TECHNICAL LIBRARY

 

VIBROSCREEN Separators Save Laundries Dollars

As EPA regulations became more stringent, commercial and industrial laundries have had more impetus to minimize the amount of biological oxygen demand (five-day average) (BOD5) and total suspended solids (TSS) that they discharge to municipal wastewater treatment systems (most BOD5 in laundries is insoluble). At about this same time, the rising cost of energy forced laundries to look for ways to maximize efficiency of their waste heat exchange systems. The laundering industry has had difficulty raising heat exchanger efficiency because fibers and insoluble organic matter settle on the heat exchanger walls, reducing heat recovery efficiency.

Even though the changes in regulations and energy costs are not new, laundries still cope with their effects. For example, more than two dozen laundries have found that VIBROSCREEN circular separators help remove significant amounts of TSS and insoluble BOD5. Most of these operations have found that 60 mesh to 120 mesh screens give best results.

The accompanying flow charts schematically describe an industrial operation (Fig. A) and a typical commercial laundry operation (Fig. B). By way of example, industrial operations would be typified by jean manufacturers that wash jeans after partial dye removal and stone washing, and before pressing and folding operations. Jean manufacturers typically use double deck VIBROSCREEN units with a 4 mesh top screen and 120 mesh bottom screen. The 4 mesh top screen helps separate the large pieces of pumice from the slurry. Other industrial applications involving dewatering fibers include the manufacture of bandages, fiberglass, and nylon fibers for other businesses.

The following table which was excerpted from "Management of Wastewater Solids" (Kason bulletin TL-8), shows BOD5 and TSS data from a laundry wastewater analysis done with and without a vibratory screener. The analysis makes a strong case for using a VIBROSCREEN unit to remove the TSS and BOD5. In percentages, the 80 mesh screen (178 micron openings) removed an average of 45 percent of TSS and 37 percent BOD5. With a 325 mesh screen (45 micron openings), 67 percent of the TSS and 68 percent of the BOD5 were removed.

Date Screen BOD5 TSS* Approx Cost
Used
(mg/L)
(mg/L)
3-Month Period
3/83
None
1669
1003
$10,000
12/83
80 mesh
1063
460
$5,000
10/84
80 mesh
1039
672
$6,000
6/85
325 mesh
504
320
$3,000
9/85
325 mesh
573
338
$3,000


* Analysis was done by the Sanitary District of Rockford, IL. Average 200 GPM of water through a 60 in. Kason VIBROSCREEN Circular Screen Separator. For higher flow rates with heavy solids loading, Kason customers achieved excellent results by using a recycle deck. A recycle deck allows an important process to occur. As the top screen floods (high hydraulic gradients), the overflow can easily pass down to the lower screen, providing twice the area for separation of liquid and solids within the same plan area.

In a typical medium to large-size laundering operation where the company has fully assumed responsibility to remove lint and pumice, two or more VIBROSCREEN units separate these materials from wastewater. For purposes of illustration, there might be a series of three tanks. Wastewater from a washing machine and extractor trenches flows into the tanks by gravity. The pH is measured in the first tank and adjusted with alkali or acid if beyond any specified limits. A pump located near an intake pipe near the bottom of the first tank propels the wastewater to VIBROSCREEN Circular Screen Separators. Each VIBROSCREEN unit is equipped with two screens and each has a Kleen Screen ring assembly which is self-cleaning. Each VIBROSCREEN unit also uses a velocity breaker that reduces influent velocity, and a rubber coated plow blade to assist solids discharge. A velocity breaker lengthens screen life by reducing impingement of abrasive solids in the slurry.

While the optimum screen mesh will depend on the nature of the wastewater and entrained solids, typically a 4 mesh (.20 in. x .20 in. holes) MG screen serves well for the top screen and a 120 mesh (.0058 in. x .0058 in.) tensile bolting cloth screen gives good results for the bottom screen.

The pump located near the intake pipe receives signals from two float level switches in the tank, switching on and off as needed to keep the wastewater level within predetermined limits.

The VIBROSCREEN unit's gyrator motor shakes the frame and screens and the shaking screen moves the lint, pumice, and stones to the edge of a coarse-mesh upper screen and a fine-mesh lower screen. The vibratory action of the screens propels the solids off the screen, into the discharge spout. Waste passes through discharge spouts into mobile containers. Since these oversize materials aren't completely dry, water drips from the mobile containers into the second tank. In a typical operation where a conventional wash formula is used, the oversize material fits the category II refuse description and can be disposed of in a sanitary landfill.

Water passing through the VIBROSCREEN units flows to a hot water holding tank, the second on the series of tanks. To conserve energy and lower the effluent water's temperature so it won't harm bacteria in natural streams and lakes, hot water heats the incoming cold water.

If, for example, the hot water was 118°F, the influent cold water temperature will
rise to about 115°F, leaving the effluent water at about 70°F. After this heat exchange, the effluent water flows to a third tank. Here a "T" area is required by most municipalities to trap any grease, oil or other liquid lighter than water. A final automatic pH balancing system in the third tank is needed to meet most municipalities' regulations. Some communities may even require a continuing record of the pH.

After meeting any relevant local, state and federal regulations, the effluent may be discharged to a municipal sewer system or septic tank/treatment system. If dye leaves the effluent too colored, then dissolved air flotation, reverse osmosis, ionization, evaporation and micro-filtration may reduce it to acceptable levels.

A choice of VIBROSCREEN unit diameters enables laundries of different sizes to select a unit that meets their wastewater flow requirements. See the chart at right for the choices.


VIBROSCREEN CHOICES

Screen DiametersVolumeMotorSpeed
(inches)
(gal/min)
Size
@60 Hz
@50 Hz
Voltage
18
20
one-third hp
1200
1000
one phase 115v or three phase 230v/460v
24
30
one-third hp
1200
1000
30
55
one-third hp
1200
1000
40
100
one-third hp
1200
1000
48
175
one hp
1200
1000
60
300
one hp
1200
1000



Vibroscreen Separators Save Laundries


Screen Tips - Volume 6, Number 3 Fall 1991