While somewhat a challenge to run, total Kjeldahl nitrogen (TKN) has always been one of my favorite tests. I started my water treatment career examining petroleum refinery wastewater where operators complained that ammonia removal was not as expected after treatment in the first aeration basin. Inlet ammonia after the EQ tank was often 25 mg/L and after the first aeration basin was still in the 20 - 22 mg/L range. So what was happening?
To understand what was happening, we need to look at the nitrogen cycle. Wastewater contains a blend of nitrogenous compounds including proteins, amines, amino acids, urea, nitrate, nitrite, and ammonia. Because of permitting, we normally just look at ammonia, nitrite, and nitrate. But at the influent to a wastewater system we should pay more attention total nitrogen.
Total Kjeldahl nitrogen (TKN) contains all organic, ammonia, and ammonium in the influent. Through a digestion step, organic nitrogen is converted into ammonium which is then extracted via distillation. With all organic nitrogen converted into ammonia, we can calculate total organic nitrogen (TON), ammonia and total Kjeldahl nitrogen. In biological treatment, the TON is eventually converted into biomass and ammonia/ammonium.
So in my early refinery case, the influent TKN was actually 45 mg/L and through testing and performing a mass balance, we found that we were removing an average of 24 mg/L across the first aeration basin - we did this by evaluating TKN in & out, Ammonia in & out, and confirming results by looking at nitrite & nitrate outlet concentrations.
So if your ammonia removal efficiency appears low or even if ammonia increases across a basin, you should look to TKN along with ammonia numbers to better describe what happens inside the biological treatment unit.
Extracellular polymeric substances (EPS) - their role in flocculation, settling rates, dewatering, and non-filamentous bulking
i have been doing a lot or work on floc formation and how EPS fluctuates in wastewater systems. Since floc and biofilms are both microbial aggregates of living and dead microbial cells held in a matrix of extra cellular polymeric substances (EPS), I though a general guide to EPS would be useful to a lot of people faced with floc settling problems including non-filamentous or viscous bulking.
To microbes, the aggregate (colony) serves four distinct functions:
The EPS is composed of multiple compounds including (from highest to lowest percentage):
Due to the cells and the EPS having a net anionic charge, divalent cations (Ca++. Mg++) are very important to forming dense, stable aggregates. In most wastewater having 14 - 40 mg/L Ca++, and 8 - 24 mg/L Mg++ is sufficient. However, recent research has found that keeping a Divalent to Monovalent cation ratio greater than 0.5 is more important for good floc formation.
How can adding peroxide upstream increase my influent BOD5? Why BOD5 vs BOD20 is key to understanding the phenomenon.
Wastewater contains a blend of many organic and inorganics compounds. In operating a treatment facility, we look at the oxygen demand from the pollutants. Wastewater oxygen demand can be estimated by tests such as BOD5, BOD20, COD, and TOC. The oldest tests are the microbial based BOD5 and BOD20. In these tests, a diluted sample of the influent is allowed to react with a microbial seed @ 20 Deg C for either 5 or 20 days. During this time, the microbes consume oxygen while digesting organics in the influent. Compounds that are not readily biodegradable or soluble are not fully consumed in the 5 day test. While the 20 day test will pick up more of the long run oxygen demand from insoluble compounds such as cellulose, long chain fatty acids, and grease. Always remember, COD > BOD20 > BOD5.
Now back to the case of hydrogen peroxide being used in a process upstream from the plant. While peroxide can be used to decrease BOD/COD by directly oxidizing the organics, in lower doses it can also increase the proportion of soluble vs insoluble organics. The peroxide reacts indiscriminately with the organics in solution. As a result, some of the recalcitrant and insoluble compounds become more bioavailable (or soluble). If there is not enough peroxide to actually fully oxidize the organics - you end up with higher levels of soluble organics that will show up in a BOD5 test. To see if this phenomenon is occurring in your system, you can check the waste stream with and without peroxide addition. You an also look at COD, filtered COD, and BOD20 - if these are relatively unchanged, it means BOD5 was impacted by increased organic bioavailability.
While researching my last post on loading anaerobic digesters with grease, I found many informational bits that can be useful for digester operators. Hydrolysis, the first step in anaerobic digestion, relies on enzymes and microbes to convert solids into organic acids used by the acetogenic bacteria. Influent makeup has a big impact on hydrolysis rates.
From the table, you can see that primary sludge with its mix of organics has more products suited for hydrolysis than activated sludge biological solids. The waste biosolids contain a mix of extracellular polymers, slowly solubilized organics, inorganic compounds, and bacteria cells. All of this takes more time for hydrolysis than the unstabilized primary solids.
Next, you should note the hydrolysis rates drop with increasing amounts of lignin in common cellulosic materials. I found this an important factor when accepting various plant wastes for the digester.
Erik Rumbaugh has been involved in biological waste treatment for over 20 years. He has worked with industrial and municipal wastewater facilities to ensure optimal performance of their treatment systems. He is a founder of Aster Bio (www.asterbio.com) specializing in biological waste treatment.
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