How To Test A Sewage Treatment Plant Properly.
By: Peter Maier, PhD,PE
Date: February 2017
When EPA in 1973 established secondary sewage treatment standards to implement the Clean Water Act (CWA) or 85% treatment, it used mainly two testing methods for its NPDES (National Pollution Discharge Elimination System) permits.
1. The BOD5 (Biochemical Oxygen Demand after 5 days) test.
2. The SS (Suspended Solids) test.
Since both test values in raw sewage were assumed to be 200 mg/l, the treated sewage, after secondary treatment, had to be less than 30 mg/l.
Most NPDES permits only set limits for BOD5, while those, familiar with BOD testing, know that BOD5 testing without any nitrogen data is not only technically incorrect, but can be very misleading. As EPA found out in the seventies, when many NPDES permit holders exceeded their BOD5< 30 mg/l limit and were fined, while in fact these alleged violators were treating their sewage better than was required by their permits.
Acknowledging the testing problem and in spite of a correct description of BOD testing in the 1983 Federal Register, EPA by administrative rule in 1984 allowed the C-BOD5 or inhibited BOD5 test to meet the secondary treatment standards, except lowering the limit to < 25 mg/l, still without any standards for remaining C-BOD and N-BOD, as well ignoring the fact that nitrogen in all its presences is a fertilizer for algae.
It was estimated that 60% of the prior alleged permit violators got into compliance by adding a few cents of a special chemical to their BOD5 test, killing those bacteria that oxidize ammonia, hence not any longer measuring N-BOD5 in the BOD5 test.
By doing so, EPA, without informing Congress, not only officially still ignored about 50 % of the pollution in sewage exerting an oxygen demand, but now also ‘officially’ ignored all the nitrogenous (urine and protein) waste, while this waste, besides exerting an oxygen demand, also is a fertilizer for algae. All clearly in violation of the intend of the CWA.
For the history and a description of the BOD test read : wp.me/p5COh2-25.
Why Correct Testing Is Important.
Although many sewage treatment do not have any nitrogen test data, as it is not required by their permits, this data is essential to establish how sewage is treated and to know how nitrogenous waste is broken down through the different breakdown processes.
Urine and proteins are organic forms of nitrogen that first hydrolyze into ammonia. This, when bacteria are present, is then oxidized into nitrites and nitrates, hence the N-BOD. However, in all these forms, being a reactive nitrogen (single atom), it is a fertilizer for algae. Only if bacteria denitrify the nitrates into nitrogen gas, is this form of reactive nitrogen not any longer available to synthesize organic matter.
Proper testing is not only important to evaluate how sewage is treated, but also to determine what the actual waste loading is on receiving waterbodies, especially important in TMDL (Total Maximum Daily Load) programs.
Even more important to know what is in treated sewage, because many receiving open waters are now used as as a source for drinking water, while its treatment still is limited to the removal of suspended solids. The presence of DBP’s (Disinfection By Products), PPHP’s (Pharmaceutical and Personal Health Products) and nitrates in drinking water originate also from sewage treatment plants. Not only damaging for humans, but equally damaging to aquatic life, the main reason EPA dropped the national effluent disinfection requirement in 1978.
The Flint water crisis was initially caused by using the Flint River, although the consequences (first odor and taste, followed by lead and iron problems) quickly overshadowed the fact that the Flint River is still polluted, as unfortunately still are most open waters.
Treatment Capacities of Sewage Treatment Plants.
Sewage treatment plants are designed for two different treatment capacities.
Hydraulic capacity, based on the peak hourly flow the plant can receive.
Pollution capacity, based on the average daily flow and the average daily concentration of pollution.
When plant capacities are mentioned, their capacity is mostly based on the average daily flow, while many of the components are designed on the maximum hourly flow expected, often expressed as a factor (2,3 or more) times average daily flow. When a city operates a 7 MGD (Million Gallons per Day) sewage treatment plant it probably can handle a peak hourly flow of 21 MGD, all depending on the sewer system.
The design of the pollution capacities of a sewage treatment plant, is still often based on the assumed raw sewage concentration of 200 mg/l for both BOD5 and SS, while on many situations did is not the case, hence the reason why proper testing of influents is important in establishing a sewage treatment plants treatment efficiency.
Essential Testing To Evaluate How Sewage Is Treated.
Besides some other testing, more appropriate for daily operation (flow rate, pH, COD,TSS), both influent and effluent should be periodically tested for:
C-BOD5 (inhibited BOD5 test), TKN (Total Kjeldahl Nitrogen), Ammonia, NO2 (Nitrite), NO3 (Nitrate), TP (Total Phosphorus) and fecal E-coli test.
This data only will provide the correct data to evaluate how sewage is treated in a sewage treatmen plant and what the effluent waste loading is on receiving water bodies.
It will also provide the essential information dealing with the breakdown processes of nitrogenous waste, important to know which process happened in the sewage treatment plant and which one will happen after the effluent is discharged into open waters.
In case TDS (Total Dissolved Solids) test is used, one has to make sure that the method used also includes organic solids. Most TDS meters, based on the electrical conductivity, only measure the concentration of minerals.
Salt Lake City Case.
In 1983, Salt Lake City passed $125 million sewage treatment expansion plan, because the facility was overloaded, all based solely on BOD5 and TSS data. When in 1984 Utah State Science Council unanimously recommended that nitrogen data was required (a recommendation rejected by the Governor) one city councilman demanded that the City would test their facility properly. They allowed one day of proper testing, but that was already enough to prove that the facility was not overloaded, but actually under-loaded for the waste it was designed for and not capable to treat the actual nitrogenous waste it was receiving. Half the incoming BOD5 was already N-BOD5. Since the proposed expansion also would not have been be able to treat the incoming nitrogenous waste, the City never expanded their facility.
Similar influent testing results in Chicago and San Diego, showed similar results, proving that many sewage treatment plants were designed to treat the wrong waste. Probably one of the main reasons of the reluctance to test properly.