Algae in Water Supplies: An Illustrated Manual on the Identification, Significance, and Control of Algae in Water Supplies. Page: 32
viii, 88 p. : ill. (some color) ; 31 cm.View a full description of this book.
Extracted Text
The following text was automatically extracted from the image on this page using optical character recognition software:
CHAPTER V
FILTER CLOGGING ALGAE
AS WATER passes through a sand filter in the treatment
plant the spaces between the grains of sand become filled
with colloidal and solid particles which had been dispersed
in the water. If the raw water comes from a surface supply
such as a reservoir, lake, or stream, the algae which are invariably
present will be well represented in the material
collected by the sand filter. They are frequently the primary
causes for the clogging of the filter.
In most places the algae and other particulate materials
are sufficiently numerous throughout most of the year to
require that the water be treated by coagulation and sedimentation
previous to filtration through sand. Without
this preliminary treatment the filter would clog so rapidly
that it would be uneconomical to use, except when relatively
clear water is available. Certain plants in California and
in Canada have been usiig the rapid sand filter without
prior coagulation at least during portions of the year (1).
Efficient coagulation and sedimentation may remove up to
90 or 95 percent of the algae from the water. The algae
remaining in the water mnay still be sufficient to cause gradual
or even rapid loss of head in the sand filter. The
clogged filter must then be taken out of service and cleaned
or backwashed. Normal filter runs commonly extend for
from 30 to 100 hours before cleaning is required, while short
filter runs, caused by the presence of algae, are even less than
10 hours in length (2). In extreme cases the clogging may
recur so frequently that the amount of water required to
backwash the filter is greater than the amount of filtered
water which reaches the distribution system. Thus the
presence of algae can slow up tile process of water treatment
and add materially to its cost.
CLOGGING PROCESSES
Both the slow and rapid sand filters may become clogged
with algae, but in the former the algae and other aquatic
microorganisms may play a useful part in the treatment
process. They form a loose, slimy layer over the surface of
the sand and act as a filter. The algae in this layer release
oxygen during photosynthesis, and the oxygen in turn is
utilized by aerobic saprophytic bacteria, fungi, and protozoa
which establish themselves in and on the filter. This permits
the decomposition or stabilization of the organic material that
was present in the raw water. Diatoms, however, due to their
rigid walls, may do more harm than good by speeding the
clogging of the filter, but it has been possible to use slow sand
filters when diatoms have put rapid filters out of commission
(3). The water that has passed through a slow sand filter is
relatively free of bacteria, algae, and other organisms as well
as of dead organic matter.It is not yet fully understood why certain algae are more
effective than others in reducing the movement of water
through the filters. Ability to develop in large numbers
is certainly one essential. The rigid wall, such as is found
in the diatoms, the copious mucilaginous material around
the cells as in the case of Pal7?e7la, and the tendency to
form flakes or a network of strands, as in Fraqilaria and
Tribonema are other factors.
Diatoms are present during all seasons of the year and
are by far the most important group of organisms which
clog filters. The most serious offenders are Asterionella.
Fragilaria, Tabe7ll7aia and Synedira. Other diatoms that
may occasionally cause this trouble include Navicula, Cyclotella,
Diatoms, and Cymherlla. all of which are illustrated on
plate 2. The rigid cell wall of diatoms is composed principally
of silica and is not subject to decomposition. Therefore,
even though the diatoms may die off rapidly on the
surface of the filter, their silica wamlls remain to plug the
pores in the sand.
In England a relatively pure growth of Fragilaria developed
in a reservoir to the extent that it. was necessary
to remove huge quantities of this diatom at the water treatment
plant. Counts of another filter clogging diatom, Asferionel!a,
indicated that the organism lias reached a density
as high as 20,000 per ml. (4).
In Chicago, when the water to be filtered contained approximately
700 microorganisms per ml., principally Tabellaria
and Fragilaria. the filter runs were only 4.5 hours.
Three days later, when the count was down to approximately
100 per ml., the filter run increased to 41 hours (5).
In Washington, D.C., filter runs were reduced from an
average of 50 hours to less than 1 hour due to the sudden
influx of the diatom Synedra. which had a concentration in
the raw water reaching 4,800 cells per ml. (6).
The filter clogging blue-green algae are represented on
plate 2 by Ananrys.is (C/hrooecccw? type), Rirulnia, Anabaena.
and three species of Oscillatoria. Anabaena is
known to have caused filter trouble in Illinois and Minnesota.
while Oreillatoria is one of the offenders in Switzerland.
Dinobryon and Tr/clihelomonas are pigmented flagellates,
the former being common in the Great Lakes and
in the soft waters of the eastern ITnited States. Chlorella.
Palmella. Spirogyra. and the desmid, Closterium, are green
algae while Triboneina is a yellow-green filament. Ch7orella
is the alga which lhas given trouble by clogging filters
made of glass wool. In addition, it grows on the inner
surfaces of the glass bottle in water coolers and in water
lines constructed of plastic tubing, when these have been exposed
to light. In the tropics and warm temperate zone,22
Upcoming Pages
Here’s what’s next.
Search Inside
This book can be searched. Note: Results may vary based on the legibility of text within the document.
Tools / Downloads
Get a copy of this page or view the extracted text.
Citing and Sharing
Basic information for referencing this web page. We also provide extended guidance on usage rights, references, copying or embedding.
Reference the current page of this Book.
Palmer, Charles Mervin, 1900-. Algae in Water Supplies: An Illustrated Manual on the Identification, Significance, and Control of Algae in Water Supplies., book, 1959; Cincinnati, Ohio. (https://digital.library.unt.edu/ark:/67531/metadc9129/m1/32/: accessed April 23, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.