Dissolved Oxygen, BOD and COD

Introduction

A water sample can contain oxygen now yet also contain material that will consume oxygen later. Dissolved oxygen, biochemical oxygen demand and chemical oxygen demand answer different questions about that state and future pressure. In a stream receiving organic waste, microbial respiration may lower dissolved oxygen before air exchange and photosynthesis replenish it, producing an oxygen-sag profile downstream.

Core explanation

Dissolved oxygen (DO) is the amount of O₂ actually present in the water at sampling, often expressed in mg L⁻¹. It supports aerobic organisms and varies with temperature, mixing, atmospheric exchange, photosynthesis and respiration. Cold water can generally hold more dissolved O₂ at saturation than warm water, all else equal. A single daytime DO reading may be high because of photosynthesis even if nighttime respiration causes low values, so timing matters.

Biochemical oxygen demand (BOD) measures how much O₂ microorganisms consume while decomposing biodegradable material under stated incubation conditions. The common BOD₅ test follows oxygen decline over five days in a diluted sample, typically at 20 °C in the dark under a standardized method. If a sample begins with 8 mg L⁻¹ DO and ends with 4 mg L⁻¹ after five days, its observed bottle depletion is 4 mg L⁻¹; dilution and blank corrections may be required to report the original sample's BOD₅. The US EPA monitoring guide explains BOD as oxygen consumed by microbial decomposition in stream water, while EPA laboratory guidance describes the five-day measurement.

Chemical oxygen demand (COD) is an oxygen-equivalent measure of substances oxidizable by a specified chemical test, commonly involving a strong oxidant. It does not wait for microorganisms and can include substances that are chemically oxidizable but not readily biodegradable. The US Geological Survey definition treats COD as an approximation of organic and other reducing material. COD and BOD should not be equated numerically without calibration for a particular wastewater: their procedures, reaction completeness and susceptible substances differ.

High BOD can deplete stream DO if microbial consumption outpaces oxygen resupply. Immediately after a waste discharge, mixing changes concentrations. Further downstream, microorganisms decompose material and DO may fall. Still farther downstream, dilution and reaeration from the atmosphere can restore O₂ as the biodegradable load diminishes, giving an oxygen sag followed by recovery. The EPA river-model guidance includes carbonaceous and nitrogenous oxygen demand, reaeration, sediment demand, photosynthesis and respiration in DO modeling.

The sag is not determined by BOD alone. Flow speed, turbulence, water depth, temperature, sediment oxygen demand and photosynthesis affect DO. An aerated rapid stream may recover sooner than a slow deep channel with the same incoming organic load. Nitrogenous demand from nitrification can create additional O₂ consumption beyond carbonaceous decomposition. For a full oxygen budget, one must include these sources and sinks rather than drawing a universal curve with fixed distance to its minimum.

DO deficit is saturation DO minus observed DO under the current temperature and salinity. A positive deficit provides a driving force for atmospheric O₂ dissolution. If photosynthesis raises DO above saturation, gas exchange can instead release O₂. The oxygen-sag minimum occurs near the point where O₂ supply catches up with consumption in a simplified reach; after that, recovery dominates. The USGS reaeration account emphasizes the balance between oxygen use and replacement.

These metrics guide treatment and monitoring. Lowering BOD in wastewater before discharge can reduce pressure on receiving-water DO. COD offers a faster chemical measure of oxidizable load, but neither metric identifies every toxic contaminant or pathogen. A sample with low BOD is not automatically safe drinking water, and a sample with adequate DO at one moment may still experience nighttime oxygen depletion.

Step-by-step reasoning

Identify whether a question asks for oxygen present (DO) or potential future oxygen consumption (BOD/COD). For BOD₅, account for initial and final DO, dilution and controls. For a stream reach, list microbial, sediment and nitrification demands alongside reaeration and photosynthesis. Predict DO decline where demand exceeds supply and recovery where supply exceeds remaining demand. Check temperature, flow and sampling time before interpreting a field profile.

Visual explanation

Draw distance downstream on the horizontal axis and DO on the vertical axis. Start near saturation upstream, show a decline after organic discharge, a minimum farther downstream and gradual recovery. Above the falling portion draw a strong BOD-demand arrow; above the recovering portion draw a stronger reaeration arrow. A separate bottle sketch shows DO before and after a five-day incubation.

Real-world analogy

DO is the cash currently in an account; BOD is a scheduled series of future withdrawals by microbes. COD is a different audit estimating how much of the account could be consumed if a strong chemical process oxidized eligible material. Reaeration is income that can replenish the account. A stream sags when withdrawals exceed incoming oxygen for a time.

Real-world example

After an organic-rich wastewater discharge, a stream may look clear but show rising microbial respiration and falling DO downriver. A properly treated effluent with lower BOD can reduce that oxygen stress. Monitoring at several distances and times is more informative than testing a single grab sample at the discharge point.

Why?

Aerobic decomposition transfers electrons from organic matter to O₂, consuming dissolved oxygen. Chemical oxidation assays estimate a broader oxidizable load through laboratory reagents. A stream's DO reflects the balance of consumption with gas exchange and photosynthesis, so high demand can create a spatial decline and later recovery.

Common misconception

BOD and COD are not dissolved oxygen concentrations. They describe potential oxygen consumption under different procedures. COD is not automatically a direct measure of biodegradable material, and BOD₅ is not the ultimate oxygen demand of every slowly decomposing substance. A single DO measurement cannot reveal a whole stream's oxygen-sag profile.

Worked example

Question: An undiluted simplified BOD bottle starts at 8.2 mg L⁻¹ DO and after five days has 5.0 mg L⁻¹, with blank corrections negligible. What is BOD₅? Reasoning: Oxygen consumed is initial minus final DO: 8.2 − 5.0 = 3.2 mg L⁻¹ . The result is valid only if oxygen remained sufficient for the test and stated assumptions hold. Answer: Approximate BOD₅ is 3.2 mg O₂ L⁻¹.

Quick check

1. Which quantity measures oxygen already dissolved in a stream at sampling time? Answer: Dissolved oxygen concentration, DO, measures the O₂ presently in the water.

Exam focus

Keep DO, BOD₅ and COD definitions separate. Apply dilution and blank corrections when supplied. In an oxygen-sag sketch, include both deoxygenation and reaeration, and consider nitrogenous demand and temperature if the problem mentions them. Do not infer potability or ecological safety from one metric alone.

Advanced insight

The classic Streeter–Phelps oxygen-sag model idealizes biochemical demand decay and reaeration as competing first-order-like processes along a flowing reach. Real rivers add sediment demand, algal cycles, tributaries and changing hydraulics. Model calibration therefore needs measured flow, temperature and DO profiles, not only a laboratory BOD value.

Summary

DO is oxygen present in water; BOD₅ is microbial oxygen consumption over a standardized five-day test; COD estimates chemically oxidizable load by an assay. Organic inputs can make respiration exceed reaeration, causing downstream DO to fall and later recover. Interpretation requires attention to dilution, temperature, flow and other oxygen sources and sinks.

Practice questions

1. Why can COD exceed BOD₅ for a wastewater sample? Answer: The chemical assay can oxidize substances that microbes do not consume within five days.

2. What physical process helps a stream recover DO downstream? Answer: Reaeration transfers O₂ from the atmosphere into undersaturated water, aided by mixing.

3. What extra oxygen-demand process can follow oxidation of ammonium in a stream? Answer: Nitrification creates nitrogenous oxygen demand.

4. Why may daytime DO fail to show a nighttime oxygen problem? Answer: Photosynthesis raises daytime DO, while nighttime respiration continues without photosynthetic O₂ production.