Managing Maintenance

About the author:

Murray Heywood is water & wastewater product development manager Eastern North America for Sherwin-Williams Protective & Marine Coatings. Heywood can be reached at [email protected].

Maintenance is a necessary evil that all wastewater facility owners and operators must address at various points in their facilities’ life cycles. Facility managers do their best to maintain optimum operating conditions to minimize maintenance needs. However, the increasingly corrosive wastewater entering sewer systems is accelerating the deterioration of concrete, steel tanks, vessels, pumps and other equipment, which increases the frequency of scheduled and emergency repairs and downtime.

The ideal method for managing wastewater facility assets is to implement a preventive maintenance plan that includes scheduled downtime for repairs, such as rehabilitating coating and lining systems that protect concrete and steel surfaces from corrosion. In a perfect world, this approach should reduce the likelihood of a facility needing an emergency repair and experiencing unscheduled downtime. Unfortunately, plant operators and managers do not work in a perfect world. Operating budgets are tight and have not kept pace with system demands, forcing operators to push facility limits and capacities, which can increase the likelihood of an emergency repair. Whether maintenance is scheduled or unscheduled, facilities operating at maximum capacity have little tolerance for shutdowns of any kind, as they impact service demands and expenses.

Standard maintenance and emergency repairs carry a high cost associated with temporary loss of the system. Shutting down a system often requires a facility to implement expensive bypass operations to divert the waste stream to other tanks or temporary holding areas until the primary system is repaired. Bypass operations can cost more than $30,000 per day for a larger plant, so every minute saved in getting the system returned to service creates significant cost savings. Fortunately, wastewater facility managers can save precious time when performing maintenance on coating and lining systems. Some newer coating technology options enable a facility to rehabilitate and repair concrete and steel surfaces and return them to service in less than 24 hours.

A Growing Need for Repairs

Over the past 50 years, many urban areas have experienced “urban sprawl” without adding new wastewater treatment facilities to manage the increased wastewater volume. While some areas have plans to build additional facilities, budget cuts from a slower-than-anticipated economic recovery have delayed or canceled construction plans. Some municipalities have found the cash to expand existing facilities, but many communities have addressed sprawl simply by adding new pipes to existing systems to carry the cargo to existing treatment plants from new subdivisions and commercial and industrial sites. 

As that cargo must now travel greater distances to reach the treatment facility, this approach has caused slower waste stream flows with longer sewage dwell times in the lines. Additionally, the amount of industrial byproducts entering the waste stream has increased, creating a more eclectic mix of organics and, subsequently, a more septic waste stream. All of these factors contribute to increased corrosion rates on wastewater infrastructure.

Concrete is the typical substrate used in waste stream flows and wastewater facilities, accounting for approximately 85% of the surface areas that will require maintenance or repair. Typical concrete assets include manholes (brick, precast or poured), wet wells, lift stations, grit chambers, influent channels, primary and secondary clarifiers, and digesters. The largest contributors to the corrosion and degradation of the concrete are the increased levels of hydrogen sulfide (H2S), organic wastes (e.g. fats and grease), and industrial waste traveling through the waste stream. 

Microbiologically induced corrosion (MIC) also is a major contributor to the deterioration of concrete because the microbes essential in the breakdown of the waste (Acidithiobacillus thiooxidans) ingest the H2S and excrete sulfuric acid (H2SO4). This reaction lowers the pH of the concrete and deteriorates the cement paste, leaving the aggregate exposed. Abrasion from debris and grit in the waste stream also contributes to the degradation of the concrete by eroding the surface and exposing the larger aggregate within the cement matrix.

Steel surfaces throughout a wastewater facility face similar threats as toxic waste streams and gases react with unprotected steel and accelerate corrosion.

Advancement Acceleration

In the past, the industry primarily used coal tar epoxies for severe wastewater applications to provide protection against concrete and steel corrosion. For many years, there was no viable alternative, as coal tar provided the chemical resistance needed and was reasonably easy to apply. However, there are many well-documented issues with coal tar, including its limited film-build characteristics (typically 48 to 50 mils, max), classification as a carcinogen and potential to cause burns during application. Coal tar products also experience reduced service lives in today’s more corrosive waste streams. Most significantly, coal tar epoxies can take seven to 10 days to cure before being put into service—a wait that is unacceptable given today’s maintenance challenges and budgetary constraints.

Newer fast-curing technologies have reduced return-to-service times to as little as 24 hours, depending on the size of the job and the crew. Facilities can perform surface preparation, apply repair mortar, apply a fast-curing coating material, and place a system back in service within a 24-hour period. Of course, this may not include the time to empty, clean and rinse a tank or vessel, as well as remove any existing coating or lining. This efficiency could reduce a facility’s bypass operation costs by $180,000 to $270,000.

The new coating technologies’ most intriguing component is that despite the idiom that “quick and fast means it just won’t last,” they now can deliver protection with little compromise on quality, even when curing in a fraction of the time of older technologies. This can be advantageous on construction, rehabilitation or repair projects, which are often plagued by delays and setbacks.