By Andrew Kleisinger, P.Eng., and M. Jason Stusick, P.Eng.
Historically, water treatment systems have incorporated mineral acid feed systems for pH suppression. Their use, while effective when the process is properly controlled, presents numerous challenges to system operation. Challenges related to shipping, handling and storage of the large volumes of acid that are required are significant. Effective process control to ensure accurate dosing is critical. Overdosing of a strong acid can lead to overshooting pH targets, creating significant process related issues.
As an alternative to acids, carbon dioxide (CO2) may be used for downward pH adjustment. When in solution, CO2 produces carbonic acid, which is less corrosive and hazardous than sulphuric or hydrochloric acid.
Enhanced coagulation allows improved removal from source waters of natural organic matter (NOM), as measured by total organic carbon (TOC), by adjusting pH prior to the coagulation reaction. This is necessary to optimize the coagulation process, reduce residual aluminum when using aluminum-based coagulants, regulate disinfection reactions, and reduce the formation potential of trihalomethanes (THMs).
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In surface waters characterized by high pH and high concentrations of dissolved organic material, enhanced coagulation is implemented to destabilize the organic molecules. This enables them to flocculate and be removed during the filtration process. Acid is added to consume alkalinity and lower the pH of the raw water to a level at which the coagulant is most efficient at removing NOM. Coagulant is added to the pH adjusted process stream prior to rapid mixing.
In applications where enhanced coagulation is practiced, alkalinity can be depleted and pH can be suppressed to levels below acceptable ranges. If water lacks alkalinity, it can be subject to large pH swings and become aggressive and corrosive. Aggressive potable water can cause premature failure of mechanical equipment in the water treatment or distribution systems. Therefore, caustic is typically required after filtration to increase the pH and stabilize finished water.
With source waters containing high alkalinity and pH, it can be difficult to use mineral acids in enhanced coagulation without exceeding the maximum usage limits (MUL) of the chemical. A benefit of CO2 is that it can effectively adjust pH without consuming alkalinity. It can also be applied using dosage rates that typically do not exceed the MUL.
Handling and safety
Acid must be transported to the site, transferred to bulk storage tanks, and pumped to the point of injection. Spills or leaks can be hazardous to operators, as well as the public. As strong acids are corrosive and can be volatile, damage can occur to mechanical and electrical equipment from fumes and leaks.
CO2 can be stored outside, eliminating the space requirements for storing acid within a facility and costly ventilation requirements. It is neither corrosive nor volatile and does not produce harmful fumes. Any leaks can be vented to the atmosphere without posing a significant risk to the public. Although it is not poisonous, it is an asphyxiant and can displace oxygen in confined areas. Therefore, a CO2 area monitor is recommended when installing a CO2 feed system.
An additional advantage of using CO2 is that it is often produced as a byproduct from industrial facilities, such as fertilizer, gas, coal and ethanol plants, breweries, and distilleries. Its application in water treatment provides a sustainable use for a product that would otherwise be wasted to the environment.
Carbon dioxide feed system design
The main components of a CO2 feed system include: bulk chemical storage tank, gas feed control panel, dissolution assembly (where required), injection equipment, and pH analyzers for process monitoring / control.
CO2 is most often stored in a liquid state in bulk storage vessels outside of the water treatment facility. Tanks range in size and come in different configurations, customized to specific site requirements. The bulk storage system typically includes a vaporizer and refrigerator to maintain CO2 in its proper state for both storage and utilization. Where required, an element on the gas feed control panel is used to heat the gas as it is drawn from the tank.
A gas feed control panel is required to feed CO2 to the process stream at a controlled rate. The panel consists of a gas flow measurement device, typically a thermal mass flow meter or Coriolis meter, a modulating flow control valve, gas temperature measurement device, CO2 area monitor, and isolation and pressure relief valves.
CO2 dissolution in the process stream is required to ensure the gas becomes entrained to form carbonic acid. If sufficient mixing and length of pipe runs exist, the injection system can be as simple as injecting the gas directly into the process stream through a porous diffuser. In other cases, dissolution assemblies may be required. These may consist of a motive stream of raw or treated water, in which CO2 gas is dissolved to form a carbonic acid solution. The solution is then reinjected into the process stream.
Dissolution assemblies may require a flow measurement device, an eductor to draw CO2 gas into the motive stream, a static mixer, and additional pumping. The selection of the appropriate injection equipment is a function of the desired CO2 gas utilization efficiency, equipment capital cost, and available plant footprint.
In order to properly design the feed system, it is recommended that bench top testing be undertaken, to determine the range of CO2 feed rates required to adjust the pH to the desired level. In lieu of testing, water quality data and theoretical modeling software can be utilized to approximate the required feed rate.