COD and BOD Discharge Limit Standards: 2026 Global Guide

What COD and BOD5 Actually Measure in Wastewater

COD and BOD5 discharge limit standards set the maximum Chemical Oxygen Demand and 5-day Biochemical Oxygen Demand allowed in treated wastewater before release. In 2026, typical surface-water limits are BOD5 ≤30 mg/L and COD ≤250 mg/L, with municipal-plant limits as tight as BOD5 ≤10 mg/L and COD ≤40 mg/L under China GB 18918-2002 Grade 1A, and EU Directive 91/271/EEC targeting BOD5 ≤25 mg/L and COD ≤125 mg/L for sensitive areas. Industrial projects should design for the strictest applicable standard among EPA, EU, China, India CPCB, and WHO to avoid retrofitting.

BOD5 (5-day biochemical oxygen demand) is the dissolved oxygen consumed by aerobic microbes while they degrade the biodegradable organic fraction of a sample, measured under APHA 5210B over five days at 20 °C. The result is expressed in mg O₂/L and tracks the load that the receiving water must absorb without dropping below the dissolved oxygen threshold for fish survival. COD (chemical oxygen demand, APHA 5220D) is the oxygen consumed when a strong oxidant — typically dichromate in sulfuric acid — is boiled with the sample. COD oxidizes both biodegradable and refractory organics, so the reported number is always equal to or higher than BOD5 for the same wastewater.

The two metrics are regulated in parallel because they answer different questions. BOD5 is the regulator's proxy for oxygen depletion and eutrophication risk; COD is the proxy for total oxidizable pollution, including dyes, solvents, and other slowly biodegradable compounds that microbes ignore during the five-day test. The typical COD:BOD5 ratio sits between 2:1 and 4:1 for municipal sewage, 3:1 to 5:1 for food and dairy effluent, and 5:1 to 10:1 for textile and chemical streams, where a large fraction of the COD is non-biodegradable (general industry baseline; per Senkar and Alumichem framing). A high COD/BOD5 ratio is the engineer's first warning that biological treatment alone will not meet a tight COD effluent limit, and that a polishing step such as MBR or RO is required.

Global COD and BOD Discharge Limit Standards Compared

Five frameworks set the practical compliance envelope for industrial and municipal discharges in 2026: the U.S. EPA secondary treatment regulation under 40 CFR Part 133, the EU Urban Wastewater Treatment Directive (91/271/EEC), China GB 18918-2002, India CPCB Schedule VI, and WHO drinking-water source guidelines. The table below places the most commonly enforced thresholds side by side so a buyer can identify which standard is the binding constraint on a project.

Three patterns are worth flagging. First, China GB 18918-2002 is the strictest numeric standard at Grade 1A (BOD5 ≤10 mg/L, COD ≤40 mg/L) and is the one most commonly copied into the procurement specifications of multinational buyers. Second, the EPA does not set a national COD limit in 40 CFR Part 133; COD limits live in individual state NPDES permits, and a typical envelope is 100–250 mg/L depending on receiving-stream assimilative capacity. Third, WHO does not publish a hard effluent COD/BOD5 number; its guidelines target downstream raw-water quality, so WHO compliance is achieved indirectly by designing the effluent to be clean enough that the receiving source still meets WHO raw-water targets after reasonable dilution. If your plant exports product to a country with tighter limits, the receiving-country standard is the design basis — see the Mexico NOM-001-SEMARNAT suspended-solids discharge guide for a worked example of destination-country specification.

Treatment Technologies Ranked by Achievable COD and BOD Reduction

Once the target effluent band is fixed, the next question is which process train can deliver it reliably from the actual influent. The table below ranks the most common unit operations by typical influent-to-effluent COD and BOD5 performance, with footprint relative to conventional activated sludge (CAS).

CAS remains the workhorse for municipal-strength influent, with 80–85% COD removal and effluent BOD5 of 20–30 mg/L — enough to clear a typical inland surface-water limit of BOD5 ≤30 mg/L. SBR tightens that to 85–92% COD removal at 15–25 mg/L BOD5 in a smaller footprint, useful where land is constrained. MBR — a submerged PVDF membrane module at 0.1–0.4 µm pore size coupled to an activated-sludge tank — delivers 95–98% COD removal with effluent BOD5 below 10 mg/L, and its 60% smaller footprint comes from the elimination of the secondary clarifier and the higher mixed-liquor suspended solids (typically 8,000–12,000 mg/L vs. 2,000–4,000 mg/L in CAS). An MBR membrane bioreactor system is the most space-efficient single-step path to China Grade 1B and most EU sensitive-area thresholds.

UASB is the right call for high-strength streams above 2,000 mg/L COD — sugar, distillery, pulp & paper, and starch — but its 60–80% COD removal leaves an effluent that is still well above any surface-water limit, so it must always be followed by aerobic polishing (CAS, SBR, or MBR). A DAF pre-treatment unit is most often used upstream of the biological step to strip 50–80% of suspended COD, FOG, and fibres; it is rarely a stand-alone compliance solution because it does not touch dissolved organics. RO or NF is reserved for the polish step on a tight reuse or Grade 1A spec, where it brings COD below 10 mg/L and total dissolved solids low enough for boiler feed or process reuse, at a higher CAPEX/OPEX than biological trains alone.

Matching the Process Train to the Strictest Applicable Standard

The procurement rule of thumb in 2026 is to design for the strictest of three constraints: the source-country discharge limit, the destination-country limit if the discharge is for water reuse or transboundary export, and the customer or supplier ESG specification. Under-specifying against the loosest of these is the single most common cause of mid-life retrofits in Indian, Chinese, and EU member-state projects.

For a target of BOD5 ≤30 mg/L and COD ≤250 mg/L (typical inland surface water, India CPCB Schedule VI, EU non-sensitive areas), CAS or SBR is usually sufficient for municipal-strength influent under 800 mg/L COD. For a target of BOD5 ≤20 mg/L and COD ≤60 mg/L (China Grade 1B, EU sensitive areas, many industrial reuse specs), MBR is the most space-efficient single-train answer; CAS followed by a polishing sand filter or ultrafiltration can also reach the band but at a larger footprint and higher operator attention. For a target of BOD5 ≤10 mg/L and COD ≤40 mg/L (China Grade 1A, EU drinking-water source protection, pharmaceutical or textile reuse), MBR alone is enough if the influent COD is held below 300 mg/L; otherwise an MBR + RO polish train is required, as in the dairy wastewater DAF + MBR process guide. The 2026 trend signal is clear: India CPCB and several EU member states are tightening effluent thresholds, and a plant specified to the 2015 baseline is already at risk of a consent revocation — so design to the 2026 standard on day one.

Cost of Non-Compliance and ROI of Designing to the Strictest Standard

Non-compliance is the most expensive line item in a poorly specified ETP. India CPCB can revoke consent-to-operate and levy per-day fines during the shutdown period; EU environmental liability under Directive 2004/35/EC can trigger remediation cost orders running into seven figures; and China applies daily progressive fines up to ¥1,000,000 under the revised Environmental Protection Law, with no upper cap on cumulative liability. Against that exposure, a 100 m³/day MBR train runs roughly 30–50% higher CAPEX than an equivalent CAS train, but eliminates the secondary clarifier and the tertiary sand filter, which trims civil works and operator hours (general industry range; see the ETP cost guide for a worked CAPEX example). The cleanest ROI argument is this: retrofitting a CAS plant to a tightened standard typically costs 1.5–2× what it would have cost to build the stricter train first, because the civil works, blowers, and tankage have to be rebuilt. A water-reuse credit of $0.50–$2.00 per m³ of recovered water — realistic in Jordan, the Gulf states, and northwest India — offsets OPEX within three to five years in arid regions and turns the compliance line into a working-asset line on the balance sheet.

Frequently Asked Questions

What is the difference between BOD5 and COD in wastewater discharge standards?
BOD5 (APHA 5210B) measures only the biodegradable fraction over 5 days at 20 °C; COD (APHA 5220D) measures total oxidizable organics, so COD is always higher than BOD5, typically by 2:1 to 10:1 depending on industry. Regulators track BOD5 for oxygen depletion and COD for total pollution load, including refractory compounds.

Which standard sets the strictest BOD5 and COD discharge limits in 2026?
China GB 18918-2002 Grade 1A, at BOD5 ≤10 mg/L and COD ≤40 mg/L, is the strictest numeric effluent standard; EU Directive 91/271/EEC sensitive areas (BOD5 ≤25 mg/L, COD ≤125 mg/L) and India CPCB Schedule VI inland surface water (BOD5 ≤30 mg/L, COD ≤250 mg/L) sit further down the scale.

What effluent COD can an MBR reliably achieve?
A submerged PVDF MBR (0.1–0.4 µm) typically delivers 95–98% COD removal, with effluent COD of 30–50 mg/L and BOD5 below 10 mg/L from a 400–800 mg/L COD influent — enough to meet China Grade 1B and most EU sensitive-area thresholds without a tertiary polish step.

How much does an online BOD analyzer cost in 2026?
Respirometric and UV-spectral online BOD analyzers for compliance monitoring typically run $8,000–$25,000 instrument cost with $1,500–$3,000/year in consumables; see the online BOD analyzer cost guide for full pricing and specification detail.

Are there comparable heavy-metal limits I should also check before specifying a discharge system?
Yes — metals limits bite independently of COD/BOD5, and several Asian frameworks are tightening in 2026; the Vietnam QCVN zinc discharge limit compliance guide is a useful cross-reference for transboundary procurement specifications.