Basics of the Biological Nutrient Removal Process

 The goal of this article is:

  1.  To Introduce the reader to the basic framework of the Biological Nutrient Removal (BNR) Process. 
  2. To provide basic guidance in using this basic framework in optimizing and troubleshooting BNR processes.

Learn Theory! Learn Theory! Learn Theory! If you know theory you can use it! If you don’t know theory you can’t use it!

Thinking in terms of complex systems is difficult. Many people, even after studying for years, do not understand basic principles. Systems thinking is often counterintuitive at many points. It takes many years to gain any degree of skill, and it still often eludes us. (Extraordinary Leadership: Thinking Systems, Making a Difference, 2009, Reberta M. Gilbert, M.D, pg 5)

This article is not a primer or refresher on nitrification, denitrification or phosphorous removal. It assumes the reader already has a basic understanding of these processes. If you feel you need a refresher, please refer to any of the Water Environment Federation’s (WEF) Manual of Practices (MOP) for Wastewater. A copy of MOP 11 can be found for free download on the internet. Key concepts crucial for understanding the BNR process will be provided at specific places in the discussion.

For example, it is important to remember that the activated sludge is a cyclic process as well as linear one. The fact that each tank has an influent and an effluent is a sufficient reminder of the linear aspect of the process. What is more difficult to remember is the cyclic aspect of the process. Even the most basic activated sludge process has a sludge recycle (see Figure 1).

FIGURE 1



The purpose of any recycle is to return a portion of the waste stream from one part of the process to another part of the process. The return sludge in a conventional activated sludge process ensures that the settled biomass is recycled back to the head of the aeration basin. Without this recycle, the aeration basin would never develop a large enough biomass to remove the substrate (BOD, phosphorous or nitrogen).

Interestingly, the most basic BNR process, and one of the first activated sludge processes developed early on, did not have any recycle. That process is the Sequencing Batch Reactor (SBR). In the BNR mode, wastewater enters the tank at the beginning of the process (Mix/Fill) while the MLSS is in an anaerobic condition, then aeration begins (React/Fill) placing the MLSS in an aerobic condition, after which the filling stops (React On/Off). When the MLSS is almost fully oxidized the next two phases of the SBR process begins (Settle), and finally (Decant/Idle). The MLSS is in an anoxic condition at this point. Then the process starts over again. Since it is a batch process and everything proceeds sequentially in a single tank, there is no need for recycle (Figure 2).

FIGURE 2

The beauty of the SBR process is the BNR process follows the logical sequence required by nature. For this reason, it is the perfect place to begin learning about the BNR process. Figure 3 illustrates the basic stages of the BNR process. The anaerobic section (Mix/Fill) of the SBR process is Stage One of the luxury phosphorous uptake process. The aerobic section (React/Fill & React) of the SBR represent Stage Two of the luxury phosphorous uptake process and Stage One of the Nitrogen Removal Process (Nitrification). The anoxic (Settling and Decant) sections represent Stage Two of the Nitrogen Removal Process (Denitrification). Carbonaceous BOD removal occurs throughout the whole SBR process.   

FIGURE 3


Included in Figure 3 is a secret that many operators are not aware of. Optimum BNR occurs only if the wastewater remains in each of the three major respiratory modes (anaerobic, aerobic and anoxic) long enough to complete the process. If the Mix/Fill phase is not long enough, the first stage of the luxury phosphorous uptake process (phosphorous release) will not be complete. The same is true for stage two of the luxury phosphorous uptake process (phosphorous uptake), nitrification, and denitrification. The percentages located below the three respiration modes represent the relative amount of the total treatment time required for that particular mode. For example, stage one of luxury phosphorous uptake requires somewhere between 15 to 20% of the total treatment time in the SBR process. Another way to visualize it is represented in Figure 4.

FIGURE 4

Why is this important?

When SBRs have difficulty with phosphorous, ammonia or nitrate removal, in many cases it is due to a deficiency in treatment time in one of the three respiration modes. An increase in effluent phosphorous can often be traced back to either 1) not enough time in the anaerobic mode or 2) not enough time in the anoxic mode. 1) can be corrected by lengthening the Mix/Fill phase. 2) results in incomplete denitrification during the Settling/Decant phase which allows nitrates to enter the Mix/Fill phase of the SBR process which is supposed to be “anaerobic.” The reason for the quotes is, as we all know, “anaerobic” is defined as “zero DO, zero nitrates”. If nitrates are in the “anaerobic” phase of the process, that phase is not anaerobic, but anoxic, no matter what we may call it. In the second scenario either a) the settle/decant phase is not long enough, b) the DO is too high at the end of the aerobic (react) phase, or c) the (React) phase is too long.

Just remember that when making changes to the time of any one of the phases it will affect the times of the other phases as well.

What Happens in Each of the Three Respiration Modes?

Figure 5 illustrates the relative concentrations of BOD, phosphorous, ammonia, nitrate/nitrite and alkalinity levels during the anaerobic, aerobic and anoxic modes.  

FIGURE 5 Relative Nutrient Concentration During the BNR Process


The BOD consumption begins almost immediately during the anaerobic respiration period and is fairly steady throughout the activated sludge process. Phosphate accumulating organisms (PAOs) consume volatile fatty acids during the anaerobic period, the same substrate as filamentous organisms, which is why adding an anaerobic phase to the activated sludge process reduces the potential for filamentous issues.

Under normal conditions the phosphate concentration goes up during this period, often as much as four times the influent concentration. During the aerobic period of the process the PAOs will take up as much as seven times the amount of PO4 they originally released. The PO4 concentrations at the end of the anaerobic period should be below 0.5 mg/L. Tracking the PO4 concentrations at the influent and effluent end of the anaerobic period is a good indicator of the health and effectiveness of the PAOs.  

Nitrogen is found in the influent in two primary forms; approximately 40% is in the form of ammonia (NH3) and approximately 60% is in the form of complex organic nitrogen measured as Total Kjeldahl Nitrogen (TKN). The TKN portion of the nitrogen is converted to ammonia during the anaerobic and aerobic periods of the BNR process. Thus, if just ammonia is measured in the influent at a concentration of 12 mg/L, the amount of TKN could be around 30 mg/L. The actual ammonia that would have to be converted is around 40 – 45 mg/L. This is reflected by the rising yellow line during the aerobic period.

During the aerobic period the nitrifiers are actively converting ammonia to nitrite/nitrate (NOX). This is represented by the falling yellow line (NH3) and rising purple line (NOX) during the aerobic period. By the end of that period the NH3 should be well below 1.0 mg/L and the NOX should be in the 30 – 40 mg/L range.

During the anoxic period of treatment, nitrate/nitrite (NOX) will be converted to nitrogen gas (Ná).  It is important that as much (NOX) be converted to (Ná) as possible before the process is returned to the “anaerobic” period of treatment. Any (NOX) entering the “anaerobic” period shortens that period. Remember, (NOX)’s in the MLSS means it is still anoxic.  

If the BNR process is working properly, the alkalinity (and eventually the pH) will decline during the aerobic period due to the nitrifying organisms (remember they are autotrophic – they consume inorganic carbon such as carbonate). During the anoxic phase, however, the denitrifying bacteria will recover approximately 50% of the original alkalinity.

Process Control

So, based on the above discussion, it should be pretty obvious what process control samples need to be pulled and where. Figure 6 illustrates the process control samples that should be pulled for optimizing and monitoring the process. The picture includes a clarifier and internal recycle for a system that is not an SBR, but the principles are the same.

FIGURE 6 Putting It All Together


Process Control vs Troubleshooting

The primary different between sampling and testing for process control and troubleshooting is frequency. When everything is working well the amount of sampling can be greatly curtailed. However, if the system is experiencing problems, the type and severity of problem, will dictate the location, frequency and duration of the sampling and testing. For example, it is not necessary to test for ammonia if the system is experiencing elevated PO4 values.

Feel free to contact me should you serve a population >10,000 or have any questions or issues that the GRWA is unable to assist you with. Also, if you would like to contribute an article feel free to email me at the address below. I am always looking for contributors that have an interesting perspective, topic or has an interesting case that they would like to share – especially if the solution is a direct result of applying the principles from this forum.

Dennis Brown, Wastewater Specialist and Trainer, Retired

dbrown.grwa@gmail.com

678.750.3996 













Comments

Post a Comment

Popular posts from this blog

Image
  Hidden Dangers of Improperly Abandoned or Constructed Wells Steve Walker, P.G.,  Source Water Specialist,  Georgia Rural Water Association Even though approximately 70% of the earth is covered in water, only about 1-2% of that water is considered drinkable. With humans inhabiting almost every corner of the earth, its easy to see how safe drinking water has always been somewhat of a concern. Here in Georgia, early settlers collected rain water, utilized springs or hand dug wells to obtain drinkable water. Dug wells gradually became more common and allowed people to settle in areas further from drinking water springs. As medical science advanced through the 1800s, people became more aware of the link between unclean drinking water and illness. This knowledge coupled with an ever increasing technical/mechanical ability, fueled the use of machines to drill wells deeper in search of cleaner, more prolific groundwater sources. These steel-cased (or other tubular casing ...
Read more
  Troubleshooting Fixed Film Systems   The goal of this article is: 1.     1. To illustrate through actual case studies how to use the Four Basic Concepts of Wastewater in actual practice for fixed-film systems. 2.     2. To illustrate the importance of using the O&M Manual to gain an accurate understanding of the design parameters of the system in operation. Learn Theory! Learn Theory! Learn Theory! If you know theory you can use it! If you don’t know theory you can’t use it! ____________________________________________________________________________________   The Georgia Rural Water Association is always available to assist any municipality that serve a population of <10,000 with process control, troubleshooting, training, lift station maintenance, smoke testing, leak detection, standard operating procedure development, asset management development, or any other compliance and operational issues that may crop up. To request...
Read more
Image
  Specific Oxygen Uptake Rate: The Fifteen Minute BOD   The goal of this article is: 1.        To introduce readers to the marvelous benefits of the Specific Oxygen Uptake Rate Learn Theory! Learn Theory! Learn Theory! If you know theory you can use it! If you don’t know theory you can’t use it! Thinking in terms of complex systems is difficult. Many people, even after studying for years, do not understand basic principles. Systems thinking is often counterintuitive at many points. It takes many years to gain any degree of skill, and it still often eludes us. (Extraordinary Leadership: Thinking Systems, Making a Difference, 2009, Reberta M. Gilbert, M.D, pg 5) ____________________________________________________________________________________   The Georgia Rural Water Association is always available to assist any municipality that serve a population of <10,000 with process control, troubleshooting, training, lift station ma...
Read more