A small biological sand filter (slow sand filter or Biosand filter) is maintained or cleaned by actively managing only the top 5 cm of sand. This top layer is either removed and replaced when the flow rate becomes unacceptable (typically 4 to 6 months) with clean sand or “wet harrowed” meaning the sand is gently agitated causing partial break up of the biological surface. This may be slightly different for a very large slow sand filter; more sand and more work.
A rapid sand filter is maintained and cleaned by forcefully injecting water backwards through the entire sand bed every 2 or 3 days. Backwashing a biological sand filter will destroy it and may result in people getting very sick if water from it is being consumed. A Frequently asked questions (FAQ) on biological sand filters can be found here.
The terminology used to describe water filters that uttilize sand and gravel to filter water can be confusing. A “slow sand filter” uses sand and biological methods to purify water. A “rapid sand filter” uses sand to filter water. Rapid sand filters do not make use of biological methods, although some biological action may take place between backwashing (cleaning). A “Biosand filter” is a modified slow sand filter, invented by Dr. David Manz. A Biosand filter uses biological and sand filtering and is designed to operate intermittantly. A “sand filter” is a general term used to refer to a water filter that uses sand as the filter media.
Effective rapid sand filtration requires that chemicals and coagulants be added to the water. The coagulants attract particles including some bacteria. When the water with these coagulated pieces flows through sand, the coagualted particles are physically blocked by the sand. The water that emerges must be treated with chemicals such as chlorine, or ozone because most of the harmful bacteria, viruses and protozoa are still in the water. A rapid sand filter fills up in several days and must be cleaned by “backwashing” – forcing air and water back through the sand with high pressure. This breaks loose the coagulated contaminants. The resulting muck must then be drained off and disposed of. This material is hazardous waste. The bacteria is not killed, it is contained. When the chlorine or ozone is added to the water from a rapid sand filter the water is sterilized but not purified. Sterilized means the pathogens are inactivated, but their remains are still in the water. Purified means the pathogens are removed from the water.
A slow sand filter, and a Biosand filter work much differently than rapid sand filters. To explain this it is helpful to think about biology class. To purify means to remove, to sterilize means to inactivate. From a biological standpoint, rapid sand filter systems sterilize water, biosand filters and slow sand filters purify water. All water, with the exception of distilled water, will have organisms in it. Because of this, when water passes through sand due to only the force of gravity, these organisms, that live in oxygen rich water (most water has some dissolved oxygen), will cling to the sand particles and start to feed on other organisms. In about 3 to 4 weeks a living layer of organisms forms in the top 5cm of sand. This layer continues to grow and will eventually spread throughout the filter with most of the activity concentrated in the top 5 to 10 cm of sand. Harmful organisms are “eaten” by this biological layer, which is often called the “Schmutzdecke” which is German for “dirt cover”. This results in purification. Eventually the layer on top gets so concentrated that it slows down the flow of water. To restore the flow of water, this biological layer must be “moderated” to allow more water to pass through at a higher rate. This is done in one of two ways. Either the top 2 cm of sand can be removed, or the surface can be “wet harrowed”. “Wet harrowing” means gently disturbing the biological layer and then allowing the resulting cloudy water to drain off. This is not backwashing. The lower layers of sand and organisms remain un-disturbed. Aside from biology, there are physical properties of a slow sand filter. As water and gravity act on the sand, a compaction and settling occur, which results in the sand becoming a “filter cake” which increases the effectiveness of the physical filtering properties of the system. Backwashing a slow sand filter will destroy this property and disrupt the biological action in the lower areas of the filter – in other words backwashing a slow sand filter or a biosand filter from the bottom of the filter will destroy it. Perhaps the confusion on this method comes from the fact that some large slow sand filters use a form of backwashing to effectively wet harrow the filter. Water is forced through the top 20 centimeters ( 8 inches ) of the 4 foot deep sand bed to break loose the top layer of biological activity. The bottom 3 feet 4 inches is left completely undisturbed. This is not the same as what is used on a rapid sand filter. In a rapid sand filter the entire sand bed is forcefully disrupted regularly every 2 or 3 days. There is a huge difference here. Constructing a small slow sand filter, in a 5 gallon bucket or a 55 gallon barrel and then backwashing it by forcing water backwards through the sand from the bottom up, to clean it, is just plain stupid. If the water from this situation is being used for consumption, people will most certainly get very sick.