What Dissolved Oxygen?

The elevated temperatures that we have seen the last few days in the Commonwealth can cause major changes to wastewater treatment. More sludge can be wasted in summer months than winter months due to fewer microorganisms needed to treat the same amount of biochemical oxygen demand (BOD) entering the plant daily. A less concentrated mixed liquor suspended solids (MLSS) is required. This is directly related to the activity of the microorganisms that treat the waste.

If you do decide to lower the MLSS concentration by wasting, you are also lowering the sludge age, which can affect the nitrogen removal process. By wasting you are also lowering the food to microorganism ratio (F/M) which can affect the bacteria’s ability to remove the BOD in the influent as efficiently as they should.

A 10o Centigrade (18o Fahrenheit) rise in water temperature can cause the activity of the microorganisms to double. If this happens it can cause an increase in BOD removal rates, oxygen use and sludge production. The water temperature has a major effect on the amount of dissolved oxygen (DO) that is available for use by the bacteria.

Cooler water holds more DO than warmer water due to the saturation level of cooler water being higher than warmer water. The saturation level is the maximum amount of oxygen a given body of water can hold at a given temperature. As the water temperature rises, the saturation level decreases, meaning the DO level and microbial activity/BOD removal decreases.

As operators we can’t control the water temperature in our plant but we can work to keep the bacteria alive and working by providing the needed dissolved oxygen to remove the BOD.

In larger facilities this may mean that during the heat of the day you have to start the third blower (or what ever you have) to supply enough DO for the organisms. As the temperature drops in the evening, you may be able to shut down that third blower to save energy cost.

In smaller plants it may mean an adjustment to the timers that operate the blowers used for aeration and pumping. It may take aeration 24 hours a day for these smaller facilities due to smaller tanks and higher temperatures. To reduce energy cost at package treatment plants aeration may be set for 45 minutes on and 15 minutes off for several hours each evening after the temperature drops.

As mentioned above water temperature and dissolved oxygen are tied together. As the temperature rises, the level of oxygen decreases due to increased bacterial activity and the water unable to accept more oxygen. This is due to it having reached its oxygen saturation point. You as operators must make the necessary decisions and changes to maintain your facilities in these heat stressful times.

 

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