Hi there, as most have ...

Hi there, as most have indicated you seem to be over dosing and should definitely be achieving complete disinfection under these conditions...in fact you appear to be wasting chlorine that is costly!

Some factors that could be influencing, retention time in the chlorine contact time - industry norm of around 15-20 mins at PDWF, WRC TT389 & WISA Design Guide referenced,

Poor effluent quality, although TSS is stated as 20mg/L, presence of algae (or other organics) and ammonia can affect, so too can temperature and pH

Of course one needs to remember that the sample bottle must be sterile and the sample collected under sterile conditions - very NB

And the timing of reading the residual chlorine -must be as soon as the reagent has dissolved, the longer it remains the further it will react! And glass vials must be clean! I don't recommend utilising colour indicator strips if that is the method of testing you are utilising.

Shout if you need to discuss to troubleshoot further

Regards