Practical Guide to Chloramination Monitoring and Control - Point of Analysis Recommendations

Dokument-ID

Dokument-ID TE12822

Veröffentlichungsdatum

Veröffentlichungsdatum 21.01.2021
Frage
Practical Guide to Chloramination Monitoring and Control - Point of Analysis Recommendations
Zusammenfassung
Rules to sustain equilibrium of chloramination control with prevalence of monochloramine. Rules to achieve best analysis results in chloramination control.
Antwort
The reaction between ammonia and free chlorine is fast, but it takes time to establish equilibrium between all chloramination species to ensure sustainable concentration of monochloramine.
 
NH3 + HOCl ↔ H2O + NH2Cl – predominant monochloramine formation, Cl2:N mass ratio <5
NH2Cl + HOCl ↔ H2O + NHCl2 – predominant dichloramine formation, Cl2:N mass ratio >5
NHCl2 + HOCl ↔ H2O + NCl3 – unstable nitrogen trichloride (trichloramine)
2NCl3 → N2↑ + (3Cl2 + 3H2O ↔ 3HCl + 3HOCl) – free chlorine formation
 
Main Takeaway: Ammonia, Free chlorine, Monochloramine, Dichloramine are always present in chloraminated water (in equilibrium). Therefore, main goal of chloramination control is to sustain equilibrium with prevalence of monochloramine (first equation above).
 
Main parameters influencing the equilibrium
  • sample pH, temperature, and time/distance after mixing the ingredients (chlorine and ammonia).
  • conductivity and hardness should be considered also, especially for ground water (too high is bad).
Main rules to sustain monochloramine and achieve best analysis results
Ideal conditions
  • Higher pH helps to stabilize monochloramine (sustains the first equilibrium). Preferred pH is 8.2-8.3, however it should be at least >7.5-7.7
  • Optimal water temperature is ~14°C. Deviation in either direction is not good, and it is usually better when the Temp is lower, than higher.
  • With preferred pH and Temp, it should be ~2 minutes retention time after last chemical injection point (either chlorine or ammonia) and it should be longer as pH and temperature deviate farther from ideal.
Non-ideal conditions
  • If pH is <7.5 and Temp is <5°C or >20°C, it is recommended to introduce a holding tank (with mixing) after the chemical injection and before the analyzer or grab sampling point.
  • If there is no way to achieve adequate retention time before the analysis – consider moving the sampling point downstream as far as possible, say to the closest pump station to achieve the stable sample and sustainable equilibrium.
If nothing can be done
  • Install the 5500sc AMC analyzer or choose grab sampling location as far as practical/possible from the chemical injection point and try to put up with the noise in the readings that may be significant. It is possible, of course, to average the readings in the output to reduce the noise, however, it is not the best practice and should be employed as the last resort, only after a validation study.
  • The biggest drawback besides noisy process readings will be a higher potential for discrepancy:
    • between grab sample and process analyzer readings and/or
    • between core parameters (monochloramine, total chlorine, ammonia).
  • In case of the discrepancies, consider more frequent grab sample analysis downstream from the process analyzer, where the sample content is more stable (case a).
  • Remember to always measure at least free ammonia in addition to total chlorine, and preferably the entire chloramination suite of parameters, e.g. using the Hach SL1000 (cases a & b).
When in trouble, ask your local Hach Sales representative or Hach Technical Support – we will help!

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