In past articles, we discussed the Eight Growth Pressures that impact secondary wastewater treatment. The fourth growth pressure is Dissolved Oxygen and the role it plays in biological treatment.
The successful operation of a wastewater treatment facility requires the control of many variables. EBS identified eight critical growth pressures that, when kept within identified target ranges, optimize biological treatment and minimize the risk of noncompliance. Dissolved oxygen is one of the most important growth pressures in aerobic treatment systems as it drives bacterial metabolism while being one of the largest cost drivers of the process.
There are two basic mechanisms of adding dissolved oxygen to wastewater treatment systems: chemical and mechanical. Chemical addition can be effective but typically requires additional engineering controls to maximize product effectiveness. The more common, and typically the more cost-effective method of dissolved oxygen addition, is mechanical aeration such as surface aeration or diffused air introduced via blowers and membranes. Mechanical addition methods typically require a more in-depth maintenance program but perform well over the life of the treatment plant when properly maintained.
From an operations perspective, dissolved oxygen should be present in excess so that a residual can be measured. Typical residuals recommended are 1-2 mg/L of dissolved oxygen, but residual recommendations vary from plant to plan and depend on specific treatment system requirements and limitations. Low dissolved oxygen can create problems with BOD removal, sludge settling due to filamentous bacteria blooms and/or deflocculated biomass, elevated effluent TSS, odor, safety due to sulfide generation, and much more.
Although DO is primarily created via mechanical means, there are times when oxygen supply cannot meet oxygen demand putting biochemical oxygen demand (BOD) permit limits at risk of being violated. Oxygen deficiency can arise for a variety of reasons, such as a power outage, aerators being down for maintenance, scavenging of oxygen by chemical compounds containing sulfur, or upstream production spills that increase organic loading to a WWTP. That is when the method of chemical addition can play a temporary but important role. EBS CN-9 offers the bacteria the second most energetically favorable electron acceptor, nitrate, to help BOD degradation continue and the plant to stay within permit limits. For more information on EBS CN-9, please see (USING CALCIUM NITRATE TO ENHANCE BOD REMOVAL article).