Amperometric Analyzer Solves Wastewater DeChlorination Problem at LNG Terminal

An international oil & gas company that operates a liquid natural gas (LNG) terminal along Italy’s Adriatic Sea coast recently encountered a problem with its wastewater treatment process. Prior to releasing water back into the Adriatic Sea after its use at the LNG terminal operations, the water must be treated to ensure it is safe and won’t harm or pollute the environment.

Coastal LNG terminals and processing facilities require a significant amount of water for use in their operations. The water is typically drawn from rivers or the sea and is used as:

• Heat exchanger cooling water for liquefaction processes (gas to liquid state)

• Gas re-vaporization water heating systems (liquid to gas state)

• Storage tank bottom wastewater resulting from condensation

• Vessel ballast water that must be periodically discharged

• Employee housing, cooking and sanitation

• Storm water run-off

• Fire system water supplies

The wastewater treatment process requires disinfection with chlorine to eliminate any disease-causing pathogens that might harm aquatic life. Once the water has been disinfected, any residual chlorine must be removed prior to releasing the effluent. A DeChlorination process analyzer samples the treated water to make sure it is safe for discharge prior to final release.

The Problem

The LNG terminal operator was experiencing data accuracy problems and high maintenance costs with its existing conventional DeChlorination analyzer. Inconsistent chlorine level readings were requiring additional treatment cycles, which the plant’s technicians suspected were really unnecessary and caused by a faulty analyzer.

Under the best of circumstances, disinfecting treated water is a challenge. Chlorine remains the most popular disinfection method around the globe, though there are other popular disinfection processes. Care must be taken to ensure discharged effluent is not overly chlorinated.

At the LNG terminal wastewater treatment operation, Chlorine is added to the water in a chlorine contact chamber prior to the final outfall of the plant. The free chlorine residual level needs to be kept high enough in the contact chamber to kill any disease-causing bacteria and viruses. High chlorine levels, however, can be toxic to aquatic life both in fresh and salt water environments.

Additional treatment must be performed to reduce chlorine levels prior to discharge from the plant. Once De-Chlorinated the water leaving the treatment plant can be reused safely (landscaping and irrigation) or released back into the environment (oceans, rivers, streams, and lakes).

In order to assure that the DeChlorination process is effective and to stay within government set discharge limits, the water leaving the plant needs to be monitored for total chlorine. Total chlorine is made up of the sum of free chlorine and combined chlorine left in the water.
 

The Solution

Measuring total Chlorine in low chlorine applications such as wastewater treatment final effluent is a difficult and maintenance intensive application. The operations team at the LNG Terminal contacted the applications group at Electro-Chemical Devices, Inc., in Anaheim, Calif., through its manufacturer’s representative and system integrator responsible for Italy for discussions about its water effluent problem.

After thoroughly reviewing the issues and then discussing alternative options, the ECD applications team recommended the use of amperometric chlorine sensors (Figure 1). They are a low maintenance and cost effective technology to accurately measure both free chlorine (in the contact chamber) and total chlorine (in the final effluent).

Chlorine sensors designed with amperometric technology offer a number of potential advantages for the continuous measurement of both free and total chlorine. At the LNG terminal’s wastewater treatment plant, the chlorine discharge limit must meet stringent regulatory requirements for total chlorine levels that are low when the water is being introduced back into an environmental body of water.