Graeme Rodden, Editor, Pulp & Paper magazine, RISI
Aug. 31, 2007
SCA Östrand mill claims it may have the cleanest pulp mill effluent in Sweden. And, with the installation of a new recovery boiler, air emissions have plunged to new lows as well.
The mill's environmental manager, Roine Morin, points to a combination of factors that have led to SCA Östrand's success. First, there is the totally-chlorine free (TCF) bleaching system using multi-stage, high-consistency ozone. Originally designed to be a closed loop system, scaling problems prevented this dream from being realized totally. However, the volume of effluent water is only 6-8 m3/tonne of pulp. Morin adds that the mill has some unique wash equipment that contributes to low wash losses entering the system. Despite the measures, Morin says that production costs are no higher than for a conventional elemental-chlorine free (ECF) mill.
Finally, there is the five-stage effluent treatment system, the Multibio system from VA-ingenjorerna of Sweden. "We have things no one else in the world has," notes Morin.
The incoming effluent enters a primary clarifier. The effluent is a mix of the mill's kraft and chemithermomechanical pulp (CTMP) effluent as well as that from the nearby M-real Wifsta fine paper mill. Originally scheduled to close in June, this mill remains open temporarily.
Following the clarifier, the effluent goes to a cooling tower prior to entering the five-stage system. From here, the wastewater goes to a final tank where the biosludge is taken out and the water released to the sea.
There are also two tanks for condensate, which is treated by biofilm (like the second stage of the treatment system). Most of the COD in the condensate is methanol, Morin explains, and if it were sent to the main effluent treatment system, there would be a risk of wild growing bacteria because it is so easily biodegraded.
The system treats about 30,000 m3/day of effluent and has an average hydraulic retention time of 12-14 hours. The reduction is equivalent to about 40 tonnes/day of chemical oxygen demand (COD), or more than 70% based on COD loading. COD levels are in the region of 10 kg/tonne of pulp. The mill releases no AOX.
The fish are healthy
Following extensive testing after startup of the system, a recently released report shows that the mill's effluent is not only non-toxic but also non bio-accumulative. Morin says there used to be a problem with fat soluble systems entering the eco chain, a common problem for most effluent treatment systems, but the Multibio process has solved this.
The amount of extractable/gas chromatographable organic material (EGOM), and of potentially bio-accumulating substance (PBS) were not measurable and below the detection limit of 40 mg/L.
The Zebra fish is the standard test fish for the EU for ecotoxicological testing. It was shown that the Östrand effluent had no effect on: growth rate, sex ratio (usually, males predominate if effluent is present), reproduction for two generations (previously, there were reproduction disturbances outside of pulp mills). "This is the first time the Swedish Environmental Research Institute (IVL) has not seen any effects at all," says Morin.
Also, during acute toxicity tests, there was no mortality among crustacean and embryo/larvae of fish exposed to undiluted effluent.
In 2007, SCA expects to produce 420,000 tonnes of bleached kraft pulp along with 75,000 tonnes of CTMP. The kraft pulp production is the allowable limit for Östrand under the environmental permit it has.
In 2008, SCA expects to petition the environmental authorities to allow it to increase kraft pulp production incrementally. For the effluent treatment system, it means adding more fillers in the second stage (biofilm).
"If we want to increase pulp production by a lot," Morin says, "then the floor of the new recovery boiler will need to be extended (provision has been made to be able to do this). And, the effluent treatment plant will need to be bigger."
Since startup, the treatment system has been relatively trouble-free. Earlier this year, there was a problem with filaments. COD efficiency was the same but there was a problem with suspended solids. It was believed that the level of ammonia in the system was too low. When the level of nitrogen (as a nutrient) was increased, the problem was solved.
Morin explains that many effluent treatment systems in Sweden suffer from "winter sickness". That is, one of the reasons it is harder to treat water in the winter is that the logs are often frozen and mills have a higher level of extractives in the system that are also fresher because of the low temperatures.
Low as you can go
Morin does not believe the results Östrand are achieving can be improved, barring a step change improvement in technology. As far as he is concerned, the mill has installed all the best available technology.
If production is increased, then obviously the total level of emissions will rise, but the level per tonne of pulp will remain the same.
"That's why I say we are unique today. If we talk about the footprint from the forest to the print shop, no producer leaves as small a footprint as SCA," Morin states.
When PPI visited Östrand in late 2004, it was promised that the new recovery boiler would start at exactly 11:00 on October 9, 2006. Unfortunately, SCA was late...by 53 seconds.
The Andritz-supplied recovery boiler is now running at a level of 2,400-2,500 tonnes of dry solids/day, far below its capacity of 3,300 tonnes of dry solids (equivalent to 575,000 tonnes/yr of pulp). If the mill does increase pulp capacity substantially, the recovery boiler could handle 4,400 tonnes of dry solids/day (800,000 tonnes/yr of kraft pulp).
The new recovery boiler removed the biggest bottleneck in the mill. However, because the Swedish government awards green certificates for electricity produced by biofuels, SCA also installed a Siemens 75-MW back pressure turbine-generator to use all the high-pressure (105 bar) steam produced in the recovery boiler.
Steam temperature is 515°C and flow is 383 tonnes/hr. High-pressure steam production was maximized to ensure as much electricity as possible could be produced.
Air is fed to the boiler at seven different levels (one primary, three secondary and three tertiary). This helps to attain high combustion with low NOX emissions. Diffuse gases are sent to the recovery boiler and burned as secondary air. Stripper gases are sent to the mill's two limekilns for combustion.
NOX emissions from the recovery boiler are 0.8 kg/tonne of pulp. Total mill emissions, including the limekilns, amount to 1.0 kg/tonne. Dust levels have also been reduced "dramatically", says Morin, from about 65 mg/m3ntg in 2004 to about 15 mg/m3ntg. There have been minimal odor disturbances since the new recovery boiler has come online.
Electrical energy production up 87%
Maximizing the amount of high-pressure steam helped Östrand increase power production from 28 MW to 52 MW. The turbine is fed with steam at 105 bar (recovery boiler) and 57 bar (power boiler). The mill can take out steam at four pressure levels and also has counterpressure steam at 3.5 bar. The first steam extraction is at 27 bar for soot blowing and feed water pumps. The other extraction pressure levels are at 16, 11.5 and 7 bar.
The new recovery island allows Östrand to produce 500 GWh/yr of power. However, because of the CTMP plant, the mill must still purchase 32 GWh/yr. But, as Morin adds, the mill can send all its power production to the national grid in exchange for green certificates and then buy back its power needs.
SCA Östrand accounts for 25% of the power generated by the various hydroelectric power stations along the Ljungan River. Although its turbine is only half as large as the largest turbine along the river, it cannot produce at its full level continually, whereas the 150-MW unit is often constricted by low water flow.
A final benefit is the reduction in fuel oil needed. Now, oil is only used for startups and Morin expects consumption will be about 2000 m3 in 2008. "We produce the biofuel equivalent of 310,000 m3."
What's next for the mill? A few bottlenecks persist. With a bleaching system as closed as Östrand's, more evaporation capacity is always needed. The digester was built for 900 tonnes/day and is now producing 1,300 tonnes/day. Regulatory approval will be needed before the mill can totally address this issue as it will mean increased production.
Otherwise, Morin and his co-workers take satisfaction in their belief that they have the cleanest bleached kraft pulp mill in Sweden and perhaps anywhere.
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