|
WATER AND WA Published in Pulp Paper International,
January 2004 The timescale of these articles usually extends back
to cover what's been happening on the water and waste front over about the
last 12 months or so, but this time we are going back to what, for most
mills, would have been the pioneering days some 50 years ago. It's likely
that many mills will not be able to recall exactly what they were doing on
the paper machine this long ago, let alone in terms of water and waste
management, so I'm going to tell the story of a fictitious paper mill that
has total recall of its history over this time. It does not matter too much
where the mill is located, but let's assume that's it's somewhere in The mill obtained (and still obtains) its fresh
water from the local river, requiring only filtration and disinfection before
use. With no real incentive to economise on water use at the time, the mill
discharged some 10,000 m3/day to the river from a very open water system,
but at least this kept the not inconsiderable losses of fibre, filler and
starch (in total about 7% of production) down to concentrations of about 650
mg/l total suspended solids (TSS) and 50 mg/l Biochemical Oxygen Demand
(BOD). Wastes were the typical mixture of old felts/wires, packaging, etc and
went to At that time, sedimentation was easily the most
popular option for primary treatment and the raw effluent settled well
without the need for help from coagulants or flocculants due to presence of a
useful excess of aluminium compounds left over from the use of alum within
the process. Shortly after the start-up of the wastewater treatment plant,
one of these irksome ironies that can befall the best-laid plans started to
make itself felt. Up until that time, the river bed had been smothered in a
mixture of papermaking fibres and fillers, which were periodically swept away
after rainfall. All the settleable suspended solids
were now removed leaving a slightly cloudy water
with a low level of dissolved organics for discharge. Unfortunately, the
make-up of the organics proved suitable as a food source for a particular
microbiological community commonly referred to as sewage fungus. Despite its name, this community is usually
dominated not by fungi, but by bacteria. It has since been shown that these
filamentous bacteria in the river can in fact only use certain types of
organic matter within the BOD fraction, notably low molecular mass compounds
such as glucose, the main source of which would be starches. Over the years,
this problem has afflicted a good number of pulp as well as paper mills due
to the presence of wood-derived degraded carbohydrates in wastewaters. It is
a rather more intractable problem for pulp mills as it is difficult to
eliminate such compounds by process modification whereas paper mills can
attempt to do so my improving starch retention. After much chemical detective
work, the mill showed that the mill broke with its high load of
surface-applied starch was indeed responsible for the sewage fungus growth,
albeit aided by the microbial action at the wet end which converted the
non-available parent starches to available glucose and maltose. As a means of
minimising this problem, the mill was one of the first to switch to cationic
starch at the size press due to their better retention in recycled broke, but
other benefits related to the cleaner wet end helped to compensate for their
higher purchase price. This measure reduced the effluent BOD to about 50
mg/l, but this improvement was made somewhat irrelevant a few years later by
the mill's decision to broaden its product mix by converting one machine to
the manufacture of packaging grades from recovered paper. As for other
recycled mills at this time, the environmental aspect of this change (in
terms of minimising the dumping of wastepaper) was not a major factor, but
the mill did have to face up to the reality of using recovered paper in terms
of site emissions. At this time, there was no practical alternative to the
installation of an aerobic activated sludge plant to deal with the higher BOD
loads, although the mill were concerned at the high energy/chemical inputs
required and the high output of secondary sludge to add to the cleaning
rejects. Biological filtration was rejected due to high land requirements. The activated sludge plant selected was a typical
design used at the time for industrial wastewater treatment, comprising a
completely-mixed aeration tank with surface aerators, which is excellent for
coping with BOD load variations from the mill. Unfortunately, this design is
also excellent for growing within the treatment system the same type of
bacteria that used to be in the river causing sewage fungus. As many mills
have since found to their cost, these filamentous bacteria impair
consolidation of the activated sludge (a problem commonly known as sludge
bulking) and, without some form of control, cause the loss of biomass solids
and eventually also of process efficiency in terms of BOD removal. Not surprisingly, resource efficiency appeared to
drop off with the use of recovered paper mainly due to the rejects from
cleaning, which still contained a lot of potentially-usable fibre. However,
the definition of resource efficiency needs a fresh look when what would be
otherwise be a waste is re-used. Most of the primary
sludge was being recycled to the packaging machine with just enough passing
to the existing vacuum filter to ease the poor dewaterability of the surplus
activated sludge (3-4 tonne solids/day). Overall therefore, about 6% of the
mill's raw material input was not being utilised. During the 1970s, the mill struggled along with
periods of excellent performance giving discharge TSS and BOD concentrations
below 30 mg/l, but interspersed with bad bulking periods that were difficult
to control even with lime or chlorine dosing to kill off the undesirable
filaments. The mill continued to expand, albeit with the same absolute fresh water
consumption and the higher TSS and BOD loads necessitated an expansion of
treatment capacity. Although the existing plant could deal with the flow
throughput, it was decided to replace the large primary sedimentation tanks
with a more compact flotation system and also include a small flow balancing
tank to deal with flow variations in the raw wastewater. The use of flotation
improved the quality of the sludge so that it could be re-used on the
packaging machine with no problems. The mechanical aeration plant was
augmented with oxygenation and the opportunity also taken to retrofit a small
aeration compartment ahead of the main aeration/oxygenation tank. Research
had shown that such a tank could help the microbial population in the
activated sludge select against the filamentous types and for the good floc-formers. As with most mill modifications, a number of
different things had been changed all at the same time so it was rather
difficult to be unequivocal about which of these was responsible for the
decrease in the frequency of bulking (but not its elimination) that followed.
It was quite likely that all had contributed in some way - the more stable
flow regime, the less anaerobic character of the primary effluent, the new
selector tank and the absence of any pockets of low dissolved oxygen in the
main bioreactor. The mill's change of direction away from printings/writings
had improved its profitability and so the decision was made in the 1980s to
convert the other machine to fluting/liner production as well.
Despite the greater use of recovered paper, which
now represented the only fibre source, the overall raw material efficiency
stayed constant at about 94% because of the continuing re-use of primary
sludge and the use of the methane from the UASB process to replace purchased
fuels. The wastewater treatment plant was very efficient in terms of overall
removals, but the discharge concentrations were somewhat above previous
levels due to the higher input loads. However, the mill was fortunate in
having a regulator who recognised that discharge loads (rather than
concentrations) determined the environmental effect in the recipient and that
these were still on a declining path. At the start of the 1990s, the mill foresaw a
significant ramping up of interest in its environmental performance because
of the general pressure on the paper industry stemming from the continuing
concerns over chlorine bleaching and the specific pressure on packaging of
all types. Although the mill had considered itself somewhat ahead of the
times over the previous 20 years, not least in setting itself discharge
standards more stringent than demanded by the regulator, it had not got round
to formulating an official environmental policy. As soon as the then new
environmental management system ISO 14001 appeared, the mill quickly realised
that this provided an ideal framework to continue improving its environmental
performance in all areas. As the mill also felt that it wanted to let people
know what it had achieved environmentally, the decision was made not only to
get certified to ISO 14001, but also to the EU's Eco-Management and Audit
Scheme (EMAS) as this imposed additional reporting requirements. On the production side, the mill also saw the
opportunity to move into somewhat higher added-value grades such as white-top
liners, which necessitated getting involved in the technology of deinking. It
was envisaged that, with the better retention of dissolved substances like
starch due to greater water closure, the existing biotreatment
plant could cope with the extra BOD loading from the continuing higher
production. This was born out in practice, but another contributing factor
was the ability of the UASB system to operate at well above its original
design loadings. Of greater potential significance was the big increase in
sludge wastes to about 100 tonne/day, but the mill had been experimenting for
some time with an alternative to landfill for its relatively small quantities
of mixed primary/secondary sludges. This was the application of sludge to
local agricultural land, which being somewhat sandy in nature, benefited from
the presence of fibrous material for water retention, of calcium carbonate
for its liming contribution and from the nutrient content of the biomass. As the 20th century drew to a close, the
mill could look back on nearly 50 years of manufacturing during which it had
increased paper production by over an order of magnitude whilst at the same
time turning its main raw material from 100% virgin pulp to 100% recovered
paper and lowering its emissions to water (see Table 1 The
possibility of something similar has also suggested itself on the wastewater
side due to the worsening problem of calcium carbonate precipitation stemming
from inadequate control of acidification reactions within the machine
systems. Some precipitation occurs within the biological treatment stages,
but the wastewater is still unstable leading to rising TSS levels in the
discharge and in the wastewater recycled to the mill. This problem has been
partially solved by further aeration in a non-biological reactor, but it is
recognised that this issue (plus that of other salts) needs tackling closer
to its source within the paper machines. The future is thus likely to see
greater integration of the wastewater treatment processes (probably new, more
compact versions of what is being used already) within the paper machine
systems allied to better circuit separation to route undesirable substances
(ie anything water soluble) away from the thin stock and whitewater
systems. Even together with the opportunities for in-house use of solid
residues, the overall raw material efficiency figure cannot be bettered that
much, but it should make achieving close to 100% utilisation more
sustainable. Table 1 Summary of mill changes
Note: Raw material efficiency defined as % of total papermaking raw materials being used beneficially in some way |
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
