GB2035284A - Trickle filter medium for sewage treatment - Google Patents

Abstract

Bio-oxidation and nitrification modules (12) for treating sewage and industrial wastewater are formed of alternating flat sheets (15) perforated with holes (20) which may be up to 0.8 cm. in width and 1.2 cm. in length and that produce a percentage void area in the sheet of up to 50 percent and corrugated sheets (14). <IMAGE>

Description

SPECIFICATION Bio-oxidation and nitrification module for treating sewage and industrial wastewater The present invention relates generally to modules for the treatment of sewage and industrial wastewater.

The bio-oxidation and nitrification of sewage and industrial wastewater normally is carried out in towers that are "packed" with treatment modules that provide a large surface area upon which micro-organisms can grow. For a number of years, such treatment towers usually were filled with rocks having an average diameter of about 5 to 10 centimeters over which the sewage or industrial wastewater was "trickled". The aerobic bacterial growth that formed on the surfaces of the rocks converted organic material and inorganic nutrients in the wastewater into relatively stable products (such as biological solids, carbon dioxide, nitrates and nitrites). The solid materials are removed from the treated wastewater before the wastewater is treated further or returned to the environment.

In more recent times, the rock packing in the treatment towers has been replaced with media in the form of modules comprised of alternating flat and corrugated sheets. The sheets usually are made of rigid plastics material, such as polyvinyl chloride, polyethylene, polypropylene or polystyrene, bonded together using a suitable adhesive or by solvent bonding. A typical module may be about 60 centimeters wide, about 120 centimeters long, and about 60 centimeters high. A number of modules are stacked on top of each other within the tower, the stacked modules often reaching a height of 10 to 15 meters. The thickness of the sheets and their rigidity must be sufficient to withstand collapse during normal use of the treatment tower.The corrugations of the corrugated sheets extend across the entire dimension of the sheet and preferably are sinuous to avoid straight fall-through of the waste-water through the module.

One type of module which found early use in the treatment of industrial wastewater is described in U.S.

Patent 3347381. A later type of module that uses corrugated sheets in which the corrugations are curvi-linear and have transverse ribs to provide additional rigidity is described in U.S. Patent 3618778.

The efficiency of the treatment tower depends in large part on the amount of surface area per cubic meter of module volume available for supporting the growth of aerobic bacteria. Modules formed of alternating sheets of flat and corrugated media can provide 85 square meters or more of surface area per cubic meter of module. Since a number of modules are stacked one on top of another, the combined weight that must be supported by the modules at the bottom of the stack becomes substantial. Reduction in weight of the modules, without sacrificing the efficiency of the module for treating wastewater or the structural load-bearing strength of the module, would relieve the overall weight which must be borne by the modules at the bottom of the stacked tower.

According to the invention there is provided a module for treating sewage and industrial wastewater comprising a plurality of flat plastics sheets and a plurality of corrugated plastics sheets having a plurality of corrugationstherein and being positioned in alternating arrangement with said flat plastics sheets, said flat plastics sheets having openings therethrough, said openings having a width up to 0.8 centimeter and a length up to 1.2 centimeter, said perforated flat plastics sheets having a percentage void area up to 50 percent, and said flat plastics sheets and alternating corrugated sheets being attached together.The perforations preferably are uniformly spaced over the face of the sheet. (The "percentage void area" of a perforated surface is derived by dividing the area of the surface removed as a result of perforating the sheet by the area ofthetotal surface priorto being perforated and multiplying the quotient by 100).

While it would be expected that the perforations would reduce the overall weight of the module, one would expect that as a result of perforating the flat sheets of the module the "vertical crush strength" of the module would be reduced so that thicker flat sheets would need to be used to compensate for the anticipated reduction in "vertical crush strength". However, it has been found, contrary to what would be anticipated, that the "vertical crush strength" of the modules is not reduced if the perforations do not exceed a certain width and the "percentage void area" of the flat sheets resulting from the perforations is restricted up to 50 percent.

Also, it would be expected that the efficiency of the module for treating wastewater would be reduced as a result of perforating the flat sheets of the module, since the surface area of the module upon which the aerobic bacteria would grow would be reduced. However, it has been found that if the width of the perforations does not exceed about 0.8 cm., the aerobic bacteria growth will "bridge" the openings in the flat sheets of the module and as a resu It will provide the equivalent surface of bacteria growth for bio-oxidation and nitrification purposes as if the flat sheets had not been perforated.

Thus, a significant reduction in the weight of the module with attendant economic advantage can be realised by utilization of the present invention without sacrificing the efficiency of the module for treating wastewater and without reducing the "vertical crush strength" of the module.

By way of example, two embodiments of a module according to the invention for treating sewage and industrial wastewater will now be described with reference to the accompanying drawings, in which: Figure 7 is a schematic perspective view, partly broken away, illustrating a tower for treating sewage and industrial wastewafer and being "packed" with treatment modules; Figure 2 is a schematic perspective view, partly broken away, of a module embodying the present invention; Figure 3 is an elevation view, partly broken away, of a perforated flat sheet of a module such as shown in Figure 2 and which illustrates the preferred embodiment of this invention: Figure 4 is an elevation view, partly broken away, of a perforated flat sheet of a module such as shown in Figure 2 and which illustrates a second embodiment of this invention; and Figure 5 is a section on the line 5-5 of Figure 3 and showing the growth of aerobic bacteria on the surfaces of the perforated flat sheet.

Referring to the drawings, the treatment tower 10 shown in Figure 1 includes a wall 11 within which are housed modules 12. Air vents 13 extend through wall 11 at its base and allow air to be drawn into tower 10 at its bottom and flow up through the openings of modules 12 in counter-current flow to the sewage or industrial wastewater inroduced at the top of the tower 10 and permitted to flow downward through the same openings through which the air is flowing.

Since modules 12 are structurally self-supporting, wall 11 of tower 10 generally is not subjected to significant structural loads and need be only of sufficient strength to withstand wind loads. In larger towers, wall 11 may be formed of precast concrete sections about 5 to 8 centimeters thick, while in smaller towers wall 11 may be fabricated or rigid corrugated fiberglass-reinforced plastic sheet about 0.5 centimeter thick supported on a lightweight metal frame.

As shown in Figure 2, modules 12 are formed of corrugated sheets 14 that alternate with flat sheets 15.

Adjacent sheets 14 and 15 of the module can be fastened together by bonding adjacent sheets 14 and 15 together using a suitable adhesive or by a solvent bonding or other technique along areas at which adjacent sheets 14 and 15 contact, or adjacent sheets 14 and 15 can be secured together by mechanical attachment.

One manner of mechanical attachment that can be used is shown and described in U.S. Patent 3,260,511.

The corrugated sheets 14 and flat sheets 15, may be fabricated of any plastic material having a suitable rigidity. Suitable plastic materials include polyvinyl chloride homopolymers, polyvinyl chloride copolymers (such as polyvinyl chloride-polyvinyl acetate copolymer and.polyvinyl-polyvinylidene chloride copolymer), polyvinylidene chloride homopolymer, polypropylene, high-density polyethylene, chlorinated polyvinyl chloride homopolymer, chlorinated low-density polyethylene, chlorinated high-density polyethylene, polymethyl methacrylate, polystyrene, and polyoxymethylene polymers and copolymers. The thickness of sheets 14 and sheets 15 may vary.Usually, thicker sheets. are used in modules that are to be positioned near the bottom of the tower since these modules will be required to support a greater load than those modules positioned near the top of the tower. Typically, the thickness of the corrugated sheets 14 will be from 15 to 70 mils thick and the thickness of the flat sheets 15, will be from 15 to 40 mils thick, depending upon the intended-placement of a particular module in a tower. When installing the modules in a tower, alternate layers of the modules usually are rotated 900 from the position of the layer of modules immediately beneath, The corrugations of corrugated sheets 14 of module 12 preferably are sinuous (zigzag or curvilinear) to avoid a straight fall-through of the wastewater through the module.As shown in Figure 2, the corrugations 17 of corrugated sheets 14, 14 preferably have flat apexes 18 and flat spaces 19 between adjacent corrugations to provide a greater surface area for bonding the corrugated sheets 14 to their adjacent flat sheets 15. Corrugations 17 generally have a depth "d" of about 2.5 to 7.5 centimeters, although sheets with corrugations 17 having a depth up to 10 centimeters may be used in some installations.

The flat sheets 15 of module 12 are perforated to provide openings 20, thereby reducing the weight of the module. The openings 20 may have a width up to 0.8 centimeter and a length up to 1.2 centimeters. The openings 20 desirably are equally spaced from adjacent openings and may be present in sufficient number to create a "percentage void area" of up to 50 percent. The perforations 20 preferably are circular and preferably are arranged in staggered rows, as is shown in the preferred modification illustrated in Figure 3.

When circular openings 20 are used the diameter of the openings should not exceed 0.8 centimeter. The embodiment shown in Figure 4, illustrates the use of non-circular openings 20 in flat sheet 15. When non-circular openings 20 are used, the width "w" of the opening should not be greater than 0.8 centimeter and the length "1" of the opening should not exceed 1.2 centimeter. To provide optimum vertical crush strength (when non-circular openings are used), the perforated sheet 15 desirably is positioned within the module 12 so that the major axis of the openings 20 will be vertical when the module is installed within a treatment tower.

The "vertical crush strength" of modules constructed with perforated flat sheets was compared to the "vertical crush strength" of modules formed with non-perforated flat sheets to ascertain whether the "vertical crush strength" of a module constructed in accordance with the present invention would be objectionably reduced as a consequence of perforating the flat sheets of the module. Twelve modules were constructed, each consisting of six flat polyvinyl choride sheets alternating with six polyvinyl chloride corrugated sheets. The flat sheets of six of the modules were perforated with uniformly spaced circular openings arranged in staggered rows and measuring 0.26 centimeters in diameter and producing a "percentage void area" of 27 percent. The flat sheets used in the twelve modules were 25 mils thick, while the corrugated sheets were 15 mils thick. The finished dimensions of each module was about 30 centimeters wide, about 30 centimeters deep and about 60 centimeters high. The modules containing the perforated flat sheets were numbered 1 through 6 and those formed with non-perforated flat sheets were numbered 7 through 12. In addition, modules Nos. 4,5 and 6 and modules Nos. 10, 11 and 12 were trimmed at their edges to produce more even end portions. The adjacent sheets of the modules were bonded together using solvent bonding. The "vertical crush strength" of the modules was determined using a Tinius Olson testing machine. The module to be tested was placed on the base of the testing machine with a 30 xm. x 30 cm.

piece of 1.9 cm. thick plywood covering the top of the module. The compression plate of the testing machine was lowered until it just contacted the surface of the polywood cover. The load scale of the machine was set so that a 100 percent short reading would be above the "vertical crush strength" of the module. A load reading of 5 percent on the chart was exerted on the module as a basis for beginning the test. The initial compression load of 5 percent exerted on the module eliminated any unevenness that might exist in the top and bottom faces of the module. The deflection at the 5 percent compression load was arbitrarily designated "zero" deflection. The load was held for 2 minutes after which time the loading was increased until a 10 percent loading was obtained. Deflection was recorded immediately prior to increasing the loading.After a 2 minute interval the loading was increased until the loading reached 15 pecent. After another 2 minute interval the loading on the module again was increased until the loading reached 20 percent. The procedure was continued until the module could not maintain the loading for 2 minutes without deflecting further (indicating failure or crushing of the module).

Table I lists the "vertical crush strengths" obtained on the twelve sample modules described above: TABLE I Vertical Crush Strength Module Kg per M2 1 4700 2 4200 3 Perforated Flat Sheets 5300 4 4700 5 4220 6 4700 7 2945 8 4220 9 Non-perforated Flat Sheets 3530 10 4220 11 3530 12 4700 The average "vertical crush strength" of modules Nos. 1 through 6 was 4640 Kg per M2 while the average "vertical crush strength" of modules Nos. 7 through 12 was only 3858 Kg per M2. The tests indicate that the perforations in the flat sheets of the modules not only did not objectionably reduce the "vertical crush strength" of the module but, in fact, improved the "vertical crush strength" of the module. It is theorized that the improvement in "vertical crush strength" may be due to an improved bond between the sheets of the module that contained the perforated flat sheets and an improved distribution of the load throughout the module.

Figure 5 illustrates a perforated flat sheet 15 with a growth of aerobic bacteria 21 adhering to the flat surfaces of sheet 15. When the growth 21 first forms, it bridges the perforations 20 in the sheet 15 and initially forms a dimpled surface contour as shown in solid lines. After the module has been in use for a period of time, the dimples fully fill in as is indicated by the "dot-dash" lines of Figure 5 While the invention has been described with reference to certain specific embodiments, it will be understood that there are many variations which would come within the spirit and scope of the invention as defined by the appended claims.

Claims (7)

1. A module for treating sewage and industrial wastewater comprising a plurality of flat plastics sheets and a plurality of corrugated plastics sheets having a plurality of corrugations therein and being positioned in alternating arrangement with said flat plastics sheets, said flat plastics sheets having openings therethrough, said openings having a width up to 0.8 centimeter and a length up to 1.2 centimeter, said perforated flat plastics sheets having a percentage void area up to 50 percent, and said flat plastics sheets and alternating corrugated sheets being attached together.

2. A module as claimed in claim 1 wherein the openings in said flat plastics sheets are equally spaced from each other.

3. A module as claimed in any preceding claim wherein the openings in said flat plastics sheets are circular and have a diameter up to approximately 0.8 centimeter.

4. A module as claimed in any preceding claim wherein the openings in said flat plastics sheets are arranged in staggered rows.

5. A module as claimed in any preceding claim wherein the said flat plastics sheets and the said corrugated plastics sheets comprise a polyvinyl chloride homopolymer.

6. A module according to any preceding claim wherein the said flat plastics sheets have a thickness of 15 to 40 mils and the said corrugated plastics sheets have a thickness of 15 to 70 mils.

7. A module for treating sewage and industrial wastewater substantially as hereinbefore described with reference to and as shown in Figures 1,2,3 and 5 or in Figures 1, 2 and 4 of the accompanying drawings.