River water quality in Japan is governed by a set of measurements that appear straightforward on a laboratory report but carry layers of meaning that matter enormously when a facility is close to a discharge limit or when a river reach shows signs of ecological stress. Biochemical oxygen demand and chemical oxygen demand are the two figures that appear most often on those reports, and they are frequently misread in the same predictable ways. BOD tells you something about the biological activity in a water sample; COD tells you something broader about its total oxidisable load. Knowing which figure to trust, and when the gap between them is significant, is the kind of judgement that comes from understanding the measurement methods as well as the numbers themselves. This article works through both parameters from method to interpretation, with reference to the Japanese Industrial Standard JIS K 0102 and the effluent standards set under the Water Pollution Prevention Act.

What BOD is actually measuring

Biochemical oxygen demand measures the quantity of dissolved oxygen consumed by microorganisms as they break down organic matter in a water sample over a fixed incubation period. Under JIS K 0102 section 21, that period is five days at 20 degrees Celsius, which is why the value is often written as BOD5. The microorganisms doing the work are the naturally occurring bacteria already present in the sample, supplemented in some protocols by a seeding solution when the sample is known to be low in active microbial population, such as heavily treated effluent or cold mountain water. The result is expressed in milligrams of oxygen consumed per litre of sample. What the figure captures is only the biodegradable fraction of the organic load, and only the portion that those specific organisms can oxidise within five days at that temperature. Compounds that degrade slowly, or that are toxic to the seed bacteria, will be underrepresented. This is not a flaw in the method; it is a characteristic that shapes what BOD is useful for.

What COD adds to the picture

Chemical oxygen demand takes a different approach: it uses a strong chemical oxidant, typically potassium permanganate under the JIS K 0102 method 17 (the permanganate method, designated CODMn), to oxidise organic and some inorganic compounds in the sample. The result is again expressed in milligrams of oxygen equivalent per litre. Because a chemical oxidant is far less selective than bacteria, COD captures a wider range of substances, including many that resist biological degradation. For rivers and lakes in Japan, the permanganate-based CODMn is the standard regulatory parameter under the Environmental Quality Standards for Water Bodies, rather than the dichromate-based CODCr commonly used in other countries. This distinction matters when comparing Japanese data with international literature: CODCr values are typically two to four times higher than CODMn values for the same sample, so direct numerical comparison without knowing the method used leads to errors in interpretation.

The BOD/COD ratio as a diagnostic tool

When both figures are available for the same sample, their ratio carries diagnostic weight. A BOD5 to CODMn ratio above roughly 0.5 suggests that most of the oxygen-demanding load is biodegradable, which points toward domestic or food-processing wastewater as a likely source. A ratio well below 0.3 suggests a significant proportion of chemically oxidisable but biologically resistant material, which may indicate industrial discharge containing surfactants, certain dyes, or slowly degrading synthetic compounds. In river monitoring along an industrial corridor, a reach that shows CODMn rising steadily while BOD5 remains relatively flat is a signal worth investigating further, not simply averaging into a quarterly report. The ratio also informs treatment plant design decisions: a low BOD/COD ratio means biological treatment alone will not achieve target effluent quality, and a tertiary chemical or physical stage will be needed.

How JIS K 0102 governs the measurement process

JIS K 0102, titled Testing Methods for Industrial Wastewater, is the reference standard used by accredited laboratories throughout Japan for both BOD and COD determination. For BOD5, the standard specifies sample collection in glass-stoppered bottles with no air gap, incubation in the dark at 20 plus or minus 1 degrees Celsius, and dissolved oxygen measurement by the Winkler titrimetric method or an approved membrane electrode method at both the start and end of incubation. Dilution ratios must be chosen so that the oxygen depletion falls between 2 mg/L and 7 mg/L over the five days; if depletion falls outside this range, the test must be repeated at a different dilution. For CODMn, the standard specifies a reflux digestion under acidic permanganate conditions for 30 minutes, followed by back-titration with oxalic acid. Each of these procedural steps is a potential source of variance, and understanding where variance enters the measurement is essential for interpreting results near a regulatory threshold.

Reading results against Japanese effluent standards

Japan's Water Pollution Prevention Act sets national uniform effluent standards for facilities discharging to public water bodies. For BOD, the standard for most facility types is 160 mg/L as a daily maximum and 120 mg/L as a daily average. For COD, the same threshold structure applies: 160 mg/L daily maximum and 120 mg/L daily average, though prefectures can and regularly do set stricter values under their own ordinances. Tokyo, Osaka, and Kanagawa prefectures each have supplementary standards for specific watercourse categories. The Environmental Quality Standards for rivers set the target for river water itself at CODMn of 5 mg/L for Class A water bodies (suitable for water supply with conventional treatment) and BOD of 2 mg/L for the same class. A facility whose treated effluent reads 80 mg/L BOD is well within the national discharge limit but is still contributing a load that, if the receiving river is low-flow in August, can push the river reach above its quality standard. Compliance with discharge limits and protection of river quality are related but not identical objectives.

Seasonal and flow conditions that change what results mean

River water quality parameters in Japan do not exist in a stable environment. Snowmelt runoff in March and April in Tohoku and Hokkaido significantly dilutes both BOD and COD in receiving waters, sometimes masking a persistent discharge source that becomes plainly visible during the low-flow conditions of late summer. Water temperature affects BOD measurement because the 20-degree incubation temperature is standardised, but it also affects the actual biological activity in the river: a river reach at 28 degrees Celsius in August will exhibit faster in-situ oxygen depletion from a given organic load than the laboratory figure suggests. Dissolved oxygen in the river itself is the ultimate outcome variable, and BOD is best understood as a proxy for the rate at which that oxygen will be consumed. A BOD5 result from a February sample and a BOD5 result from an August sample taken at the same sampling point describe the same river under very different conditions, and should be compared with that in mind.

Common sources of error and how to account for them

Several recurring errors undermine BOD and COD measurements in practice. For BOD, the most common is inadequate seeding when samples are heavily chlorinated or contain industrial biocides: the seed bacteria are suppressed, oxygen depletion is artificially low, and the result understates the true biodegradable load. The corrective step is to verify seed activity with a glucose-glutamic acid check solution, which should yield a BOD5 of 198 plus or minus 30.5 mg/L under JIS conditions. For COD, interference from chloride ions is a known issue in samples from coastal discharge points or where seawater intrusion occurs in estuarine rivers; the standard correction is to add silver sulfate to the digestion mixture to precipitate chloride before oxidation. Sample preservation is critical for both parameters: samples should be analysed within six hours of collection or acidified to pH below 2 and refrigerated at 4 degrees Celsius, with analysis completed within 24 hours for BOD and 48 hours for COD.

BOD and COD figures on a laboratory report are the beginning of an interpretation, not the end of one. Read alongside the method used, the season, the flow conditions, and the regulatory context specific to the receiving water body, they are genuinely informative. Silt Lab Zone issues results with full method notation precisely because the number without its method is a partial statement.