Environmental laws and regulatory aspects


Allowed Free residual Chlorine at plants outfalls given by Governments’ Environmental Laws and Regulations is also related (and extendable) to Chlorine dioxide, with the concentrations of the two species being in a fixed ratio, i.e. 1.9025.

So it means that, to be, for instance, within 0.1 mg/l threshold, the allowed ClO2 concentration is 0.19 mg/l, or, to be within 0.05 mg/l threshold the allowed ClO2 concentration is 0.10 mg/l.

In order to have a simple guideline, for a standard cooling seawater with:

  • a demand of 0.35ppm of ClO2, 
  • in principle, the maximum dosage level upstream the circuit of ClO2 is around 0.45 mg/l (on daily basis)

However, it’s to be noted that in many case the limit is referred to a “Daily basis”: this is the case where ClO2 is more advantageous than Hypochlorite, due to the paramount fact that the dosage can be shot, i.e.:

  • Supposed a total dosage of 8hrs/day of Chlorine Dioxide, 
  • on a daily basis the allowed threshold of 0.05mg/l of free chlorine is equivalent to 0.15mg/l
  • In fact: 0.15mg/l for 8 hours and 16 hours with 0.00mg/l of concentration gives the average daily 0.05mg/l.

Therefore, in terms of Chlorine Dioxide, for a 8-hour long shot, the allowed threshold at the outfall is around 0.30mg/l, that is much higher than usual values found in the outfall.

About this environmental issue on free chlorine limits, it is worthy to report a notable example in a GGC country : in order to comply with the environmental regulations, the residual free chlorine in the effluent that is released to the sea is allowed to be within 0.5 mg/litre on a monthly average and not to exceed 2.0 mg/litre as a maximum value at any time: both thresholds are on the safety side for chlorine dioxide, less for chlorine treatment, just considering that the former can be dosed in shot mode, so that the actual monthly average on the discharge can be significantly lower than permitted.

This effect of “dilution by time” could not be achieved with continuous Hypochlorite treatment that, for this respect, can be considered more impacting than ClO2.