By Simon D. Marshall
Summary: Even though many municipal water treatment facilities and other large applications have utilized on-site generation of sodium hypochlorite since the 1970s, the need to reduce salt and energy costs continues to be emphasized. Benefits of the process over other alternatives as well as a unique approach to an old idea are presented here.
For more than 30 years, on-site generation of sodium hypochlorite has been a key process in water treatment for municipal and industrial applications.
The design and operation of one particular system is straightforward. Salt (NaCl) is dissolved in a tank using softened water to produce a 30 percent brine solution. The brine solution is reduced in concentration with further softened water to 3 percent (thereby cutting scaling of cathodes and stretching the intervals for acid cleaning from 2-to-3 weeks to 4-to-6 months) and pumped through an electrolytic cell. This 10:1 dilution of the concentrated brine solution is required because basic reactions in an electrolytic cell become inefficient above a 1 percent concentration, hence the requirement to electrolyze a 3 percent solution to 1 percent vs. 30 percent to 1 percent which would waste a tremendous amount of salt.
The electrolytic cell consists of numerous titanium plates divided into arrays or cell packs consisting of an equal number of anodic and cathodic plates. The cell packs are configured electrically in parallel, while the overall cell is configured electrically in series. The cell is fed DC power from a rectifier. The cell electrolyzes the diluted brine into a sodium hypochlorite solution.
In simple terms, chlorine is evolved at the anode surface, while hydrogen is evolved at the cathode surface. The secondary reaction of chlorine, sodium and the hydroxyl ion nets sodium hypochlorite or chlorine bleach at a 0.8 percent solution. As a reference, bottled household bleach is typically delivered at a 5-to-6 percent solution. The process can be described as follows:
NaCl + H2O + 2e = NaOCl + H2
Salt + Water + Electrical Energy = Sodium Hypochlorite + Hydrogen
Sodium hypochlorite solution is held in storage for 24 to 48 hours (as a backup to the generation process to allow for maintenance and repair) before being dosed directly into the supply using a metering pump. The benefits of this process over chlorine gas and bulk hypochlorite have been well documented and include:
- Eliminates bulk storage of chemicals
- Reduced risk to plant personnel because of drastically reduced hazardous material (HazMat) storage and handling requirements;
- No transportation liabilities
- Reduced threat to public safety
- Occupational Safety and Health Administration (OSHA) and U.S. Environmental Protection Agency (USEPA) exempt
- Lower disinfection by-products (DBPs)*
- Consistent solution concentration
Aware of ever-increasing water quality standards and under pressure to reduce plant operating costs, a research and development (R&D) team studied how to better understand the operating characteristics of the cell to further optimize the product performance. The result is a split flow cell with a guaranteed 20 percent reduction in consumable operating cost—15 gallons of water, 2.0 kilowatt hours (kWh) and 3.0 pounds (lbs) of salt per pound (lb) equivalent chlorine (Cl2).
During the electrolytic process, the solution temperature increases approximately 30°F across the cell as a result of resistive losses associated with flowing current through the 3 percent electrolyte. The study determined that with an average influent temperature of 65°F, the cell efficiency reduced notably as the solution rose to 95°F on exit from the cell. In addition, the creation of by-products* was also concentrated at the higher temperature. The team established that if the solution temperature was held below 80°F, not only was there a significant reduction in salt and power consumption to produce the 0.8 percent sodium hypochlorite solution, but also the creation of by-products became negligible. That’s because chlorate formation is very dependent upon temperature and increases geometrically above 80°F.
Having recognized the dynamics of the individual cell compartment’s temperature and its effect upon efficiency and DBPs, the team redesigned the hydraulic process to allow the entire cell to operate within the optimum temperature range of 65-80°F.
Being bound by the thermodynamic principles associated with these various reactions, it became evident that it would be necessary to bleed energy from the cell during operation or to somehow chill the cell electrolyte. To “bleed” energy means to displace that energy in some fashion such as heat exchange, automobile radiator or a refrigeration system, etc., which pumps the energy from one medium to another. Your home refrigerator, for instance, chills food in part by bleeding the energy from the air contained within the refrigerator.
Dividing the water
After extensive pilot plant work, the decision was made to split the flows to the cell. One hundred percent of the brine and one-third of the softened water is injected at the front of the cell resulting in near perfect electrical efficiency in the first third of the cell due to the three-fold excess in salt present. The split flow cell functions to dilute the concentrated brine from 30 percent to 10 percent in the first cell passes and further dilutes the brine to the net process goal of 3 percent. The first passes of the cell at 10 percent are far more efficient electrically than the later passes, but would be very wasteful of salt if the secondary dilution didn’t take place, hence the split flow design. The balance, or two-thirds, of the softened water is then directed to a water chiller where it’s cooled and then injected into the last two-thirds of the cell. The net effect of this is a cell that always operates within the optimum efficiency window of 65-80°F (see Figure 1).
The team’s work has taken advantage of two separate phenomena associated with conventional electrolytic cell design to optimize the process within a single electrolytic device. First, the efficiency gain associated with the excess salt in the first compartment and resulting dilution. Second, the ability to effectively bleed energy from the process via the chilled secondary electrolyte.
In summary, the split flow cell system performance reduces salt consumption from 3.5 lbs per lb. of Cl2 to 3.0 lbs per lb of Cl2, and energy consumption from 2.5 kWh/lb to 2.0 kWh/lb of Cl2. Therefore, it produced an operational cost reduction of 14 percent in salt consumption and 20 percent in power (see Figure 2).
The R&D team maintains an ongoing program of product refinement and improvement. Current research is focusing on the minimization of the bromate DBP and its potential sources.
The split flow cell is available at the same cost as traditional competitive on-site hypochlorite generation cells and, with no increase in capital cost, the savings are delivered as a net benefit to end users.
The new process discussed here consumes 20 percent less in raw materials than competitive products. Plus, with salt and energy costs at $0.05 per pound and $0.06 per kWh, respectively, the operating cost reductions over the life of the equipment can amount to hundreds of thousands of dollars.
The process provides all of the advantages of on-site generation while actively minimizing the costs of production and potential DBPs. With the much anticipated chlorate/chlorite rules being negotiated both in the United States and Europe, the ability to actively manage DBP concentration to less than 2 percent of the total disinfection production will prove to be a tremendous benefit to everybody.
About the author
Simon D. Marshall is the ClorTec Product Manager for Severn Trent Services, of Fort Washington, Pa., an international provider of solutions to municipal and industrial water and wastewater treatment operations. The ClorTecTM OSHG and CT Split Flow Cell are discussed in this article. Simon can be reached at (800) 524-6542 or email: http://[email protected].