Combating Sanitation System Corrosion
Sanitation corrosion is a worldwide problem that permeates through a system of moving waste to processing stations, and often ending with water reclamation. As these systems become outdated, governments are looking for cost-effective, long-term repair and protection solutions.
Governments, particularly major cities, are faced with a growing demand on aging sanitation infrastructures and escalating cost of maintenance. (An introduction to the problems faced by municipalities can be found in Public Institutions Corrosion Prevention Efforts Leave a Lot to Be Desired.) For instance, in Singapore, 85% of citizens live in 900,000 publicly built and governed housing units. These began in 1960 and many of the concrete refuse chutes are now deteriorating. The residents’ waste falls through these hoppers into collection bins, which are then removed. This garbage disposal method is installed everywhere for convenience, but can become an unhealthy environment for bacteria growth, foul odors and vermin as corrosion eats away at the sides.
Hong Kong has a sewer system that stretches about 900 miles (1,448 km) going to 280 treatment facilities. Over 700 million gallons (2.6 billion L) are processed daily. They rely on crumbling brick and mortar manholes ranging from four feet (1.2 m) to 40 feet (12.2 m) deep to work on problem lines.
A growing number of water reclamation plants are at the other end of sanitation systems. Los Angeles facilities are capturing and processing wastewater to produce over 80 million gallons of reclaimed water per day. As plants are being constructed, now is the time for a long-term view to prevent damage from corrosion and ground movement.
Common Sanitation System Problems
These sanitation components share common issues. First, many are concrete or brick and mortar, which are prone to corrosion from chemicals, water and the environment. Also, steel rebar that stabilizes the large slabs of concrete can be exposed through a crack and will begin to rust; the rust expands and cracks concrete even further. (For more information about rebar corrosion, see The Effects of Corrosion on the Shear Behavior of Materials.)
A second common problem is the wet nature of sanitation systems. This comes from both the liquid materials flowing through or being held, and the humid atmosphere. Many coatings for concrete are not designed to endure.
The third factor is the ongoing use of these areas. The challenge is often how to clean, dry and repair surfaces enough to ensure a successful coating application. Each project has to be evaluated for its unique circumstances.
Trash Disposal Solutions
A number of renovation projects have been initiated in Singapore for the repair of concrete trash disposal hoppers. The high-rise apartment buildings, called flats, are designed with one or more trash chutes running the height of the 12 to 25 stories-tall buildings.
In consulting with local applicators, ArmorThane quickly evaluated the problem of high humidity, which often causes polyurethane and epoxy polymers to lose their properties needed for long-term repairs and structural integrity. Because it would be unrealistic to wait for the optimum spraying conditions, they recommended a 1:1 ratio elastomeric polyurea. This polymer is specifically designed to cure properly in highly humid environments found in areas such as Southeast Asia. With the decreased moisture sensitivity, crews could spray the chutes on any set schedule.
For this project, high-pressure spraying equipment was raised to the roof of the flat for optimum access. Once a hopper was cleaned, repaired and inspected, the equipment was set up. Applying the coating involved lowering the trained technician down the chute, spraying all four sides as he went.
Sewer Manhole Rehabilitation
As the access point for inspecting and repairing sewer lines, manholes must be kept safe for worker entry. For a Hong Kong project, ArmorThane proposed a long-term solution using a polyurea spray coating method to stop deterioration and strengthen walls instead of patching existing problems or completely rebuilding parts of the system.
To earn approval from government officials, Rob Anderson in our USA headquarters flew to Hong Kong to demonstrate the process and define results. High-pressure spraying equipment was shipped in and mounted on a flatbed truck for mobility.
To accommodate the audience and make for easy inspection, Anderson cleaned and sprayed a large concrete box culvert used to divert stormwater under the roads. Although the deterioration was not as extensive as brick and mortar manholes, the coating method was nearly the same. Receiving approval, a second trip was scheduled to demonstrate the manhole coating process and train local applicators. Anderson worked with sanitation staff to tackle numerous challenges created from both the sewer environment and complexity of the underground sanitation grid.
The first challenge was circumventing the flow of sewage to work on manholes and tunnels without creating significant problems upstream. An airbag was inserted upstream and inflated to stop the flow. Next, the flow had to be diverted around the area to a downstream line using a trash pumping system.
Once the trial sewage line was cleared, all surfaces were power washed, thoroughly dried and repaired. Next, the 510 elastomeric polyurea was sprayed. This is specially formulated to not react to residual moisture, and it outperforms other coatings. It does not crack even as conditions cause the surface to expand and contract, and it is extremely resistant to corrosive chemicals and temperature change. A second coating was applied to yield an approximate total thickness of 120 mils.
Wastewater Reclamation Infrastructure
With growing demands for water, especially in areas that have problems maintaining reserves, governments are turning to wastewater treatment options. An important message learned in the sanitation construction of long ago is that systems eventually corrode. Steps taken now greatly extend the life of these processing units and protect the environment along the way.
By coating the processing containment tanks and ponds with special polymers, applicators are able to create a seamless membrane that is crack and corrosion resistant. Reactants and corrosive materials sit harmlessly against the coating and have no effect on tanks or pipes. With no volatile organic compounds (VOCs) or solvents to leach into or absorb water or chemicals, these coatings are safe for people, animals and plants. US ANSI NSF-61-certified coatings are also available for holding potable water.
In areas such as California, ground movement is a threat. Developed with tough elongation and tensile strength characteristics, these coatings often hold the barrier even when the underlying surface cracks.
Benefits to the Sanitation Industry
The most successful sanitation protection and rehabilitation projects use elastomeric coatings to keep existing structures and equipment in working order. The chemical reacts quickly to form a seamless protective barrier.
- Increased Wear Protection – The service life is greatly improved, in some cases even by a factor of four or more.
- Humidity Resistance – The polymer doesn’t react to high humidity and residual moisture, allowing proper adherence and structural integrity.
- Corrosion Resistance – Corrosive elements that could potentially cause structural failure over time are blocked by the chemically-resist polyurea coating.
- Easier to Clean – Even a simple washing of a coated surface can remove refuse and organic build-up; cracks and crevices are sealed.
- Reduction of Bacterial Growth – The combination of the inert and impermeable nature of a cured ArmorThane coating reduces the ability of bacteria to gain a foothold and subsequently, the easier-to-clean environment slows accumulation of bacterial strands.
- Sound Dampening – The movement and impact of falling refuse to the structure walls has a lower level of noise with coated refuse chutes.
- Crack Resistance – The flexibility and elongation of tough polymers offer extra protection against ground shift and other factors that can cause uncoated surfaces to crack.
The use of special polymers for sanitation projects offers a two-fold advantage. One is the protection of both the environment and human health that rely on the accurate construction and maintenance of these waste removal systems. Leaks into groundwater or the spread of disease can have long-lasting and devastating effects, plus can cost millions in liability.
The other factor is a reduction in maintenance costs. That’s good news for budgets that are stretched thin with growth but face escalating repairs or replacement. Whether new construction or rehabilitation of an existing structure, coating projects extend the life of components throughout a sanitation system. The key is the proper design of these projects with field-proven chemicals and procedures.