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Incorrect Application of the BOD Test

  • By Peter Maier
  • Jun 07, 2010

Peter Maier, Ph.D., P.E.

The two waste components of sewage are carbonaceous (fecal) and nitrogenous (urine and protein). Fecal waste feeds heterotrophic bacteria, which causes a carbonaceous biochemical oxygen demand (C-BOD), while urine and protein are picked up by autotrophic bacteria, resulting in a nitrogenous demand or N-BOD. The latter also can be measured with the Total Kjeldahl Nitrogen (TKN) test.

The original technical literature assumed that, during the first five days of the BOD test, only heterotrophic bacteria are active and that the average strength of raw sewage has a BOD5 value of 200 milligrams per liter (mg/l) with a TKN value of 40 mg/l. The total BOD exerted by raw sewage, if discharged into open water, is 1.5 x 200 (300) + 4.6 x 40 (184) = 488 mg/l and not 200 mg/l as some still think.

This is well described in any older textbook or from the U.S. Geological Survey (pdf). Unfortunately, when regulations were initiated, many assumed that the oxygen required to stabilize all organic matter in sewage was equal to the BOD5 value or 200 mg/l, thereby ignoring not only part of the oxygen required by heterotrophic bacteria, but all of the oxygen required by the autotrophic bacteria. This was and still is commonly done in most countries.

The Clean Water Act sought to eliminate all pollution by 1985, which had to be achieved by demanding best available sewage treatment. Since "complete" or 100 percent treatment was not economically feasible, Congress settled for "secondary treatment," which was considered to be equal to 85 percent treatment. With the assumption that raw sewage contains 200 mg/l BOD5, the U.S. Environmental Protection Agency in 1972 used the BOD5 test in its definition of secondary treatment and set the BOD5 effluent standard at less than 30 mg/l (85 percent of 200), without realizing that the BOD5 test only represents 40 percent of the total BOD. In fact, instead of demanding 85 percent treatment, EPA actually settled for 34 percent (85 percent of 40 percent = 34 percent) BOD treatment in its NPDES (National Pollution Discharge Elimination System) permits.

That the autotrophic bacteria are not active during the first five days may be valid for very fresh sewage, but it is not valid when the test is applied on sewage exposed to nature and on treated sewage. Many plants in the '70s violated their permits because of high BOD5 test readings, while in many cases (60 percent), this was caused by the fact that a large portion of the BOD5 test reading was N-BOD5, a pollutant not required to be treated.

In 1984, EPA acknowledged the problems caused by the BOD5 test and, by administrative rule, allowed the use of C-BOD5 test. This inhibited test adds a special chemical that kills autotrophic bacteria so that they do not contribute to the test result. This administrative ruling not only lowered treatment from 100 percent to a measly 34 percent BOD treatment but also officially ignored all the water pollution caused by nitrogenous waste. This actual human waste (urea, amino acids, ammonia, and nitrates) not only exerts an oxygen demand (N-BOD), but it also is a fertilizer for algae growth that contributes to eutrophication, causing dead zones and red tides.

The only correct application of the BOD test is:

Total BOD = 1.5 x C-BOD5 + 4.6 x TKN

In 1993, local environmental groups and I petitioned EPA to modify its secondary treatment requirement as it is technically feasible and suggested the following:

C-BOD5 less than 15 mg/l

TKN less than 5 mg/l

NH3-N less than 1.0 mg/l

NO3-N less than 5 mg/l

Total P less than 2 mg/l

(All measured over 7-day averages, representing better than 90 percent BOD treatment.)

Compared to present treatment requirements, this seems very strict, but these values can be achieved easily using simple oxidation ditch processes, with the additional advantage that, due to its long solid retention time, disinfection is not necessary, eliminating cost and such issues as carcinogenic disinfection byproducts.

Incorrect BOD testing sadly is not the only problem as most of the engineering used to design and built new sewage treatment plants, is based on tradition. A tradition that, during the past century, was solely focused on the prevention of nuisances, while not recognizing that sewage is an essential part of the natural element recycling processes of carbon, hydrogen, oxygen and nitrogen, which are the basis of all biological life on Earth.

Canada is considering new regulations and requires an effluent C-BOD of less than 25 mg/l, (the U.S. C-BOD5 limit is 25 mg/l) while again ignoring N-BOD and the fact that nitrogenous waste is a fertilizer and increasingly a major problem. Hopefully, the Canadian authorities will bite the bullet and correct this essential water pollution test so we finally will know how sewage plants operate and what their effluent waste loadings are on receiving waters.

About the Author

Peter Maier, Ph.D., P.E., has been speaking against the incorrect use of BOD since 1983. To learn more, visit www.petermaier.net.

Comments

Fri, Jun 11, 2010 Dr Edo McGowan

Dr. Maier mentions spread of pathogens, and the state regulator, above, notes that bacteria criteria have to be met at the end of the pipe. Thus the questions remain unanswered, do sewer plants really release effluent that is generally safe. I would contend not---not safe for humans and not safe for the environment, be it riverine, lacustrine, or marine. This was basically confirmed by the US EPA itself. In the later 1970s, the US EPA did do a large study on survival of bacteria and in particular survival and generation of antibiotic resistant bacteria as part of sewage processing, see : http://aem.asm.org/cgi/reprint/43/2/371.pdf. That study noted----“It is evident that antibiotic resistant coliforms are entering the aquatic environment via treated municipal wastewater effluence. These organisms may subsequently transfer this resistance to pathogenic organisms, resulting in reduced efficacy of antimicrobial chemotherapy. Several researchers have pointed out that waste water, treated or untreated, is a primary contributor of bacteria to the aquatic ecosystem. Studies have demonstrate that significant numbers of mutli-drug-resistant coliforms occur in rivers, bays, bathing beaches, and coastal canals. Conventional wastewater purification methods without disinfection are not adequate for removal of antibiotic-resistant bacteria. If we look at finished reclaimed or recycled water from sewage effluent, which by the way receives far greater treatment than typical effluents, we find that the standards normally used fail to protect public health. The Rose report notes such, see: http://www.werf.org/pdf/00PUM2T.pdf. Harwood, et al (http://aem.asm.org/cgi/content/short/71/6/3163) republished the results and the WERF study and in discussing finished recycled water demonstrated the following:------- "microorganisms were detected in disinfected effluent samples at the following frequencies: total coliforms, 63%; fecal coliforms, 27%; enterococci, 27%; C. perfringens, 61%; F-specific coliphages, 40%; and enteric viruses, 31%. Cryptosporidium oocysts and Giardia cysts were detected in 70% and 80%, respectively, of reclaimed water samples. The failure of measurements of single indicator organism to correlate with pathogens suggests that public health is not adequately protected by simple monitoring schemes based on detection of a single indicator, particularly at the detection limits routinely employed." Matthew W. Chang looked at chlorine's impact on Staph aureus and noted that chlorine enhanced virulence. He dealt mainly with MRSA. Thus, we find that while US EPA has known about the failure of proper BOD testing and the failure to curb serious antibiotic resistant pathogens as produced by current sewage processing, it has apparently sat on its hands and done nothing. I think that is the critical message here, US EPA is clientele captured by the very industry it was to regulate.

Wed, Jun 9, 2010 Dennis Sarpen York,PA

Most of the WWTPs we are familiar with have been required to discharge effluent with very low ammonia nitrogen and C-BOD concentrations for some time. Recently, the Chesapeake Bay Strategy required further reductions in TP and TN as well which usually results in further reductions in C-BOD. Agricultural contributions of TN, TP and suspended solids to the Bay from Pennsylvania however, are much higher than what is contributed by WWTPs. Until the agricultural contribution issue is resolved it's difficult to see how the water of the Bay will improve.

Tue, Jun 8, 2010 Peter Maier Utah

Human waste (municipal) sewage, when dumped into open waters will besides causing turbidity (measured as Suspended Solids): 1. Exert a biochemical oxygen demand (BOD), affecting the dissolved oxygen concentration. 2. Stimulate algae and other aquatic life, due to nitrogen and phosphorus. 3. Potential spread of pathogenic bacteria. 4. Can cause toxicity for fish, due to high NH3 concentrations. 5. Pharmaceutical and Personal Health Products. When Congress passed the Clean Water Act, its intent, as Senator Muskie stated in 1972, not to use our rivers to treat our sewage any longer. I therefore have to assume that the goal of the Act should have addressed (eliminate) basically all the above mentioned negative impacts. I know hindsight is 20/20, but when we now evaluate how effective EPA’s implementation of the CWA has been on these four impacts, than have to come to the following conclusions, besides the turbidity issue (SS). 1. Due to the incorrect use of the BOD5 test we ignored the N-BOD, which represents 40% of this type of pollution. 2. This issue was simple ignored when EPA implemented the CWA 3. EPA originally did have an e-coli bacteria standard, but dropped this in 1978 due to the fact that chlorination of treated sewage caused disinfection by products (DBP), some considered carcinogenic. 4. This issue was only addressed for very sensitive waters and often incorrectly addressed, since toxicity of fish depends on the free NH3 concentration (not NH4), which in turn depends on the pH of the water. 5. This issue obviously was at that time never even on the radar screen, but still hardly is getting any attention. We, as we now do, can keep blaming other causes (non-point sources, farmers), but wouldn’t it be better to correct what we are doing now and at least implement the CWA as it so clearly was intended and promised to the American public. Comment on disinfection: E-coli bacteria live in the intestines of vertebrates, while fecal e-coli live in the intestines of warm blooded vertebrates. Their presence is only used as an indication of potential contamination with human feces. Long SRT processes create a lean and very competitive environment for these bacteria to survive and that is probably the reason that effluents of oxidation ditches show a very low count. The test still does not prove any non presence of pathogenic bacteria, but that is another issue.

Tue, Jun 8, 2010 State Regulator

The comment about not needing to disinfect long detention treatment processes is questionable. EPA does not allow mixing zones for bacteria. Therefore, bacteria criteria have to be met at the end of the pipe. With E. coli as the indicator, those values range from around 100 - 500 cfu/100 mL. We do not routinely see bacteria concentrations that low without supplemental disinfection. Therefore, we would be surprised to see any designer not include supplemental disinfection if disinfection is required.

Tue, Jun 8, 2010 cleanwater USA

While Dr. Maier is primarily correct ,he is ignoring the fact that most state EPAs, including Ohio are requiring tertiary levels of treatment. There are NH4 levels set at less than 5 mg/l both summer and winter and the C-BODs are less than 10 mg/l. I think the full story should be told not just half.

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