I was privileged to present at the 2010 Pumper & Cleaner Environmental Expo International in Louisville on operation and maintenance of aerobic treatment units.
The presentation was on the basics, addressing the vast business opportunities in maintaining the many technologies emerging in our industry. In attendance were some seasoned O&M professionals, and they had good technical questions. So here is a follow-up for those who are dealing with a common problem.
Why does it happen?
One of the most common questions is how to deal with scum on the clarifiers. Sometimes the scum consistency is heavy and greasy, and sometimes it is light and fluffy. You can check this by running it between the fingers of your gloved hand.
If you suspect fats, oils and grease (FOG), a laboratory analysis can verify. It’s important to work with a certified water-quality lab, as they use special glass containers with preservatives that stabilize the samples.
Either way, we need to ask: Why is it happening? A number of situations can cause scum formation in the clarifier. We first need to look upstream and ask about the use of the system. Is the system serving a commercial facility such as a restaurant, deli, laundry or auto shop? Or, if a residence, what are the cooking habits? Do the occupants run a home business? Or is it an intermittently used vacation property? Any one of these situations can cause this condition.
Try education
If the upstream findings are bad management practices resulting in FOG down the drain, we can begin with employee or homeowner education. Good practices include diverting high-strength waste by pouring oils and greases into the trash, and wiping hands, surfaces and utensils with disposable towels.
In some cases, even good practices can’t make up for systems that were not designed for the facilities they serve. A grease interceptor installed between the facility and the treatment unit is a good option in most cases. Some ATUs are designed with minimal trash trap storage before the aeration chamber.
But if the facility uses high-temperature cleaning systems, FOG does not have time to solidify in primary treatment and ends up surfacing in the clarifier. Another common problem in commercial facilities is the use of chemical degreasers. These products emulsify greases, sometimes sending FOG downstream into the soil treatment area. That is not a good scenario, as the soil pores will eventually plug, causing failure.
Bulking sludge
If we determine that FOG is not the problem and the scum is light and fluffy, then we most likely have a condition known as bulking sludge. Traditionally ATUs operate with a continuous air supply. In times of low or no flow, dissolved oxygen levels increase in the clarifier. This causes an imbalance affecting the nature of the sludge, which typically settles into an anaerobic state. With the higher oxygen levels, nitrogen gas releases at a high rate, carrying settled sludge to the surface — thus the fluffy effect.
ATU manufacturers are addressing this issue in various ways. Most common is to program a vacation mode into the control panel, where the air pump can be sequenced on and off to control the DO level.
Others are looking at adding an alternate food supply when the facility is not in use. I have heard of some using dog food, commercial fertilizer, corn syrup or other carbon sources that keep the bacteria healthy and consuming both food and oxygen, thus maintaining the needed balance, or food-to-micro-organism (FM) ratio.
This approach is intended to and will increase BOD, so it is important to find the balance and avoid overloading the system. The best first step is to consult with the manufacturer for advice. If it is an older unit and you can’t get technical support, it might help to add a time clock for the air supply, and an automatic food source, or do both.
If you go this route, management needs will greatly increase, especially during setup and fine-tuning. The bacteria in aerobic processes are very sensitive and can easily be killed off if starved of food or oxygen for extended periods.
Achieving the proper balance of air on/air off to maintain the optimum 2 mg/l DO will require frequent monitoring. I have heard of sequencing from 20 minutes off/30 minutes on, to 2 hours off/6 hours on. It will depend on the type of air supply and operating system: diffusers, airlift, recirculation or others.
Settling test
In any case, the scum on the clarifier needs to be removed by vacuum truck, pumping, or manually transferring it to the primary chamber of the treatment system.
In addition, controlling the solids levels in the system is critical. Routine O&M should always involve a settling test. A sample of mixed liquor is drawn from the aeration chamber and placed in a straight-walled container with graduated markings. A clear beaker or graduated cylinder is best.
The sample should be set aside for 30 minutes and checked for settleability. The desirable range of solids should be between 20 and 60 percent. This test is like a snapshot of conditions in the clarifier.
If the solids are above 60 percent, a portion of the sludge should be removed (wasted) from the clarifier. The minimum 20 percent is also important, as this sludge is considered activated and contains hungry bacteria that will be returned to the aeration chamber as part of the system’s function.
Of course, a number of other operating conditions can cause treatment problems. Toxic loading, overuse and inflow/infiltration are topics in themselves. In summary, here are some of the key points for evaluating and controlling scum in clarifiers:
• Determine the characteristics of the scum
• Perform laboratory analysis if necessary
• Evaluate the use of the system
• Educate system users
• Consult with system manufacturer
• Upgrade the system with appropriate technologies, such as a grease interceptor, air-supply controller, or alternate food source
• Control solids in system
• Perform all necessary maintenance
• Record all findings and activities
• Learn, learn, learn
Learning places
There are good programs for professionals interested in learning more about the operation, maintenance and monitoring of advanced treatment systems. The Consortium of Institutes for Decentralized Wastewater Treatment has an excellent program offered as a national certification by the National Association of Wastewater Transporters. Check the NAWT Web site at 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.
