5.2 BOD, TSS, Solids, and Residual Tests

What the main wastewater tests actually tell you

Laboratory analysis is a separate WPI content area, but the exam uses lab data throughout process-control questions. The operator is expected to know what a result measures, where the sample belongs in the treatment train, and whether the number supports a control action. EPA's Clean Water Act methods framework also matters: compliance data must be produced by approved or permit-accepted methods, not by casual field shortcuts.

Biochemical oxygen demand (BOD) estimates the oxygen microorganisms consume while degrading biodegradable organic matter over the test period. It is a strength and loading signal. High influent BOD means more food and oxygen demand. High effluent BOD may point to under-aeration, short sludge age, toxicity, hydraulic short-circuiting, solids carryover, or sampling error. Carbonaceous BOD (CBOD) suppresses nitrogenous oxygen demand so the result focuses on carbonaceous material.

Total suspended solids (TSS) measures particles retained on a filter. It is closely tied to clarification, filtration, sludge wasting, and effluent compliance. High effluent TSS after secondary clarification may be biological floc washout, high sludge blanket, hydraulic overload, poor settling, filamentous bulking, or mechanical problems at weirs and scum removal. TSS can also carry BOD, phosphorus, and pathogens, so a solids problem can become a multi-parameter problem.

BOD and CBOD interpretation

A BOD number is most useful when paired with flow. A 250 mg/L influent at 0.4 MGD is a very different load than 250 mg/L at 8 MGD. For removal calculations, use influent and effluent values from corresponding locations and periods. If influent BOD is 220 mg/L and effluent BOD is 11 mg/L, removal is ((220 - 11) / 220) x 100 = 95 percent. That is strong performance, but the operator still checks the permit limit because permits may regulate concentration, mass, percent removal, or a combination.

Exam traps around BOD include reading COD as BOD, assuming high BOD always means high TSS, and forgetting that effluent BOD can rise from solids carryover. If the secondary clarifier is washing out biological solids, the lab may report both high TSS and high BOD even though the aeration basin biology was removing soluble substrate.

TSS, solids, and sludge data

Solids testing has several layers. Total solids include everything left after drying. Suspended solids are retained on a filter. Dissolved solids pass through the filter. Volatile solids are the portion lost on ignition and are often used as an organic or biological indicator; fixed solids remain as mineral material. In activated sludge, MLSS measures mixed-liquor suspended solids, while MLVSS estimates the more biologically active fraction.

Settleability tests are process-control snapshots. A 30-minute settleometer reading that leaves a compact sludge blanket with clear supernatant suggests better clarification than a bulky blanket, floating solids, or turbid supernatant. Use SVI to compare settling relative to MLSS. A high settled volume by itself is not enough; the same mL/L reading means something different at 1,500 mg/L MLSS than at 4,000 mg/L MLSS.

Residuals, DO, pH, and field timing

Chlorine residual, dissolved oxygen (DO), pH, temperature, and sulfide-related readings can change quickly after collection. EPA sampling guidance treats grab samples as appropriate for instantaneous conditions and parameters not suited to compositing, including pH, temperature, DO, chlorine, oil and grease, and coliform bacteria when required by permit. That means the exam may ask why a delayed bottle result is unreliable: the sample no longer represents the process condition.

For disinfection, separate chlorine dose, chlorine demand, residual, contact time, and dechlorination. Dose is what is applied. Demand is what wastewater consumes. Residual is what remains. Dechlorination removes residual before discharge when required. A low fecal indicator result with excessive residual is not automatically acceptable if the NPDES permit has a residual limit.

Lab-result sanity checks

Good operators do not chase one surprising number without context. Confirm sample point, sample type, preservation, holding time, instrument calibration, blanks, duplicates, and process trend. Then compare the result with field observations: odors, foam, clarifier blanket, sludge color, filter headloss, UV transmittance, or chlorine contact-basin condition. A result that contradicts all other evidence may still be true, but it deserves verification before a major process adjustment.

Matching the test to the decision

Choose the test that answers the operating question. If the question is whether aeration has enough biomass for the incoming food, use BOD loading with MLSS or MLVSS, not only effluent TSS. If the question is whether the final clarifier is losing solids, use effluent TSS, sludge blanket, settleometer, and visual clarity. If the question is whether disinfection is complete and safe for discharge, use the required bacteriological indicator, contact time, residual, and any dechlorination limit. Good exam answers usually respect these boundaries instead of using one familiar number for every problem.

Also separate compliance testing from process-control testing. A quick settleometer, bench residual check, or handheld DO reading can guide operations, but the permit may require a specific method, sample point, frequency, and reporting basis for NPDES compliance.