Earth Filtration System Operation

Continuous addition of filter aid (body feeding) is accomplished either by feeding filter aid as a slurry or by dry feeding. Slurry feeding is usually done with plunger or diaphragm pumps. If filtration is a batch process, the filter aid can be added directly to the batch. 

In the operation of a filtration system, the filter is first precoated by circulating a mixture of filter aid and clear or filtered liquid from the precoat tank through the filter and back to the precoat tank. This is continued until all the filter aid is deposited on the filter. The body feed injection system is then started and the filter is changed over, with minimum fluctuations in pressure, from precoating to filtering. 

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Diatomaceous Earth Filtration System Design Diatomaceous Earth Filtration System Operation Diatomaceous Earth Filtration System Maintenance Types of Precoat Filters Auxiliary Parts and Equipment Bulk Filter Aid Handling... 

Conventional treatment, which includes coagulation, flocculation, clarification (sedimentation or flotation), and filtration, along with disinfection, can achieve 99.9% inactivation of Giardia cysts and 99.99% inactivation of enteric viruses when properly designed and operated. Direct filtration, slow sand filtration, and diatomaceous earth filtration systems, each combined with disinfection, have also achieved these reductions. 

Diatomaceous earth filtration, also known as precoat filtration or diatomite filtration, relies on a layer of diatomaceous earth about 0.3-cm (1/8-in.) thick placed on a septum or filter element. The septums may be placed in pressure vessels or operated under vacuum in open vessels. A schematic diagram of a typical pressure system is shown in Fig. 2. 

Moderate decreases in viscosity in polishing filtration do not necessarily imply blopolymer loss. So long as polymer is not Incorporated In filtercake as it would be in dlatomaceous earth filtration (or if it can be backwashed off, even if it is), it should be possible to operate with a feed of higher viscosity than necessary for injection, and to recycle feed through the filtration apparatus, perhaps after dilution. Another possibility would be to recycle polishing filtration blowdown into the microscreening system. 

Table 3.9 is a comparison of operating costs for a 70,000 GPD plant replacing the conventional filter press with UF. The major savings are associated with the elimination of diatomaceous earth filtration and the reduction in enzyme (pectinase) requirements. Even with amortization, the operation costs with UF are only 45% of the conventional system costs. The ROI is 42% with a payback in 2.4 years. 

Precoat filters in amine filtration. serviee use a diatomaceous earth or perlite filteraid pre-coat (typically about 10 pounds of filteraid per 100 square feet of filter surface area) and a body feed of 1 pound of filteraid per pound of solids removed. They usually filter a 10% slipstream, and depending on the type of filter aid, can remove particles as small as 1 micron. Precoat filters were widely used in large amine systems (greater than 1000 gpm circulation) in the 1070s. However, precoat filters require significant operator attention, and amine losses can be substantial unless they are carefully operated. For these reasons, precoat filters are no longer widely used for amine filtration. 

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