Anaerobic biological treatment of wastewaters from the pulp and paper industry

Bioveah Advs Vol.2,pp 273-299, Printed

1984 0734-9750/84 $0.00 + .50 All Rights Reserved. Copyright k Pergamon Press Lid

in Great Britain.

ANAEROBIC BIOLOGICAL TREATMENT OF WASTEWATERS FROM THE PULP AND PAPER INDUSTRY L. J. WEBB PIRA. Randal|s Road, Leatherheod. Surrey KT22 7BU. U K

INTRODUCTION

Traditlonallv, affected

wastewater

by physical

(eg combustion svstems

that

of strong

used

processes

for

have

they

treatment

processes

been

have

pulping

treatment

of

effective

slmplv

in the pulp and paper

(eg sedimentation) liquors weaker

and

in controlling

transformed

the various

aqueous

the

in,lustrv has been

and by oxidative aerobic

wastewaters). pollution,

emission

from

processes biological

Whilst

it could

one

form

these

be argued

to another.

For example:

- in combustion compounds - in

of pulp

liquors,

the release

of various

malodourous

sulphur

production

of

amounts

of

surplus

(the so-called

"clean"

to the atmosphere,

aerobic

blotreatment,

the

large

biomass for land disposal.

In terms of "internal" or "'low-/non-waste" proven of

benefits

raw

been

more

strength

encountered

reducing the

and

bacteria

present widely

time, known

(2,3):

than

(i).

these

negative the

and

waters

with

include

problems

of mill water

However,

reclrculatlng

associated

and odour these

volumes/loads

energy

of

problems

abatement

the closing-up

effluent

including

various

of pollution

technologies),

of reduced

materials

temperature systems,

methods

owing in

anaerobic

enhanced

potential 273

systems

to

higher

substant~allv

closed

micro-organlsms from

and organic

of anaerobic

bacteria

benefits

more

of

has

conservation the

corrosion

from sulphldes

aspects

improved

cost

have

sulphate

acids. are

At

perhaps

effective

274

L.J. WEBB

pollution control and energy production from wastes.

Anaerobic differs respect carbon

treatment

of

from

most

-

produces

it

dioxide),

conventional anaerobic

organic

other a

valuable

rather

aerohlc

wastewaters

wastewater

than

but

these

by-product,

pulpin~

options methane

negatlvelv-valued

processes.

processes~

from

treatment

There are

are

and in

gas

solids

benefits

counterbalanced

to

important

(together

surplus

other

papermaklng

one

some

with

as

from

in

using

extent

by

a

number of potential operating problems:

ADVANTAGES

DISADVANTAGES

Net energy production

Sensltivltv to toxic materials and

Low surplus sludge production

shock loads

Low nutrient requirements

Slow start-up

High loadlngs possible

Lack of proven experience and

Intermittent operation possible

The

pros

and

cons

of

using

discussed further in this paper

credibilltv

anaerobic in

the

treatment

technologies

light of previous

will

laboratory,

be

pilot

and full scale experience with pulp and paper industry wastes.

BACKGROUND: SCIENTIFIC AND ENGINEERING ASPECTS

Microbiology and Biochemistry

In the presence of adequate inorganic nutrients a n d a

pure

culture

substrate would new

of

an

aerobic

such as glucose

provide energy

cellular

bacteria

breakdown

to carbon dioxide and water.

for maintenance

material.

can

perhaps trace organics,

It

has

requirements and

been

calculated

a

simple

organic

This dissimilation for the svnthesls of

(4)

that,

during

aerobic

degradation, about 40% of the glucose energy is converted to biomass and the remainder released as heat.

This with and

relatively the

situation

carbon

bacterial

simple

for complete

dioxide.

species

distinct groups

of

transformation

(5),

This but

different

under

anaerobic

conversion requires bacteria

aerobic

breakdown of glucose

cannot

at

conditions

least

(Fig.

be

affected

two

and

I).

As

a

in

by some

result

contrasts to methane one

single

cases of

the

four very

ANAEROBIC BIOLOGICAL TREATMENT OF WASTEWATERS low

partial

acids

pressure

such

as

relationship very

vleld

clearly

hvdrogen

proplonic

to

that

acetic

is

to

the

overall

from

the

conversion

established

(6) that

process of

about

reoulred

acid

between the hvdrogen-produclng

important

energy

of

and

conversion

bvdrogen,

performance.

of

275

the

the

of

svntroplc

acetogens and the methanogens

acetic

70%

for

acid

Despite

low

free

it

has

been

to methane,

the methane

is

the

produced

is derived

from acetic acid and the remainder from carbon dioxide reduction by hydrogen.

glucans glucose pyruvic acid J acetic acid

~

fatty acids

hydrogen/

=

~

carbon dioxide

methane/ y carbon dioxide

GROUP I

BACTERIA

Hvdrolvtlc and fermentative bacteria wblch varletv of metabolic products from the substrate.

GROUP II

BACTERIA

Hydrogen producing acetogens which convert acids above C 2 to acetic acid and hvdroRen.

form a initial

volatile

GROUP III BACTERIA

Homoacetogens which can produce acetic acid from a wide varletv of multi- or slngle-carbon compounds without producing hydrogen.

GROUP IV

Metbenogens which can produce methane from acetic methanol, formic acid and hvdrogenlcarbon dioxide.

Flg.

i.

BACTERIA

acid,

Biochemical Transformation of Glucan Polysaccharlde to Methane

The biochemical

pathways

relatlvelv

well

understood,

still

well

detailed

not

(Co-enzvme

M

and

F420),

used by the hvdrolvtlc but

apart which

the from

route the

appear

from

known to

be

and acetogenlc acetic

acid

involvement unique

to

bacteria are

to methane of

the

two

is

enzvme~

methanogenlc

70

L.J. WEBB

bacteria

(7).

has

It

content of glucose about

equally

however,

been

is conserved

divided

assume

a very

ATP mole -I glucose) ATP

mole -I

the

observed

compared

glucose

(about

that

about

90~

and

heat.

to

the more

the

energy

the remainder

This

from ~]ucose

of

conversion

widely

being

calculation

does,

to methane

quoted

value

(2

of

5.5

(8).

Notwithstanding this, the predicted low -I blomass kg glucose removed) does agree with

0.05 kg

vlelds

biomass

low ATP vleld

mole

(4)

in the product methane,

between

mole

biomass yield

estimated

of methane,

which

are

often

close

to

the

theoretical

value of 3 5 0 % kg -I COD removed.

Process Kinetics

For

most

soluble

fermentation acetic

is

acid.

suhstrates,

the

final

For solid

the initial

hvdrolvsls

ill-deflned,

evidence

rate

of

as

nature

limiting

methane

substrates, to soluble

heterogenous

conflicting

the

stare

step

during

production

particularly

from

anaerobic

intermedlarv

lignocellulosic

materials,

compounds may be rate-llmiting; of

li~nocelluloses

to whether

the

initial

would

the often

account

hvdrolvsls

fnr

the

or final methane

reduction is rate-llmitin~ for such solids (9,10).

Using models

based

on Monod

kinetics,

the

maximum

spec~flc

growth

the methanogens

depends both on the nature of the substrate and

of methanogenic

bacteria present.

of

aerobic

bacteria

methanogens

vary

Compared

(tvpicallv

from

0.01-0.06

0.003-0.03

hr -] growing

population

of active

hr

(ii).

bacteria must

exceed

from

The

of

svstem.

residence

t~me

blolo~ical that, the

is

treatment

under

steadv

reciprocal In

retention

time

equals

time

under

substrate factors

established

svstems.

Using

the

bacteria.

retention

well

net

conventional the

into

accommodate

the shock

design loads,

would of

the

growth

any

retention

conditions be

4-5

dioxide

it

rate

can

of

to

a viable rate

of

the

of (12)

exceed

rate-llmltinR the

solids

minimum

sollds

simple

system or

cell

shown

carbohydrate

incorporation

treatment

fluctuations

he

(where

time), a

mean

operation

time must tbe

digester

The

or

and

retention

with

days.

biological

temperature

rates

mesophilic

growth

time

design

approach, solids

completelv-mlxed

mesophillc

such as glucose

the

retention

for

this

hvdraulic

for

hydrogen/carbon

process,

of

the rate at which they are removed

the

specific

specific growth

values

In order to achieve

solids

state conditions,

of a

concept

now

on

in a continuous

the slowest-growing the

hr-l),

growing

on acetic acid

~acteria

to maximum

0.1-0.2

rate

the species

the

(in

of

safetv

order

presence

to of

ANAEROBIC BIOLOGICAL TREATMENT OF WASTEWATERS l~hlbitorv materials)

necessitates

the use of design

of at least 10-30 davs for stable operation. solids

retention

behind

the

solids

retention

times

in

tradltlonal

digesters

of

new

conf~guratlons,

time

is achieved

with

a

retention

times

The need for high oDeratlonal

development

reactor

277

solids

low

has

been

the

in

hydraulic

motlvat~on

which

the

retention

high

time

and

hence small reactor si~e (see section on Process Englneerln~).

Environmental Factors

Apart

from

the

fermentation,

inherent

amenabilltv

of

the

there are four major factors

organic

substrate

influencing

to anaerobic

the viab~lltv of the

overall process:

Substrate utillse

the

wastewater

temperature)

aerobic

that

anaerobic

the suhstrate for conversion to methane,

determines

volume.

Assuming

Concentration.

As

a

concentration

result

systems,

of

of

new

biomass

the

potential

the

low blomass

a low substrate

energy

are

able

to

the substrate concentration

produced vleld

vleld

concentration

bacteria

from

and per

(together unlt

anaerobic

(
a very high separation efflclencv or retention of suspended

with

wastewater compared

COD ~-I)

to

requires

solids (~99%) to

prevent washout of active blomass (Fig. 2).

100 -

Assumptions Aerobic Anaerohic MLSS Y|(kg kg-lTss removed) eld (gl'l) COO

%TSS

removal 7080

~naerobic ,

0.7

F~g. 2.

Anaerobfc

the psvchrophillc ~

q

1 10 1()0 COD removal (g I"1)

Critical Sedimentation/Retentlon Anaerobic Treatment Plants.

Temperature.

common

is

3.0 0.4 t 10.0 0.1

processes

can

be

Efflciencles

operated

range (<20°C) to the thermophilic mesophlllc

operation

at

30-35°C.

at

any

in

Aerobic

temperature

range (50-55°C), Essentially,

and

from

but the

th~s

is

a

278

L.J. WEBB

compromise nature

between

of

the

psvchrophillc substrate

the

potentlallv

thermophillc

process

temperatures

(13).

concentration

also

anv operating temperature

high

requirements

the

slow

~astewater

determines

(Fig.

heatlnR

against

the

rate

and

of

temperature

potential

sensitive

treatment together

net

energy

at

with

vlel~

at

3).

40

Minimum 30 waste temperature

oC

net energy production

2o,

10

n• t

• n ergy consumption

0 _ 0

i

4

6

8

J

,

10

12

Minimum C O D r e m o v a l

Fig. 3. Effect of COD Removal Balance (assuming 35°C conversion efflciencv). Alkalinltv. important dioxide

The due

alkalinltv

to

the

methanogens wastes,

to DH

the

in the

liquor.

liquor

phase

During

form

of

the

of

and

anaerobic

ammonium

wastes must

For process

within

quanltlties

the acetoge~ic

but carbohydrate

of the feed

of

variations.

alkalinltv

treatment,

and Waste Water Temperature on Net Energy process operation and 70% methane to energy

considerable

produced during

14

(g 1-1)

anaerobic

organic

acids

the poor

of

is

rely o~ the natural

stabilltv,

and

tolerance

treatment

bicarbonate

reactor

the biacrbonate

is

carbon of

the

protelnaceous

released

during

buffer caDacltv alkalinltv must

at least exceed the organic acid alkalinity and should preferably he 2.5-5.0 g

£-i

as

CaCO 3

(14).

The

buffer

capacity

of

low

alkalinltv

should be augmented with lime or, preferablv,

sodium 5~carbonate.

Toxic

Materials.

can

toxic

materlalq,

disturbances problem

of

arises

depression, consumption.

due

All but this from to

biological

processes

be

affected

anaerobic

systems

are

kind

aerobic

systems.

The most

hydrogen

ions

than

accumulation an

imbalance

of

between

undoubtedly

organic

Whilst at one time it was believed

more

and acid

liquors

adversely

bv

sensitive

to

common

toxlcitv

consequent production

that high concentrations

pH and of

ANAEROBIC BIOLOGICAL TREATMENT OF WASTEWATERS acetlc

acid were

toxic

that

a well-balanced

acid

concentrations

are chlorinated cations

at

the methanogens

anaerobic

system

(10-20 g L-l).

hydrocarbons,

low

concentrations

to

can

The

awav

with

from

the methane

methane

undesirable

aqueous

phase.

for growth

producers

production.

equally

either

as

High

30 mg

wastes

for

The

bacteria

with

S £-i),

but

thus

low

at

thus

product

high

relative

reducing

bacteria electrons

produced

gas

sulphide

levels

organic

heavv metal

diverting

sulphide

clear

compounds

concenratlons

in the

higher

high

toxic

metals

as the sulphate

require

is now

free ammonia,

hvdrogen,

hydrogen

it

directly

sulphate

a contaminant

Methanogenlc

(about

treat

alkaline/alkaline-earth

sulphur compounds.

279

7.0-7.2,

prlnclp]e

to that or organic matter are undesirable compete

pH

anionic detergents,

concentrations,

and

at

or

in

is the

concentrations

(>200 mg S £-I)

may

be

inhlbltorv (15).

Process Englneerln~

The traditional of

sewage

anaerobic

sludge

digestion

for many

years,

process

but

been limited by the long retention

has been used

application

to

time required.

for the treatment

industrlal

wastes

This constraint

has

has been

overcome hv a number of reactor designed over the past 30 years (Fig. 4):

Anaerobic aerobic been

Contact

Process.

activated

applied

The

anaerobic

sludge process

on

a

full-scale

to

use

of

shock'

Lamella (18).

Anaerobic 1960's

the

1950's

conventional and

has since

meat-processlng

plants,

The problem of poor solids settlement

separators

(17)

and

bv vacuum degasslng

the

application

of

(16), a

bv the

'thermal

Commercial designs are available.

Filters.

(19)

for

Upflow

the

suspended material, full-scale rotating

from

liquor has been overcome

plate

of

in the

wastewaters

starch factories and sugar plants. the actlvelv-gasslng

analogue

was developed

but this

treatment. biological

anaerobic

treatment

of

filters

industrial

process has been

Commercial contactors

designs

has

also

were

developed

wastewaters little are

been

during

containing

explolted

available. evaluated

to date The

under

the

little

use

for of

anaerobic

conditions.

Upflow

Anaerobic

Lettlnga during

and

the

Sludge

co-workers

1970's

Blanket

Reactors.

at the University

(21) and

relies

on the

This

process

of Wagenlngen formation

of

was

developed

bv

in the Netherlands a granular,

compac~

280

L,J. WEBB

sludge

hed obviating

the need

filter)

or external

settlement

UASB

concept

throughout

is the

now

being

world

and

constructed, mainly for the reactor

also

forms

the

for attachment facilites

to Internal

(as

exploited

commercially

a

of

number

bv

full-scale

(as

in a

process).

The

various plants

of campaign sugar wastes

treatment

second

surfaces

in the contact

stage

of

the

two-stage

process

companies have

(22).

bee~ A UASB

developed

in

Belgium and now being marketed.

ONCE-THROUGH

REACTOR

f_. tl°2 • ".':.'.'.v.-.-.'.

-,-.',-

-

surplus solids

gas

ANAEROBIC

FILTER

g

~ .

Flg. 4.

.

.

U~-FLOW 1 SLUDGE BLANKET

Anaerobic Treatment Processes

Fluidised/Expanded out on fluldised during

.

Beds. bed

During

systems,

the later 1970's

and

the 1970's, much research has

mainly

1980's,

for a e r o b i c for anaerobic

treatment

been carried

(24),

treatment.

but also,

Few,

if any,

ANAEROBIC BIOLOGICAL TREATMENT OF WASTEWATERS Pull-scale

plants

have

been

installed,

hut

commercial

281 designs

such

as

Dorr-Ollver's Anltron reactor (25) are available.

APPLICATION TO PULP AND PAPER INDUSTRY WASTES

Background

Compared to work that has been conducted on wastes from other industries

(eg

food

few

manufacturing

investigations

and

into

agriculture),

anaerobic

there

treatment

of

wastes

Industry, at least until the last 2-3 years. have

concerneA

various

waste

streams

from

from paper and hoard making and, apart classical systems four

digesters

treating

mills

are

(26,27),

there

installing

of

which

be

pulping

no

at

(28-30),

one

installed

for industrial wastewater treatment.

amount of work thus far carried out, 'state-of-the-art' (31-37). the

of

The current

following

anaerobic

of

pulp

the

the

treatment

and

present

plants

in

largest

paper

the malorltv rather

time.

However,

Europe

anaerobic

pulp

this

vear

units

vet small

have reviewed

and

paper

mill

llst

of work

published

the

wastes

(to May 1983) will be summarlsed

a chronological

of

anaerobic

Despite the relatlvelv

of

than

examples

'hlgh-rate'

a number of articles

'state-of-the-art'

sections;

the

operations,

full-scale

treatment

one

from

relatlvelv

Of these studies,

wastes

anaerobic

will

been

from one or two possible

are

pulp or paper mill

have

in

Cs detailed

in Table 1 and performance data in Table 2.

Mechanlcal Pulp/Fibreboard Wastewaters

Few

studies

about of

have

been

the blodegradabilltv

toxic

removal) anaerobic

wood are

of

varlahle

reactor

Helslnkl to

of

extractlves

(40),

but

On the it

has

upon

category,

methane

Loading

perhaps

and due and

vlelds

appear

the work

estimated

anaerobic

(41)

at

this

Darticular

Pilot

scale

presence

(50-g0%

BOD and

to

the

be

well

the Technical

of

closure the

the same as a conventlonal

example).

doubts

type of wastewater

zero operating costs due to the value of the methane in

to

to the known

that

treatment

due

performance

the partlcular

basis of

been

cost of about

this

such effluents

15 m 3 tonne -I with

have a capital

in

(38).

depending

used,

theoretical maximum.

svstem

conducted

trials

of

the

water

wastewater

would

aerobic

produced at

below

Unlversltv

but

(US$3OOk p.a.

this

mechanical pulp/newsprlnt mill are currently in progress (42).

plant,

integrated

282

l.J. WEBB Table I.

Chronology of Anaerobic Treatment on Pulp and Paper Industry Wastes

Date

Type of Waste

Scale of Operation

Country

Reference

19091915 1932 1936 19471954 1948

Strawboard waste

Full

Netherlands

26

Strawboard waste sludge Spent sulphite pulp liquor Strawboard wastewater sludge

USA USA USA

66 71 31,60

USA

43

1949 1952 1952 1952 1958

Straw pulping waste Papermill wastewater Papermill sludge Rag, Rope, Jute pulp liquors Spent neutral sulphite pulp liquor Papermill sludge Straw pulping liquor Spent sulphite liquor Mechanical pulping wastewater Kraft black liquor evaporator condensate Fibreboard wastewater Desugared spent sulphite liquor Pulp/papermill mixed wastewater Kraft bleaching liquor Papermill wastewater Pulp/papermill sludge Fibreboard wastewaters

Laboratory Laboratory Laboratory, pilot Laboratory, (full) Pilot Laboratory Laboratory Laboratory Laboratory

West Germany USA USA USA USA

72 61-63 67 59 31

Laboratory Full Laboratory Laboratory Laboratory

UK France Canada Sweden Sweden

68 27 50 39 39

Full Laboratory

West Germany USA

44 51,52

Spain Finland USA Japan Sweden

28,29,45, 46 73,74 64 55 45

Papermill sludge TMP wastewater

Laboratory, pilot, full Pilot, full Pilot, full Pilot Laboratory, pilot Laboratory Laboratory

Italy USA

69 33,36

Kraft black liquors

Laboratory

USA

33,36

NSSC Pulp/papermill wastewater Sulphite liquor evaporator condensate Sulphite liquor evaporator condensate Sulphite liquor evaporator condensate Sulphite liquor evaporator condensate Paper/board mill wastewater/ sludge Mechanical pulp/newsprlnt mill wastewater NSSC Pulp/papermill wastewater Unbleached Kraft mill wastewater Kraft and sulphite condensates Pulp/papermill wastewater 3 papermill wastewaters Sulphite liquor evaporator condensate

Pilot

USA

48

Laboratory, pilot Laboratory

Sweden

45,54

USA

56

Laboratory

West Germany

57

Pilot

Japan

55

Laboratory

UK

32,34

Laboratory, pilot Laboratory

Finland

40-42

Canada

37,49

Laboratory

USA

53

Laboratory, pilot Pilot, full Pilot Laboratory

USA

36

Sweden Netherlands West Germany

29 30,65 58

1962 1966 1973 1977 1977 1978 1979 19791983 1981 1981 1981 1981 1981 19811982 19811982 1981 19811982 1981 1981 1981 1982 1982 1982 1982 1982 1983 1983 1983

Fibreboard wastewater

ANAEROBIC

BIOLOGICAL

~ulphlte

Pulping Wastewaters

Several

studies

wastewaters

have

been

undertaken

from semi-chemical

pulping.

the

industry

first

pulp and paper

treatment ~31).

in the

work

demonstrated

organics

and

at

loadlngs.

ions)

that

could

(45,46).

The

at

a

reasonable

contrasts

with

in

studies

a

other

full-scale

wastewater BOD d

In a Canadian study a 3 day

retention

sulphlte

liquor

latter.

toxic

the

(50),

Polytechnic

were

in North

very

low 51,

plant

is

albeit

and

at

paper

study

(110-120 Following

now

refractory

being

mill

(310

£ kg -I the

and

low

organic

in the pilot plant

this

values 52).

America

digesters,

(sulphate/sulphIte

pulp

from

one of

for anaerobic

containing

successfully,

yield

mixed

two on the spent

liquors

investigated

materials

Integ=ated

(49,

£

study by in

Spain

kg -I

COD

COD

removed)

successful

constructed

but would also

may have

concentrations

occupy

about

less

demonstrated

to

pilot

treat

a

with

(7-10 g A-I). of

5% of long

the

same

as

the

area

of

the

land

accllmatlsatlon

lack of adequate results

from

in the novel

been caused

by the short

low methane

sulphite/sulphate

period

efficiency

low retention

The

the

less successful

have

spent

or about

treatment;

low removal

SSL (51,52) may

combined

concentrations

a

in the

at 35°C and

25% of a conventional

anaerobic

the

fJltratlon

system

than

that

been a factor Certainly,

ozonated

accllmatlsation

only

for efficient

(eg 49).

by high

cost

evaporatlon/Incineratlon

is necessary

studies

it was shown that anaerobic

time would

study of desugared,

caused

be

and

to about 7000 tonne fuel oil will be produced annually.

work

accllmatlsation

(30d)

but only

wastewaters

be treated

Anamet

(SSL)

lagoon,

This

(150-200d)

calcium

to

liquors

volume of 4550 m 3 d -I containing 150 tonne COD d -I and 50 -I (28, 47). ~le capital cost of this plant is about US$4M but

methane ecuivalent

an aerated

pulp

NSSC pulp

Virginia

strong

methane

reported

other

wastes the

large

removed)

tonne

pulping,

In fact,

This has been confirmed more recently

Biotechnlques

trials,

spent

of any system based on once-through

potentlallv

sodlum/calclum

C

1940's

Within the limitations

this

A

late

on

sulphlte

liquor from full sulphlte

283

TREATMENT OF WASTEWATERS

time

(3.3d)

yields

relative

were

and high probably

to fermentable

organics eg 4.3 g £-I total S and 8 g £-I BOD (51,52).

Kraft Pulping Wastewaters

Until ~d

recently,

been carried

no studies

of Kraft

black

liquor or more dilute

wastewaters

out, bu~ resulr.s from three studies have now been published

Treating

Simulated wastewater Bleach liquor PULPING CONDENSATES Kraft liquor evaporation SSL evaporation SSL evaporation

pulp/paper wastewater NSSC/wastewater pulp/paper wastewater NSSC pulp/paper wastewater Spent sulphite liquor Desugared, ozonated spent spent sulphite liquor KRAFT PULP MILLS Black liquor, diluated Unbleached wastewater

SULPHITE PULP MILLS NSSC pulp liquor NSSC pulp liquor NSSC straw/hardwood

Lab. Digester Lab. Contact Pilot

8.8 BOD 6.0 BOD 20-37 COD, I0-19 BOD

38 35 35-37

Filter Lab. Digester Lab. Contact Pilot

0.5-1.8 COD 20 COD 10-15 COD, 4-5 BOD

35 37

1.8 BOD Digester Lab. 0.5 BOD, I.9 COD USAB Lab.

Lab. Fluid bed Full 30-40

35

Filter Lab. Digester Lab.

58 COD, 8 BOD 4.3 total S

-

27

Digester Pilot 30-32

20-25 35-37

30-32 35-37

30-38

38

Temp (°C)

7.5 COD, I.9 BOD Digester Lab.

1.2 SO I.I BO~

Digester Lab. 3| BOD, 1.9 SO 4 Digester Lab. 18 COD, 6 BOD Contact Pilot

Lab.

-

-

Filter Lab.

Reactor Type Size

0.32 COD 1.0 COD 4-5 COD

0.5-4.7 COD 1.5-2.5 BOD

0.08 BOD 3.4 COD

2.4 BOD

1.1 COD

0.1BOD

0.2 BOD 1.9 BOD 5 COD

5-6 hr retention 2.1BOD 2.2 BOD 1.5 BOD I-2 COD

0.4-0.6 COD

Loadin~ . (kg m-Jd -z)

Pulp and Paper Mill Wastes

Integrated pulp/newsprint mill wastewater TM9 pressate Fibreboard wastewater Fibreboard wastewater Fibreboard wastewater

(g r-l)

Waste Characteristics

of Anaerobic

MILLS 1.5-3.4 COD

Performance

MECHANICAL PULP/FIBREBOARD TMP wastewater

Waste Type

Table 2.

-1

COD

320

300 £ kg

32% COD, 85% BOD

m

1 ~

COD

kg: COD

Variable

95%

50 £ kg -1 COD removed

120 £ kg -1 COD removed

II0 £ kg -I COD removed

310 £ kg -I COD Removed

200 £ kg removed

260 £ kg -I COD removed

Methane Production

90% COD

40% COD 78-86% BOD 38% COD 55-35% BOD 60-70% BOD

49% COD 70% BOD 75-85% BOD 19% COD 36% BOD

85% BOD 67% BOD

70-80% BOD 40-75% BOD 50% COD

50-80% BOD 80% BOD 50% BOD 65% COD, 79% BOD

60-70% BOD

73-50% COD

Performance (% removal)

39 45 54

36 73,74

33 53

50 51,52

49

48

31 31 45,46

33,36 43 44 45

40

39

Re f.

~

12-13 COD, 6 . 3 BOD 5.0 COD

SSL evap/yeast liquor

25 VS 44TS, 32 VS

30-38 VS

Papermill primary

Prlmary/secondary (1:1)

Screen reject/secondary (1:1) Papermill primary

Evaporated wbltewater 2 . 1 30D + yeast extract W a s t e p a p e r - b a s e d f l u t i n g 1 . 9 - 2 . 3 BOD mill Wastepaper-based tissue 1.0 COD mill MILL SLUDGES Strawboard primary I0 DS Boardmill primary 20 DS

Magntfite/Kraft evap. SSL e v a p o r a t i o n 10-20 COD SSL e v a p o r a t i o n 4 . 5 COD SSL e v a p o r a t i o n 4 . 5 COD NON-WOOD PULP/PAPER MILLS Rag pulp liquor 2.6 BOD Rope pulp liquor 2.3 SOD Jute pulp liquor 1.8 gOD Strawboard w a s t e w a t e r 0 . 6 - 0 . 9 BOD Strawboard w a s t e w a t e r 3.0 EOD Straw pulp l i q u o r 40 BOD PAPER/BOARD MILL NASTEWATERS Evaporated whitewater 2.1BOD

SSL evaporation

Waste Characteristics (g t - l )

20 36

Batch Lab. D i g e s t e r Lab.

D i g e s t e r Lab.

Contact P i l o t

C o n t a c t Lab.

D i g e s t e r Lab.

33

USAB Pilot

35

52

52

35

50

45

Lagoon P u l l

30

D i g e s t e r Lab. 30

35 35 35 30 -

D i g e s t e r Lab. D i g e s t e r Lab. D i g e s t e r Lab. " C o n t a c t " Lab. D i g e s t e r Pilot Digester Full

D i g e s t e r Lab.

5.0 COD 3.0 COD 0.3 COD

37 35

1.0-1.9 VS

3.0 VS

4.2 VS

3.1VS

0 . 1 6 VS

4 COD

0 . 4 BOD

1.4 BOD

0.7 gOD

0.43 BOD 0.58 BOD 0.90 BOD 0 . 4 BOD 0 . 6 BOD 2.4 NOD

-

3.2-16 COD

3.5-4.0 COD

Loading (kg m-3d- l )

-

35

52-54

(°C)

Tamp

F i l t e r Lab. C o n t a c t Lab. F i l t e r Lab. D i g e s t e r Lab.

Filter Lab.

Contact Pilot

Reactor Type S i z e s

BOD BOD BOD BOD BOD

60-485 VS

71% VS

50-55% VS

43% VS

39% VS

50-54% VS 565 VS

52% COD, 695 BOD

80-84% BOD

68Z BOD

715 BOD

835 77Z 72X 87g 75Z -

95% COD 90% COD 90X COD

76% BOD

82% COD, 91% BOD 90-79% COD

Performance (% Removal)

Performance of Anaerobic Systems Treating Pulp and Paper Mill Wastes

Waste Type

Table 2 (Cont'd).

530 t total kg -I VS removed - I 690 £ total kg removed 600 t t o t a l kg -1 removed 270 t kg -1 VS removed 314 t kg -I VS removed 220-280 t kg -I VS removed

-

420 t total kg -I VS removed

-

-

-

-

-

360 £ kg -I COD removed 330 t kg -I COD removed

Methane Production

69

55

55

68

66 67

65

64

63

61

59 59 59 60 31 27

35 57 58 58

56

55

Ref.

286

WEBB

L.J.

(33,36,53).

Two

of

the

studies

(33,36)

used

traditional

digesters at low loadings,

but BOD removal was also

interesting

anaerobic

Kraft hour

case

mill

concerned

wastewater

retention

was

low (50 £

due

to

the

time,

(BOD BOD

removal

was

kg -I COD removed).

presence

of

treatment

500 mg £-I)

of a

in a UASB

high

The

once-through

low (35-55%). rather

weak

reactor

(78-86%),

but

but

unbleached

(53).

At

a

the methane

latter was probably at

sulphate/sulphlde,

The most

least

concentrations

13

yield

in Dart

of

sulphur

compounds were not quoted.

Pulping Condensates

Condensates

from

chemical

pulping

represent

attractive

feedstocks

for

anaerobic treatment as the magor part of the organic matter present consists of

simple

chemicals

(sulphlte

methanogenic evaporation loadin~s,

such

pulping),

bacteria. of

as

whlch

Kraft

methanol

can

Onlv black

be one

liquor

(sulphate

converted

pulping)

directly

detailed

study

has

carried

been

or

to

on

the out

when

concentrations

of

acid

by

the

condensate (39);

COD removal and methane yields were generally good,

significantly

acetic

methane

at

from

moderate

but diminished

sulphur-contalnlng

organics

(mercaptans, dimethvl sulphide) were present.

From the data for the various reported

studies of SSL evaporator condensate,

the addition of certain m l c r o - n u t r l e n t s would COD

removal

le

mesophillc

thermophIllc

treatment

compared

the

to

(55)

with

mesophillc

ferric

chloride

sulphide sulphur

up

to

dioxide

treatment

for

in

during

efflclencies

the

wastes (54)

precipitation, blogas

evaporation and

with

added

all

gave

no

with

trace 80%

added

+

were

but. concentrations

still

observed.

of SSL condensates

methane

yields,

but

most

The

may

lead

for high

metals COD

trace

The latter pilot plant work did,

sulphide

4.6%

other

treatment

giving only 32% COD removal.

seem to he essential

(56-58)

treatments

and

removal

materials

however, of

add

hydrogen

carryover

of

to variable

concentrations

(where

measured) seem to be in the range 0.5-1.0 g total S ~--i.

As a result in

Japan

of

the

successful

(55),

an

economic

demonstrated activated

that

sludge

thermophillc plant

completely aerobic system.

pilot

scale

comparison

would

for

anaerobic save

about

trials a

300

of thermophillc treatment -i tonne d sulphlte mill

treatment ~0.bM

followed

annually

bv

a

compared

small to

a

ANAEROBIC ~on-Wood

Apart

from

the

previouslv-mentioned

at

Wastewaters) pulping

an

NSSC

was the major

undertaken

at

anaerobic France

287

BIOLOGICAL TREATMENT OF WASTEWATERS

Pulp/Paper Mill Wastewaters

pulp

and

treating

in the mid-1960's

high BOD removals

(section

(where

the

to effluent paper

mills.

straw pulp

liquor

(27).

(75-87%),

work

Mill

contribution

non-wood

digester

straw

Straw

on

Sulphlte

organic

COD),

this,

to have

board wastewaters

but at low loadings

Pulping

from

few studies

Despite seems

load

have

a

been

full-scale

been

(31,60)

straw

built

have

in

given

(0.4-0.6 kg BOD m -3 d-l).

Paper/Board Mill Wastewaters

Apart from two recent studies

(one unpublished),

no work on paper/board

m111

wastewaters

has been carried out since that by Rudolfs and Amberg at Rutgers

University

in

the

concentration water,

the

early

(1.5

g

effect

investigated

In

1950's

£-i of

as

(61-63).

SO4)

In

sulphate

some

detail.

In

the

reduction The

view

of

simulated on

the

process

progressive

high

sulphate

(concentrated)

increase

in

was shown to have little effect on the acetogenic

but gas

ceased

mg

S £-i.

Treatabilltv

completely of

the white

of sparglng with nltrogen/carbon addition of yeast extract

A

North

American

represents up-ratlng pilot

BOD removal conditions £-i.

the

50%

mills

de-sludglng)

for

wastepaper-based

mllls

weak wastewaters

(down t o

Installed

by a t

least

(65).

facilities

by

an

existing

of

150-200

a combination

sulphide

and the

secondary

fibres

the

are

2 mills.

lagoon

5.5d retention

reduced

US~O.1M

system time,

discharge from

per

in

successful was

80-85% consent

2000 to 60 mg

annum

(aeration

to the original fully aerobic system.

currently

treatment

Good r e s u l t s

1 g COD £ - i )

processes

Following

treatment,

the BOD being

compared

of anaerobic

aerated

aerobic

average

from

(64).

At an average

costs

Netherlands

UASB process

improved

media

the potential

further

with,

operating

In the

of

of

With

are now complied in

was

to remove hydrogen

corrugating

treatment

operation.

Is achieved.

power, nutrients,

of

example

experiments,

Savings

Various

producing

aerobic

to anaerobic

water

dioxide

concentrations

as

phase,

(as an adjuvant).

an excellent existing

scale

converted

mill

sulphide

at

was

sulphide

digestion proceeded production

white

performance

and

investigating of

have

settled

the

wastewaters

been obtained

full-scale

suitability

plants

on are

from

relatlvelv now b e i n g

288

L.J. WEBB

l~I~] 1 S l u d g e s

Anaerohic

decomposition

occurring

in

of

stomachs cellulose methane

breakdown

In

sludges

(67)

the

from

solids

sediments,

ruminating

production

(70).

but

water

of cellulosic

has

been up

pulp

and A

paper

removal

of

study

in

at 50°C

has

solids,

Japan

indicated

savings

sludge

treatment

compared

the

and

in

limiting

step

in

primary

specific at

the

sludges,

5

in

days

clarlfier

of

volatile

thermophilic

digestion

£0.7M

to an

process

sewage

destruction

of a contact

of

of

and

a higher

than

s~tes

occurring

studies 40-60%

natural

of

rate

removal

mesophillc

laboratory and pilot scale performance

anaerobic

be

indicate

between

volatile

to

specific

mills

has shown that digestion lower

disposal

digestion

cellulose

range,

comparison

the

not

70%

is a common

waste

In

shown

to

mesophilic

(66-69).

landfl]l

animals.

with

materials

svstems

methane

36°C.

vleld,

Based

on

system (55), an economic

per

annum

for

the rmophillc

dewatering/incineration

existing

system at a 100 t o n n e d -1 integrated pulp and paper mill.

FUTURE PROSPECTS

Waste Feedstocks

It

seems

likely

processes sludges

is or

that

the other

economics

would

cellulose

present,

least

scope

cake

of

onlv

but also the

potential

pulp/paper

residues.

not

on

the

the

reactor.

with other technologies

70-75%

return

for

mill

For

on

sludge

treatment, content

of

the

moisture,

low

the

and

a

figure

which

investment

treatment.

extractives probably

such

as

As

losses

resin

be a compromise

of

this

little

during

adequate would

acids,

between

also

mechanical

the

maximum

tend optimum

residues

(77).

pulping,

to be fairly closed

anaerobic

of

to install

products from solid mJll

strength

wastewater

type

allows

needed

(and paper) mill system would probably need mixed

of

process

the

produce

a

than

sludge digestion would also have to compete

producing valuable

relatlvelv

anaerobic

for landfill disposal. At -i £16 tonne dry sollds in 1980 for a

eg single cell protein (75), energy (76) or compost

In view

of

rather

costs

on the capital

Anaerobic

application

wastewaters

organic

local

latter averaged

containing

for an acceptable

an anaerobic

greatest

solid

depend

in the UK,

dewatered

tbe

treatment

to

(and

toxic

consumption

strength/temperature

pulp

in order to

temperature)

concentrate

water

the

("low"

for wood would water

ANAEROBIC BIOLOGICAL TREATMENT OF WASTEWATERS consumptlon) mechanical treating solids

and

minimal

pulp/paper

there

rather

than

the unsettled

would

have

a

toxicltv

mills,

high

("high" would

the

llgnln

water

appear

settled

content

289

consumption). be

no

wastewater

as

with

to

a

For

benefit the

consequent

from

suspended

low

rate

of

presence

of

anaerobic degradation.

In

the

treatment

of

wastewaters

sulphur compounds

is a common

the

of

treatability

discussed chemical

paper/hoard

separately pulp mill,

from

potential mill

chemical

pulping,

the

problem;

as this

could

wastewaters,

this

(see section on Sulphur Compounds). there are potentla]

benefits

also affect

subject

will

be

For any integrated

from treating

the combined

wastewater from pulp and papermaking rather than individual streams, vlz: i)

dilution of potentiallv toxic components in the pulp mill wastewater

if) fortification of the paper mill effluent.

This possibility would be particularlv with no liquor burning facilities, have

to

compete

by-product llkelv being

other

but, once more, anaerobic

biological

eg the Pekilo Process.

to be no better slgnlflcantlv

condensates of

with

greater

sulphur

than

COD

to high

compounds

systems

Colour removal

than bv aerobic

is well-suited

volatile

relevant to integrated

rate

may

BOD The

anaerobic

limit

treatment would

producing

a

by anaerobic

systems,

removal.

sulphite mills

removal organic

treatment thus

but

is

normally

composition

treatment,

treatability

valuable

of

carryover

and

micro-nutrlent

the

possibility

addition may he necessary.

For

wastewaters

using

anaerobic

considered.

benefit

would

depend

suspended

that

operating Development weak,

these

with

wastewaters

the

could

to

concentration, In

for

be achieved

(78)

most

With

nature

easily

of

for

is

the

extend

of be

clarified of

wastewater

treatment,

(
organic

largely

increasing

should

the

settleabillty

anaerobic

substantially

dissolved

of

it

by wastepaper-based

systems

processes

mills

and

anaerobic

water

would The

wastepaper-based from starch.

sedimentation treatment

view

viable

closed

practicable

wastewaters.

mills,

primary

present.

wastewaters

from

paper/board

to anaerobic

requirements

viable,

mill

which is derived

on

substantlally

of

sub-30°C

treatable

prior

compared

solids

strength~temperature llkelv

non-lntegrated

treatment

The

wastewater the

from

m3

mills

tonne-l).

treatment the

carbon

carbohydrate,

is

range (DOC) much

of of in of

closure of the water system,

In-mill anaerobic activity converts carbohydrates

to organic volatile

acids,

290

L.J. WEBB

which

can account

systems

are

for

UD

employed,

to

50%

of

sulphate

the

DOC

(79).

concentrations

When

alum/rosln

exceeding

3000

SO4) can be generated {n closed water svstems, but these -I <500 mg £ when svnthetlc neutral sizes are used (3).

The incorporation technique

to

discharge) chemical 20,000

of an anaerobic

facilitate

without

exper~enclng

composition mg

i-I

feed-stock

for

and

anaerobic

of

the

water

the problem of organics' combined (up

temperature

systems

50°C)

svstem

that

possible

zero

(34).

an

to

be one

(le

build-up

represents

(as

reduced

with their high strength

to

provided

can be effectively

can be

£-1

unit at such mills would

closure

of such waters

COD)

from slimlcldes

treatment

complete

sizing

mg

The

(up to

excellent

toxlcltv

problems

controlled.

Sulphur Compounds

Operating

dlfflcultles

experienced pulping

reducing

sulphate. bacteria

co-exlst hydrogen

whilst

notably

a

wide

sulphide hut

removal.

has

At

sulphate-free

successful

molasses-based

electrons

effect

systems

bacteria

on

paper

inhibit

practical

in anaerobic control

has

distillery

terms,

been

effluent

the

contaminated

be an

reactors

in

important selective

might

claimed containing

Production

up

lowering

terms

of

successful

with

high

factor

in

inhibition

be expected (81)

in to

methane

bacteria

production,

efflclencv

wastewaters,

waters may

methanogenlc

methane

from

process

mill

in

chemicals In

can

concentrations.

for

economics.

reducing

sulphate

sulphate

of

use of

and acetic acid bv sulphate

range

least

process

of

be

sulphlte

over use by the methanogena

and

sizing inorganic

levels

could

and

of the widespread

reducing

diverts

and

by virtue

of hydrogen

compounds

from Kraft

sulphate

little

organic

of sulphur those

thermodynamically

high

completely, over

vlelds,

The utillsation is favoured

However,

production

but

bv the presence

wastewaters,

and with paper mill wastewaters

aluminlum

(80).

caused

with pulping

of gas

BOD/COD use

of

levels

of

the of

can

overall sulphate

to be difficult, the

7500

treatment

mg

of

£-i

sulphate

of sulphate

reducing

(as SO4).

The severltv

of sulphide

bacteria would depend mixing

regime.

Sulphide

naturally-present iron compounds

has

toxicity

on reactor

iron been

levels salts

following conditions in many

working

precipitating

successfully

the action

in terms of pH,

applied

digesters

ferrous for

temperature are mlnlmlsed

sulphlde.

the pilot

Addition

scale

and by of

treatment

ANAEROBIC ofsulphlte

evaporator

is preferable the

to allowing

presence

corrosion

BIOLOGICAL TREATMENT OF WASTEWATERS

condensate

of

(82).

(54).

hydrogen

hydrogen

In-situ

sulphide

sulphide

to be liberated

during

When concentrations

291

precipitation

exceed

gas

sulphide

in the blogas as

burning

about

of

1% v/v,

can

accelerate

hydrogen

sulphide

should be removed by scrubbing.

Energy Production

Theoretlcallv, containing

50% methane

is normally Allowing

60-70%

for

reasonable removed

some

i.i

value

of

50% carbon

substrate yield

In energy

the

and

theoretical

assuming

gas),

fermentation

terms,

kg

of

during

COD

each

to

this kg -I

tonne

is equivalent organic

of

waste

and

organic

3256

of

to 13 MJ

solids

GJ -I

matter

is

tonne -I. small

the

methane

yield

For waatewater

fraction

available

to

temperatures

of

that

offset

~35°C,

would

be

temperatures used

during

purchased

the energy

m3

tonne -I

this

COD

kg -I

COD

COD

removed natural £43

of

requirements.

when 35

about

0.4 only

would

For

kg

anaerobic

although

manufacture,

still be available

kg -I

about

or

energy,

pulp/paper

energy

could

of waste heat could be utillsed

The way in which biogas would

9.8

<35°C,

a

(as

converted to methane. For a mill with a specific COD load -i tonne paper produced and achieving 80% COD removal on treatment,

heat,

3506

kg -I

£3

liquor.

waste

is

yield

gas

content

in the

and

treatment

biomass

a

methane

dioxide

biomass

anaerobic

neglecting

produces

The actual

of carbon

conversion

value

carbohydrates

dioxide.

due to the dissolution

methane

(cf

removed). and,

anaerobic

GJ a be

wastewater

if existing

sources

for wastewater heating.

be used at pulp and paper mills

would

depend

on the fuel already in use, vlz: i)

at mills

already

existing

boiler system could be modified

using

gaseous

fuels

such as natural to accept

gas,

part

of

the

blogas or a blend of

natural gas and blogas li) at

mills

installed

using

Combustion

of blogas

burner

content

(83).

or

liquid

to use biogas only,

local requirements

but

solid

rigs

do

fuels,

a

special

boiler

could

be

the steam or hot water being used to serve

such as for space heating. is compatible need

slight

with existing modification

natural due

to

gas the

installations, carbon

dioxide

L.J. WEBB

292 Process Economics

Relatively provided

few

of

the

detailed

studies

estimates

that of other possible treatment

of

recentlv

SSL

(84),

evaluating

the

(principally not

vet

those

using

between

results

feaslbilltv

anaerobic

treatment/dlsposal

studv

and

(32,34)

other

options

falrlv economic

were

provide

costed

for

techniques

programme

water

useful

made

general

on

m~lls

systems)

evaluation

settled

to

been

paper/board

treatment a

have

alternative

closed

a

(aerobic)

have

plant costs for the

from

an

64)

compared

filtration

bv

with

of

55,

from the PIRA research

wastewaters

wastepaper

the

50,

treatment

anaerobic

for treatment

of

45,

Comprehensive by

results

treatabilltv

available,

pre-project

systems.

Whilst

(41,

of anaerobic

condensate

but costs

have not been provided.

above

the cost

treatment

evaporator

published

described

of

are

in

the

comparison

systems.

wastewater

Four wlth

the

following characteristics: Mean flow

1600 m 3 d -I

Maximum flow

i00 m 3 hr -I

Total COD

2500 mg £-i

Temperature

35-40°C

Although the cellulosic mills

could

primary

be

solids present

anaeroblcallv,

degraded

sludge

solids

treatment/dlsposal

in the unsettled wastewater from such

to

the

options

mill

plus

the

present

would

costs

are

still

practise

be

summarlsed

cases using anaerobic treatment% ~t has been assumed achieved

and

that

requirements. solids

plant.

The cost

to the

costs

for

options (NPV) and

from

allowance equipment.

produced

value

of

the gas and

4 are more

should

for

cash be

piping

This

aerobic

stage

benefits of anaerobic

or discounted It

methane

the final

nutrients

3 and

2.

the

cost

of

The

attractive flow

noted,

(DCF)

in

or

but

also

financial terms

return

however,

blogas

could

treatment

produced,

energy.

are

that

of

than the

in

be significant,

Table

recvcllng

3.

surplus

to

than In option

recycled

to

can he seen to

the

analvsls either

The In

the

costs

but would

do

3 as the

to be due

reduced shows net

not

not

operating that

present

both value

options

include

to existing depend

is

process

the anaerobic

the conventional

any modifications

the size and layout of each mill site.

of

economic.

tht 80% COD removal

is

The gas yield in option 4 is greater

surplus

only

all

more

i

any

gas-flred

critically

on

ANAEROBIC Table 3.

(a) Individual

293

BIOLOGICAL TREATMENT OF WASTEWATERS

Treatment/Disposal (1981 Prices)

Costs for Settled Mill Wastewater

Cost Elements

COST Capital

OPTION 1 (£k)

Operating (£k pa): Sewer Chemicals Energy Labour Sludge Disposal Total Operating Gas Value

(£k pa)

(£k pa)

OPTION 2

OPTION 3

OPTION 4

0

700

400

700

192 0 0 0 0

0 30 33 8 0

79 6 0 12 0

0 i0 5 15 0

192

71

97

30

0

0

37

46

(b) Analysis of NPV (Net Present Value) and DCF (Discount Cash Plow) Costs Capital Operating (£k) (£k pa)

Scheme

0 700 400 700

-192 - 71 - 60 + 16

NPV o v e r 20 years at discount rate of 5% 10% 20% -2390 -1580 -1150 - 500

-1640 -1300 - 910 - 840

Incremental DCF rate of return (%) compared to Option 1

Option Option Option Option

I 2 3 4

- 930 -1050 - 690 - 780

16.5 33.0 29.7

Option Option Option Option

1: Sewer discharge of settled wastewater 2: River discharge of settled and aerobically-treated wastewater 3: Sewer discharge of settled and anaeroblcally-treated wastewater 4: River discharge of settled, anaeroblcally-treated and aerobicallytreated wastewater

CONCLUSIONS

Despite paper

a long period when anaerobic mills

intensified pollution effluents, be caused ±his,

was over

control

rarely the

considered

last

5

and energy

years

by the presence

a

of wastewaters

prsctlcable

motivated

production.

the main concern centres

the construction

treatment as

by

For both

on the potential

of sulphur-contalnlng

in 1983 of at

least

from pulp and

option,

research

twin

ob~ectlves

the

pulping

and

difficulties

chemicals.

4 full-scale

has of

papermaklng that could

Notwithstanding

s~aerobic

reactors

294

L.J. WEBB

treatlng various pulp and paper mill wastewaters should establish sufficient credibility and confidence for many others to be constructed in the future.

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