Biotechnology and Bioinformatics (1+1)

6.2.4.7. Submerged bed filters

An under gravel filter is a classic down flow submerged bed filter. Submerged bed filters have been used extensively for small scale aquaculture and backyard water feature systems. These filters can be operated in up flow, down flow or cross (horizontal) flow.

The classic (old) systems

• Consisted of gravel with an under drain system.
• An improvement to these systems was the addition of air piping underneath. The air was used to 'bump' the filter to dislodge solids and restore full flow. The problems with these filters is the large size, low void fraction, tendency to plug and extremely high weight. In gene ral, these old gravel based systems are not suitable for modern aquaculture.

Modern submerged bed filters

• Are very efficient, have low head loss and are very easy to build and maintain.
• The key difference is the type of media and the water flow path.
• A modern submerged filter uses structured media in a cross (horizontal) flow mode. This type of biofilter probably comes closer to the ideal biofilter than any other type.

A typical installation would be configured similar to a raceway. The filter media is installed in a long trough. The length of the flow path can vary based on the retention time required. By using a relatively high velocity, it is possible to insure plug flow. This is a big advantage over well mixed systems or systems with short retention times. If it is not possible to remove the entire BOD before the biofilter, one will establish different zones in the filter. As nutrients are absorbed or removed in the first sections of the filter, different types of organisms will establish dominance in the zones where they enjoy optimum conditions. There are a variety of ways to configure a raceway type system.

Submerged filters can operate with or without aeration. If the flow path is long and the nutrient loading is high, it is wise to have aeration in the filter. One of the easier methods is the traditional aeration system with large silica air stones.

There is always the possibility to install the submerged biofilter media in the culture tank. This has the advantage of saving the cost a separate vessel and associated piping. The big disadvantage to this system is that it is difficult to remove the suspended solids before the water enters the biofilter. Because there are too many different configurations to draw them all, here is a brief description of a few of the possibilities.

1. Air lift the water into one end of a filter designed as a raceway and air lift it back into the culture tank at the other end.

2. Pump the water into a particulate filter such as a rotary drum and then flow through the biofilter.

3. Locate tubes or columns of packing throughout the culture tank and induce a flow through them with air stones.

4. Locate the filter media around the walls of the culture tank and induce a flow up through the media with air stones.

Sometimes it is not possible to use a raceway type biofilter system. If existing tanks must be used, it might be easier to build a system with internal recirculation. The advantage of internal recirculation is that it increases the velocity of water past the media and adds oxygen to the water. Increasing the velocity helps insure a more even distribution of water throughout the filter media and reduces the possibility of dead zones that are not receiving nutrients and oxygen. It also helps to keep particles in suspension. Suspended solids tend to settle out in areas of low water velocity. This is a problem because accumulations of solids can become anaerobic and contribute to poor water quality. Here are a couple of examples of internal recirculation systems. The cone bottom tank is preferred over the flat bottom tank because any solids that settle out will be removed immediately.