Click here to enlarge imageFor blending to meet arsenic levels other than 8 ppb, use the equation:
Q ratio = (AsLag - AsBlend) / (AsBlend - AsLead) where
Q ratio = Lag well flow / Lead well flow
AsLag = Lag well arsenic (ppb)
AsLead = Lead well arsenic (ppb)
AsBlend = Blended maximum arsenic (ppb)
For example, if the higher Arsenic Lag Well is 12 ppb and the lower Arsenic Lead Well is 2 ppb with a desired 5 ppb blended level the Q ratio (Lag flow / Lead flow) = (2-5) / (5-12) = .43. If the Lead Well delivers 1000 gpm, then the Lag Well must be limited to 1000 x 0.43 = 430 gpm in order to achieve an arsenic level of 5 ppb going to distribution.
Partial Treatment Blending
If a community does not have any water sources that are below the limit, and cannot or will not absorb the cost of treating all of their water, they may be able to use a partial treatment approach where only a portion of the water source is treated to remove most of the arsenic and remixed with untreated water with higher arsenic content to meet the prescribed EPA levels.
When employing the partial treatment blending approach, a ratio control valve can be used to proportionally split the flow so that one portion is treated to remove arsenic and the remaining raw water is remixed after the treatment process to provide water for the distribution system that meets regulatory requirements. If the treatment process removal rate changes with time, the ratio factor may need to be adjusted periodically. Monitoring raw and treatment arsenic levels will determine the split ratio.
Other variations of this strategy include source blending in combination with partial treatment blending. Regardless of the method selected, the objective is to minimize treatment costs while limiting arsenic to an acceptable level.
Additional Functions
In addition to electronic ratio control, hydraulic pilots can be incorporated into the valve to control upstream and downstream pressure. That way, the valve can also be set up to operate hydraulically in the event that power or input signal from the meter is temporarily lost. WW
About the Author:
Roger Lah has been with Cla-Val Co. of Newport Beach, CA, for more than seven years and was instrumental in the development of the company’s electronic valve control product line. His education includes a Bachelor of Science in Agricultural Engineering with an emphasis on irrigation systems and hydraulics from Purdue University, and a Master of Science degree from the University of Illinois, also in Agricultural Engineering