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Secondary Effluent Depth Filtration Medium Capacity

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The filter medium has a maximum accumulation capacity for the particulate material removed from the suspension that is being filtered. The maximum accumulation capacity (generally referred to as maximum specific deposit [MSD]) of the filter is basically an equilibrium condition, when the binding strength between the particles and the collecting medium (collecting medium can be the medium itself or the particles previously attached to the collecting medium) is equal to the shear forces exerted on the particles by the fluid. Even if a particle comes into contact with the surface of the collecting medium, it cannot be attached to the collecting medium beyond the equilibrium condition.

The determination of the MSD has been an interest to researchers in depth filtration field as MSD is frequently used to predict the Total Suspended Solids (TSS) concentration depth profile as a function of filtration time (and hence time to reach filter breakthrough). The MSD was defined in this study as the maximum mass of TSS particles that can be accumulated within a unit-filter volume. Some filtration models define the MSD as used in this study, whereas others define the MSD as the maximum volume of TSS particles that can be accumulated within unit-filter volume. Both definitions are acceptable, as long as a proper conversion is used between the two parameters, when necessary.

The determination of the MSD for the filtration of settled secondary effluent, and the effects of the filtration variables (i.e., collector size, porosity, filtration rate, and medium depth) on the MSD are presented in this paper. The MSD experiments were conducted using a Compressible Medium Filter (CMF) at medium-compression ratios between 15 percent and 40 percent and at filtration rates from 410 L/m2·min (10 gpm/ft2) to 820 L/m2·min (20 gpm/ft2). The CMF, which has also been designated as “Fuzzy Filter” because of the appearance of the filter medium, was formed by shrinking synthetic fibers into a quasi-spherical shape.

The most straightforward way of calculating the MSD of the medium is to monitor continuously the specific deposit (mass amount of particles accumulated within the unit volume of the filter medium) until the specific deposit reaches an asymptotic value, which corresponds to the MSD. The MSD value of the medium can be calculated using the following expression: MSD = QV t = 0 t = t ( C inf C x ) d t Where

MSD = maximum specific deposit, mg/L;

Q = flow rate, L/min;

V = total volume of the filter medium between medium depths of 0 and x, L;

Cinf = TSS concentration at the influent to the filter, mg/L;

Cx = TSS concentration at medium depth of x, mg/L;

t = time, min; and

ts = saturation time, min.
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Document Type: Research Article

Publication date: 2006-01-01

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