For more than half a century, municipal councils, as well as industrial and mining companies throughout Australia and New Zealand have invested billions of dollars into equipment and infrastructure to process waste water and sewage. Corrosion of this infrastructure, and in some cases the subsequent leakage, costs industry in excess of $1B each year. The main assets that are impacted by corrosion in waste water treatment plants (WWTP) are the pipelines, storage tanks, clarifier ponds and sewage channels.
As much of the WWTPs infrastructure is ageing, it is starting to require refurbishment or replacement. One method of refurbishment of these assets is to carry out surface repairs and then apply protective coatings. These coatings must be strong, flexible and resistant to chemical attack.
Corrosion particularly affects the submerged parts of structures in WWTPs. “Special consideration has to be given when coating structures in sewage treatment plants,” stated Dennis Baker, Special Projects Engineer at Gold Coast-based Rhino Linings Australasia (RLA). “One of the more corrosive by-products of sewage is hydrogen sulphide gas.”
“Hydrogen sulphide reacts with moisture on surfaces in a waste water plant and bubbles up to form sulphuric acid which really loves concrete,” added Baker. Hydrogen sulphide (H2S) attacks the cement, copper, iron and silver which gradually degrades the structure. In the case of a pipeline, this may ultimately result in the collapse of the pipe wall. The modern requirement for capping storages in order to control excessive odours has the drawback of also increasing gas concentrations.
One type of coating from RLA that is ideally suited for waste water treatment is spray applied Polyurea. The company has been working with this material since the early 1990s and now manufactures in Australia a range of consistent formulations which are suitable for a variety applications.
Pure Polyurea is a relatively modern material that has been developing rapidly during the past 10 – 15 years. “Polyureas and particularly Pure Polyurea came to the forefront in 1980 when the entire outer surface of the Alaskan oil pipeline was coated in polyurea,” Baker said.
Pure Polyureas are formed when a liquid isocyanate is mixed with an amine based resin solution. Isocyanates are reactive because the double covalent bond attaching the carbon atom to nitrogen and oxygen atoms is easily broken to form single bonds in the more stable tetrahedral configuration around the carbon atom.
The Rhino Linings Pure Polyurea comes as a two-part solution that is mixed under high temperature and pressure (3000 psi at 65ºC) in a specially designed spray apparatus. When applied, the excellent chemical cross linking produces a dense but flexible surface. The high density makes the coating almost impervious to abrasion, water and chemicals.
One company that is keenly aware of the benefits of polyureas is Queensland-based Satintouch, one of the premiere applicators accredited by Rhino Linings.
The chemicals utilised in the Polyurea and Pure Polyurea coatings means that most of the work Satintouch, and other applicators, carries out is defined as an Environmentally Relevant Activity (ERA) under government legislation. The main activities undertaken fall under ERA 17, 38 and 57. These regulations require a licence for the abrasive cleaning and spray coating work to be conducted as well as special constraints on the subsequent cleanup in order to minimise the impact on the natural environment around the work site.
Pure Polyurea coatings ‘snap cure’ to form a solid surface in a few seconds and can be walked on without damage in less than a minute. Another advantage is the ability for it to be sprayed at very high thickness (6000 microns and greater) on a sloping or vertical surface without sagging or running. The surface of a RLA coating is easy to maintain, clean and recoat if necessary.
Whereas epoxies and paints form a solid rigid shell, the flexibility of polyurea coatings allows them to move with the expansion and contraction of the underlying structure as temperatures change.
Spray coating enables quicker application and less disruption to a client’s operations. With a rapid cure time of three to six seconds and the ability to walk on it in a matter of minutes means that a facility can be back on line much quicker. “Unlike all other coatings, Pure Polyureas are not affected by ambient moisture or temperature,” stated Baker. “We can return a facility to service in four hours with full cure in 24 hours.”
One unusual project Blair’s company was involved with was at a cyanide production plant in Queensland. The company cleaned existing tanks and concrete structures to remove the original coatings before priming them and applying the new Pure Polyurea covering. “It was a major challenge to recoat surfaces at an operating cyanide plant,” Blair said. “There were very strict health and safety guidelines we had to comply with, both in terms of the team preparing for the work each day and the actual operation of our equipment.”
“The majority of our work is onsite at a client’s facility, with about 40 per cent done in one of our yards in Mt Isa, Dalby or suburban Brisbane,” Blair stated. For one mining project, the larger tanks were cleaned and coated onsite, but transportable structures such as smaller tanks were trucked to one of the company’s yards where the blasting systems are in buildings that are design to limit the spread of the abrasive material and debris.
Satintouch’s teams usually consist of three technicians and a supervisor working on-site along with an independent QA/QC inspector who oversees projects and ensures all procedures are followed and documentation prepared according to requirements and specifications. This includes the Rhino Linings procedures as well as the rules and guidelines of the ERA legislation.
The durability of Pure Polyurea and Polyurea as surface protection means that money can be saved because the structure has a longer repair/replace cycle. “Polyurea coatings are also easy to repair,” Baker said. “The area around a damaged surface can be re-activated using special primers and then covered with a new layer of polyurea.”
For projects involving heavy traffic and wear, Blair said that Satintouch will usually install an indicator layer which is bright red before the final top coat. Once the red layer starts to show through the main coating, it is clear that the structure or surface will need re-covering soon. Recording these wear rates enables better protection planning.
Rhino Linings coatings are covered by long guarantees, with water treatment facilities extending to 20 and 25 years warranty. Blair has a personal routine for client follow-up where he contacts as many customers as practical every two years. “Even though our products and applications have extended warranties, I try to speak with all my clients every two years just to make sure everything that we installed and coated still meets their requirements and that there have been no failures,” he stated.
Spray applied Pure Polyurea and Polyurethane from RLA offer superior solutions for liquid containment. All coatings developed by the company for the water industry are continually tested to ensure they comply with the latest standards and have been certified safe for applications such as lining potable water storage tanks, marine aquariums, food freezers and grain silos. RLA coatings are tested for compliance with AS4020 at the Australian Water Quality Centre in Adelaide.
RLA’s proprietary formulations, combined with innovative substrate preparations, result in excellent adhesion and a seamless surface over virtually any shaped structure. “Applied correctly, our spray applied polymer coatings have attachment loads of at least 6 to 10 mPA (750 to 1250 psi),” said Baker. “In most cases, the concrete substrate would give way before the coating peeled off.”
In addition to hydrogen sulphide being corrosive to structures and pipes at WWTPs, the flammable, colourless gas also poses a health risk to workers. The typical rotten egg smell can be detected by people at concentrations ranging from 0.0005 to 0.3 parts per million (ppm).