Filter sizing, clarifying filtration

Settling does not give a complete separation one product is a con-centratea suspension and the other is a hquid which may contain fine particles of suspended sohds. However, settling is often the best way to process veiy large volumes of a dilute suspension and remove most of the hquid. The concentrated suspension can then be filtered with smaller equipment than would be needed to filter the original dilute suspension, and the cloudy liquid can be clarified if necessaiy. Settlers can also be used for classifying particles by size or density, which is usually not possible with filtration. 

This wastewater treatment plant of a medium-size urban area (about 200,000 inhabitants) is located at Nimes, in the South of France. It includes two biological processes with fixed biomass, with a clarifier in between. The first step is a trickling filter using plastic material, and the second one is a biological filtration on immersed bed (with pouzzolane). 

Apricot was used as a model for studies of fruit puree clarification. Various puree concentrations were treated for 30 min at 50° C with 0.4 cc/L, each, of commercial cellulase and pectinase enzyme and clarified by filtration through a 0.45(im pore size ceramic microfilter. Sparkling clear apricot juice was produced at flux rates from 90-190 L/m2h. Above 13° starting Brix. juice flux showed little increase with increasing starting Brix. Dissolved solids flux increased substantially with increasing starting Brix. Permeate remained clear and retained most of its flavor and aroma when concentrated by vacuum evaporation to 58° Brix. There appears to be some retention of enzymes by the filter. Retained enzymes were successfully utilized in a 4 h trial in which untreated puree was continuously added to retentate. in amounts equal to permeate removed, after startup on enzyme-treated puree. 

Membrane Filters can be used for final filtration or prefiltration, whereas depth filters, lacking a precisely defined pore size, are used in e g. clarifying processes, where quantitative retention is not required, or as a prefilter to prolong the life span of a downstream membrane. 

In order to clarify the role of adhesion in the filtration process, let us examine the deposition of particles on isolated cylindrical fibers placed in an aerosol stream. They way in which the dust deposit is formed on individual cylindrical fibers of a permeable filter barrier, with a flow velocity of 1 m/sec, is shown in Fig. XII. 1. The clearly visible local side growths of lead and zinc oxide dust particles (particle size approximately 1 jim) are directed at an angle of 110-120° to the flow axis. When more aerosol passes through the filter, the growths may meet, forming a continuous layer that acts as a secondary filter medium [322]. 

The silicate solution is somewhat diluted, aged, clarified, and ultrafiltered in a rapidly stirred pressure cell at 50 psi. By following the concentration of SiO- and Na20 in the filtrate one can calculate by difference how much of the silicate is polymerized to a size that will not go through the pores. Also, it can be shown that Si(OH)4 and HSiOj pass freely through the membrane so that the initial concentration of these species in the filtrate is the same as in the initial solution. The commercial sodium silicate solutions were diluted with an cqui volume of water, aged about a week, filtered through fine filter paper to remove traces of suspended matter, and then ultrafiltered. 

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