12.2 BOD, CBOD, DO, and Oxygen Demand

Oxygen Demand Concepts

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

Biochemical oxygen demand (BOD) is the oxygen used by microorganisms while degrading biodegradable organic matter over a specified test time, commonly five days. Carbonaceous biochemical oxygen demand (CBOD) is the portion caused by carbon-based organic material. A CBOD test uses a nitrification inhibitor so that ammonia oxidation does not add nitrogenous oxygen demand. Dissolved oxygen (DO) is the oxygen actually present in the water column, usually reported as mg/L.

BOD, CBOD, and DO Compared

A high BOD discharge does not mean the discharge contains oxygen. It means the discharge will consume oxygen after it enters a biological reactor or receiving water. That 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 the same in and out, concentration removal gives the same result. If flow changes, use loads.

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

BODt = L0(1 - e^(-kt))

where L0 is ultimate oxygen demand and k is the deoxygenation rate constant. If the problem gives this equation or the needed constants, follow the units carefully. Larger k means demand is exerted faster, not necessarily that the total ultimate demand is larger.

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 oxygen problem downstream.

Stream Oxygen Sag

After a biodegradable discharge enters a stream, oxygen usually follows a sag-and-recovery pattern. Near the outfall, mixing and dilution occur. Downstream, microorganisms consume oxygen as waste decays, increasing the DO deficit. Reaeration from the atmosphere adds oxygen back to the stream. The critical location is where the deficit is greatest, meaning deoxygenation and reaeration are momentarily balanced.

Important cues:

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

Exam Strategy

When a question mentions 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. Above all, keep the sign straight: oxygen demand consumes DO, while reaeration restores it.