chapter 21 revised_6th edition

7/22/2019 Chapter 21 Revised_6th Edition

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Chapter 21

Trickling Filters, RotatingBiological Contactors,

and Combined Processes

21-1Copyright 2007 Water Environment Federation.

Introduction 21-2

Concepts 21-4

Load Variations 21-4

pH and Alkalinity 21-5

Toxicity 21-5

Nutrients 21-5

Temperature 21-5Dissolved Oxygen 21-6

Microorganisms 21-7

Trickling Filters and Biotowers 21-7

Alternatives 21-8

Low-Rate Filters/Biotowers 21-10

Intermediate-Rate Filters 21-11

High-Rate Filters 21-12

Roughing Filters 21-11

Description of Process 21-12Description of Equipment 21-13

Distribution Systems 21-13

Filter Media 21-16

Underdrain System 21-16

Containment Structure 21-18

Filter Pump Station 21-18

Secondary Clarifier 21-18

Process Control 21-19Flow Patterns 21-19

Distribution Rates 21-19

Clarifier Operation 21-21

Troubleshooting 21-22

Planned Maintenance 21-28

Distributor Bearings 21-28

Safety 21-28

Rotating Biological Contactors 21-31

Alternatives 21-32Description of Process 21-35


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21-2 Operation of Municipal Wastewater Treatment Plants

Copyright 2007 Water Environment Federation.

Description of Equipment 21-38

Tankage 21-38

Baffles 21-38

Filter Media 21-39

Covers 21-40

Rotating Biological Contactor 21-40

Drives

Influent and Effluent Lines 21-43

and Valves

Instrumentation 21-43Process Control 21-43

Other Processes 21-44

Staging and Trains 21-45

Supplemental Aeration 21-45

Step Feeding or Enlarged 21-46

First Stage

Recirculation 21-46

Rotational Speed 21-47

Secondary Clarifier 21-47Nitrification 21-47



Mechanical Drive Systems 21-50

Air-Drive Systems 21-50

Combined Processes 21-52

Alternatives 21-52

Activated Biofilter 21-53

Trickling Filter Solids Contact 21-53

Roughing Filter Activated 21-57

Sludge

Biofilter Activated Sludge 21-58

Trickling Filter Activated 21-58

Sludge

Description of Processes 21-58

Description of Equipment 21-59Trickling Filter or Biotower 21-59

Filter Pump Station 21-59

Contact Channel or Aeration 21-59

Basin

Aeration Equipment 21-60

Clarification 21-61

Process Control 21-61

Process Changes 21-61

Biotower 21-61Contact Channel or Aeration 21-61

Basin

Clarification 21-62



References 21-64

INTRODUCTION

Tricking filters, biotowers, and rotating biological contactors (RBCs) are generally

known as fixed-film treatment processes. Of these three processes, the trickling filter

process predates biotowers, RBCs, and combined fixed-film and suspended growth

(FF/SG) processes.


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Trickling Filters, Rotating Biological Contactors, and Combined Processes 21-3

In fact, trickling filters predate most of the treatment methods considered in

other chapters of this manual, and they are still a viable process. New types of filter

media are now used; therefore, rock media systems are labeled trickling filters, and

plastic media systems are labeled biotowers. Trickling filters are being incorporated

into wastewater facilities using new methods or processes, and many rock filters are

being refurbished for continued use. This chapter helps both operators and engineers

grasp the operations and maintenance requirements of trickling filters as used in ex-

isting and new systems.

Although their performance has been good, RBC equipment operation has been

plagued by failures. This chapter discusses many of the changes that occurred in both

design and operation. It includes discussions of predicting operating problems or plant

overload and emphasizes methods of upgrading or improving RBC operations.

Combined processes [i.e., trickling filters, biotowers, or RBCs (fixed-film) coupled

with suspended-growth (activated sludge) processes] now number several hundred in

the United States. Combined FF/SG processes are designed to take advantage of the

strengths and minimize the weaknesses of each process. In many cases, the practice is

used to reduce construction costs by avoiding the need for additional tankage. In some

industrial or high-strength-waste applications, the FF/SG processes have helped elim-

inate shock loads to the activated sludge process. The coupling of biological processes

has solved many problems, but also produced new control criteria or concerns. This

chapter addresses operations and maintenance concerns associated with the coupling

or combining of biological processes.

Fixed-film biological processes remove dissolved organics and finely divided or-ganic solids from wastewater. Removal occurs primarily by converting soluble and

colloidal material into a biological film that develops on the filter media. Raw do-

mestic and industrial wastewater typically contains settleable solids, floatable mate-

rials, and other debris. Failure to remove these solids before the wastewater enters

the fixed-film reactors can interfere with their oxygen-transfer capabilities, plug the

filter media, result in high solids yield, or create other problems. Therefore, both

fixed-film and combined-growth processes are typically preceded by screenings and

the grit removal process. Primary treatment processes should be used to reduce the

fixed-film process load.

The fixed-film or biological media removes soluble biochemical oxygen demand

(BOD) and produces biological solids. Most often, the process removes carbonaceousBOD; however, sometimes the fixed-film systems can be loaded so slow-growing,

nitrogen-converting autotrophic bacteria (nitrifiers) can compete with the more rapid-

growing heterotrophic bacteria used for carbonaceous BOD removal; therefore, the

fixed-film process can be used to nitrify the wastewater.



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In combined FF/SG processes, the second-stage process may remove varying

amounts of soluble BOD (SBOD), depending largely on the loading and performance

of the first-stage process. Combined processes are given different names, depending on

which stage removes the most SBOD and at which point biological solids are intro-

duced to the treatment scheme.

To ensure that treatment is successful, the following principles must be considered

in both the design and operation of fixed-film and combined processes.

CONCEPTS. Removal of organic materials through the use of fixed and combinedreactors is accomplished by means of a biological film on the fixed-film media. This

filma viscous, jellylike slimetypically is composed of a large and diverse popula-

tion of living organisms (e.g., bacteria, protozoa, algae, fungi, worms, and even insectlarvae). Most of the mass of this population is aerobic organisms. An aerobic organism

requires oxygen to function properly.

As the slime layer thickens, the adsorbed organic matter is metabolized before it

can reach the microorganisms near the media face. As a result, the microorganisms

near the media face enter into an endogenous phase of growth and lose their ability to

cling to the media surface (Metcalf and Eddy, 1979). This allows the flowing waste-

water to scour the slime from the filter mediaa process is known as sloughingand a

new slime growth begins. The sloughing process continues at various stages through-

out the filter. However, sloughing can be encouraged either by increased hydraulic

loading, supplemental aeration, temperature changes, other operator-induced changes,

or environmental conditions.The removal of soluble organic material is a relatively rapid process. Good re-

moval of soluble organics can typically be achieved at low to moderate loading of the

fixed-film reactors. However, the stabilization or breakdown of biological solids gener-

ated in removing the soluble organics is a longer process. The time required for com-

pletion of this process will vary, depending on the type of filter media being used, rate

of organic loading to the fixed-film process, hydraulic shear, temperature, and other

factors (Water Pollution Control Federation, 1988).

LOAD VARIATIONS. Fixed-film reactors (trickling filters or RBCs) vary in their

ability to absorb either seasonal or shock industrial wastewater loads. For either pro-cess, extremes may cause a bleed-through of BOD or even severe sloughing or biologi-

cal kill. Recycled filter effluent is often used to dilute incoming raw waste and add

oxygen to the trickling filters, biotowers, or first stages of the RBCs. Using recycle tech-

niques, treatment of wastewater with 5-day BOD (BOD5) concentrations greater than




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10 000 mg/L is possible. This occurs especially in the treatment of food processing

waste, for which trickling filters are often used.

pH AND ALKALINITY. Bacteria associated with fixed-film reactors typicallythrive in a pH range between 5.5 and 9.0; the best pH range is between 6.8 and 7.2. Of-

ten, treatment problems result from rapid changes in pH values rather than from ex-

treme long-term average values.

Although low pH may result from the discharge of industrial wastes or poorly

buffered natural waters, a drop in pH may also occur in the treated wastewater if the

fixed-film or combined reactors are lightly loaded or when temperatures are moderate.

This indicates that nitrification is occurring. Nitrification oxidizes ammonia, resulting

in the loss of alkalinity and a corresponding pH drop.

TOXICITY. Both fixed-film and combined-growth processes are typically less sus-ceptible to toxicity or shock loads than suspended-growth (activated sludge) systems.

However, complex organic substances, heavy metals, pesticides, inorganic solids, and

even surges of disinfectants (e.g., chlorine) could either greatly reduce the performance

of, or cause a biological kill within, both fixed-film and combined processes. The toxic-

ity causes either poor treatment performance or massive sloughing. Source control

through industrial waste management is necessary if industrial discharges are causing

the toxicity problem.

NUTRIENTS. Wastewater that is primarily from domestic sources typically has morethan sufficient nutrients to ensure that bacterial growth is not inhibited because of thelack of essential nutrients. However, some industrial wastes (especially food process-

ing) lack sufficient nutrients to promote normal bacterial growth. Mixtures of domestic

and industrial waste can also be nutrient deficient when the industrial portion domi-

nates the municipal plants organic load.

The most commonly deficient nutrients are nitrogen (N) and phosphorus (P). To be

available for bacterial growth, nitrogen must be in the form of soluble ammonia, and phos-

phorus must occur in the orthophosphate form. Empirical ratios based on the amounts of

nutrients needed for producing biological cells suggest that for each 45 kg (100 lb) of BOD5,

2.3 kg (5 lb) of nitrogen and 0.5 kg (1 lb) of phosphorus must be available for proper cell

growth. In equation form, this ratio is 90 BOD5

4.6 N

1 P (100 BOD5

5 N

1 P).

TEMPERATURE. Biological activity (hence BOD removal) in all treatment systemsdeclines as temperatures decrease. However, fixed-film bacteria appear to be more

sensitive to the temperature drop than the bacteria in suspended-growth (activated




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sludge) systems. For example, a drop in temperature from 26 to 20 C (78 to 68 F) in a

suspended-growth system will typically halve the coefficient of removal. The same

temperature drop in a fixed-film reactor could decrease the removal rate coefficient by

two-thirds or more. As a result of reduced efficiency during winter months, several lit-

erature sources indicate that filters should be sized 25% larger in the northern United

States than in southern areas. Attempting to conserve heat in the wastewater as it pro-

ceeds through treatment may sometimes reduce the adverse effects of cold tempera-

tures. Following are some ideas on how this can be accomplished:

Removing a unit from service,

Covering the fixed-film process,

Reducing recirculation,

Using forced rather than natural ventilation,

Reducing the settling tanks hydraulic retention time,

Operating in parallel rather than in series,

Adjusting orifice and splash plates to reduce spray,

Constructing windbreaks to reduce wind effects,

Intermittently dosing the filter,

Opening dump gates or removing splash plates from distributor arms, and

Covering open sumps and transfer structures.

DISSOLVED OXYGEN. Dissolved oxygen is needed to sustain the aerobic micro-

organisms in the fixed-film process. As water flows over the fixed-film media, oxygentransfers to the water. When water is recycled in the fixed-film process, the presence of

a high concentration of dissolved oxygen in the fixed-film underflow or treated efflu-

ent does not necessarily mean that this same concentration is available in the RBCs

first stage, the trickling filters interior, or the biotowers various levels. Oxygen defi-

ciencies with trickling filter and biotower media have been less troublesome than those

with RBC media.

Biotowers with high-rate (plastic or redwood) media are frequently designed for

BOD5 loadings between 320 and 480 kg BOD5/100 m3d (200 and 300 lb BOD5/1000 cu

ft/day) before concerns are raised about low dissolved oxygen in the filter underflow.

However, some biotowers are designed for loadings less than 240 kg BOD5/100 m3d

(150 lb BOD5/1000 cu ft/day) to prevent odor problems. Loadings with rock filter me-dia are often maintained at less than 90 kg BOD5/100 m

3d (50 lb BOD5/1000 cu ft/day)

to ensure adequate dissolved oxygen and low odor potential. A survey of highly loaded

trickling filters or biotowers indicates that odors result less frequently from high or-

ganic loading or low dissolved oxygen in the filter underflow than from constituents in




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the wastewater (e.g., high industrial loading). Some odors always emanate from trick-

ling filters and biotowers; the proximity and sensitivity of nearby residents will often

determine whether the odor is a nuisance.

Dissolved oxygen can be a limiting factor in RBC performance, if improperly de-

signed. Organic loadings to RBCs should be limited to 2.9 to 3.9 kg BOD5/100 m2d (6.0

to 8.0 lb BOD5/1000 sq ft/day) or 1.2 to 2.0 kg SBOD5/100 m2d (2.5 to 4.0 lb SBOD5/

1000 sq ft/day) for the first-stage units in service. First-stage units are typically loaded

at three to four times the recommended BOD5 load of the total units in service.

To reduce the oxygen limitations of RBC facilities, a number of plants have been

upgraded by preceding or combining preaeration, trickling filter, activated sludge, or

solids contact with RBC reactors. Alternatively, supplemental aeration has been added

to the RBC units in some facilities. These approaches to RBC upgrading are discussed

in the Combined Processes section of this chapter.

MICROORGANISMS. The treatment of wastewater by fixed-film processes pro-duces a biological (zoogleal) slime that coats the surface of the media. When fixed-film

reactors are used for BOD removal, the microbial population consists of various species

of heterotrophic bacteria with smaller populations of protozoa and fungi. If these reac-

tors are used for nitrification, autotrophic nitrifying microorganisms predominate,

with smaller numbers of heterotrophic bacteria.

Under conditions of low dissolved oxygen, nutrient deficiencies, or low pH val-

ues, organisms that either remove BOD slowly or exhibit poor settling characteristics

can dominate the fixed-film reactor. These organisms are predominately filamentousbacteria and sometimes fungi and are a nuisance.

Eliminating filamentous bacteria typically involves identifying the source of the

nuisance and eliminating the condition that allows them to dominate the system. In

addition, the recycle of suspended-growth bacteria over the fixed-film reactor has often

reduced the presence of filamentous bacteria. This mode resembles the selector acti-

vated sludge process described in Chapter 20. This approach for fixed-film reactors is

discussed in more detail in the Combined Processes section of this chapter.

TRICKLING FILTERS AND BIOTOWERS

Trickling filters attempt to duplicate the natural purification process that occurswhen polluted wastewater enters a receiving stream and trickles over a rock bed or

rocky river bottom. In the natural purification process, bacteria in the rock bed re-

move the soluble organic pollutants and purify the water. For more than 100 years

(since the late 1880s), trickling filters have been considered a principal method of




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wastewater purification. The principle of using a rock bed for purification was ap-

plied in filter design, with the rock beds typically ranging from 0.9 to 2.4 m (3 to 8 ft)

deep. After declining use in the late 1960s and early 1970s, trickling filters regained

popularity in the late 1970s and early 1980s, primarily because of new media types. The

new high-rate media were typically preferred over rock media because they offer more

surface area for biological growth and improved treatment efficiency. The advent of

high-rate media minimized many of the rock media problems (e.g., plugging, uncon-

trolled sloughing, odors, and filter flies). Consequently, almost all trickling filters con-

structed in the late 1980s use high-rate media (Water Pollution Control Federation,

1988); they are called biotowers.

This section focuses on the operation and maintenance of both types of trickling fil-

ters. Their operations and maintenance needs may vary greatly, depending on the type

of filter media used and how the filter was designed (Albertson and Eckenfelder, 1984).

ALTERNATIVES. There are four basic categories of filter design, based on theorganic loading of the trickling filter/biotower. In the first three categorieslow-,

intermediate-, and high-rate filtersthe filter removes all or essentially all of the BOD

applied (Table 21.1). In the fourth category (the roughing filter), the filter is typically

combined with another biological treatment step (typically activated sludge, RBC, or

another filter), where a substantial amount of BOD removal occurs.

The categories of trickling filters/biotowers are typically based on BOD5 loading

to the filter divided by the volume of filter media, calculated as follows:

(21.1)

Where

BOD5 applied kg primary effluent BOD5/d;

(primary effluent BOD5, mg/L)(flow, ML/d) and

Volume of media

Or (in U.S. customary units)

BOD5 applied

lb primary effluent BOD5/d; (primary effluent BOD5, mg/L)(flow, mgd)

(8.34 lb/gal); and

Volume of media horizontal (plan) area, sq ft media depth, fft

1000

horizontal (plan) area, m media depth, m

100

2

Organic (BOD ) load BOD applied, lb d

Volum55

/

ee of media, 1000 cu ft




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Although filters/biotowers are typically not classified by hydraulic loading, the hy-

draulic or wetting rate is a useful loading parameter and is calculated as follows:

(21.2)

Where

Total flow incoming filter recycle flow;

sum of filter pump capacity, gal/min; and

Horizontal plan area , for circular unit

length width, for rectangular unit.

3 14. (diameter )

4

2

Hydraulic loadwetting rate

Total flow (including recycle), pumped, gal min

Horizontal (plan) area, sq ft

/



TABLE 21.1 Trickling filter categories.

Trickling filter categories

Operating characteristics Low Intermediate High Roughing

Organic loading, 25 2540 40100 100300lb BOD/d/1000 cu ft/day

Filter mediaa Rock or Rock or Rock or High ratehigh rate high rate high rate

Nitrificationb Yes Partial Unlikely No

Combined process requiredc

For secondary treatment No Unlikely Likely Yes

For tertiary treatment Yes Yes Yes Yes

Type typically usedd TF/SC TF/SC TF/SC TF/AS

ABF ABF TF/RBC BF/AS

2-stage RF/ASfilters 2-stage

filters

aHigh rate plastic or redwood.bAt 26 C (78 F) and without a second-stage or combined process.cIndicates if combined or dual process is typically used.dTF/SC trickling filtersolids contact; TF/AS trickling filteractivated sludge; ABF activatedbiofilter; TF/RBC trickling filterrotating biological contactor; BF/AS biofilteractivated sludge; andRF/AS roughing filteractivated sludge.


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Example 21.1. Calculate the organic and hydraulic loadings of a filter.

Given:

Q (Flow) 5 mgd,

Primary effluent BOD5 120 mg/L,

Filter diameter 110 ft,

Media depth 8 ft,

Number of filters 1, and

Total filter pumping capacity 7000 gal/min.

Solution:

Horizontal plan area

9499 sq ft

Filter volume

76(1000 cu ft)

BOD5 applied (120 mg/L)(5 mgd)(8.34)

5004 lb BOD5/d

Organic load filter

65.8 lb BOD5/d/1000 cu ft

Hydraulic load

0.74 gpm/sq ft

Organic loading, typically expressed as total BOD5 in the primary effluent, is often re-

ferred to as total organic loading (TOL). Another common way to evaluate BOD loading

depends on the amount of soluble filter/biotower BOD. This loading is referred to as

the soluble organic loading (SOL).

Low-Rate Filters/Biotowers. Low-rate filters/biotowers typically include rock trickling

filter media. At loadings of less than 40 kg BOD5/100 m3d (25 lb BOD5/d/1000 cu ft),fewer problems from filter flies, odors, or media plugging (ponding) are expected than

with filters operating at higher loading rates.

Low-rate trickling filters with rock media range in depth from 0.9 to 2.4 m (3 to

8 ft). Most low-rate filters are circular with rotary distributors. However, a number of

70000

9499

gal min

sq ft)

/

5400

6 1000

lb BOD d

7 cu ft)5/

(

(9499 sq ft)(8 ft)

1000

3 14. (110)

4

2




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rectangular rock filters remain in operation. With rock media, low-rate filters are typi-

cally not hydraulically limited; application rates ranging from 0.01 to 0.04 L/m2s (0.02

to 0.06 gpm/sq ft) are common. Both circular and rectangular filters are sometimes

equipped with dosing siphons or periodic pumping to provide a high wetting rate for

short intervals between rest periods.

With high-rate plastic filter media, a minimum wetting rate (accomplished via

recirculation or dosing) is typically maintained to prevent the media from drying out

and to ensure good contact. A rate of 0.4 L/m2s (0.7 gpm/sq ft) is typically considered

good practice. Potential loss in BOD removal performance should be evaluated at

lower rates.

Sloughed solids from a low-rate filter are typically well-digested, so these filters

yield less solids than higher rate filters. Solids yields of 0.5 kg total suspended solids

(TSS)/kg (0.5 lb TSS/lb) secondary influent BOD5 are not uncommon, especially with

rock media.

To provide tertiary treatment or polished effluent, combined processes [e.g., trick-

ling filter/solids contact (TF/SC) or activated biofilter (ABF)] may be used. Combined

processes are typically not required to achieve effluent of conventional secondary treat-

ment quality. Secondary quality effluent is readily attainable if the low-rate trickling

filter design incorporates filter media with bioflocculation capabilities or good sec-

ondary clarification (Harrison et al., 1984). For more information, see the Combined

Processes section of this chapter.

Intermediate-Rate Filters. Intermediate filters may be loaded up to 64 kg BOD5/

100 m3d (40 lb BOD5/d/1000 cu ft). Recirculation of trickling filter/biotower effluent

is typically practiced to ensure good distribution and thorough blending of filter and

secondary effluents to prevent bleed-through or short-circuiting of BOD with the

treated effluent.

Biological solids that slough from an intermediate trickling filter are not as well-

digested as those from a low-rate unit. Yields ranging from 0.6 to 0.8 kg TSS/kg (0.6 to

0.8 lb TSS/lb BOD5) are common, depending on the filter media type.

Nitrifying bacteria have difficulty competing with heterotrophic bacteria, and am-

monia removal via nitrification is typically incomplete. However, carbonaceous BOD

removal is nearly complete. Therefore, with good clarification following the filter, use

of a combined process to achieve secondary treatment is almost never necessary. Both

the TF/SC and ABF processes can be used to improve effluent quality, as described in

the Combined Processes section of this chapter.

High-Rate Filters. The maximum organic removal of most filter media ranges from

48 to 96 kg SBOD5/100 m3d (30 to 60 lb SBOD5/d/1000 cu ft), depending on tem-




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perature, wastewater characteristics, and other conditions. High-rate filters, typically

loaded at their maximum organic loading capabilities, receive total BOD5 loadings

ranging from 64 to 160 kg BOD5/100 m3d (40 to 100 lb BOD5/d/1000 cu ft). Achieving

secondary effluent quality with high-rate filters reliably without a second-stage

process is less predictable than with low- or intermediate-rate filters. Therefore, high-

rate trickling filters are typically used with combined processes (Table 21.1).

With the recirculation typically used with high-rate filters, hydraulic loading rates

typically range from 0.3 to 0.4 L/m2s (0.5 to 0.7 gpm/sq ft), depending on the type of

filter media used.

Roughing Filters. Roughing filters are typically designed to allow a substantial amount

of SBOD to bleed through the trickling filter. An RBC or second, smaller trickling filter

follows the first-stage filter to complete the BOD oxidation. The second stage of treat-ment is typically 30 to 50% of the size required without a roughing filter. An activated

sludge process that follows a roughing filter typically has to assimilate the sloughed

solids and remaining BOD; therefore, a significant load has not been reduced.

Roughing filters typically have a design load ranging from 160 to 480 kg BOD 5/

100 m3d (100 to 300 lb BOD5/day/1000 cu ft). A further description of the process

modes follows in the Combined Processes section of this chapter.

DESCRIPTION OF PROCESS. Regardless of the type of trickling filter/biotowerused, the pollutant-removal mechanisms remain the same. Microorganisms cover a fil-

ter consisting of rock (river or crushed aggregate), plastic, or redwood media. The

wastewater enters the filter medium at a controlled rate (trickled), causing intimatecontact between waste, the air, microorganisms, and other organisms.

The termfilter is misleading, because it suggests physical separation of the solids

from the liquid via straining action. This does not occur, even with closely packed rock

media, and certainly not with the more open high-rate media used in biotowers. Instead,

treatment occurs when the microorganisms absorb and use dissolved organics for their

growth and reproduction as the wastewater cascades randomly through the voids

(spaces between the media).

The complex population of microorganisms is predominately aerobic. It absorbs

oxygen from air circulating through the media. Circulation can be enhanced by a

forced ventilation system consisting of a series of fans and an air-distribution system.

However, most trickling filters/biotowers rely solely on natural ventilation to supply

the oxygen necessary for aerobic treatment.

High-rate biotower media offer more surface area than rock media for microbial

attachment per cubic meter (cubic foot) of media. Also, biotowers have more void




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space than rock media, allowing sloughed solids to exit the biotower and improving

air circulation.

The ability to control the unit process will vary greatly, depending on the facilities

and equipment provided by the design engineer. Also, the operating strategy used for

process control may result in significant changes in the process character and its abil-

ity to remove pollutants. These issues are considered in the following two sections.

DESCRIPTION OF EQUIPMENT. The structure, distribution, and support sys-tem used with the media are collectively named either a trickling filter or a biotower. The

term trickling filter typically applies to filters that use rock media and are relatively shal-

low [1.2 to 3.0 m (4 to 10 ft) deep]; processes that use plastic or redwood media with

depths greater than 3 m (10 ft) are typically referred to as biological towers or biotowers.

A similar term, biofilter, sometimes refers to filter towers in which biological solids

from an activated sludge system are recycled over the media.

The following six basic components are common to all trickling filter and biotower

systems:

Distribution system,

Filter media,

Underdrain system,

Containment structure,

Filter pump station or dosing siphon, and

Secondary clarifiers.

These basic components are illustrated in Figure 21.1. The purpose of these parts is

described in Table 21.2. A more detailed description of the basic components follows.

Distribution Systems. The two basic types of distribution systems are fixed-nozzle

and rotary distributors. Fixed-nozzle distributors were frequently used during the

early to mid-1900s, but their use on new trickling filters is limited. Fixed-nozzle dis-

tributors consist of a piping system, often supported slightly above the top of the trick-

ling filter media, that feeds wastewater and recycled wastewater from a pumping sta-

tion or siphon box through spray nozzles. A number of advancements in fixed-nozzle

design include springs, balls, or other mechanisms to evenly distribute wastewater at

various flows. Even with these improvements, obtaining even distribution with a fixed-

nozzle distribution system is more difficult than with rotary distribution systems. Fixed-

distributor systems have also declined in use because of difficult access to the nozzles

for cleaning raising safety concerns. Rotary distributors consist of a center well (typi-




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cally metal) mounted on a distributor base or pier. The distributor typically has two or

more arms that carry the pumped or siphoned wastewater to varying sized orifices for

distribution over the media surface. The thrust of the water spray drives the filter armsforward. Speed-retardant back-spray orifices are often used to adjust the distributors

rotational speed, while maintaining the desired flowrate to the filter.

Recently, some rotary distributors have been equipped with motorized drive units

to precisely control the wastewater flow distribution speed. Distributors may be set up



FIGURE 21.1 Trickling filter parts. (Copyrighted material from Operation of Wastewater

Treatment Plants, Volume 1, 6th Edition, Chapter 6, Trickling Filters; reproduced by

permission of the Office of Water Programs, California State University, Sacramento.)


15/66



TABLE 21.2 Parts of a trickling filter (California State, 1988).

Part Purpose

Inlet pipe Conveys wastewater to trickling filter

Distributor base Supports rotating distributor arms

Distributor bearings Allows distributor arms to rotate

Distributor arm Conveys wastewater to outlet orifices located along the arms

Outlet orifice Controls flow to filter media; adjustable to provide evendistribution of wastewater to each square metre (square foot) offilter media

Speed-retarder orifice Regulates speed of distributor arms

Splash plate Distributes flow from orifices evenly over filter media

Arm dump gate Drains distributor arm and controls filter flies along filterretaining wall; also used for flushing distributor arm to removeaccumulated debris that might block outlet orifices

Filter media Provides a large surface area on which the biological slime grows

Support grill Keeps filter media in place and out of underdrainage system

Underdrain system Collects treated wastewater from under filter media and conveysit to the underdrain channel; also permits air flow through media.

Underdrain channel Drains filter effluent to the outlet box

Outlet box Collects filter effluent before it flows to the next process

Outlet valve Regulates flow of filter effluent from outlet box into outlet pipe;closed when filter is to be flooded

Outlet pipe Conveys filter effluent to next treatment process

Retaining wall Holds filter media in place

Ventilation port Allows air to flow through the media

Stay rod Supports distributor arm

Turnbuckle on stay rod Permits adjusting and leveling of distributor arm to produce aneven distribution of wastewater over the media

Center well Provides for higher water head to maintain equal flow todistributor arms; typically a head of 45 to 60 cm (18 to 24 in.) ismaintained on the orifices

Splitter box Divides flow to the trickling filters for recirculation or to thesecondary clarifiers

Recirculation pump Returns or recirculates flow to the trickling filters


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to be mechanically driven at all times or just when stalled. These operating provisions

are aimed at selecting a distributor speed to increase biomass sloughing. Decreasing

distributor speed may prevent plugging, decreased performance, and odors, particu-

larly in heavily loaded filters. Adding motorized drives also can increase the perfor-

mance of an exiting tricking filter or biotower.

The distributor support bearings are either at the top of the mast or at the bottom

of the turntable. Both types of bearings are widely used.

Another newer method of more precisely controlling the wastewater flow distrib-

ution to the trickling filter is a system of pneumatically controlled gates to open and

close the orifices on both sides of the distributor arms. As flow to the trickling filter

varies, the speed is maintained by automatically adjusting the gates over the orifices.

Filter Media. Of the many types of media materials used to support biological growth,

the most common types are shown in Figure 21.2. Media are typically classified as

either high-rate (high surface area and void ratio) or standard rock media.

Filter media types made of plastic sheets include vertical, 60-degree crossflow, and

45-degree crossflow. Random media are open-webbed plastic shapes. For carbona-

ceous BOD removal, their surface area typically ranges from 89 to 105 m2/m3 (27 to 32

sq ft/cu ft) of media, and their void percentage is between 92 and 97% (open-space per-

centage of unit volume). Filter media for nitrification (post-BOD removal) are available,

with surface areas in excess of 131 m2/m3 (40 sq ft/cu ft). Based on numerous studies to

compare trickling filter media, the present consensus is that cross-flow media may offer

better flow distribution than other media, especially at low organic loads. Comparedwith 60-degree cross-flow media, vertical media provide nearly equal distribution and

may better avoid plugging, especially at higher organic loadings.

Rock media may consist of either graded material from natural river beds or

crushed stone. Most rock media provide approximately 149 m2/m3 (15 sq ft/cu ft) of

surface area and less than 40% void space.

A significant difference between rock media and plastic media is that most loose

stone aggregates have a dry weight of approximately 1282 kg/m3 (80 lb/cu ft) com-

pared with a density of 32 to 48 kg/m3 (2 to 3 lb/cu ft) for plastic media. Additional

provisions required for plastic media include UV protective additives on the exposed

top layer of plastic media filters; thicker plastic walls for media packs installed in the

lower sections of the filter, where loads increase; and, under certain conditions, ameans for shielding the top layer from the effects of the distributors hydraulic force.

Underdrain System. The underdrain system supporting rock media typically consists

of precast blocks laid over the entire sloping filter floor. Underdrain and support




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systems for high-rate media typically consist of a network of concrete piers and sup-

port stringers placed with their centers 0.3 to 0.6 m (1 to 2 ft) apart. Redwood or pres-

sure-treated wood is also used as underdrain material.Underdrains for plastic or high-rate filter media are typically 0.3 to 0.6 m (1 to

2 ft) deep to allow air movement to the interior of the filter. Floors typically slope

downward to a collection trough that carries wastewater to an outlet structure. The

collection trough also serves as an air conduit to the interior of the trickling filter. Ac-



FIGURE 21.2 Trickling filter media (Harrison and Daigger, 1987).


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cess to the filter underdrain system should be available at the outlet box to allow peri-

odic inspection. The confined space entry procedures described in Chapter 5 govern

the inspection.

Containment Structure. The housing for rock media typically consists of poured-in-

place concrete. Filter towers are lightweight containment structures consisting of pre-

cast concrete, fiberglass panels, or other materials. These structures are used with high-

rate media that are self-supporting (exert no wall pressure).

Ventilation ports, typically located at the base of the filter tower, are designed to

prevent the filter tower from being stained by the heavy splash of distributed waste-

water cascading to the filter floor. Closed louvers, if available, allow the vents to be

closed during cold weather. The filter structure may include low-pressure fans and air

ducts (typically fiberglass) to distribute air in the filter underdrain.

The wall of the containment structure often extends 1.2 to 1.5 m (4 to 5 ft) above

the top of the filter media. This prevents spray from staining the sides of the filter

tower, reduces wind effects that may reduce wastewater temperatures or stall distrib-

utors, and provides a structural base for domed covers.

Filter Pump Station. As an integral part of the trickling filter or biotower system, the

pumping station typically lifts the primary effluent and the recirculated filter effluent,

if any, to the top of the media. Sometimes a siphon dosing tank or gravity flow feeds

the distributor. The filter/biotower feed pumps most typically used are vertical-turbine

units mounted above a wet well. Submersible pumps and dry-pit centrifugal pumps

may also be used in the filter pump station.The trickling filter/biotower is typically elevated so the hydraulic grade line al-

lows gravity flow to the secondary clarifier or other downstream treatment units. If re-

circulation is used, the downstream treatment unit or clarifier typically controls the

water level in the pumping station wet well, so a control valve is not necessary to mod-

ulate the amount of underflow returning to the pumps.

Secondary Clarifier. In the past, clarifier design often received insufficient attention.

Although wastewater treatment professionals typically recognize the need for im-

proved clarifier design criteria associated with suspended-growth or activated sludge

plants, this need also exists with the design and operation of secondary clarifiers used

in the fixed-film trickling filter/biotower process.Performance of the trickling filter/biotower process is typically not limited by

SBOD removal, but by the secondary clarifiers ability to separate suspended solids

from treated wastewater. This is especially true for low-, intermediate-, and high-rate




19/66

processes that remove most of the SBOD. Therefore, effluent quality depends largely

on the particulate BOD associated with solids remaining in the clarifier effluent.

With the trickling filter/biotower process, past practices have resulted in sec-

ondary clarifiers with high hydraulic overflow rates and shallow sidewater depths

[2.4 to 3.0 m (8 to 10 ft)]. Corresponding suspended-growth systems were often de-

signed with clarifiers having much lower hydraulic overflow rates and sidewater

depths of 3.0 to 3.7 m (10 to 12 ft). As trickling filter/biotower plants are now required

to achieve secondary or even higher treatment levels, the clarifier sidewater depth

must be larger to provide a greater separation zone for solids removal. Likewise, re-

duced overflow rates may be needed to achieve the required effluent quality (Tekippe

and Bender, 1987).

PROCESS CONTROL. Many operating problems may be avoided by changingone or more of the following process control variables: flow patterns, distribution rates,

and clarifier operation.

Flow Patterns. Although operators do not control the arrangement of the major treat-

ment units, opportunities may exist to take units offline, operate in stages (parallel or

series filters), or recycle settled biological solids over high-rate filter media. For exam-

ple, staging or operating filters in series sometimes increases the overall systems BOD

removal because of increased efficiency in the higher-loaded first stage. If both rock

and high-rate filters are available, operators should consider operating the rock filter

in the first stage to achieve a high reduction in produced biological solids. Then, the

second-stage filter would be operated in a polishing mode, taking advantage of thehigh-rate filter medias bioflocculation capabilities.

With little or no modification, many existing trickling filters/biotowers with high-

rate media may incorporate small amounts of biological solids recycle over the filter

media to enhance the flocculation of particulate solids. Recycled solids may be re-

turned to the filter underflow of the rock filter media, as is done in trickling filter and

solids combined processes.

Distribution Rates. As a principal process control measure, operators can control the

rates at which wastewater and filter effluent are distributed to the filter media. Recir-

culation can serve several purposes, as follows:

Reduce the strength of the wastewater being applied to the filter;

Increase the hydraulic load to reduce flies, snails, or other nuisances;

Maintain distributor movement during low flows;




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Produce hydraulic shear to encourage solids sloughing and prevent ponding;

Dilute toxic wastes, if present;

Reseed the filters microbial population;

Provide uniform flow distribution; and

Prevent filters from drying out.

The most common recirculation patterns used for trickling filters/biotowers are shown

in Figure 21.3. If odors emanate from the primary clarifier or headworks, recycling fil-

ter effluent to either location may help control them. When the recirculated water

passes through either the primary or secondary clarifier, operators need to prevent ex-

cessive hydraulic loading of the clarifier.

A typical control strategy for highly loaded or roughing filters is to frequently

(once per week) maintain the maximum pumping rate possible for a 2- to 3-hour period.

This encourages sloughing, so solids buildup is less likely, and uncontrolled sloughing

is minimized.

Another approach is to slow the distributor arm using back-spray nozzles, so the

media receives a greater instantaneous flush. When a trickling filter/biotower accumu-

lates excess solids, aerobic surface area decreases, which, in turn, reduces oxygen trans-

fer. Because oxygen does not penetrate more than 1 to 1.5 mm of film thickness, there is

no benefit with more than 0.76 mm (0.03 in.) biomass on the media (Albertson, 1989).

A German process parameter that has been considered in the United States is

Spulkraft flushing intensity (SK), which is defined by the following equation:

(21.3)

Where

SK flushing intensity, mm/pass of arm;

q r total hydraulic rate, m3/m2h;

a number of distributor arms;

n rotational speed, rev/min; and

1.0 m3/m2h 0.41 gpm/sq ft.

Some recommended SK values for design and flushing flowrates are given in Table 21.3.

Compared with the activated sludge treatment process, trickling filters can use

30 to 50% less energy if the pumping rate is optimized. Thoughtful consideration needs

to be given to balancing the need to maintain a minimum wetting rate versus the po-

tential energy savings of lower recirculation rates.

SK 1000 mm m

(60 min h)

( )( / )

( )( ) /

q r

a n




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Clarifier Operation. The manner in which secondary clarifiers are operated can sig-

nificantly affect trickling filter performance. Although clarifier operation with fixed-

film reactors is not as critical as that with suspended-growth systems, operators muststill pay close attention to final settling.

Sludge must be removed quickly from the final settling tank before gasification oc-

curs or denitrification causes solids to rise. Use of the secondary clarifier as a principal

means of thickening (rather than simply for solids settling) may not produce the best



FIGURE 21.3 Recirculation patterns for trickling filters.


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effluent quality, especially during summer months, when denitrification is likely to oc-

cur. The sludge blanket depth in the secondary clarifier should be limited to 0.3 to 0.6 m

(1 to 2 ft). Continuous pumping or intermittent pumping with automatic timer control

are used to accomplish solids wasting.

TROUBLESHOOTING. Even though the trickling filter/biotower process is con-sidered one of the most trouble-free means of secondary treatment, the potential for

operating problems exists (Table 21.4). The source of mechanical problems is often ob-

vious. However, less obvious causes of problems may stem from operations, design

overload, influent characteristics, and other non-equipment-related items.

Good records and data associated with the trickling filter are essential in locating,

identifying, and applying the proper corrective measure to solve problems. Tracking

SBOD, suspended solids, pH, temperature, and other parameters may be necessary to

recognize trends that result in adverse trickling filter/biotower effects.

Common operating problems may result from increased growth, changes in waste-

water characteristics, improper design, or equipment failures. Regardless of the source,

these problems eventually become categorized into either operation or maintenance

areas. In summary, the problems addressed in Table 21.4 are as follows:

Operations:

Increase in secondary clarifier effluent suspended solids,

Increase in secondary clarifier effluent BOD,

Objectionable odors from filter,

Ponding on filter media,

Filter flies, and

Icing.

TABLE 21.3 Design and flushing Spulkraft (SK) values for distributors (revised, higherSK values reflect new, post-publication data) (Albertson, 1989).

BOD5 loading (lb/d/cu fta) Design SK (mm/pass) Flushing SK (mm/pass)

25 2575 10050 50150 15075 75225 225

100 100300 300150 150450 450200 200600 600

aBOD biochemical oxygen demand; lb/d/cu ft 16.02 kg/m3d.


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TABLE 21.4 Troubleshooting guide for trickling filters (continued on next page).

Problem/possible cause Corrective action

Operations

Increase in secondary clarifier effluent suspended solids

Clarifier hydraulically overloaded Check clarifier surface overflow rate; if possible,reduce flow to clarifier to less than 35 m3/m2d(900 gal/d/sq ft) by reducing recirculation orputting another clarifier into service

Expand plant

Denitrification in clarifier Increase clarifier sludge withdrawal rate

Increase loading on trickling filter to prevent

nitrification; skim floating sludge from entire surfaceof clarifier or use water sprays to release nitrogengas from sludge so sludge will resettle

Excessive sloughings from biofilter because of Increase clarifier sludge withdrawal ratechanges in wastewater

Check wastewater for toxic materials, changes in pH,temperature, BOD, or other constituents

Identify and eliminate source of wastewater causingthe upset

Enforce sewer-use ordinance

Equipment malfunction in secondary clarifier Check for broken sludge-collection equipment andrepair or replace broken equipment

Short-circuiting of flow through secondary clarifier Level effluent weirs

Install clarifier center pier exit, baffles, effluent weirbaffles, or other baffles to prevent short-circuiting

Increase in secondary clarifier effluent biochemical oxygen demand (BOD)

Increase in effluent suspended solids See corrective actions for Increase in secondaryclarifier effluent suspended solids

Excessive organic loads on filter Calculate loading

Reduce loading by putting more biofilters in service

Increase BOD removal in primary settling tanks byusing all tanks available and minimizing storage inprimary sludge tanks

Eliminate high-strength sidestreams in plant

Expand plantUndesirable biological growth on media Perform microscopic examination of biological

growth

Chlorinate filter to kill off undesirable growth


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Objectionable odors from filter

Excessive organic load causing anaerobic Calculate loadingdecomposition in filter

Reduce loading by putting more biofilters in service

Increase BOD removal in primary settling tanks byusing all tanks available and minimizing storage orprimary sludge in tanks

Encourage aerobic conditions in treatment unitsahead of the biofilter by adding chemical oxidants(e.g., chlorine, potassium permanganate, or hydrogen

peroxide) or by preaerating, recycling plant effluent,or increasing air to aerated grit chambers

Enforce industrial waste ordinance, if industry issource of excess load

Scrub biofilter offgases

Replace rock media with plastic media

Expand plant

Insufficient ventilation Increase hydraulic loading to wash out excessbiological growth

Remove debris from filter effluent channels andunderdrains

Remove debris from top of filter media

Unclog vent pipes

Reduce hydraulic loading if underdrains are flooded

Install fans to induce draft through filter

Check for filter plugging caused by breakdown ofmedia

Ponding on filter media

Excessive biological growth Reduce organic loading

Slow down distributor to increase 5K value

Increase hydraulic loading to increase sloughing

Flush filter surface with high-pressure stream of water

Chlorinate filter influent for several hours; maintain

1 to 2 mg/L residual chlorine on the filterFlood filter for 24 hours

Shut down filter until media dries out

Enforce industrial waste ordinance if industry issource of excess load


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Poor media Replace media

Poor housekeeping Remove debris from filter surface, vent pipes,underdrains, and effluent channels

Filter flies (psychoda)

Insufficient wetting of filter media (a continually Increase hydraulic loadingwet environment is not conducive to filter fly

Unplug spray orifices or nozzlesbreeding and a high wetting rate will wash flyeggs from the filter) Use orifice opening at end of rotating distributor

arms to spray filter walls

Filter environment nodule conducive to filter Flood filter for several hours each week during fly

fly breeding season

Chlorinate the filter for several hours each weekduring fly season; maintain a 1- to 2-mg/L chlorineresidual on the filter

Poor housekeeping Keep area surrounding filter mowed; remove weedsand shrubs

Icing

Low wastewater temperature Decrease recirculation

Remove ice from orifices, nozzles, and distributorarms with a high-pressure stream of water

Reduce number of filters in service, providedeffluent limits can still be met

Reduce retention time in pretreatment and primarytreatment units

Construct windbreak or covers.

Maintenance

Rotating distributor slows down or stops

Insufficient flow to turn distributor Increase hydraulic loading

Close reversing jets

Clogged arms or orifices Flush out arms by opening end plates; flush outorifices; remove solids from influent wastewater

Clogged distributor arm vent pipe Remove material from vent pipe by rodding orflushing

Remove solids from influent wastewater

Bad main bearing Replace bearing

Distributor arms not level Adjust guy wires at tie rods


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Distributor rods hitting media Level media

Remove some media

Dirt in main bearing lube oil

Worn bearing dust seal Replace seal

Worn turntable seal or seal plate Replace seal; inspect seal plate and replace if worn

Condensate not drained regularly or oil level too low Check oil level, drain condensate, and refill if needed

Water leaking from distributor base

Worn turntable seal Replace seal

Leaking expansion joint between distributor and Repair or replace expansion joint

influent piping

Broken top media

Foreign material Flush out with a high-pressure stream of clean water

Rod out with wire or hook

Disassemble and clean

Secondary clarifier sludge collector stopped

Torque overload setting exceeded Reduce sludge blanket; withdraw excess sludge

Check if skimmer portion of collector hung up onscum trough; free and repair or adjust skimmer

Drain tank and remove foreign objects

Loss of power Reset drive unit circuit breaker if tripped (after cause

for trip is identified and corrected)Reset drive unit, motor control center, or plant maincircuit breakers as necessary when power is restoredto plant after interruption

Check drive motor for excessive current draw; ifcurrent excessive, determine reason

Check drive motor overload relays; replace if bad orundersized

Failure of drive unit Check drive chains and shear pins; replace asnecessary and use proper size shear pin, or damagewill occur

Check and replace worn gears, couplings, speedreducers, or bearings as needed; lubricate and

provide preventive maintenance for units as permanufacturers instruction

Recirculation pumps delivering insufficient flow

Excessive head Open closed or throttled valves

Unplug distributor arms, headers, and laterals


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Maintenance:

Rotating distributor slowing down or stopping,

Dirt in main bearing lube oil,

Water leaking from distributor base, Nozzle or orifices plugged,

Top media broken,

Secondary clarifier sludge collector stopped, and

Recirculation pumps delivering insufficient flow.



TABLE 21.4 Troubleshooting guide for trickling filters.


Unplug distributor nozzles and orifices

Unplug distributor vent lines

Pump malfunction Adjust or replace packing or mechanical seals

Adjust impeller to casing clearance

Replace wear rings if worn excessively

Replace or resurface worn shaft sleeves

Check impeller for wear and entangled solids;remove debris; replace impeller if necessary

Check pump casing for air lock

Release trapped air

Lubricate bearings as per manufacturers instructions

Replace worn bearings

Pump drive motor failure Lubricate bearings as per manufacturers instructions

Replace worn bearings

Keep motor as clean and dry as possible

Pump and motor misalignment; check vibration andalignment

Redesign as needed

Burned windings; rewind or replace motor

Check drive motor for excessive current draw; if

current draw is excessive, determine reasonCheck drive motor overload relays; replace if bad orundersized

Reset drive motor, motor control centers, or plantmain circuit breakers after cause for trip is identifiedand corrected, or when power is restored afterinterruption


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PLANNED MAINTENANCE. Planned maintenance will vary from plant to plant,depending on unique design features and equipment installed. Although this chapter

cannot address all of these items, a summary of the most common and important main-

tenance tasks follows.

Table 21.5 is a guide to planned maintenance for the following:

Rotary distributors,

Fixed-nozzle distributors,

Filter media,

Underdrains,

Media containment structure,

Filter pumps,

Secondary clarifiers, and

Appurtenant equipment.

The information provided in Table 21.5 is not equipment- or plant-specific. Therefore,

both the manufacturers literature and engineers operating instructions should be

consulted and followed. The frequency of maintenance procedures depends on site-

specific conditions. However, until operating experience is gained, frequent plant in-

spections and maintenance should continue. Maintenance schedules should consider

the increased performance of trickling filters in warm weather months, which may re-

duce the effect of removing process units from service.

DISTRIBUTOR BEARINGS. Distributor bearings typically ride on removableraces (tracks) in a bath of oil (Figure 21.4). The oil, typically specified by the manufac-

turer, is selected to prevent oxidation and corrosion and to minimize friction. Because

the oil level and condition are crucial to the life of the equipment, they need regular

checking in accordance with the manufacturers recommendations (typically weekly).

A common procedure is to check the oil by draining approximately 0.6 L (1 pt) into a

clean container. If the oil is clean and free of water, it is returned to the unit. If the oil is

dirty, it is drained and refilled with a mixture of approximately one part oil and three

parts solvent (e.g., kerosene). Then, the distributor is operated for a few minutes, the

mixture is drained, and the distributor is filled with clean oil.

If water is found in the oil, then either the seal fluid is low or the gasket in the

mechanical seals requires replacement.

SAFETY. Work on distributors may proceed only after the arms have been stoppedand locked in place, and the distributor pumps or control valves electrical switch has




29/66



TABLE 21.5 Planned maintainance for trickling filters (continued on next page).

Rotary distributors

Observe the distributor daily. Make sure the rotation is smooth and that spray nozzlesare not plugged.

Lubricate the main support bearings and any guide or stabilizing bearings according tothe manufacturers instructions. Change lubricant periodically, typically twice a year. Ifthe bearings are oil-lubricated, check the oil level, drain condensate weekly, and add oilas needed.

Time the rotational speed of the distributor at one or more flow rates. Record and filethe results for future comparison. A change in speed at the same flow rate indicates

bearing trouble.

Flush distributor arms monthly by opening end shear gates or blind flanges to removedebris. Drain the arms if idle during cold weather to prevent damage via freezing.

Clean orifices weekly with a high-pressure stream of water or with a hooked piece of wire.

Keep distributor arm vent pipes free of ice, grease, and solids. Clean in the same manneras the distributor arm orifices. Air pockets will form if the vents are plugged. Airpockets will cause uneven hydraulic loading in the filter, and nonuniform load andexcessive wear of the distributor support bearing.

Make sure distributor arms are level. To maintain level, the vertical guy wire should betaken up during the summer and let out during the winter by adjusting the guy wire tierods. Maintain arms in the correct horizontal orientation by adjusting horizontal tie rods.

Periodically check distributor seal and, if applicable, the influent pipe to distributorexpansion joint for leaks. Replace as necessary. When replacing, check seal plates forwear and replace if wear is excessive. Some seals should be kept submerged even if thefilter is idle or their life will be severely shortened.

Remove ice from distributor arms. Ice buildup causes nonuniform loads and reducesmain bearing life.

Paint the distributor as needed to guard against corrosion. Cover bearings when sand-blasting to protect against contamination. Check oil by draining a little oil through a nylonstocking after sandblasting. Ground the distributor arms to protect bearings if welding ondistributor and lock out the drive mechanism at the main electrical panel. Adjust secondaryarm overflow weirs and pan test wastewater distribution on filter as needed.

Fixed nozzle distributors

Observe spray pattern daily. Unplug block nozzles manually or by increasing hydraulicloading. Flush headers and laterals monthly by opening end plates. Adjust nozzle springtension as needed.

Filter media

Observe condition of filter media surface daily. Remove leaves, large solids and plastics,grease balls, broken wood lath or plastic media, and other debris. If ponding is evident,find and eliminate the cause. Keep vent pipes open, and remove accumulated debris.Store extra plastic media out of sunlight to prevent damage via ultraviolet rays. Observemedia for settling. After they are installed, media settle because of their own weight


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TABLE 21.5 Planned maintainance for trickling filters.

and the weight of the biofilm and water attached to its surface. Settling should beuniform and should stabilize after a few weeks. Total settling is typically less than 0.3 m(1 ft) for random plastic media, less for plastic sheet media, and nearly zero for rock. Ifsettling is nonuniform or excessive, remove some of the media for inspection.

Observe media for hydraulic erosion, particularly in regions where reversing jets hit themedia.

Underdrains

Flush out periodically if possible. Remove debris from the effluent channels.

Media containment structure

Maintain spray against inside wall of filter to prevent filter fly infestation and to preventice buildup in winter.

Practice good housekeeping. Keep fiberglass, concrete, or steel outside walls clean andpainted, if applicable. Keep grass around structures cut, and remove weeds and tallshrubs to help prevent filter fly and other insect infestations. Remember, usinginsecticides around treatment units may have adverse effects on water quality or the

biological treatment units.

Filter pumps

Check packing or mechanical seals for leakage daily. Adjust or replace as needed.Lubricate pump and motor bearings as per manufacturers instructions. Keep pumpmotor as clean and dry as possible. Periodically check shaft sleeves, wearing rings,and impellers for wear; repair or replace as needed. Perform speed reducer, coupling,and other appurtenant equipment maintenance according to manufacturersinstructions.

Secondary clarifier Lubricate drive motor bearings, speed-reducing gear, drive chains, work and spur

gears, and the main support bearing for the solids-collection equipment according to themanufacturers instructions. Flush scum troughs and grease wells daily. Maintain solids-withdrawal equipment. Clean effluent wells and baffles at least weekly. Paint orotherwise protect equipment from corrosion as needed.

Appurtenant equipment

Maintain piping, valves, forced draft blowers, and other appurtenant equipmentaccording to the manufacturers instructions.

been disengaged and locked out on the electrical panel. The filter medium should notbe walked on, because it will be slippery. Plastic grating is often placed as a permanent

walking surface to provide safe access to the distributor.

Covered trickling filters have special safety considerations, because they are con-

sidered confined spaces. The possibility exists for the atmosphere under the dome to


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contain little oxygen or much hydrogen sulfide or ammonia. Maintenance in these

areas must include proper confined space entry procedures. Chapter 5 discusses other

safety considerations.

ROTATING BIOLOGICAL CONTACTORS

A rotating biological contactors filter media consist of plastic discs mounted on a

long, horizontal, rotating shaft (Figure 21.5). A biological slime similar to that of the

trickling filter/biotower grows on the media. However, rather than being stationary, the

filter media rotate into the settled wastewater and then emerge into the atmosphere,

where the microorganisms receive oxygen that helps them consume organic materials

in the wastewater.

Rotating biological contactors have been extensively used at hundreds of locations

in the United States to treat municipal and industrial wastewater. It is estimated that

more than 600 RBC plants are now used for industrial and municipal wastewater treat-

ment. Most of the plants are designed and used for BOD 5 removal and a few for both

BOD5 and nitrogen removal.When RBCs were initially introduced for wastewater treatment during the late

1970s and early 1980s, mechanical problems and organic overloading occurred fre-

quently. By the mid-1980s, both equipment manufacturers and consulting engineers

had developed standards that minimized most of the problems, but some systems are



FIGURE 21.4 Trickling filter bearing (Copyrighted material from Operation of Waste-

water Treatment Plants, Volume 1, 6th Edition, Chapter 6, Trickling Filters; reproduced

with the permission of the Office of Water Programs, California State University,

Sacramento).


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still mechanically unsound. This section discusses the use of RBCs principally for car-bonaceous BOD5 removal and ammonia nitrification.

ALTERNATIVES. The flow pattern for RBC treatment of wastewater resembles thatfor most other biological systems, because good preliminary and primary treatment

are essential to remove solids that would otherwise interfere with RBC performance

(Figure 21.6).

A secondary clarifier must be provided to remove sloughed solids from the

treated wastewater. Solids that settle in the secondary clarifier can either be recycled to

the primary clarifier for cosettling or pumped directly to a solids-handling system (Fig-

ure 21.6).

The term shaft typically is used to describe both the metal support and the filter me-dia discs. The discs are made of high-density circular plastic sheets, typically 3.6 m (12 ft)

in diameter (although larger sizes are available from some manufacturers). The sheets,

bonded and assembled onto the horizontal shafts, are typically 7.6 m (25 ft) long. Each

shaft typically provides approximately 9300 m2 (100 000 sq ft) of surface area for micro-



FIGURE 21.5 Rotating biological contactor shaft and media.


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organism attachment. Lower density media are typically used for carbonaceous BOD5removal, and higher density media are typically used for ammonia nitrification.

Alternative selection associated with RBCs consists primarily of the number and

arrangement of shafts (Figure 21.7) that support the RBC discs (Zickefoose, 1984). A

common arrangement includes a separate shaft for each stage, especially when theflow is perpendicular to the shafts, as shown in plan A of Figure 21.7. A single shaft can

be divided into two or more stages by adding a baffle at one or more sections along the

flow pattern (plan B of Figure 21.7). This arrangement typically applies when the

wastewater flow pattern parallels the shaft. A stage may also be eliminated by removing



FIGURE 21.6 Rotating biological contactor process flow schematic.


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a baffle, as shown in plan C of Figure 21.7. The baffles may be constructed of either per-

forated concrete or slotted boards. To reduce the organic loading of a stage, baffles areoften removable. This allows two or more shafts to operate in a single stage, as illus-

trated in plan D of Figure 21.7. Staging is often used to improve effluent quality. Four

or more stages, combined with lower organic loading, are typically used to obtain a ni-

trified or well-treated effluent.



FIGURE 21.7 Stage arrangements and flow patterns for rotating biological contactors.


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Staging is sometimes used to overcome oxygen-transfer problems in the first shafts

(first stages) of an RBC facility. Often, baffles are removed on the first several stages of

multistage facilities to distribute loading among several shafts. This method, among

others, has been used to reduce the organic loading and overcome oxygen-transfer dif-

ficulties in heavily loaded RBC reactors.

A train (several parallel series of stages) is also typically used to reduce loadings

on the first RBC stages. Baffles may also be removed between shafts to increase the sur-

face area available in the first stage. Figure 21.7 illustrates the use of both trains and

stages in either parallel or series flow modes. Good designs will include a number of

gates and baffles to provide for system flexibility. This allows operators to vary the

flow pattern to accommodate facility-specific load and effluent quality criteria.

Designers should consider including one or more of the following to ensure that

the process will have adequate operating flexibility:

Supplemental aeration to increase dissolved oxygen levels in the first and sec-

ond stages;

A means for removing excess biofilm growth (e.g., supplemental aeration, rota-

tional speed control, and reversal);

Multiple treatment trains;

Removable baffles between all stages;

Variable rotational speeds in the first and second stages;

Load cells for first- and second-stage shafts;

Alternate flow distribution systems (e.g., step feeding); and

Recirculation of secondary clarifier effluent.

A significant number of RBCs have encountered oxygen limitations or overloads in the

first stages. In other cases, the RBCs have simply reached their design loads. Regard-

less of the reason, RBC upgrades are typically accomplished by adding more RBCs or

by constructing the following:

New processes in parallel (i.e., side-by-side) with existing RBCs (e.g., activated

sludge processes, trickling filters, or aerated lagoons); and

New processes in series (i.e., preceding or following existing RBCs), such as ac-

tivated sludge processes, trickling filters, aerated lagoons, preaeration processes,

or solids contact processes.

DESCRIPTION OF PROCESS. Rotating biological contactor systems consist ofplastic media, typically a series of vertical discs, mounted on a horizontal shaft that




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slowly rotates, turning the media into and out of a tank of wastewater. Rotating bio-

logical contactor shafts are rotated by either a mechanical or a compressed air drive so

the media on up to 40% of its diameter are immersed in the wastewater. The waste-

water being treated flows through the contactor by simple displacement and gravity.

Bacteria and other microorganisms that are naturally present in the wastewater adhere

and grow on the surface of the rotating media. The biological film sloughs off when-

ever the biomass growth becomes too thick and heavy for the media to support. The

sloughed biofilm and other suspended solids are carried away in the wastewater and

removed in the secondary clarifier.

The biological slime on the first stages is typically 0.15 to 0.33 cm (0.06 to 0.13 in.)

thick. A healthy biomass on the first stage tends to be light brown, while the biomass

on later stages tends to have a gold or reddish sheen. Lightly loaded units may be

nearly devoid of visible biomass. A white or gray biomass indicates domination by fil-

amentous (Beggiatoa, Thiothrix, or Lepothrix) bacteriaan unhealthy sign.

Like the trickling filter process, many of the process choices are fixed during de-

sign, and RBC operators have limited opportunities to make changes. The design choices

discussed in the following section will help both operators and designers better under-

stand the RBC process.

Loadings for RBCs are typically based on BOD5 loading to the RBC units divided

by the medias surface area. Organic loading is typically calculated for all units online

or simply for the first stages, where an oxygen limitation may exist. The organic load

may be based on either the soluble or total BOD.

(21.4)

Where

BOD5 applied kg of primary effluent BOD5/d;

(primary effluent BOD5, mg/L)(flow, ML/d); and

Media surface area, 100 m3

Or (in U.S. customary units)

BOD5 applied lb of primary effluent BOD5/d;

(primary effluent BOD5, mg/L)(flow, mgd)(8.34 lb/gal); and

Media surface area,

1000 sq ft

Surface per shaft, sq ft number of shafts

10

000

Surface per shaft, m number of shafts

100

2

Organic (BOD ) load BOD applied, lb d

Area55

/

oof media, 1000 sq ft




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Although RBCs are typically not classified by hydraulic loading, the hydraulic load is

a useful operating parameter and is calculated as follows:

(21.5)

Example 21.2. Calculate the organic and hydraulic loads of an RBC system.

Given:

Q 3 mgd,

Primary effluent BOD5 (TOL) 120 mg/L,

Primary effluent soluble 80 mg/L, andBOD5 (SOL)

Total number of RBC shafts 8 at 100 000 sq ft/shaft. Four of the eight

shafts are in the first stage (four trains of two

shafts each).

Solution:

Total system 100 000 sq ft 8 shafts

Surface area 800 000 sq ft

First stage 100 000 sq ft 4 shafts

Surface area 400 000 sq ft

TOL (120 mg/L)(3 mgd)(8.34) 3002 lb BOD5/d

SOL (80 mg/L)(3 mgd)(8.34)

2002 lb SBOD5/d

System TOL

3.75 lb BOD5/d/1000 sq ft

System SOLb

2.5 lb SBOD5/d/1000 sq ft

First-stage TOL


3002 lb SBOD d

400 units5/

2002 lb SBOD d

800 units5/

3002 lb BOD d

800 units5

a

/

Hydraulic load, gpd sq ftTotal flow into plant, gpd

Surface area per shaft, sq ft number/




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First-stage SOLb


System hydraulic load

3.75 gpd/sq ft

(Notes: aEach unit is equal to 1000 sq ft. bFor system SOL and first-stage SOL, organic

loading exceeds the acceptable limits often used to gauge the ability to operate without

problems. System upgrading may be required to improve performance or prevent ad-

verse effects.

DESCRIPTION OF EQUIPMENT. Rotating biological contactor systems typi-cally include the following six equipment items (many also include instrumentation):

Tankage,

Baffles,

Filter media,

Cover,

Drive assembly, and

Inlet and outlet piping.

Figures 21.8 and 21.9 illustrate various equipment components that are typically used in

the RBC process. The names for individual equipment components may differ slightly,

depending on the manufacturer. The purpose of each part is described in Table 21.6

and discussed further in the following sections.

Tankage. Containment structures or tanks for RBC equipment may consist of metal

tanks for small pilot plants or single-shaft units. However, multishaft units almost al-

ways include tankage made of concrete basins (Figure 21.8). The tank volume typically

provides approximately 1 hour of hydraulic contact time; this typically corresponds to

4.9 L tank volume/m2 (0.12 gal/sq ft) of standard-density filter media.

Baffles. Internal baffling or weir structures separate the stages of the RBC reactors.Baffling along one shaft is accomplished by removing a section of discs and replacing

it with a stationary bulkhead (Figure 21.9). Baffling used to separate multiple shafts

may be made of either concrete or wood. Removable baffles are often used to allow

process changes after the facility is constructed.

(3 mgd)(1 10 gpd mgd)

800 000 sq ft

6 /

2002 lb BOD d400 units

5 /




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Filter Media. The media used for RBCs is composed of high-density polyethylene.

Manufacturers vary the thickness and shape of the material to conform to their ownstandards. Similar variations occur in the shapes and sizes of the shafts and structural

frames used to support individual discs.

Two broad categories of RBC media exist: standard-density media (Figure 21.10) and

high-density media (closer disc spacing). Standard-density media have approximately



FIGURE 21.8 Air-driven rotating biological contactor.


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9300 m2 (100 000 sq ft) of surface area/shaft. High-density media are typically used in

the later or nitrification stages of RBCs and may have between 11 200 and 16 700 m 2

(120 000 and 180 000 sq ft) of surface area/shaft (Gross et al., 1984).Covers. Covers or enclosures are used with RBCs to:

Protect biological slimes from freezing;

Prevent rain from washing off slime growth;

Prevent media exposure to sunlight, which results in algae growth;

Protect the media from UV rays, which can weaken them; and

Provide protection from the elements.

Covers are often made of fiberglass or other reinforced resin plastics. Another ap-

proach involves housing a number of shafts in a building. In either case, the RBC en-

closure must have ventilation, humidity and condensation control, and heat lossprovisions.

Rotating Biological Contactor Drives. The discs can be rotated by either mechanical

or air drive units. Both types of drives have bearings to support the RBC shafts. Every



FIGURE 21.9 Mechanically driven rotating biological contactor.


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RBC shaft has at least one bearing designed to accommodate thermal expansion as the

shaft heats and cools; most shafts have one expansion and one non-expansion bearing.

Mechanical-drive RBCs use a chain and sprocket assembly (Figure 21.9) to rotate

the shaft. The motors, typically rated at 3730 to 5590 W (5 to 7.5 hp)/shaft, may be

equipped to allow changing shims or sprocket sizes and installation of an electronic

speed controller to vary rotational speed.

Air-driven RBC units have a blower and air diffuser at the bottom of each RBC

shaft (Figure 21.8; California State University, 1988). Air cups pinned to the edge of the

plastic disc trap air bubbles released from the air header. As the air bubbles rise, they

cause the RBC shaft to rotate.T

chapter 21 revised_6th edition

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