Troubleshooting Fixed Film Systems

 

The goal of this article is:

1.    1. To illustrate through actual case studies how to use the Four Basic Concepts of Wastewater in actual practice for fixed-film systems.

2.   2. To illustrate the importance of using the O&M Manual to gain an accurate understanding of the design parameters of the system in operation.

Learn Theory! Learn Theory! Learn Theory! If you know theory you can use it! If you don’t know theory you can’t use it!

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 The Georgia Rural Water Association is always available to assist any municipality that serve a population of <10,000 with process control, troubleshooting, training, lift station maintenance, smoke testing, leak detection, standard operating procedure development, asset management development, or any other compliance and operational issues that may crop up. To request their assistance, please contact them at Contact Us - Georgia Rural Water Association (grwa.org).

The next few posts will focus on Activated Sludge Concepts. After that, I will focus on some water related topics of interest.

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This first troubleshooting article is going to cover fixed film devices. Fixed film technology was the original concept behind trickling filters. The concept has evolved to the point where a thin film of biological “slime’ is grown on a wide variety of ‘media’.  The media is either submerged in the wastewater to be treated or the wastewater is poured, trickled or sprayed over the media in a constant stream. Over time the biological ‘slime’ thickens to a point where it will slough off and can be separated from the treated wastewater.

The media can consist of rocks, wood slats, flat plastic, corrugated plastic, plastic balls (similar to ping pong balls), and various configurations of plastic media that can be produced in sheets (like an air conditioner filter) or small aggregate material placed into a sump.

Years ago, when I worked for a chemical company, I was asked to come and evaluate chemicals for a wastewater system that served a religious camp. The camp was used at various degrees throughout the year, and the waste stream was, obviously, very erratic. Wastewater flowed from cottages around the camp into large sumps that captured paper and other solids before flowing to a large box. The box was equipped with submerged corrugated plastic media for slime growth. There was a space of around three feet under the media for the sloughed off material to settle and a sludge pump for pumping off the settled sludge. It was also equipped with a bower and the diffuser headers were located just underneath the plastic media. Air was constantly blowing through the headers to assist in controlling the thickness of the biological growth on the media.

The concept is simple: during the warmer months and periods of higher organic and hydraulic loading, the blower speed would be increased to increase agitation of the media to maintain the biological growth at a specific thickness on the media. During the colder months and during those periods when the organic and hydraulic loadings were less, the air flow would be reduced to reduce the agitation and allow the bacteria more time to develop the proper thickness.

When I arrived on the scene, the first thing I did was to inspect the facility to get and understanding of the principal components. The second thing I did was to review their flow and BOD loading. The third thing I did was to ask for the M&M Manual and compare the design specifications to the actual flow and BOD loadings of the plant.

During my walkabout of the plant, I noticed the SCFM air flow of the air distribution system was at the maximum 20 SCFM. When I looked at the O&M Manual, the recommended SCFM for their hydraulic and organic loadings was 5 SCFM. I asked the operator if he had ever adjusted the air flow of the unit. Of course, the answer was no, that the air flow was where it was at when they took over operation 13 years earlier. I asked if we could turn the ari flow to 5 SCFM, and the answer was no. I then asked if they felt comfortable if I adjusted it back to 10 SCFM. They were agreeable to doing that. Som while there, I adjusted the air flow back to 10 SCFM.

Two weeks later I called to see how the system was operating and was told this was the first time in 13 years the plant met permit. What was happening?

The way the system is designed to operate is this: the amount of turbulence in the treatment chamber is conditioned upon the relationship of hydraulic water flow and air flow. There must be enough turbulence to slough off the biological growth and maintain the correct thickness of the growth, but not too much to keep the sloughing’s from settling. The problem in this case is the turbulence was too high to allow a thick enough growth and to permit settling. So, they always had a high TSS leaving the plant during the winter, and a high BOD and TSS leaving the plant during the summer. Turning down the air reduced the agitation in the chamber, permitting a thicker growth, which produced a heavier slough that settled better, and reduced the turbulence in the tank enough to allow the sloughing’s to settle.

This is similar to what we had to do when operating a high-rate trickling filter. During the winter months we had to reduce the internal recycle to allow the biological growth to thicken on the filter walls. This is equivalent to increasing the sludge age in an activated sludge system. During the summer months we increased the internal recycle to speed up the sloughing so we did not get snails, which is an indication of too high of a ‘sludge age’ on the filter.

CONCLUSIONS

The O&M Manual should provide an accurate understanding of the design parameters of the system in operation. The design parameters are provided at 100% of flow. Most systems do not operate anywhere near full capacity, so the operator must make adjustments to the system to ensure the system is operating according to the correct design parameters. In some cases this may be taking process units out of service. In others, adjusting the air requirements or recycle rates based on organic, hydraulic or environmental (temperature) conditions.

Feel free to contact me should you serve a population >10,000 or have any questions or issues that the GRWA is unable to assist you with. Also, if you would like to contribute an article feel free to email me at the address below. I am always looking for contributors that have an interesting perspective, topic or has an interesting case that they would like to share – especially if the solution is a direct result of applying the principles from this forum.

Dennis Brown, Wastewater Specialist and Trainer, Retired

dbrown.grwa@gmail.com

678.750.3996