09/06/2021 | Wastewater | 8 MINUTE READ

Biological Oxygen Demand (BOD) – How it Affects Wastewater Treatment Plants

Treating water is an essential aspect of many different applications. Whether you’re looking to create bottled water or require clean and disinfected water for industrial processes, the water treatment process requires many moving parts. Among the most important components of the wastewater treatment process involves biological oxygen demand. BOD refers to the total amount of oxygen that bacteria and other forms of microorganisms will consume while decomposing any organic matter that’s present in the water.

By performing a BOD test, it’s possible to determine how much oxygen a sample of water requires to effectively decompose organic matter. Keep in mind that this test takes the current water conditions into account. In order for this test to work properly, the water needs to maintain aerobic conditions, which indicates that there are enough oxygen-consuming bacteria in the water to decompose organic matter. It’s possible for organic matter to get into a sample of water from numerous sources, which can include everything from algal blooms to sewers.

In order to understand how biological oxygen demand works, it’s also important to know how dissolved oxygen is involved in this process. By measuring the amount of dissolved oxygen in the water, you’ll be able to perform a BOD test. You can determine how much biodegradable organic compounds are currently present in the water by measuring dissolved oxygen concentration over a five-day period. If dissolved oxygen levels drop over this five-day period, this means that bacteria and other microorganisms are effectively degrading organic matter.

Because bacteria requires some dissolved oxygen to break down organic matter, DO levels could drop significantly if a high amount of organic matter is degrading in the water. The issue with this process is that the rest of the aquatic life in the water sample requires dissolved oxygen to live.

When dissolved oxygen levels reduce by a substantial amount, the aquatic life will no longer receive the amount of DO they need. Keep in mind that the environment can also affect biological oxygen demand. For instance, when excess fertilizers seep into a body of water, dissolved oxygen levels can lessen. DO levels will also drop when temperatures increase.

In order to comply with set BOD limits, all manufacturing and commercial production industries must implement some kind of wastewater pre-treatment program. If you receive high BOD readings, this indicates that too much organic matter is present in the water, which will reduce overall water quality. This article goes into detail about biological oxygen demand and what it means for wastewater treatment plants.

Factors that Determine Biological Oxygen Demand (BOD)

There are numerous factors that can determine biological oxygen demand. It’s important to understand that the factors that affect biological oxygen demand are the same ones that can lessen dissolved oxygen levels. As mentioned previously, DO levels can drop in the event that excess fertilizers and similar contaminants get into a body of water. Environmental stresses like high temperatures may also lead to a reduction in the amount of dissolved oxygen in the water, which will invariably cause an increase in BOD readings.

If you want to determine the current biological oxygen demand for a source of water, there’s a specific test that must be performed. You can determine BOD by measuring how dissolved oxygen levels differ across a period of five days. To receive accurate readings, it’s important to take an initial dissolved oxygen measurement for a sample of water. Once five days have passed, you can take an additional measurement from the same sample of water. Keep in mind that dissolved oxygen levels are typically displayed in parts per million, which means that you could receive readings that look like 50ppm.

If you receive high BOD readings after performing this test, these results tell you that the amount of dissolved oxygen in the water is low, which is bad for the quality of water. When dissolved oxygen levels drop, this indicates that the oxygen-consuming bacteria in the water are unable to obtain the amount of DO they require to degrade organic matter. If BOD readings are high, you can be confident that the water is unsafe to drink or use for industrial processes until it has been effectively treated.

In the event that you receive low BOD readings, this means that dissolved oxygen isn’t being removed from the water at a high rate. As such, the water quality is high and shouldn’t contain too many contaminants. Cold water is able to retain oxygen more effectively than warm water, which means that dissolved oxygen levels are typically lower in the warm summer months.

If a BOD test is performed on a river that hasn’t been polluted, you can expect your readings to come in at less than 1PPM. On the other hand, BOD readings for untreated sewage can extend from 200-600PPM. Likely the biggest factor in determining BOD involves time. Once a water sample is collected, the BOD test needs to be performed within 48 hours to receive accurate readings. If you want reliable results, it’s essential that the sample of water contains enough healthy bacteria.

Origins of BOD

There are many sources of BOD that you should be aware of. If you receive high BOD readings, it’s likely that an ample amount of organic matter is present in the water. When organic matter gets into water, it has to come from a source of some kind. The primary sources associated with biological oxygen demand include:

Woody debris and leaves
Animal manure
Dead animals and plants
Effluent from paper and pulp mills
Septic systems that are malfunctioning or failing altogether
Food processing plants
Wastewater treatment plants
Storm water runoff

Any of these sources could lead to high BOD levels. While it’s always possible to treat highly contaminated water in a manner that will increase its purity and get rid of nearly all contaminants, it’s also highly recommended that you identify the source of the high BOD readings and make sure that it doesn’t continue affecting the quality of your water.

While many sources of BOD are associated with industrial facilities and various environmental factors, the average American household also plays a significant role in BOD sources. In the last few decades, phosphate pollutants have become increasingly common in American households. These pollutants are present in pet waste, certain detergents and soaps, and fertilizers. If you don’t properly dispose of the chemicals around your home, it’s possible that these pollutants will seep into the water and create high BOD readings.

Biological Oxygen Demand in Wastewater Treatment Plants

Wastewater treatment plants are essential for cleaning wastewater before it gets sent into nearby rivers and streams or is reused. These facilities have been integral towards making sure that municipal water quality remains at a high level. There are two primary stages that wastewater treatment plants use when purifying water. The first stage of treatment occurs right when the sewage gets into the facility. The sewage will be taken through a porous screen that’s able to capture larger contaminants. By getting rid of these contaminants at the beginning of the process, the additional treatment methods will be more effective.

Sewage water is then taken through a grit chamber that can effectively separate some of the heavier materials from the water. These materials will then sink directly to the bottom of the grit chamber. Before the second stage of the treatment process begins, the wastewater will flow through a spacious sedimentation tank, which ensures that most of the remaining solids will drift towards the lowermost portion of the tank. Even though most of the contaminants have been filtered out of the wastewater at this point, additional purification is necessary, which occurs during the second stage.

The second stage of treatment can remove nearly 85% of organic matter that’s still present in the water. There are numerous methods that can be used to properly purify the water, which include the activated sludge process, the chlorination process, ozonation, or UV filtration. No matter which method a wastewater treatment facility uses, the water will then be deemed safe enough for plants, animals, humans, and the surrounding environment.

Since wastewater treatment plants must get rid of nearly all contaminants that are found in the water, BOD can be a highly important aspect of making sure that the water has been properly filtered. In most cases, wastewater treatment plants use BOD to determine how much organic pollution is present in the wastewater.

Keep in mind that organic matter represents just one type of pollution that can affect wastewater. When an industrial facility discharges untreated wastewater into waterways or sanitary sewers, they could face strict penalties. These penalties are based on BOD readings of the discharged water.

Biological oxygen demand has become an increasingly important metric for the majority of wastewater treatments plants. By taking these measurements at different intervals throughout the treatment process, it’s possible for a plant to identify how effective its methods are. If the BOD levels are high, it’s likely that the treatment process the plant uses isn’t producing satisfactory results. On the other hand, low BOD readings indicate that the treatment process is working as intended.

Posted by Sensorex on September 6, 2021

Sensorex is a global leader in the design and manufacture of quality sensors for water quality and process applications. The company offers more than 2000 sensor packages for pH, ORP, conductivity, dissolved oxygen, free chlorine, chlorine dioxide, UV transmittance and other specialty measurements, as well as a full line of sensor accessories and transmitters. Its expert technical support engineers solve analytical sensor challenges with custom designs and off the shelf products.

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