Aerobic Treatment Units: Carful Evaluation to Ensure Consistent Results

Careful evaluation of complex operating systems is required to ensure excellent results over the life of the unit.

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Last month I explained general operating characteristics and air supply concerns of operating aerobic treatment units. This month I’ll look at what service providers should check in the aeration chamber of a unit.

Ensuring the proper operation of an ATU requires several evaluations. Good mixing of materials in the chamber is required to achieve the expected level of treatment. This puts the organic material in contact with the microorganisms in the chamber. Just like when we use soil or other media for treatment, having contact and time are important in the aeration chamber as well.

If there is not good contact and the waste is not there long enough, the treatment will not happen. Visual inspection in the chamber while operating should show a rolling appearance of the liquid and you should hear the sound of mixing water; if these are not present, a likely aeration problem needs to be addressed - perhaps orifices are clogged, air supply pipes are broken; something needs attention.

THREE FACTORS

Other items need to be evaluated, requiring taking a sample from the chamber. These parameters are dissolved oxygen, pH and temperature.

Dissolved oxygen concentration in the aeration chamber can be checked by collecting a water sample and using an oxygen meter or colorimetric test to determine the oxygen content. The dissolved oxygen content should be greater than 2 mg/L. This demonstrates there is adequate aeration for the microorganisms to facilitate breaking down the waste.

If the dissolved oxygen content is high, above 6 mg/L, then the unit may have just been turned back on before the time of the service call. The service provider should discuss the need for continuous operation with the homeowner and explore why the unit may have been turned off. Issues related to either noise or odor should be addressed. For instance, homeowners may turn off a compressor if it’s running outside a bedroom window.

A pH meter can be used to test the pH of the water. A pH in the neutral range, around 7.0, is good for microbial growth. If you are a service provider, recognize a pH meter needs to be calibrated periodically to insure proper readings. Most meters do not do well carried around in the cab of a pickup, where they are subjected to extreme heat and cold, dust, etc. They need to be protected and cared for properly.

Temperature in the unit can vary, though a moderate temperature is best. A temperature too high or low will not be conducive to microbial growth. This is one reason why – particularly in cold climates – a unit that is only used periodically in the winter will not perform well. The bacterial action decreases as the temperature decreases resulting in reduced treatment. In cases like this, an aerobic treatment unit as part of the treatment train is not the best choice.

SLUDGE LEVEL

In systems with a separate aeration chamber, you can use a probe to check for accumulation of sludge in the bottom of the system.  

The amount of biomass is measured using a 30-minute settleability test. For this you’ll need a beaker or other container – it doesn’t have to be glass – with 10 equal graduations on the side. A mixed liquor sample is pulled from the aeration chamber and allowed to settle for 30 minutes in the beaker. At the end of the 30 minutes, the solids and liquids have separated. The solids should be between 20 to 60 percent of the volume. If the volume is greater than 60 percent, you’ll need to pump solids from the system.

This leads to an interesting point that gets brought up a lot – there have been suggestions that only a portion of the material be removed. Any good pumper will tell you that if you want to get rid of the excessive solids you will need to pump the whole amount because in a tank with liquid and solids, the last thing to come out are the solids. So if the test indicates the unit needs to be cleaned all of the material has to come out.

PLUGGED MEDIA

The color of the biomass material on the media should be checked. The color should be brown and not black. If the biomass appears black, aeration should be checked and the chamber pumped.

Also check for plugged media, which is caused by excess biomass growing on the media over time. Not only will the biomass cause plugging, but it may also lead to a condition where the biomass is floating as it grabs onto air as it is being aerated. When this condition occurs – or there is bridging of the biomass across the media – it is time to clean the media.

Blow air through the media using a wand from underneath to clean it. You can also pressure wash the media. In some instances, the media may be beyond cleaning and will need to be replaced.

USE THE RIGHT PARTS

One final note if you are considering repairs or replacing parts: You should check with the manufacturer to see what other maintenance is required and how it should be performed. The manufacturer can provide guidance specific to the product. The recommendations given by the manufacturer need to be followed. This may involve specific manufacturer training.

Additionally, use only replacement parts approved for the particular ATU model. These are engineered systems designed to function using specific parts or types of equipment. Failure to follow these guidelines will likely result in a loss of performance. If the unit is installed to meet specific BMP requirements this may void the operating permit.



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