Eye on Alkalinity

Simple tests can help you determine whether too little influent alkalinity may be keeping you from meeting a minimum chlorine residual
Eye on Alkalinity

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Troubleshooting treatment plant problems can be a daunting task — it seems there are so many variables to look at when making a diagnosis.

One such problem is the inability to meet a minimum chlorine residual standard. There are several reasons why a treatment plant might not be able to maintain chlorine residual: excessive chlorine demand, improper effluent pH, disinfectant feed equipment malfunction, and interferences with the residual chlorine testing reagents.

But another possible cause is a lack of alkalinity, and alkalinity is one of the most overlooked process control tests. Alkalinity — the ability of water to buffer against a pH change or to neutralize acids — is measured in mg/l as calcium carbonate (CaCO3). Through a complex chain of events, alkalinity can affect the chlorine residual in plant effluent.

Alkalinity testing is easily done in a lab or in the field without expensive lab equipment, and the results are very useful. Lab supply companies make easy-to-use test kits, and some can even be purchased at swimming pool supply stores.


Where alkalinity comes from

Raw wastewater contains some alkalinity. How much depends on a few factors, including the source of the water. Water from a deep aquifer, reservoir, river, lake or seawater contains different amounts of alkalinity. To see how much is in your local source water, you can contact your drinking water supplier, read the consumer confidence report they send out, or run an alkalinity test yourself.

How can alkalinity contribute to wastewater treatment plant problems? Normally having too much alkalinity is not the issue — it’s having too little to complete biological and chemical treatment. Wastewater treatment processes that consume alkalinity include:

• Biological nitrification in aeration tanks, trickling filters and RBCs.

• Gas chlorination for effluent disinfection.

• The acid formation stage of anaerobic digestion.

• Biological nitrification in aerobic digesters.

• Chemical addition of aluminum or iron salts.

If your influent is already alkalinity-limited, these processes will use up what is available and then may decrease or stop altogether.


How it all works

Inside a wastewater treatment plant, autotrophic bacteria (nitrifiers) consume alkalinity and oxidize ammonia to gain energy for reproduction. With enough dissolved oxygen (DO), adequate temperatures, the right detention time and several other items, nitrifiers convert ammonia and ammonium to nitrite (NO2-), and then nitrate (NO3-).

Under optimum conditions, almost all the ammonia and ammonium may be converted to nitrate. But what if things are not optimum? If there is not enough influent alkalinity to support nitrification, you may find an increase in nitrite exiting the biological treatment process. This might be at certain times of the day, or on certain days of the week, depending on your flows and influent characteristics.

To fully convert one pound of ammonia/ammonium to nitrate, it takes about 7.1 pounds of alkalinity to support the nitrifiers. If your influent ammonia is around 30 mg/l, you need 213 mg/l of influent alkalinity to complete nitrification. Remember that this is a minimum — you still need some for acid buffering in downstream processes, like disinfection.


Making a diagnosis

It’s not uncommon to see influent ammonia/ammonium higher than 30 mg/l, and some facilities see 50 to 75 mg/l at certain times of the day. If you think your plant might be facing an influent alkalinity deficiency, you should test the clarifier effluent for alkalinity, ammonia, nitrite and nitrate. Collect samples at the same time, or a little before the time, you see the effluent total chlorine residual drop.

Suppose you suspect that too little alkalinity may be keeping your plant from meeting the minimum chlorine residual. That

suspicion will be confirmed if you see that the alkalinity is very low (<40 mg/l as CaCO3), the ammonia/ammonium levels have increased, nitrite is present, and nitrate nitrogen has decreased. Just 1 to 2 mg/l of nitrite is all it takes to lose 5 to 10 mg/l of chlorine residual.

Under normal operating conditions, when influent alkalinity is sufficient, you would see clarifier effluent alkalinity greater than 50 mg/l, the ammonia and nitrite numbers at or near zero, and nitrate nitrogen elevated — say, above 15 mg/l as N.

Nitrite has a large demand on effluent chlorine residual. One mg/l of nitrite can consume about 5 mg/l of total chlorine residual, reducing the chlorine to chloride and rendering it useless as a disinfectant. Another byproduct of this reaction is the oxidation of the nitrite to nitrate. If you measured just nitrate in the chlorine contact chamber, you would see a spike in nitrate. This happens because the nitrite was oxidized by the chlorine to nitrate and used up in the reaction.

You need to measure nitrite and nitrate just before the chlorine injection point. If you add chlorine to the clarifier for algae control, collect your sample upstream of this chlorine feed. Even running the tests on settleometer supernatant would be acceptable.


What to do

If the analysis confirms your thoughts of an influent alkalinity deficiency, there are a few things you could do to correct the situation. One is to turn up the chlorine feed to maximum and even manually feed dry granular chlorine in the form of high-test hypochlorite (HTH) to the tank to boost the chlorine residual. That works to some extent, but it could be costly and unreliable.

You could boost influent alkalinity by adding lime (slaked into solution), sodium hydroxide (NaOH) or soda ash (sodium carbonate, Na2CO3), especially during times when normal influent alkalinity is not enough to cover the 7.1 to 1 ratio of alkalinity to influent ammonia/ammonium.

If your facility is not required to nitrify, you could inhibit nitrification by limiting dissolved oxygen, reducing aerobic retention times, or reducing sludge age. If your facility is required to nitrify, you need to ensure complete nitrification.

Another possibility is allowing denitrification to take place somewhere in the plant. Denitrification is the reduction of nitrate to nitrogen gas in the presence of facultative heterotrophic bacteria in an anoxic (dissolved-oxygen-free) environment.

During the biological reduction of nitrate, a small amount (about 3.5 mg/l) of bicarbonate alkalinity is re-established, helping to increase effluent alkalinity, and the interference of nitrite is reduced as well.

So there you have it. You have several solutions to look at, and some in-plant research to do to find out which would be the most effective, both for cost and ease of application. Always follow safety rules when handling chemicals, read the MSDS before using chemicals, and wear appropriate personal protective equipment. Be safe out there!


About the author

Ron Trygar is senior training specialist in water and wastewater at the University of Florida TREEO Center and a certified environmental trainer (CET). He can be reached at rtrygar@treeo.ufl.edu.


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