Question of the Day: Holding Time Index

Question

What is Holding Time Index, how is it calculated, and what is the impact on a water system in regards to the inhibitors used and biocide program?

Apostolos Kavadias · Answer

The impact of HTI depends on the system and the additive characteristics. Antiscaling agent is loosing it's force during time, and cannot be efficient on very low HTI (once through system). Very high HTI usually lead to very high concentration and LSI (Langelier Saturation Index), needing higher antiscaling dosage. 
Oxidizing biocides are consumed almost immediately, which should be taken into account. Generally biicide dosing and the type of application should be followed by measuring the Total Bacteria Count in order to determine the best method for it's application (shock or continuous dosing).
Periodical analyzes of water ions and TBC is a must in order to verify the system operating conditions.

Shesharam Chouhan · Answer

ln2*system volume/ blowdown 
 
it it indicates the time take to drop the conc of a species to half of its initial concentration. It helps to determine suitablity of dispersing polymer/ scale inhibitor to act before its degrade. In case of biocide which can flashes away, it helps to determine residence/ retention time which is further compared with reaction time to calculate required dosage.

Apostolos Kavadias · Answer

t(1/2)= 0,69*V/MU
where:
t(1/2)= half holding time (h). It is the time needed to lower the additive concentration at half (1/2), after stopping it's addition (dosing). 
V= water system total volume (m3)
MU= Make-up water (m3/h). Equal to water looses, including blow down.

Question of the Day: Holding Time Index

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