Patent Application
20140124461
The present invention relates to rotary disc filters for treating wastewater, and more particularly to a method for inhibiting biological growth on filtration media employed in rotary disc filters.
Biofouling from biological growth on filter media is a serious problem in water treatment facilities, and, in particular, filters used therein. This biological growth is usually present in the form of biofilm. Biofilm comprises bacterial colonies that attach to filter media and the excretions therefrom. Biofilm clogs and fouls filters and, without treatment, can result in total filter blockage within a period of days or weeks.
These problems are only exacerbated when filter media is comprised of nonwoven media. Such nonwoven media can be produced with openings smaller than ten microns, and may be used in rotary disc filters to improve removal efficiency and filtration rates. These filtration improvements from nonwoven filter media, however, cannot be maintained due to the formation of biofilm on the fibers comprising the nonwoven filter media. Such biofilm cannot be eliminated with a standard 8 bar backwash typically used in disc filter backwashing operations. Indeed, backwash up to 80 bar is insufficient to eliminate such biofouling. After approximately one to two weeks of utilization for tertiary water treatment, in many cases nonwoven filter media will be completely blocked by biofouling.
Disclosed herein is a method or process for inhibiting bio-fouling from biological growth on filtration media of a rotary disc filter. In this method, water is directed to the rotary disc filter comprising at least one filter disc. The filter disc has filter media positioned to permit water filtration. The water is directed through the filter media to produce a filtrate. The filter media is then positioned for cleaning by rotating at least a portion of the filter media to a backwashing position. A backwash is provided and a biocide is mixed therewith to produce a backwash-biocide solution. The backwash-biocide solution is then sprayed onto the filter media during a backwashing operation, and inhibits and eliminates biological growth on the filter media.
The current invention is directed towards methods for inhibiting biological growth on rotary disc filters. Rotary disc filters are well known and widely used to provide water filtration. As used herein, the term ‘water’ encompasses all forms of feedwater, to include wastewater. Rotary disc filters are shown and described in patents and other published materials. For example, reference is made to U.S. Pat. No. No. 7,597,805 and U.S. Patent Publication No. 2008/0035584. The disclosures of these two publications are expressly incorporated herein by reference. A complete and unified understanding of disc filters, their structure, and operation can be gained by reviewing these materials.
A brief overview of the structure and operation of a typical disc filter may be beneficial.
The number of rotary filter discs 14 secured on the drum and rotatable therewith can vary. Basically, each rotary filter disc 14 includes a filter frame 16 and filter media 18 secured on opposite sides of each rotary filter disc 14. A holding area is defined inside each rotary filter disc 14 for receiving influent to be filtered by the rotary filter disc 14.
The disc filter 10 is provided with a drive system for rotatively driving the drum and the rotary filter discs 14 mounted thereon. There is provided a drum motor 64 that is operative to drive a sprocket or sheave (not shown) connected to the drum. See
Continuing to refer to
Disc filter 10 also includes an effluent holding tank 26. Effluent holding tank 26 is disposed about a downstream end portion of the disc filter 10, and as shown in the drawings, extends around at least a lower portion of the rotary filter discs 14. As the influent moves outwardly through the filter media 18, this results in the water being filtered, and it follows that the filtered water constitutes an effluent. It is this effluent that is held within the effluent holding tank 26. There is also provided an effluent outlet associated with the effluent holding tank 26 for directing effluent or filtered water from the disc filter 10.
Therefore, it follows that influent water to be treated or filtered is directed into the influent inlet 22 and into the influent holding tank 24 where the water accumulates to a selected height therein so as to provide a head pressure for effectively causing the water to move from the inner portions of the rotary filter discs 14 outwardly through the filter media 18. Influent held within the holding tank 24 eventually is directed into the drum, and from the drum through openings therein into the interior areas of the rotary filter discs 14. Now, the water within the rotary filter disc moves outwardly through the filter media 18 into the effluent holding tank 26, and eventually out the effluent outlet.
The present application focuses on methods for preventing biological growth on disc filters. One way to prevent, eliminate, or inhibit biological growth is to utilize a biocide. Biocides are substances (or in some cases organisms) that kill currently growing biological contaminants and deter growth of new biological contaminants. For example, the biocide chlorine has long been added to swimming pools and spas to both kill bacteria present in the pool water and prevent new bacterial growth therein.
The methods disclosed herein may be used with any biocide that can remove biofilm from filter media. In preferred embodiments, the biocide is one that does not cause environmental harm. One preferred type of biocide is peroxy acids. One example of a peroxy acid is peracetic acid. Peracetic acid inhibits growth of a broad range of biological contaminants. After treatment, peracetic acid breaks down into hydrogen peroxide and acetic acid, which are non-toxic and environmentally friendly. In one exemplary embodiment, the concentration of the peracetic acid used is approximately 2-15% by weight. Another example of a peroxy acid (biocide) is performic acid. Performic acid effectively inhibits growth of, inter alia, bacteria, fungi, viruses, and other microorganisms. Because performic acid degrades to carbon dioxide, oxygen, and water, it is an environmentally friendly biocide.
The present invention envisions incorporating a biocide application into the backwashing system of a rotary disc filter. One such backwashing system is shown in
As shown in
In one embodiment, the biocide is dosed to the backwash at a concentration of approximately 10-300 ppm, with dosing occurring approximately 5-100% of the time. The concentration of the biocide dosed and the frequency of dosing will vary depending upon conditions and particular application. In one embodiment, for example, it is believed that in many applications a concentration of approximately 20-80 ppm of biocide with a dosing frequency of 10-20% will be sufficient to control biofouling on the filter media. The term “frequency of dosing” is a term that compares the frequency of applying the biocide relative to the frequency of backwashing. For example, a dosing frequency of 50% means that the biocide is being mixed with the backwash one-half or 50% of the time. In order to backwash the filter media 18, the drum can be continuously or intermittently rotated such that the filter media or filter panels 18 enter the accumulated effluent in the effluent holding tank 26. It is appreciated that only a bottom portion of the filter media 18 is effective at any one time to filter the influent. From time-to-time the drum and rotary filter discs 14 will be rotated, and when this occurs, some portions of the filter media 18 will be rotated to an upper portion and in this position the filter media 18 will not be in a position to filter the effluent.
During a backwash cycle, high pressure backwash-biocide solution is sprayed from the nozzle arrays 46 onto the outer surfaces of the filter media 18 to clean them. This can occur when the drum and rotary filter discs 14 are stationary or being rotated. The backwash-biocide solution sprayed on from the nozzle arrays 46 impacts the outer surface of the filter media 18, vibrating the filter media and even penetrating the filter media. This causes debris caught on the inner side of the filter media 18 to be dislodged or removed from the inner surface of the filter media 18. This debris and the backwash water fall into the underlying trough extending through the drum. Thereafter the debris and backwash water are channeled out the outlet 50. It is appreciated that, while upper portions of the filter media 18 are backwashed, disinfected, and cleaned, the lower submerged portions of the filter media can continue to filter the influent.
In another embodiment, the biocide could be applied to the filter media independently of the backwash system. In this case, a separate set of nozzles could be utilized to spray the biocide onto the filter media. The biocide could be chemically diluted and applied at a relatively low pressure, for example, 1-2 bar, while the filter discs are rotated relatively slowly. An example of this embodiment is shown in
Although the present methods have been shown and described in considerable detail with respect to only a few/particular exemplary embodiments thereof, it should be understood by those skilled in the art that it is not intended to limit the methods to the embodiments since various modifications, omissions, and additions may be made to the disclosed embodiments without materially departing from the novel teachings and advantages of the methods, particularly in light of the foregoing teachings.
The present invention may, of course, be carried out in other specific ways than those herein set forth without departing from the scope and the essential characteristics of the invention. The present embodiments are therefore to be construed in all aspects as illustrative and not restrictive and all changes coming within the meaning and equivalency range of the appended claims are intended to be embraced therein.
