Online measurements and permit-monitoring samples can both be useful, but they are not automatically interchangeable. They may represent different times, sample volumes, locations or analytical methods. The discharge permit and the site’s approved monitoring arrangements determine what is required for formal reporting.
The Environment Agency’s guidance on selecting a monitoring approach describes periodic and composite sampling. A composite result represents an average over its collection period; the appropriate approach depends on the discharge and permit context. Its separate CEN and ISO methods index identifies recognised analytical methods for relevant determinands. Check the current permit and regulator guidance for the site rather than inferring requirements from a general article.
Ask what each data point represents
For an online trend, record the instrument’s sample point, measurement cycle, conditioning and reporting basis. For a grab or composite sample, record the collection window, flow basis, preservation, laboratory method and reporting units. These details explain why the values may not be a direct like-for-like comparison.
Use online data for its intended operational role
Trend information can help an operator see when conditions changed and decide whether further investigation is warranted. It does not, by itself, establish compliance, replace prescribed sampling or override the permit. Confirm unusual results using the method and process required by the site’s approved arrangements.
Link the two evidence streams carefully
Plan comparison samples in advance where possible. Match location and timing to the question, document method differences, and keep the audit trail for both online and laboratory results. Escalate through site procedures when data conflict.
The current 3S-CL brochure lists wastewater and several nutrient parameters as possible application categories; suitability remains method- and sample-specific. See the wastewater nutrient hub, the online-versus-laboratory guide and PMA contact.