Biological Oxygen Demand

1^st February, 2024.

In this post, we will see the concept of biological oxygen demand (BOD).

When you are working in a sewage water treatment process, there are a number of parameters which needs to be measured for determining the quality. Each and every parameter has its own role and importance. One of the most important parameters to measure is BOD (biological oxygen demand). Many plants even fully rely on this parameter to check whether their system is functioning properly or not. So, it is necessary to understand the whole idea around BOD and why it is important. In this post, we will see the concept of biological oxygen demand or BOD.

BOD Wastewater:

First of all, let us see why BOD is important. Sewage or wastewater contains human excreta, urine, rainwater, plant leaves, faeces, oils and greases, runoff from streets, heavy metals or some toxic chemicals. Basically, it is a type of domestic waste water which is hazardous for environment. That water must be treated before discharging it in rivers, creeks or seas. Sewage water mostly contains organic waste. These organisms arise due to outside environment conditions, waste present inside, or any chemical inlet. The main reason out of all these is human waste, which itself contains organisms. These can be bacteria, fungi, protozoa or other forms of living organisms. Out of these, there are two types - good and bad. Bad ones will pollute the water and good ones will prevent the water from pollution.

How? The good ones will feed on bad organic material for their growth and decompose or digest them. This significantly reduces the amount of organic waste from water. The organic matter is food for them and to eat the food, they will need to survive in the water. For survival, they will require oxygen as we require. With the presence of proper oxygen, they will able to take action on the organisms like decomposing, breaking, digesting or metabolizing. So, the base point is that oxygen must be present in water for them. This oxygen demand is thus important to measure as it determines how much oxygen is required in the water for the microorganisms to survive and take action.

BOD in wastewater treatment:

BOD, as discussed earlier, stands for biological oxygen demand in wastewater treatment processes. It is a measure of how much oxygen is required in the wastewater for microorganisms to survive. Simply put, if the pollution is high, it means more organisms are present in it. This will require a high amount of oxygen. Less pollution means the organisms are few and this will require a less amount of oxygen. So, high BOD means high level of pollution and low BOD means low level of pollution. If BOD is high and sufficient oxygen is not given, then the microorganisms will suffocate, stress themselves or die in the end. If BOD is low and oxygen is not removed if present in high levels, then too the organisms will die due to over-oxygen. So, BOD levels must be properly maintained in the waste water for proper treatment. Sewage treatment plants will take an inlet of high BOD, treat them and discharge the outlet with a low BOD. This means the water has now been cleaned properly.

Pollution control boards usually BOD levels of the treated water. If the BOD level is not as per desired results, then stringent actions are taken against the water treatment firm or the municipality firm using it. So, BOD plays a very important role in water treatment. BOD is measured usually in mg/L (milligrams of oxygen per litre of water).

BOD treatment:

The three methods used for BOD treatment or reduction are wastewater clarification, wastewater separation and anaerobic microbial decomposition. Of this, the most effective method used to maintain BOD for treating water is aerating the water (anaerobic microbial decomposition). In aeration, an air blower is used to blow air in the water, which provides oxygen to the water and enables the microorganisms to decompose the waste. The proportion of air is maintained by varying the speed of the blower. If the oxygen level increases, then it means too much oxygen is present in the water and this will command the blower to reduce the speed. If the oxygen level decreases, then it means less oxygen is present in the water now and hence, the blower speed will increase automatically. This maintains the desired oxygen levels in the water. As the process goes on for a period of time, a stage will be reached where the oxygen level will be maintained for a long period of time and this indicates that the microorganisms have properly treated the water. This reduces BOD automatically and the final outlet water is a pure treated one.

I have covered some general principles related to biological oxygen demand. I have also not attempted to cover all the methods, as it can vary from case to case and there is no end to it. Once you are familiar with this measurement technique, you can easily tackle all types of problems in it.

Thank you for reading the post. I hope you liked it and will find a new way in this type of technology.