts with hydrated calcium silicates and lime in the cement structure and reduces it to a soft swelling paste.

• Conditions are optimal for this process of Sulphide attack when there is high
biological oxygen demand, presence of sulphates (Common in all water systems), higher ambient temperatures (quickening the kinetics of breakdown and reduction), long detention times (Flat grades – low velocities).

The options

Various methods have been used and tried in the past, these include:

Overdesign of Sewerage Systems, including greater wall thickness in pipes and tunnels and the provision of ventilation shafts and machine ducted air to reduce and remove the Hydrogen Sulphide gas.

These methods are very expensive and are only partially effective.

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Bacteriocides: These are added into concrete, using a special process of Octration with the express aim of preventing bacteria forming on the concrete surface. The effect is strictly limited and is practically ineffective, being, at best, a palliative measure.

Provide special concretes: (Calcareous aggregates, Type 5 Seawater cement, Hi Alumina cement such as Ciment Fondue) These can sometimes delay the onset of serious Hydrogen sulphide corrosion (in case of calcareous aggregate) but they are purely palliatives, in the medium term.

Liquid applied coatings: Such as Bitumens, Coal-Tar epoxies and even expensive urethane elastomers.

These all eventually fail owing to the phenomenon of ‘pin-holing’ that occurs when coatings are sprayed onto a surface. These small asperities in the body of the applied film provide an easy route to the concrete substrate for the small Hydrogen Sulphide molecule. Also concrete provides a difficult and uneven substrate to paint or coat, with unviable surface tension effects.

The solution

The solution was found in 1947 when the first T-ribbed liner were made in the USA.

Being cast into concrete, at the time of manufacture, the T-ribbed lined pipes were fully protected by mechanically anchored PVC plastic liners.

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• Provision of a lining made of flexible plastic that is chemically inert when subjected to Sulphuric Acid attack. The lining shall be made from thermo-plastic PVC or other flexible plastic, provided there are no pin-holes and that an effective way of attaching the plastic to the concrete is devised.

• Paint or coatings do not adhere very well to concrete – neither do glues or adhesives - PROBLEM

• These linings were extruded under heat and pressure to form a high grade plastic of high molecular weight, intrinsically free of voids and pin-holes in its structure. Further, the lining was thermo-plastic, meaning that it could be heated to melting point, allowed to cool, and it would remain unchanged in all physical and chemical characteristics. This meant that the material could be fused to itself by the application of heat, thus rendering it weldable into continuous lengths.