One of the major reasons for sewer rehabilitation is corrosion, and one of the major corrosion inducers is hydrogen sulfide. Surprisingly hydrogen sulfide can reduce the effective service life of a sewer pipeline to less than half the lifespan it was designed for. The cost of rehabilitation escalates when municipalities fail to undertake periodic inspections, and condition assessment of buried pipelines. (Read Your Comprehensive Guide to Condition Assessment of Buried Pipelines.)
The cost associated with controlling corrosion and avoiding damage to infrastructure is much less compared to the cost of rehabilitation by repair or replacement.
Sulfide Formation in Sewers
Sulfur is abundantly found in sewers. When sulfate reducing bacteria oxidize organic matter with sulfate in anaerobic conditions, such as the absence of oxygen or nitrate, it forms sulfide. In gravity sewers, production of hydrogen sulfide takes place at low flow velocity and temperatures above 15 to 20 degrees Celsius. The high temperatures increase the biological activity, consequently increasing consumption of oxygen, and production of sulfide.
The low flow velocity doesn't create enough turbulence for re-aeration and thus aids in the buildup of biofilm. Bacteria living below the waterline in the sewer converts sulfate into ionic sulfide which then forms dissolved hydrogen sulfide. The stench associated with sewers is a result of this gas being released from the wastewater during turbulence or splashing.
Bacteria that live above the waterline convert the released hydrogen sulfide gas to sulfuric acid, which drips back down into the waste water and is neutralized into sulfate, allowing the reaction to begin all over again.
Sulfide Corrosion Mechanism
There are two ways that hydrogen sulfide corrosion takes place.
The main cause of internal corrosion of sewers is acid attack, which is a result of hydrogen sulfide gas converting to sulfuric acid by biological activity, in the presence of moisture.
Direct Chemical Reaction
Hydrogen sulfide gas reacts directly with metallic components of the sewer system such as copper and iron pipes and fittings. This can cause the failure of mechanical equipment and electrical systems that are used for transportation and treatment of sewage.
Hydrogen sulfide corrosion takes place when conditions in the sewer are ideal for hydrogen sulfide to react with its surroundings.
Some of them are:
- Low flow velocity causing low levels or absence of oxygen in the wastewater.
- Excessive detention time due to debris build up or flat slope.
- Presence of excessive debris buildup in the sewer.
- Excessive flow velocity or turbulence leading to stripping of hydrogen sulfide gas causing further oxidation.
- Characteristics of the wastewater.
- Presence of moisture on the material encouraging acid attack.
- Presence of material such as iron and copper that isn't corrosion resistant.
- Presence of dissolved sulfide through chemical additives.
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Detecting Hydrogen Sulfide Corrosion Problems
There are many locations in a sewer system where hydrogen sulfide corrosion can be detected. Some of them are:
- Sewers having flat slopes resulting in longer detention periods due to low velocity is one of the main areas ripe for hydrogen sulfide corrosion.
- Sewers with steep gradients resulting in turbulent flow or flow exceeding critical velocity.
- Wet walls of pump stations.
- Headworks of treatment facilities.
- Electrical, ventilation and heating systems.
- Structures and equipment that handle sludge from the wastewater.
- Drop manholes, junction chambers and force main discharges.
- Wastewater with high (>250 mg/L) biochemical oxygen demand (BOD).
Controlling Hydrogen Sulfide Corrosion
Though hydrogen sulfide corrosion is one of the major causes of pipe damage and failure, it's not something that can't be treated or controlled. The first step to controlling corrosion is to carry out periodic condition assessment, and ensuring that all pipelines are thoroughly inspected for signs of corrosion damage. Treating the damage at the start can help in reducing time and cost associated with the repair. (Read Methods for Preventing Corrosion in Infrastructure.)
Below are a few ways that hydrogen sulfide corrosion can be controlled:
Reduce Dissolved Sulfide in the Wastewater
Chemical oxidants such as hydrogen peroxide or introduction of air can induce oxidation in the wastewater. Dissolved sulfide can be precipitated by the addition of metallic salts, and pH can be elevated by adding caustic soda to reduce the amount of dissolved sulfide present in the sewer.
Dissolved oxygen in the wastewater should be >0.5 mg/L, dissolved sulfide levels should be <0.3 mg/L, and atmospheric hydrogen sulfide levels should be below 5 ppm.
Maintain Slope of Sewer
The slope of the sewer should be maintained such that the flow never reaches a low-flow velocity leading to accumulation of debris; neither should it cross the critical velocity causing turbulent flow. Both these conditions are detrimental for the sewer pipe.
The best way to prevent corrosion is to use pipe and fitting material such as polyvinyl chloride (PVC), high density polyethylene (HDPE) and fiberglass reinforced plastic (FRP). These materials are chemically neutral when exposed to sulfuric acid.
Regular Inspection and Maintenance
Inspection of sewer lines helps pinpoint problem areas and allows for effective cleaning and selection of appropriate repair or rehabilitation methods. Periodic and regular cleaning of sewer lines can also effectively deter the problem caused by hydrogen sulfide since debris accumulation reduces the flow velocity and increases its detention time in the sewer. The increased biological activity as a result of this depletes oxygen and generates sulfide.
Pressure washing and pigging can be done for gravity sewers to clean them up.
What We've Learned
Fortunately, engineers of trenchless technology have been innovative in researching better and effective ways to rehabilitate sewers than the traditional dig-and-repair method. Cured-in-place pipe (CIPP), sliplining, pipe bursting and mechanical spot repair are just some of the methods that can be used to rehabilitate pipes that have been damaged by hydrogen sulfide corrosion.
Trenchless inspections using closed-circuit television camera (CCTV), and robotic crawlers with mounted cameras help detect specific problem areas, which can then be repaired or replaced using one of the rehabilitation methods. (Read An Overview of Robotic Pipe Repair.)
Since hydrogen sulfide corrosion is a rampant problem in most of our wastewater collection systems, a corrosion monitoring method should be established to carry out regular inspections, including collating data to determine different causes of hydrogen sulfide corrosion.
Municipalities can save a lot of cost and time on costly repair by incorporating a planned approach to corrosion control by carrying out condition assessment of pipelines.