Commercial Swimming Pools.

Commercial Swimming Pools.

DPD Online Instruments Versus the Halogen Sensor

In Ballast Water Treatment Systems (BWTS) for larger ships, generated residual oxidant systems, resulting from either ozone or electrolytic generation, inactivates invasive species. In both cases, the residual oxidant is bromine.

Sensors and/or analyzers are used both to control the level of oxidant generated during ballasting and to detect the residual oxidant levels before deballasting (ballast water pumped out) operations occur. During ballasting, water is loaded, treated and monitored to ensure inactivation of invasive species is complete.  During deballasting, sensors monitor the residual oxidant level to ensure that the residual concentration is below 0.2 (0.1 in some cases) ppm. This is a regulatory level required by International Maritime Organization and the US Coast Guard.

Most BWTS systems use N,N-diethyl-p-phenylenediamine, (DPD) Instrumentation at various locations on board the ship. These units use regent feed colorimeters to indicate Total Oxidant levels. They require a relatively long cycle time of 75 seconds before displaying a reading.  In addition to the cycle time, discarding the first or second measurement is typical due to initial inaccuracies. This delay is further compounded when the sample water must flow hundreds of feet to the colorimeter location before analysis can occur on larger ships. Sampling pumps, water lines and automated valves must be added to system which complicate and CAPEX costs.

While no analytical method is perfect, DPD Instruments suffer from a number of limitations in this application. They require:

  • Use of tubing as small 1mm (ID) which has a tendency to clog (Via dried reagents or biofouling)
  • Filtration of the sample (Increase turbidity causes inaccuracies)
  • Filter cleaning
  • Cleaning of the colorimeter with acid every 60 days
  • Regent replacement every 60 days
  • Disposal of a waste stream

In general, these systems are complicated, difficult to troubleshoot and require considerable maintenance. This is problematic on a ship since there is typically a small crew with many duties during ballasting and deballasting (when these systems are used).

Halogen Systems, Inc. manufactures Water Treatment sensors for Seawater, drinking water, swimming pools and cooling towers. It’s Oxidant/ Deoxidant Sensor for use in Ballast Water Treatment Systems is currently on-board ships.  The sensor is installed directly in the ballast pipe and measures TRO every 50 seconds. These measurements are accurate within two, and sometimes one, cycle.

Halogen Systems’ Oxidant Sensor has some advantages in this application:

  • Long interval between calibrations
  • Self- cleaning electrodes
  • Flow independent measurement
  • Direct pipe insertion
  • Integrated salinity measurement
  • Auto ranging oxidant measurement from 0.07 ppm to 15 ppm
  • No electrolytes or membranes
  • Biofouling resistant

Halogen Systems sensor was designed with this challenging application in mind. It has a solution that can potentially save on both OPEX and CAPEX costs while simplifying operation by ship’s crew.