Hach Disinfection Series - Step 8

8. Determining an Ammonium Feed Rate for Chloramination

In past Hach Disinfection Series articles, we have discussed the differences between free chlorine and monochloramine in disinfection. We have also covered how to optimize the chloramination process in different situations, but many operators are questioning how to determine the amount of Liquid Ammonium Sulfate (LAS) that is required to achieve a chloraminated (monochloramine) result. This article focuses on how to determine the LAS feed rate based on chlorine feed rate and the measurement of ammonia levels.

Proper Measurement is the Key to Proper Control.

• Measure total ammonia with a test kit or lab method, for example the salicylate method, to detect/control the ammonia feed rateⁱ. We can refer to this as a "coarse control" to monitor the ammonia feed rate.
• Measure monochloramine/free ammonia with a monochloramine/free ammonia kit or lab methodⁱⁱ. This we can refer to as a "fine control." Fine control is used to adjust the chlorine to ammonia ratio to achieve chloramination with monochloramine — keep free ammonia greater than but close to zero. For more discussion on ratios, please reference Hach Disinfection Series Article Step 5 - Optimizing Monochloramine Production.
  1. If one has too much free ammonia, either slightly increase the chlorine feed or decrease the ammonia feed.
  2. If one has too little free ammonia, either increase ammonia feed or decrease chlorine feed.
  3. Typically one should change only one variable. That is, one should set either ammonia or chlorine feed rate and vary the other one. Different utilities will have their own preference as to which one to fix and which one to vary. This preference will be based on chemical feed schemes and other operational considerations.

What is Liquid Ammonium Sulfate (LAS)?

According to the General Chemical Product Data Sheet, LAS is a 38-40% aqueous solution of ammonium sulfate, (NH₄)₂SO₄. Each gallon of LAS has a specific gravity from 1.216 to 1.228 or 10.15 to 10.25 #/gallon and thus is typically 10% as ammonia (NH₃). For coarse control, see step one above; we can simplify this using the approximation that LAS is 10#/gallon. Since LAS is 10% as ammonia, each gallon (10#) contains approximately 1# of ammonia or 0.84# as N:

Or, a 40% solution of LAS has a weight of 10#/gallon and contains 0.84# as N.

Example: If the expected chlorine feed rate is 4 #/hour, how much LAS should be fed per hour to achieve an approximate ratio of 4.5:1 (chlorine: nitrogen)?

Now for fine control measure monochloramine and free ammonia. Is the free ammonia close to but greater than zero? If yes, great! If free ammonia is zero, decrease chlorine feed (or increase ammonia feed) until a slightly positive free ammonia is obtained. Remember, it is best to fix one variable and make changes with only one.

Use of Aqua Ammonia (ammonium hydroxide), Granular Ammonium Sulfate or Anhydrous Ammonia

If these chemicals are being used to achieve a chloraminated result, calculate using a procedure similar to that outlined above.

Online Analysis Tools

Experience proves that the chloramination process can be extremely effective, but only if the concentration of ammonia and chlorine in the system remains in proper balance. For optimal control of the chloramination process, a low range ammonia and monochloramine analyzer is an essential toolⁱⁱⁱ. And, for reporting total residual chlorine, use an online chlorine analyzer. Many utilities utilize Hach’s market leading colorimetric method^iv. In some situations, such as where where a reagent stream may cause disposal issues, it may make sense to use an amperometric method^v. If you are making a decision between methods, it may be helpful to visit Hach's Online Chlorine Selection Guide.

Instrument References:

ⁱ Ammonia Pocket Colorimeter II Test Kit - Many of Hach’s market leading multiple wavelength colorimeter or spectrophotometer instruments can also be used.

ⁱⁱ Free Ammonia Pocket Colorimeter II Test Kit - Many of Hach’s market leading multiple wavelength colorimeter or spectrophotometer instruments can also be used.

ⁱⁱⁱ APA 6000 Ammonia and Monochloramine Analyzer

^iv CL17 Total Residual Chlorine Analyzer (colorimetric DPD method)

^v CL10 sc Total Chlorine Analyzer (amperometric method)