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AGR-WATE71-05 · Lesson 8 · 15 clock hours · lab

Wastewater Parameters: BOD, TSS, and Settleable Solids

Objective. Students will prepare and incubate BOD5 samples, filter and dry TSS samples, and measure settleable solids using an Imhoff cone, recording all data in the correct format and calculating final results in standard reporting units.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Why These Parameters Matter

Biochemical Oxygen Demand (BOD), Total Suspended Solids (TSS), and settleable solids are three of the most important measurements in wastewater treatment. Regulatory agencies use them to set permit limits, operators use them to judge plant performance, and licensing exams test them heavily. Understanding how to collect, prepare, and analyze samples correctly — and how to record results in standard units — is a core competency for any Class I or Class II wastewater operator.

Biochemical Oxygen Demand (BOD5)

BOD measures the amount of dissolved oxygen that microorganisms consume while breaking down organic matter in a water sample over five days at 20°C. The subscript "5" refers to the five-day incubation period, which is the standard test duration. A high BOD reading means a lot of organic material is present; a low reading means the water is cleaner. Wastewater treatment plants are designed to reduce BOD before discharge.

Sample preparation. Raw wastewater contains far too much organic material for a direct measurement — the bottle would run out of oxygen before five days are up. You must dilute the sample with dilution water: aerated, clean water that has been saturated with dissolved oxygen. Dilution water is sometimes "seeded" with a small volume of treated effluent or a commercial seed preparation to ensure enough active bacteria are present to perform the biodegradation. Choose a dilution ratio so that the final dissolved oxygen (DO) at day 5 is at least 1 mg/L but no more than 7 mg/L has been consumed; this ensures the test is valid.

Procedure. Fill duplicate 300 mL BOD bottles completely (no air space) with your diluted sample. Measure and record the initial DO (DO_i) with a calibrated DO meter immediately after filling. Seal the bottles and place them in a 20°C incubator. After exactly five days, remove the bottles, measure final DO (DO_f), and record. Calculate BOD5 using the standard formula:

> BOD5 (mg/L) = (DO_i − DO_f) × Dilution Factor

The dilution factor equals the total bottle volume divided by the volume of sample added (e.g., if you added 6 mL of sample to 294 mL of dilution water to fill a 300 mL bottle, the dilution factor is 300/6 = 50).

Safety. Wastewater is a biohazard. Wear nitrile gloves, safety glasses, and a lab coat at all times. Wash hands thoroughly after handling samples. Dispose of wastewater samples according to your facility's standard operating procedures.

Total Suspended Solids (TSS)

TSS is the mass of solid material retained on a glass fiber filter after a measured volume of sample is passed through it under vacuum. Results are reported in milligrams per liter (mg/L). TSS is a direct measure of particulate matter in the water and is a standard permit parameter for wastewater effluent.

Procedure. Pre-weigh a glass fiber filter (Whatman GF/C or equivalent) on an analytical balance after it has been dried and cooled in a desiccator; record this as the tare weight (W1). Assemble the vacuum filtration unit, place the filter, and filter a measured volume of well-mixed sample — typically 100 mL, though you may adjust for solids-rich samples. Transfer the filter to a drying oven and dry at the temperature and duration specified in the approved method (commonly around 103–105°C for at least one hour). Cool the filter in a desiccator to room temperature, then reweigh it and record as W2.

Calculation:

> TSS (mg/L) = [(W2 − W1) in mg] ÷ Volume filtered in liters

Record all weights to at least four decimal places on the balance (0.1 mg readability). Label your filters clearly. Duplicate or triplicate samples improve reliability.

Safety. The drying oven is hot; use heat-resistant gloves when handling filters. Do not use the desiccator while it contains solvents. Confirm the desiccant (usually silica gel) is still active (blue, not pink, for indicating silica gel).

Settleable Solids — Imhoff Cone Test

Settleable solids are the portion of suspended material that will gravity-settle from one liter of sample in 45 minutes. The Imhoff cone is the standard apparatus: a graduated, cone-shaped glass or plastic vessel marked in milliliters. Results are reported in mL/L.

Procedure. Pour a well-mixed, 1-liter sample into the Imhoff cone. Allow it to settle undisturbed for 40 minutes. At 40 minutes, gently stir or rotate the cone walls with a glass rod to dislodge material clinging to the sides; then allow to settle for an additional 5 minutes (45 minutes total elapsed time). Read and record the volume of settled material at the bottom of the cone in mL/L.

This test is fast, simple, and requires no electricity or chemicals. Many industrial and municipal pretreatment permits specify settleable solids limits. A result above the permit limit signals a problem upstream in treatment or in the industrial discharge feeding the plant.

Recording Data Correctly

All raw data — weights, volumes, DO readings, times, temperature, instrument calibration records — must be entered directly into your lab notebook in ink at the time of measurement. Never record on scratch paper and transcribe later. Initial and date each page. Include units on every value. These practices are required by Standard Methods and by state regulatory agencies when reviewing lab records.

Connecting to the License Exam

State Class I and Class II wastewater operator exams include questions on BOD calculation, TSS methodology, and interpretation of settleable solids results. You must know the formula, the correct units, and what a high or low result means for plant operations. Accurate sampling and documentation protect the public and protect your license.

Key terms8

BOD5Biochemical Oxygen Demand measured over a five-day incubation at 20°C; the amount of dissolved oxygen consumed by microorganisms as they break down organic matter in a sample, reported in mg/L.
Dilution FactorThe ratio of the total bottle volume to the volume of sample added; used in the BOD5 formula to convert the oxygen depletion in the diluted bottle back to the concentration in the original sample.
Total Suspended Solids (TSS)The mass of particulate material retained on a glass fiber filter after filtering a measured sample volume, dried and weighed gravimetrically; reported in mg/L.
Gravimetric AnalysisA laboratory technique that determines the quantity of a substance by precise weighing before and after a process such as filtration and drying.
Imhoff ConeA graduated, cone-shaped vessel used to measure settleable solids; one liter of sample is allowed to settle for 45 minutes and the volume of settled material is read directly in mL/L.
Settleable SolidsThe portion of suspended material in a wastewater sample that settles by gravity in 45 minutes in an Imhoff cone; reported in mL/L.
Dilution WaterAerated, high-quality water saturated with dissolved oxygen and sometimes seeded with active bacteria; used to dilute raw wastewater to an appropriate strength for BOD5 analysis.
DesiccatorA sealed container holding a drying agent (desiccant) used to cool weighed filters or other samples to room temperature without absorbing moisture from the air, ensuring accurate mass measurements.

Worked examples4

1. A BOD bottle (300 mL) was filled with 6 mL of raw wastewater and 294 mL of dilution water. Initial DO = 8.6 mg/L. After 5 days at 20°C, final DO = 2.4 mg/L. Calculate BOD5.
  1. Step 1 — Find the dilution factor: Dilution Factor = Total volume ÷ Sample volume = 300 mL ÷ 6 mL = 50.
  2. Step 2 — Find the DO depletion in the bottle: DO_i − DO_f = 8.6 − 2.4 = 6.2 mg/L.
  3. Step 3 — Apply the BOD5 formula: BOD5 = (DO_i − DO_f) × Dilution Factor = 6.2 × 50 = 310 mg/L.
  4. Step 4 — Check validity: DO consumed = 6.2 mg/L (between 1 and 7 mg/L remaining, so final DO = 2.4 mg/L ≥ 1 mg/L). Test is valid.
Answer: BOD5 = 310 mg/L
2. A TSS test uses a 100 mL sample. The pre-dried, tared filter weighs 1.5423 g. After filtering and drying, the filter weighs 1.5561 g. Calculate TSS in mg/L.
  1. Step 1 — Find the mass of solids collected: W2 − W1 = 1.5561 g − 1.5423 g = 0.0138 g.
  2. Step 2 — Convert grams to milligrams: 0.0138 g × 1000 mg/g = 13.8 mg.
  3. Step 3 — Convert filtered volume to liters: 100 mL ÷ 1000 = 0.100 L.
  4. Step 4 — Calculate TSS: TSS = 13.8 mg ÷ 0.100 L = 138 mg/L.
Answer: TSS = 138 mg/L
3. A BOD test uses a 300 mL bottle filled with 15 mL of sample and 285 mL of dilution water. Initial DO = 9.1 mg/L; final DO after 5 days = 5.3 mg/L. Calculate BOD5.
  1. Step 1 — Find the dilution factor: 300 mL ÷ 15 mL = 20.
  2. Step 2 — Find DO depletion: 9.1 − 5.3 = 3.8 mg/L.
  3. Step 3 — Calculate BOD5: 3.8 × 20 = 76 mg/L.
  4. Step 4 — Check validity: Final DO = 5.3 mg/L ≥ 1 mg/L; DO consumed = 3.8 mg/L ≤ 7 mg/L. Test is valid.
Answer: BOD5 = 76 mg/L
4. A TSS test filters 250 mL of secondary effluent. Tare weight of the filter = 0.8812 g; weight after drying = 0.8834 g. Calculate TSS in mg/L.
  1. Step 1 — Mass of solids: 0.8834 g − 0.8812 g = 0.0022 g.
  2. Step 2 — Convert to milligrams: 0.0022 g × 1000 = 2.2 mg.
  3. Step 3 — Convert volume to liters: 250 mL ÷ 1000 = 0.250 L.
  4. Step 4 — Calculate TSS: 2.2 mg ÷ 0.250 L = 8.8 mg/L.
Answer: TSS = 8.8 mg/L

The math in this lesson1

Learners compute a 300 ÷ 6 = 50 dilution factor and BOD₅ = 6.2 × 50 = 310 mg/L.
6.RP.3
In class: Set up BOD5 analysis: dilute a prepared wastewater sample to appropriate dilution using DO-saturated dilution water, fill duplicate BOD bottles, seal, and place in 20°C incubator; measure and record initial DO; after 5-day incubation, measure final DO and calculate BOD5 in mg/L using the standard formula (BOD = [(DO_i – DO_f) × dilution factor]). · Perform TSS gravimetric analysis: pre-weigh glass fiber filters, filter measured volume of sample under vacuum, dry filters in oven at the method-required temperature, cool in desiccator, reweigh, and calculate TSS in mg/L; record all weights and volumes in lab notebook. · Measure settleable solids from the same wastewater sample using an Imhoff cone: pour 1 liter of sample, allow to settle for 45 minutes with a gentle stir at 45 minutes, and read the settled volume in mL/L; compare result to a typical permit limit and document.
Materials: BOD bottles (300 mL), DO meter, 20°C BOD incubator, dilution water (aerated, seeded as needed), vacuum filtration unit, glass fiber filters (Whatman GF/C or equivalent), analytical balance (0.1 mg readability), drying oven, desiccator, Imhoff cones and stand, volumetric glassware, lab notebooks, Standard Methods BOD and TSS method reference pages
Check: BOD calculation formula applied correctly with correct dilution factor; TSS result within ±10% of instructor reference value; settleable solids reading recorded with units; lab notebook entries complete; pass = correct calculation with no arithmetic errors and complete documentation.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Wastewater Treatment — Laboratory Procedures
1. A 300 mL BOD bottle contains 10 mL of wastewater sample and 290 mL of dilution water. The initial DO is 8.8 mg/L and the final DO after 5 days is 3.8 mg/L. What is the BOD5 in mg/L?
Wastewater Treatment — Laboratory Procedures
2. Why must the final dissolved oxygen in a BOD bottle be at least 1 mg/L after the 5-day incubation?
Wastewater Treatment — Laboratory Procedures
3. During TSS analysis, a glass fiber filter is cooled in a desiccator before weighing. What is the primary purpose of this step?
Wastewater Treatment — Laboratory Procedures
4. An Imhoff cone settleable solids test reads 12 mL/L. In what units should this result be recorded on the laboratory data sheet?
Wastewater Treatment — Laboratory Procedures
5. During BOD5 setup, a technician forgets to fully fill a BOD bottle and leaves an air space under the stopper. How does this most likely affect the result?
Wastewater Treatment — Laboratory Procedures
6. Which of the following best describes the correct sequence for TSS gravimetric analysis?
Wastewater Treatment — Laboratory Procedures
7. A wastewater operator is performing the Imhoff cone settleable solids test. At what point during the 45-minute test should the walls of the cone be gently stirred?
Wastewater Treatment — Laboratory Procedures
8. A sample of raw wastewater is analyzed for TSS. A 100 mL volume is filtered; the tare weight of the filter is 1.2200 g and the weight after drying is 1.2455 g. What is the TSS in mg/L?
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