Corrosion is a serious problem plaguing the sewer system all over the world as old pipelines are ignored and in a state of deterioration and collapse.
New pipelines are being continuously laid over the old system while the old pipelines are still functional and lost in the maze of underground pipelines.
The stench emanating from sewers is an indicator of what’s taking place within the pipeline. It is not entirely the smell of generated waste such as fecal and industrial discharges flowing through it.
In fact, according to Jim Joyce, PE of V&A Consulting Engineers, the strongest, the most offensive smell is generated in the sewer itself from inorganic compounds produced in the sewer from natural biology.
These odor-generating conditions are also the main cause behind corrosion leading to degradation and eventual failure of sewer pipelines.
Sulfate-reducing bacteria oxidize organic matter with sulfate in anaerobic conditions i.e. the absence of oxygen or nitrate and form sulfides. The production of hydrogen sulfide takes place at low flow velocity and temperatures above 15 to 20°C in gravity sewers.
Biological activity increases at high temperatures, increasing the consumption of oxygen and the production of sulfide.
In a previous article titled: How to Detect and Control Hydrogen Sulfide Corrosion Problems in Sewer Pipes, we have discussed the corrosion mechanism of sulfide in sewer pipes.
Here we will take a look at how corrosion due to hydrogen sulfide in sewer pipes can be controlled.
Factors Influencing Sulfide Production
Naturally occurring sulfur-reducing bacteria reduce sulfates to aqueous hydrogen sulfide (H2S) in the absence of oxygen. The aqueous H2S migrates as a gas into the air space where sulfur-oxidizing bacteria oxidize it into sulfuric acid (H2SO4).
This sulfuric acid — H2SO4 — is what attacks concrete and metal sewer pipes leading to their corrosion and eventual failure if left untreated.
How much sulfide is generated in a sewer system depends on some factors, such as:
- The pH of the wastewater.
- The flow rate of sewage in the pipeline — velocity less than 2 fps (feet per second) will allow inorganic grit to accumulate at the bottom leading to sulfide buildup.
- High retention time within the collection system.
- Excess concentration of nutrients and organic material in the sewage.
- Temperatures above 15 to 20°C.
- The concentration of dissolved oxygen.
Controlling Sulfide Production
A layer of slime covers the surfaces below the waterline, the bottom half of the pipe and on settled debris in a sewer pipeline.
As wastewater flows through the pipe, materials such as grit and debris settle to the bottom over the existing slime layer, increasing the surface area of the slime layer, consequently increasing the production of dissolved sulfide.
This sulfide cycle can be interrupted using some simple methods. Below a few of these methods are discussed.
Control Accumulation of Debris
As mentioned above, the settling of debris over time increases the surface area of the already existing slime layer. Regular inspection and cleaning can get rid of this problem. Debris accumulating over time can also cause other sewer problems such as backflow and blockage of the sewer pipes.
The best way to prevent corrosion in pipelines due to H2S is to inspect pipelines on a regular basis. Pipelines exhibiting signs of damage due to debris accumulated overtime should be rehabilitated or replaced. Rehabilitation and repair are now conveniently carried out using trenchless rehabilitation methods such as cured-in-place pipe (CIPP), pipe bursting and sliplining.
Maintain Slope of Sewer
Designers should consider proper flow velocities in sanitary sewer collection systems. Usually, gravity sewers are designed to prevent collection of debris with the minimum flow rate of 2 fps. Flows greater than 2 fps will minimize and often eliminate H2S problems.
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 to the sewer pipe.
Chemical Addition to Control Dissolved Sulfide
Chemical oxidants such as hydrogen peroxide or the 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. Certain chemicals such as nitrate, hydrogen peroxide, sodium hydroxide, magnesium hydroxide are commonly used as additives to control the concentration of dissolved sulfide.
The benefits and disadvantages of chemical additives should be studied well before considering an application to a sewer system.
Reduce Turbulent Flow in Sewers
Falling water is turbulent and water splashing from outfalls in the collection system strip H2S gas from the water and release it into the air. This release causes an odor and also further oxidizes it to sulfuric acid which can begin the process of corrosion.
It is helpful to reduce turbulence at drop manholes, Parshall flumes, and outfalls to control the production of odor and corrosion.
The slope of the sewer also should not cause turbulent flow.
Use of Inert Pipe Materials
The best way to prevent corrosion is to use pipe and fitting materials made with polyvinyl chloride (PVC), high-density polyethylene (HDPE) and fiberglass reinforced plastic (FRP).
These materials are chemically neutral when exposed to sulfuric acid. Old pipes that need rehabilitation can also be rehabilitated using pipes made with these materials with the help of trenchless rehabilitation techniques. Sturdy liners are also available that can easily line the old pipe and make it as good as new.
These pipe materials also have excellent hydraulic properties, are resistant to corrosion and chemicals, and prevent the accumulation of biological matter.
New sewer pipelines can be installed with a proper slope and a wide range of inert pipe materials that will reduce or eliminate the problems of corrosion significantly.
Existing installations, however, have to be maintained with the limited options available. The best method to keep sulfide production and therefore corrosion in check is to regularly use chemicals to break the sulfur cycle once the sulfide production factors are controlled, such as by oxidation, reduction and pH control.
Corrosion problems in existing systems can also be resolved by increasing ventilation, applying protective coatings such as bituminous and coal tar products, vinyl and epoxy resins, plasticized PVC sheets and more.