Determination of Chemical Oxygen Demand (COD) in Livestock and Aquaculture Wastewater: Methods, Stan

Chemical oxygen demand (COD) is the key indicator of the organic pollution load in wastewater generated by livestock, poultry and aquaculture operations, and its accurate determination is essential for both regulatory compliance and treatment process control. This article provides a technical overview of COD testing for breeding wastewater, covering sample types, applicable standards such as HJ 828-2017, ISO 15705 and ASTM D1252, digestion and titration procedures, interference control, and typical industrial applications.

Technical Overview

Chemical oxygen demand (COD) quantifies the mass of oxygen, expressed in milligrams per litre (mg/L), consumed when the oxidizable matter in a water sample is digested by a strong oxidant, typically potassium dichromate in hot, concentrated sulfuric acid with a silver sulfate catalyst. In livestock, poultry and aquaculture wastewater, this oxidizable matter consists mainly of manure solids, urinary organics, feed residues, bedding fibers, cleaning agents and residual veterinary products, producing COD concentrations that range from tens of mg/L in aquaculture pond effluent to more than 10,000 mg/L in raw swine slurry.

Because discharge permits and treatment performance are judged primarily against COD, its determination is a routine task for accredited third-party laboratories, environmental monitoring stations and large-scale farm operators. The principal referee technique worldwide is the dichromate method (HJ 828-2017 in China, ASTM D1252 and ISO 6060 internationally), supported by rapid sealed-tube spectrophotometric procedures (HJ/T 399-2007 and ISO 15705) for screening and process control. Direct discharge of breeding wastewater in China is governed by GB 18596-2001, which sets a maximum allowable COD concentration of 400 mg/L, while many provincial and watershed standards impose stricter values for sensitive receiving waters.

• Measured parameter: CODCr, reported as mg O2/L
• Typical matrix: high-strength wastewater with elevated suspended solids, nitrogen and phosphorus
• Key interferences: chloride, high solids content, sample non-homogeneity
• Primary standards: HJ 828-2017, HJ/T 399-2007, HJ/T 70-2001, ISO 15705, ASTM D1252, GB 18596-2001

Testing Samples

Representative sampling is the most challenging step in COD testing of breeding wastewater, because the matrix is heterogeneous and solids settle or float rapidly. Samples are usually collected as grab samples for process diagnosis or as 24-hour flow-proportional composites for compliance verification, in accordance with HJ 91.1-2019 and HJ 493-2009.

Swine Farm Wastewater

Includes raw slurry from underfloor pits, flushing water and separation liquor. COD typically ranges from 5,000 to 15,000 mg/L, so thorough homogenization and large dilution factors are required before digestion.

Cattle and Dairy Farm Wastewater

Comprises milking parlor wash water, barn flushing and yard runoff, with COD commonly between 1,500 and 8,000 mg/L. Fibrous solids and milk fats demand vigorous mixing during subsampling.

Poultry Farm Wastewater

Originates from layer, broiler and duck houses and washing operations, with COD generally between 1,000 and 6,000 mg/L. Feathers, feed particles and grease can bias results if not uniformly dispersed.

Aquaculture Effluent

Pond drainage water and recirculating system backwash usually show low COD of 20 to 200 mg/L. Marine and brackish systems introduce chloride interference, which must be addressed during method selection.

Treatment Process Streams

These include solid-liquid separation filtrate, anaerobic digester influent and effluent, lagoon and constructed wetland inflow and outflow, oxidation ditch mixed liquor and the final monitored discharge outfall.

Testing Items

The core testing item is the CODCr concentration at each critical point of the wastewater system, from raw influent to final discharge. COD testing is normally bundled with related parameters because breeding wastewater permits and process evaluations are multi-parameter in nature.

• CODCr concentration (mg/L) of raw wastewater, each treatment stage and the final outfall
• COD removal efficiency of individual treatment units and of the overall system
• Biodegradability assessment expressed as the BOD5/COD ratio
• Chloride content, which determines whether a chlorine-correction procedure is needed
• Supporting parameters typically measured together: BOD5, ammonia nitrogen, total nitrogen, total phosphorus, suspended solids and pH

Testing Methods

Sample Handling and Preservation

Samples are collected in clean glass containers, homogenized with a blender or magnetic stirrer, and acidified with concentrated sulfuric acid to pH ≤ 2 to suppress microbial degradation. They are stored at 4 °C and analyzed within the holding time prescribed by the governing method, commonly within 7 days.

Dichromate Reflux-Titration Method (HJ 828-2017, ASTM D1252, ISO 6060)

A measured sample portion is reacted with a known excess of potassium dichromate in strongly acidic medium, using silver sulfate as a catalyst and mercuric sulfate to complex chloride. The mixture is refluxed for two hours, cooled, and the remaining dichromate is back-titrated with standardized ferrous ammonium sulfate using ferroin as the indicator. The method has a detection limit of approximately 4 mg/L and an undiluted working range of roughly 16 to 700 mg/L; high-strength slurry must be diluted into this range. Samples with chloride above 1,000 mg/L require the chlorine-correction procedure of HJ/T 70-2001. Mercury-containing waste must be collected and disposed of as hazardous waste.

Rapid Digestion Spectrophotometric Method (HJ/T 399-2007, ISO 15705)

The sample is digested in a sealed tube with pre-dosed dichromate reagent at 165 ± 2 °C for 15 to 20 minutes. Absorbance is measured at 600 ± 20 nm for the high range of approximately 100 to 1,000 mg/L, or at 440 ± 20 nm for the low range of approximately 15 to 250 mg/L. The approach is fast, generates less reagent waste and suits large batch screening and daily process control; results intended for official reporting should be periodically verified against the referee titration method.

High-Chloride Wastewater Variant (HJ/T 70-2001)

For saline aquaculture effluent or streams carrying disinfectant residues, the chlorine-correction method quantifies and subtracts the dichromate consumed by oxidized chloride, providing reliable COD values where the standard method would otherwise yield a significant positive bias.

Online and Automated Monitoring

Large farms subject to continuous discharge monitoring may install automatic COD analyzers at the outfall, based on dichromate digestion or photochemical oxidation. Data are used for permit compliance and early warning, provided the analyzer is periodically validated against laboratory reference measurements. UV254 surrogate sensors can be applied for trend monitoring but require site-specific calibration because breeding wastewater color and solids vary widely.

Quality Control

Each analytical batch should include a method blank, duplicate samples with relative deviation typically within 10 percent, a potassium hydrogen phthalate check standard such as 500 mg/L with recovery between 90 and 110 percent, and matrix spike recoveries to verify that dilution and homogenization have not distorted the result.

Applications

COD determination supports nearly every stage of breeding wastewater management, from farm design to regulatory reporting.

• Compliance monitoring against GB 18596-2001, local discharge standards and individual discharge permits, including environmental protection tax assessments
• Environmental impact assessment and construction completion acceptance monitoring for new or expanded farms
• Design and sizing of treatment processes such as solid-liquid separation, anaerobic digestion, aeration basins, lagoons and constructed wetlands
• Operational control of biological treatment, including organic loading rate, aeration demand estimation and detection of shock loads
• Biodegradability evaluation through the BOD5/COD ratio, where values above 0.3 generally indicate readily biodegradable wastewater suitable for biological treatment
• Research on manure resource recovery, biogas production and advanced treatment technologies

FAQ

What COD concentrations are typical in breeding wastewater?

Raw swine slurry commonly reaches 5,000 to 15,000 mg/L, cattle and poultry wastewater 1,000 to 8,000 mg/L, anaerobic digester effluent 500 to 2,000 mg/L, and aquaculture effluent 20 to 200 mg/L. The national discharge limit for livestock and poultry breeding under GB 18596-2001 is 400 mg/L, with stricter local limits in sensitive regions.

How should samples with very high suspended solids be handled?

The sample must be homogenized with a blender until uniform, and the subsample withdrawn with a wide-bore pipette while stirring continues. The aliquot is then diluted so that the final COD falls within the working range of the selected method, and the dilution factor is applied to the calculation.

How is chloride interference controlled?

Mercuric sulfate in the digestion mixture masks chloride up to about 1,000 mg/L. Above this level, the chlorine-correction method of HJ/T 70-2001 should be used, or the sample may be diluted so that chloride no longer interferes while COD remains above the method detection limit.

What is the difference between COD and BOD5?

COD measures everything chemically oxidizable, including refractory organics, while BOD5 measures only the fraction biologically degraded over five days. For breeding wastewater both are measured together, since their ratio indicates biodegradability and helps validate biological treatment performance.

Can rapid spectrophotometric COD results be used for compliance reporting?

Rapid sealed-tube methods are acceptable where the permit or the regulator explicitly recognizes them. In most official and dispute situations, the dichromate reflux-titration referee method remains the reference, so laboratories should confirm the reporting requirements of the applicable standard.

How frequently should COD be monitored at a breeding farm?

Frequency follows the discharge permit, typically weekly to monthly manual sampling at the outfall for medium and large operations, supplemented by daily or continuous readings from online analyzers. Additional in-process measurements are recommended after any change in feeding, stocking density or treatment operation.