How to Choose a COD Analyzer for Industrial Wastewater
and meet QCVN 40:2025 / QCVN 14:2008
Chemical Oxygen Demand (COD) is the single number environmental authorities use to cap how much organic pollution your discharge can carry. If you treat or discharge industrial wastewater in Vietnam, your permit is almost certainly expressed as a COD limit under QCVN 40:2025 (industrial wastewater) or QCVN 14:2008 (domestic sewage). This guide explains how to pick an analyzer that measures COD accurately, survives your matrix, and keeps you inside that limit — without over-paying for capability you do not need.
1. What COD actually measures (and why regulators trust it)
COD is the amount of oxygen (mg O₂/L) required to chemically oxidize the organic and oxidizable inorganic matter in a water sample. Unlike BOD, which relies on slow microbial respiration (5–20 days), COD is a fast, reproducible chemical test — typically 2 hours for the standard digestion. That speed is exactly why permitting bodies worldwide, including Vietnam's MONRE, set discharge limits in COD rather than BOD alone.
2. Choose the oxidation method first
There are two standardized COD methods, and the right one is dictated by how dirty your water is:
- CODCr — dichromate method (HJ 828 / ISO 15705 / US EPA 410.4). Potassium dichromate is a strong oxidant, so CODCr covers the full pollution range of industrial effluent — from a few mg/L up to ~20,000 mg/L. This is the method permitting labs and online analyzers use for process wastewater.
- CODMn — permanganate method. Potassium permanganate is a much weaker oxidant, so CODMn is only meaningful for relatively clean water (surface water, drinking water, well-treated final effluent), typically 0–10 mg/L. It under-reports the true oxygen demand of industrial wastewater and must not be used to demonstrate compliance with an industrial COD permit.
Rule of thumb: if the sample comes from a factory drain, a treatment tank, or any process stream, you need CODCr. Reserve CODMn for clean-water monitoring.
3. Online vs. laboratory analyzer
| Factor | Online (in-situ / bypass) analyzer | Laboratory (batch) analyzer |
|---|---|---|
| Readings | Continuous, real-time | Per sample, periodic |
| Best for | Process control, alarm on permit breach, automatic dosing | Compliance reporting, method validation, low throughput |
| Consumables | Reagents / digestion chemicals, periodic replacement | Reagents per batch, lower ongoing cost |
| Typical precision | Good for trend & control | Reference-grade, lowest uncertainty |
Most plants want both: an online CODCr analyzer for live control, backed by periodic lab CODCr (HJ 828) measurements for the compliance report. If budget is tight, start with a lab analyzer and add online monitoring once you understand your diurnal variation.
4. Specs that decide whether the analyzer survives your matrix
- Measurement range — match it to your expected COD, not your permit limit. A 0–200 mg/L instrument will saturate on a 1,500 mg/L equalization tank; a 0–20,000 mg/L instrument is sluggish at the low end. Many sites use one range for raw/equalization and a lower range for final effluent.
- Chloride interference — chloride is oxidized by dichromate and inflates COD. Above roughly 1,000 mg/L Cl⁻ (common in food, seafood, and saline industrial streams) you need chloride masking (sulfate) or a chloride-compensation algorithm, or your reading will be falsely high.
- Detection limit — for final-effluent permits near 50–100 mg/L, confirm the instrument's MDL sits well below the limit so you can prove compliance.
- Digestion time & automation — closed-reflux 2-hour digestion is the reference; "fast COD" (15–30 min) wet-chemistry variants trade some accuracy for speed in online duty.
- Sampling & pre-treatment — suspended solids settle and clog; look for auto-cleaning, filtration, or homogenization if your stream is dirty.
5. Match the instrument to your discharge limit
Vietnam's QCVN 40:2025 sets COD limits that depend on your discharge flow (the permit "column" A / B / C) and whether you discharge to a receiving water or to a sewer. The correct limit is not a single number — it is a function of your flow. Use our QCVN 40:2025 COD limit calculator to convert your permitted flow into the exact column limit, then size the analyzer range around that value with headroom on both sides.
The same logic applies to domestic-sewage permits under QCVN 14:2008 and to CODMn-based clean-water limits — pick the method (CODCr vs CODMn) the permit actually names.
6. Quick comparison
| Method | Typical range | Best for | Reference |
|---|---|---|---|
| CODCr (dichromate) | ~0–20,000 mg/L | Industrial & process wastewater | HJ 828 / ISO 15705 / EPA 410.4 |
| CODMn (permanganate) | ~0–10 mg/L | Clean / surface / drinking water | ISO 8467 / APHA 5220 |
7. Related buying guides
Frequently asked questions
What is the difference between CODCr and CODMn?
CODCr uses potassium dichromate (a strong oxidant) and covers the full industrial range up to ~20,000 mg/L; CODMn uses permanganate (a weak oxidant) and only works for clean water up to ~10 mg/L. Use CODCr for any process or industrial wastewater; use CODMn only for clean-water monitoring.
How does chloride affect COD measurement?
Chloride is oxidized by dichromate and adds a positive error to the COD reading. Above about 1,000 mg/L chloride — typical of food, seafood, and saline streams — you need chloride masking or a chloride-compensation algorithm, otherwise the measured COD is falsely high.
Do I need an online or a laboratory COD analyzer?
An online CODCr analyzer gives continuous control and an early alarm on permit breach; a laboratory CODCr analyzer (HJ 828) provides the reference-grade measurement for compliance reporting. Most plants run both, starting with lab if budget is limited.