Over the past three decades, since introducing the Energy Policy Act in 1994, the plumbing engineering community has seen higher incidents of drain line blockage and pipe corrosion. To address the issue, cast iron pipe manufacturers have offered epoxy or zinc-phosphate, electro-deposited products to stand up to corrosion. Other manufacturers have promoted existing corrosion-resistant materials to replace cast iron such as thermoplastics polyvinylchloride (PVC) and chlorinated polyvinyl chloride (CPVC). The cause of waste pipe corrosion has been studied and well documented, and this article provides some background and explains the various causes of blockages and corrosion and what is being done about it.

EPA Energy Policy Act, USGBC LEED Certification and General Water Use Reduction Goals

A gradual but steady evolution of the water conservation and sustainability landscape in the United States urged manufacturers of plumbing products to develop new fixtures that consumed less water for operation while the plumbing engineering community continued to keep abreast of the latest code requirements, technologies, and consumer expectations.

The market introduced water-conserving fixtures in the early 1990s after the passage of the Energy Policy Act of 1994, which set the maximum water consumption of toilets to 1.6 gallons per flush (gpf), a reduction from 5.0 to 3.5 gpf. Public lavatory faucets have been limited to flow rates of 0.5 gpm at 80 psig or 0.25 gallons per cycle since the BOCA National Plumbing Code, 1993 edition.

The concept of metered and touchless (sensor-operated) low-flow faucets emerged in the late 1990s as a response to the need for more efficient water usage in public spaces, commercial buildings and health care settings. These faucets initially gained traction in high-traffic environments where minimizing water waste was a priority. In the early 2000s, touchless faucets became more prevalent in the commercial sector, particularly in airports, shopping malls, restaurants, health care facilities and other public facilities. Also, in the early 2000s, the USGBC’s LEED building certification program incentivized the specification of ultra-low flow fixtures as part of achieving indoor water use reduction points.

EPA’s WaterSense program came out in 2006 in an effort by the federal government to promote the use of water-conserving fixtures as a part of a broader effort to raise public and market awareness of sustainable and energy-efficient building designs. This voluntary program limits, among other water-consuming products, water closets to 1.28 gpf and urinals to 1.0 gpf for product certification. The 2014 version v4 of LEED first referenced EPA WaterSense-labeled fixtures as the baseline for water consumption to meet the prerequisite for Indoor Water Use Reduction credits.

Today, many states and/or local energy or conservation codes require new construction or significant renovations to meet a LEED Certified or Silver Certification and will typically require indoor water consumption to meet at least the WE prerequisite of reduction in fixture use by 20%. Depending on the jurisdiction, this may apply to public facilities or any construction over a certain square footage (5,000 to 10,000 square feet).

The increased use of low-consumption plumbing fixtures has led to concerns about the impact on the corrosion rates of drainage pipes, particularly those made of copper and cast iron. Corrosive wastewater in drainage pipes causes the iron in the cast iron to deplete, leaving behind the graphite structure, which is much weaker and more brittle. Visually, the affected cast iron may appear intact, but it becomes soft and crumbly, often resembling a gray or black powdery material. Copper tubing used in wastewater drainage also corrodes through oxidation. Small holes or pits in the pipe surface form called pitting corrosion. Greenish or bluish deposits on the pipe exterior indicate copper corrosion while the progressive thinning of the pipe wall leads to leaks or structural failure. This issue arises due to several factors.

Factors Contributing to Increased Corrosion Rates

Reduced water flow rate and volume: Low-consumption fixtures reduce the amount of water flowing through the drainage system. This lower volume can lead to decreased velocities and reduced carrying distances, resulting in solids settling and accumulating in pipes.

The accumulation of solids can create environments more conducive to corrosion, particularly in uncoated metal pipes. This increased retention time can enhance the corrosive interactions between the wastewater and the pipe material. Lower water volumes may be insufficient to flush away debris, sediments, and biofilms that can contribute to corrosion. These materials can create localized corrosion cells, leading to pitting and other forms of corrosion.

Increased concentration of corrosive agents: Reduced water usage can lead to higher concentrations of dissolved oxygen, carbon dioxide, organic acids, sulfates, chlorides and other corrosive agents in the wastewater. These agents can exacerbate the corrosion of metal pipes. Higher concentrations of these agents can accelerate the corrosion process in copper and cast-iron pipes.

pH Changes: The pH of the wastewater can be more variable with reduced water flow. Extremes in pH (either too acidic or too alkaline) can increase the corrosion rate of both copper and cast-iron pipes.

Extended contact time: Reduced water flow means that water and its contents remain in contact with the pipe walls for longer periods. Prolonged contact time can lead to more corrosive chemical reactions.

Hydrogen sulfide production: Low water volumes can create anaerobic conditions in the pipes, promoting hydrogen sulfide production by sulfate-reducing bacteria. Hydrogen sulfide combined with moisture forms sulfuric acid and is highly corrosive, especially to cast iron pipes, leading to increased corrosion rates.

Evidence from Studies and Industry Observations

Research has shown that low-flow conditions can exacerbate corrosion in both copper and cast iron pipes. For example, studies have observed increased pitting corrosion in copper pipes and accelerated deterioration in cast iron pipes under low-flow conditions. Reports from plumbing and water treatment industries have highlighted issues with pipe longevity and increased maintenance requirements in systems utilizing low-consumption fixtures.

Heriot-Watt University’s research on water conservation has led to the development of advanced drainage simulation tools like DRAINET, which help predict flow and solid transport in low-flush systems. This work has supported the creation of design solutions that ensure effective drainage without relying on increased flow rates, contributing to UK legislation that could reduce water consumption in buildings by up to 25%. Through collaboration with international industry, these innovations aim to balance water conservation with efficient design and installation in modern plumbing systems.

ASPE, in collaboration with the University of Cincinnati and IAPMO, has been focusing on the rightsizing of plumbing systems, particularly through the development and expansion of the Water Demand Calculator. Initially created for residential applications, this tool, introduced in the 2018 Uniform Plumbing Code, represented the first significant update in peak flow rate calculations in nearly 80 years. Further research builds upon these calculations for commercial buildings, aiming to optimize plumbing system designs for efficiency and sustainability.

Mitigation Strategies

Design: Ensure that drainage systems are designed to maintain sufficient flow velocities, even with low-consumption fixtures. Oversizing drainage piping is counter-productive, and the current pipe sizing tables of model codes and state-maintained codes have yet to be adequately adapted to lower flow discharges. Allowing smaller pipe diameters seems counter-intuitive to preventing blockage, but the key to effective drainage is to have high flow velocities with the pipe sufficiently full and at greater pitch to increase carrying distances. Therefore, one effective strategy is to design for the greatest drain line pitch possible rather than defaulting to the minimum code-allowed slope. If a 4-inch pipe can be pitched at 2% rather than 1%, carrying distances and velocities can be increased.

Regular maintenance and cleaning: Encourage facilities managers to implement regular inspection and cleaning schedules to remove accumulated solids and biofilms that can contribute to corrosion.

Corrosion-resistant materials: As discussed at the beginning of this article, consider using corrosion-resistant materials such as PVC, CPVC, coated cast iron, or stainless steel for drainage pipes, especially in areas where low flow is a significant concern. These materials are more resistant to corrosion and have smoother inner bores to allow for longer carrying distances.

Conclusions

In summary, while low-consumption plumbing fixtures contribute to water conservation, they can also increase pipe blockages and the corrosion rate of copper and cast iron drainage pipes under certain conditions. Addressing this requires careful design, maintenance and sometimes the use of alternative materials or chemical treatments to mitigate the potential negative effects. To achieve more appropriate pipe sizing and pitch in waste system designs, organizations such as ASPE, IAPMO, Heriot-Watt University, and the University of Cincinnati conduct ongoing research as a basis for modifications to model codes and standards.