Level 2: Time to Endogenous Respiration
Many activated sludge systems do not achieve endogenous respiration by the time the mixed liquor leaves the bioreactor, indicating that treatment is not complete. When this is the case, extended oxygen uptake rate testing can be used to determine how long it takes for the system to reach a state of "complete" treatment.
Endogenous respiration, also referred to as endogenous metabolism, is defined here as “a situation where living organisms oxidize some of their own cellular mass instead of new organic matter they adsorb or absorb from their environment” (California State University, Sacramento, Volume II). When endogenous respiration has been achieved, the microorganisms (primarily bacteria) have begun to use their own cellular mass because the food supply, the five day BOD or COD, has been exhausted. In other words:
Treatment is complete when endogenous respiration occurs.
Surprising, to me at least, is that many wastewater treatment systems do not achieve endogenous respiration by the time the MLSS overflows the bioreactor. When there is too much food (COD) entering the bioreactor it can be several hours or more after the MLSS has left the bioreactor before endogenous respiration is achieved. When this happens, the oxidation of organics is continuing in the secondary clarifier, interfering with the rate (slowing the rate) at which the MLSS settles, perhaps increasing effluent turbidity, suspended solids, and COD, as illustrated in graphic below.
Repeated OUR measurements on the same MLSS sample (being aerated) can be used to determine when an activated sludge has reached an endogenous state (when all the oxidizable substrate has been used up by the microorganisms). The time it takes the mixed liquor to reach an endogenous state should not exceed the detention time in the bioreactor, otherwise treatment will be incomplete. The OUR measurement of activated sludge or the MLSS is a measure of the utilization rate of the readily biodegradable chemical oxygen demand (RBCOD) that remains in the wastewater. More RBCOD in the wastewater correlates directly with a higher oxygen consumption rate. The oxygen consumption rate falls to a minimum value only when all of the RBCOD has been consumed and this is the endogenous respiration rate.
In the graphic below we can see a biological reactor that has been properly sized for the applied organic load. At the exit end of a plug-flow reactor the COD concentration has been reduced to a minimum value and the dissolved oxygen (DO) concentration has been steadily increasing along the length of the aeration basin. Even with more aeration time we would see only a minimal (or no) further reduction in the COD concentration. The bacteria, having consumed the readily available food, have reached an endogenous state. After the bacteria have settled in the clarifier and been returned to the bioreactor they will be hungry and will start oxidizing the organics in the wastewater immediately. This describes an optimized activated sludge treatment system.
Test Procedure for Conducting Extended OUR Analysis
In order to conduct extended OUR testing you need to collect at least a liter or more (I usually collect about three gallons in a five gallon pail, making sure to bring a lid for the pail) of MLSS, bring it to the lab, and start aerating the MLSS. You need to run your first OUR (Level 1 OUR Test) as soon as you get your sample aerating.
You will need an aquarium air pump when doing extended oxygen uptake rate testing. These pumps are inexpensive, costing approximately $30 at any pet store in the United States. I prefer the Top Fin air pumps sold by PetSmart. They offer several models with 1, 2, and 4 air outlets. I like the Air-8000 with four outlets, as shown in the image below, because it provides not only a lot of air, but good mixing in a 5-gallon pail. Pump air flow rates are tabulated below the photograph.
You will also need airline tubing and air stones. The air stones provide fine bubbles and good oxygen dispersion through the mixed liquor. The tubing costs about $6 for 25-feet and a packet of six air stones costs $5 from PetSmart. The image below is from Amazon.com where a pack of 10 air stones can be purchased for $5.99.
Ideally, you will generate an OUR value every 30 minutes or so and graph the values so you can observe what is happening in the MLSS sample over time. If the organic compounds in the wastewater are not too complex you will see a steady, gradual, reduction in the OUR as shown in the graph below.
For this heavily loaded industrial plant (graphic above), a petrochemical plant, the well-acclimated biomass required more than 2.5 hours to achieve endogenous respiration. Don't be surprised if it seems like you will never reach endogenous, as shown in the graph below of a municipal plant MLSS.
This is also a heavily loaded wastewater plant, even though it's a municipal plant, because it has a large contribution from industrial sources as well as numerous slug loads of high-strength leachate containing complex and refractory (hard to oxidize) organics. At 124 minutes it looked like we might finally be on our way to endogenous respiration but the OUR increased, and stayed high, at the 209 minute mark, indicating complex organics slowly being broken down to a size allowing for oxidation by the bacteria. At the 420 minute mark, fully seven hours without food, the MLSS still had a high OUR at 16.9 mg/L oxygen/L/hr. So it was no surprise that the secondary clarifiers were very turbid due to the activity taking place from the continued oxidation of organics in clarifiers.
To read about Level 3 OUR Testing please click here.