Things you need to know about sewer network maintenance
Are you having trouble getting approval for a routine inspection program for your sewer network? Here is some information to bolster your business case.
The Challenge
Councils and utilities are rarely in a position to inspect 100% of their sewer or stormwater networks. This means that careful selection is required when planning which pipes, manholes, and pits to inspect. This should involve risk-based decisions that often include a focus on high consequence of failure locations. However, risk is a combination of both consequence and likelihood of failure. For sewer networks, there is often a significantly increased likelihood of failure immediately downstream of sewer rising mains (pumped mains) due to sulphuric acid generation
What causes hydrogen sulphide corrosion in sewers?
Sulphate in sewage is converted to hydrogen sulphide (H2S) by bacteria present in the sediment/biofilm layer. The H2S can move from the liquid to gas phase which is often the cause of customer odour complaints. Sulphur oxidising bacteria (SOB) above the water level convert this gas to sulphuric acid which can be highly corrosive to the surface of concrete manholes and pipes.
Why are inspections necessary?
The cost and the associated impact of a partial or full failure of these assets can be significant. Understanding where these issues are likely to occur, the current condition of these assets, and the rate of condition change over time allows better management of risk and can save large reactive repair expenses.
What can I do about managing these assets?
Periodic inspection of the first few manholes downstream of rising main outlets and the receiving gravity pipework (especially if concrete/asbestos cement) is a prudent investment and recommended as a subset of your broader condition inspection programming. The image above is an example of proactive rehabilitation of two manholes just downstream of a rising main. This planned work is a fraction of the cost of dealing with a reactive repair after failure.
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