12.2 BOD, CBOD, DO, and Oxygen Demand

Oxygen Demand Concepts

The NCEES PE Civil WRE specification names stream degradation and oxygen dynamics under surface water and groundwater quality. The core idea is simple: waste materials consume oxygen, while streams and treatment systems need enough dissolved oxygen to support aquatic life and biological processes. The exam will not ask for a research-grade water-quality model, but it can ask you to connect BOD, CBOD, ammonia, reaeration, and DO deficit defensibly.

Biochemical oxygen demand (BOD) is the oxygen consumed by microorganisms while degrading biodegradable organic matter over a specified time, commonly five days (BOD5) at 20 degrees C. Carbonaceous biochemical oxygen demand (CBOD) is the portion caused by carbon-based organic material. A CBOD test adds a nitrification inhibitor (such as allylthiourea or TCMP) so ammonia oxidation does not add nitrogenous oxygen demand. Dissolved oxygen (DO) is the oxygen actually present in the water column, usually mg/L; clean cool surface water saturates near 9 to 11 mg/L, and most aquatic-life standards require a daily minimum of about 5 mg/L.

BOD, CBOD, and DO Compared

A high-BOD discharge does not contain oxygen. It means the discharge will consume oxygen once it enters a biological reactor or receiving water. That single distinction eliminates many wrong choices on conceptual questions.

Typical Calculations

For treatment performance, start with load:

BOD load (lb/day) = Q (MGD) x BOD (mg/L) x 8.34

Percent removal = (influent load - effluent load) / influent load x 100%

If flow is equal in and out, concentration removal gives the same result. Secondary treatment must typically achieve at least 85 percent BOD5 and TSS removal with a 30-day average effluent of 30 mg/L under the federal secondary-treatment rule.

For first-order BOD exertion, a common form is:

BODt = L0 (1 - e^(-k t))

where L0 is ultimate oxygen demand and k is the deoxygenation rate constant (base e, day^-1). A larger k means demand is exerted faster, not that the ultimate demand is larger. Watch whether the problem reports k in base e or base 10; they differ by a factor of 2.303.

For ammonia oxidation, use the stoichiometric oxygen demand when supplied:

Oxygen demand = 4.57 x mg/L NH3-N oxidized

This factor explains why a discharge with moderate CBOD but high ammonia can still cause a serious downstream oxygen problem.

Stream Oxygen Sag

After a biodegradable discharge enters a stream, oxygen follows a sag-and-recovery pattern described by the Streeter-Phelps model. Near the outfall, mixing and dilution dominate. Downstream, microorganisms consume oxygen as waste decays, raising the DO deficit. Reaeration from the atmosphere returns oxygen. The critical point is where the deficit is greatest, where deoxygenation (kd L) and reaeration (kr D) are momentarily balanced.

Important cues:

• Warm water holds less oxygen than cold water (lower saturation).
• Turbulent shallow flow reaerates faster than deep sluggish flow.
• High BOD or ammonia increases oxygen demand.
• Low stream flow reduces dilution and can create a critical condition.
• Treatment that lowers BOD and ammonia reduces downstream oxygen stress.

Exam Strategy

When a question reports low DO below an outfall, decide whether the driver is carbonaceous demand, nitrogenous demand, poor reaeration, low dilution flow, or a combination. When the question gives influent and effluent BOD, compute removal before interpreting permit compliance. When it gives saturation DO and actual DO, compute the deficit directly as D = DOsat - DOactual. Keep the sign straight: oxygen demand consumes DO, reaeration restores it. A frequent trap is treating CBOD5 and BOD5 as identical for a nitrifying sample, which understates total oxygen demand.

Worked Ultimate-Demand Example

A wastewater sample has a measured BOD5 of 200 mg/L and a deoxygenation rate constant k of 0.23 day^-1 (base e). Estimate the ultimate carbonaceous demand L0. Rearranging BODt = L0(1 - e^(-kt)) gives L0 = BOD5 / (1 - e^(-0.23 x 5)) = 200 / (1 - e^(-1.15)) = 200 / (1 - 0.317) = 200 / 0.683 = 293 mg/L. The five-day test therefore captures about 68 percent of the ultimate demand at this rate constant, a typical fraction for domestic wastewater. If the same sample also contained 25 mg/L of oxidizable ammonia-nitrogen, the additional nitrogenous demand would be 25 x 4.57 = 114 mg/L, raising the total ultimate oxygen demand to roughly 407 mg/L.

A candidate who reports only the carbonaceous portion will badly underestimate aeration or receiving-water impact.

Reaeration and Saturation Cues

Dissolved-oxygen saturation falls as temperature rises and as elevation (lower atmospheric pressure) increases. The table below shows the trend exam writers exploit when they raise water temperature in a scenario to tighten the DO margin.

Reading these cues lets you decide quickly whether a low-DO scenario is driven by demand (high BOD or ammonia), by reduced supply (warm sluggish water, poor reaeration), or by reduced dilution (low stream flow), which is precisely what conceptual sag questions reward.