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Effect of Alum Sludge & Anaerobic Bacteria

There have been articles concerning the use of alum to capture phosphorus within an individual waste water treatment system septic tank. One of the articles appeared in an EPA publication, click here to view the link. The link describes: “The controlled addition of chemicals such as aluminum, iron, and calcium compounds with subsequent flocculation and sedimentation has had only limited success because of inadequate operation and maintenance of mechanical equipment problems and excessive sludge production.”

I offer the results of my research to dispute this statement. During the 4 years of using alum to condition the wastewater from my house, the septic tank sludge level was monitored on a monthly frequency. Two weeks before the alum testing started, the tank was pumped and washed clean by a septic pumping contractor. The sludge level increased dramatically for the first 6 months but then decreased and leveled out over the subsequent years. The record of these measures is shown on the following graph.

I also recorded sludge accumulation after tank cleaning without alum and the initial six months of sludge increase was almost identical to the alum sludge profile. After some thought, I realized that sludge level represents volume of organic waste at the bottom of the tank. The volume increases over time as a result of the daily addition of waste from the house. This increase in volume ceases once the anaerobic bacteria digestion rate “catches up” to the rate of organic matter deposition. Furthermore it appears that the digestive bacteria regulate the level of sludge through their environment selection process. The environment within the sludge is highly anaerobic but the interface between sludge and liquid may be a transition zone to aerobic digestion – a totally different type of bacteria.

From my experimentation and observations, I would conclude that too frequent pumping of septic tanks may inhibit the digestion process due to lack of sufficient bacteria colonies to develop.

Tanks that do not have a healthy sludge condition may very well pass undigested organic matter into the absorption field that would otherwise be dissolved by bacteria breakdown. My TSS measurements however, do not strongly support the idea that more solids pass out of the tank in tanks without a healthy sludge accumulation. My recommendation to all homeowners is to not hose down tanks after pumping and rather leave an inch or two of sludge in the bottom of the tank in order to more quickly reestablish the organic digestive capacity.

The question might be asked, did my testing show that sludge accumulates to a greater amount when alum is used. The graph shows the sludge depth over 42 months. The average depth remained almost constant. Over the 42 month testing of alum, approximately 35 gallons of 50% liquid alum was used.

The use of alum does not inhibit efficient bacterial digestion because of the sludge accumulation volume. When no alum is used, the sludge accumulation volume is almost the same.

My testing and research has shown the following results:

  1. Alum, when used in discreet amounts to remove the phosphorus from the entering wastewater, does not inhibit anaerobic digestion.
  2. Sludge accumulation with alum over 42 months averaged 12 inches with no cap.
  3. Sludge accumulation without using alum over 10 months averaged 8 inches with 1 ½ inch cap.
  4. The use of alum increased the total volume of solids in the tank from 9 ½ to 12 inches over 42 months.
  5. The pH of the wastewater remained essentially the same with the use of alum as without the use of alum.  TSS with alum was 42% lower than TSS without alum.
  6. The volume of alum added to the wastewater was 0.04%
  7. Washing out the septic tank after pumping delays the anaerobic bacteria formation and results in an initial excess buildup of solids. Some sludge (1-2 inches) should be left in the tank after pumping to provide early bacteria formation.
  8. The pump out frequency for a 2 member household can be 3 years in order to conform to most septic regulations. With more in the household, and a 1000 gallon tank, the frequency should be conservatively increased to every 2 years.

There was no excessive buildup of sludge when using alum. In fact the sludge volume was very predictable. Whether alum or no alum is used, any introduction of harsh chemicals (those that are bacteria cleansing agents) may alter the normal formation of sludge volume. Hence in normal practice, cleaning agents, particularly chlorine types should never be mixed with the waste water in a household septic system. The above comments refute the comments in the EPA article about use of alum.