Understanding Alkalinity

Knowledge of this principle of wastewater chemistry can help you control your process effectively – and perform well on licensing exams.
Understanding Alkalinity
An operator uses a portable pH meter (Oakton pHTestr 30), buret and magnetic stir plate to titrate a sample to pH 4.5 during an alkalinity test.

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In my TPO article last December (“What Exactly Is pH?”), I explained pH in detail, including in-depth descriptions of atoms, atomic structure and molecules. The article’s subtitle mentioned “a look at what acidity and alkalinity mean at the molecular level.”

I didn’t mention alkalinity too often in that article. This article focuses on alkalinity, where it comes from and how it can affect wastewater treatment processes.

What is alkalinity?

Alkalinity is the ability of a liquid or substance to resist a change in pH, or the capacity of water to buffer against an acid. However you might word it, the main principle is that alkalinity protects against acids.

Alkalinity is present in liquids as dissolved minerals like calcium and magnesium. These alkali metals are found everywhere in nature, especially in the earth’s crust. Potable water treatment plants sometimes use groundwater as a source, and this water may contain many milligrams per liter (mg/L) of dissolved calcium and magnesium. Some people notice this in their household fixtures and refer to it as hard water, or lime-scale buildup. Commercial products that dissolve this scale usually contain strong acids.

Once drinking water reaches a sink or shower drain, it becomes wastewater. Thus, the amount of alkalinity in wastewater treatment plant influent is usually close to the alkalinity in the potable water supply. There are exceptions, especially considering the source and type of drinking water treatment, industrial contributions to the sewer system and rainwater inflow and infiltration.

Measuring alkalinity

We measure alkalinity using test kits that contain reagents and dilute acid, or directly measure the pH while adding acid to a sample. Let’s look at measuring total alkalinity using the latter method, which is referred to in Simplified Laboratory Procedures for Wastewater Examination. It can be used to measure the alkalinity of samples like plant influent wastewater, plant effluent, mixed liquor suspended solids (MLSS), trickling filter and RBC fluids, and plant biosolids.

Starting with a 100 mL sample, measure the pH of the sample using a properly calibrated pH meter and probe assembly. Once the reading has stabilized, titrate the sample with 0.1 N or 0.02 N (N = normality) sulfuric acid (H2SO4) using a buret and stirrer until the pH reading reaches 4.5. Read the amount of acid used to reach pH 4.5. The amount of alkalinity in the sample is calculated using the formula shown:

Alkalinity, mg/L as CaCO3 = (mL H2SO4 x NH2SO4 x 50,000) mL of sample

In the equation above, 50,000 represents the equivalent weight of CaCO3 (50) multiplied by 1,000 mg. Fifty thousand is a constant used in the formula. When alkalinity is reported, it is expressed as calcium carbonate, or CaCO3-.

What if the sample used in the test above was distilled water? Distilled water has essentially nothing in it other than hydrogen and oxygen (H2O) and has no way to buffer the acid, so the pH drops rapidly toward 4.5 with little acid addition. Just 1 mL or 2 mL of acid might be enough to drop the pH to 4.5. The reported value of alkalinity for these samples might be expressed as 0 mg/L alkalinity as CaCO3-.

If the sample described contains high amounts of calcium and magnesium, it may take a lot of sulfuric acid to get the pH to drop. In some areas, the drinking water could contain 200 to 400 mg/L of alkalinity, as CaCO3-. Influent wastewater that contains roughly 200 to 250 mg/L of alkalinity as CaCO3- usually contains enough buffering capacity to prevent low pH values at the end of the treatment process.

Alkalinity and treatment

Calcium and magnesium are essential elements bacteria need to carry on metabolic functions and reproduce. Other essential elements include nitrogen, phosphorus, potassium, iron, sulfur, oxygen, carbon and hydrogen. Without these elements present, the bacteria in treatment plants would not function properly, and could result in an over-abundance of nuisance bacteria — the kinds that cause odors or inhibit settling.

We need some alkalinity to buffer against drops in pH values. Processes that biologically produce acids or acidic chemicals include:

  • Biological nitrification (the conversion of ammonium to nitrite then nitrate)
  • Anaerobic conditions in sewer systems
  • Anaerobic conditions in primary clarifiers
  • Anaerobic sludge digestion processes
  • Anaerobic fermentation basins in biological phosphorus removal systems
  • Chemical coagulant addition (aluminum sulfate, ferric sulfate, ferric chloride)
  • Pure gaseous chlorine for disinfection

When these biological conditions occur in a treatment plant, or when acidic chemicals are added, the free hydrogen (H+) in the acid reacts with the negatively charged alkalinity, and the two effectively neutralize each other. If the water contained only the exact amount of alkalinity required to neutralize the acids, there would not be enough alkalinity remaining to protect the final pH value from falling if any additional acid were formed or added downstream.

Biological processes like nitrification and anaerobic digestion rely on alkalinity. Without alkalinity, organic acids formed during these processes would drive the pH down to a point where the bacteria would be inhibited or could no longer survive.

For instance, during the acid-formation stage of anaerobic digestion, volatile fatty acids are produced as acid-forming bacteria feed on the viscous, nearly septic sludge. A second group of bacteria, methanogens, then consume the volatile fatty acids. From this reaction, methanogens produce methane and bicarbonate alkalinity. The alkalinity they produce helps buffer the acid produced by the volatile acid formers.

In a properly operated anaerobic digester, the ratio of volatile acid to alkalinity is between 0.1 to 0.25 parts acid for every one part alkalinity per liter. If a digester is overfed and volatile acids are rapidly increasing, the methane formers can’t consume the acids fast enough. This causes the alkalinity to become depleted.

For example, if the volatile acid climbs rapidly to 1,500 mg/L and the alkalinity is steady at 3,000 mg/L as CaCO3, then the ratio becomes 0.5:1, and methane production slows down or even stops. If the operator does not take corrective action, the digester may become sour and stop working completely. In fact, if the operator did not pay attention to alkalinity and used pH as the sole operating process control parameter, the digester could become sour before the pH finally indicated an operating problem.

Supplementing alkalinity

If the alkalinity present in the influent is not sufficient, or if there is a need to increase alkalinity in the treatment plant, chemical addition can help correct the deficiency. Common chemicals used to increase alkalinity and pH include:

  • Calcium oxide or calcium hydroxide (as lime slurry)
  • Sodium hydroxide (caustic soda)
  • Sodium carbonate (soda ash) or sodium bicarbonate
  • Magnesium hydroxide or magnesium bicarbonate

Sodium hypochlorite (bleach) and calcium hypochlorite (granular chlorine) will raise the liquid pH and alkalinity while performing as disinfectants. Care must be taken when using these chemicals, not only because of their very high pH and corrosive effects, but also because of the dangerous chemical reactions that occur when they are added to low-pH liquids and biosolids. Always handle chemicals with caution; read and follow the recommendations found on the MSDS documents and labels.

In summary, alkalinity can be a useful process control tool. Keeping an eye on the alkalinity coming into the treatment plant, through the various unit processes and in plant effluent can provide clues to biological and chemical changes, sometimes helping prevent process upsets. Remember that pH and alkalinity are not the same thing — they are measurements of two distinct and separate chemical conditions.

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.  

References

Simplified Laboratory Procedures for Wastewater Examination, Third Edition, Water Environment Federation, 1985.

Operation of Wastewater Treatment Plants, Volume II, Sixth Edition, California State University, Sacramento.

Wastewater Residuals Stabilization, MOP FD-9, Water Environment Federation, 1995. 



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