Multimedia filters usually contain several layers of filter media, with different density and size distribution, organized one on top of the other. Some high performance specific media only use one type of material, generally a zeolite.
A typical configuration is, accommodated from top to bottom in the following order: anthracite, granite and sand. Gravel is used at the bottom of the tank as support for the rest of the media. In order to expand the bed, remove particles and accommodate the layers, a backwash is performed after one cycle is completed, generally at a flow 20 to 25% higher than the service flow rate. To increase turbulence and accelerate the expansion of the bed, air can be injected at the bottom of the tank. Each cycle can last from 12 to 24 hours depending on the filter design; each backwash last between 15 and 60 minutes and no water is produced during this operation. For this reason, and when water is needed 24 hours a day, it is recommended to relay on a buffer tank to hold treated water to feed the rest of the system.
When treating chlorinated water that needs to be dechlorinated, it is preferable to do so after the filter to maintain propitious bacteriologic conditions on the filter.
SPECIAL FILTER MEDIA
Due to its unique density this media can be used on multimedia filters. Its 0,8 gr/cm3 of density allows it to remain on top of heavier media such as sand and granite, acting as a prefiltration layer. This translates on longer runs and lower pressure drop as well as less frequent backwashes, reducing water consumption on the operation.
This granular filter media presents high density and hardness. On downflow multimedia filters it's the finest filtration stage. To achieve this condition of bigger grains on top of smaller ones, material with different size and different specific weight are used. Granite has a specific weight of 4.0 so it is the lowest layer; its effective size of 0.3 mm allows a range of filtration from 10 to 20 microns.
TURBIDEX / FILTER AG.
A new filtration media, much more effective than the previous ones is Turbidex; a mineral aluminosilicate with irregular surface and higher porosity, which serves as a filter for suspended solids, with excellent performance.
Turbidex allows a much better filtration than conventional media, retaining particles on a range of 3 to 5 microns. This is the result of not only a physical separation but an electrostatic absorption, flocculation and ion exchange. As a result of the filtration through Turbidex, water with less than 0,1 NTU is obtained.
One of the main advantages of the use of Turbidex is that it requires no additional filtration methods after its treatment, because no extra filters are needed, against sand or multimedia filters, which may require a polishment stage.
This technology makes it possible to reduce the consumption of chemicals and cartridge filters, while extending the life of membranes and reducing the cleaning frequency of reverse osmosis equipments installed after it. It is also much simpler to work with a single media, instead of two or three different ones with different densities, weight, packaging, prices, etc.
It is important to keep in mind when designing a filter, the flow rate of water being treated (between 12 and 30 meters/hr), the height of the bed (around 1 meter), the backwash frequency and the space available for backwashing (30 - 50%). The feed pump should be designed considering both the service and backwash flow rate.
For further information please visit: www.turbidex.com
ADSORBSIA is a DOW development for the reduction of Arsenic and other heavy metals. The granular media made mostly of Titanium Dioxide is disposable and keeps the kinetic properties of its nanocrystalline structure, maintaining a physically stable structure. Studies in The United States have proven that ADSORBSIA presents kinetics ten times higher than regular media, made of Iron oxides.
One of the main advantages of the media is its robustness on removing arsenic while it's exposed to variable pH conditions. Even under these conditions, it consistently removes As, which is extremely important when there are seasonal changes on feed pH.
ADSORBSIA is a one-time use media, usually operated downflow. Periodical backwashes may be needed, with the single objective of expanding the bed, although the typical frequency is very low. Since the media is disposed after exhausted, no arsenic containing flows are generated, so no additional treatment is required. Due to its high affinity with arsenic, it is safe and easy to dispose off the media as landfill, since it´s a non hazardous waste.