17 DEC 2003
Particle Size. Particle size can be very difficult to accurately measure and de-
scribe. Generally, particles are irregular in shape and often angular. In practice, how-
ever, they are often described as spheres or sphere equivalents. Because it is impossi-
ble to accurately measure and describe each particle in a waste stream, numerous
methodologies have been developed to estimate the size of typical particles. As shown
in Figure 3-1, the methodology used can result in sphere diameter equivalents that can
vary by more than a factor of two for the same size particle. When comparing particle
removal estimates from different manufacturers, the design professional should make
sure that the particle sizes used are determined using comparable methodologies.
3-2.1.1 Liquid particle counters use photozone light blockage, where a pulse is gener-
ated proportioned to projected area.
3-2.1.2 Feret diameter is the perpendicular projection onto a fixed direction of the dis-
tance between two parallel lines.
3-2.1.3 Stokes diameter is determined through settling and projecting particle diame-
ter through use of stokes law (ASTM D 422-63).
3-2.1.4 Colture contours measure particle diameter through changes in conductivity
of particles in a week electrolyte solution.
3-2.1.5 In general, granular media filters can remove particles in the 4 to 6 micron
range and pressure filters can remove particles as small as 10 microns. However, the
efficiency of particle removal in these micron ranges is very low compared to cartridge
and bag filters and the efficiency likely will diminish over the length of the filter run.
Therefore, if it is necessary to efficiently remove particles in the range of 4 to 6 microns
from a waste stream, bag or cartridge filtration will be needed. The designer should also
recognize that, for a bag or cartridge filter to operate efficiently, they may require pre-
treatment by a media filter. This is particularly true where significant amounts of solids,
considerably larger than the desired particle size in the effluent, are present. However,
where a significant concentration of large particles is present, smaller micron-size parti-
cles may be trapped in the schmutzdeck, which forms at the top of the filter. Hence,
granular filtration may still warrant consideration. Figure 3-2 shows typical particle size
ranges within the filtration separation spectrum.
Relative Costs. Typically, the capital cost for media filtration is higher than
the cost of cartridge or bag filtration. Operation and maintenance (O&M) costs increase
as the volume of waste to be treated increases. The long-term cost for granular media
filters may be less than the cost needed for membrane replacement in bag or cartridge
filters or precoat filter media because of the costs of expendable media and potentially
hazardous waste disposal.
3-2.3 Metals Removal. In many systems, metals are removed by precipitation,
sedimentation, and filtration. For waste streams low in solids and metals to be removed,
continuous backwash filters can enhance the metals removal by providing a constant