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.
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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