In order to remove nitrogen at your wastewater treatment facility you must understand the different forms of nitrogen and some commonly referred to terms that you will be dealing with.
Total Nitrogen (TN) is the sum of all nitrogen forms or;
Total Nitrogen = TKN + NO2 + NO3
TKN stands for Total Kjeldahl Nitrogen which is the sum of; NH3 + Organic Nitrogen
NH3 stands for Ammonia Nitrogen
Organic Nitrogen is derived from amino acids & proteins:
Good examples are urea and uric acid
NO2 stands for Nitrite
NO3 stands for Nitrate
N2 stands for Nitrogen Gas
Refractory Nitrogen is the nitrogen that can’t be biologically decomposed.
Alkalinity is easiest defined as the ability to resist a drop in pH. This is very important because for every 1 part ammonia (NH3) converted to nitrate (NO3) – 7.1 parts of alkalinity are depleted, and for every 1 part nitrate (NO3) removed – 3.6 parts alkalinity are recovered.
An anoxic zone is a basin, or portion of, which is mixed but not aerated. The dissolved oxygen levels must be less than 1.0 mg/L but never reach 0.0 mg/L. In an anoxic zone the bug’s oxygen source is derived from the nitrate (NO3) compounds.
Nitrification and denitrification are two terms that are commonly misunderstood. Both are individually distinct processes.
Nitrification is the conversion of ammonia (NH3) to nitrate (NO3). How is this done? This is a two-step process that is done with oxygen and two types of bacteria, Nitrosomonas and Nitrobacter, known collectively as the nitrifiers.
Ammonia (NH3) + Oxygen (O2) + Alkalinity + Nitrosomonas = Nitrite (NO2)
Nitrite (NO2) + Oxygen (O2) + Alkalinity + Nitrobacter = Nitrate (NO3)
Nitrite (NO2) is the unstable form of nitrogen and is easily converted because it does not wish to be in this form.
The total conversion of ammonia (NH3) to nitrate (NO3) takes 4.6 parts oxygen and 7.1 parts alkalinity to convert 1 part ammonia (NH3).
Denitrification is the conversion of nitrate (NO3) to nitrogen gas (N2). How is this done? Heterotrophic bacteria utilize the nitrate (NO3) as an oxygen source under anoxic conditions to break down organic substances.
Nitrates (NO3) + Organics + Heterotrophic bacteria = Nitrogen Gas & Oxygen & Alkalinity
Now that you understand the different forms of nitrogen and terms that you will be dealing with, the next questions are; What forms of nitrogen do you test for and what can you use to test for them.
Total kjeldahl nitrogen (TKN) is an involved test that many wastewater treatment facility laboratories are not equipped to perform. If you can’t perform this test, do not use this as an excuse to ignore monitoring the nitrogen cycle. The ammonia (NH3) values are approximately 60% of the Total kjeldahl nitrogen (TKN) values, and the organic nitrogen is generally removed in the settled sludge. Also, Total kjeldahl nitrogen (TKN) generally equals 15 – 20 % of the Biochemical Oxygen Demand (BOD) of the raw sewage.
The following tests are a must to monitor and control the nitrogen cycle: pH, alkalinity, ammonia (NH3), nitrite (NO2) & nitrate (NO3)
All of the major lab supply companies sell field test kits that are inexpensive, easy to use, and provide quick relatively accurate results. Some of the things to keep in mind when purchasing field test kits are;
- Proper ranges should be determined
- Results should all be expresses as nitrogen:
Ammonia – (NH3 as N), Nitrite – (NO2 as N), Nitrate – (NO3 as N)
All results are expressed as nitrogen so that you can total them up to determine the Total nitrogen values.
Now that you have your test kits the next questions are: Where do we test in our wastewater treatment facility and what numbers should we expect?
To establish a good handle on nitrogen in your wastewater treatment facility, you must develop a good sampling program that will give a complete profile of your system.
The first sampling point would be the raw influent or primary effluent if you have a primary clarifier. Typically, what is entering the facility will be high in alkalinity and ammonia (NH3) with very little to no nitrite (NO2) or nitrate (NO3). A quick way to determine if you might need to perform alkalinity addition is to multiply the ammonia (NH3) by 7.1 mg/L. If this number exceeds the influent alkalinity concentration, you will probably have to add something such as sodium hydroxide or lime to the aeration tank.
Why is this important? When you start converting ammonia (NH3) to nitrate (NO3) in the aeration tank many hydrogen ions will be released. When alkalinity drops below 50 mg/l your pH can drop dramatically. You should never allow the pH of the aeration tank to drop below 6.5. Biological activity will be inhibited and toxic ammonia (NH3) can bleed right through your system.
The next sampling points to establish are in your aeration tank. The length of the tank will dictate how many sampling points are required. Generally, I use three locations: The front, middle and end of the aeration tank.
If a suitable environment is maintained in the aeration tank most of the ammonia (NH3) will be converted to nitrate (NO3) by the time it leaves the tank.
Why would you use three locations if all you care about is what is leaving the tank? You must determine how much tankage is required to perform this conversion.
If this conversion is completed in the middle of the tank this will give you 1/3 of the tank to establish an anoxic zone. An anoxic zone will enable you to not only convert the nitrogen but to remove it.
The final sampling point will be the plant effluent prior to chlorination. There should never be less than 50 mg/L of alkalinity. The pH should never be out of the permitted range. Ammonia (NH3) should have extremely low concentrations. Nitrite (NO2) should be very low to non-detectable and the majority of the nitrogen will be in the nitrate (NO3) form.
Throughout all of your testing the nitrite (NO2) levels should be very low. Why bother testing for them then? High levels of nitrite (NO2) in the system indicate there is, or about to be, a problem with the nitrification cycle. Nitrosomonas bacteria are harder to kill than Nitrobacter bacteria. If the Nitrobacter bacteria are killed off, the Nitrosomonas bacteria will continue working on the ammonia (NH3) and you will have a jammed cycle with high levels of nitrite (NO2). An effluent with high nitrite (NO2) concentrations will be difficult to disinfect because of the tremendous chlorine demand it poses.
What type of problems might you encounter while performing nitrification? A decrease in the aeration tank pH due to insufficient alkalinity causing ammonia (NH3) to bleed through the system which will cause a decrease in the microbiological activity. An inability to completely nitrify due to a lack of dissolved oxygen; mixed liquor suspended solids, mean cell retention time, and cold temperatures.
All these factors can inhibit the nitrification cycle. High ammonia (NH3) discharges can affect your toxicity testing. High nitrite (NO2) levels will cause a tremendous chlorine demand making disinfection difficult – jeopardizing your fecal coliform limits. Leaving sludge that is high in nitrate (NO3) too long in the secondary clarifier can cause the sludge blanket to rise to the surface when the nitrogen gas is released. This will make quite a mess and will jeopardize your TSS limits.
Why bother to nitrify at your wastewater treatment facility if there can be this many problems? Aside from permit limits, ammonia (NH3) is toxic to fish and other aquatic life. Ammonia (NH3) discharges also place a very high oxygen demand on the receiving streams. Most operators perform nitrification due to the desire to produce a highly stabilized effluent at their wastewater treatment facility.
Now that we have converted all this ammonia (NH3) to nitrate (NO3), how can we remove it from the system or more specifically perform denitrification?
An anoxic zone will have to be established within the wastewater treatment facility. Regardless of where and how you do this, the principles of operating an anoxic zone will always be the same.
The dissolved oxygen levels must be as close, without reaching, 0.0 mg/l as possible. A safe target point to avoid septicity while starting your zone would be 0.5 mg/L.A good operating point would be 0.2 mg/l.
There must be a carbon source. Raw influent usually works fine but some plants have to supplement the carbon source by injecting methanol or ethanol. It takes about 2.0 – 2.5 parts methanol for every part nitrate (NO3) that is denitrified.
The mixed liquor suspended solids concentration must be kept in balance with the food supply. In other words, the Food to Microorganisms should be in the proper range (on the lower end) for the type of process you are operating.
The pH of the anoxic zone should be close to neutral (7.0) and never drop below 6.5.
How will this all come together to work? Heterotrophic bacteria need a carbon source for food. They obtain their oxygen the easiest way possible using the following sequence: free and dissolved oxygen, nitrate (NO3), and then sulfate (SO4). If your zone has no free or dissolved oxygen, the “bugs” will have to obtain their oxygen source by breaking down the nitrate (NO3) that are returned to the anoxic zone in the activated sludge. As the “bugs” utilize the nitrate (NO3) as an oxygen source to break down the carbon, their source of food, nitrogen gas will be released to the atmosphere.
Bugs + Carbon + Nitrate (NO3) = Nitrogen Gas (N2) + Oxygen (O2) + 3.6 parts Alkalinity
Many wastewater treatment facilities have been performing successful single tank denitrification by creating and utilizing anoxic zones. Some examples that I have seen work are:
Constructing a dedicated anoxic zone at the head of the aeration tank by installing a baffle and mechanical mixers
Utilizing the first 1/4 to 1/3 of the aeration basin as an anoxic zone by throttling the aeration system diffusers valves to allow mixing without transferring dissolved oxygen
A dissolved oxygen probe in the aeration tank tied into a variable frequency drive that sends a signal to the blowers, providing a continuous dissolved oxygen level as determined by the set points
Utilizing timers to cycle the aeration system on and off which allows the whole aeration basin to be used intermittently as an anoxic zone (always check with your equipment manufacturers before implementing this method so that no damage will occur)
You may find that the return activated sludge pump cannot return enough nitrate (NO3) to the anoxic zone quick enough. If that is the case, a high yield submersible pump can be lowered into the effluent end of the aeration tank.
The discharge piping can be attached to the aeration basin walls, returning mixed liquor high in nitrate (NO3) back to the anoxic zone. The outlet of the discharge piping will need to be slightly submerged to avoid splashing and the introduction of dissolved oxygen.
How will you know when all of the nitrate (NO3) is used up? The next place the bugs will go for their oxygen source is the sulfate (SO4). As the sulfates are used up, sulfides will combine with hydrogen to form hydrogen sulfide and this stinks like rotten eggs.
Why would you want to perform denitrification at your facility? The obvious reason would be Total nitrogen limits in your discharge permit, others include; alkalinity and oxygen recovery, the desire to produce a highly stabilized effluent, and a reduction of problems with rising sludge in your clarifier.
Establishing and successfully operating a nitrogen removal process at your wastewater treatment facility will take some time and effort on your part. A lot more process control testing will have to be performed and system upsets may occur. The benefits will by far outweigh the headaches and great pride can be taken when you discharge a highly stabilized effluent from your wastewater treatment facility.
This article was written by Robert Scott the Wastewater Technician for the Atlantic States Rural Water & Wastewater Association.