Corrosion Problem in Water and Wastewater Treatment
Prof A S Khanna
IIT Bombay (Retd)
Chairman SSPC India

Water is the scarcest resource on Earth these days. About 71 percent of the Earth's surface is water-covered, and the oceans hold about 96.5 percent of the water available on Earth in aggregate. If we move to drinking water source, the total share comes from large rivers, lakes, ponds, rain, and snow. Still it is very unfortunate to know that there are numerable places in the world which are devoid of natural drinkable water. Gulf and Saudi Arabia are heavily dependent on desalinated sea water for all their daily needs and cultivation. Many cities are dependent on the annual monsoon which provides enough water collected for the whole year. Mumbai is the best example for that. Nonetheless, this article is not to discuss the global water problem and how to make it available; instead, the author here has concentrated on the corrosion issue of water distribution system to household, establishment, and utility periphery.

Water from ponds, rivers, and lakes is first cleaned at one place and then it is distributed to houses, factories, and other utilities. The distribution is usually done through huge pipelines placed either underground or above the surface. In most of the cases, these pipelines are constituted by cemented pipes, cast iron pipes, or steel pipes. For the steel pipe usage, both inside as well as outside corrosion problem need to be taken care of. The primary concern about the water quality is due to the presence of toxic metals such as lead and mercury, deterioration of household plumbing, usual problems such as poor taste, and staining on laundry wash basins and drains.

Fig. 1 A typical water treatment plant

In order to know whether the water will cause corrosion to a pipe or not, there is a method to measure called Saturation Index (SI). Langelier Saturation Index is to evaluate water quality data to determine if the water has a tendency to form a chemical scale. In order to use this index, laboratory analysis is needed for pH, conductivity, total dissolved solids, alkalinity, and total hardness. Then the actual pH of water is compared to the theoretical pH (pHt) based on the chemical analysis. The Saturation Index is then given by:

SI = pH - pHt

The Saturation Index is typically either negative or positive and rarely zero. A zero Saturation Index indicates that the water is “balanced” and is less likely to cause scale formation. The scale, typically a carbonate residue, may clog the pipe thus reducing the flow; and may build up on hot water heaters thus imparting an alkali taste to water, reducing the water heater efficiency, and causing other aesthetic problems. Table 1 presents a typical range of SI for drinking water, a respective description about the nature of the water, and general treatment recommendations. A negative SI suggests that the water is under saturated in context of carbonate equilibrium with greater corrosive potentiality.

Reasons for Pipeline Corrosion: To dig deep, the factorials responsible for pipeline corrosion surfaced out as:

1. The pH of water
2. Oxygen content in water as it facilitates the rusting of metal thus causing degradation.
3. Water's chemical make-up, as the minerals present in water influence the corrosion level. For instance, high levels of calcium can cause build-up level.
4. Water temperature is another prime contributor owing to its corrosion rate enhancing ability.
5. High water velocity, water turbulence, and sudden directional changes in terms of turns and elbows can lead to erosion and corrosion.

Treatment: The treatment to protect the steel pipes from corrosion caused by water consists of:
1. Addition of inhibitors influencing the SI value to remain in the range of 0.5 to (-1).
2. Application of suitable coating.
Use of inhibitor chemicals is the most effective method to control internal corrosion of steel pipes. Many of these inhibitors are either organic or inorganic chemicals which react with steel walls and form a thin layer thus protecting the pipe from further corrosion to happen. One such important inhibitor used for drinking water distribution is water soluble alkali silicates. It's worth mentioning that such distribution is made of steel, stainless steel, copper, or nickel. The silicate reacts with available calcium in water and forms insoluble calcium-silicate layer on the top of the corrosion products, thus prevents further corrosion causing reaction due to aggressive water, carbonic acid, chlorine, and chloramine. The amount of silicates used are in ppm level which forms a thin mono layer and therefore stops the formation of iron oxide and hence stops the corrosion [2].

These days many of the new water pipelines, getting installed at many cities in India, are choosing a polymer coating layer, especially a food grade epoxy coating with a suitable "C" value which controls the drag and also helps in controlling the corrosion.

From external side, the steel pipe used either above the surface or underground needs corrosion protection in the form of coatings. For above the surface pipelines, a suitable long term system (6-8 years) is shot blasting followed by inorganic Zinc Silicate primer and one/two coat of epoxy with top aliphatic polyurethane coat with total thickness ranging from 200 to 250 μm, depending upon whether the environment is C3 or C4.

For underground water-pipelines, till 2000, reinforced concrete was used as external protection to the steel pipe. However, many failures of such pipelines were reported due to ingress of chemicals, chlorides etc thus causing the pipeline damage; and presently a three layered PE coating has been accepted as the best solution. To add to it, use of coal tar is a coating option on specific areas.

Waste Water Treatment

In context of wastewater, a great concern is the treatment and transportation of the same. It is the biggest source of corrosion as it has pH ranging from 1 to 14, toxic solvents, and bio-organics which can deteriorate the steel pipes. There are wide range of different wastewater components, such as Organic components - fats, greases, proteins, surfactants, oils, pesticides, phenols, and the inorganic components of raw sewage viz heavy metals, nitrogen, phosphorus, sulphur, acids, and a variety of strong alkalis. In addition, gases such as hydrogen sulphide, methane, ammonia, oxygen, carbon dioxide and nitrogen are commonly found in wastewater. Anaerobic decomposition of organic materials containing sulphur and nitrogen produces, hydrogen sulphide, amines, and volatile fatty acids. Chlorine and ozone, which are usually added in the final stages for disinfecting can also cause corrosion. Biological bacteria such as sulphur -reducing bacteria (SRB) reduce sulphates to sulphites in an anaerobic environment and produce hydrogen sulphide (H2S) gas. Other aerobes, most commonly different strains of Thiobacillus, will oxidize the sulphur to sulphuric acid bringing pH to as low as 1.0, which can cause corrosion to metallic or concrete container.

Corrosion control of internal wastewater pipelines

Steel pipes transporting sewage or waste water can corrode easily due to highly corrosive water ranging from pH 1 to 14, and also due to presence of various toxic gases especially hydrogen sulphide. Hence the internal surface of the steel or concrete or even ductile iron pipes. Bituminous enamel or coal tar enamel were at one time used to apply for internal and external surface of waste water pipelines [3], the details of material specifications can be seen in AWWA C203. However, due to emission of carcinogenic fumes during application, use of coal-tar has been discontinued. Thus the best coating systems for internal coating are :

1. 100 percent solvent-free epoxy coating lining to a thickness of 500-1000 μm.
2. Solvent-free epoxies or polyester coatings modified with glass-flakes can handle highly toxic waste.
3. High solid and high build, fast cure polyamide epoxy protects the internal surface very effectively.
4. 100 percent solids, spray-applied, aromatic poly-urea coating and lining system is suitable for use in immersion and internal lining.

In order to apply these coatings, proper surface preparation is required preferably Sa21/2 or better, followed by application of this coating. For bigger pipes diameters and tanks, these coatings can be applied using airless spray guns but for smaller pipe diameters, automatic pressure spray, and abrasive blasting can be used. It is also important to properly inspect the pipes by checking holidays if any, bond strength etc.

Corrosion control of External Waste water Pipelines

Though a simple coal tar enamel coating can be enough for giving protection to a waste water pipeline, a polymer based coating usage is required to stop the movement of moisture and pollutants from the soil to the steel surface by slow permeation. It is therefore expected to use all those coatings which are required for underground crude/gas pipelines:

1. Single epoxy coating
2. Three layer polyethylene coating
3. Liquid Epoxy/elastomeric PU coating

Though there is no use of applying cathodic protection on water pipelines as it is mandatory in case of crude/gas pipelines, some users have already started putting cathodic protection also.

1 Brian Oram, Corrosive Drinking Water(Lead, Copper, Aluminum, Zinc and More ), -water-lead-copper-aluminum-zinc-and-more
2 A Corrosion Protection solution for drinking water distribution piping systems, -for-drinking-waterdistribution-piping-systems.htm
3 Shiwei William Guan, Corrosion protection by coatings for water and waste water pipelines, Water and Wastewater Program West Virginia University, PA, May 15, 2001
4 A.S.Khanna, Corrosion Protection of Underground Gas/Crude Pipelines, in "Corrosion and Materials in the Oil and Gas Industries", CRS publisher, 2016 p 204-215