Saturday, June 25, 2011

Nitrogen in wastewater and more real world results

I spend quite a bit of time on this blog discussing nitrogen in wastewater and thought I would take a minute and explain a little about the species of nitrogen present in domestic wastewater.  

Nitrogen is present in wastewater in several forms:

Organic nitrogen, primarily as urea.  Urea is the end product of the metabolism of nitrogen-containing compounds in most mammals and is excreted in the urine (Solomons 1988)


Ammonia:  Ammonia is generated by ammonification of urea.  You can see the amino groups (NH2) in the urea that degrade to ammonia. (Grady, Daigger and Lim, 1999)

Ammonia (NH3)

If the pH of the wastewater is neutral to acidic the ammonia will be in the form of ammonium.

Ammonium Ion

Nitrate and Nitrite: The ammonium is converted to nitrite and then nitrate in the presence of oxygen.

Nitrate (NO3)

Nitrite (NO2)

Those are the main forms of nitrogen in domestic wastewater.  Total nitrogen (TN) is the sum of all species:

Total nitrogen (TN) = Organic nitrogen (urea) + Ammonia/Ammonium + Nitrate + Nitrite.

You may have heard of Total Kjeldahl Nitrogen (TKN), this is not TN.  TKN is the sum of organic nitrogen (urea) + ammonia/ammonium.  TKN can be equal to TN if no nitrate or nitrite is present in the sample.  TKN cannot be greater than TN.  With the definition of TKN, total nitrogen can be expressed as: 
TN = TKN + nitrate +nitrite.

I spoke with an onsite maintenance provider who maintained a competing system and was responsible for collecting wastewater samples for analysis.  He was very proud of the fact that the total nitrogen for his systems was under 20 mg/L and felt the need to prove it to me by showing me a copy of his sample results.  The nitrogen analysis he was having run by the lab, however, was for total kjeldahl nitrogen (TKN) not total nitrogen (TN).  He heard the word, "total" in TKN and assumed he was getting the correct sample analysis.  The problem was that once the nitrate and nitrite were added to the results, to obtain a true TN value, his systems were actually exceeding the state standard by a huge margin.  You can see it is important to understand what you are doing.

This brings up an interesting point.  We have all heard of Biochemical Oxygen Demand (BOD) and most know that BOD is a measure of the amount of oxygen required to oxidize biologically degradable compounds completely to water and carbon dioxide.  The most common form of this test measures the dissolved oxygen concentration remaining in a sample over a period of 5 days, hence the BOD5 test.   What you will see fairly often are manufacturers presenting cBOD as BOD5.  a cBOD test is a different test though. 

In a cBOD test, the bacteria responsible for converting ammonia to nitrate and nitrite are killed. This effectively negates the oxygen demand imposed by the nitrogen species.  The cBOD test provides useful information but should not be confused with the standard BOD5 test.  I think manufacturers use cBOD because if you collect a sample from a treatment system and analyze it for BOD5 and cBOD5 you will see that the cBOD5 sample produces a lower value.  The reason the value is lower is because a significant portion of the oxygen-demanding species have been negated by killing the microorganisms that are responsible for oxidizing it.  

Real World, Cold Weather Results (Not Waco in the summer)

I am including some recent Eliminite samples from five residential systems in Montana.  These are cold weather samples taken from standard residential systems.  The nitrogen results represents an 80 to 90% removal.

Lot Total Nitrogen BOD
22 14 18
26 14 17
28 15 17
29 10 11
32 17 11
Average 14.00 14.80

Montana has had a long cold winter this year but as the results show, these values are superior to most systems best warm weather data.   Our nitrogen numbers are not the only values that are much lower than competing onsite systems; our prices are significantly lower also.  It's just a fact.

Highway Rest Areas
I'll be discussing the highway rest area systems in a day or two.  We are currently achieving about a 90% removal of nitrogen.  The influent nitrogen is a little higher than previously thought;  400 to 500 mg/L TN and the BOD is over 1000 mg/L.  We don't even have the system dialed in yet and are currently removing 93% TN.   Oh...these are cold weather results also!

Blog Traffic
Should I mention what company has visited this blog more in the last month than anyone...even me!?  If you guys would like to buy some systems, we can work something out.