Clarifying the Wastewater Treatment Choices

Originally Published 1994 - (Costs Are Outdated)The disadvantages include slow system throughput
Companies should consider such factors as space andand high costs associated with the filter media, which
capital requirement, desired filtrate quality, and levelmust be applied continuously to the batch
of automation when selecting water and wastewaterpretreatment tank immediately before vacuum
filters.filtration. In addition, the filtering media increase total
Filtration is the easiest way to remove solids fromsludge volume as much as 60%. Because vacuum
water and wastewater streams. In wastewaterfilter cost usually includes the pretreatment system, a
systems, filtration is used to decrease suspendedunit that processes 19 m3/d (5000 gal/d) can cost
solids entering the publicly owned treatment works or$60,000.
water-reuse systems or to increase solids in sludgeCyclone Separators
dewatering applications.Cyclone separators are relatively new. Because they
Filter selection begins after a company determineshave higher rotating speeds than vacuum filters, they
the required level of filtration or filtered water quality;create a significant amount of centrifugal energy to
decides whether filtrate will be disposed or reused;filter wastewater through various media sizes. The
identifies how much space is available; calculates therotating filter uses a fixed-micronsize medium, such as
amount of capital available to purchase, operate, anddiatomaceous earth, to filter the wastestream. The
maintain the system; decides whether the systembenefits of cyclone separators include their ability to
should be automatic or manual; and considers anyhandle variable throughputs, filter varying particle
pertinent water conservation requirements.sizes, and produce drier sludge cake than vacuum
The level of solids removal or filtered-water qualityfilters for about the same cost (around $60,00). heir
desired defines the type and size of filter necessary;drawbacks are similar to those of vacuum filters,
most are available with varying automation levels andnamely high filter media costs and sludge volume.
price ranges.Plate-and-frame Presses
Proper filter size is determined by flow rate and thePlate-and-frame presses are batch processes in which
amount, size, and type of solids to be removed.fixed plates held together in a hydraulic press are
Particle sizes and filtration levels can be determinedpre-coated with diatomaceous earth filtering media.
by filtering water and solids through varying sizes ofThe press size depends on the total solids volume
filter paper and measuring the total solids content ofper load. Liquid is loaded into the presses with an
filtered water.air-driven diaphragm pump in increments of 172, 345,
Solids remaining in filtered water will be the same size517, and 690 kPa (25, 50, 75, and 100 psi). The press
or smaller than the last filter paper used and willis pressurized at 690 kPa (100 psi) causing water to
indicate which filter media opening size to use. (Abe squeezed out. After water is removed, the filter
filter manufacturer or laboratory can be hired tois "blown down" with air and opened so sludge can
perform this procedure.) The quantity of solids inbe removed from the plates. The cake usually is at
water or wastewater can be determined by aleast 35% solids. The limitations of plate-and-frame
laboratory analysis of total solids; flow can bepresses include a processing time of at least 4 hours,
determined from plant records or an in-line flowa large footprint, an ability to dewater only the
meter.volume of the plates, and the operating time required
The following equation is used to calculate solidsto clean the plates. Therefore, if wastewater
loadings (how quickly a filter will load with solids):contains high solids levels, the press must be large.
solids (lb/h) = flow (gal/min) x total solids (ppm) xFilter presses range in size from 0.014 to more than
(8.34/60/1,000,000).2.8 m3 (0.5 to more than 100 ft3). An.028-m3 (1-ft3)
After solids loading, filter area, and filter mediapress costs $3,500, and a 2.3-m3 (80-ft) press costs
opening size have been determined, the amount of$100,000. Large presses also require automatic loading
time required before cleaning, or back-washing, shouldcontrols, plate shifters, sludge carts, and other
be calculated, and the fate of filtrate waste must heequipment that can cost an additional $100,000. The
determined.advantage of filter presses is their ability to produce
In other words, after a filter is backwashed, wheresteady, dry cake, which often is required for landfill
will wastewater go? Backwashing often requires updisposal.
to 10% of the total filtered volume, a 10:1 ratio. ForMultiplate Spring-loaded Filters
example, 379 m3 (100,000 gal) of filtered waterMultiplate spring-loaded filters use specially fitted disks
would generate 38 m3 (10,000 gal) of backwash thatwith machined gradations that provide the required
contains concentrated solid and may require additionalmicron-size opening. The disks, which are held
treatment.together with a preloaded spring, allow water to flow
Some wastewater must be pretreated beforethrough while holding back filtered particles as small as
filtration. Oily wastewater, for example, tends to2 microns. These filters are backwashed with water
agglomerate quickly between or over filter media andbased on a preset pressure differential and come
requires conditioning before filtration. Fine-gradewith an option for all- water or airassisted
filtration systems are susceptible to fouling frombackwashing, which saves water and greatly
biological growth, yeast, and other microorganisms;increases filter-plate scouring. These continuous
performing bench tests or consulting with filteron-line filters usually are configured in pairs (one for
manufacturers about specific applications isfiltration, one for backwashing). The amount of water
recommended.used for backwashing is about 10% of the total
Y-Strainersfiltered flow. Air-assisted filter backwash saves an
Y-strainers are the simplest, least expensive ($30 toadditional 3% to 5% of water but requires
$100) filters, but they have a small filter area andcompressed air. The programmable logic
require frequent maintenance. They typically are usedcontrol-based filters enable adjustments in the time
on small pipes to protect such equipment as seals,between backwashes and backwash duration. The
pumps, and nozzles from solids buildup.filters cost $4,500 per pair, and the controller costs
Bag Filtersan additional $5,000 but can control a bank of up to
Bag filters basically are larger Y-strainers. These filters12 filters.
are bags of selected pore sizes that are enclosed inMultimedia Sand Filters
a pressure housing with a removable cover forMultimedia sand filters traditionally are used to treat
cleaning. They are easy to use, come in variablewater but also can be used to treat wastewater
filtering sizes, and are relatively low in cost, especiallythat has non-uniform solids loadings. These
for final filtration or applications with low solidsthreedimensional systems provide much greater
loadings. Bag filters do not exceed 203 mm (8 in.) infiltering volume than other filters because they use
diameter; for larger diameters, an inline bank withboth the bed surface and depth to remove particles
multiple filters in one housing is used.as small as 0.45 microns.
The price of carbon or stainless steel bag filterAn on-line suspended solids analyzer measures the
housings varies according to size, from $100 for aamount of particles coming through the filter. As
51-mm (2-in.) housing to $3000 for a 1219 mm (48-in.)solids levels increase past, or break through, the filter,
housing. Filter bags range in price from $6 to $10 fora preset suspended solids level initiates backwash.
pore sizes of 25 to 800 microns. Bags are availableBackwash can be air assisted to save water, but
straight or pleated. Pleated bags provide addedcare must be taken not to blow the media out with
surface area but cost more than twice as much asthe filtrate material. These filters usually come in pairs
flat ones. The disadvantages of bag filters includeto provide continuous on-line service. They work well
high maintenance requirements for systems that areas final filters for water with uniform solids. Water
underdesigned or have high solids loads and the costsavings is accomplished when the right control
of bag replacement.scheme and air-assisted backwashing system are
Vacuum Filtersused, A 0.19-nil/min (50-gal/rnin) pair of multimedia
Vacuum filters use a slowly rotating vacuumsand filters with a controller and backwashing header
(negative pressure) drum that pulls dirty liquid fromassembly costs approximately $15,000.
the outside of the drum and media to the inside ofWith the vast array of available filters and control
the drum. The vacuum filter usually is packaged withscenarios, companies willing to perform some
a pre-batch treatment system to coagulate theadvance research on their wastestreams should have
wastestream before filtration. The advantage of thisno difficulty finding a system to meet their needs.
filter is its ability to filter a variety of wastewaters.