Paper and board production are
among the most water-intensive of industrial processes, consuming an average
of 20 m3/tonne. Waste reduction methods and closed loop recirculation
have already cut consumption in Europe by more than 90%, but are frequently
accompanied by losses in quality and productivity. In the European Commission-funded
PAPER KIDNEY project, a team co-ordinated by German research institute
Stiftung (PTS) explored two innovative purification processes, with
the aim of achieving efficient effluent-free manufacture.
Closed-loop water circuits in a papermaking plant
can be compared to the flow of blood in the human body, which requires
regular cleansing by the kidneys to prevent the build up of toxins and
contraries. In a paper mill, the recycling of recovered paper leads to
an accumulation of contaminants arising from paper additives and foreign
matter such as food particles entering the water flow. These can give
rise to severe deterioration in end-product quality, as well as to productivity
problems caused by slime formation, odour and machine failure. Consequently,
manufacturers are often obliged to reopen their closed systems.
In the three-year PAPER KIDNEY project, a consortium comprising research
institutes, water treatment companies and paper mills representing five
countries co-operated to investigate thermophilic (elevated temperature)
anaerobic and aerobic biotechnological treatments, polishing steps and
membrane technology with the potential to permit trouble-free recycling.
The ultimate objective of this Commission-funded initiative is to close
water circuits in paper and board production as far as possible. However,
as well as causing contamination problems because of 'contraries', closing
the circuits leads to significant increases in process water temperature.
To avoid the energy consumption required for pre-cooling and post-heating,
the aim was to develop advanced process-integrated treatment systems that
would allow reliable purification at higher-than-usual temperatures.
In the first approach,
thermophilic anaerobic 'kidney' technology was combined with an
aeration step and membrane ultra-filtration. The anaerobic process
relies on the action of naturally occurring bacteria that feed upon
the pollutants in an oxygen-free environment. Preliminary pilot
trials carried out by PTS in collaboration with Dutch environmental
protection specialist Paques
demonstrated for the first time that a thermophilic version of this
treatment could provide effective purification over several months.
Excellent degradation rates were achieved at temperatures around
55°C. After parameter optimisation, COD (chemical oxygen demand
- a measure of pollution) could be reduced by 50 to 80%, and sulphate
levels by 30 to 50%. This was despite the fact that the volumetric
and sludge loadings were much higher than those typically encountered
in the routine mill environment.
PTS also showed the advantage of a subsequent aeration step as a
means of removing calcium - concentrations of which could be cut
by more than 50%.
Anaerobic treatment not only gave excellent purification results,
but also proved to have a positive effect on the downstream ultra-filtration
unit by increasing the retention capability of the membranes used.
These trials form the basis for pilot and full-scale testing now
underway at the Oudegem-VPK
paper mill in Belgium. Meanwhile, the exploration of further process
variations continues, with the aim of arriving at an overall concept
for closed-loop plant operation.
The second approach
- thermophilic aerobic processing of the effluent from anaerobic
degradation - was tested on a laboratory-scale by Centro
de Estudios e Investigaciones Técnicas de Guipúzcoa
(CEIT) , Spain, over a six-month period.
COD reduction rates were found to be lower than those for mesophilic
(25° to 35°C) treatment under comparable conditions. Filtration
of the effluent produced water with good characteristics for reuse
- but at present, the approach was deemed to be economically unfeasible
for high flow rates.
First pilot-scale trials of a process comprising thermophilic aerobic
treatment, followed by sedimentation and biomass separation by ultra-filtration,
will nevertheless be conducted later in the year at the SAICA
mill in Spain.
PAPER KIDNEY is due
to be completed in November 2001. Its results to date have been
extremely encouraging, particularly via the first approach. The
performance of the innovative thermophilic anaerobic system, together
with a downstream aeration step, proved to be significantly better
than could be achieved with the mesophilic anaerobic reactors previously
considered to be state of the art in the paper industry. Moreover,
a long-term trial lasting more than one year has validated its operational
Even if this system proves not be cheaper to use in the long term,
it offers strategic advantages. For example, many mills are limited
in their absolute effluent load by discharge legislation - which
can put a brake on production. The kidney approach could make it
possible to increase production while simultaneously keeping effluent
discharge at former levels, or even reducing them.
This method also has important benefits beyond that of facilitating
optimal water reuse. Bacterial decontamination converts the carbon
from any organic compounds present into methane, which can be burned
as a cost-saving fuel for the plant. The ability to function at
a constant higher temperature also shortens paper and board drying
times, potentially increasing productivity by around 5%
Under the Innovative
products, processes and organisation key action, PAPER KIDNEY
is determining the feasibility of minimising paper mills' water
consumption without compromising product quality.
KIDNEY Advanced water treatment technologies for kidney operating
of zero effluent water systems for paper and board production