Biotechnology and Bioinformatics (1+1)

6.2.4.6. Trickling filters

Trickling filters are one of the oldest types of biological filters. Trickling filters filled with rock or coal were built in the late 1800's for sewage treatment.

• Trickling filters use a solid support media to provide surfaces on which bacteria grow and accumulate.
• In a trickling filter waste water is sprayed over a rock or synthetic media bed. Waste water is applied on an intermittent basis and collected in a sump underneath the media.
• Bacteria attach and grow on the media.
• Natural air currents supply oxygen.
• In some systems, air is forced into the filter with a fan. However, most filters rely on natural convection and diffusion to move air throughout the filter.

The first step in the design of a trickling filter is to pick the right packing or media. Over the years many different materials have been used for trickling filters but today, the best packing is structured media. Structured media is composed of sheets of rigid PVC that are corrugated and glued together to form blocks.

One of the advantages of structured media is it's flexibility and ease of use. Structured media can be used to build a convenient biofilter without a vessel. Since the vessel is typically the major cost of a biofilter, a biofilter with no vessel can be a real money saver. Structured media can be stacked on a frame work or any flat surface. It can be located over a culture tank or have its own water collecting sump. No sides are required because the packing is self supporting.

The most important requirement in the design of any trickling filter is a good water distribution system at the top. There are two common ways to do this. A pressure spray system with splash guards at the top is probably the simplest. The only drawback is the additional pressure drop required to operate the nozzle. The other system involves the construct ion of a shallow water distribution pan with several gravity flow target nozzles in the bottom of the pan. Here are some typical arrangements for a "vessel-less" trickling filters.

Designing a trickling filter is to balance the competing requirements on the design.

Advantages

• Trickling filters are rugged and easy to operate.
• They have the ability to treat a wide variety of nutrient levels. Properly designed systems can handle solids very well.
• One of the big advantages of a trickling filter is that the water can leave with more oxygen than it entered.
• Because trickling filters have a large - air water interface, they also act as strippers to remove CO₂, H₂S, N₂ or other undesirable volatile gases.

Drawback

• Energy cost required to pump the water to the top of the filter.
• A high narrow filter will save space but take more pumping energy.
• A wide low filter will use less energy but take up more space.
• In order to keep the energy costs to a minimum, the pumping head for the filter should be as low as possible. The maximum plan area covered by the filter is determined by the minimum water loading.
• In order to minimize the floor space used by the filter, the filter should be as tall as possible. The practical limitations are the height of the building, the head limits on the pump and the structural and stability considerations of the vessel.
• A taller filter will have a longer flow path for the water. This means a more complete treatment of the water with each pass.
• Taller filters will have higher specific water loadings. This means better flushing action, more turbulent water films and higher ammonium removal rates.

Trickling filters for industrial applications are sometimes 30 ft. tall. This is not practical for aquaculture systems. In gene ral, trickling filters for aquaculture are between 4 and 10 ft. tall.