The Importance of Dissolved Oxygen (DO) in Wastewater Treatment

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Beyond the normal activities of consuming, bathing, and the natural world, we often overlook the quality of this liquid life. We’re talking about high-quality H2O, water that drives everything we do from athletics to making a morning cup of joe. We often expect high-quality water, never thinking twice about it, and inherently taking it for granted. However, from an environmental and manufacturing standpoint, the importance of maintaining water quality cannot be taken as lightly.

This article will focus on an important aspect of water quality in supporting aquatic life and health conditions for all: dissolved oxygen (DO). We will primarily focus on wastewater treatment and effluent flow (discharge from wastewater plants), but the concepts of dissolved oxygen and water quality measurements can be applied to all water applications and water systems. For example, if you’d like to learn more specifically about dissolved oxygen and measuring dissolved oxygen, look no further. 

What is dissolved oxygen in water?

Oxygen in water seems like an oxymoron, but it’s true. Particles of oxygen gas will dissolve into liquid water just like sugar or salt will in a classic middle school solubility experiment. This fundamental action in nature coins the dissolved oxygen term and helps us understand the general quality of water. Any water, tap water, lake water, ocean water, and our hot topic of the day, wastewater.

You may be thinking, but isn’t water made up of oxygen? You are correct as well, since water is made up of two parts hydrogen and one part oxygen (H2O) it does have oxygen in the base liquid. 

However, the big distinction here is that this one oxygen atom is bonded to two hydrogen atoms. So it cannot go floating around willy nilly and provide life to aquatic organisms. Therefore dissolved oxygen is pure oxygen gas (usually in the form of O2) that dissolves and fits between the individual H2O water molecules. This phenomenon can even be seen with the naked eye.

To continue, this free-floating dissolved oxygen is what supports aquatic life in natural waterways, and needs to be considered in practically all water applications (hydroponics, environmental sampling, fish farming, aquariums, brewing, etc). 

Why is dissolved oxygen important in water?

As stated above, aquatic life relies heavily on enough oxygen to survive just like you and me. If dissolved oxygen levels drop too low there can be a large loss of fish and plant population. Therefore, dissolved oxygen measurements can provide a great insight into water quality, especially when paired with pH, temperature, and other probe measurements.

As a rule of thumb, according to the Environmental Protection Agency (EPA), dissolved oxygen levels approaching 3 mg/L are in the danger zone for supporting common aquatic life, and then levels below 1 mg/L cannot support any aquatic life.

Dissolved oxygen levels vary for each organism, but levels around 8-9 mg/L will support all life (fish or plants) and approach the oxygen saturation level of water. This high level of dissolved oxygen should be closely monitored in hydroponic setups, as large changes can have detrimental effects on the plants.

Generally, dissolved oxygen will naturally balance itself in moving waterways full of fish and plant life. Since natural aeration, agitation (wind, animal movement) increase dissolved oxygen levels, one should pay closer attention if these elements do not exist like in a basement hydroponic tank. Other factors that affect dissolved oxygen include temperature, pressure, and salt concentration (salinity).

  1. Temperature – as temperature rises, DO levels decrease
  2. Pressure – as pressure decreases, DO levels decrease
  3. Salinity – as the salt content increases, DO levels decrease

These 3 factors define the oxygen saturation level of a particular water system. In most cases, the dissolved oxygen level will not increase, naturally, beyond 13-14 mg/L but as stated above 8-9 mg/L is a high-quality water condition.

As a result, proper monitoring of dissolved oxygen levels is extremely important to prevent loss of life. Whether you are considering hydroponics, an aquarium, or environmental work, you’ll want to have an accurate and reliable dissolved oxygen sensor. This particular kit from Atlas Scientific will provide everything you would need to begin measuring and monitoring dissolved oxygen. 

What is wastewater treatment in a wastewater plant?

As one can imagine, all the dirty dishwater, toilet water, shower water that goes down the drain has to go somewhere. In most cases, this place is usually the sewer or septic tank. But where does our wastewater go after that?

To a wastewater treatment facility!

Like plants, trees, and other greens clean the air of carbon dioxide without praise, wastewater facilities perform an amazing task of cleaning our water for safe reuse—which is often overlooked and taken for granted in everyday life.  

So, how does a wastewater facility treat our water to make it reusable? The science behind water cleaning can become complicated quickly, but basically, there is a primary treatment and secondary treatment that incoming wastewater undergoes before becoming effluent flow, or natural discharge.

Primary wastewater treatment consists of removing large solids out of the water through filtration. These techniques vary but they all boil down to an industrial-size pool cleaner scooping leaves out of the water (except there are a lot more than leaves in our case).

Secondary wastewater treatment involves a more chemical process, using aeration, flocculation, and activated sludge, to trap and collect all the harmful bacteria we cannot see with our eyes. Different wastewater plants will not use those exact techniques mentioned above, but either way, the goal is to filter out tiny microorganisms that cannot be filtered with physical filtration (like in the primary treatment).

Some wastewater facilities continue with a third, tertiary treatment, but most just add chlorine as a disinfectant which will kill any remaining bacteria. This disinfection step ensures that the water stays clean along the whole journey to a municipal home or natural environment. Chlorine is a fantastic disinfectant chemical but has recently found itself in hot water over health concerns to humans and the environment. If you’d like to learn more about chlorine and how to measure it in your water, check out this article by Atlas Scientific. 

What is the dissolved oxygen (DO) range in a wastewater facility?

To put some color into the whole situation, let’s discuss dissolved oxygen levels alongside the whole wastewater treatment process and why it’s important—even in wastewater.

For starters, when wastewater enters the treatment facility it is usually pretty “dead” in the sense that minimal life exists and the dissolved oxygen content is near zero. (Remember: levels around 8-9 mg/L is optimal for aquatic life, where this water is close to 0 mg/L dissolved oxygen). This is fine for primary treatment where large solids are filtered out as one would strain spaghetti noodles in boiling water. 

However, maintaining and monitoring dissolved oxygen levels in secondary wastewater treatment is crucial to achieving the desired output: clean water. We recommend using a continuous monitoring process like the industrial DO sensor to adequately measure and monitor the process. 

Recall in the secondary treatment, aeration and activated sludge are used. The aeration helps increase and maintain consistent dissolved oxygen levels for the bacteria in the activated sludge to live and perform its job to collect all the contaminants. 

In secondary wastewater treatment, it is recommended to have at least 2 mg/L of dissolved oxygen to avoid dead zones in the biological floc by keeping the critical bacteria alive, instead of dying and settling out of the treatment mixture. 

On the other side, levels of dissolved oxygen can vary from process to process but dissolved oxygen levels that increase beyond 5-6 mg/L could indicate a problem. Since dissolved oxygen increases with mixing and flow, if the mechanical mixer rates are too high (thus increasing DO) they could shear the activated sludge apart which is undesirable. This scenario is more unlikely but something to keep in mind when beginning or adjusting the secondary treatment process.

In summary, dissolved oxygen needs to be considered and maintained even in a wastewater plant. Levels around 2 mg/L should suffice for the biological process (but this is low compared to natural, healthy waters that support aquatic life). Additionally, having some amount of dissolved oxygen in the process also aids the effluent discharge, as not to disrupt nature with “dead” water or water low in dissolved oxygen.

What is effluent in wastewater treatment?

Effluent is the term used to describe wastewater, treated or untreated, that is discharged into the world—be it a lake, river, land, or reused. In most cases, treatment facilities discharge their treated effluent into a river or nearby body of water. Since the water flows and usually falls slightly into the natural water body, it creates better-than-none dissolved oxygen conditions. (Remember, mixing and movement create water motion and aeration that increase dissolved oxygen.) However, wastewater effluent can be a concern for aquatic life and the environment near the dump zone if not properly treated and monitored.

How does wastewater effluent affect the environment?

Even though many wastewater treatments require some level of dissolved oxygen in the secondary treatment section (around 2mg/L), these levels are inadequate to support larger aquatic organisms like fish and plants. As a result, rivers that receive wastewater effluent usually have “dead zones” directly downstream from the effluent since the effluent discharge is so low in dissolved oxygen. This lack of oxygen is called hypoxia and is very harmful to aquatic life.

However, in swift and large rivers this is not a large issue since the water will rebalance and reoxidize rather quickly. Although it draws an important point about choosing a safe spot to dump effluent wastewater without wiping out aquatic life in the area (i.e. stagnant lake, small slow-moving river).

Additionally, adding disinfectants like chlorine and chloride create more health concerns for the environment, namely variable reproduction rates and mortality in aquatic species, and other alterations to the local ecosystem. 

Conclusion

Dissolved oxygen is an extremely important characteristic of water quality for any application (hydroponics, environmental sampling, fish farming, aquariums, brewing, etc) including wastewater and wastewater treatment. In this way, DO should always be monitored during secondary treatment and effluent discharge to ensure safe levels for sanitation and the external environment. Be mindful of the conditions (temperature, pressure, salinity) and other natural factors that influence dissolved oxygen. In the end, always use your calibrated dissolved oxygen probe to accurately measure and monitor water quality to prevent unhealthy conditions.

If you would like to learn more about other water quality measurements, characteristics, or applications for dissolved oxygen, do not hesitate to reach out to the world-class team at Atlas Scientific.

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