(BOD) Biological (Or Biochemical) Oxygen Demand & Wastewater System


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BOD, a benchmark that enables septic professionals to assess, design and adjust on-site treatment for effective commercial wastewater treatment.        

In this article, we’ll look at the differences between residential and commercial wastewater and discuss biological (or biochemical)oxygen demand (BOD). This numeric measure of wastewater strength is a key factor in guiding septic system design and processes for commercial operations, including the need for on-site treatment plants.                                                      

Not all wastewater is created equal. Residential wastewater is all pretty much the same; we all shower and wash dishes, do laundry and make use of the toilet in similar ways. But when commercial wastewater comes into the mix, we discover that there is tremendous diversity in its makeup, based on the size and nature of commercial enterprises. The diversity of commercial wastewater is a product of the diversity of commercial operations. Think about that diversity: hotels, car washes, restaurants, beauty parlours, funeral homes, photography studios, print shops, nursing homes, campgrounds, department stores… the list of commercial operations, each with sinks, toilets and drains, is a long one! And for our discussion here, we can add non-commercial operations, like schools and churches, too.

Let me state right up front that in some online material you will see BOD defined as “biological oxygen demand” and in other places “biochemical oxygen demand.” They are equivalent terms. Way back when, some water-quality scientists came up with one term, and some other water-quality scientists came up with the other term, and apparently they never got around to agreeing to one or the other. So now everybody just says “BOD.”

Commercial wastewater means anything coming from a business or non-profit institution. In some cases, the nature of commercial wastewater may simply be a matter of volume, that is, more toilets, sinks and showers, and more people using them, as with hotels, for example, or schools, churches, campgrounds or fitness centres.

There are other types of businesses that send a range of specific waste into the septic system. For example, car washes and dry cleaners send chemical cleaners, oils and grease into the wastewater stream. Hospitals and nursing homes yield increased volume and a variety of pharmaceuticals, biological hazards and harsh cleaning agents. Restaurants also put cleaning agents down the drain along with food waste, fats, grease and oils, while funeral homes, beauty salons and photography studios each have their own set of solvents and strong chemical agents as a significant component of the wastewater.

It’s fairly straightforward to describe a range of commercial enterprises in terms of the wastewater they are likely to produce and what’s likely to be in it. We can predict the solvents that a dry cleaning shop might use and that a restaurant will have grease in its waste stream. But how do we quantify wastewater makeup for a given enterprise and then ensure that the septic system serving that enterprise is effective? This is where BOD, a common measure of water quality, comes in to play. A BOD measure for wastewater effluent will reveal the level of treatment needed, and whether or not a given treatment system is operating effectively for wastewater being produced, for a specific commercial enterprise.

Remember, the effluent is what is released from the septic tank following primary treatment, the somewhat cleaned-up wastewater that goes into secondary treatment in the leaching field. Remember also that effluent has to be effectively treated to specific standards before it reaches the interception zone, that area where it joins with groundwater or surface waters. We expect that treated wastewater is as clean as natural water by the time it is joining with groundwater or surface waters.

Ok, BOD is an easy-to-remember acronym, but what is it exactly, and what does it tell us?

Definition time: BOD is a measure of the amount of dissolved oxygen in the water that is used by aerobic microbes when decomposing organic matter.

Um… huh? That seems like a rather involved definition, so let’s unpack it a bit. First, water contains dissolved oxygen. That means that water contains a certain concentration of free oxygen molecules, produced by aquatic plants and algae (just as terrestrial plants pump oxygen into the air), and also from physical exchange with air (turbulent water has lots of dissolved oxygen because it is mixing freely with air; still or stagnant water has little-dissolved oxygen).

Dissolved oxygen is important for aerobic aquatic life, from fish to macro-invertebrates like insects, crayfish, mussels or snails, on down to bacteria and other microbial life. Aquatic organisms breath dissolved oxygen in water just as we land-dwelling organisms breath oxygen in the air. They are also adapted to certain levels of dissolved oxygen. For example (two tasty examples, at that), trout live in cold, fast-moving and turbulent streams that are highly oxygenated, while catfish are bottom-dwelling fish, perfectly happy in warm, still, poorly oxygenated water.

While we’re talking about oxygen, this is a good time to define two important terms that come up a lot in discussing septic systems: aerobic and anaerobic. Aerobic refers to oxygen-using processes. So, aerobic organisms, like most bacteria, insects, fish, land animals, and of course, us, use oxygen in respiration to fuel our bodily processes. Without oxygen, we all die.  Anaerobic means processes that occur in the absence of oxygen. Some organisms, notably anaerobic bacteria, use other chemical compounds instead of oxygen in their respiration. These bacteria are found in environments with little or no oxygen (i.e., anoxic environments), such as mudflats, the deep ocean, and, of course, septic tanks. If you’ve ever smelt the sulfurous, rotten-egg smell of low tide at the seashore, your smelling anaerobic bacteria, living their lives!

Now, back to our BOD definition. Dissolved oxygen is the first part. The second part is that aerobic microbes use dissolved oxygen while decomposing organic matter. They don’t breathe with lungs as we do, but they use oxygen to fuel the biochemical processes that keep them growing and reproducing – which is the whole point of life when you’re a microbe. Organic matter is food for aerobic microbes; in the process of consuming it they are breaking it down. It’s just like worms and insects eating your garbage in the compost pile, only on a smaller scale, and with everything happening suspended in water.

Recall from previous posts that wastewater effluent, what flows out of the septic tank after the solids have settled, contains a significant amount of suspended organic material. If this organic material is not sufficiently broken down in secondary treatment in the leaching field, it can eventually make its way into the streams, rivers, ponds and lakes that make up surface waters.

In every environment, the more abundant the food supply, the more the populations of organisms that eat that food will grow and expand. In the case of suspended organic matter, greater concentrations lead to greater microbial population growth. An excessive concentration of suspended organic material can lead to a microbial population explosion. You may ask, “So what? Microbes are tiny!” True, but these microbes are aerobic, meaning that they consume oxygen just by being alive, eating all that organic material. If the microbial population is large enough, it can consume dissolved oxygen faster than aquatic plants and the atmosphere can replenish it. And that negatively impacts other aerobic organisms, like aquatic insects, other macroinvertebrates, and fish.

In a nutshell then, too much suspended organic material leads to too many aerobic microbes, who suck up all the oxygen in the water, leaving no oxygen for fish and other living things to breathe. We call that “anoxic” conditions, and that is why we want to ensure that wastewater effluent BOD is well controlled, so as not to negatively impact surface waters and aquatic life.

We’ll see up ahead how residential and commercial wastewater streams differ in “strength,” which is measured in part by BOD, and how that results in differing septic system requirements. First, though, let’s briefly look at how BOD is measured and quantified.


BOD measurement is relatively simple as far as laboratory procedures go. A water sample is placed in a pint-sized bottle and ‘seeded’ with a small amount of bacteria. The level of dissolved oxygen in the sample is measured, and then the bottle is incubated in the dark for 5 days, after which dissolved oxygen is again measured. (Incubation in the dark ensures that any algae in the sample don’t photosynthesize and produce oxygen, which would throw off the measurements.) The difference in dissolved oxygen level between the beginning and end of the test gives the BOD5 (i.e. the 5-day biological or biochemical oxygen demand), which is typically expressed in units of mg/L (milligrams dissolved oxygen per litre of water). There are dilution factors and control samples to take into account, but that’s a general idea. Since the five-day test is the standard laboratory procedure, “BOD” and “BOD5” are generally similar terms, with the only distinction that the former is the general measure and the latter is a specific test result.

A high BOD value means that there is a lot of organic material in the water, indicating a potential for problematic microbial growth. Does that make sense? A high BOD value means that the microbes that were seeded into the sample found a lot of “food” (i.e., organic material) to eat, so they were happy, eating and growing and reproducing, and in the process used up a lot of the dissolved oxygen in the water sample during the five days of the test.  Clean, natural water from a river or stream will have a BOD of 1 mg/L or less, and polluted rivers about 10 mg/L, as points of reference. Raw sewage, as in septic system input, will have BOD values in the hundreds of mg/L, while effectively treated sewage effluent will be below 20 mg/L.

So why not just measure dissolved organic material in wastewater effluent? Wouldn’t that be a more direct measure of our concern about suspended organic material? The answer is that there are thousands of chemical compounds that could serve as a source of microbial organic food. It takes more complicated equipment and very expensive protocols to identify and quantify individual chemicals, and what we really want to know is, is there too much “food” that will cause the microbial population to get out of control? We’re not so interested in the specific compounds that make up the “food.” So, the direct and inexpensive method is to measure microbial responses to available “food” in the form of BOD.

Commercial wastewater, because it, in general, involves higher volumes and a greater diversity of chemical inputs, and because harsh chemical inputs may reduce the effectiveness of the treatment process (e.g. by harming the bacteria that normally break down wastewater material in the septic system), in general, has the potential to yield effluent with high BOD. Because of this, special on-site systems are useful to reduce the BOD of the effluent, for example, grease interceptors (for food establishments), re-circulating sand filters, and other system variations that manipulate hold times or hydraulic loading to increase the breakdown of suspended organic material. The best way to assess the treatment needs for specific commercial operations is to inventory the chemicals and other material being sent into the wastewater stream and to take effluent samples to directly measure BOD.

The U.S. state of Vermont published a useful guide that specifically addresses non-residential and commercial wastewater considerations in terms of wastewater strength. While BOD is the main measure, measures of total suspended solids (TTS) and fats, oil and grease (FOG) also are used to characterize effluent strength. Focusing just on BOD, residential wastewater effluent from a septic tank is considered “low strength”  with a BOD5 less than 230 mg/L, which is then sufficiently treated by conventional septic systems. Commercial enterprises generally lead to “moderate strength” effluent having a BOD5 up to 400 mg/L and therefore requiring specific system modifications as mentioned above.

Healthy microbial populations are critical components of septic treatment. However, we need to ensure that they do their job effectively within the septic system, otherwise insufficiently treated effluent will lead to unhealthy microbial populations out in the environment.

BOD is a simply determined baseline measure of wastewater effluent strength. For the very diverse and complex range of possible wastewater compositions generated in commercial enterprises of all kinds, it is a benchmark that enables septic professionals to assess, design and adjust on-site treatment for effective commercial wastewater treatment.

Luis Goncalves, ROWP, IN, PL

I fell into this amazing trade quite by accident, I have always loved being around heavy machinery and moving earth and in contrast love working with designing things.  This trade in the septic field has led me down a wonderful path of exploring all of my skills and passions while working hard at achieving success.

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