Use of Vacuum Filtration

The principle operational variable is the sludge to be filleted, since chemical conditioning can be considered a means of changing the sludge. The type of sludge filtered is within the control of the operator, and it is also within his province to dictate what happens to the sludge before it gels to the vacuum filter. 

A method of improving filter efficiency is to use compounds that can be mixed with the sludge to improve its filterability. Filter aids were also described earlier. The disadvantage of filter aids is that the quantity of sludge will increase. Tlie advisability of usmg filter aids is strongly dependent on die local cost, as well as the increase in filtration efficiency. 

Precoat filters are widely used in the chemical process industry. In this operation, instead of mixing the filter aid with the sludge, the filter aid is placed onto the filter fabric before the sludge is introduced. This prevents rapid premature clogging of the smaller pores and allows for a greater filter yield. Diatomaceous earth is an effective but expensive filter aid for pre-coat filters. 

Waste activated sludge, unless mixed with primary sludges, is not normally dewatered on vacuum filters. An exception to this is pure oxygen sludges that can be filtered successfiilly with the use of ferric cMoride conditioning in a filter leaf. 

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