Biochemical Oxygen Demand (BOD) Test
NEWEA - New England Water Environment Association
Tim Loftus, Chief Chemist and Industrial Pretreatment Coordinator at the Webster-Dudley Wastewater Treatment
and a member of NEWEA Lab Practices Committee, states, "Obviously, the BOD test is not the most scientifically
based test we perform. As such, it has many problems associated with it. The most significant problem is that the
results come five days after the fact. By that time you’ve already discharged any problem wastewater. Then the
variability of the seed must be constantly monitored. Sometimes the bacteria wants to work, sometimes they don’t.
Again, by the time you find out, it is five days too late. Some analysts support the use of respirometry to give more
timely results, but regulatory agencies have not fully accepted the methods yet so NPDES permits continue to specify
that we perform the standard 5-day BOD test."
Loftus adds, "There is no absolute BOD value of a sample as there would be for say copper or lead. BOD results are
test defined. In other words, BOD values are based on the parameters of the test method, not on any “true” BOD
value."
Peter Maier, retired profession waste water engineer:
It would only take half an hour to verify how the BOD test should be done. So, why than is everybody looking the other
direction, especially since we have no idea how sewage treatment plants perform and what their waste loading is on a
receiving water body.
First the composition of sludge from a sewage treatment plant depends on the solids that are in the raw sewage.
When you have an activated sludge process, only the biodegradable solids are used by the bacteria and converted in
carbon dioxide and partly used to grow new bacteria. The solids that are inert or non-biodegradable and removed by
the activated sludge settling process remain what they are.
Only part of the activated sludge is actually living bacteria (probably less then 20%), most is some organic matter (like
glue) bacteria use to capture and hold on to their food. How far the biodegradable solids are used is also a function
of the biological process maintained in the system, but in general the longer the activated sludge is kept in the system
the more biodegradable matter is used.
Shorter detention processes result in less biodegrading of biodegradable matter and thus more of it left in the sludge,
where it will rot and stink.
The largest mistake wastewater engineers (private and government) made in the past was not to apply the BOD test
correctly and as consequence we never have been able to evaluate the real treatment performances of the different
sewage treatment systems. You could do what you want, because a real cost-benefit ratio analysis was impossible.
Especially as in many countries. like in the USA, nitrogenous waste is as another consequence is not addressed. So
now we deal with the eutrphicaton of our open waters and refuse to consider how much we are causing this by using
our rivers still as urinals.
The second largest mistake, especially in the US is the fact that Congress allocated federal funding (85%) for the
construction of new sewage treatment plants. This led to cities building facilities, supposedly for the future, without any
justification, while offering this additional capacity to industries. The claim now is that these industries have to provide
a certain pre-treatment, but those requirements often have little to do, with what type of waste is in their wastewater.
Most cities are very secretive about the arrangements they have with industries and these industries now refuse to
treat their own wastewater.
If any matter is discharged in municipal systems, then, in preventing general contamination of sewage and sludge, this
matter should be removed.prior to discharge. As head of the industrial wastewater section of a large engineering firm,
we first looked at the husbandry of an industry and often were able allocate the major culprits and reduce in general
their wastewater flows, before even designing a treatment system..
I had heard about the excuse of using only the 5 day value of the BOD test, because that was the time it took for the
water in the Thames River to flow from London to the North sea. The BOD test was developed around 1920 to
determine the dissolved oxygen sag in rivers when sewage was dumped in a river. This was in the twenties in
England, but the BOD test is very well described in the 'old' technical literature and if you like to know more visit my
website www.petermaier.net and pull up the BOD test description in the Technical PDF section.
The reason that at that time the first five days of the test only measured the oxygen use by the heterotrophic
bacteria (carbonaceous = fecal waste) and not the autotrophic bacteria (nitrifiers) using oxygen for feeding on
nitrogenous (urine and part of the proteins) waste was probably because they were using very fresh sewage
(probably produced in the lab) and there are a lot of heterotrophic bacteria in our fecal waste, but our urine does not
contain any bacteria, they have to come from the outside. It is still assumed that the first 5 days of the BOD test
(BOD5) result from the oxygen use by heterotrophic bacteria, while tests have shown that this is not always the case,
as EPA finally assumed in the 1984 ruling based on test results on effluents, when a large portion of the 5 day
reading was already nitrogenous BOD and which prior to the ruling had penalized sewage treatment plants of being
out of compliance with the NPDES permits.
It was estimated that after EPA allowed the inhibited BOD test( the same BOD test, except with a certain chemical
added to kill selectively only the autotrophic bacteria (nitrifiers)) 60% of all the swage treatment plants out of
compliance got into compliance by adding this few cents of chemicals to the test. By doing so, EPA lowered the goal
of the Clean Water Act from elimination (100% treatment) to a measly 85% of 40% = 35 % treatment and ignored all
the pollution caused by nitrogenous (urine and part of the protein) waste. EPA did lower the goals without even
informing the Congress.
All this is very easy to verify, but nobody seems to care. And the 1920 assumption that the 5 day value is solely
carbonaceous is also still maintained, while several test on raw sewage, especially in warmer climates and sewer
systems with long detention times, showed that this assumption is wrong, with the consequence that sewage treatment
plants are designed to treat the wrong waste and this represents a horrendous waste of public funds, but again
nobody seems to care.
Utah State's Science Council in 1984, unanimously recommended to the Governor to correct the BOD test procedure,
but fearing he would lose a lot of federal funds, the Governor did not accept the recommendation. Ever since I have
asked newly elected governors to accept the 1984 Councils' recommendations, but every time my request was
passed on to the Dept. of Environmental Quality, who simple refuses to do so. I had hoped that Drew Bohan would do
something as chairman of the Ocean Project board, but he too clearly does not want to do anything either. Hopefully
somebody will show enough interest in our environment and do something about this.