Boiler Feed Water Pre Treatment

Physical Treatment

The hydrogen gas will be allowed to pass from the source of the feed water to the membranes in a closed loop without letting the water get in touch with the air or any oxidizing agent. This step will keep the hydrogen sulfide in gaseous form where it will pass the membranes and leave with the product water unaffected where it will be removed as a post treatment step after the RO process.

Sand filtration

Multi Media Filter  

Removal of Suspended Solids and Turbidity

Filtration is the process of removing suspended solids from a fluid by passing it through a porous medium. Coarse, medium, and fine porous media have been used depending on the requirement. The filter media are artificial membranes, nets, sand filter, and high technological filter systems. The choice of filters depends on the required filtering speed and the cleanness requirement.

Media filters use a filtration bed consisting of one or more layers of media granules which are sand and anthracite. The grain size for fine sand filter is in the range of 0.35 to 0.5 mm or 8×12 mesh to 60 mesh and for anthracite filter 0.6 to 0.9 mm. They will remove particulate solids down to 12 microns.

A multimedia filter is designed to make better use of the bed depth in the removal of a greater volume of suspended solids and reducing the feed water turbidity. This is achieved by loading larger (irregular shaped) media granules of lower density such as anthracite over a smaller media granules of higher density such as sand. The larger granules at the top remove the larger suspended particles leaving the smaller particles to be filtered by the finer media, thus result in more efficient filtration and longer run between cleaning.

The design depth of the filter media is normally about 0.6 m (24 inches) with 50% freeboard for media expansion during backwash. The multimedia filters are usually filled with 0.4 m (16 inches) of sand covered with 0.2 m (8 inches) of anthracite.

The multimedia filters can be operated by either gravity or a pressure. A higher pressure drop can be applied for higher filter beds or smaller filter grains or higher filtration velocities. The design filtration service flow is usually 20 – 36 m/hr/m2. The back wash rates are in the range of 30 -40 m/hr/m2. For feed water with higher fouling potential, flow rates of less than 20 m/hr is preferred.

The available pressure is about 2 bar (30 psi) to 4 bar (60 psi) for pressure filters. Periodically, the filter is backwashed and rinsed to remove the deposited matter manually once or more a day. If the fouling rate is high, then the filter should be backwashed whenever the differential pressure increases between the inlet and the outlet of the filter by about 0.3 to 0.5 bar. Backwash time is normally about 10 min. frequent shut-downs and start-ups should be avoided, because a velocity shock will release previously deposited particulate matters.

3.1.3 : Cartridge filtration

Another step of pre-treatment is the cartridge filtration. It helps in reducing the suspended solids quantity in order to protect the membranes not to get damage.

The micro filtration will be accomplished in two stages in series using 5 & 1 micron pore size cartridges successively

 Filter Cartridge need to be changed regularly or if there is increase in pressure drop more than 15 psi. Replacing cartridges depend on the quantity of suspended solids in the water. It is advisable to change cartridges when there is an increase in the differential pressure between the inlet and outlet of the filters.

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