July 5, 2017

 

Is Your Lab Ready to Support Nutrient Removal with Quality Data?

Basic Nutrient removal in a Difficult Matrix

 

Advances in nutrient removal and recovery seem to be coming at breakneck speed. This is especially true, perhaps, to those of us in a wastewater lab; associated with a treatment facility but not actually on the front lines of treatment. There are many new technologies available, new configurations for aeration basins, new acronyms, new (read lower) permit limits, even ‘new’ microbes. And there is always a new theory to test, a new idea on the horizon. Most of the new ideas and technologies that I hear about are accompanied by a request from the lab for data: more data on what we are currently doing, more data from a new system that is being tested, a sampling campaign to get critical data for a new technology. Flows and returns, diurnal patterns, changes in concentration and speciation of nutrients, It all must be tested, and the data needs to be as good as possible. It all got me thinking about the basics of nutrient removal, and the need for labs to be able to accurately perform analyses on the complicated matrix of aeration basins.

 

By basics of nutrient removal, I am referring to traditional biological ammonia removal, in this case in a secondary basin using activated sludge. This is in itself a series of complicated reactions occurring in a complex and delicately balanced system-far from actually being basic!

 

Most nitrogen coming into wastewater treatment plants is in the form of ammonia (NH₃). Biological removal of ammonia from wastewater involves oxidizing it to nitrite (NO₂) and nitrate (NO₃) and then ultimately to elemental Nitrogen gas (N₂).

 

This is done by first using bacteria known as ammonia-oxidizing bacteria (AOB), which are autotrophic chemolithotrophs, and are also obligate aerobes. That is, they can make organic molecules from elements such as sunlight or chemical bonds in their environment, can oxidize inorganic substrates (NH₃ in this case) for energy, use CO₂ for a carbon source, and require an oxygen rich environment. These are also known as Nitrosomas, and they provide the first step in the Nitrification process that converts ammonia to nitrate:

 

 

2NH₄⁺ + 3O₂ --------à 2NO₂- + 2H₂O + 4H⁺ + Biomass                     (Equation 1)

 

 

The next step is performed by Nitrite-oxidizing bacteria (NOB), which convert nitrite to nitrate. These are also autotrophic chemolithotrophs, as well as obligate aerobes. Their source for energy is the inorganic substrate of nitrite (NO₂):

 

2NO₂^- + O₂ --------à 2NO₃- + Biomass                                               (Equation 2)

 

 

These two reactions together are known as Nitrification, the process of converting ammonia to nitrate.

 

Notice that nitrification is a purely aerobic process.

 

Nitrification in a suspended growth/sludge secondary depends on many factors: pH, Alkalinity, Dissolved Oxygen (DO), the presence of any toxic chemicals, temperature, the COD:TKN ratio, and the fact that nitrifying bacteria are outcompeted by heterotrophic bacteria (bacteria that use organic Carbon, and not CO₂, for growth). A pH of less than 7 is detrimental to the process of nitrification. Notice how the equation of ammonia oxidation (Equation 1) adds acidity to the basin. This is where alkalinity comes in. In fact, for each gram of ammonia nitrified, 7.2 grams of CaCO₃ alkalinity are required. Each gram of ammonia nitrified also requires 4.6 grams of O₂. So oxygen must be added constantly, but it must be at a controlled level since adding DO above a level of 3ppm, in general, provides no benefit and is a waste of energy and money. Toxic chemicals reduce nitrification ability, and keeping a secondary basin free from these requires a robust pretreatment program. The COD:TKN ratio is also a factor, as influent loads that are biased higher in organic load (COD) tend to decrease nitrification rates by providing an environment more conducive for aerobic heterotrophs. The slow rate of growth of nitrifiers generally means that activated sludge processes that denitrify have longer sludge retention times (SRT) than ones that only treat for carbon/BOD.

 

The process of converting nitrate to nitrogen gas is known as denitrification. Denitrification completes the conversion of ammonia to nitrate to nitrogen gas. Denitrification is also bacterially driven, this time by heterotrophic (get their carbon source from organic sources, not CO₂), facultative (able to use oxygen or other substrates as terminal electron acceptors) bacteria known as denitrifiers (i.e. Pseudomanas, Thiobacillus denitrificans). Denitrification-in contrast to nitrification-occurs in an anoxic environment (where nitrate is available but oxygen is not). This means that aeration basins must have different zones with different oxygen levels to accommodate the growth of both nitrifiers and denitrifiers.

 

Denitrification follows the general reaction (which uses methanol as a general source of organic carbon/BOD/food):

 

6NO₃^- + 5CH₃OH   --------à 3N_{2 (gas)} + 5CO₂ + 7H₂O + 6OH^-                   (Equation 3)

 

For every 1 gram of nitrate that is converted to dinitrogen gas, 2.9 grams of BOD are consumed and 3.6 grams of alkalinity (as CaCO₃) are produced. Ideally, operators can configure their basins so that they can take advantage of the alkalinity produced from denitrification to supply some of the alkalinity needed for nitrification. To do this however, denitrification must occur prior to nitrification-another complication.

 

There is clearly a lot going on even in this basic example of ammonia removal. To assist plant operators, a lab must be able to provide quality and timely values for pH, ALK, BOD, NH₃, NO₂, NO₃, COD, TKN, and Temperature, as well as quite possibly the composition and abundance of microbes. Many of these values can be provided via inline instruments now, but even so, the fact that the lab must have accurate analyses of these parameters does not change since the instruments are calibrated to lab values.

 

Thus it is very important that any lab analyzing activated sludge samples thinks about the difficulties of analyzing this difficult matrix. Bad data causes bad decision making, and an activated sludge is no place for that!

 

Here, in no particular order, are some ideas to consider: Do nutrient samples need to be digested/distilled? Do standard hold times even apply in such a biologically active matrix?  Does your standard of known concentration actually represent this matrix, or are you using a clean standard that gives a false idea of how well you are doing? Can you spike a mixed liquor sample and get recovery? Are you diluting samples so much that you are raising your method detection limit to the point it is not practically useful? Does the matrix itself cause colorimetric interference? Can you digest and run the same sample twice with comparable results? Can different analysts run the sample with comparable results? Are you pH preserving the sample correctly, or is the alkalinity in the solids in the sample slowly neutralizing acid and raising the pH over time? Is your DO/pH/etc... meter subject to fouling? Are there interferences in the matrix (do reported values increase with dilution)?

 

Being able to support basic nutrient removal with quality data is paramount for wastewater labs.

 

Remember, it only gets more complicated from here!

 

References:

J. Rodziewicz, A. Mielcarek, W. Janczukowicz, and U. Filipkowska. Effect of COD/TKN ratio on the effectiveness of nitrogen compounds transformation in a reactor with immobilized biomass. University of Warmia and Mazury in Olsztyn, Department of Environment Engineering.

 

R. Sharma and S. K. Gupta. Influence of Chemical Oxygen Demand/Total Kjeldahl Nitrogen Ratio and Sludge Age on Nitrification of Nitrogenous Wastewater. Water Environment Research. Vol. 76, No. 2 (Mar. - Apr., 2004), pp. 155-161.

 

S. Okabe, Y. Aoi, H. Satoh, and Y. Suwa.  2011. Nitrification in Wastewater Treatment, p 405-433. In  B. Ward, D. Arp, and M. Klotz (ed), Nitrification. ASM Press, Washington, DC.

 

Steve Polson, P.E. Nutrient Removal 101- Process Fundamentals and Operation. JTAC Presentation May 18, 2017 at AWWA Headquarters, Denver, CO.

 

Richard MacAlpine holds an MS in Environmental Science (WQ Emphasis) from CU-Denver, is on the Education Subcommittee of RMWQAA, and has worked in the lab at Metro Wastewater Reclamation District for the last decade plus.