Trickling Filters

Trickling filters have been used to treat wastewater since the 1890s. It was found that when settled wastewater was passed over rock surfaces, slime grew on the rocks and the water became cleaner. Today, we still use this principle, but in many installations we use plastic media instead of rocks. In most wastewater treatment systems, the trickling filter follows primary treatment and includes a secondary settling tank or clarifier, as shown in Figure 14.2. Trickling filters are widely used for the treatment of domestic and industrial wastes. The process is a fixed-film biological treatment method designed to remove BOD5 and suspended solids.

A trickling filter consists of a rotating distribution arm that sprays and evenly distributes liquid wastewater over a circular bed of fist-sized rocks, other coarse materials, or synthetic media (see Figure 14.3). The spaces between the media allow air to circulate easily so aerobic conditions can be maintained. The spaces also allow wastewater to trickle down through, around, and over the media. A layer of biological slime that absorbs and consumes the wastes trickling through the bed covers

Figure 14.2 Simplified flow diagram of trickling filter used for wastewa-ter treatment.
Figure 14.3 Schematic of cross-section of a trickling filter.

the media material. The organisms aerobically decompose the solids, producing more organisms and stable wastes, which either become part of the slime or are discharged back into the wastewater flowing over the media. This slime consists mainly of bacteria, but it may also include algae, protozoa, worms, snails, fungi, and insect larvae. The accumulating slime occasionally sloughs off (sloughings) individual media materials (see Figure 14.4) and is collected at the bottom of the filter, along with the treated wastewater, and is passed on to the secondary settling tank where it is removed. The overall performance of the trickling filter is dependent on hydraulic and organic loading, temperature, and recirculation.

Figure 14.4 Filter media showing biological activities that take place on surface area.

To clearly understand the correct operation of the trickling filter, the operator must be familiar with certain terms. (Note that the following list of terms applies to the trickling filter process. We assume that other terms related to other units within the treatment system are already familiar to operators.)

Biological towers—A type of trickling filter that is very deep (10 to 20 feet). Filled with lightweight synthetic media, these towers are also know as oxidation or roughing towers or (because of their extremely high hydraulic loading) super-high-rate trickling filters.

Biomass—The total mass of organisms attached to the media. Similar to solids inventory in the activated sludge process, biomass is sometimes referred to as the zoogleal slime.

Distribution arm—The device most widely used to apply wastewater evenly over the entire surface of the media. In most cases, the force of the wastewater being sprayed through the orifices moves the arm.

Filter underdrain—The open space provided under the media to collect the liquid (wastewater and sloughings) and to allow air to enter the filter. It has a sloped floor to collect the flow to a central channel for removal.

High-rate trickling filters—A classification (see Table 14.1) in which the organic loading is in the range of 25 to 100 lb BODs per 1000 ft3 of media per day. The standard-rate filter may also produce a highly nitrified effluent.

Hydraulic loading—The amount of wastewater flow applied to the surface of the trickling filter media. It can be expressed in several ways: flow per square foot of surface per day (gpd/ft2), flow per acre per day (MGAD), or flow per acre-foot per day (MGAFD). The hydraulic loading includes all flow entering the filter.

Media—An inert substance placed in the filter to provide a surface for the microorganism to grow on. The media can be field stone, crushed stone, slag, plastic, or redwood slats.

Organic loading—The amount of BODs or chemical oxygen demand (COD) applied to a given volume of filter media. It does not include the BOD5 or chemical oxygen demand (COD) contributed to any recirculated flow and is commonly expressed as lb BOD5 or COD per 1000 ft3 of media.

Recirculation—The return of filter effluent back to the head of the trickling filter. It can level flow variations and assist in solving operational problems, such as ponding, filter flies, and odors.

Roughing filters—A classification of trickling filters (see Table 14.1) in which the organic loading is in excess of 200 lb BOD5 per 1000 ft3 of media per day. A roughing filter is used to reduce the loading on other biological treatment processes to produce an industrial discharge that can be safely treated in a municipal treatment facility.

TABLE 14.1 TRICKLING FILTER CLASSIFICATION

Filter Class

Standard

Intermediate

High Rate

Super High Rate

Roughing

Hydraulic loading (gpd/ft2)

25-90

90-230

230-900

350-2100

>900

Organic loading (BOD per 1000 ft3)

5-25

15-30

25-300

Up to 300

>300

Sloughing frequency

Seasonal

Varies

Continuous

Continuous

Continuous

Distribution

Rotary

Rotary fixed

Rotary fixed

Rotary

Rotary fixed

Recirculation

No

Usually

Always

Usually

Not usually

Media depth (ft)

6-8

6-8

3-8

Up to 40

3-20

Media type

Rock

Rock

Rock

Plastic

Rock

Plastic

Plastic

Plastic

Plastic

Wood

Wood

Wood

Wood

Nitrification

Yes

Some

Some

Limited

None

Filter flies

Yes

Variable

Variable

Very few

Not usually

BOD removal

80-85%

50-70%

65-80%

65-85%

40-65%

TSS removal

80-85%

50-70%

65-80%

65-85%

40-65%

Sloughing—The process in which the excess growths break away from the media and wash through the filter to the underdrains with the wastewater. These sloughings must be removed from the flow by settling.

Staging—The practice of operating two or more trickling filters in series. The effluent of one filter is used as the influent of the next. This practice can produce a higher quality effluent by removing additional BOD5 or COD.

Guide to Alternative Fuels

Guide to Alternative Fuels

Your Alternative Fuel Solution for Saving Money, Reducing Oil Dependency, and Helping the Planet. Ethanol is an alternative to gasoline. The use of ethanol has been demonstrated to reduce greenhouse emissions slightly as compared to gasoline. Through this ebook, you are going to learn what you will need to know why choosing an alternative fuel may benefit you and your future.

Get My Free Ebook


Post a comment