USING AN AMMONIA PROBE FOR PROCESS CONTROL TROUBLESHOOTING
INTRODUCTION
An ammonia probe is a handy tool for troubleshooting process control problems. Now that most plants nitrify plants, even small ones, have ammonia probes. The probes themselves cost less than $400, and the selective ion meter (a fancy pH meter) used to measure the ammonia probe output may cost as little as $250.
Why is an ammonia probe a good troubleshooting tool?
- Many plant labs will have an ammonia probe and staff trained in its’ use.
- The nitrification reaction occurs at a steady rate throughout the secondary treatment process. This allows the operator to track the ammonia depletion with the ammonia probe.
- The nitrification reaction is sensitive to many operating problems, such as lack of DO; recycle side streams and lack of detention time.
- The ammonia probe is easy to use and can measure ammonia in all types of samples, The samples can be from the refrigerator or one grabbed only minutes ago.
Thes qualities allow the alert operator to detect many problems that may otherwise go unfound. Examples of problems that can be sniffed out by an ammonia probe are:
- Short-circuiting in an aeration tank.
- Poor influent or RAS distribution among multiple aeration tanks
- Lack of DO in one tank, or in part of a tank
- RAS going septic in final settling tanks
- Poor air distribution among aeration tanks
- Supernatant or dewatering filtrate bleeding through the plant partially treated.
- Trickling filter media or snail problems
- Solids decay in “tertiary” lagoons
This paper will demonstrate how the ammonia probe can be used for troubleshooting and include real-world problems detected by ammonia probes.
CHARACTERISTICS OF NITRIFICATION
Nitrification is a two-stage biological process which oxidizes the ammonia to nitrate ion. Compared to carbonaceous BOD consuming organisms, nitrifiers are relatively slow growing, strict aerobes that consume ammonia at a steady rate in the aeration tank. This difference in food consumption rate is shown in Figure 1.
FIGURE 1
BOD AND NITRIFICATION DEPLETION RATES
IN A PLUG-FLOW AERATION TANK
This steady rate of ammonia depletion can be seen in the real world by doing a “bucket test”. In a bucket test, RAS with nitrifiers present and secondary influent wastewater are combined at a ratio to approximate the MLSS concentration, poured into a large bucket and aerated to maintain aerobic conditions. Samples are taken from the bucket at regular intervals and analyzed for ammonia concentration. A typical bucket test result is shown in Figure 2.
FIGURE 2
BUCKET TEST AMMONIA PROFILE
Time, Minutes
This test shows what happens in a plug flow aeration tank as the mixed liquor flows through the tank. (Note: results are site-specific and will vary due to temperature, mixed liquor dissolved oxygen (DO) concentration and other factors. The rate of depletion will remain fairly constant during the test)
WHAT CAUSES NITRIFICATION PROBLEMS IN ACTIVATED SLUDGE?
The most common causes of incomplete nitrification are:
- Poor flow splitting between aeration tanks,
- Lack of sufficient DO in some tanks, parts of tanks, or at certain times of the day,
- Plant side streams high in ammonia concentration, such as anaerobic digester supernatant, filtrate or centrate,
- Short circuiting caused by tank geometry, or
- A combination of all or some of the above.
WHAT MAKES THE AMMONIA TEST A VALUABLE TROUBLESHOOTING TOOL?
Because nitrification removes ammonia at a constant rate through an aeration tank allows the ammonia concentration to be used as a “tracer”, just like a dye tracer.
Ammonia depletion will reveal patterns of flow through aeration tanks. Ammonia removal, or lack thereof, can identify less-than ideal conditions, such as low DO or side streams.
WHERE CAN THE TEST BE USED?
The ammonia probe can be used to analyze refrigerated grab or composite samples, and fresh grab samples. Samples from all plant processes can be analyzed, including:
- Influent wastewater
- Primary effluent
- Mixed liquor
- Return activated sludge (RAS)
- Waste activated sludge (WAS)
- Aerobic and anaerobic supernatant
- Belt press filtrate and centrifuge centrate
- Gravity thickener (GT), gravity belt thickener (GBT) and dissolved air floatation (DAF) thickener overflows
- Secondary effluent
- Final effluent
- Water from any other plant process
HOW TO TAKE AN AERATION TANK PROFILE
Taking an ammonia profile of an aeration tank is just the same as taking a DO profile of an aeration tank. Grab samples of mixed liquor are taken at the aeration tank inlet, outlet and points in between. For a single pass tank, take a total of 3 or 4 samples. For a multiple-pass tank, take at least two samples in each pass. . The samples do not have to be settled and supernated before analysis. Grab samples of mixed liquor should be analyzed within 30 minutes of being taken, and preferably sooner than that. The operator should calibrate the ammonia probe before collecting the sample to reduce analysis time.
Figure 3 shows typical plug flow tank sample locations
FIGURE 3
AMMONIA PROFILE SAMPLE LOCATIONS
DIFFERENCES BETWEEN PLUG-FLOW AND COMPLETE MIX PLANTS
The ammonia probe is best used in a plug-flow tank, because a plug-flow tank tends to have an ammonia depletion profile similar to a bucket test, as shown in figure 4.
FIGURE 4
AMMONIA PROFILE IN A PLUG-FLOW TANK
A complete mix reactor tends to have uniform ammonia concentration, such as the one in this oxidation ditch in figure 5.
FIGURE 5
AMMONIA CONCENTRATION IN AN OXIDATION DITCH
EXAMPLES OF HOW TO USE THE TEST
The following examples are real-world examples of how use of an ammonia probe revealed process problems.
POOR FLOW SPLITTING
Plants that have multiple aeration tanks operating in parallel always have the potential for unequal flow splits. Tanks that receive more flow will have less detention time. All other things being equal, an aeration tank receiving more flow will have more ammonia at the tank effluent than one receiving less flow. An example of this is shown in figure 6.
FIGURE 6
POOR FLOW DISTRIBUTION, CASE 1
The tank effluent ammonia results show that tanks 1 and 8 receive more flow than tanks 2-7. In this instance, the poor distribution did not cause a problem because effluent limits were met.
SHORT-CIRCUITING (TANK GEOMETRY)
Tank geometry affects aeration tank detention time. In a complete mix tank, or rectangular “plug-flow” tanks with a length to width ratio or 4:1 or less, the average aeration tank detention time may be less than 25% of the theoretical detention time. Switching a multiple-tank system where the tanks operate in parallel to one where the tanks operate in series flow will reduce short-circuiting and improve actual detention times. Figure 7 shows the difference in results between parallel and series operation
FIGURE 7
PARALLEL VS. SERIES FLOW
SIDE STREAMS
Side streams with high ammonia concentrations can affect effluent ammonia concentration due to poor flow distribution, irregular flows and overloading an aeration system. High strength side streams that are not distributed evenly across multiple aeration tanks can overload one section of the plant and cause an increase in effluent ammonia concentration. Figure 8 shows an example of this. In this case, belt filter press (BFP) filtrate with an ammonia concentration of 300-400 mg/L was recycled back to only half of the plant, Two sequential samplers were set up to sample the mixed liquor leaving one tank affected by the recycle and one not receiving any BFP filtrate flow. The results are shown in Figure 8. Additional evidence was found by measuring the ammonia concentration going to the different tanks. The primary effluent going to aeration tanks numbers 3-6 was 19 mg/L; the ammonia concentration to tanks numbers 7-10 was 27 mg/L. As a result of this and other research, the belt press filtrate was re-routed to the primary settling tank influent so it would be distributed evenly to all tanks.
FIGURE 8
EFFECT OF SIDE STREAMS, CASE 1
AERATION TANK DO
Nitrification problems caused by low DO can be very difficult to find, especially in multiple-tank systems, plug flow systems, and in plants with poor DO control and/or monitoring. Nitrifiying bacteria are strict aerobes, and will not nitrify unless there is adequate DO.
What is an “adequate” DO? There is no magic number for an adequate DO. The aeration tank DO at which nitrification begins is site specific, and will change throughout the year. Nitrification will usually begin in a plug-flow aeration tanks when the DO is somewhere between 0.5 and 2.0 mg/L. Variables affecting the minimum DO for nitrification are:
- Aeration tank detention time
- Wastewater septicity
- Wastewater concentration
- Temperature
- MLSS concentration
Low DO can affect an aeration tank at certain times of the day when the flow and/or organic loading is higher. Figure 10 shows the effluent ammonia variation caused by low DO during some parts of the day in some aeration tanks.
FIGURE 10
EFFLUENT AMMONIA VARIATIONS
CAUSED BY PERIODIC LOW DO
In the case above, the cyclic trend of the ammonia concentration in the “south” aeration tanks corresponds with wide variations in mixed liquor DO The “north” tanks did not have low DO and were less affected by changes in load
Low DO problems can be very difficult to detect due to limited monitoring. Many plants do not monitor aeration tank DO, or, if they do, only take grab samples from the end of each aeration tank twice a day. Such sampling is of little value for troubleshooting and can be misleading because it does not reveal anything about the DO in the aeration tank at all the other times of the day, or in the other parts of the aeration tank. Low DO in the portions of an aeration tank can inhibit nitrification as well,, because there may be a minimum DO required to start nitrification, especially in long, plug flow tanks where the initial section is heavily loaded.
Still another use of an ammonia probe is to determine the difference in efficiency, in a practical way, between diffusers in two different tanks. Figure 11 shows the ammonia profiles in two aeration tanks. One tank has new diffusers; the other, seven year old diffusers from another manufacturer. The tank with new diffusers requires about 35% less air to nitrify approximately the same amount of ammonia as the tank with the old diffusers.
FIGURE 11
DIFFUSER EFFICIENCY
AEROBIC DIGESTER OPERATION
Aerobic digester ammonia concentration is a good indicator of aerobic digester health. Most aerobic digesters nitrify, converting the ammonia liberated from cellular destruction into nitrate and acid. The digester will generally have a site-specific baseline ammonia concentration. Rising ammonia concentration is an indicator that the digester air supply is not sufficient to nitrify all the ammonia released by digestion. This test is especially useful when the aerobic digester feed sludge is thickened before entering the digester. When the sludge is thickened an increasing ammonia concentration can indicate that the sludge is too thick and should be thinned down. In these cases the digester may soon develop odors and, if the digester temperature is above 90o F, foaming may occur. The ammonia concentration can be used as a process control tool, taken at least twice a week and tracked for changes.
SUMMARY
An ammonia probe is a handy tool for tracking and troubleshooting the activated sludge process, and aerobic digesters. The ammonia probe and selective ion meter are inexpensive and easy to use, and will yield results in a few minutes.
ACKNOWLEDGEMENTS
I would like to thank Steve Hallett, Mike Carson and Chris McGibbeney from the City of Toledo Bay View Water Reclamation Plant for helping develop this troubleshooting process, and for allowing me to use examples from plant operations spanning over 10 years’ time. I would like to thank Doug Keller from the Village of Carey, Ohio the use of his plant data and turning an offhanded remark I made one day into a successful project. Last, I would like to thank Angelo Klousiadis from the City of Mansfield OH and Lynnius Maximus Marshall for the use of their data to illustrate problems.
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