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The Eight Growth Pressures of Wastewater Treatment: #1 BOD Loading

Growth Pressure #1 BOD Loading

Industrial wastewater treatment involves physical, chemical, and biological approaches. Physical treatment removes suspended solids, settleable solids, and oils and grease. Chemical treatment is often required for effective or enhanced removes dissolved and colloidal solids, nutrients, heavy metals, and oils and grease. And biological treatment removes biodegradable organics and nutrients.

For biological processes, EBS focuses on what we refer to as The Eight Critical Growth Pressures when we assess a secondary system for efficiency and effectiveness. Any factor that affects the microbial community is called a growth pressure. These consist of BOD loading, pH, toxic or inhibitory compounds, dissolved oxygen, retention time, nutrients, viability of the bacteria, and temperature. For the next several newsletters, we will communicate the importance of each growth pressure and how it impacts the biological wastewater treatment process.

Growth Pressure #1: BOD Loading

A fundamental requirement of wastewater treatment is the removal of biochemical oxygen demand (BOD) by the bacteria, biomass, or “bugs” (Figure 1).

BOD Food

Figure 1

Hence, the first growth pressure we will discuss is BOD loading (Figure 2), often called “food” for the biomass. BOD is a measure of the quantity of biodegradable material in the water. Yet not all BOD is equal. Some compounds like methanol and sugars are more easily degraded than others such as turpentine and soaps. Consequently, the biological population responds differently to the various food sources. Changes in food source types or quantity will also affect the ability of the biomass to quickly assimilate and degrade BOD.


The standard BOD test for permitting is the 5-day BOD test, written as BOD5. The rationale for the five-day test duration dates back to the early 20th century when a couple of Commissions in Merry Old England selected the 5-day BOD test as the definitive test for organic pollution of rivers. While BOD is the “true” measure of food loading to a secondary wastewater system, it is not a practical measurement for operational purposes of measuring or predicting the loading to a system, since learning five days after the fact makes real-time monitoring and control quite challenging. Consequently, many wastewater operators rely on an alternative measurement, or surrogate, to help estimate what the present food loading to the plant is. The two most common surrogate tests for food loading are Chemical Oxygen Demand (COD) and Total Organic Carbon (TOC).

Why is a discussion on BOD loading important? First, virtually all NPDES permits include a discharge limit on BOD5. It is also a method of gauging the efficiency of biological systems. The ability to effectively and efficiently remove BOD is dependent on the other growth pressures that will be discussed in future articles. The more BOD entering a treatment system, the more dissolved oxygen, nutrients, and/or biomass will be required to achieve complete BOD conversion. Utilizing a surrogate such as COD or TOC can allow the operator to make timely process changes to avoid loss of treatment, poor biomass quality, or even permit violations.

In future articles in the series, we will discuss each of the other Growth Pressures in the context of how they impact the overall biological health and viability of the system.