Chemical oxygen demand (COD) testing is the core water quality analysis used to quantify the organic pollution load in livestock, poultry, and aquaculture wastewater before treatment and discharge. This guide explains the COD test principle, applicable standards such as HJ 828-2017, HJ/T 399-2007, ISO 15705, and GB 18596-2001, sampling requirements, analytical methods, and quality control practices for breeding farm effluent. Accurate COD measurement is essential for regulatory compliance, treatment process optimization, and environmental protection tax verification at every regulated farming operation.
Technical Overview
Chemical oxygen demand (COD) is the mass of oxygen, expressed in milligrams per liter (mg/L), required to chemically oxidize the organic matter and oxidizable inorganic compounds in a water sample under strongly acidic conditions using potassium dichromate as the oxidizing agent. For livestock, poultry, and aquaculture operations, COD is the primary indicator of the organic pollution carried by manure-laden wastewater, which typically contains feces, urine, residual feed, bedding fibers, fats, detergents, and disinfectant residues.
Breeding wastewater is among the most organically concentrated effluents generated by any industry. Raw swine slurry commonly exhibits COD values between 5,000 and 20,000 mg/L, flushed manure from cattle and dairy operations frequently ranges from 2,000 to 10,000 mg/L, and even aquaculture pond drainage can carry organic loads of several hundred mg/L from residual feed and metabolic waste. Because GB 18596-2001, the discharge standard for intensive livestock and poultry breeding in China, caps the allowable daily average COD concentration at 400 mg/L, verified laboratory testing is indispensable for every regulated facility.
- Verifies compliance with statutory discharge limits under GB 18596-2001 and stricter local standards
- Quantifies pollutant loads for discharge permits and environmental protection tax assessment
- Monitors the performance of anaerobic digesters, lagoons, constructed wetlands, and aeration systems
- Supports environmental impact assessment acceptance and commissioning of new treatment plants
Testing Samples
Raw Swine Farm Wastewater
Pig house flushing water and raw slurry carry the highest organic loads in the breeding sector, with COD frequently exceeding 5,000 mg/L. These samples must be thoroughly homogenized and accurately diluted before analysis so that results fall within the working range of the selected method.
Poultry Farm Effluent
Washing water from layer and broiler houses contains manure, feather residues, and fat particles. High suspended solids content makes representative sampling and homogenization critical, and pre-treatment such as blending of coarse particles may be required.
Cattle and Dairy Parlor Wastewater
Milking parlor and barn cleaning water contains milk residues, animal fats, and cleaning agents, with COD typically ranging from 1,000 to 6,000 mg/L. Fat layers must be dispersed before subsampling to avoid underestimating the true load.
Aquaculture Drainage
Treatment Process Streams
Sampling points include biogas digester influent and effluent, sedimentation tank outlets, oxidation pond and constructed wetland discharges, and the final compliance discharge outlet. Paired inlet and outlet sampling allows removal efficiency to be calculated for each treatment stage.
Testing Items
The primary test item is COD determined by the dichromate method (CODCr), which is the legally recognized compliance parameter. A complete regulatory testing suite for breeding wastewater under GB 18596-2001 typically includes the companion parameters below, since all must meet their respective daily average limits simultaneously.
- CODCr: maximum allowable daily average of 400 mg/L
- Five-day biochemical oxygen demand (BOD5): limit of 150 mg/L
- Suspended solids (SS): limit of 200 mg/L
- Ammonia nitrogen (NH3-N): limit of 80 mg/L
- Total phosphorus (TP): limit of 8.0 mg/L
- pH, total nitrogen, and fecal coliform counts as supplementary indicators
Testing Methods
Dichromate Reflux Titration Method (HJ 828-2017, GB 11914-89, ISO 6060)
A measured sample is mixed with a known excess of potassium dichromate solution, silver sulfate catalyst, and concentrated sulfuric acid, with mercuric sulfate added to complex chloride at a mass ratio of approximately 10:1 relative to chloride. The mixture is refluxed for two hours, after which the remaining dichromate is titrated with standardized ferrous ammonium sulfate using ferroin indicator, with the color change from yellow-green to reddish-brown marking the endpoint. The method offers a working range of roughly 16 to 700 mg/L for undiluted samples with a detection limit of about 4 mg/L, and high-COD breeding wastewater must be diluted appropriately. This titration procedure is the reference and arbitration method for regulatory reporting in China.
Rapid Digestion Spectrophotometric Method (HJ/T 399-2007, ISO 15705)
The sample is digested with pre-packaged reagents in a sealed tube at 165 °C for 20 minutes, and the absorbance is then measured at approximately 440 nm for the low range based on hexavalent chromium depletion, or at approximately 600 nm for the high range of about 100 to 1,000 mg/L based on trivalent chromium formation. The technique is fast, reagent-efficient, and well suited to screening and daily process monitoring, although results intended for compliance submission should be cross-validated against the reflux titration method. ASTM D1252 provides equivalent dichromate-based macro and micro procedures for North American programs.
High-Chloride Wastewater Method (HJ/T 70-2001)
When chloride exceeds approximately 1,000 mg/L, as may occur where saline aquaculture water or chlorine-based disinfectants are used, chloride ions are oxidized by dichromate and produce falsely elevated results. HJ/T 70-2001 applies a chlorine gas correction procedure in which the chlorine generated during digestion is absorbed and quantified separately, allowing the true COD to be corrected. This method should be selected whenever elevated chloride is confirmed by prior screening.
Sampling, Preservation, and Pretreatment
Samples should be collected in accordance with HJ 493 and HJ 494 sampling specifications, using clean glass or chemical-resistant containers at properly located discharge outlets. Because breeding wastewater settles and stratifies rapidly, vigorous homogenization before subsampling is mandatory. Samples are preserved with concentrated sulfuric acid to pH below 2 and stored at 4 °C, with analysis preferably completed within 24 hours and no later than seven days under proper preservation.
Quality Assurance and Quality Control
- Method blanks carried through the full digestion and titration procedure
- Parallel duplicate samples with relative deviation generally not exceeding 10 percent
- Calibration verification using potassium hydrogen phthalate reference solution, for which 0.4251 g dried and diluted to one liter corresponds to a theoretical COD of 500 mg/L
- Matrix spike recoveries within the range of 90 to 110 percent
- Analysis performed by accredited third-party laboratories operating under an ISO/IEC 17025 quality management system
Applications
COD testing data for breeding wastewater directly supports regulatory, operational, and commercial decision-making across the livestock and aquaculture value chain.
- Statutory discharge compliance verification against GB 18596-2001 and local receptor standards such as GB 3838
- Discharge permit reporting and environmental protection tax declaration based on pollutant equivalent loads
- Completion acceptance of wastewater treatment facilities under environmental impact assessment approvals
- Optimization of anaerobic digestion, aeration intensity, and organic loading rates to reduce energy and chemical costs
- Verification of treated effluent quality prior to reuse for farm irrigation or flushing water
- Independent evidence for regulatory inspections, dispute resolution, and buyer or certification audits
FAQ
What is the COD discharge limit for livestock breeding wastewater?
Under GB 18596-2001, the maximum allowable daily average COD concentration is 400 mg/L for discharge from intensive livestock and poultry operations. Provinces and sensitive water protection zones may enforce stricter local limits, so the applicable regional regulation should always be confirmed.
Which COD method is most suitable for high-strength breeding wastewater?
The dichromate reflux titration method under HJ 828-2017 is the reference method for compliance testing, provided the sample is homogenized and diluted into the working range. The rapid digestion spectrophotometric method under HJ/T 399-2007 is preferred for rapid screening and daily operational monitoring.
How is chloride interference controlled during COD testing?
For chloride below approximately 1,000 mg/L, mercuric sulfate is added to complex chloride at a 10:1 mass ratio. For higher chloride concentrations, the chlorine gas correction method of HJ/T 70-2001 should be applied to remove the positive bias caused by chloride oxidation.
How often should COD testing be performed?
Frequency is dictated by the discharge permit, but small and medium operations typically test monthly to quarterly, while large intensive facilities may be required to install automatic online COD monitoring with periodic laboratory verification. Increased frequency is recommended during treatment plant commissioning or after process changes.
What is the difference between COD and BOD5?
COD measures all chemically oxidizable matter within about two hours, while BOD5 measures only the biologically degradable fraction over five days of incubation. The COD to BOD ratio indicates wastewater biodegradability and helps determine whether biological treatment will be effective, but COD remains the fast, legally recognized compliance parameter.
How long can wastewater samples be stored before analysis?
Unpreserved samples should be analyzed within 24 hours. When acidified with sulfuric acid to pH below 2 and refrigerated at 4 °C, samples may be held for up to seven days, although the shortest practicable holding time always yields the most reliable result.