Chemical Water Treatment Equipment Information
Chemical water treatment equipment includes chemicals, monitors, and other equipment used to detect and treat common process water problems.
Process Water Overview
Process water is defined as water that cannot be classified as drinking water and is used in industrial equipment and processes. It is commonly used in boilers, cooling towers, or heat exchangers.
While water's low cost, wide availability, and heat-exchange abilities make it an ideal heat exchange medium, its characteristics as a solvent can cause unwanted side effects in industrial systems. The most common harmful effects can be classified into the four main categories discussed below.
Corrosion is an electrochemical process in which a metal—commonly mild steel or another lower-cost material—returns to its natural oxide state, causing metal thickness loss or full penetration through a metal tube wall. Corrosion begins when metal ions dissolve into an electrolyte (water, in this case), leaving behind electrons. These free electrons flow through the metal and gather at a cathode point, where electron-consuming reactions occur. The reactions result in metal loss and possible formation of deposits.
A corroded pipe.
Image credit: Vsolymossy / CC BY 3.0
Corrosive attacks may be general and affect the metal surface uniformly, or localized more severely within a small area. The latter action, commonly called pitting, often affects slower-corroding metals such as copper, aluminum alloys, and stainless steel. A third type, galvanic corrosion, occurs when two dissimilar contacting metals corrode at nozzle connections or fittings.
Scale is a primarily inorganic coating formed by water-soluble minerals. Common scale minerals are calcium carbonate, calcium phosphate, magnesium silicate, and silica. A mineral that is less soluble in a certain water sample is more likely to precipitate on surfaces as scale. A water sample's ability to form scale is determined by four factors:
Temperature: Minerals become more soluble as temperature increases (direct solubility) or decreases (inverse solubility).
pH: Changes in alkalinity or acidity greatly affect a mineral's solubility. For example, calcium carbonate decreases in solubility as pH increases, becoming much more likely to form scale.
Total amount of scale-forming material present in the sample.
Influence from other dissolved materials.
Fouling is the accumulation of solid contaminants, other than scale, that contributes to equipment deterioration or hampers system performance. Foulants include dirt, silt, sand, organic materials, microbial masses, and certain phosphates. This phenomenon is generally more prevalent in water containing large amounts of suspended material, but environmental factors such as temperature, flow velocity, microbial growth, corrosion, and contamination can also influence fouling.
Biofouling in a cooling water system.
Image credit: MERUS
Water's natural life-giving properties encourage biological growth of micro-organisms. Unchecked growth of these organisms in process systems results in microbial slimes, sticky masses of micro-organisms that encourage fouling and corrosion. While these organisms typically enter a system within the original water source, airborne organisms may also be blown into a cooling tower or other system.
Effects of Corrosion, Scale, Fouling, and Biological Contamination
The problems above can have a direct negative impact on systems using process water. Common problems include:
Increased maintenance, repair, and shutdown costs, leading to higher total cost of operation (TCO)
Reduced heat transfer efficiency, leading to higher fuel and energy costs
Increased emissions and environmental compliance issues
Possible plant shutdowns, equipment loss, and product quality issues
In extreme cases, catastrophic boiler failure and possible loss of life
Chemicals are added to raw process water at several points during the treatment process. Chemical treatment of boiler feedwater, cooling tower water, and other process water is commonly performed in conjunction with a mechanical treatment program using ion exchange equipment, dealkalizers, and deaerators.
Chemicals play a significant role in boiler and cooling water treatment processes.
Image credit: Naigi Chemical Products Co.
Chemical suppliers often classify their products based on the categories below.
Chemicals such as algaecides, biocides, and other disinfectants kill unwanted organisms in process water.
Algaecides kill algae and cyanobacteria (blue-green algae) in a water sample. While introducing an algaecide into a system eliminates all algae present, it does not remove the toxins released by the algae prior to death.
Biocides and other disinfectants are toxic to micro-organisms and rapidly reduce their numbers, causing the population to die out. The most common biocides are oxidizing types such as chlorine, chlorine dioxide, and ozone. When oxidizing biocides are ineffective in a particular water system, non-oxidizing agents such as copper salts, amines, chlorinated phenolics, or quaternary ammonium salts may be used. For more detailed information on biocides and their reactions, visit Lenntech's biocides page.
Each disinfectant has inherent advantages and disadvantages. For example, ozone is highly effective in both liquids and on surfaces and is commonly used in pharmaceutical applications and process water treatment, but is expensive.
Chemical corrosion inhibitors react with metallic surfaces to provide protection against corrosion. There are five types:
Passivity inhibitors, or passivators, shift metallic surfaces into the passive range, causing a shift in corrosion potential. These include oxidizing agents chromate, nitrite, and nitrate as well as non-oxidizing ions such as phosphate and molybdate. Passivators are the most effective type of corrosion inhibitor.
Cathodic inhibitors may operate using one of two reactions. Chemicals such as arsenic and antimony make the recombination and discharge of hydrogen more difficult. Conversely, inhibitors such as calcium, zinc, and magnesium are deposited as oxides on a metal surface, forming a protective coating.
Organic inhibitors act on the entire surface of a metal part, forming a hydrophobic coating.
Precipitation-inducing inhibitors include silicates and phosphates, and form a protective coating on the metal surface.
Volatile corrosion inhibitors (VCI) include morpholine, hydrazine, and salts of cyclohexylamine or dicyclohexylamine. When these compounds contact a metal surface, vapor condenses and is hydrolyzed, forming protective ions.
Flocculants promote the formation of bonds between suspended solids in a water sample. When suspended solids bond to each other they form a larger particle called a floc. Flocculants have one or more assigned charges that generally determine their function. Cationic (positive) polymer flocculants are based on nitrogen, while anionic ones are based on carboxylate ions. Polyampholyte are specialized compounds that carry both positive and negative charges.
Chemical oxidants reduce chemical and biochemical oxygen demand. In more extreme uses they also completely oxidize organic or inorganic contaminants, reducing them to carbon dioxide and water. The most common oxidants overlap with oxidizing disinfectants and corrosion inhibitors.
Hydrogen peroxide is one of the most common oxidants due to its versatility, effectiveness, and safety. It is deployed in situations requiring odor control, corrosion control, organic oxidation, or metal oxidation.
Ozone is widely used as both a disinfectant and oxidant. It effectively oxidizes common contaminants such as nitrite, halogens, iron, cyanide, manganese, PCBs, and pesticides.
Oxygen can be used to oxidize minerals such as iron or manganese.
Oxygen scavengers may be thought of as the opposite of oxidants: they prevent oxidation reactions that cause corrosion. Organic solutions such as carbohydrazine, hydroquinone, and diethylhydroxyethanol are used as scavengers because they have a slightly negative charge; because oxygen molecules have a slightly positive charge, the organic scavengers absorb them and prevent oxidation. Non-organic oxygen scavengers include hydrazine and sodium sulfite.
pH control chemicals include neutralizing agents and pH conditioners. Neutralizing agents move water's pH level from overly acidic or basic. Sodium hydroxide, calcium carbonate, and lime increase pH, while sulphuric or hydrochloric acid, both diluted, lower it. Neutralization reactions typically cause a rise in temperature, which may affect other process water variables.
pH conditioners, such as hydrogen chloride and natrium hydroxide make finer adjustments. The amount of substance used depends on the necessary adjustments.
Resin cleaners chemically clean used ion exchange resins. These resins can be reused following regeneration, but this process causes serious fouling; resin cleaners oxidize these foulants. Common resin cleaners include sodium chloride, potassium chloride, citric acid, and chlorine dioxide.
Scale inhibitors are negatively charged polymers that attach to scale-forming mineral formations after those minerals exceed their solubility limits. The polymers disrupt the crystallization process, preventing scale. Common scale inhibitors include phosphate esters and phosphoric acid.
Monitors and Controllers
Manufacturers may also offer monitors and controllers related to chemical water treatment. Monitors contain sensors or probes that return data on key variables, including pH, water quality, temperature, and the amount of chemicals already contained in process water. Further information on these instruments and sensors can be found in the following Engineering360 specification guides:
Specialized controllers may be used to control chemical treatment. Devices with a "feed and bleed" function can automatically feed disinfectants and other chemicals into a process water system based on elapsed time or as a percentage of water makeup.
Chemical water treatment equipment and chemicals may be designed and used according to published standards, many of which are listed on the Engineering360 Standards page. Examples include:
NACE 11206 -- Biocide monitoring and control in cooling towers
EN 12952-12 -- Requirements for boiler feedwater and boiler water quality
SAI - SAA AS 5059 -- Power station cooling tower water systems—Management of Legionnaires' disease
Cleaver-Brooks—Boiler water treatment (pdf)