Biofilm reactors in anaerobic wastewater treatment

Biotech. Adv. Vol. 5, pp. 257-269. 1987 Printed in Great Brilain

0734-9750/87 $0.00 + .50 Pergamon }ournals Lid

BIOFILM R E A C T O R S IN A N A E R O B I C WASTEWATER TREATMENT E. R. HALL Environment Canada, |.Vastewater Technology Centre. Burlington, Ontario, Canada L7R 4A6

ABSTRACT Attached biofilm reactors provide the means for anaerobic wastewater treatment at f u l l

scale.

implementing energy-efficient Progress has been made in the

development of fixed, expanded and fluidized bed anaerobic processes by addressing fundamental reactor design issues.

Several new biofilm reactor concepts have

evolved from recent studies.

KEYMQRDS wastewater treatment, anaerobic, biofilm, fixed bed, expanded bed, fluidized bed I - INTRODUCTION Biofilm reactors u t i l i z e a fixed film approach for the development of the high biomass concentrations required for waters.

efficient

anaerobic treatment of

waste-

An inert medium or carrier is added to the treatment vessel and

process is operated to favour the growth of microorganisms attached to the surface.

T h i s physical attachment prevents biomass washout and leads to

microorganism concentrations and long solids

retention times

(SRT).

It

the media high also

allows these reactors to be operated with liquid upflow velocities which exceed the settling velocities of non-attached bioparticies.

257

2s~

ERU\LL

Although r e l a t i v e l y high microorganism concentrations can be achieved in b i o f i l m processes, transfer

the

negative

effects

characteristics

of

the

m u s t also

accumulated

be minmized.

biomass Jewell

on

reactor

(1982)

mass

provided

a

d e t a i l e d discussion of how upflow v e l o c i t y can be adjusted in anaerobic reactors to

control

accumulated

biomass through media expansion.

anaerobic process development which moved from fixed-bed fluidized

bed systems,

illustrates

the

search

microorganism concentrations, without increased

for

The

progression

in

systems, to expanded and

reactor designs with

higher

mass t r a n s f e r l i m i t a t i o n s .

This paper reviews the degree to which these goals have been achieved in recent studies

of

treatment. biochemical

fixed, It

expanded and

fluidized

has been assumed that

beds

for

anaerobic

process optimization through control

and microbiological parameters could be carried out

fashion in any of these reactors.

wastewater

in

of

a similar

Therefore, the following discussion focuses on

the issues of biomass concentration and mass transfer as reported from p i l o t or full

scale

facilities.

For

a more comprehensive discussion

of

anaerobic

technology the reader is referred to reviews by Henze and Harremoes (1983), Speece (1983) and Demunynck et al. (1984). also

summarized the

principles

Jewell (1982) and Heijnen et al. (1986) have of

expanded and

fluidized

bed

anaerobic

biochemistry and microbiology of

anaerobic

microorganisms has allowed process designers to optimize many of the

environ-

technology, while Wilkie et a l . (1984) discussed fixed bed systems.

DESIGN AND OPERATION

Biomass Concentration and Process Loadin9 An improved understanding mental factors However, this

that

of

effect

the

anaerobic

microorganisms

knowledge has not resulted in

(Speece et

any substantial

al.,

1986).

increase

in the

maximum substrate removal rates attained in mixed anaerobic cultures (Table 1). In most biofilm systems, the maximum substrate removal rate of I kg COD/kg VSS.d found by Young and McCarty (1967) s t i l l value have been reported.

applies, even though exceptions to this

Therefore, dramatic

increases

in

the volumetric

loading rates of biofilm anaerobic reactors may only be obtained by developing higher reactor biomass concentrations.

259

BIOFILMREACTORSIN ANAEROBIC WASTEWATER TREATMENT

Table 1.

Reported reactor biomass concentrations and a c t i v i t i e s

Pr__-~__$

VSS ~ n c .

B£msmmActlvity

kg/m

kq COD/kg VSS .d

Fixed Beds (Pilot Scale) Upflow 14.2 0.77 Upflow 5. I 0.23 Upflow 4.9 3.8 Opflow 35 0.14 - 0.57 Downflow 20 0.25 - 1.0 E ~ ~ (Imboratory Scale) 40 60 0.26 - 1.2 F l u l d l a e d B 4 w ~ (Pilot S c a l e ) 11.5 4.3 30 0.17 - 0.83 31.2 1.3 F 1 u l d l s e d Beds (Ft~1 ~ e ) 20 0.9 22 0.9

Approz. f l i t

Referm~e

days

13 43 2.6 173 - 17.5 40 - 10

Young & D a h a b Young & Dahab Donovan et al. Hall 1984 Envir. Canada

1983 1983 1984 (Unpublished)

38 - 8.3

S w i t z e n b a ~ & Danskin Schraa & Jewel1 1984

2.4 60 - 12 7.6

Donovan et al. 1984 Envir. Canada (Unpublished) Heijnen et al. 1985

11 12

1982

Heijnen et al. 1985 Dorr-Oliver (Unpublished)

The nature and size of the biofilm support medium plays a major role in determining the concentration of biomass that can be retained in a fixed film reactor (Young and Dahab, 1983).

For fixed bed reactors, the smaller media used in labor-

atory scale studies (Aivasidis, 1985) may lead to unrealistic estimates of solids retention and process loadings in comparison to f u l l

scale systems.

In addition,

the presence of the medium makes determination of the reactor biomass concentrations d i f f i c u l t .

Nonetheless, several authors have reported data from p i l o t scale

experimental studies which may be reasonable estimates of the values to

be

expected in f u l l scale plants. (Table 1). The results summarized in Table i indicate that fixed fi|m anaerobic reactors have the capability of achieving biomass concentrations that approach those reached by sludge blanket reactors (Henze and Harremoes, 1983). concentrations processes.

(4.9

to

35

kg

VSS/m3)

are

In general, the lowest

associated

with

the

fixed

bed

V e r y high values (40 to 60 kg VSS/m3) have been reported for labor-

atory scale expanded bed systems. However these concentrations have not yet been confirmed at a larger scale. ing since data from f u l l

The fluidized bed systems are p a r t i c u l a r l y interest-

scale plants indicates that the high biomass concentra-

tions observed in p i l o t studies can also be developed at a commercial scale.

26O

E.R. HALL

The product

of

biomass concentration

and a c t i v i t y

from Table 1 provides

indication of the volumetric removal rates achieved in the studies cited.

an

These

values range from a low of 2.0 kg COD/m3.d in a fixed bed system to a high of 49 kg/m3-d in a p i l o t scale fluidized bed. that

it

is

possible to obtain

The difference supports the contention

a large reduction

in the size of an anaerobic

f a c i l i t y by designing for a maximum retained biomass concentration. The biomass concentrations can also be used to calculate an approximate SRT for each system. This was done for Table 1 by assuming a y i e l d coefficient of 0.1 kg VSS/kg COD removed for the removal data reported by the authors.

The daily solids

growth rate was then divided into the reported reactor biomass concentration to estimate the SRT.

The results indicate that the prevailing SRT in a fixed film

process is dependent on both the biomass concentration in the reactor and on the imposed removal rate. indicate

that

Values which ranged from

fixed

film

2-3 days to over 170 days,

anaerobic processes provide

considerable

design

f l e x i b i l i t y using the SRT approach. Mass Transfer Considerations Although fixed film processes can be used successfully to produce high reactor biomass concentrations, an "active"

form.

i t is essential that the accumulated microorganisms be in

Generally,

this

is

understood to

mean that

the

biOmass

actively participates in the metabolism of substrate from the bulk liquid phase. In order to ensure that contact between the microorganisms and the substrate is maximized i t is necessary to design reactors which are optimized with respect to: o

biofilm surface area,

"

biofilm thickness, and,

o

liquid phase hydraulics.

Mass transfer

into

surface-to-volume

fixed

ratio

biofilms

is

high.

will

occur most rapidly when the

Once an attached

film

biofilm

has developed, the

exposed biomass surface area is similar in relative terms to the surface area of the packing.

From Table 2 i t

is apparent that expanded and fluidized bed systems

present substantial advantages in theoretical surface area.

BIOFILM REACTORS IN ANAEROBIC WASTEWATER TREATMENT

Table 2.

261

Surface to volume ratios of fixed film support media

Voidage

Process

S/V

%

(m2/m3)

Downflow Fixed Film

50% - 95%

70 - 100

Upflow modular medium

90% - 95%

85 - 100

Upflow random medium (9-15 cm)

90% - 95%

90 - 200

Expanded bed (0.3 mm)

45% - 55%

9000 - 11000

Fluidized bed (0.3 mm)

50% - 300%

4000 - i0000

The specific surface area provided by most fixed bed packings is r e l a t i v e l y low because of the requirements in these systems for high voidage to reduce rapid plugging.

The surface area contributed by non-attached solids in upflow fixed bed

reactors is not included in the estimates of Table 2. Substrate penetration into a fixed anaerobic biofilm was examined by Henze and Harremoes (1983).

As a result of the high Ks values associated with anaerobic

methane fermentation,

i t was concluded that biofilm diffusional resistance would

not become significant until the film thickness reached values between 0.3 and 1.0 mm.

This was taken to indicate that diffusional resistance was of no practical

significance values.

since

anaerobic biofilm

thickness

J e w e l ] (1985) supported this

was usually

thickness in mesophilic expanded beds rarely exceeded 0.02 mm. systems, the

same author observed film

Heijnen et a l . , bed reactors

t h a n these that

biofilm

In thermophilic

depths of up to 0.17 mm.

Similarly,

(1986) indicated that biofilm thickness in p i l o t scale fluidized

varied between 0.06 and 0.20 mm.

more d i f f i c u l t

less

assumption by reporting

to make in fixed bed reactors.

F i l m thickness measurements are However, observations made in

Canada with fixed bed reactors at laboratory scale (Kennedy, 1985) and at p i l o t scale at the Wastewater Technology Centre (WTC) indicate that biofilms of up to 5.0 mm thickness can develop after prolonged periods of operation. This tends to suggest that over the long term, biofilm diffusional resistance could contribute to a deterioraton in

performance in

periodic solids removal.

fixed bed reactors

which do not undergo

262

E.R. HALL

In

contrast

to

the

discussion

above,

Switzenbaum

(1983)

concluded

that

the

superior performance of expanded and fluidized bed reactors could be attributed to reduced mass transfer surface. bulk

resistance

in

the stagnant liquid

layer at the biofilm

An alternative explanation may be derived from an examination of the

liquid

hydraulic

behaviour in

biofilm

anaerobic reactors.

Regression

analysis of residence time distribution data (Hall, 1985) showed that fixed bed reactors may develop significant quantities of flow bypass (short-circuiting) and dead volume.

Similar non-idealities could not be detected in

a pilot

scale

fluidized bed which was operated in parallel to the fixed bed systems. Tracer studies

in a f u l l

scale Canadian downflow fixed

indicated that the mixing characteristics first

2 years of operation (Figure 1).

film

anaerobic reactor

Non-ideal flow in fixed bed reactors

could be the cause of the comparative result discussed by Switzenbaum.

~,0RESIOEN( E TIME OISTRIBUTIOI~ - THEORETICAL CSTF

2,8-

m

2,62:4-

-

-

.

~.?

IDDII

I~1

2 9 JUNE 1984 6 MAre.r1 ~ o :

i

222,0-I.e-

C~o

1.6-

IA1.2I.O-

.4.2-

"'...

0-' 0.0

i.O

also

deteriorated substantially over the

20

e(+l$) Fig. 1 - Residence time distribution data from a full scale downflow stationary fixed film reactor (Beak Engineering Ltd., 1985).

30

4O

BIOFILMREACTORSIN ANAEROBICWASTEWATER TREATMENT

263

Comparative Performance The l i t e r a t u r e contains

no comparative studies

of the performance of

fixed,

expanded and fluidized bed anaerobic treatment processes. Donovan et a l . (1984) reported the results of a p i l o t scale program in which an anaerobic f i l t e r , an upflow sludge blanket reactor, and a fluidized bed system were compared during the treatment of evaporator condensate from a paper mill in the United States.

It was

concluded that the fluidized bed produced the best effluent quality at much higher loadings than the other two processes. Unfortunately , with the short experimental period available to the authors, the three systems could not be compared directly under parallel conditions. Jovanovic et al. (1986) described a comparative study of p i l o t scale fixed bed and fluidized bed reactors.

A series of influent concentration changes and flow rate

changes were imposed on the p i l o t reactors to study their transien~ and short term pseudo-steady state behaviour.

An analysis of the experimental results shows

distinct differences in the performance of the processes (Figure

2).

COD removal

efficiency in the upflow and downflow fixed bed reactors was found to be affected by both organic loading and hydraulic retention time (HRT). Percent COD removal

I00

SO

40

W~rN.

~T.O.,~ ........ w t i . I ~ ~ wiT.,.oq

•

' .

0

n~rf

WeT.e , v ........ w 4 I . i e v . w~ 4od

r

AmlWON

~

20 I00

/~'~'"

so

• 40 r

IO

S . 1 . .''°'''

U#dm

2

.....

•

0

e

lo

12

vm.o ~ ov

04

ORGANIC

Fig. 2.

16 LOADING

2 (kg

4

6

e

io

12

* a Wen,

1,1

Is

•

le

COO/m~'.d)

Observed relationship between percent COD removal and organic loading rate in p i l o t

scale anaerobic f i l t e r

(ANFIL), downflow stationary

fixed film reactor (DSFF), upflow anaerobic sludge blanket (UASB) and fluidized bed (ANITRON),(Jovanovic et a l . , 1986).

264

E . R . HALL

varied between 60% and 100% under the operating conditions studied. fluidized

b e d , COD removal efficiency

conditions.

Furthermore, only organic loading exerted

the performance of the fluidized bed. it

remained close

to

For the

100% under all

a significant effect on

As in the study by Donovan et a l . (1984),

was concluded that the fluidized bed could h a v e operated s a t i s f a c t o r i l y at

much higher organic ]oadings. RECENT DEVELOPMENTS IN BIOFILN REACTORDESIGN

In the last two years there have been simple, but important changes in the design of upflow fixed bed anaerobic reactors.

Guiot et al. (1984,1986), Hall (1984) and

Reynolds and Co]leran (1986) discussed how the removal or the lower 50% - 75% of the media in anaerobic f i l t e r s could produce a hybrid sludge b l a n k e t / f i l t e r .

The

resulting hybrid design (Figure 3) has the potential of eliminating the hydraulic problems found in fixed bed reactors, while incorporating the advantages of both fixed film and upflow sludge blanket treatment.

The most significant benefit of

the hybrid reactor concept may be the reduced cost of the support media required. Commercially available packing materials contribute between US $ 75 and $ 200/m3 to the capital cost of a fixed bed anaerobic reactor.

The hybrid configuration is

~-':=:~l GASDOME "-#-r

214/

1

i

IV"

V

EMERGENCY OVERFLOW EFFLUENT OUTLET

EVLUENT COLLEC~ON5YSTE.EM

/ / / / / / / / / / / / / / / / /

-h " "su..oR, " <''''':'C" " " " " : ' ' ' ' ' ' , ' ' " " " " ' " " , ".

PERIPHERAL WITHDRAWAL FOR MIXING

,

II

4"~ /"

Fig. 3

STR°CTU.E

'

SUSPENDED

--

GROWTH

,.

ZONE

Schematicof hybrid anaerobic reactor for treatment of thermal sludge conditioning liquor (Crawford, 1984).

..

.. SAMPLING ~ SYSTEM SEEDING AND DRAINING

BIOFILM REACTORSIN ANAEROBIC WASTEWATER TREATMENT

in

use in three f u l l

265

scale treatment plants in Canada (Roe and Love, 1984;

Crawford and Teletzke, 1986) and similar designs have been applied in Finland (Rekunen, 1985) and Belgium. The full scale 2-phase fluidized bed system developed by Gist-brocades (Heijnen et a l . , 1985; Enger, 1986) in The Netherlands incorporates several unique features. To reduce recycle power requirements, the sand used in this plant is smaller than that used in previous full p i l o t studies.

scale systems (Sutton,

1985) and in most published

In addition, the height/ diameter ratio of the fluidized beds is

relatively high - producing biogas shearing of the attached biofilm.

Perhaps of

most interest is the incorporation of a 3-phase separator at the top of the reactor (Figure 4) which functions in a similar fashion to that of a sludge blanket system. fluidized

I t is interesting to note that recent designs of both fixed and

bed processes appear to

be moving toward the UASB configuration

described by Lettinga and Vinken (1981).

Two

stage

anaerobic

fluidized

bed w a s t e

water

,.,o-. ......

~,-

F ll**diled beg

I

A

treatment.

L

~.o,-'~ .......

4._e- .....

o.o

.1

Fluldlzea bed

A

Purified Waste

water

RI A¢ld*|,catton

Fig. 4

R2 Met hl*nahon

Flow sheet of Gist-brocades 2-stage fluidized bed anaerobic treatment process (Heijnen et a l . , 1985).

waslewatef

266

E R HALL

The u t i l i z a t i o n of high rate anaerobic treatment may soon be extended by two additional

developments.

For

applications

involving

toxic

or

inhibitory

wastewaters, the use of adsorptive media as the biofilm support shows promise. Activated carbon has been used in this manner in fixed and fluidized beds (Suidan et a l . ,

1983) and expanded bed systems (Wang et a l . , 1984) for the treatment of

coal gasification effluents.

Fixed film process efficiency may also be increased

significantly by operation at thermophilic temperatures.

Schraa and Jewell (1984)

reported that expanded beds can be operated successfully at loadings of up to 150 kg COD/m3.d.

55°C at

organic

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anaerobic

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BIOFILM REACTORSIN ANAEROBIC WASTEWATER TREATMENT

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