Laboratory Analysis and Process Monitoring
Laboratory and monitoring
Laboratory data drive every process decision, so sampling integrity matters. Grab samples represent a moment; composite samples represent a period. Flow-weighted composites are required when concentration varies with flow. Preservation and holding times: BOD 48 hours at 4 C, TSS 7 days, ammonia 28 days acidified, chlorine and DO analyzed immediately, coliforms within 8 hours (24-hour incubation at 35 C).
Calculations you must be able to do cold: loading (lb/day) = flow (MGD) x concentration (mg/L) x 8.34. Percent removal = (in - out) / in x 100. BOD5 uses seeded dilution water with a blank correction; nitrification inhibition matters when effluent ammonia is high. COD is always at least BOD because it oxidizes nearly everything. TSS is retained on a glass fiber filter at 103-105 C; VSS is the portion burned off at 550 C.
QA/QC: duplicates, method blanks, matrix spikes, and calibration standards; a control chart outside plus or minus 2 SD signals a trend, outside 3 SD signals an investigation.
Traps: mixing mg/L with percent; forgetting the 8.34 factor; comparing BOD to COD directly; analyzing an unfiltered sample for ammonia; and ignoring significant figures on reports.
Worked example: influent BOD 220 mg/L at 1.5 MGD = 1.5 x 220 x 8.34 = 2,752 lb/day. Effluent 8 mg/L = 100 lb/day. Removal = (2,752 - 100) / 2,752 x 100 = 96.4%, the same as (220 - 8) / 220 when flows are equal.