Despite the stamp of legitimacy from bodies like US EPA, and proven benefits, the closed reactor method for measuring Chemical Oxygen Demand in wastewater seems to be getting a raw deal in the region. 'To COD or not to COD is passé - today, the burning question is how quickly and accurately can you do it, so the wise bench chemist spoke. But in a world wedded to standards, his was a lone voice.'
For years, wastewater treatment plants (WWTP) have relied on the determination of oxygen demand of wastewater to measure organic waste loadings (to and from the plant), and evaluate the organic removal efficiency of the treatment processes. The end-objective is to limit or prevent the organic pollution of water bodies that receive the WWTP's effluent discharge stream, while ensuring that regulatory requirements governing such discharges are not breached.
"All our waste has to be disposed off somewhere. Liquid waste should ideally be treated in a WWTP before discharge, but in the worst case scenario, may be dumped (untreated or partially treated) into a river or lake or the sea. The organic content in the discharged effluent will compete with organisms downstream for oxygen, and the resultant oxygen starvation will destroy marine and plant life," said Dr Andy Buck, Regional Technical Manager-Chemistry, Bodycote Laboratories, adding, "It is very important to control and limit the impact of wastewater discharges on the environment. Measuring oxygen demand is an effective way of doing that."
The three tests used to determine oxygen demand are:
1. COD (Chemical Oxygen Demand): is the total measurement of all organic compounds that can be oxidised.
2. TOC (Total Organic Carbon): is the measurement of organic carbon.
3. BOD (Biological Oxygen Demand): is the measurement of organic carbon that bacteria can oxidise.
The BOD test is regarded as a good indicator of pollution value in water because it closely parallels the natural process, measuring the organic material that can be oxidised with oxygen in the sample when catalysed by bacterial enzymes. In contrast, COD methods use chemical oxidants to oxidise organic matter. However, WWTPs and testing laboratories prefer COD over BOD for a number of reasons, a significant one being the speed of analysis. Christopher (Chris) R, Business Development Director, Al Hoty Stanger Laboratories, pointed out that the COD test, on average, takes two hours while the BOD test requires five days at the minimum. Obtaining constant repetitive values with BOD is difficult. By the time the WWTP receives the BOD value, the waste stream it characterises has already left the plant.
The BOD test cannot provide the real-time information that WWTP technicians need to respond to changes in oxygen demand and adjust the treatment process. In the absence of such timely adjustments, among the many risks a WWTP has to face, is a possible plant shutdown due to bacteria build up. From that viewpoint, COD is a quick and easy way to get a snap in-time picture of what is going on in the treatment.
In the case of industrial effluents, bacterial or chemical matter present in the effluent can interfere with the BOD test. While COD is not a direct substitute for BOD, a ratio can be correlated between the two tests over a period of time.
The COD test can be run as a check on other test results as well. Andy said, "When I get a report with TOC, COD and BOD values, I will start with the value of COD because that will tell me what to expect for BOD and TOC. It is given that BOD value cannot be higher than COD because the number of compounds that can be chemically oxidised is greater than those that can be degraded biologically. If I see any discrepancies, I will start asking questions."
BOD will be zero for non-biodegradable compounds. For biodegradable compounds, the value of BOD will tend to approach COD as the analysis time or incubation period increases. However, final BOD will be less than COD, because some proportion of the compounds biodegraded are not oxidised but utilised for cell growth. However, in one or two rare cases where highly biodegradable compound/s are only partially oxidised in the COD test (ex. benzene), BOD may be greater than COD.
Between COD and TOC, the former is more effective because the amount of oxygen necessary for oxidation differs from one organic compound to the other; TOC, on the other hand, measures only the organic carbon.
Open or closed
The dominant methodology for COD determination is the dichromate reflux method laid down in the American Public Health Association's 'Standard Methods for the Examination of Water and Wastewater.' Here, the wastewater sample is mixed with excess potassium dichromate and sulphuric acid, and the mixture is heated under total reflux conditions for a period of two hours. During digestion, the chemically oxidisable organic material reduces an equivalent amount of dichromate.
In the open reflux method, also called the standard method, COD is calculated by titrating the excess dichromate. The sample is cooled after the two-hour digestion period, a few drops of indicator (ferroin) solution are added and the excess dichromate is titrated with a solution of ferrous ammonium sulphate (FAS) of known concentration, until the colour changes from green to reddish brown. The titration reaction corresponds to the oxidation of the FAS by the dichromate. The final calculation of COD is obtained in terms of mg/L of oxygen used. A reagent blank with an equivalent volume of distilled water (in place of wastewater) is also refluxed and titrated as a check on the procedure.
The alternative to open reflux is the closed reflux or closed reactor method. Here, a small volume of wastewater sample is heated with concentrated dichromate solution in pre-packaged 'test in tube' kits. The COD is estimated with a spectrophotometer which measures the absorbance of the dichromate left over after the reflux.
While both tests share identical chemistry, they differ substantially in terms of final analysis and reagent preparation. Chris elaborated, "You still need to go through the two hour digestion process for the two methods. Moreover, both measure colour change. But with open reflux, it is the human eye that interprets the colour change, while you have its electronic equivalent in the closed reactor where the spectrophotometer is an integral part. In fact, titration adds another 20-30 minutes to getting the analysis out, whereas with a spectrophotometer, it is a matter of 2-3 minutes. You also don't need to spend time preparing the dichromate or the FAS solutions or adding the end-point indicator. The closed reactor method saves a lot of time because it eliminates the laborious titration and separation procedures of the standard method."
Titration being a very drawn out process involving minute amounts, the risk of errors is quite high. Andy elaborated: "With a burette, you have only got an accuracy of +/- 0.1 mL. Let us assume that 0.1 mL of the FAS solution is equivalent to x mg of COD. If the person doing the titration is off even by 0.4 mL, he could end up inducing quite a large error. You haven't got that problem with the closed reactor method as measurement is carried out in the spectrophotometer." He noted that titration is a visual indicator; therefore, if four chemists were to look at the same end-point, the outcome could well be four different results. In some cases, the error can be quite significant depending on the person's perception of colour change. However, in a spectrophotometer, the numbers would be the same every time, regardless of who is operating the machine.
Chris pointed out that the titration stage in open reflux is also susceptible to external influences like lighting in the laboratory, the eyesight of the chemist, and cross contamination.
Water analysis major Hach has gone one step further by putting barcodes incorporating all the values including the wavelength on its cuvettes or test in tubes. These values are picked up automatically for measurement by the spectrophotometer. "That is another source of human error eliminated because if the technician doesn't set the right wavelength, he will get incorrect reading," said David Hewitt, Sales Manager - Middle East, Hach Lange, which pioneered the launch of the world's first ready-to-use reagent packages for photometric analysis in the 1960s.
Time consuming exercise
Open reflux demands three hours or even more time for preparing and standardising the dichromate and FAS solutions, including weighing, pipetting and transferring chemicals. In contrast, the closed reactor method uses pre-measured, ready-to-use reagents in fresh vials, which eliminates problems associated with preparation, standardisation and uniformity found in open reflux. Andy said, "In the laboratory, reagents are made manually in small batches, due to which even a small difference between the batches could impact results."
Chris argued that preparation of reagents for open reflux is ultimately a costly exercise because customers pay only for the analysis. He said, "If my technician is spending three hours a day preparing the acid dichromate for the reflux, the FAS for titration, it is three hours of work that he is not getting paid for. I would prefer him spending productive time doing analysis and generating revenues."
A major disadvantage of the open reflux method is cross contamination that can result from the presence of even minute traces of organic matter on the glassware, causing errors in the test results. However, this is a non-issue for the closed reactor method where ready-made and relatively fresh reagents and the glassware are used.
Another disadvantage of open reflux is that it measures the dichromate indirectly. "In the classical method, we are measuring the residual dichromate reacting with the titrant to produce the colour change. Instead of reading the residual dichromate directly, we are looking at the reaction between yet another reagent with the residual dichromate, adding yet another bit of chemistry into the equation," said Andy.
Simple and quick
Chris noted that the closed reactor method (with its spectrophotometers and compact heating blocks), has stripped away the complexity and human error associated with the standard method and brought it down to the simplicity of 'school boy chemistry,' so much so that entry level chemists can do COD tests with minimal training. Open reflux, on the other hand, requires technicians versatile in using glassware, and precise in performing titrations and manipulations with the maths.
If the throughput of COD tests is very high, the open reflux method can be a headache. David elaborated, "For example, if a lab does 25 tests of COD a day, seven days a week, 365 days a year, open reflux can be very time consuming. Using the Hach system, in two hours, you can digest 25 samples simultaneously in the heating block; you also have repeat robustness. But in open reflux, you can only have a block of six at a time -five samples plus one blank. Thus, for 25 samples, you will need five cycles of two hours which implies a minimum 10 hours for digestion compared a single cycle of two hours for test-in-tube. With open reflux method, laboratories are restricted in terms of the number of samples they can test at a time."
Chris pointed out that laboratories today want to get back to their customers as quickly as possible without compromising the integrity of the results. At the same time, WWTP operators are concerned about running their plants efficiently and meeting regulatory on discharges because there is a cost element attached to both aspects. "For example, if a WWTP designed to accept 3,000 COD gets 4,000 COD, the system will automatically shut down. In many countries, breaching the COD limits in wastewater discharge invite extensive fines. The faster plant operators understand the problem, the quicker they can rectify it. COD test using closed reactor method serve their needs best," said Chris.
David noted that WWTPs lacking the funds, equipment and personnel for carrying out open reflux method or having small laboratories stand to gain using the closed reactor method. He said, "By using Hach, they can do a quick test and obtain a fair idea of existing problems or those in the making. Of course, they may need to send their samples to a bigger laboratory to validate the test results."
Andy made a pertinent point that WWTPs are run by technicians who are not chemists. "They don't have the competence of a trained chemist that is essential to conducting the open reflux test. Of course, they may do it after a lot of training, but again, open reflux is a classic chemistry-based methodology. On the other hand, the closed reactor method is simple enough for a WWTP technician or boiler operator to do COD tests and produce reliable results," he observed.
However, the school chemistry simplicity of the closed reactor doesn't mean that bench chemists are redundant. Chris pointed out that equipment constitutes only five per cent of the effort, and can never substitute the knowledge and experience that chemists bring into the analysis. He said, "It is up to the chemist to make the equipment work to his requirement and not the other way round."
Ultimately, the tests and methods for determining oxygen demand are interlinked. Chris explained, "If you have a COD of 2,000, you cannot have a BOD of 5,000. It comes down to how intelligently the chemist analyses the final results, and his grasp of the ball park numbers. Once a chemist is fully into COD testing, he can come up with the right results, irrespective of the method used. But with the closed reactor method, you get results faster."
The dedicated tests inherent to the closed reactor system help avoid mix-ups. Andy said, "If you mix up samples during titration, the only option available is a repeat of the digestion process. The problem can be avoided with the closed reactor system with its pre-packaged reagent vials."
Slow to catch up
Europe and the US are using the closed reactor method to measure COD and also parameters such as nitrate, nitrite, phosphate, and ammonia. In fact, the closed reactor method is US EPA approved, accepted or equivalent. However, it hasn't found takers in the region especially among the influential regulatory authorities. Dubai Municipality as well as the Regulation & Supervision Bureau (RSB) in Abu Dhabi are reluctant to analyse results obtained by the closed reactor method on the grounds that the test method does not incorporate the issue of homogeneity.
David explained, "COD measurement is made as settled COD or Total COD which includes suspended solids. In this region, samples are taken directly at the inlet prior to any screening or grit removal, resulting in high COD value. The rest of the world considers only settled or filtered COD (The sample is so prepared that the solids are either removed or homogenised) because the wastewater treatment plant process is only concerned about the water flowing through the plant, and not the solids that get removed in the pre-treatment stage through screens, settling tanks, filtration beds, clarifiers."
Typically, if a sample contains suspended solids, it can be filtered with a 0.45 micron filter that takes out almost every thing that is not in-solution. Another option is to grind the sample in an agitator, which will release all the organics, till a stage is reached where they are indistinguishable from the sample liquid. The homogeneity principle is equally applicable to both closed and open reflux methods.
Chris observed that the chemist's approach to tackling the sample is important. He said, "The chemist is trained to eliminate errors associated with settled and total COD. If the sample has suspended solids in it, he is expected to homogenise it and perform both settled and total COD tests. We compare both the results because the suspended solids have chemicals that are released into the water from time to time."
The argument advanced by the authorities in the region about measuring total COD while the rest of the world measures either settled or filtered COD is that they are unable to fathom what is coming into the treatment works. For example, tanker drivers from many industry sectors dispose their waste at the same plants. Therefore, they believe that measuring COD before the process will give them a true reading.
Andy felt that another reason why regulatory bodies here are cautious to shift to the closed reactor method is that it is a relatively new technology to the region. Though well established and accepted in US/Europe, in the Middle East, the closed reactor method represents a step into the unknown. He explained, "The regulatory bodies here are still evolving, and understandably, don't want to make mistakes and select the wrong or inappropriate method. The classic approach, in the case of COD, is the open reflux method. It works every time, is robust, and adaptable to the environment.
Andy also pointed to the high salinity in the water as a restraining factor against the closed reactor method for COD. He said, "In this region, we often get water samples with very high salinity. The presence of chloride in the sample interferes with the COD test. At elevated levels, chloride becomes an additive interference. During reflux, chloride is oxidised by dichromate to chlorine, giving rise to apparently high COD. Anything over 1,800-ppm starts the add-on effect; for every 20 mg of chloride, you get approximately 50 mg apparent increase in COD. The only solution is to add extra masking agents for the chloride."
In the open reflux method, mercuric sulphate can be added to suppress the oxidation of chloride ions. But in closed reflux, the amount of reagents is pre-measured and fixed, and if mercury-free reagents are being used, there would be no masking at all. Hence, regulatory authorities are concerned that in closed reactor method there might not be enough masking agents to nullify the chloride interferenc.
Andy said, "The challenge is to convince them that the closed reactor method gives results that are equivalent to the open reflux, the only proviso being that chemists should take into account the interference ions. For example, we have instituted a system wherein the first thing checked for in all the samples is salinity."
Closed mindset?
David said that local authorities, while amenable to using the closed reactor for lower values of COD, fear that the laboratories would end up confused on using the two methods depending on where the measurement is taken. This concern has led to a rigid approach on the part of some municipalities to accrediting labs only for the open reflux method. "For labs to start using closed reactor method, regulatory approval and notification is a must because at present, authorities here can reject results other than the ones obtained through open reflux," said Chris.
Another mindset issue with the authorities is equating length of time spend on tests with seriousness and better quality results. Such a perception poses a change management hurdle to the acceptance of the closed reactor method over open reflux.
However, choosing between open and closed reflux is an open decision in private contracts. Chris said, "We carried out boiler water tests for silica, nitrate, nitrite and iron using the closed reactor method for an IWPP in Abu Dhabi because the test volumes were high. Using the standard method would have taken us several years."
Chris also clarified that open reflux isn't outdated; rather, it is the standard technique which is used and accepted all over the world even today; the same applies to the closed reactor method too, barring exceptions like the UAE. He stressed that companies like Hach Lange would have to play a major role in educating their clients on the advantages and disadvantages related to using the closed reactor method.
The preference for closed reactor over open reflux in the developed countries has an environmental twist too. A key issue with the standard method, apart from the time and preparation involved, is environmental pollution as the method uses large quantity of mercury compared to closed reactor method. While mercury is used in both open reflux and closed reactor methods, the latter uses less of the sample (water), hence, it uses less mercury. This again raises the issue of whether the closed reactor method deliver true analysis of the water being measured, as they use little amounts (5-mL) whereas open reactor uses 90 per cent more. However, David pointed out that both the Americans and Europeans are happy with Hach's results and have officially recognised the method in their guide lines.
He felt there is no reason why a method which has been approved by bodies like the US EPA and UK EA and recognised by accreditation bodies in most countries cannot work here. "We want to bring organisations here up to date with what is accepted across the world. Telling the laboratories that there is only one way to do the test is unfair. I believe accredited laboratories should be trusted to decide which method - o-pen reflux or closed reactor - is best suited for the work in hand, taking into account the test volumes and urgency at the customer's end," he said.
Chris added that what the client plans to do with the test results is also a deciding factor. He explained, "If the client intends to use the results for litigation, I would advise them to do both the closed reactor and the standard methods. The person sitting across the table or in judgement should be made to understand that the closed reactor method is internationally accepted, and the lab issuing the result is doing so based on its responsibility as an accredited/independent laboratory."
Apart from hurdles posed by regulatory ambivalence and high salinity levels in the sample, the closed reactor method must also contend with tag of being costly while open reflux is seen as more economical. Andy said, "We buy a 2.5 litre bottle of acid dichromate and typically use 18-mL in the standard method; in closed reactor, you only get 2-3 mL of dichromate in the reagent vials, yet they cost a lot of money. While open reflux is labour intensive, in the UAE labour costs are low."
David argued that for a single COD test, open reflux is the cheaper option, but if the throughput is high, closed reactor is a more economical option. He blamed underutilisation of spectrophotometers by the laboratories for the high-cost perception. "Using a basic spectrometer like Hach DR5000, you can analyse a whole range of inorganics, in addition to COD and BOD. You can do small volumes of metals too. Laboratories can also use our cuvettes for round robin tests that are an important element of external quality assurance. By making full use of the capabilities and range of tests available, they can get a good return on investment," he said.
Chris pointed to difficulties associated with procuring reagents locally and the expiry of pre-packaged chemicals as barriers to adoption of the closed reactor method. David admitted that bringing chemicals into this region is not easy as customs clearance requires substantial paperwork, which interferes with the distribution logistics. Hach is working with customers to determine the right stocking options, and with regional distributors to ensure there is sufficient stocking of fast moving reagents like COD, nitrite, nitrate, phosphate and ammonia. On the shelf life issue, David admitted that the shelf life of products depends on whether they are stored at the correct temperature. "But all of the above will have at least 12 month shelf life or longer," he assured.
In the end, no analytical method or its variants can claim to be error-free. Andy said, "All analytical procedures carry a measurement uncertainty factor, and the closed reactor method cannot be an exception. Even if you put two samples that - in an ideal world - are identical, you will still get some differences between them." Chris said that a test result represents a 95 per cent confidence level. "We are always striving to better that, but sometimes, the environment or the chemicals or the chemists themselves bring in an element of uncertainty. With the closed reactor method, we try to cut down on these errors."
© H2O 2009
