Biotechnology and Bioinformatics (1+1)

6.2.2.2. Particulate Filters

• Once sufficient oxygen is provided, the next easiest way to improve water quality is to remove suspended solids.
• This is a more difficult task since particles come in all shapes, sizes and densities.
• Suspended solids consist primarily of uneaten food and feces which are slightly denser than water. Large particles, above 100 microns, will settle out quite easily. Particles above 50 microns can be filtered out with a screen. Particles below 10 microns are difficult to filter and are generally removed by some other means.

There are different types of particulate filters that can remove suspended solids. They generally fall into three broad categories.

• The first type are settling basins, tube settlers, plate settlers, swirl separators and similar systems that allow the particles to drop out of the flowing stream by gravity. They are relatively simple devices and they work well on large particles. Settling systems generally have very low pump head requirements.
• The second type are sand filters, sock filters, drum filters, disk filters, belt filters and similar systems that mechanically remove the particles from a flowing stream. These types of systems "screen" the particles. The size of particle removed is dependent on the size of the screen or sieve. Pump head requirements can vary from low to very high. Some biofilters such as bead filters claim to do both particulate filtration and biological filtration.
• The third type of particulate filter is the floatation or bubble seperator. These are commonly known as protein skimmers. In this device, air is bubbled into a column and the fine particles become attached to the surface of bubbles. The resulting froth or foam is collected and removed from the system. These devices require a certain amount of surfactant type compounds in the water in order to work properly. Although they are not typically designed for solids removal, some submerged biofilters will tend to collect fine particles due to the sticky nature of biofilms. This can be both a benefit and a maintenance problem. If the biofilter is not designed for easy cleaning, solids collection can represent a maintenance headache.

Removal of suspended solids is important since suspended solids comprise the majority of the BOD ( Biological Oxygen Demand ). The BOD not removed by the particulate filtration system must be removed by the biofilter before effective ammonia removal will occur. Thus the size of the biofilter is influenced by the effectiveness of the particulate filter.

The way that solids are removed is also important. The best systems remove solids quickly without degrading them in any way. If the solid particles are broken or reduced in size, it makes it easier for nutrients to dissolve into the water. These nutrients must then be removed by another part of the water treatment system or flushed out by water exchange. Time is also important because the longer solids are held in the system, the more degradation will occur. Floating bead filters are particularly bad in this regard since they hold the solids for long periods of time before backflushing.