Some municipal wastewater treatment plants do not run chemical oxygen demand (COD) analysis on their waste and key process streams. But those that do have found that it provides numerous process control, quality control and time- and labor-saving advantages.
For the Salt Lake City Water Reclamation Plant, COD analysis is helping to reduce biological oxygen demand (BOD) dilution, better analyze industrial pretreatment flow, and assess and help maintain the “health” of trickling filters.
“I don’t know why any municipal wastewater treatment plant would not run scheduled COD analysis,” said Cas Knies, a chemist for the Salt Lake City Public Utilities Department. “But I think the rationale for many plants may be that because COD analysis is not part of their permit status, they would simply rather not bother with it.”
Nearly all municipal wastewater treatment plants perform BOD testing to meet their NPDES reporting requirements. To measure oxygen demand, the BOD method relies on enzymes produced by bacteria to catalyze the oxidation of organic matter during a five-day incubation period. In contrast, COD methods use chemical oxidants to oxidize organic matter. BOD simulates the actual treatment plant process by measuring the organic material that can be oxidized with the oxygen in the sample when catalyzed by bacterial enzymes. Although COD is comparable to BOD, COD actually measures chemically oxidizable matter. The test takes two hours to perform.
Many municipal wastewater treatment plants currently do not conduct COD analysis, primarily because it’s not required by their NPDES permits. Also, COD analysis had once been a fairly difficult test to perform. The older, macro digestion method required a considerable amount of bench space, equipment and volume of reagents for each test, and the required sample volumes were very large.
Since the early 1980s, COD analysis has been an easy procedure to perform with the introduction of the Hach closed-reflux micro method, which uses small, pre-dosed reagents.
COD typically correlates to BOD, which is one of the reasons the COD method was initially developed.
“We use COD analysis extensively,” Knies said. “It’s a quick and accurate test that immediately gives us an idea of where we are going, as far as our process control is concerned. We can’t say that about our required BOD test because it takes five days to get the results.”
The Salt Lake City Department of Public Utilities Laboratory provides analytical services for the city’s drinking water facility and distribution system, water reclamation facility and reservoir water. The city’s water reclamation plant is a state-of-the-art facility that processes an average of 32 mgd. COD testing has played an integral role in the lab’s scheduled analytical procedures at the water reclamation facility for more than a decade.
Eliminates multiple BOD dilutions
A ratio can typically be established between the two analytical methods once COD and BOD data have been gathered over time.
For the Salt Lake City lab, parallel COD and BOD testing is beneficial partly because the COD test can be used to target a specific BOD range, thereby eliminating the need for multiple BOD dilutions. The lab will run a COD analysis on the plant’s raw influent or final effluent whenever there are questions regarding the waste stream.
“With COD analysis, we can perform fewer BOD dilutions,” Knies said. “It saves time, is convenient, and there’s less chance of missing a particular BOD test, which is often critical because some are reportable, and we have only one shot to complete them. Because we always run COD analysis, we only need three BOD dilutions, which is our required minimum for each sample. Occasionally, when there is evidence associated with a dump, such as samples having a questionable appearance and/or odor, we will run a COD test on our raw influent to determine if we need to adjust our solutions for setting up correct BOD dilutions. It saves us a lot of time.”
In addition, the lab always runs COD analysis on industrial pretreatment samples that require BOD testing.
Linking efficiency to
soluble nutrient uptake
The lab also conducts scheduled COD analysis to monitor and maintain the “health” of the plant’s eight trickling filters.
“COD tests are run and data collected on the soluble influent and soluble effluent of the plant’s trickling filters,” Knies said. “COD reduction is plotted to determine if there is an upward or downward trend. This quickly alerts us to any problems that need to be addressed.”
The plant runs soluble BOD and COD tests on two of its trickling filters every Tuesday and Thursday. They are composite samples for both influent and effluent. In addition, the plant takes grab samples once a week from each filter for soluble influent and effluent COD analysis.
“The purpose of COD testing here is to obtain an immediate response,” Knies said. “We get results in about two hours, whereas we would have to wait five days for soluble BOD analysis to tell us if one or more of our filters were experiencing problems. With COD analysis, we’re able to quickly identify short-term trends, which is very useful for process control.”
Knies added that COD analysis and trending helps in linking the efficiency of the filters with soluble nutrient uptake.
Using prepared COD reagents
The laboratory uses the closed-reflux micro method using Hach digestion vials for COD testing. The pre-dosed reagents are contained in 16-mm glass tubes, or vials, that fit directly into all commercially available COD reactors and spectrophotometers. The prepared COD reagents minimize handling of corrosive and toxic chemicals and eliminate pipetting and measuring hazardous reagents.
The lab uses 20-1,500 mg/L COD reagents on the plant’s raw water influent and 3-150 COD reagents for the final effluent and for the CODs and soluble CODs it runs for the trickling filters. After 2 mL of sample is added to a vial, the vial is capped and placed in the reactor to digest at 150°C for two hours.
After the vial is cooled, results are then read directly on a spectrophotometer, which features preprogrammed software for storing procedures that allow lab personnel to simply load samples into the instrument’s carousel module and walk away while the instrument performs the measurements and calculates the appropriate COD concentration.
“The pre-dosed COD reagents are accurate and very simple to use,” Knies said. “One of the really useful features of these reagents is that if we exceed the 150 mg/L for the effluent, or the 1,500 mg/L for the raw influent, the vial immediately turns green, which alerts us to the fact that we need to either switch to a higher strength vial or set up a dilution.”
Testing offers
early knowledge
For municipal wastewater treatment plant operations, the benefits of scheduled COD testing are often significant. Knies said facilities that do not perform COD analysis on their waste and key process streams are missing a number of important opportunities for gaining better process control at their plants and better efficiencies in their laboratories.
“BOD analysis is performed at our plant primarily to satisfy permit requirements. Because of its lengthy incubation period, it does not provide much value for process control purposes,”
Knies said. “With COD analysis, however, early knowledge of the strength of incoming water and flows entering and exiting key process streams allows plant operators to optimize treatment processes. The COD:BOD correlation, coupled with a simple, fast test procedure, allows our lab to be more efficient in carrying out its duties.”
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