Leaf Filters Working Principle and Coagulation

Leaf filters provide a large surface area by using various filter leaves and do not require complete disassembly for cleaning necessary with plate and Frame Filter Press. The general filter lead consists of a heavy wire drawings screen mounted in a tabular frame that acts as a support and filtrate conduit.

The slurry to be filtered fills the space around the leaf and is forced to flow through the leaf by applying pressure on this slurry or vacuum with the leaf. In either case, the filter cake builds upon the outside of the leaf on the cloth, and filtrate passes from within the leaf to the filtrate discharge system.

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Note: Leaf filters may be grouped as vacuum leaf filters and pressure leaf filters. The Moore filter is a vacuum leaf filter while the Kelly filter and Sweetland filter are pressure leaf filters.

Advantages of Leaf filters

Leaf filters are great for removing large particles. They have several advantages over other types of filters.

1. They are easy to install.

2. They are affordable.

3. They are effective at removing large particles and pollutants.

4. They are easy to clean.

Disadvantage of Leaf filters

They have many advantages, but there are also some disadvantages.

1. Dirt and dust can build up on the filter surface over time, which can impair its performance. In addition, leaf filters can be difficult to replace if they become damaged.

2. Leaf filters is that they can be relatively expensive. 

3. The leaf filters is they require regular maintenance to keep them functioning optimally.

Coagulants and Role of Coagulants in Filtration 

The most common coagulants in use are aluminum sulfate (usually called alum or filter alum) and ferrous sulfate.

Alum is the most commonly used for the coagulation of water because of its excellent floc formation, its relative economy, its stability, and ease of cleaning. To react with alum to form a precipitate water must have some alkalinity. For water having low natural alkalinity, it is necessary to add soda ash or lime (calcium carbonate powder). It is found that alum coagulants are best in the pH range of 6.0 to 8.0. It may be added in powdered form or in the form of a solution. 

Chemical coagulation consists of adding small amounts of coagulants to water from flocculant precipitates which coalesce the suspended impurities and cause them to sink rapidly.

It is not possible to remove such finely divided and colloidal particles, micro-organisms, and color-producing compounds from water by use of sand filters. To remove these from the water coagulants are added to the water before filtration. When alum is added to water, it gets hydrolysis by natural or artificially created alkalinity of water with the matter, etc. Gets adhere to this precipitate and are removed in sand filters. 

Take these Notes is, Orginal Sources: Unit Operations-II, KA Gavhane