Microbiology of trickling filter

VEERMATA JIJABAI TECHNOLOGICAL INSTITUTE.

MATUNGA, MUMBAI

A

Seminar Report

On

“MICROBIOLOGY OF TRICKLING FILTERS”

Presented By,

Mansi J. Kakani

MTech (Environmental Engineering)

Under the guidance of,

Dr. Aruna Joshi

Microbiology of Trickling Filter

1

INDEX

Sr. No.

Contents Page No.

1. Trickling Filters: Introduction

2

3. Filtration 4

2. Filter Classification

4

3. Trickling Filter: Process description

6

4. Nitrification

7

5. Biology of Trickling Filters

9

6. Organisms present in trickling filter biofilms

10

7. Removal of Pathogens and Parasites by Trickling Filters

12

8. Advantages and disadvantages of trickling filters

13

9. Operational Problems

14

10. References

19


2

MICROIOLOGY OF TRICKLING FILTER

INTRODUCTION:

A trickling filter (TF) is a wastewater treatment system that biodegrades organic matter

and can also be used to achieve nitrification. The wastewater trickles through a circular bed of

coarse stones or plastic material. A rotating distributor (a rotating pipe with several holes across

it) evenly distributes the wastewater from above the bed. The microorganisms in the wastewater

attach themselves to the bed (also known as the filter media), which is covered with bacteria. The

ssbacteria break down the organic waste and remove pollutants from the wastewater.

When excess nutrients became a concern, it became necessary to adapt “conventional”

sewage treatment systems to meet the increased oxygen demand placed on receiving waters by

high ammonia nitrogen concentrations in wastewater effluents.

The trickling filter consists of several major components including distribution system,

media, underdrains, effluent channel, secondary settling tank, and recirculation pumps and

piping. Each of these components has one or more purposes.

In operation, wastewater is distributed evenly over the surface of the trickling filter

media. As the wastewater flows over the surface of the media the organisms in the slime remove

the organic matter from the flow.

The organisms aerobically decompose the solids producing more organisms and stable

wastes, which either become part of the zoogleal slime or are discharged back into the

wastewater flowing over the media. The wastewater continues through the filter to the

underdrain system where it is collected and carried out of the filter. At the same time air flows

through the filter (bottom to the top or top to bottom depending on temperature). Oxygen is

transferred from the air to the wastewater and slime to maintain the aerobic conditions.

Periodically the slime on the media becomes too heavy and portions will be released. This

material known as sloughings is carried out of the filter with the wastewater flow and is removed

in the settling tank following the filter.


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Trickling filters are very efficient at removing B.O.D. and ammonia from wastewater,

and they use a minimal amount of power. The cost to remove B.O.D. is only a few dollars per

ton.

The common hardware and components of trickling filter are shown in the schematic

diagram shown below:

Fig. No.:01 Schematic Diagram of Trickling Filter


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FILTRATION:

In the secondary treatment processes of sewage water, the effluent from sedimentation

tank (primary settling tank) should be stabilized aerobically or anaerobically to get clearer

effluents. The effluent from primary settling tank which contains unstable organic matter should

be converted into stable forms like nitrates & sulphates by oxidation for which filters are used.

Influent is sprinkled over the open beds of coarse aggregate. Effluent from filters again settled in

secondary settling tank & sludge in the secondary clarifier tank is digested in sludge digestion

process, if required.

FILTER CLASSIFICATIONS: Trickling filters are classified on the basis of their hydraulic and organic loading rate

respectively.

The “Hydraulic loading rate” is the total flow including recirculation applied on unit area of a filter per day. While, the “Organic loading rate” is the 5-day 20°C BOD, excluding the BOD of the recirculant, applied per unit volume in a day.

They may be classified as low or standard, intermediate, high, or super high rate

1. Standard Rate Filter

Standard rate trickling filters normally are designed for hydraulic ratings of 1.1 to 4.3 cu

m/sq m • d and organic loadings of 0.08 to 0.41 kg/cu m • d. These filters are normally 6 to 8 ft

(1.8 to 2.4 m) deep and rectangular or circular in shape. They usually are dosed intermittently by

dosing tanks with automatic siphons or by periodic pumping. The interval between dosings will

vary with the rate of wastewater flow, but should be short enough to prevent filter growths from

becoming dry. Some recirculation may become necessary to achieve this. During normal

operation, a thick growth develops in the filter until a temperature change or the flow through the

filter causes a large portion to slough off. This sloughing usually occurs in the spring or fall and

is known as “unloading.” The sloughed material is a stable, easily settled, humus-like material,

frequently containing many worms and filter fly larvae.

2. Intermediate Rate Filter

Intermediate rate filters normally are designed to treat hydraulic loadings of 4.3 to 10.8

cu m/sq m • d and organic loadings of 0.25 to 0.49 kg/cu m • d including recirculation. In the

past, there have been some cases in which the organic loading in the intermediate range


5

stimulated considerable biological filter growth and the hydraulic loading was not sufficient to

eliminate clogging of the trickling filter medium. Other plants operating in this range have had

few operational problems. In some cases, intermediate rate filters actually are underloaded high

rate filters.

3. High Rate Filter

High rate filters are normally designed for substantially higher loadings than are standard

rate units. A filter receiving a BOD loading between 0.41 to 4.88 kg/cu m • d. These filters

usually are 3 to 8 ft (0.9 to 2.4 m) deep and circular in shape. They are designed to receive

wastewater continuously. The high rate of application is achieved by recirculating wastewater

that already has passed through the filter, and the heavy flow of wastewater over the filter

medium produces continuous rather than periodic sloughing of the filter growths. Because the

solids are not retained in the high rate filter as long as they are in the standard rate unit, they are

less stable and continue to exert BOD after they leave the filter. The solids also are much lighter

and more difficult to settle than those sloughed from a standard rate filter.

4. Roughing Filter

Roughing filters are basically high rate filters treating an organic load of more than 1.6

kg/ cu m • d. In many cases, these filters are used to pre-treat the waste before its feeding to an

activated sludge plant. Most roughing filters designed today use synthetic media.

5. Super High Rate Filter

The major differences between super and high rate filters are greater hydraulic loadings

and a much greater filter depth. Some super high rate filters are designed to handle hydraulic

loads of more than 162.3 cu m/sq m • d. Most of these filters are in the form of packed towers

with depths

to 40 ft (12m). It is the use of synthetic media that permits the high loading rates and greater

filter depth. The microbial layer that grows on the trickling filter media is the most important

part of a trickling filter. The organisms in the microbial layer feed on the pollutants in the sewage

and convert them to solids that will settle out of the sewage. As the microbial layer grows, a

portion detaches or sloughs from the trickling filter media.


6

TRICKLING FILTERS: PROCESS DESCRIPTION

The trickling or percolating filter was introduced in 1890 and is one of the earliest

systems for biological waste treatment. It has four major components (Fig. 1):

1. A circular or rectangular tank containing the filter medium with a bed depth of

approximately 1.0–2.5 m. The filter medium provides a large surface area for microbial growth.

The ideal filter medium should provide a large surface area to maximize microbial attachment

and growth. It should also provide sufficient void space for air diffusion as well as allowing

sloughed microbial biofilm to pass through. It should not be toxic to microorganisms and should

be chemically and mechanically stable.

The filter media used in trickling filters are stones (crushed limestone and granite),

ceramic material, treated wood, hard coal, or plastic media. Selection of filter media is based on

factors such as specific surface area, void space, unit weight, media configuration and size, and

cost. The smaller the size, the higher the surface area for microbial attachment and growth, but

the lower the percentage of void space. Plastic media, introduced in the 1970s, are made of PVC

or polypropylene and are mainly used in high-rate trickling filters. They have a low bulk density

and offer optimum surface area (85–140 m2/m3) and much higher void space (up to 95 percent)

than that of other filter media. Thus, filter clogging is considerably minimized when these

media are used. Plastic is also a light material that requires less reinforced concrete tanks than do

stone media. Therefore, biological tower reactors containing these materials can be as high as 6–

10 m.

2. A wastewater distributor, which allows a uniform hydraulic load over the filter

material. It has one to four arms and its configuration and speed depends on the filter media

used. Hydraulic load varies from less than 5 m3/m2/day for low-rate filters to more than 25

m3/m2/day for high-rate filters. Wastewater is percolated or trickled over the filter and provides

nutrients for the growth of microorganisms on the filter surface.

3. An underdrain system for collection of liquid and introduction of air. The underdrain

collects treated wastewater as well as biological solids (i.e., microbial biomass) that have been

sloughed off the biofilm material.

4. A final clarifier: This is also called the humus tank, for separation of solids from the

treated wastewater


7

.

NITRIFICATION:

Nitrogen exists in many forms in the environment and can enter aquatic systems from

either natural or human- generated sources. Some of the primary direct sources or transport

mechanisms of nitrogen from sewage are listed below:

• Untreated sewage—direct discharge

• Publically owned treatment works

(POTW) effluent—direct discharge, land application

• POTW waste solids—direct discharge, land application

• Septic tanks and leach fields—groundwater movement

Nitrification/denitrification is the process of converting nitrogen into a form in which it

can ultimately be removed. In the first step of this process, influent ammonia nitrogen is oxidized

to nitrate nitrogen. At this point, the nitrogen has merely changed forms and has not yet been

removed from the wastewater. In the second step, denitrification, nitrate nitrogen is converted


8

into nitrogen gas that vents to the atmosphere naturally. The three general types of TF

nitrification configurations are single pass mode and alternating double filtration (ADF) mode

and recirculation mode.

Single pass mode: The single pass mode has an organic loading rate of 0.06–0.12 kg

BOD m23 day21.

Alternating double filtration (ADF) mode: The ADF mode involves the alternate use

(1–2 weeks interval) of two sets of filters and humus tanks and allows higher organic

loading rates with no problems of filter clogging.

Recirculation mode: Trickling filter effluents are partially recirculated through the filter

to increase the treatment efficiency of the filter media. A portion of the treated effluent is

returned to the filter. The recirculation ratio, R, is the ratio of the flow rate of recirculated

effluent to the flow rate of the wastewater influent:

R= QR/Q

Where, QR = the flow rate of recirculated trickling filter effluent; and Q= the flow rate of

wastewater influent.

Changes in the quality and quantity of wastewater can be handled by adjusting the rate of

recirculation. Recirculation improves contact between wastewater and the filter material, helps

dilute high-BOD or toxic wastewater, increases dissolved oxygen for biodegradation of organics

and for tackling odor problems, improves distribution of the influent on the filter surface,

prevents the filter from drying out during the night when the wastewater flow is low, and avoids

ponding (i.e., puddles at the surface of the filter).

Although numerous types and combinations of treatment units exist, in general, a single-

stage TF has to remove organic material in the upper portion of the unit and provide nitrifying

bacteria for nitrification in the lower part.

Since the influent is necessarily of high organic strength (i.e., receiving primary treatment

only), it has to be applied at a rate low enough to achieve both organic removal and nitrification

sufficient for required effluent quality. A two-stage system allows for greater process flexibility

since each stage can be operated independently and the flow regime can be varied to achieve the

best results.


9

BIOLOGY OF TRICKLING FILTERS: A trickling filter essentially converts soluble organic matter to biomass, which is further

removed via settling in the final clarifier. Organic loading is the rate at which BOD is applied to

the trickling filter, and is expressed in kg BOD applied per m3 of filter per day. A typical organic

loading is 0.5 kg/m3/day and may vary from 0.1–0.4 for lowrate filters to 0.5–1 kg BOD/m3/day

for high-rate filters. This parameter is important to the performance of the trickling filter and

may dictate the hydraulic loading onto the filter. Removal of BOD by trickling filters is

approximately 85 percent for low-rate filters and 65–75 percent for high-rate filters.

BIOFILM FORMATION:

The biofilm forming on the surface of the filter media in trickling filters is called the

zoogleal film. It is composed of bacteria, fungi, algae, protozoa, and other life forms (Fig. 10.4).

Scanning electron micrographs of biofilms forming on surfaces are displayed in Figure 10.5. The

processes involved in biofilm formation in wastewater are similar to those occurring in natural

aquatic environments. After conditioning of the substratum with organic materials, the surface is

colonized by bacteria, followed by other life forms. Bacterial adsorption to the substratum


10

requires the formation of a polymer-containing matrix, named glycocalyx. These extracellular

polymers help anchor the biofilm microorganisms to the surface of the filter material (Bitton and

Marshall, 1980). The sequence of events that lead to biofilm formation is described in more

detail in Chapter 16. The glycocalyx also provides a surface that is rich in polyanionic

compounds that complex metal ions. Biofilm microorganisms degrade the organic matter present

in wastewater. The increase in biofilm thickness leads, however, to limited oxygen diffusion to

the deeper layers of the biofilm, creating an anaerobic environment near the filter media surface.

Microorganisms in the deeper layers face a reduced supply of organic substrates and enter into

the endogenous phase of growth. They are subsequently sloughed off the surface. Sloughing is

followed by the formation of a new biofilm

ORGANISMS PRESENT IN TRICKLING FILTER BIOFILMS: Trickling filters are notable for the diversity of life forms that participate in wastewater

treatment, making this process relatively more complex than the activated sludge process. In

addition to prokaryotic and eukaryotic microorganisms, trickling filters contain higher life forms

such as rotifers, nematodes, annelid worms, snails, and many insect larvae. The reader should

consult a color atlas of the organisms most frequently observed in wastewater. The major groups

of organisms encountered in trickling filters are the following:

Bacteria: These are active in the uptake and degradation of soluble organic matter.

Colloidal organic matter is also trapped in the filter and degraded by extracellular enzymes.

Facultative bacteria are the predominating organisms in trickling filters, and decompose the

organic material in the wastewater along with aerobic and anaerobic bacteria. Zooglea,

Pseudomonas, Flavobacterium, Achromobacter, Alcaligenes, are among the bacterial species

commonly associated with the trickling filter. Within the slime layer, where adverse conditions

prevail with respect to growth, the filamentous forms Sphaerotilus, natans and Beggiatoa will be

found. In the lower reaches of the filter, the nitrifying bacteria (Nitrosomonas and Nitrobacter)

will be present. Nitrifying bacteria convert ammonia into nitrate via nitrite and are outcompeted

by heterotrophs for oxygen and ammonia in the presence of organic materials.

Fungi: These organisms are also active in the biofilm in connection with waste

stabilization. They predominate, however, only under low pH conditions, a situation that can be

created by the introduction of some acidifying industrial wastes. At times, fungi growth can be


11

so rapid that the filter clogs and ventilation becomes restricted. Some examples of fungi found

in trickling filters are Fusarium, Penicillium, Aspergillus, Mucor, Geotrichum, and yeasts. The

hyphae growth is helpful for the transfer of oxygen to the lower depths of the biofilm.

Algae: Algae can grow only in the upper reaches of the filter where sunlight is available.

Many types of algae also grow on the biofilm surface (e.g., Ulothrix, Phormidium, Anacystis,

Euglena, Chlorella). Generally, algae do not take a direct part in waste degradation. They

produce oxygen during daytime following photosynthesis, and some species of blue-green algae

are also able to fix nitrogen. In contrast to activated sludge, algae and fungi are important

components of biofilms in trickling filters. From an operational standpoint, the algae may be

troublesome because they can cause clogging of the filter surface, which may cause odours.

Protozoa: These unicellular eukaryotic organisms feed on biofilm bacteria. Continuous

removal of bacteria by protozoa helps maintain a high decomposition rate. The protozoa

occurring in biofilms are flagellates (e.g., Bodo, Monas), ciliates (e.g., Colpidium, Vorticella,

Opercularia and Epistylis) and amoeba (e.g., Amoeba, Arcella). Their function is to feed on the

biological films and as a result, effluent turbidity decreases and the biofilm is maintained in a

higher growth state. The higher animals such as worms, snails, and insects, feed on the biological

film. Snails are specially troublesome in trickling filters used mainly for nitrification, where they

have been known to consume enough of the nitrifying bacteria to significantly reduce treatment

efficiency. Rotifers (e.g., Rotaria) are also encountered in biofilms.


12

REMOVAL OF PATHOGENS AND PARASITES BY TRICKLING FILTERS:

The removal of pathogens and parasites by trickling filters is generally low and erratic.

Bacterial removal is inconsistent and varies from 20 to .90 percent, depending on the operation

of the trickling filter. Removal of Salmonella by trickling filters is lower than the activated

sludge process and may vary from 75 to 95 percent. The removal of viruses by trickling filters is

also generally low and erratic. Filtration rate affects the removal of viruses, and probably other

pathogenic microorganisms. Laboratory experiments showed that at medium filtration rate (10

MGD/acre), the removal of viruses, coliforms, fecal streptococci, and BOD and COD, was

greater than at a higher rate (23 MGD/acre). The mechanism of removal of viruses by trickling

filters is poorly understood. Some investigators have suggested that viruses are removed by

adsorption to the biofilm material. Cysts and oocysts removal by trickling filters is also generally

low and erratic, with removals varying from 10 to 99 percent. The removal efficiency of trickling

filters is generally lower than that of activated sludge as cysts are frequently detected in

effluents. However, no significant difference was found between activated sludge and trickling

filters as regards the removal of Giardia cysts and Cryptosporidium oocysts.


13

ADVANTAGES AND DISADVANTAGES OF TRICKLING FILTERS:

Some advantages and problems related to trickling filters are the following:

Advantages:

Trickling filters are attractive to small communities because of easy operation, low

maintenance costs, and reliability. They are used to treat toxic industrial effluents and are able to

withstand shock loads of toxic inputs. The sloughed biofilms can also be easily removed by

sedimentation.

Disadvantages:

High organic loading may lead to filter clogging as a result of excessive growth of slime

bacteria in biofilms. Excessive biofilm growth can also cause odor problems in trickling filters.

Clogging restricts air circulation, resulting in low availability of oxygen to biofilm

microorganisms. However, modifications have helped improve the BOD removal of trickling

filters. The following are some of these improvements:

1. Alternating double filtration (ADF), which consists of alternating two filters for receiving the

waste.

2. Slowing down wastewater distribution

3. Use of plastic materials in the filter for increased surface area and improvement of air

circulation

4. Management of odours problems by increasing air flow by means of forced ventilation


14

OPERATIONAL PROBLEMS: Even though the trickling filter process is considered to be one of the most trouble-free

types of secondary treatment, the potential for operating problems still exists. Troubleshooting a

trickling filter treatment facility involves basically three steps:

1. Locate the problem.

2. Identify the cause of the problem

3. Apply the proper corrective measures.

Care is a necessary ingredient when attempting to correct operational problems because what

appears as a simple problem actually may be a symptom of a more complex problem.

The trickling filter is not normally the sole treatment unit, but rather is a component of total

sewerage systems. Each component of the system can have an impact on the operation of the

other components. For this reason, all components should be considered when an operational

problem exists. When troubleshooting, the operator should examine the following:

1. The treatment plant’s operation and maintenance manual;

2. The design and operating flexibility of the treatment facility;

3. The daily operating records and log book;

4. The current operating procedures; and

5. The possibility of upset caused by acids, alkali, oil, or other toxic materials from industrial

sources.

Common operating problems and their possible corrective

measures are discussed in the following paragraphs.

A. FILTER PONDING

Causes:

Filter ponding may be caused by:

1. Rock or other media are too small or not sufficiently uniform in size.

2. Rock medium breaks up, because of extremes in temperatures, producing fines that clog the

voids.

3. Primary treatment units are operated improperly, resulting in excessive SS in the influent to

the filter.

4. Excessive sloughing takes place, clogging the voids between the medium.


15

5. Organic loading on the filter is excessive for the liquid loading.

Prevention and Cure:

1. Rake or fork the rocks on the filter surface. Heavy equipment should not be allowed on the

filter.

2. Wash the filter surface with a stream of water under high pressure.

3. Stop the distributor over the ponded area and allow a continuous flow of wastewater to wash

the growths out of the filter.

4. Dose the filter with heavy applications of chlorine (5 mg/l free chlorine in filter influent) for

periods of several hours at weekly intervals. This should be done during periods of low flow

(possibly at night) to minimize the chlorine requirement. Chlorine is particularly effective for

control of fungus growths, with doses as low as 1 mg/l.

5. Take the filter out of service for a period of one day or longer to allow it to dry out. This

should be done only if there is adequate provision for treating the waste in other units.

6. Flood the filter and allow it to stand for at least 24 h. This is applicable only to filters with

water-tight walls and provisions for sealing underdrains.

7. Replace the filter medium if all other methods fail. It may cost less to replace an old medium

with a new than to clean the old one.

B. FILTER FLIES:

The filter fly (psychoda) is a nuisance frequently associated with the filter operation. The fly

(moth) passes through ordinary window screens and gets into the eyes, ears, mouth, and nostrils

of anyone coming within its flight range. Its natural flight range is only a few hundred feet, but it

can be carried much farther by the wind. Its life cycle varies from 22 days at 60°F (15°) to 7 days

at 85°F (29° C). Filter flies develop most frequently in an alternately wet and dry environment.


1. Dose filter continuously, not intermittently.

2. Remove excessive biological growths by methods similar to those used to reduce ponding.

3. Flood the filter for 24 h at weekly or biweekly intervals. To be continually effective, the filter

must be flooded at intervals frequent enough to prevent the fly from completing its life cycle

between floodings.


16

4. Wash vigorously the inside of the exposed filter walls. This sometimes can be accomplished

by adjusting or modifying the ends of the distributor arms to provide wastewater for splashing on

the wall. If the walls are kept wet, flies cannot survive.

5. Maintain the whole plant and the filter itself so as not to provide sanctuaries for flies.

6. Chlorinate the wastewater to produce a chlorine residual of 0.5 to 1 mg/l at intervals frequent

enough to prevent the fly from completing its life cycle between chlorine additions (1-2 week

intervals).

7. Apply insecticide. An insecticide applied to the filter surface and walls will kill the adult flies

that come in contact with it. If applied at intervals of 4 to 6 weeks, the flies usually are

controlled. The application of insecticides does not have any noticeably adverse effect on normal

filter operation. Frequently, a strain of flies will develop immunity to the materials used; if this

happens, other environmentally acceptable insecticides may be tried. The effects of an

insecticide on the receiving waters should be considered before application.

C. ODORS

The trickling filter is basically an aerobic process; therefore, no serious odour problem

should exist. The presence of “rotten egg” odour is an indication of anaerobic condition.


1. Maintain aerobic conditions in all units, including settling tanks and wastewater system.

2. Reduce accumulations of sludge and biological growths.

3. Chlorinate filter influent for short periods, preferably when flow to plant is low.

4. Recirculate to filters.

5. Practice good housekeeping throughout the plant.

6. Clean underdrain systems of all stoppages.

7. Clean all filter vents.

8. Increase ventilation by forcing air into filter drain system.

9. Reduce usually heavy organic loadings (such as milk processing or canning wastes) that

produce heavy accumulations of solids in filter or cause ponding.

10. Cover filter and deodorize the off gases (usually beyond operator control and will require

major cost for improvements).


17

D. ICING FOR FILTER SURFACE:

Cold weather not only reduces the efficiency of trickling filters by decreasing the activity of the

microorganisms, but in severe cases actually can cause the wastewater to freeze on the medium

surface. Ice can build up enough to render the filter ineffective and thus require its removal from

service.


1. Decrease the number of times that waste is recirculated. It may be necessary to eliminate

recirculation until weather moderates.

2. When two-stage filters are used, operate filters in parallel, with little or no recirculation, until

weather moderates.

3. Adjust orifices and splash plates to improve uniformity of distribution on the filter. Unequal

distribution may result in application of wastewater in concentric bands. Spraying and splashing

at the margin between wet and dry zones will create ridges of ice.

4. Erect a wind screen at the filter in the path of prevailing winds.

5. Break up and remove ice frequently.

E. Clogging of Distribution-Arm Orifices

Causes: Non-uniform flow of wastewater over the filter medium may cause plugging, thus

reducing the area to which the wastewater is applied and, therefore, the filter’s efficiency. In

severe cases in which a large number of the distributor-arm orifices are clogged, excessive

pressure can be generated in the distributor and cause the seal to blow.


1. Clean all orifices and flush the distributor piping occasionally.

2. Improve grease and SS removal in the primary settling tank.

3. Maintain adequate hydraulic load on filter.

4. Lubricate the distributor according to the manufacturer’s instructions.

F. Snails, Moss, and Roaches

These problems normally are restricted to certain regions of the country, especially the South.

Any one or a combination of these three materials could become severe enough to cause ponding

or other operational problems. Snails in limited numbers normally do not hamper trickling filter

operation. Because of their prolific nature, a few snails can become many in a relatively short


18

period of time. The organisms themselves are not the problem, but the shell remaining after they

die may damage the distributor and pumps and may even create problems in digesters.


1. Chlorinate heavily (10mg/l or more) to produce a chlorine residual of 0.5 to 1 mg/l noted in

the filter effluent for several hours.

2. Flush the filter using maximum recirculation rate.


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REFERENCES

[1] Metcalf and Eddy, ‘Wastewater Engineering, Treatment and Reuse’, Fourth Edition, Page

No.: 891-896, Tata McGraw-Hill (http://books.google.co.in)

[2] Gabriel Bitton, ‘Wastewater Microbiology’, Third Edition, Page No.: 291-303, A John Wiley

& Sons Publications, Department of Environmental engineering Sciences, University of Florida,

Gainesville, Florida. (http://www2.hcmuaf.edu.vn)

[3] http://www.waterworldce.com/courses/31/PDF/trickling.pdf

[4] http://www.nesc.wvu.edu/pdf/WW/publications/eti/TF_gen.pdf

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Microbiology of trickling filter

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