Trickling filters and CO2 strippers are common devices in recirculating aquaculture systems. They are often referred to as packed towers. The performance of these towers is controlled by: The type of packing or media. The amount of packing. The shape of the vessel. The ratio of air to water. The temperature. The distribution of air through the tower. The distribution of water through the tower.
The distribution of water is the most commonly neglected and least understood aspect of the design of packed towers. Poor water distribution is often the primary cause for poor performance and high maintenance costs for packed towers. A well designed tower will have an even water distribution across the entire packing surface. For a biofiltration system, this gives all of the microorganisms an equal and consistent supply of nutrients and oxygen. For a stripping tower, even water distribution provides a uniform water film on all of the packing for maximum gas transfer.
Poor water distribution can lead to the following problems:
- Loss of performance due to dry surfaces in the packing bed.
- Loss of performance due to air bypass through the dry areas.
- Erosion of the packing due to locally heavy hydraulic loadings.
- Plugging of the packing due to scale deposits in the intermittently wetted areas
If the water distribution is not done correctly, no other changes will bring the tower up to full performance. On the other hand, improving the water distribution in a tower is a very cost effective way to get more performance out of a system. Although the water distribution system is a key element in the design of a tower, it is a relatively small part of the system cost. Usually, it is less than 10% of the cost. If a distribution system only wets half of the media, the tower must be twice as large to get the same performance as a system that gives complete coverage. It cannot be overemphasized that an even distribution of water is essential to obtain full performance from a packed tower.
Water distribution designs that are less than optimum include the use of the following methods:
- A single pipe that dumps water in the center of the vessel.
- A pipe with a few large holes drilled into the bottom or side.
- A pipe with multiple small holes drilled into the bottom or side.
- A grid or drip plate.
- A system of troughs or launders with notches cut into the edge.
- A single, hollow cone nozzle.