Win the pH Balancing Act

Choose your septage or grease waste stabilization material carefully and then follow appropriate safety procedures to prepare loads for land application or disposal at a the treatment plant.
Win the pH Balancing Act
Jim Anderson, Ph.D., is an emeritus professor at the University of Minnesota Department of Soil, Water and Climate and recipient of the pumping industry’s Ralph Macchio Lifetime Achievement Award. Email Jim questions about septic system maintenance and operation at

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Acolumn I wrote several months ago about alkaline stabilization of septage spurred a few reader questions, so it’s a good time to revisit the use of lime for pathogen reduction, vector control and odors.

To meet the pathogen reduction requirements to land-apply septage, the pH must be raised to 12 or greater and held for a minimum of 30 minutes. For grease traps where the grease is hauled to a municipal treatment plant, there are usually pH requirements for the material delivered to the plant. These requirements are a part of the federal National Pollutant Discharge Elimination System permit to operate and discharge.

Since grease trap waste can be very acidic — pH less than 5 — it may be necessary to raise the pH before it can be accepted at the treatment plant. This is because as grease breaks down, fatty acids are formed, lowering the pH. The longer the period of time between clean-outs, the lower the pH will go. It is not uncommon to find pH around 4 in a grease trap that is pumped every 3 months.


The most common materials used to raise pH are calcium hydroxide (hydrated lime) and calcium oxide (quicklime). Other materials can be used, but then, in general, more material will be required. These are most often a manufacturing byproduct. There is also the potential for those materials to carry other pollutants, such as heavy metals. Before using other materials, get an analysis to make sure they will not cause other problems.

A question often asked is whether agricultural lime can be used. The purpose of agricultural lime is to raise the pH of the soil to between 5.5 and 6.5 for certain crops. It is not appropriate for raising the pH of septage to 12. Lime materials should be purchased through a chemical supplier — not the local agricultural store or garden center. Some pumpers make this mistake because it is labeled lime and is a cheaper material.

Another common question is how to add the lime to the septage. The most common method I have seen is to make a slurry out of the dry lime material and draw the slurry into the truck at the time a tank is pumped. The slurry is drawn in at an equivalent rate of 20 to 30 pounds of dry material to 1,000 gallons of pumped septage. The actual amount required varies depending on the amount of solids in the septage. More solids require more dry material equivalent.

Usually, this slurry is carried on the truck in 5-gallon buckets, each one holding the 20 to 30 pounds of dry equivalent. The slurry can be drawn in either before or after the tank is pumped; most pumpers I have talked with prefer to draw the slurry in before pumping the tank.


Some pumpers have reported they add the lime in the dry form. This can be done through the top ports of the truck or through the vacuum line. Many pumpers relate that sucking dry material through the vacuum line has resulted in excessive pump wear, so this practice should probably not be followed on a continuous basis. The dry material may clump in the bottom of the tank and then not mix well with the septage. If calcium oxide is used, it can react with moisture in the vacuum hose, causing damage to the hose.

Calcium oxide is more reactive than calcium hydroxide; it is important to take safety precautions when handling the material. It can cause serious burns if it gets onto the skin or in the eyes. Rubber gloves, respirators, protective eyewear and clothing covering the skin are a must. There is also a fire danger when storing bags of calcium oxide, which can ignite if it gets wet. If a fire does start, it cannot be put out with water. It must be put out using a carbon dioxide extinguisher. For these reasons, most pumpers prefer using calcium hydroxide even though it takes a little more material to raise the pH.

Other questions often asked are whether the material can be added directly to the septic tank, and then if all the contents can be pumped into the truck. The concern is that this disrupts the biology of the tank and affects treatment after the tank is pumped. This is probably not the case. When sewage flows back into the tank after cleaning, there are more than enough bacteria to restart the tank, so any reduction in treatment would be very temporary. The main question should be whether the method chosen allows the septage to mix with the material so the pH is raised to the proper level.


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