method which simplifies subsequent cleaning is leaving mounds of material from the
last fall cleaning about 1 foot high and 4 to 8 feet apart to support the ice. The mounds
are completely removed for spring cleaning.
(3) Maintenance. When pools of wastewater remain on a filter several
hours after dosing, the filter should be taken out of service until dry and then cleaned.
When settled wastewater is applied, a mat that dries, cracks, and curls forms on the
surface. The mat is readily removed with forks or rakes. When the effluent from
secondary treatment is applied to sand filters, a mat may form slowly or not at all and
clogging may take place between the top sand grains. Cleaning is then necessary at
less frequent intervals. About 1/2 inch of the top sand must be removed with shovels or
hand scraper when required.
(a) Remove mat. Hand removal is customary, but at plants without
underdrains, tractor-drawn scoop-type scrapers may be used. Heavy equipment cannot
be used on beds having underdrains because the tile will be crushed.
(b) Level surface. After the surface has been cleaned, it should be
loosened with a garden rake and leveled to grade. Harrowing a sand filter should be
resorted to only for deep clogging because harrowing fouls the sand to greater depths.
(c) Add sand. New sand must be added periodically to maintain
proper sand depth over the underdrain. For filters built in natural sand deposits of
considerable depth without underdrains, sand need rarely be added except for small
(d) Avoid ponding. Sand filters must not be allowed to pond or to be
used as lagoons after clogging except in extreme emergency because solids continue
to accumulate on the surface. It then becomes difficult to restore the filter to normal
operation. Control of grease content, plus frequent cleaning and resting of filters,
usually prevents ponding.
d. Activated Sludge Unit.
(1) General. The activated sludge system is a biological unit used for
secondary treatment. This system or process removes dissolved and suspended solids
from wastewater, changing it to more stable substances for removal later in a final
settling basin or clarifier. This process is accomplished by rapid oxidation of organic
matter by aerobic microorganisms. When wastewater is agitated continuously in the
presence of oxygen, the nonfiltrable residue agglutinates or flocculates. This floc is
called activated sludge.
(2) Properties of activated sludge. Good quality activated sludge is usually
brown in color, is granular in appearance with sharply defined edges, and has a slight
musty odor. It is a biologically active material containing numerous microorganisms and
a mixture of organic solids in various stages of decomposition. Some materials are