Specific Oxygen Uptake Rate: The Fifteen Minute BOD

 

The goal of this article is:

1.       To introduce readers to the marvelous benefits of the Specific Oxygen Uptake Rate

Learn Theory! Learn Theory! Learn Theory! If you know theory you can use it! If you don’t know theory you can’t use it!

Thinking in terms of complex systems is difficult. Many people, even after studying for years, do not understand basic principles. Systems thinking is often counterintuitive at many points. It takes many years to gain any degree of skill, and it still often eludes us. (Extraordinary Leadership: Thinking Systems, Making a Difference, 2009, Reberta M. Gilbert, M.D, pg 5)

____________________________________________________________________________________

 The Georgia Rural Water Association is always available to assist any municipality that serve a population of <10,000 with process control, troubleshooting, training, lift station maintenance, smoke testing, leak detection, standard operating procedure development, asset management development, or any other compliance and operational issues that may crop up. To request their assistance, please contact them at Contact Us - Georgia Rural Water Association (grwa.org).

The next few posts will focus on Activated Sludge Concepts. After that, I will focus on some water related topics of interest.

************************************************************************************ 

Specific Oxygen Uptake Rate (SOUR)

The Most Valuable yet underutilized test available for operators of the Activated Sludge Process (University of Tennessee) [capitals and emphasis Dennis Brown]

 

The SOUR calculates the specific amount of oxygen utilized in a specific time period (usually five minutes) by the specific amount of MLVSS in your system [Dennis’ definition] .

 

The BOD calculates the specific amount of oxygen utilized in a specific time period (usually five days) [Dennis’ definition].

“if you were being sent to an island in the Pacific and was told you would only be allowed two process control tests to take with you, what would you take?”

This is a question that has floated around the wastewater industry for years. If you are like me, you oscillate between tests depending on the day of the week or the specific issues you are currently experiencing. One test would I take would definitely be the settleometer test.  It provides a wealth of information in its own right, but even then I sometimes wonder if I would not be better served with the centrifuge test? I can generally gauge the water clarity by looking down in the clarifier. Determining the solids concentration, however, is a different matter and the centrifuge would be a better test for that purpose. But if I had a choice between the BOD and the SOUR test, it would, hands down, be the SOUR test.

WHAT IS AN SOUR?

The basic SOUR test consists of taking a sample of biosolids, aerating it vigorously for 1 – 2 minutes to saturate the sample with oxygen, pouring the highly oxygenated MLSS into a BOD bottle and taking DO readings with a DO meter over the next ten to fifteen minutes. If plotted over time, it would look like figure 1a (after around 2 minutes for the excess oxygen to leave the bottle). Figure 1b illustrates OUR tests at different F/M ratios.

FIGURE 1. Ten Minute Dissolved Oxygen Profile

 

FIGURE 1b OUR at different F/M ratios

The next step is to convert the value to mg/L of DO per hour. So, for the ten (10) minute DO profile in figure 1 above, subtract the final reading from the initial reading and multiply it by six (6). The final part of the equation would be to multiply that figure by 1000 then divide that figure by the MLSS or MLVSS if you prefer. The proper equation is shown in FIGURE 2a. with an example in 2b.

FIGURE 2a Calculations for the SOUR

 

FIGURE 2b Example

 

The practicality of effluent BOD test is almost limited to demonstrating the plant meets compliance. As far as a process control tool, by the time the results arrive the plant could already be in a tailspin. The influent BOD is more practical in that it is used in the Food/Microorganism Ratio calculation. But, like the Settleometer test, the SOUR test can be used in many different ways in many different locations to provide a wide range of information. For example, it can be used:

1.       to determine the amount of food entering the biomass at any given time

2.       to determine the quality of food

3.       to detect the presence of and degree of toxicity

4.       to determine the health of the biomass

5.       to determine the degree of oxidation

Let’s take these in order.

TO DETERMINE THE AMOUNT OF FOOD ENTERING THE BIOMASS

The SOUR test lends itself to one very important characteristic. The sample can be ‘spiked’ with varying amounts of different substances to see what effect those substances will have on the MLSS. A SOUR test run with MLSS alone under normal operation will provide valuable information about the health of the organisms and the degree of oxidation. If a portion of the sample is mixed with influent, the increase in the oxygen uptake can tell the staff something about the amount of food entering the system. In order for this to be valuable, several tests would have to be run on samples with different ratios of Influent/MLSS (for example, 1%, 2.5% and 5%). Plotted over a period of time, the SOUR’s, spiked with the different ratios, should enable the staff to determine the relative influent BOD concentration from the final SOUR values.

The staff should also be able to determine how much food is entering the system at any given time. Let’s say an egg cracking plant is known to discharge high strength wastes on occasion. Unless the plant is equipped with a DO meter in the aeration basin, the staff is unlikely to know something entered the plant until they took an instantaneous DO reading with the field meter. Even then they would not know the full extent of the damage. The SOUR could provide some rapid feedback on how the plant was impacted. It would also provide valuable feedback on the plant’s recovery – much faster than waiting for a 5-Day BOD test.

Let’s take this one step further and assume the plant has an equalization basin (EQ). Yes, a luxury, but one that some plants do have. The plant could divert flow from the aeration basin to the EQ basin, conducting a SOUR test on the MLSS, spiking it with a portion of the influent to determine when the high-strength BOD has passed.

Later, when the influent returns to normal, the SOUR test could be spiked with the high-strength waste from the EQ (blended with the influent at different ratios) to determine how many gallons per day of the EQ waste can be returned back to the head of the plant.

In municipal systems with varying industrial waste streams, the SOUR can be used to determine the relative strength of the influent at any given moment. 

TO DETERMINE THE QUALITY OF FOOD

Once again, assuming a municipality that either receives flow from industrial plants, receiving septic tank waste, leachate or receiving a discrete waste load from any other location, the SOUR sample can be spiked with the unknown product to determine the impact of discrete waste streams on the biomass. Some industries may have discharges that are not consistent. An industry that produces different products on different shifts, or may have an occasional discharge that is significantly different from normal. The SOUR test can be used to determine the impact on the biomass prior to discharge. It can also be used to determine the amount of a specific substrate (technical name for ‘food’) that the biomass is capable of assimilating.

TO DETECT THE PRESENCE AND DEGREE OF TOXICITY

The SOUR test can be used to detect the presence of toxic substances in discrete waste streams or the influent. Here are two examples. Hopefully these may give you ideas for how you may integrate the SOUR test in your facility.

Cyanide in the Influent

I worked at a facility (Facility B) that was downstream from another facility (Facility A) run by the same municipality. Facility A had a plating shop that discharged into its collection system. Late one evening the plating shop discharged a waste stream containing excessive levels of cyanide that killed the biomass in Facility A. The operation staff was alerted to an issue when they conducted one of their normal DO tests (conducted once every 6 hours round the clock) and noticed the DO was excessive, indicating no microbial activity. They suspected something was wrong and called their Plant Manager (PM), who called the Director. The director immediately had the staff close the influent gate to Facility A which directed all their influent to Plant B. The director then called the PM of plant B (me). I called my staff and, knowing it took about six hours for the flow from Facility A to reach Facility B, called in an additional person and instructed them to start running SOUR tests on the MLSS spiked with 5% influent. When the DO of the SOUR started to flatline, the staff diverted the influent into our EQ basin. This continued until the spiked SOUR test no longer indicated any inhibition.

Samples from Plant A’s MLSS and the EQ basin at Plant B were sent to a local lab for analysis. It was determined that both waste streams contained high levels of cyanide. Once we know what the culprit was we could develop a plan of action.

In the early 1980’s I had also worked in a plating shop where they used cyanide in the plating process. I was the wastewater operator. In addition, I have a chemistry degree and therefore knew some things about cyanide. It tends to gas off rapidly at a neutral pH. Also, chlorine is used to ‘destroy’ cyanide in wastewater plants. Therefore, all we had to due to neutralize the cyanide is either add chlorine or wait.

Based on the low levels of cyanide in the basins at both plants, it was decided we would wait and let the cyanide gas off on its own. It took four days for the cyanide to dissipate and we were able to bring the flow from the EQ back into the plant. At the same time activated sludge was hauled from some of our other facilities to reseed Plant A.

                SEPTIC TANK DISCHARGES

I was working as an operation specialist for a contract operation firm and visited a facility in New England that serviced 15 – 20 septic tank trucks per day. On occasion a septic truck would dump something that would have an adverse impact on the plant. The procedure they put in place to manage it was to save a 250 mL sample from every truck, which they saved for two days. If they had an adverse reaction they suspected was from a septic truck, they would run an SOUR spiked with 10% septic tank waste, beginning with the most recent sample and working backwards. The culprit was then banned from dumping.

Another way to approach the problem, it the plant only receives two to four trucks per day, would be to conduct a SOUR with a small sample from the septic truck prior to discharge.

TO DETERMINE THE HEALTH OF THE BIOMASS

If a SOUR is run on a sample taken at the same location and time of day it can tell the staff a lot about the health of the biomass. Figure 3 offers typical SOUR values for different type of activated sludge plants.

FIGURE 3

 

 High SOUR biomasses will settle slowly, not compact well and the effluent can be high in BOD, TSS, ammonia and phosphorous.

Very low SOUR biomasses will settle fast, compact well and have noticeable pin floc.

TO DETERMINE THE DEGREE OF OXIDATION

A sample taken at the end of the aeration basin just before going to the clarifier can let the staff know if the waste stream is sufficiently oxidized. A sufficiently oxidized biomass will use little oxygen and indicated the effluent will be low in BOD. If the SOUR uses more oxygen than normal, it may mean the plant is not carrying enough solids, or has received a slug of high-strength waste.

If a sample taken in the middle of the aeration basin is highly oxidized (the DO slope is flat), it is an indication the plant is carrying too many solids. If the aeration basin is not oxidized enough (the DO slope is steep) the plant needs to carry more solids, or the hydraulic detention time needs to be increased.

Refer back to figure 3 for typical values.

USING THE SOUR AS A PROCESS CONTROL TOOL IN BNR PLANTS

Let’s use the configuration in figure 4.

FIGURE 4. Typical BNR Plant

 

Carbon (food, BOD) is required in every part of the process. No matter what the configuration, the last tank will receive the least amount of BOD. This can have a negative impact on that specific process.  If our example, the anoxic tank is last. It is possible, depending on the amount of phosphorous entering the anaerobic tank and the biomass concentration in the system that the anoxic tank might be BOD deficient.

The SOUR test is the best method for determining if there is enough BOD in the system for the completion of the denitrification process. However, the ideal SOUR value will have to be determined by means of trial and error.

The spiked SOUR test is also the best method for determining how much BOD needs to be added to the system to complete the denitrification process. The specific substrate (food) that will be used for the biomass to must be used in the SOUR. Different ratios can be added to the SOUR to determine the ideal amount of substrate to be added to the basin.

SUMMARY

Successful Wastewater Treatment is all about optimizing the relationship between Food and Population (F/M). This relationship has a direct effect on the respiration rate of the Organisms. The SOUR Test takes advantages of these relationships to provide almost instantaneous feedback to the operator. It can be used to indicate toxicity, to determine the degree of oxidation of the treated waste, as a titration method to determine how much of a waste stream a system can accept or how much substrate to add for optimal treatment.

Feel free to contact me should you serve a population >10,000 or have any questions or issues that the GRWA is unable to assist you with. Also, if you would like to contribute an article feel free to email me at the address below. I am always looking for contributors that have an interesting perspective, topic or has an interesting case that they would like to share – especially if the solution is a direct result of applying the principles from this forum.

Dennis Brown, Wastewater Specialist and Trainer, Retired

dbrown.grwa@gmail.com

678.750.3996