Page 445 - Fundamentals of Water Treatment Unit Processes : Physical, Chemical, and Biological

400 Fundamentals of Water Treatment Unit Processes: Physical, Chemical, and Biological

Streamlines

Sand grains

FIGURE 13.4 Scraped sand bed at Empire, Colorado, contrasted
with schmutzdecke after 30 days operation, comprised of carbon-
aceous deposit. (Adapted from Hendricks, D. W. (Ed.), Manual of FIGURE 13.5 Streamlines within a sand bed. (Adapted from
Design for Slow Sand Filtration, AWWA Research Foundation and Hendricks, D. W. (Ed.), Manual of Design for Slow Sand Filtration,
American Water Works Association, Denver, CO, p. 13, 1991.) AWWA Research Foundation and American Water Works Associ-
ation, Denver, CO, p. 9, 1991.)
Schuler et al. (1991) describe similar observations in filter-
ing a water in Pennsylvania having turbidities 0.1–5.8 NTU. perspective of the same idea from Yao et al. (1971). The smaller
The schmutzdecke was ‘‘tightly packed and unattached to the the sand grains, the higher the probability of an impingement;
sand.’’ In pilot filters at Colorado State University (Bellamy there are simply more bifurcations per unit distance for an
et al., 1985a,b), a well-defined schmutzdecke was not visible, interstitial stream with smaller sand. Also, the lower the inter-
but the headloss increased with time consistent with the stitial velocity, the higher the probability of impingement;
development of a schmutzdecke. Scraping the surface resulted lower velocity permits more ‘‘steps’’ of random motion by
in recovery of the clean-bed headloss. diffusion per unit distance and more time for an impingement
Whatever the character of the schmutzdecke, a deposit of to result from gravity acting on a particle and altering its
some sort always occurs in every slow sand filter and causes trajectory. By the same token, higher temperature gives more
headloss to increase. Removing the schmutzdecke by scraping random motion ‘‘steps’’ per unit time (for small particles) than
will cause the headloss to recover to the ‘‘clean-bed’’ level lower temperature, and hence there is a higher probability of
(plus some incremental headloss due to deposits or biofilm impingement (Section 12.3.3.3). Removals by interception are
development within the sand bed). not affected, however, by velocity (in the laminar flow regime).
The three transport mechanisms, interception, sedimentation,
13.2.1.2 Depth Filtration and diffusion, are discussed subsequently.
Within the sand bed, ambient raw water particles (viruses, As implied in Figure 13.5, a particle within the interstitial
bacteria, cysts, mineral turbidity, etc.) that are not removed by stream will most likely, at some point during its path, impinge
the schmutzdecke have some probability of being transported upon a sand grain (due to one of the three transport mechan-
to a sand grain surface during its passage through the inter- isms). Whether it attaches or not depends, in the case of
stices. If a biofilm has developed on the grains comprising the biofiltration, on whether a biofilm exists on the sand grain
sand bed, such particles may attach (and thus be removed). surface. Plain particles may attach to bare sand grains in some
Such removal within the sand bed is, by definition, depth cases, depending on inter-particle forces. Evidently such
filtration. The two facets of depth filtration are (1) transport attachments occur, especially for microorganisms, since bio-
and (2) attachment (Section 12.3.3). films do develop.

13.2.1.2.1 Interstitial Flow 13.2.1.2.2 Attachment Coefficient and the Role
To better visualize the transport step of depth filtration of Biofilm
(Iwasaki, 1937), Figure 13.5 depicts a packed bed of sand Unless attachment occurs, there is no removal. The fraction of
grains with associated streamline configuration assumed by a particles that attach, relative to the number of collisions, is by
flow of water from top to bottom. As seen, within a packed bed definition, the coefficient, a. Research suggests that biofilm
with many sand grains, the streamlines have a tortuous config- development on the sand grains provides an adsorptive sur-
uration. The stream tubes bifurcate and rejoin and bifurcate face for such attachment. Another idea is that extracellular
again at random points. This continuous bifurcation creates enzymes will coagulate some biological particles to permit
opportunity for collisions between particles and sand grains. attachment (i.e., the enzyme alters the zeta-potential of the
The probability of an impingement within a given distance of particle to permit attachment). If so, these particles become
travel depends upon the size of the sand grains, the interstitial the biofilm. Once attachment has occurred, the biofilm may
velocity, and temperature. Figure 12.19 provides another metabolize biological particles and organic contaminants.