Which device’s readings would be affected by oil and grease in wastewater?

• A. pH meter
• B. pH meter
• C. ORP probe
• D. Turbidity meter

Answer: (C)

Oil and grease can foul sensor surfaces and change the environment around an electrode, which directly affects electrochemical measurements like oxidation-reduction potential (ORP). The ORP reading comes from the potential at the sensing electrode as redox species exchange electrons with its surface. If a film of oil forms on the electrode or oil droplets create a localized, oil-rich microenvironment near the tip, mass transport of oxidants and reductants is impeded and the surface chemistry is altered. This can slow the response, cause drift, or yield a reading that reflects the oil-film region more than the true bulk water chemistry. In oily wastewater, the oil layer can also introduce organics that participate in redox reactions at the electrode, further skewing the measured potential. So, readings from an ORP probe would be particularly susceptible to interference from oil and grease.