Understanding ATUs

Aerobic treatment units work on the same basic principles as municipal wastewater treatment plants. They require special knowledge and good maintenance.
Understanding ATUs
Cross-section diagram of a typical single-compartment septic tank. (Photos courtesy of Kit Rosefield)

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In September Onsite Installer, we discussed media filters as one advanced treatment option to deal with site and soil limitations. Now we will expand that topic to include the many configurations of aerobic treatment units (ATUs).

Before we get into the types of units, we need to look at the basic operating principles. Almost all ATUs operate with the common municipal wastewater treatment process known as activated sludge, where beneficial microbes live in a solution called mixed liquor. This is a combination of the microbes, oxygen and their food – pathogenic organisms and organic and inorganic compounds – biochemical oxygen demand (BOD).


Two basic types

ATUs use two basic configurations: suspended growth and attached growth (or fixed film). Both involve fully saturated environments (submerged in liquid). In suspended growth systems, an air supply maintains the desired dissolved oxygen (DO) level and mixes the solution to maintain suspension of the activated sludge – the solids – during aeration.

The activated sludge is formed as the beneficial microbes and food adhere to each other and become heavy. As the wastewater moves from aeration into a clarification phase, the heavy particles settle to the bottom and form a sludge blanket in the clarifier.

As the microbes and their food settle in the clarifier, now deprived of oxygen, they become stressed and in some cases die off. In a well-operated system, some of the sludge is returned (return activated sludge) to the aeration basin. There, the oxygen-deprived microbes begin eating again, and at a higher rate than normal. The clear liquid at the top of the clarifier (supernatant) either flows over a weir or is pumped off in a timed sequence.

In attached growth processes, the microbes attach to the surface of a submerged media, where they form layers as the food attaches to them. As these layers of activated sludge become heavy, they slough off and settle to the bottom. Many attached growth processes get their mixing and oxygen through an airlift function that moves the mixed liquor and some settled sludge up and over the media, creating a circulating affect.

In both suspended growth and attached growth processes, sludge accumulation needs to be managed to maintain the proper food-to-microorganism (FM) ratio. Periodic removal of sludge (wasting) is needed to keep the system in balance. The removed material is called waste activated sludge (WAS).


Where it all begins

Aerobic microbes don’t like old, stale food, so it is rare to see a full-sized septic tank at the head of an ATU treatment train. Many ATUs use a trash tank to collect the larger junk that gets flushed down the drain. Most trash tanks store no more than 24 hours of raw wastewater.

Maintaining these smaller tanks is important. Maintenance frequency totally depends on how well you educate the system user on what they can put down the drain. Talk dollars and cents – when you explain how much it costs to pump such a small component, the economics speak for themselves.


Is there enough food?

In normal use (a home occupied full time), an ATU should need no attention beyond routine O&M. But if the unit is on a vacation property used part time, an alternate food supply or startup period might be necessary to keep the microbes healthy and active. It is best to check with the ATU manufacturer to see if they recommend supplemental feeding for partial occupancy. I have heard of adding dog food, corn syrup, or other carbon-based products to be sure the microbes get the nutrients they need.


Is the chemistry in balance?

A couple of chemical balance considerations can make or break a healthy microbe population. One very important element for most life forms is oxygen. Most ATUs have air systems designed by the manufacturer to maintain a DO level of 2 to 6 mg/l. For most aerobic processes, 2 mg/l is the optimum DO level.

If DO levels fall much lower, microbes can die off or get sluggish. On the other hand, excessive oxygen can create conditions that encourage growth of other organisms that compete with the desirable microbes for the food. These include filamentous organisms, which look like a network of fine hairs in the mixed liquor and form a kind of net that keeps solids from settling, allowing them to pass through the clarifier and on to the next treatment train component.

If that happens to be a disinfection unit, the unwanted solids will impede disinfection. If the next component is the soil treatment unit, solids can foul the soil, pores, resulting in premature failure.

Higher oxygen levels can also cause bulking of the settled sludge, especially if excessive sludge is present. Bulking sludge rises to the clarifier surface, creating a gooey mat that typically has to be removed manually.

Another chemical consideration is pH, a measure of acidity or alkalinity. The range of pH is from 0 to 14. Seven is neutral, lower pH numbers are acidic, and higher numbers are alkaline. The lowest pH to which microbes should be exposed is about 5.5, and the highest is about 8.5. In a normally operated system, pH should not be an issue, but if users are sending inappropriate chemicals down the drain, things can go haywire pretty fast. This is where a residential evaluation survey is extremely helpful (visit the Consortium of Institutes for Decentralized Wastewater Treatment at www.onsiteconsortium.org).


Managing solids

For evaluating and managing solids, there are two basic methods. The first, which relates to the volume of solids in suspension, is a 30-minute settling test. Here, we draw a sample from the aeration chamber, fill a 1,000 ml graduated cylinder or beaker, and set it aside where it won’t be disturbed. It is a good practice to check and record the settling after about five minutes to get an idea how fast the solids are settling. After a half hour, the solids should be fully settled. We typically want to see no less than 20 percent and no more than 60 percent solids by volume.

If we are below 20 percent and no wasting has been done recently, we may have a low FM ratio. If we are above the 60 percent mark and it appears settling is complete, it is time to waste solids from the clarifier. If settling appears incomplete and there is still an undefined separation, there could be low oxygen or an unwanted condition such as filamentous growth. If filamentous growth becomes an issue, a thorough pumping and cleaning of the unit is recommended.

Another good way to decide when to waste is to use a sludge depth device. If the sludge blanket becomes deeper than 30 percent of the total clarifier depth, then wasting to achieve a 20 percent level is recommended. Wasting can be done with a submersible pump or a vacuum truck. If a truck is used, that’s a good time to clean the trash tank and possibly the discharge pump tank.


Servicing the air supply

ATUs use many types of air supplies, including diaphragm and rotary vane compressors, ring blowers, and submersible vacuum aspirators. The air supply is usually coupled with a fine- or coarse-bubble diffuser, air injection via an aspirator, or airlift and cascade air transfer.

Each option has different service needs, specified by the manufacturer. Some have air intake filters and some have air exhaust filters. Whatever the configuration, get trained by the manufacturer and be clear on what the unit is supposed to be doing. Airflow and air pressure tests are valuable to see if the air supply and distribution systems are performing as designed. Some ATUs have high- or low-pressure alarms or high-liquid-level alarms.

So, let’s summarize what we’ve covered.

1. ATUs are similar to small-scale municipal treatment plants.

2. They can be susceptible to underloading or overloading of BOD.

3. Keeping the proper oxygen level is critical to supporting a good microbe population.

4. High or low pH can adversely affect the biological process.

5. Managing solids is important to controlling a number of factors, including oxygen levels, undesirable organisms such as filamentous, and bulking of sludge.

6. Know the type of air supply and its maintenance requirements.

7. Learn how to test and verify alarm and monitoring systems.

8. Be properly trained by the ATU manufacturer.

9. Educate your users and yourself.

The next O&M Matters will feature disinfection, then keep an eye out for the final article on drip dispersal. For information on O&M training programs, visit www.nawt.org.


About the author

Kit Rosefield is an adjunct instructor at Columbia Community College and a trainer for NAWT and the California Onsite Wastewater Association. His company, Onsite Wastewater Management in Mi Wuk Village, Calif., has a consumer education service at www.septicguy.com. Reach him at 209/770-6760 or kit@septicguy.com.


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