By Oliver Grievson
Biochemical oxygen demand, or BOD, is probably one of the most- and least-known parameters in the wastewater industry. Everyone has heard of it, yet most people would have trouble defining what it is exactly; it’s mentioned a lot and very rarely defined. For the sake of this article, it can be defined as:
The amount of oxygen required by aerobic microorganisms to decompose the organic matter in a sample water – such as that polluted by sewage – and used as a measure of the degree of water pollution
We may also gain understanding by defining the test (Clesceri et al., 1998):
The molecular oxygen utilized during a specified incubation period for the biochemical degradation of organic material (carbonaceous demand) and the oxygen used to oxidize inorganic material such as sulphides and ferrous iron.
I have always understood it by the analytical method that is used to measure it. In its simplest form, you measure the amount of oxygen consumed by bacteria in a five-day period in a jar that’s been put in the dark at 20°C for five days. Taking all of this aside, it is a method that has defined the water industry for over a hundred years now, first devised by the Royal Commission in 1908. It is a measure of the organic strength of sewage via oxygen utilization rate, and it is still a very useful parameter to this day.
But the test has its limitations. Probably chief among these is that it takes five days to analyze, and thus has limited operational usefulness and cannot be used as a measurable parameter for control. Furthermore, the test has limited accuracy and repeatability at concentrations less than 5 mg/L O2 and is not valid unless at least 2 mg/L of O2 has been consumed (Clesceri et al., 1998)
This last limitation is probably the most telling as discharge standards in wastewater treatment plants are getting lower and lower with concentrations <10 mg/L; therefore the limitations of the testing method have the potential to cause more and more problems. The question that has to be asked is: What are the alternatives to measuring BOD in the modern wastewater industry?
There are several, including:
- Chemical oxygen demand (COD)
- Total organic carbon (TOC)
- Tryptophan and tryptophan-like substances
Chemical oxygen demand (COD) is a technique that has been used for many years and is a regulated parameter in Europe under the Urban Wastewater Treatment Directive. It is an indirect measurement of the amount of organic compounds in water, and in crude (domestic) wastewater is often approximately 2-½ times the BOD. It is a technique that has long been associated with BOD and, with rapid online methods arising in the past 10 years, can be used as a measurable parameter for control. The cost of the instrumentation has meant that the online measurement technique has not been particularly widespread across the domestic wastewater industry, although it has been prevalent in industrial wastewater treatment.
Respirometry is probably the closest of all of the alternative measures of measuring organic load to BOD. The technique is a direct measurement of the oxygen consumed and was devised in the UK in the 1950s. However, execution has been restricted to benchtop respirometers, as it is simply too complex for wastewater operators. This has started to change in the past five to 10 years with the onset of online sonde techniques being used and, more recently, floating respirometers for wastewater treatment plants.
Total organic carbon (TOC) as an alternative to BOD came into the wastewater industry in 2013 when the Water Environment Federation (WEF) Laboratory Practices Committee hosted a webcast where three managers in the U.S. water industry presented the case. This was done by measuring the relationship between TOC and BOD for several years to prove that the BOD could be implied by measuring TOC, and that the relationship was stable. The findings were enough to persuade local regulatory authorities that the TOC test was a valid alternative to BOD with comparable results. The advantage with TOC is that results are measurable within two hours instead of five days, allowing for online analysers and sensors as a real alternative to the BOD test.
Tryptophan and tryptophan-like substances using fluorescence spectrometry is the last potential alternative to the BOD test. It is a rapid detection technique that in the past five years has been developed as a method to imply the organic pollution, as tryptophan (amongst other amino acids) is a part of the human diet. The technique has been plagued with problems through interferences with chlorophyll and compounds such as diesel (depending on the individual sensors). At the 2015 Sensing in Water Conference, it was concluded that tryptophan sensors are generally good at detecting crude pollution in the environment where a discharge has occurred, but that further development is needed if the sensor is going to be used at lower levels typical of the effluents from wastewater treatment plants.
All of the above alternative methods of measuring the organic pollution load in wastewater have potential to replace BOD as the measurement of choice in the wastewater industry. Currently, they may offer a level of measurement that will allow water companies to at least monitor the performance of their treatment process and, in some cases, offer an element of control that can help the water industry (a) improve compliance, and (b) improve efficiency. However, some alternatives (namely the tryptophan method) need further development if they are to become mainstream techniques.