Effective corrosion management

As one would expect, the acidic strength of the wastewater (which is determined by the number of such micro-organisms) plays a crucial role in determining the type of corrosion management measures that need to be taken. If the pH level is above 3, the risk of attack can be minimised simply by using either a concrete which is based on a sulphate-resisting cement or calcareous aggregate, or by increasing the thickness of the concrete.

If the pH level falls below 3, however, some form of surface protection is invariably needed to maintain the integrity of the concrete. A wide range of surface protection coatings and linings are currently available which are typically based on acrylic, epoxy, furan, vinyl ester, polyester and polyureth-ane resins. Because of their compatibility with concrete and their excellent tolerance of variable conditions during installation, epoxy resins are most readily used in wastewater treatment facilities.

Formulated from polymer resins with a balance of modifiers, fillers, and other active ingredients, linings are usually reinforced with either glass cloth or glass mat. The coefficient of expansion between concrete and unmodified resins naturally differs, so the modifiers and reinforcements used in linings are carefully selected for their ability to adjust the coefficient of expansion. As well as adhesion, physical stability and permeation resistance, the resins must provide the flexibility to adhere to concrete in a wide range of service conditions.

Lining systems are invariably utilised in immersion applications because they can withstand these aggressive chemical environments better than coatings. Linings are typically between 2,000 and 5,000 microns thick and usually applied by trowel and roller.

Coatings are generally used for severe spillage or immersion conditions in ambient temperatures and in areas where abrasion resistance is not absolutely necessary. This is because coatings do not provide the resistance to the permeation driving force produced by elevated temperatures. High-build coatings which are filled with inert flakes such as graphite and mica normally range in thickness from 200 to 1000 microns. Generally coatings are applied using various techniques such as brush, roller, conventional sprays and airless sprays.

The quality of the concrete plays an important part in the performance of the lining or coating. It should meet the requirements specified in DIN 1045, Concrete and Reinforced Concrete: Design and Construction.

Whilst the compressive strength of the concrete should be at least 25 N/mm², the most important property affecting the performance of the coating or lining is tensile strength. If a force of at least 1.5 N/mm² is required to remove a machined-pipe cap that has been glued to the surface of the concrete this indicates excellent tensile strength. A force of 1.3 N/mm² – 1.5 N/mm² is, however, acceptable. Other important considerations relating to the concrete are:

low water content results in high compressive strength

moisture content over 3 per cent adversely affects application of the lining or coating

a slow drying time promotes high surface strength

poured concrete should be float finished and troweled to produce a dense surface

formed concrete may need defects, such as air pockets and pinholes, filled with an appropriate material prior to application of the coating or lining

concrete structures should be on gravel or sand rather than soil, especially clay, to minimise capillary flow of water from the soil through the concrete

The most commonly used surface protection systems in wastewater treatment facilities are glass cloth-reinforced linings, glass mat-reinforced linings and spray-applied, flake-filled coatings.

In particularly abrasive environments, such as thickener tanks, clarifiers and grit chambers an appropriate lining is likely to combine a resin with two or more times its weight in filler and include glass cloth-reinforcement.

Typically the system would consist of a 1.5 mm trowel-on base coat, a layer of woven glass reinforcement, a saturating layer of resin and a finish coat that has a different filler than the base coat. The application of a thin base layer (1.5 mm) minimises the total shrinkage stress of the lining system and allows the glass cloth-reinforcement to adhere to the substrate. The glass cloth-reinforcement further reduces shrinkage and strengthens the lining.

The 1.5 mm top coat fills any voids in the base and reinforcement, whilst the filler in the topcoat should provide abrasion and impact resistance.

Typically used to protect junction boxes and sludge dry beds, glass mat-reinforced linings involve the use of chopped strand fibreglass mat in combination with a permanently flexible resin system. Such a design provides crack-bridging capabilities which are unavailable with other linings or coatings.

Installation is similar to that of glass cloth-reinforced lining, except that a glass mat is applied onto the base coat and saturated with resin, which is applied by roller. A second layer of glass mat is placed on the wet surface and saturated in the same way. The mat should overlap all joints by at least 25 mm, but the laps of the two layers should not be in the same place. Before the second layer has hardened, a surface veil or tissue is laid on top and rolled, and once this has set any bumps or loose fibres are sanded off. A resin top coat is then applied as the final layer.

Spray-applied, flake-filled coatings are generally used in pumping stations, primary effluent collecting chambers and mixing boxes, effluent channels, anaerobic basins and anaerobic/anoxic common effluent channels. Application by airless spraying is preferred because, firstly, it enables the flakes to be oriented parallel to the surface and, secondly, the coatings do not require thinning prior to application.

At least two coats, with a final dry film thickness of 200-1,000 microns, are applied depending on the severity of the application. And, in order to ensure good coverage during application, different colours are used for each layer.

Coatings and linings offer a technically favourable and cost effective method for preventing corrosion in concrete wastewater treatment structures. Undoubtedly protection can be effective for at least 15 years, if the concrete structures are correctly designed and prepared and the surface protection system is carefully selected and applied.