How to Select the Right Sensors for Wastewater Monitoring

In a Hydrocarbon Engineering article, Emerson’s Andrew Smith looks at the sensors used to measure effluents in refinery wastewater.

For those people responsible for compliance with EPA and other environmental regulations, it can seem like there are many hoops to jump through, but the right instrumentation makes compliance easier.

Monitoring water quality is the primary concern of my article in the November issue of Hydrocarbon Engineering. It looks at the sources of pollutants in a typical refinery and the technologies used to detect and quantify those specific pollutants found in specific streams. This is critical since it requires a balance of looking for all the right pollutants but without overdoing it.

To begin with, there is no reason to look for things that are not there. There is a short list of potential pollutants from a given source within a facility. The caustic hydrogen sulfide and mercaptan removal section of a unit is not likely to produce ammonia, so there is no need to look for it. So, the question becomes: how does the relevant effluent at this point in the process change water characteristics in a measurable way that can also indicate the amount of harmful pollutants?

Fortunately, most of the effluents present in a refinery change one or more of these chemical characteristics: pH, oxidation-reduction potential (ORP), and conductivity. When looking at a specific waste stream, such as sour water from a steam stripper, or condensate blowdown from a boiler, it is critical to choose the right approach.

Making correct sensor selections calls for cooperation between the plant’s internal engineering staff and trusted instrumentation partners. Questions of range, repeatability, accuracy, reliability, maintainability, and other performance characteristics need to be examined in specific situations. For example, in a given application, conductivity or pH could both be highly useful measurements, but conductivity might be less maintenance-intensive. In another situation, the opposite might be the case.

Fortunately, Emerson has a wide range of sensors for liquid analysis.

ORP and pH measurement technologies are similar in nature and Emerson’s Rosemount 3900 General Purpose pH/ORP Sensor can handle both. Its double junction reference aids in the sensor’s resistance to harsh environments and helps prolong sensor life. The double junction protects the reference element from poisoning ions—such as ammonia, chlorine, cyanides, and sulfides. The robust sensor design makes the 3900 reliable in the wide range of aqueous solutions found in pipelines, open tanks or ponds.

Conductivity monitoring is also important, and Emerson’s Rosemount 400 Contacting Conductivity Sensor can accurately measure electrolytic conductivity in a broad range of applications from high purity water to clean cooling water. The Rosemount 400 sensors are ideal for use in clean, non-corrosive liquid having less than 20,000 µS/cm.

What’s the situation at your plant? Are you struggling with keeping your emissions in line, or doing a great job protecting communities around the plant and downstream?

Visit the Emerson Liquid Analysis pages at for more on technologies and solutions for monitoring pH, ORP, conductivity, dissolved oxygen, ozone, chlorine, and turbidity. You can also connect and interact with other engineers in the Oil & Gas and Chemical Groups at the Emerson Exchange 365 community.

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