N092-152 TITLE: Development of a Total Residual Oxidant Sensor Development of a Total Residual Oxidant Sensor

TECHNOLOGY AREAS: Ground/Sea Vehicles, Sensors

ACQUISITION PROGRAM: submitted thru Code 33, in support of POM 2010 FNC Shipboard Desalination

OBJECTIVE: Develop a compact, near real-time in-stream detector capable of continuously detecting and reporting the total residual oxidant (TRO) content in the hypochlorite-enhanced seawater streams that the Navy uses for periodic biofouling control flushes. Such a detector will provide the required input for measurement of TRO in concentrated oxidant streams made with a variety of source waters including potable water, natural seawater, and estuarine sources for future advanced desalination systems; allowing them to operate with autonomy requiring minimal interface with existing systems.

DESCRIPTION: A new Office of Naval Research (ONR) Enabling Capability (EC) program focuses on shipboard desalination, with an emphasis on improving ship operational capabilities in littoral and near shore seawaters. Critical to the success of this EC program is the development of technology for superior filtration of the suspended solids in the incoming seawater prior to reverse osmosis membranes. Previous efforts completed under the ONR Expeditionary Unit Water Purification program demonstrated that microfiltration membrane-based filtration can provide superior filtrate quality with very low maintenance requirements using electrolytic hypochlorite-enhanced seawater flushes. Central to the long-term, low maintenance operation of this advanced filtration equipment is the ability to monitor and control the TRO content present in the seawater flush streams for the purpose of control and protection of downstream equipment. Electrolytic chlorination systems are currently used on ships and submarines to control biological fouling (biofouling). These systems are installed in-line of a ships seawater system to impede the growth of biofouling.

In an effort to reduce the manpower burden of regulating chlorine systems, reduction-oxidation (redox) probes have been considered as a viable approach for chlorination control. Redox probes are able to accurately measure the changes in redox potential between natural, chlorinated, and dechlorinated environments. Many redox devices currently exist on the open market for monitoring TRO and feedback control to disinfection and biocide treatment systems. Predominantly, these devices were designed and have been utilized in the potable water systems. However, when used in the marine environment these devices have yet to be useful from a pragmatic sense since the electrodes and membranes frequently foul and in some cases material selection do not address this harsh environment. The end result has been that these devices require frequent maintenance in order to maintain a reasonable calibration.

A TRO detector that is of high utility for military use should have a variety of performance characteristics that are not typically found in existing chlorine detection equipment. The most obvious and key drawbacks to existing equipment include a large amount of process dead time, when analysis is not available, relatively large size, significant logistical requirements for consumables (syringes, columns, bottled gases, sample vials, etc.), and high maintenance requirements, including calibration. All of these items have desired improvements, and are addressed below in the guidelines and requirements. A practical military chlorine sensor should be compact, quick, and require nothing other than electricity, water, and potentially air. The device should be able to provide the TRO content in seawater from littoral and blue water areas. The device should be capable of operation in a high-TDS environment, with varying levels of pH, silt, dirt, sand and other impurities (including hydrocarbons and pesticides). Due to the importance of verifying TRO levels prior to meeting sensitive membrane materials, it is important that the device provide high reliability and availability in a military environment.

GUIDELINES FOR NEW TECHNOLOGY:
1. Capable of operating in a seawater environments including littoral and deep blue sea environments
a. Threshold: Seawater TDS between 25,000 and 42,000 mg/L and seawater temperatures between 35oF and 100oF
b. Objective: Seawater TDS up to 60,000 mg/L plus turbid freshwater up to 150 NTU
2. Device should hold calibration for 1000+ hours of continuous operation
3. Device accuracy should provide at least an order of magnitude linear range, and be ±5% between 50 and 500 mg/L TRO.
4. Capable of consistent, repeatable measurement even with concentration variation over the desired range
5. Capable of operation in a military environment
6. No consumables
7. Volume no larger than 1.5 ft3 total size preferred
8. Capable of CAN-BUS communication, with the ability to be configured for other military relevant controls and data buses
9. Results provided near-real-time, with total cycle time (result output to result output) goals as follows:
a. Threshold: = 3 min
b. Objective: = 1 min
10. High level of availability and reliability
a. Threshold: 1000 hours
b. Objective: 3000 hours
11. Minimal process requirements
12. Available process streams include:
a. Compressed air
b. Water
c. Electricity (110VAC)

PHASE I: Demonstration of TRO (total residual oxidant) detector efficacy in a laboratory environment, utilizing at least a model seawater mixture of relevant composition (e.g., ASTM synthetic seawater or "Instant Ocean") and bleach solutions over a TRO concentration range of 50 to 500 mg/L.

PHASE II: Demonstration of the device with natural seawater, assembly of full scale system to validate operation on an input and output flow from an electrolytic chlorination device. Deliverable will be utilized to prove performance in a Navy natural seawater test facility coupled with an operating chlorination process (deliverable is the stand-alone detector with CAN output for interface).

Phase II Option – Advanced design to improve cycle time, reliability and/or reduced system size.

PHASE III: Commercialization of device in combination with a Navy-relevant desalination system.

PRIVATE SECTOR COMMERCIAL POTENTIAL/DUAL-USE APPLICATIONS: Land-based seawater desalination is expected to become more common as large population centers need more water. California, Texas, and Florida either have built or have plans to build large municipal plants. Such plants may select to produce the required oxidant in place from seawater as the Navy does and would need this type of sensor. Also, the sensor is expected to be derived from current sensors and thus the research will likely lead to improvements in technology relevant to broader applications.

REFERENCES:
1. http://www.villagemarine.com/euwp2.html

2. ASTM D1141 – 98 (2008) Standard Practice for the Preparation of Substitute Ocean Water

KEYWORDS: sensors; chlorination; seawater; oxidant; desalination; water purification

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