WATER REUSE 1309
screen out suspended solids down to 0.5 to 1.0 μ m. This type
of filter clogs rapidly and must be backwashed frequently.
Both vacuum and pressure filters are used for tertiary
treatment. About 85% of influent turbidity is removed
in a vacuum filter. Application rates in the range of 0.5 to
1.0 gpm/ft^2 are common for both types of filters. These low
rates make both vacuum and pressure filters unsuitable in
large installations.
Microstrainers are rotating drum screens with extremely
fine stainless steel or polyester fabric having openings in the
range of 15 to 60 μ m. Flowing liquid enters the effluent end
of the horizontal drum and passes through the mesh to the
effluent end. The mesh rotates with the drum while trapping
solids. A spray of treated effluent washes the retained solids
into a hopper, from which they are lead to final disposal.
Ultraviolet radiation inhibits microbial growth. At intervals
of 7 to 28 days the mesh is washed with chlorinated water
for control of slime growth. Wash water is about 3 to 5% of
the total treated water. Microstrainer removal efficiencies
are 30 to 55% of applied BOD and 40 to 60% of suspended
solids.
Preparation of effluents of exceptional quality involves
“effluent polishing.” Plant effluent is collected in a sump,
pumped through tube settlers, and then sent through a
mixed-media filter bed. Filter effluent is collected in storagetanks. These tanks serve the dual roles of reservoir for
filter backwash water and chlorine contact chamber. The
tube-settler–mixed-media combination removes phosphate
when preceded by coagulation and flocculation. These
units are suitable for production of a high-quality effluent
when the applied influent has a suspended-solids concen-
tration as high as 2000 mg/l (0.2%).
Hard detergents such as ABS (alkybenzenesulfonate) are
not removed by conventional biological treatment. ABS has
been replaced almost completely in household detergents
by linear compounds, but foam continues to be a problem.
Foam-separation techniques have been applied for removal
of refractories such as organic hydrates and nitrogenous
compounds. Air is pumped through spargers, producing
small bubbles. Rising bubbles collect suspended solids and
surface-active substances. Foam collects at the top and is
removed and collapsed, yielding a waste concentrate. In
some installations waste flows downward, while gas spargers
at the bottom give countercurrent flow.
Activated carbon has long been used for removal of
tastes, odors, and color. It has not been suitable for treatment
of waters with high concentrations of organic matter because
the surface area available for adsorption is soon exhausted at
the influent end of a packed bed. However, a fluidized bed of
granular activated carbon gives a more uniform concentra-
tion gradient throughout the bed. Removal rates of 70% for
BOD and 80% for COD are possible with activated carbon
adsorption. Carbon is regenerated in multistage hearth fur-
naces in a stream-air mixture. About 5% of the carbon is lost
with each regeneration.
Hard waters are softened by ion exchange. Calcium and
magnesium ions are exchanged for a cation in the resin, usually
sodium, and 100% of sulfate, 95% phosphate, 85% nitrate, and
45% COD are removed by ion exchange. Color and organic
matter are removed by cationic and anionic mixed beds, but
organic matter tends to foul the beds. Cost and frequency of
regeneration are disadvantages.
Cross-flow membrane-separation technologies of micro-
filtration, ultrafiltration, reverse osmosis, and nanofiltration
may be defined on the basis of pore size or removal function.
Membranes are commonly rated by their molecular-weight
cutoff, the maximum molecular weight of a compound that
will pass through the membrane. Microfiltration removes
suspended submicron material but does not remove truly
dissolved material. Ultrafiltration removes nonionic solute.
These may be organic macromolecules. Microfilters and
ultrafilters may be regarded as sieves. Reverse osmosis
involves a solution or suspension flowing under pressure
through a membrane and the product being withdrawn on
the low-pressure side. This process can treat dissolved-solid
concentrations of 1 mg/l to 35,000 mg/l and particles ranging
in size from less than 1 nm to 600 nm. The treated material
must be nonfouling. Frequently, it is necessary to pretreat in
order to avoid membrane fouling. Recent advances have been
mostly in membrane development. Pilot studies for particular
applications may be required. Energy costs can be quite
significant in process economics. Mass transfer in reverse
osmosis comes about due to pressure difference across aTABLE 3Process Substances TreatedBiological
Conventional secondary BOD, COD, suspended solids,
microorganisms
Modifications of conventional BOD, COD, suspended solids,
microorganisms,
Secondary nutrients
Anaerobic denitrification Nitrates
Algal harvesting Nitrates and phosphates
Chemical
Ammonia stripping Ammonia nitrogen
Ion exchange Nutrients
Electrodialysis Salts
Chemical precipitation Suspended solids, phosphates
Physical
Activated carbon adsorption Organic matter, suspended solids
Sedimentation Suspended solids, microorganisms
Filtration Suspended solids, microorganisms
Microstrainers Very small particles
Reverse osmosis Salts
Distillation Salts
Foam separation Detergents
Vapor-compression evaporation Concentration of wastewater
Wasteheat evaporation Concentration of wastewater
Steam stripping Hydrogen sulfide, ammoniaC023_005_r03.indd 1309C023_005_r03.indd 1309 11/18/2005 1:04:50 PM11/18/2005 1:04:50 PM