Why COD and BOD Are Still the Backbone of Water Quality Compliance
Chemical Oxygen Demand (COD) and Biological Oxygen Demand (BOD) have been the standard measures of organic pollution in water for over a century. Despite the emergence of TOC analyzers and online nutrient sensors, COD and BOD remain the regulatory parameters in industrial discharge permits, municipal wastewater compliance, and receiving water standards across Asia, Europe, and most of the developing world.
Understanding the difference between these two parameters—and selecting the right measurement approach—is essential for environmental engineers, lab technicians, and plant operators responsible for discharge compliance and process control.
COD vs. BOD: The Fundamental Difference
Chemical Oxygen Demand (COD)
COD measures the oxygen equivalent needed to chemically oxidize all organic (and some inorganic) compounds in a water sample using a strong chemical oxidant—typically potassium dichromate (K₂Cr₂O₇) under acidic conditions and heat. The standard method (ISO 6060, APHA 5220-C) refluxes the sample with dichromate at 150°C for 2 hours. COD includes both biodegradable and non-biodegradable organic matter.
Typical ranges:
- Clean river water: <10 mg/L
- Secondary treated effluent: 20–60 mg/L
- Raw municipal wastewater: 300–600 mg/L
- Food processing effluent: 2,000–50,000 mg/L
- Landfill leachate: 5,000–100,000+ mg/L
Biological Oxygen Demand (BOD₅)
BOD measures the oxygen consumed by microorganisms while biodegrading organic matter under aerobic conditions over a standard 5-day incubation at 20°C (BOD₅). It represents only the biodegradable fraction of organic matter—COD is always ≥ BOD₅. The BOD₅/COD ratio characterizes the biodegradability of a wastewater and is essential for biological treatment design.
BOD₅/COD ratio interpretation:
- >0.6: Highly biodegradable — excellent feedstock for biological treatment
- 0.4–0.6: Moderate biodegradability — standard municipal wastewater
- 0.2–0.4: Partially biodegradable — may require acclimation or pretreatment
- <0.2: Low biodegradability — recalcitrant compounds, consider advanced oxidation
Standard Laboratory Methods
COD Analysis Methods
Open reflux (macro method, ISO 6060): The classical reference method using 250 mL Erlenmeyer flasks. Provides high accuracy but requires 100 mL sample volume, generates significant chromium waste, and takes 2.5 hours total. Suitable for R&D and regulatory disputes where traceability is required.
Closed tube method (micro/vial method, APHA 5220-D): Sealed digestion vials (2 mL or 5 mL sample) heated in a block digester at 150°C for 2 hours, followed by colorimetric reading at 600 nm (low range) or 420 nm (high range). Dramatically reduces chromium waste and enables high-throughput analysis with 25+ samples per digestion block. This is the dominant method in routine industrial and municipal labs today.
COD reagent kits from major suppliers (Hach, Merck, YSI, Palintest) come pre-filled in sealed vials with all reagents prepared. Available in ranges: 0–15, 0–150, 0–1,500, 0–15,000 mg/L. Select the range that targets 80% of full scale for best accuracy.
BOD Analysis Methods
Dilution method (APHA 5210-B): The standard reference method. The sample is diluted to bring expected BOD within the 2–6 mg/L DO depletion range, seeded with adapted microorganisms, and incubated at 20°C for 5 days in sealed glass BOD bottles. Initial and final DO are measured with a DO meter or probe. Requires 5 days turnaround—a significant operational limitation for process control.
Manometric method (respirometry): The sample is sealed with a CO₂ absorber (KOH pellets) in a pressure vessel. As microorganisms consume oxygen, the pressure drops in proportion to BOD. Electronic pressure sensors continuously log the pressure decline. Results after 5 days (BOD₅) but the full curve reveals biodegradation kinetics. This is the principle behind OxiTop® and similar instruments—20 simultaneous samples, no daily DO measurement required.
Short-cut BOD estimation: For operational process control where a 5-day wait is impractical, BOD can be estimated from COD using a site-specific COD:BOD ratio established from historical data. This approach is acceptable for internal process control but not for regulatory reporting.
Online COD Monitoring
The 2-hour laboratory COD method is too slow for real-time process control. Several online approaches are available:
UV-Vis Spectrophotometric Analyzers
Online UV-Vis probes measure light absorption across multiple wavelengths (200–750 nm) and use chemometric algorithms to correlate absorbance spectra with COD, BOD, TSS, turbidity, nitrate, and other parameters simultaneously. The probe is immersed directly in the flow or in a bypass flow cell.
Advantages: No reagents, continuous real-time output, multi-parameter capability
Limitations: Requires site-specific calibration against laboratory reference methods. Performance degrades if the water matrix (color, turbidity, industrial inputs) changes significantly. Not suitable for highly colored samples where UV absorption overlaps organic matter.
UV-Vis probes have replaced traditional online COD analyzers in many municipal influent, effluent, and sewer overflow monitoring applications because of their zero-reagent operation and minimal maintenance.
Wet Chemistry Online COD Analyzers
These analyzers replicate the closed-tube dichromate digestion in automated flow chemistry. A sample is automatically filtered, mixed with reagents, digested, and colorimetrically analyzed. Measurement cycle time: 20–120 minutes. They consume reagents and generate chromium waste but track the reference method more accurately than UV-Vis, especially for industrial effluents with complex matrices.
Common suppliers: Hach (PHOSPHAX, APA, TresCon), Endress+Hauser (CA80), YSI (IQ SensorNet). These are widely specified in industrial discharge monitoring programs requiring <±10% agreement with the reference method.
Total Organic Carbon (TOC) as a COD Surrogate
For many industrial effluents with a stable organic composition, TOC correlates linearly with COD (COD = k × TOC where k is site-specific). Online TOC analyzers provide continuous data with 5–15 minute cycle times. Once the k factor is established from at least 20 paired COD-TOC measurements, TOC can replace COD for internal process control (though regulators may require periodic direct COD verification).
Sample Handling: The Underrated Source of Error
Even perfect analytical technique fails if sample handling is wrong. COD and BOD are among the most preservation-sensitive parameters in water analysis.
COD Sample Preservation
- Acidify to pH <2 with H₂SO₄ immediately after collection
- Analyze within 28 days (refrigerated at 4°C)
- Do not freeze—phase separation of organics can cause analytical bias
- If the sample contains high chloride (>2,000 mg/L), add mercuric sulfate to mask chloride interference before acidification
BOD Sample Handling
- Collect in glass bottles only—plastic may leach or absorb organic material
- Cool immediately to 4°C; begin analysis within 6 hours for influent samples (up to 24 hours for effluent)
- Never freeze BOD samples—ice crystal formation lyses microbial cells and artificially increases BOD
- For samples with DO above saturation (supersaturated), gently aerate before sealing to avoid positive bias from degassing during incubation
Instrument Selection Guide
Bench-Top COD Systems
For routine lab analysis of 10–100 samples per day, the combination of a block digester (25-place or 40-place) and a dedicated photometer with COD program is the standard setup. Leading systems include the Hach DRB200 block digester with DR series photometers, and the WTW CR 3200 with pHotoFlex colorimeter. Using vial-based reagent kits (pre-dosed, closed tube) minimizes chemical preparation error and reduces chromium waste by 90% versus the open reflux method.
BOD Incubation Systems
Dedicated BOD incubators maintain 20°C ±1°C over 5 days in darkness. Temperature uniformity is critical—a 1°C deviation causes approximately 4% error in BOD₅. Manometric BOD systems (OxiTop, AquaLytic AL-BOD) enable unattended analysis: 20 samples sealed at Day 0, downloaded at Day 5 via infrared link. The entire BOD curve (day 1 through 28) is stored for kinetic analysis.
Portable Field Units
For field sampling at remote industrial discharge points, portable COD photometers with pre-dosed vials (Hach DR series, WTW pHotoFlex, Palintest Photometer 7500) enable immediate on-site analysis. Results in 2 hours after digestion. Combined with a portable thermal reactor (field digester), this approach eliminates sample preservation errors from long transport times.
Key Takeaways
- COD measures total oxidizable matter in 2 hours; BOD₅ measures biodegradable fraction over 5 days. Both are required for complete characterization of industrial effluent.
- BOD₅/COD ratio is the most important design parameter for biological treatment system sizing.
- Closed vial COD kits have replaced open reflux in routine labs—lower waste, higher throughput, comparable accuracy.
- Online UV-Vis probes provide real-time COD estimation without reagents; wet chemistry analyzers provide higher accuracy for permit compliance monitoring.
- Sample preservation failures cause more COD/BOD errors than analytical technique failures—follow holding time and temperature protocols rigorously.
Request a Quote
- Portable and bench COD/BOD analyzers for wastewater and industrial effluent monitoring
- Photometric and titrimetric methods available