The night the river stopped breathing
Rapid Chemical Oxygen Demand (COD) measurement on the riverbank
The night is quiet, and the river looks calm. Then, a sample goes into the cuvette. Soon after, the digester hisses, and the colors in the cuvette change quickly. Consequently, the result climbs. Chemical Oxygen Demand (COD) exceeds the safe threshold, and tension fills the air. To verify, the analyst adds a reference sample and checks the blank to rule out error. Meanwhile, the thermometer shows warm water and low flow. At the same time, the team notes the time, location, temperature, and sky conditions. In addition, a comparison with last week’s reading shows an upward trend. Therefore, a short note goes to the on-call team. Afterwards, they load more cuvettes and repeat the digestion. Finally, the second reading confirms the first…
One number, three effects: DO↓, fish↓, environmental alarm
High Chemical Oxygen Demand (COD) means a greater “appetite” for oxygen. Consequently, Dissolved Oxygen (DO) drops, and therefore fish start gasping. As a result, the environmental alarm goes off, and minutes matter. To begin with, fish gather at the surface. After that, their movements become chaotic and weaken. What’s more, warm water further accelerates any nighttime oxygen deficit. For this reason, operators limit discharges if they can. At the same time, field crews head out to check the river reach. All in all, each passing hour can, in fact, make things dramatically worse. In conclusion, high COD is often a harbinger of fish kill conditions.
Chemical Oxygen Demand (COD) in simple words
What COD measures and in what units
Chemical Oxygen Demand (COD) is a simple measure of the water sample’s “oxygen appetite.” The test uses a strong oxidant and the reagent “burns” what can be oxidized—without real fire. The result shows how much oxygen would be required to break it down. It’s reported in milligrams of oxygen per liter (mg O₂/L). A milligram is one-thousandth of a gram. A liter is a large bottle of water. The higher the COD, the bigger the oxygen appetite and the more oxidizable matter present.
COD doesn’t name compounds. It’s a single number summing everything that reacts with the oxidant. Most often these are organic compounds, but sometimes certain inorganics as well. That’s why COD is a composite indicator. A low value means a small oxygen appetite. A high value signals a risk of oxygen depletion and trouble for fish.
Test variants: COD-Cr and COD-Mn
COD-Cr uses potassium dichromate (Cr). It is a “strong” oxidant, and as such, it covers most organic compounds and some inorganics. Therefore, it is the reference method in laboratories and permits. Typically, digestion time is about two hours. Moreover, when samples contain chlorides, mercuric sulfate is added so they do not inflate the result. In addition, silver sulfate is used as a catalyst. Safety rules certainly apply: fume hood, gloves, goggles, and proper waste disposal.
By contrast, COD-Mn uses potassium permanganate (Mn). This oxidant is milder. Consequently, it “sees” simple compounds better, such as sugars or alcohols. However, it oxidizes persistent and aromatic compounds less effectively. Results are therefore often distinctly lower than COD-Cr. For this reason, this variant is sometimes used as an “oxidizability” indicator, and it works well in waters with low concentrations.
Importantly, COD-Cr and COD-Mn numbers should not be compared directly. After all, these are different tests with different oxidative “strength.” For example, the same sample may show COD-Cr = 120 mg O₂/L and COD-Mn = 30 mg O₂/L. In this case, the method is chosen based on purpose and sample type. It could be a river, COD in wastewater, or drinking water. Furthermore, “mercury-free” COD-Cr kits exist and work when chlorides are low. At high chloride levels, however, correction is needed—or chlorides must be removed before analysis.
Other wastewater pollution indicators alongside COD: BOD₅, DO, TOC, UV254
BOD₅ (Biochemical Oxygen Demand over 5 days) – shows how much oxygen bacteria will consume while degrading matter in the sample over five days. It increases where there’s “fresh,” readily degradable load. BOD₅ is usually lower than COD because it covers only the biodegradable fraction. Example: high BOD₅ with low DO is a fast track to fish kill risk.
DO (Dissolved Oxygen) – tells how much oxygen is currently available in the water. It’s the first, very sensitive signal for fish and invertebrates. When COD and BOD₅ are high, DO usually falls. Additionally, in summer at high temperatures, DO falls faster because warm water “holds” oxygen worse.
TOC (Total Organic Carbon) – sums the carbon contained in all organic compounds. It’s independent of bacteria and the oxidant used, so it’s good for load calculations and time-series comparisons. For example, a decline in TOC at constant flow means less organic matter reaching the receiver.
UV254 (absorbance at 254 nm) – increases when there are more aromatic compounds in the sample (e.g., lignin derivatives, phenols). Such compounds are harder to remove and more often require activated carbon or ozonation. Conversely, a drop in UV254 after carbon filtration usually confirms adsorption worked.
Together these indicators form a simple compass. COD and BOD₅ speak to the “oxygen appetite,” DO to the here-and-now, and TOC/UV254 to the total amount and “character” of the matter. It pays to read them together and as a series, not just a single number.
What affects the result: flow, temperature, nutrients
Flow matters most. When there’s little water, the same amount of pollution yields a higher concentration. Conversely, when there’s a lot of water, the same pollution is diluted and numbers drop. Therefore, the same facility can show different results in drought versus after rain. Remember, concentration is mg per liter, and the impact on the river also depends on load—i.e., amount per hour.
Temperature also matters. Specifically, warm water holds oxygen worse, so DO drops occur faster during heat waves. By contrast, winter can be safer because cold water holds more oxygen and degradation is slower. Additionally, at night plants and algae respire, so DO is typically lowest at dawn.
Nutrients—nitrogen and phosphorus—are important, too. These compounds “feed” algae. As a result, when algae abound, they produce oxygen by day. However, once they die off, their decay consumes oxygen at night, and COD then rises. In general, nutrients mainly come from agriculture, wastewater, and runoff from paved surfaces during downpours.
That’s why results should always be read together with weather and river conditions. For instance, droughts, low flows, cloudbursts, and snowmelt can change the picture overnight. In short, the same monitoring point can appear “different” in summer and winter, at low and high water.
Concentration vs load: a simple conversion
Concentration is mg O₂ per liter. Load tells how much matter reaches the river over time. Calculate it as:
load = COD (mg/L) × flow (m³/h) × 0.001 = kg/h.
This helps assess the actual impact on the receiver, not just the water’s “density.”
If COD = 80 mg O₂/L and the flow is 20 m³/h, the load is 1.6 kg O₂/h. If COD drops to 60 mg O₂/L but flow rises to 40 m³/h, the load increases to 2.4 kg O₂/h. Always analyze concentration and flow together.
Why this matters: rivers and tap water
High Chemical Oxygen Demand (COD) means oxygen is consumed quickly. Oxygen disappears first in slow reaches and backwaters. In summer and at low river levels this happens even faster. Bacteria and algae continue to consume oxygen at night, so dawn is often worst. The effect is sluggish fish, fish kills, musty smell, and blooms.
For drinking water plants, high COD means tougher operation. More coagulant and oxidant are needed. Ozone or activated carbon is used more often. Filters clog faster, so backwashing is more frequent. More sludge is produced for disposal. Aeration and pumps consume more energy. As a result, treatment and maintenance costs rise.
That’s why COD measurement—and COD in wastewater at the source—acts like an early alarm. It shows when the load is rising and where to look for the cause. It allows retention, stream separation, and smoothing of inflow. The river gets less “oxygen appetite,” and the plant runs more stably. In short: low COD means a calmer river and easier tap water. High values mean greater fish kill risk and higher cleanup and treatment bills.
COD vs BOD₅ vs DO vs TOC – a short compass
COD is a quick test. It shows how much oxygen a sample would “eat” upon oxidation. Results come in a few hours. It sums all oxidizable compounds.
BOD₅ is a five-day biological test. It shows how much oxygen bacteria consume while degrading matter. It covers only the biodegradable fraction and is usually lower than COD.
DO is dissolved oxygen here and now—the river’s “breath thermometer.” When COD is high and water is warm, DO falls faster. It’s the first warning signal for fish.
TOC is total organic carbon. It shows how much carbon is in organic compounds in the sample. It’s independent of bacteria and oxidant, so it’s good for load balancing and trend tracking.
Using them together: day-to-day, COD is used for quick process control and early alarms. BOD₅ confirms biological treatment performance. DO warns of fish kill conditions. TOC helps with mass balances, comparisons, and chemical dosing. Simple examples: high COD + low DO on a hot day = fast alarm and fish kill risk. Another: high COD + low BOD₅ + stable DO = poorly biodegradable compounds; “polishing” needed (e.g., activated carbon or ozone).
Where does high COD come from?
Natural sources: leaves, algae, decay
Nature can raise Chemical Oxygen Demand (COD) on its own. In autumn, leaves, needles, and twigs fall into rivers and start decomposing. Humic acids and other organic compounds enter the water. The water’s “oxygen appetite” increases, and dissolved oxygen (DO) can fall.
In spring and summer, algae and cyanobacteria bloom. After the bloom, dead biomass settles and decomposes, raising COD again. Bottom sediments also consume oxygen during decay—this is Sediment Oxygen Demand (SOD). When water is warm and flow is low, these processes speed up. In such conditions, even small pollution inputs worsen the situation.
Anthropogenic sources: wastewater, agriculture, industry, storms and overflows
High Chemical Oxygen Demand (COD) often results from human activity. Municipal wastewater brings food residues, detergents, and fats. Industrial wastewater adds alcohols, sugars, oils, dyes, and solvents. Particularly heavy contributors are the food, paper, chemical, and textile industries. Agriculture supplies suspended solids, organic residues, and nutrients from fields—especially after fertilizing. Road runoff brings fuels and fine rubber particles. All this increases the river’s oxidizable load.
Emergency discharges or combined sewer overflows can raise COD by an order of magnitude at once. During downpours, a mix of sewage and stormwater reaches the river directly. Illegal discharges and leaky septic tanks act quietly but continuously. In a small river, each such input is visible immediately.
Consequently, algae production rises, and their subsequent decay increases COD again. Bottom sediments accumulate matter and consume oxygen. The lower the flow, the stronger the downstream effect. That’s why reaches below outfalls are the most sensitive.
Seasonality and weather: low flows, heat waves, cloudbursts
Weather influences Chemical Oxygen Demand (COD). In summer, low flows and heat reduce flow and warm the water. Oxygen dissolves worse; decay speeds up. DO therefore drops faster and COD rises. Plants and algae respire at night, using oxygen, so mornings often see the lowest DO.
Cloudbursts and storms resuspend sediments and wash streets and fields—the “first flush” effect. A sudden load of suspended solids and organics reaches the river. COD can spike within hours. In cities, combined sewer overflows add to the surge. After a long drought, the first rain is often the most burdensome.
Winter can be the opposite: lower temperatures slow decay, so COD is usually lower. Ice limits aeration, and snowmelt carries dirty water from streets and fields. The load then arrives suddenly and locally. In spring and autumn, strong winds can mix the water and improve oxygen conditions, though short, intense rains still raise COD.
How we measure COD without losing the truth
Reference method PN-ISO 6060 (dichromate): when and how
This is the benchmark method—the point of reference for Chemical Oxygen Demand (COD). The sample is mixed with potassium dichromate in sulfuric acid. Silver sulfate is added as a catalyst. When chlorides (table salt is sodium chloride) are present, mercuric sulfate is added to bind them so they don’t inflate results.
The sample is then refluxed for about 2 hours. Reflux means heating with a condenser so nothing evaporates and volume stays constant. After digestion, the excess oxidant is measured—either by titration (adding solution dropwise to an endpoint signaled by a color change, e.g., with ferroin) or colorimetrically in a photometer.
This method is accurate and robust for “difficult” samples. It’s used for reporting, permits, and in accredited labs. It requires safe practices: fume hood, gloves, goggles, and proper hazardous waste disposal.
PN-ISO 15705 (vial/cuvette method): routine and limits
This is a closed, convenient day-to-day method. Reagents are pre-measured in vials/cuvettes. You add the sample, cap it, and digest in a thermoreactor (heating block) for about 2 hours. A photometer or spectrophotometer reads the color and converts it to a result. It uses little sample and fewer chemicals, and exposure to reagents is limited.
There are limits. Each vial has a defined measuring range, so very high COD requires dilution. A blank and a control sample with a known value are important—they confirm the test works properly.
Titration vs colorimetry: accuracy, time, costs
Titration is the “dropwise” approach. After digestion, ferrous ammonium sulfate (FAS, Mohr’s salt) is added to the sample until the endpoint indicated by a color change (often with ferroin). Equipment is simple and inexpensive, but the color endpoint can be subjective. It handles high COD well, but requires time and skill.
Colorimetry uses a photometer/spectrophotometer. The device measures light absorption and converts it to a result. Readings are fast and repeatable. Per-test cost can be higher, but operator exposure to chemicals is lower. In practice, labs combine both: colorimetry for series, titration for extremes and verification.
Interferences, chlorides, and safety: mercury, silver, “green” alternatives
Interferences are phenomena that distort results. The most common are chlorides. In the test they can “masquerade” as organics and raise COD. Mercuric sulfate is therefore used to bind chlorides. It’s toxic, so H&S applies: fume hood, gloves, goggles, and proper hazardous waste containers. Silver sulfate acts as a catalyst and also ends up in hazardous waste.
There are greener solutions. When chlorides are low, mercury-free kits are used. You can dilute the sample, use ion exchange (remove chlorides on resin), or apply corrections from control tests. Regardless of method, quality control is key: blank, spiked samples (known added load), and duplicates. That’s how COD results stay reliable and comparable across series.
Law and practice – what COD means in decisions and water classes
Water status classification: where COD “weighs” the most
Water status assessments focus on physico-chemical elements: Chemical Oxygen Demand (COD), Biochemical Oxygen Demand in 5 days (BOD₅), Dissolved Oxygen (DO), Total Organic Carbon (TOC), and nutrients (nitrogen and phosphorus). A simple “weakest link” rule applies: one bad result can lower the class of the entire reach. Oxygen-organic indicators often decide the assessment.
COD “weighs” the most when organic load is high and flow is low. That’s typical of small rivers, below outfalls, and in summer. Under such conditions, even a small increase in COD and BOD₅ accelerates DO decline. At night algae respire, so oxygen drops faster.
One number isn’t enough. High COD with low DO and rising BOD₅ usually means fresh, readily degradable load. High COD with low BOD₅ suggests poorly degradable compounds. A simple BOD₅/COD ratio helps: around 0.5 indicates good biodegradability; values near 0.1 signal trouble.
Water permits: typical limits at outfalls
Water permits set allowable values for COD in wastewater, BOD₅, and other indicators. Limits are given as concentration (mg O₂/L) and load (kg per day, sometimes kg per year). This controls both the “density” of pollution and its amount over time.
Thresholds depend on industry, facility size, and receiver sensitivity. Small rivers and protected areas have stricter requirements. Decisions specify how to test: PN-ISO 6060 (dichromate) or PN-ISO 15705 (vials), sampling method (24-hour composite or grab samples), and frequency (e.g., weekly or monthly).
When limits are exceeded, corrective actions start. First, cause analysis and action plan. Then flow equalization, coagulant dose adjustments, biological section maintenance, and network sealing. Repeated violations risk fines and enforced upgrades. To reduce risk, simple procedures and alarms are used. During rain, retention or a primary settler by-pass is activated, and the operator is alerted about a potential COD peak in wastewater.
Where to check results: public databases and reports
Water quality data are public. The national water quality portal shows maps of points, charts, and enables downloads. You can filter Chemical Oxygen Demand (COD), BOD₅, DO, and other indicators. Regional inspectorate reports include chapters on waters and trends. Municipal and water utility bulletins publish results from intakes and outfalls.
When comparing results, best practice is to pair points above and below outfalls, use the same season, and similar times. Check flow and temperature. The fullest picture comes from COD, BOD₅, and DO together.
When interpreting, watch for method differences (PN-ISO 6060 vs PN-ISO 15705), sample dilutions, and “<” or “>” flags showing out-of-range values. Grab samples aren’t equivalent to 24-hour composites. Rain events and combined sewer overflows distort short series. A simple log helps: date, time, station, flow, temperature, COD, BOD₅, DO. It’s easier then to spot changes and “peaks.”
Data reportage – a map of Poland’s COD “hotspots”
Urban reaches and WWTP outfalls: the “before/after” pattern
Cities clearly show the “before/after” pattern. Above the outfall, Chemical Oxygen Demand (COD) is often lower because water carries less load. Just below, the indicator often rises, sometimes sharply. At the outfall itself, the operator measures COD in wastewater. These values help assess discharge impact. Flows in sewers are larger at night and morning, so results fluctuate more. After downpours, combined overflows can push COD in wastewater and COD in the receiver up within hours. That’s why points 200–500 meters above and 200–500 meters below the outfall are compared. Mixing zones create “pockets” near banks where water stalls. DO drops fastest there and fish rise to the surface. The highest fish kill risk is right below the outfall and in coves. Farther downstream, the river dilutes the load, but elevated COD can persist for many kilometers. Narrow channels, low water level, and high temperature amplify the effect.
Agricultural catchments: field runoff raises COD and fish kill risk
In agricultural areas, the chain of events is simple. After rain, field runoff carries nitrogen and phosphorus from fertilizers (nutrients), plus soil, plant debris, and slurry. These are algae food, so blooms surge. As the bloom ages, dead algae settle. Bacteria decompose this biomass and use lots of oxygen. Chemical Oxygen Demand (COD) rises and Dissolved Oxygen (DO) falls.
In summer the problem is worse: flows are low and water is warm. Heat worsens oxygen dissolution, and algae respire at night, so DO drops fastest near dawn. In small streams, extra “chokes” like ditches and retention ponds slow water. Oxygen has fewer chances to replenish, so DO dips are deeper and longer. In short: rain → field runoff → bloom → decay → COD up → DO down. That sets the stage for fish kills along longer river stretches.
Incidents: how the indicator jumps an hour after a discharge
When a discharge occurs, Chemical Oxygen Demand (COD) jumps suddenly. Water turns darker and more turbid. Dissolved Oxygen (DO) drops soon after. Odor and foam often appear—clear warning signs.
In data, the picture is simple: a sharp COD “peak” near the discharge point. About an hour later, this wave reaches the next monitoring point. Over time, the peak lowers, but the “tail” persists. Early on, it’s worth taking samples every 15–30 minutes and noting flow, temperature, and the wave’s leading edge.
Downstream, the wave weakens through dilution and self-purification. Even so, the signature of elevated COD may still be visible the next day. If COD in wastewater at the outfall rises at the same time, it’s easier to link source and timing.
What high COD does to the ecosystem and… tap water
River cascade: oxygen deficit, fish kills, toxins, bioaccumulation
When Chemical Oxygen Demand (COD) is high, water consumes oxygen faster. Dissolved Oxygen (DO) falls—especially at night, in heat, and at low flow. Fish first rise to the surface and “gulp” air. Then they weaken and huddle near banks. Sensitive species and large individuals suffer most. Without quick improvement, fish kills may occur. Gas bubbles rise from the bottom; streaks and foam appear on the water.
Anaerobic processes then kick in. Bacteria produce hydrogen sulfide, methane, and ammonia. The water smells like “rotten eggs,” sediments darken. Iron, manganese, and phosphorus are released from the bottom. Later, blooms occur more easily, followed again by oxygen drops—a vicious cycle.
After blooms, toxins may appear. Contact with such water can be risky for people and animals. Some pollutants are persistent and stick to sediments and tissues. That’s bioaccumulation. Concentrations rise up the food chain. High COD degrades the entire river reach and surrounding life: fish, birds, mammals, and even pets.
Water treatment: more coagulant, ozone, AOP, membranes
When COD in raw water rises, the plant’s job gets harder. More coagulant and flocculant are needed. pH and alkalinity are often adjusted to make coagulation work. Flocs are larger and settle faster. Filters clog sooner and need more frequent backwashing—using water and power. More sludge is produced and must be dewatered or hauled.
To improve taste and odor, activated carbon is added. In powdered form (PAC) it works quickly; in granular form (GAC) it works longer. For stubborn odors like geosmin and MIB, ozone helps. For hard-to-treat compounds, AOPs—advanced oxidation processes—are used (e.g., ozone with hydrogen peroxide or UV). These break molecules into less troublesome fragments.
In tough cases, membranes are deployed. Ultrafiltration (UF) removes suspended solids and most microorganisms. Nanofiltration (NF) and reverse osmosis (RO) capture smaller molecules. Membranes deliver high quality but require cleaning, chemicals, and pressure. A concentrate stream is produced and must be managed.
To hit doses, jar tests are run and online sensors (e.g., UV254, TOC) are used. This helps optimize coagulant and ozone contact time. Still, costs rise: energy, reagents, maintenance, filter backwashing, and sludge disposal. The best fix is at the source: less load in the river, lower COD in raw water, fewer headaches at the plant.
The cost per mg O₂/L: how the utility’s bill rises
Each extra “mg O₂/L” of COD usually means new costs. Coagulant and flocculant doses go up first. Then more activated carbon or longer ozonation. More backwashing consumes water and power. Membranes need cleaning and faster replacement. More sludge is produced for dewatering and hauling. When COD in wastewater at outfalls also rises, the intake sees a higher load day after day.
Costs pile up: electricity, reagents, equipment service, sludge and concentrate disposal, and analytics. Taste and odor issues can increase network flushes and complaints. High COD often forces more ozone or AOP—raising the power bill. Disinfection by-products may appear and require extra carbon. The cost spiral is especially visible in summer and after storms.
A rough example: when COD in raw water doubles (e.g., from 5 to 10 mg O₂/L), plants typically increase coagulant doses by tens of percent. PAC is dosed more often and ozonation runs longer. Filter backwashes can rise from a few to a dozen or more per day during blooms. Scale depends on technology and river quality, but the direction is clear: higher load means higher unit water cost.
Bottom line: fixing the source is cheapest. Less load in the river means lower Chemical Oxygen Demand (COD) at the intake. That means less chemicals, less energy, and less sludge. Limit combined overflows, seal networks, improve WWTP performance, and add buffer strips on fields. Every mg O₂/L removed upstream pays back multiple times at the plant.
How to lower Chemical Oxygen Demand (COD)
5 steps for plants: audit, separation, load balance, SOPs, maintenance
Step 1: Audit – what, where, and when the load is generated
Start with a short audit. Technologists sketch a simple plant map: where wastewater comes from and where it goes. Operators measure COD in wastewater on main lines: production, cleaning, and condensate. They take a 24-hour composite and several grab samples: morning, noon, and night. Each time they note the hour, flow, temperature, color, and odor. Then they align results with flow to identify peak hours. Finally, they compare weekdays with weekends and production modes.
Step 2: Separation and flow equalization
Next, streams are separated. Operators split “dirty” from “cleaner” at the source. Maintenance installs screens, grease traps, and grit removal. Stormwater doesn’t enter the industrial sewer—it brings street grime and dilutes load. A buffer tank equalizes inflow so the biology is fed steadily. Mixers run slowly to keep fats from solidifying and forming plugs.
Step 3: Load balance and production planning
Calculate load with a simple formula: mg/L × m³/h = kg/d. For example, 800 mg/L at 10 m³/h is 8 kg/h, ~192 kg per day. Plan production accordingly. Shift “heavy” processes to hours with lower flow. Send part of the stream to the buffer and release it slower to keep the biological step evenly loaded.
Step 4: SOPs – simple rules for rain and spikes
Write straightforward SOPs. The manager sets step-by-step actions for storms, failures, and COD spikes. Define warning and alarm thresholds, e.g., 80% and 90% of the limit. Post contacts and responsibilities at pump and valve panels. Staff drills quarterly and logs lessons. Responses become fast and predictable.
Step 5: Maintenance and plant hygiene
Maintenance regularly calibrates probes, flowmeters, and dosing pumps. They check mixers, blowers, and valves—listening for unusual sounds. In the biological step, operators maintain stable DO, proper sludge age, and recirculation. Screens, sieves, and separators are cleaned before deposits harden. A simple maintenance log aids control and planning.
Result: fewer peaks and lower COD in wastewater
The outcome is simple: fewer COD peaks and steadier inflow. COD in wastewater at the outfall drops, and parameters approach limits less often. Treatment runs calmer, uses fewer chemicals and less energy, and alarms occur less frequently. Filtration needs fewer backwashes; less sludge is produced.
Technologies: biology, activated carbon, advanced oxidation
Biology is the first line. Activated sludge, SBR (Sequencing Batch Reactor), and MBBR (Moving Bed Biofilm Reactor) reduce the readily degradable fraction. Operators watch DO, pH, and sludge age. The buffer tank feeds evenly, so the reactor stays stable. When inflow fluctuates, aeration and recirculation control smooths spikes. COD drops at this stage.
Next comes simple “polishing.” Activated carbon binds tougher compounds and improves odor. PAC works fast and damps sudden peaks. GAC works longer and steadier. Ozonation helps with troublesome odors. If compounds are resistant, AOP—advanced oxidation (e.g., ozone with hydrogen peroxide or UV)—is used. For fats and suspended solids, DAF (Dissolved Air Flotation) with coagulant and polymer works well. Each step contributes to lowering COD.
The process is staged: first separation and equalization; then biology; finally carbon or advanced oxidation. This order cuts costs because earlier stages remove most of the load; the rest works lighter. COD drops more effectively and cheaply.
Online monitoring and KPIs: warning thresholds and auto-alarms
In monitoring, timing is everything. UV254 probes measure absorbance at 254 nm—a quick hint of aromatic compounds. TOC shows how much carbon is in organics. In parallel, flow, pH, temperature, and conductivity are measured. The system computes load by the simple formula: concentration (mg/L) × flow (m³/h) × 0.001 = kg/h. Trends emerge, not just single numbers.
In practice, warning thresholds are set at ~80% of the limit; alarms at ~90%. When COD rises past a threshold, the system texts/emails the on-call operator. Bypass to buffer and limiting inflow from the “dirtiest” lines often kick in. The on-call operator logs the event and takes verification samples. Causes are clear and actions recorded.
KPIs (Key Performance Indicators) that help here: average COD at the outfall, 95th percentile, and number of exceedances per month. Track coagulant use per cubic meter, energy per kilogram of removed load, and count of odor complaints. Monitoring COD in raw wastewater shows the upstream trend. The team responds earlier—not after the fact.
How to read results wisely
COD/BOD₅/DO relationship: interpretations and helper indicators
Chemical Oxygen Demand (COD) is the sample’s “oxygen appetite.” The higher it is, the more oxygen is required to oxidize what’s in the water. BOD₅ is the oxygen bacteria consume in five days. DO is dissolved oxygen here and now—how much oxygen fish have available.
Reading them together: if COD is high, BOD₅ is high, and DO is low, the water contains fresh, readily degradable matter. Oxygen disappears quickly; fish kill risk rises. If COD is high, BOD₅ is low, and DO is stable, poorly degradable or even toxic compounds dominate. Biology struggles and the load “drags” downstream.
A simple BOD₅/COD ratio helps. Near 0.5, most matter is readily degradable. 0.2–0.3 means a “mixed” composition. ≤ 0.1 means a poorly biodegradable or toxic-laden load.
Example 1: COD 200 mg O₂/L, BOD₅ 150 mg O₂/L, DO 4 mg/L → fresh load, rapid oxygen consumption, high fish kill risk.
Example 2: COD 120 mg O₂/L, BOD₅ 15 mg O₂/L, DO 8 mg/L → poorly degradable compounds; “polishing” needed (e.g., activated carbon or ozone).
Helper indicators are straightforward. TOC = total organic carbon—how much carbon organics contain. UV254 = absorbance at 254 nm—rises with aromatics. In those cases, carbon or ozone tends to help. Look at the set of results and at the series, not one reading.
The 7 most common reporting and controlling errors
1) Comparing concentrations without flow; load can rise while mg/L falls.
Tip: plot COD and flow side by side.
2) Mixing methods (PN-ISO 6060 vs PN-ISO 15705) without noting differences.
Tip: always state the method; don’t directly compare results from different methods in one table.
3) Ignoring chlorides and not correcting; COD can be inflated.
Tip: note chlorides in wastewater and apply proper correction.
4) Comparing a 24-hour composite with a grab sample as if equivalent.
Tip: compare composite with composite and grab with grab.
5) Drawing conclusions from a single measurement; lack of series and season leads to bad decisions.
Tip: use medians and percentiles, not only means.
6) Confusing units and indicators: COD (mg O₂/L) vs TOC (mg C/L).
Tip: when listing both, spell out full names in parentheses.
7) Analyzing DO at noon instead of dawn; fish kill risk can be invisible in daytime.
Tip: log sampling time and weather (heat, downpour).
Conclusions – one number that organizes the picture of water
One number, many effects
Chemical Oxygen Demand (COD) is a quick “snapshot” of what’s happening in water. Accordingly, when COD rises, oxygen is consumed faster. As a result, DO falls, so organisms have less “breath.” At the same time, BOD₅ shows whether the load is fresh and readily degradable. Moreover, TOC shows how much organic carbon is arriving over time. In addition, UV254 can help—it rises with aromatic compounds. Taken together, they complete the picture.
Context, however, matters: flow, temperature, and time of day. For example, in heat, water “holds” oxygen worse, and DO is lowest at dawn. Likewise, at low flow, the same pollution yields higher concentration. In practice, therefore, look at COD, BOD₅, DO, TOC, and flow together. For instance, COD 150 mg O₂/L and DO 4 mg/L on a hot day = fast alarm. Conversely, high COD, low BOD₅, and stable DO suggest poorly biodegradable load—so in this case, use carbon or ozone.
What to remember on duty and in the field
Firstly, COD and DO move in opposite directions. When Chemical Oxygen Demand rises, Dissolved Oxygen usually falls. Moreover, in summer and at low flows, this happens faster. Consequently, the DO minimum is most often observed at dawn. Values above 7 mg/L are usually safe; however, around 5 mg/L is a warning level. Lastly, below 3 mg/L, fish kill risk grows quickly. Therefore, comparisons only make sense under similar conditions: same point, similar hour, similar flow. For this reason, your log should include date, time, weather, flow, temperature, and river level. In addition, keep COD, BOD₅, and DO alongside.
Meanwhile, weather can change the picture in hours. For instance, after downpours, the “first flush” sends sediments and grime at once. In contrast, during heat, water loses oxygen faster and algae consume it at night. As a result, the lowest DO is often seen at dawn after hot days. Furthermore, tracking sources helps close incidents. COD in wastewater on key lines shows where load originates. When the outfall rises, it is easier to link the river spike to discharge time. Likewise, the BOD₅/COD ratio indicates biodegradability: around 0.5 means fresh; about 0.1 signals tough load.
Therefore, before summer, it is important to line up simple steps. To begin with, focus on monitoring: COD, DO, BOD₅, TOC, and flow. After that, apply equalization and separation to soften peaks. Then again, add biology and carbon “polishing,” and for resistant compounds—AOP or ozone. In the meantime, keep storm plans and retention ready. All in all, high COD won’t blindside you at the worst moment.
Why it pays
Lower COD therefore means a calmer river and fewer alarms. In practice, it also translates into lower chemical doses, shorter ozonation, and fewer filter backwashes. As a result, energy use falls and less sludge is produced. Ultimately, treatment bills decline, while residents, nature, and the utility’s budget all benefit.
In summary, Chemical Oxygen Demand (COD) is like a dashboard gauge. It won’t fix the engine by itself, yet it will show early that something is happening. When read together with BOD₅, DO, and TOC, the picture becomes fuller. Additionally, by considering flow and weather, decisions get better—both in the field and at the plant. Moreover, COD in wastewater at the source helps pinpoint causes. Importantly, the direction of change matters more than a single number. That’s why series, times of day, and seasons all count. Taken together, this picture warns of fish kills, failures, and cost spikes. In the end, the river breathes easier and tap water stays safe.
Chemical Oxygen Demand (COD) – FAQ
What is Chemical Oxygen Demand (COD)?
COD measures a water sample’s “oxygen appetite.” It shows how much oxygen would be needed to oxidize what’s oxidizable in the sample. Units: mg O₂/L. In Polish, COD = ChZT.
How does COD differ from BOD₅?
COD is a fast chemical test (about 2 hours). BOD₅ is a biological test that lasts 5 days. COD also “sees” poorly biodegradable compounds; BOD₅ covers only the biodegradable fraction, so it’s usually lower.
What’s the relationship between COD and Dissolved Oxygen (DO)?
When COD rises, oxygen is consumed faster and DO falls—especially during heat and at night. Low DO is an early warning for fish kill risk.
What is TOC and how does it relate to COD?
TOC (Total Organic Carbon) sums carbon in organic compounds and is independent of bacteria and oxidant. COD shows oxygen appetite; TOC shows how much organic matter there is in total. Together they describe load well.
What COD values are “low” and which are “high”?
Clean rivers: single digits to teens mg O₂/L. Below outfalls/in cities: usually tens. Raw wastewater: hundreds to thousands mg O₂/L. After treatment, values drop multiple times. Exact limits depend on the permit and receiver.
How do I convert concentration to load?
Load [kg/h] = COD (mg/L) × flow (m³/h) × 0.001.
Example: 80 mg/L × 20 m³/h = 1.6 kg/h.
How is COD measured in practice?
Two main methods: PN-ISO 6060 (dichromate)—digestion, then titration or photometry; PN-ISO 15705 (vials/cuvettes)—ready vials, digestion in a block, photometric readout. The latter is convenient for routine work.
How do COD-Cr and COD-Mn differ?
COD-Cr uses dichromate (strong oxidant) and yields higher, more complete values. COD-Mn uses permanganate (milder), better “sees” simple compounds, and often gives lower results. Don’t compare them directly.
What can inflate COD results?
Chlorides most commonly. They react in the test and increase readings. Use mercuric sulfate, dilution, ion exchange, or method corrections. Always include a blank and a control sample.
What does high COD at a plant outfall mean?
It signals a large oxidizable load that can overwhelm biology. Equalize inflow, separate fats/suspended solids, and add “polishing” (activated carbon or oxidation).
How to respond quickly to a COD spike in a river?
Check DO, flow, and the wave’s leading edge. Take dense downstream samples; verify COD in wastewater at outfalls. Limit discharges and enable retention if possible.
How to lower COD at the outfall?
Start with separation and buffering; then biology (activated sludge, SBR, MBBR); finish with polishing (activated carbon, ozone, AOP). Continuous UV254 and TOC monitoring helps.
When should DO be measured to avoid missing risk?
At dawn. At night, algae and plants consume oxygen, so DO is lowest. Noon readings can be misleading.
Which abbreviations should I know?
COD – Chemical Oxygen Demand (Polish ChZT); BOD₅ – 5-day Biochemical Oxygen Demand; DO – Dissolved Oxygen; TOC – Total Organic Carbon; UV254 – UV absorbance at 254 nm.
Does COD determine tap water quality?
Not by itself. But high river COD usually increases coagulant, carbon, and ozone use—raising treatment costs.
What are typical test times?
COD with vials: ~2 hours. BOD₅: 5 days. DO: immediate by probe. TOC and UV254: minutes. Costs depend on range and sample count.
