Wastewater Plants Chemical Oxygen Demand and Air Rate Health Indicator
Industrial Wastewater Treatment Plants (WWTPs) clean the water used in process applications before it is returned to a waterway, typically a stream or river, so that there is no impact to vegetation and wildlife in these waterways.
Industrial WWTPs with an aerobic bioreactor- more commonly called aeration or oxidation tanks- used to breakdown the chemicals in the process water rely on an injection system to dissolve oxygen into the water. The residual dissolved oxygen (D.O.) level in the water leaving the bioreactor is critical to keeping the biomass healthy and the residual amount needed varies from plant to plant depending on many factors including plant size, physical layout downstream of the bioreactor, clarification retention time, effluent permit limits, etc. Thus, air is critical to the plant’s operation and D.O. is a key performance indicator to ensure enough air is being delivered to the plant to keep the biomass healthy.
A continuous dissolved oxygen analyzer is a critical instrument in WWTPs to observe and react in real time to changes in the biomass health and to the influent wastewater composition. Early generations of these D.O. probes required routine cleaning and calibration and if they were neglected, the probe readings were erroneous and became untrustworthy. Even though new technology has improved the accuracy and reliability of these probes and they now have reduced cleaning frequency, the readings are still often discounted as inaccurate when the readings go below the plant’s target D.O. Operation of the bioreactor at low D.O. can lead to serious biohealth degradation and even death.
A way to gauge if the D.O. reading is accurate and determine if the biomass is healthy, is to develop an Air demand table that is based on the wastewater flow, the chemical oxygen demand (COD) test results, and the system’s air diffuser grid efficiency. This table can then be used by operating personnel to check if the air flow rate is high enough for the current wastewater flow and COD results.
This table can be developed in a few steps:
- Define the WWTP’s typical, historic COD and flowrate ranges
- Set the air delivery system limits per device. In this example, air blowers are used to supply the Oxygen using a submersed aeration grid, but there are other aerator types like surface or paddle aerators and jet eductor type aerators to which this method can also be applied.
- Determine the system’s air diffuser efficiency from the manufacturer’s specifications
COD is a great indication of the plant’s load and the test results only take about 2.5 hrs to get, vs 5 days for a BOD (biochemical oxygen demand) test. The results are reported in mg of O2 per liter of sample and is used to calculate the amount of air needed to deliver this amount of O2. Since COD is a concentration, the plant’s flow rate is needed to calculate the pounds of COD, aka pounds of O2, needed to breakdown the chemicals in the wastewater. The pounds of COD / O2 is converted to the air flow meter’s unit of measure for easy interpretation. Pick COD and influent water flowrates that are typical to the plant.
Some plants have multiple air delivery systems and in the example given, two air blowers are installed on the plant so that if the demand is low, only one air blower is needed, and if the demand exceeds the capacity of one air blower, then both air blowers are run. It is important to identify equipment limitations when the plant’s processing capacity has been exceeded and other adjustments are needed. For example, some plants have emergency wastewater storage tanks that can be filled so the high load water ca be processed later when the influent load is lower. Or, some units have connections where rental blowers or air compressors can be installed temporarily. When supplement air sources are installed, even if temporary, it is important to not exceed the air delivery system capacity (air grid and diffuser) or the grid and diffusers could be compromised causing decreased oxygen update efficiency and loss of D.O.
The air diffusion efficiency depends on two main factors: the diffuser type and its depth under water. There are many different diffuser types from coarse to fine bubble diffusers that also may vary in materials of construction from ceramic to stainless steel. Each manufacturer provides an uptake efficiency curve for the specific diffuser model that is also based on the water depth.
The oxygen transfer efficiency may vary with flowrate, but an average efficiency in the normal air flowrate range for the unit can be used to generate the air demand table.
Once this information is gathered, the air rate demand can be calculated based on the COD concentration, the influent water rate, and the air diffuser efficiency. The diffuser efficiency used in the table calculation can be adjusted to match historic data- especially if there is known damage to the air grid.
A few cautions to note about the table:
- The COD test used at most sites only accounts for organic compounds and inorganic compounds like ammonia and sulfides are not include in the oxygen demand result. Adjustments to or inclusion of the ammonia and nitrogen compound concentration may be needed if there are elevated amounts of these components in the feed and if there are nitrifying bacteria in the bioreactor.
- The air demand table should be verified against a healthy biomass. When the plant is stressed from an upset, the air demand will NOT match the table created and a higher air rate is needed. As such, the table can also be used to see if the plant is stressed (indicated if the actual air demand is higher than the table results).
