CLOTH MEDIA FILTRATION KEYWORDS: Cloth Media, Filtration, Reuse

CLOTH MEDIA FILTRATION Lloyd Johnson, P.E.*

Aqua-Aerobic Systems, Inc. 6306 N. Alpine Road, Rockford, IL 61111

(Ph) 815/639-4516, (fax) 815/654-2508, [email protected]

KEYWORDS: Cloth Media, Filtration, Reuse, TSS, NTU, Flux

ABSTRACT The Cloth Media Filter by Aqua-Aerobic Systems, Inc. is a unique filtration system for application in both municipal and industrial wastewater treatment. It offers significant benefits over traditional granular media filtration systems as proven by over 300 installations worldwide since 1993. Among these benefits are high throughputs in a much smaller footprint (75% smaller than conventional granular media filters) and high quality effluent (typically, = 5.0 mg/l TSS and = 2.0 NTU). The key feature which catapulted the filter technology ahead of all others is its media, a unique cloth that is carefully engineered for quality, durability, and performance.

During operation, influent enters filter tanks and flows by gravity through the cloth media on stationary hollow support structures. The filtrate exits through a hollow shaft, which supports the cloth media. As solids accumulate on the surface of the cloth media, the water level surrounding the media rises. Once a predetermined level is reached, a cleaning mode is initiated and the media surface is automatically backwashed. Heavier solids settle to the bottom of the tank and are pumped back to the head-works of the plant.

This unique cloth media has been adapted to a variety of mechanical configurations, including disks, drums and diamonds. Each provides application specific benefits.

This paper will provide a description of the cloth media filtration technology and its features and benefits.

Introduction Today we are frequently reminded of our resource limitations as they relate to a broad spectrum of applications in industry, recreation and day-to-day living. One of the most significant of all our resources is water. Increasing demands, often without equal replenishment, are forcing adaptations of use and serious management of all existing water sources.

Filtration in its variety of forms continues to play a significant role in the effective and efficient cleaning of various water streams. The focus of this paper is to discuss the emerging use of cloth media in the water reuse market. The media is uniquely developed for this application, but may also have extended application in various industries. The primary attributes of the media is the ability to apply high flux rates in combination with good solids removal and the ability to restore the media through low energy cleaning.

Copyright 2005 Aqua-Aerobic Systems, Inc.

Background Early successful commercial use of cloth media in wastewater treatment applications took place in central Europe during the 1980s. The typical filter was configured on a drum that could rotate for cleaning. The cloth media filtration unit was typically placed in a concrete tank following a rotating biological contactor (RBC) for small on-site treatment plants. The RBC/filter combination, as shown in Figure 1, provided a very small foot print because the cloth media filter replaced the clarification step

.

Figure 1: Rotating biological contactor with cloth media filter

A needlefelt style of media, as shown in Figures 1A and 1B, was used in these early applications. Needlefelt provided excellent filtration characteristics but at the expense of high energy cleaning. To clean the media, it required a frequent clean water backwash and periodic high pressure spray followed by a clean water backwash. The filter needed to be removed from service to perform the high pressure spray cleaning, therefore interrupting the filtration process.

Figure 1A: Cross Section Needlefelt Figure 1B: Schematic


In the mid 1990s, Drs. Grabbe and Seyfried, University of Hanover, developed an alternative media configuration. This new media is commonly referred to as “pile” or a velour style of construction. See Figures 2A and 2B. The unique pile constructed media preserves the filtration capability in terms of flux and filterability and, at the same time, significantly improves the solids retention capacity of the media. Additionally, the media can be restored by low energy cleaning without the need for high pressure spray. This combined improvement has led to more efficient cleaning because the filtration process in no longer interrupted and it is accomplished at a lower energy cost.

Figure 2A: Pile media cross section Figure 2B: Schematic

Operational Characteristics of Pile Cloth Media The cloth media filtration units are low-head gravity driven filters. The filtration component is completely submerged throughout the operational cycle. Influent is introduced to a tank and permeates through the cloth media. The filtrate is collected in a center column or header and flows to an effluent chamber where it is discharged from the tank. Solids contained in the influent accumulate on and within the media. Solids accumulation will cause additional head loss resistance which, at a predetermined liquid differential, will initiate a backwash cleaning event. The cleaning event can also be initiated manually or by a default time setting.

One of the primary advantages of the pile constructed media is the ability to clean and restore the media without the need for a high pressure spray. See Figures 3 and 4. In Figure 3, the pile media is shown in filtration orientation, while in figure 4, the pile media is shown in backwash orientation.

Figure 3: Pile media Figure 4: Pile media Filtration orientation Backwash orientation


The pile fibers lift and straighten during the backwash event, allowing the filtered water to actively clean throughout the media. Additionally, a suction header is designed to permit a narrow strip of media to be cleaned and also provides the subsequent realignment of the fibers following cleaning.

Filter Configuration Historically, the common method of filtration in the water and wastewater industries is by granular media in a variety of forms. Typical configurations are shallow depth single and dual media, deep bed filters with single or multimedia, and pressure filters. Granular media is restricted to a flat planer arrangement contained in various tank configurations. Cloth media is very flexible and can be configured in almost an unlimited fashion.

Three basic configurations are shown in figures 5, 6, and 7. They are a drum, disk, and diamond respectively. The configurations tend to be self limiting by their geometry as it relates to flow capacity. Each is typically applied as follows: a drum is used for flows between 10 and 100 gallons per minute; a disk filter is used for flows from 0.25 to 10 million gallons per day; and a diamond is used for flows above 10 million gallons per day. All three cloth media arrangements use low head gravity flow with a typical 12 inch level differential that imitates a backwash event.

Figure 5: Cloth Media Drum Filter Figure 6: Cloth Media Disk Filter

Figure 7: Cloth Media Diamond Filter


Performance Characteristics The performance of any given media will reflect its construction attributes in the filtered water quality. The pile media provides surface area based on the fiber choices used in the yarn selection. The fibers overlay each other and develop a three to four millimeter filtration depth. During the filtration mode, solids collect on and within the media, providing additional filtration of finer particles. This phenomenon is referred to as “ripening.” Another filtration mechanism is a result of particles that collect on the surface of the media and form a mat. This phenomenon is referred to as “auto filtration.” A perceived advantage of the pile cloth is its open structure in combination with auto-filtration that minimizes the build up of head loss as the solids accumulate during filtration. We know filtration flux contributes to head loss but another contributing factor to the rate at which the head loss occurs is influent particle size distribution; smaller particles create more head loss per mass accumulated than do larger particles.

Studies conducted at the University of California (Davis) have provided valuable insight with respect to cloth media filtration performance. The significance of these studies demonstrates the required filtration performance to meet water reuse criteria as legislated under Title 22. The primary objective was to maintain a filtrate turbidity of less than 2 NTU under all conditions. The data reflected in the charts below are based on typical secondary effluent from an extended aeration process with a mean cell residence time greater than eight days.

The low influent turbidity experiments were conducted using directly applied secondary effluent. For high influent turbidity experiments, the secondary effluent feed stream was augmented with mixed liquor suspended solids. Testing included the following three flux rates: 3 (black), 4.5 (blue), and 6 gpm/ft

2 (red).

Table 1, provides an overall comparative summary of the two cloth media configurations. It is important to note the similarity in filtration performance recognizing that the pile cloth provides two to three times more solids storage capacity. Also, the associated backwash frequency is longer in duration for the pile cloth.


Table 1: Summary of average turbidity and TSS removal for the CMDF with the needlefelt and pile fabrics

Needlefelt Pile

Turbiditya TSSb Turbiditya TSSb

Exp HLR Influent, Effluent, Removal, Influent, Effluent, Removal, Influent, Effluent, Removal, Influent, Effluent, Removal m/hr NTU NTU % mg/L mg/L % NTU NTU % mg/L mg/L %

1 7.3 2.2 0.69 69 6.7 0.49 93 2.5 0.91 64 5.0 0.57 88

2 7.3 9.3 0.46 95 21.0 0.73 96 20.6 0.53 97 57.4 0.62 99

3 11.0 2.9 0.94 98 7.1 0.56 92 3.2 0.84 74 8.2 0.94 88

4 11.0 8.3 0.70 92 18.4 0.79 96 17.7 0.52 97 53.0 0.45 99

5 14.7 4.9 1.10 78 8.8 0.82 91 4.6 1.14 75 8.2 0.89 89

6 14.7 7.2 0.79 89 19.0 0.69 96 15.4 0.71 95 31.8 0.72 98

a Turbidity values were taken from continuous measurements. b TSS values obtained from grab samples.


The particle distribution in the influent and filtrate streams is another way to measure filtration efficiency. Each wastewater will be unique and often the particle distribution will change as a result of existing upstream conditions. Filtrate quality is affected by solids retention time (SRT), process method like fixed film versus a complete mix regime and process operating conditions such as nutrient removal. Data recently gathered on two different process streams, courtesy of UC Davis, are noted in Figure 8.

Figure 8: Particle Distribution for Two Solid Retention Times

The data clearly indicates particle size variation based on process and chemical conditioning. It is commonly known that chemical conditioning can improve filtration but it can also cause additional problems. Plus, chemicals add expense, which we prefer to avoid.

Conclusion Cloth media provides a good alternative to granular media for water reuse applications and may be applicable to certain industrial applications. The cloth is unique and can be arranged in many configurations that provide good space utilization. The pile cloth provides higher solids accumulation per unit of area than does needlefelt cloth and can be cleaned without high pressure spray. The main principle of operation is gravity flow through a stationary media with a typical head differential of 12 inches to initiate cleaning. Water reuse is growing in importance throughout the world. Cloth media can provide an efficient solution. New media selections are currently being developed to meet unique application requirements.


CLOTH MEDIA FILTRATION KEYWORDS: Cloth Media, Filtration, Reuse

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