An Introduction to I&I
Most buried underground assets within the sanitary sewer collection system are concrete, brick, or some form of masonry. A simplified way to visualize concrete within collection systems is to envision it as a buried high-density sponge. Over time, this high-density sponge becomes less dense from corrosion, or fatigued and showing symptoms of cracks and other damage (some hairlines, some more obvious), allowing this substrate to absorb or channel water from the saturated earth surrounding it. Some areas or sections of soil become more saturated than others, usually from a high water table or simply rain and irrigation.
The term inflow and infiltration (I&I) refers to the undesired entry of clean water, originating from stormwater (inflow) and groundwater (infiltration), into sanitary sewer collection systems. This unintentional influx of clean water occurs due to various factors, such as deteriorated concrete, mortar, cracks, faulty seals, joint and connection failures, or other unforeseen areas that result in leaks. The entry points for this water can include manholes, pipes, wet wells, and lift stations. Essentially, all underground assets are vulnerable, particularly those situated within the water table, as they undergo hydrostatic pressure from groundwater infiltration and an inflow of stormwater from heavy rain events that persistently seek pathways for entry. One area that can be tackled to offset I&I contribution is at the manhole.
As infrastructure ages, this condition can be accelerated and compounded by corrosion from hydrosulfuric acid (H2S). The associated costs of excavation, repair, or replacement often become burdensome for municipalities. Utilizing in situ or trenchless solutions for the rehabilitation and lining of collection systems has emerged as a crucial tool for municipalities. This approach is essential in addressing the restoration of aging infrastructure and effectively sealing off the issues related to inflow and infiltration.
I&I is a condition that cannot be ignored, not only from an environmental standpoint, but also because the cost burden grows and becomes unsustainable. The “clean” water entering the system ultimately mixes with sewage and ends up at the treatment plant. Originally, older plants were designed for calculated flow rates and volume based on a projected population growth factor that often did not account for I&I at the rate we see today in many aging, older cities. If this surge of volume continues, many asset owners will end up with a sanitary sewer overflow (SSO) condition, a release of untreated or partially treated sewage from a municipal sanitary sewer. I&I is an environmental problem that must be avoided and also carries a considerable financial impact.
I&I can affect both brick and concrete substrates and, although the severity may vary, it is an issue that can wreak havoc wherever there are buried assets.
According to the American Society of Civil Engineers (ASCE), the nation’s 16,000+ wastewater treatment plants are currently operating at an average of 81 percent of their design capacities. Additionally, 15 percent of these plants have either reached or exceeded their intended capacity. These statistics shed light on the ongoing challenge posed by I&I, as it persists in pushing these percentages upward. The consequence of this increased strain on treatment plants heightens the risk of SSOs and will necessitate construction expansion of the plant if ignored.
The Rise of I&I Abatement Programs
The ASCE’s “US Infrastructure Report Card” currently assigns a grade of D+ to wastewater infrastructure in the US. However, the ASCE has found that the increased adoption of asset management plans has allowed 62 percent of surveyed utilities to shift from reactive responses to proactive management of wastewater infrastructure maintenance. This proactive approach is crucial to addressing issues before corrosion takes hold and I&I affects sanitary sewer collection systems.
The US Environmental Protection Agency (EPA) emphasizes the importance of I&I abatement programs as well, outlining minimum goals to achieve comprehensive improvements in wastewater management:
- Reduce Ratepayer Costs: Implement cost-effective I&I reduction projects to decrease the financial burden on ratepayers for treating wastewater.
- Minimize Liability: Mitigate liability arising from pollution and public health risks by eliminating SSOs during storm events.
- Avoid Wastewater Treatment Plant Expansion: Eliminate sufficient I&I to preempt the capital costs associated with expanding wastewater treatment plant capacity in anticipation of a 10% population growth over the next 20 years.
- Avoid Interceptor Expansion Costs: Eliminate sufficient I&I to avoid the capital costs linked to the expansion of interceptors, which will be necessary to support the development of certain neighborhoods.
- Offset Environmental and Regulatory Impact: Eliminate enough I&I to offset the environmental and regulatory impact of sewer system expansion and increased water demand expected over the next 15 years.
These goals underscore the EPA’s commitment to enhancing the efficiency, environmental sustainability, and resilience of wastewater infrastructure while addressing the challenges posed by I&I.
The landscape of mandates has been expanding, accompanied by consent decrees and funding initiatives to address the challenges in wastewater infrastructure. Municipalities are actively pursuing various funding opportunities stemming from federal, state, and local sources intended to bolster their efforts. In addition to external funding, some municipalities are allocating resources from their own annual budgets specifically designated for I&I maintenance. This dual approach, combining external financial support with dedicated internal budgets, reflects a concerted effort to tackle the issues associated with aged and fatigued collection systems comprehensively.
Trenchless Approach to Providing a Targeted Solution
Targeting the sources of contribution and identifying problematic areas, along with determining the factors causing the I&I condition, is the foundation for informed product and solution selection. Is the I&I primarily entering through manholes, pipes, pumping stations, or a combination? Recognizing that not every sewer line requires the same approach ensures that the chosen solutions align with the specific needs of the system.
Fortunately, trenchless methods, featuring innovative and proven lining techniques, can effectively address both EPA mandates and municipal I&I abatement goals.
Over the decades, innovation has given rise to a variety of high-quality and cost-effective trenchless solutions for buried assets, eliminating the need for traditional and more costly dig-and-replace methods. Structural liners are an integral part of these trenchless strategies.
The application of trenchless lining methods goes beyond mere compliance with mandates, offering a holistic and cost-efficient approach to improving the functionality, resilience, and longevity of wastewater infrastructure. Trenchless lining methods, aimed at sealing I&I, not only align with EPA requirements but also offer a range of additional benefits:
- Protects Health and Environment and Reduces SSOs: These methods contribute to protecting public health and the environment from contamination associated with SSOs and other potential hazards.
- Optimizes Taxpayer Funds: Trenchless lining provides a cost-effective solution by addressing I&I issues without the need for extensive and expensive infrastructure replacements.
- Delays Infrastructure Expansion: By reducing the rate of I&I and improving system efficiency, trenchless lining methods can delay the need to expand infrastructure and treatment facilities. This, in turn, alleviates the burden on capacity and associated capital costs.
- Extends Asset Life Cycle: Trenchless liners provide upgraded corrosion protection, extending the life cycle of the infrastructure and reducing the frequency of maintenance and repairs.
Trenchless solutions for vertical structures (such as manholes and lift stations) are not all created equal. Each manhole is unique and therefore finding the right solution should vary depending on the environment, and severity of I&I to name a few factors. Solutions may include cementitious linings, geopolymer concrete, calcium aluminate, epoxy-mortar high-strength, structural polyurethane/ polyurea, foam composite systems bonded polymer lining systems or ultra-high build structural epoxies. Careful consideration should be given to this decision and consulting with an experienced, trained professional can often be the key to a successful I&I mitigation solution.
Michael Caputi, Vice President of Epoxytec