Wet Well Tank Lining, Norfolk

What is a Wet Well ?

A wet well is a component of a pumping station in water, wastewater, or stormwater systems. It serves as a holding chamber for liquids before they are pumped out to a treatment facility or another part of the system. Wet wells are commonly used in sewage and drainage systems, as well as in industrial settings.

Key features of a wet well:

1. Holding tank: It collects and stores incoming wastewater, stormwater, or other liquids.
2. Pumping mechanism: When the liquid in the wet well reaches a certain level, pumps are activated to move the liquid to its next destination (e.g., treatment plant, higher elevation, or another section of the system).
3. Control system: Wet wells often have level sensors or floats to monitor the liquid level. The control system will activate or deactivate pumps based on the liquid level.
4. Design considerations: Wet wells must be designed to minimize sediment buildup, reduce odors, and prevent issues like pump cavitation or clogging.

Common uses:

• Sewage pumping stations: Wet wells are part of lift stations that help move wastewater from lower to higher elevations when gravity flow isn’t sufficient.
• Stormwater systems: They manage stormwater by collecting runoff and pumping it to drainage systems or retention ponds.
• Water treatment plants: Wet wells hold untreated or treated water for pumping to various stages of treatment or distribution.

In summary, a wet well acts as a temporary storage area for liquids and works in conjunction with pumps to control liquid flow in systems that rely on hydraulic management.

Being a below ground structure we suspected that the cause of the failure was ground water pressure against the bonded face of the lining, possibly combined with poor application.

Why does ground wate cause tank linings to fail?

Groundwater can cause tank linings to fail for several reasons, primarily related to pressure, chemical interactions, and moisture intrusion. Here’s a detailed explanation of how groundwater contributes to tank lining failure:

1. Hydrostatic Pressure

When a tank is buried or partially underground, groundwater exerts hydrostatic pressure on the tank walls and the lining. This pressure can be significant, especially if the groundwater table is high. Over time, this pressure can cause:

• Cracking or Blistering: Continuous pressure can cause the tank lining to crack, blister, or even delaminate (separate from the tank wall).
• Flexing of the Tank Structure: The pressure can cause the tank to deform slightly, leading to stress on the lining. If the tank is not designed to handle this external pressure, the lining can break down or lose adhesion.

2. Moisture Intrusion and Osmosis

Osmosis occurs when water permeates through a semi-permeable membrane (in this case, the tank lining) due to differences in solute concentrations. Even if the tank lining is designed to be waterproof, small imperfections or pinholes can allow moisture to seep through from the outside. This process can cause:

• Blistering: Groundwater may migrate through the tank lining by osmosis, causing blisters to form on the lining. These blisters are pockets of water or gas trapped between the lining and the tank wall, which can compromise the structural integrity of the lining.
• Corrosion: Once moisture penetrates the lining, it can reach the tank’s metal or concrete surface, leading to corrosion or degradation of the underlying material. Corrosion weakens the tank structure and accelerates lining failure.

3. Chemical Attack

Groundwater often contains various dissolved chemicals, including salts, acids, or other contaminants, depending on the environment. These chemicals can react with the materials used in the tank lining and cause:

• Chemical Degradation: Groundwater containing aggressive chemicals, such as sulfates or chlorides, can degrade certain types of tank linings (e.g., epoxy or polymer linings). Prolonged exposure can cause softening, cracking, or disintegration of the lining material.
• pH Imbalance: Groundwater with extreme pH levels (very acidic or alkaline) can weaken the lining materials, leading to a breakdown in their protective properties.

4. Freeze-Thaw Cycles

In regions where groundwater freezes and thaws seasonally, this can exert additional stress on tank linings. When water in the ground freezes, it expands, increasing pressure on the tank structure and its lining. The repeated expansion and contraction during freeze-thaw cycles can cause:

• Cracking: The lining material may crack or become brittle due to the mechanical stress caused by the freeze-thaw cycles, especially if water has infiltrated the lining.
• Delamination: The constant freeze-thaw movement can cause the lining to lose adhesion and separate from the tank walls.

5. Capillary Action and Wicking

Capillary action occurs when groundwater seeps upwards through tiny cracks or pores in a tank’s structure. This moisture can be drawn into the lining material or through seams and joints. Over time, this wicking action can:

• Deteriorate Adhesives: Adhesives used to bond the lining to the tank may lose their strength when constantly exposed to moisture, causing the lining to peel away or separate.
• Promote Corrosion: Moisture brought in through capillary action can reach the tank’s interior, leading to corrosion or material degradation.

6. Microbial Induced Corrosion (MIC)

In some cases, groundwater can introduce bacteria or other microorganisms that promote Microbial Induced Corrosion (MIC). Certain bacteria can produce corrosive by-products like sulfuric acid, which can attack both the lining and the tank’s structural materials, leading to premature failure.

Preventative Measures:

To mitigate the risk of tank lining failure due to groundwater, several strategies can be used:

• Proper Tank Design: Tanks should be designed with adequate structural strength to resist hydrostatic pressure from groundwater.
• High-Quality Lining Materials: Use linings that are resistant to both chemical attack and moisture penetration, such as reinforced epoxy coatings or specialized polymer linings.
• Groundwater Barriers: Installing external waterproofing membranes or drainage systems around the tank can help reduce groundwater contact.
• Regular Maintenance and Inspections: Regularly inspecting the tank and lining for signs of moisture ingress, blistering, or cracking can help catch early signs of failure before major damage occurs.

In summary, groundwater can compromise tank linings through mechanisms like pressure-induced stress, moisture intrusion, chemical reactions, and environmental conditions like freeze-thaw cycles. Understanding these risks helps in selecting appropriate materials and implementing design measures to prevent failure.