If these ports are in the same country surrounded by the same body of water then treating ballast water before discharge will not be necessary. However if these ports are in different countries then treatment will be required. If your vessel takes ballast water on in one port, then after returning from offshore, it needs to de-ballast this water in a second neighbouring port in another country, then  treatment before discharge will be required. Whilst one option is to fit treatment units on each vessel at considerable cost, you could instead install a port based single stage mobile containerised BWTS in each of the 2 or 3 ports where your vessels operate, so enabling your full fleet to utilise the shared assets when required. Limited ballast water pipe retrofit work will be required on each vessel to allow the port system to connect to the vessel’s ballast water line, however these costs coupled with the costs of 2 or 3 port based single stage treatment systems is significantly lower than the cost of having to retrofit every vessel in your regional fleet with a BWTS.