Effluent treatment heats up at mills

Faced with the application of IPPC (integrated pollution and prevention control) regulations, papermakers are looking at a variety of ways to treat their sludge and effluent

by Leslie Webb

When it comes to sludge and effluent treatment, papermakers really get to try out their own products first hand due to the amount of paperwork involved. The latest regulation to hit the industry in terms of effluent treatment is the integrated pollution prevention and control (IPPC), wich formed the basis for discussion in this review two years ago (PPI, April 1999, pp 29-32). This legislation has been some time coming, having been agreed within the European Union (EU) back in 1996, and is now being applied for the first time. The first EU member state to implement it in practice is the UK with the paper industry having the distinction of being the guinea pigs. Mills in England and Wales had to have their IPPC applications in with the Environment Agency by the end of last February while mills in Scotland have an extra four months to do their homework. The guidance note for implementing IPPC at UK mills is a lot shorter than the BAT reference (BREF) document published in its final form last year as a result of an exchange of information between EU member states. Much of the detail of what is required of UK mills under IPPC comes from the UK's forerunner version of the regulation, the IPC (integrated pollution control). This was introduced in 1996 and has now been amended and extended to cover all of the IPPC issues.

IPPC and water

Optimized water management remains a key IPPC issue, embracing a number of essential measures where the term "materials" includes energy as well as substances. Pulp and papermakers need to keep an eye on the following areas:

 ï minimizing the introduction into the papermaking system of materials that would impede closing up of water systems and that would increase the requirements for wastewater treatment
 ï operating paper machine systems to minimize normal and (abnormal) accidental losses, ie
  - minimizing transfer to the paper machine system of problematic materials
  - efficient retention aid programs, including inactivation of interfering substances
  - minimized fresh water use through optimized circuit design and equipment selection
  - integrated recovery systems for water and non-retained materials
  - efficient process management in terms of control systems and plant maintenance
 ï use of minimum input wastewater treatment processes:
  - to facilitate closing up across the whole mill
  - to recover non-retained materials for re-use on-site or elsewhere
  - to provide adequate containment of accidental releases
  - to comply with discharge standards.

The first measure is largely in the hands of pulp suppliers (for virgin pulps) and of papermakers themselves (for recycled pulps), but is outside the scope of this particular article. Materials that cause some problems in papermaking usually cause even more problems when the water system is closed. Finding the best way to deal with these materials in the system is important as eliminating such materials from the incoming pulp is not usually possible. One way is to keep them confined within a small part of the total system and well away from the paper machine itself by careful segregation of water circuits. This concept is not new, but bears reiteration as it is taking some time to gain acceptance. One of the machinery suppliers, Andritz in Austria, is trying to push this concept for mills integrated with either mechanical pulping or deinking through the use of a wash press in place of a disc filter or decker after the final bleaching stage. As well as keeping the dissolved and colloidal solids away from the papermaking circuits, the system produces a lower volume stream with high dissolved solids concentrations and temperature, opening up new possibilities for treatment as a result.

Stora Ensoís Kabel mill has installed a Hadwaco MVR pilot plant in Germany

Andritz has formed an alliance with the Finnish company, Hadwaco, to combine improved pulp washing with a process (evaporation) that can concentrate the wash water stream even further, generating clean water for re-use and a concentrate for disposal. Using mechanical vapor recompression (MVR) evaporation, pilot scale tests have been carried out at two mills so far, one of them being the stone groundwood mill of Stora Enso at Kabel in Germany. This process stream has an appreciable concentration of suspended solids as well as of dissolved organics and dissolved salts, so pre-treatment with flotation/filtration was necessary. At a concentration factor of 20 to give a concentrate with 11 percent total solids, the evaporator removed about 95 percent COD (chemical oxygen demand) and 99 percent salts. The residual COD included a contribution from methanol in the first stage condensate, which could be separated to improve the quality of condensate for re-use. A further concentration to 50 percent total solids was achieved in a steam driven, forced circulation evaporator so that it could be burnt in an incinerator such as the pressurized, oxygen-based Conox unit. Projected costs for COD removal are favorable compared to conventional aerobic biological treatment.

Getting into hot water

The temperature of combined wastewater from mills using a large proportion of mechanical pulp is high as most of the input pulping energy (approximately 5GJ/tonne pulp) is dissipated as heat. Even with the much lower thermal inputs to the water system that are typical of papermaking (0.2-0.8 GJ/tonne), process or wastewater temperatures well above 40°C are easily attainable. Biological treatment of such wastewater is not normally attempted due to fears about process instability, which, in the case of aerobic plants, is caused by the adverse effect of high temperatures on biomass diversity. As a result, cooling systems are required to bring temperatures down to within the range for mesophilic operation. These can be either of the normal cooling tower design or heat exchangers using open or closed cooling circuits. Aerobic biotreatment itself can function as an efficient cooling system during colder months, but may not be adequate during warmer periods.

This historic coolness toward biotreatment at thermophilic temperatures has begun to break down in recent years for both aerobic and anaerobic processes. A thermophilic anaerobic plant (an IC reactor from Paques) was installed a couple of years ago at a Dutch board mill, which already operated with a fully closed water system running at 50-60°C. A number of research studies on thermophilic aerobic treatment have shown promising results:
 ï Finnish work on a groundwood process water at 59°C using a suspended carrier reactor gave 40 percent COD removal at very high loadings in a single stage process and nearly 60 percent COD removal in a two-stage process
 ï Canadian work on bleached kraft mill effluent using sequencing batch reactors showed that COD removals were lower at thermophilic (55 and 60°C) compared to mesophilic temperatures. The thermophilic wastewater contained pinpoint flocs increasing the final TSS (total suspended solids) from 20 to 80 mg/l
 ï Canadian work on kraft evaporator condensates using a membrane bioreactor demonstrated that maximum COD (methanol) removal occurred at 60°C and costs were estimated to be considerably lower than for steam stripping of methanol.

It is noticeable that all of the three studies cited above have used recently-developed incarnations of the activated sludge process, where the already-high loadings can be pushed even further at higher temperatures to give very compact treatment units. In last year's article (PPI, April 2000, pp 37-43), the first membrane bioreactor (MBR) version of the activated sludge process at Papeteries du Rhin in France was described. It has not taken long for the second one to get going, this time at the small Van Houtum & Palm (VHP) mill at Ugchelen near Apeldoorn in the Netherlands. But this installation is different in one important respect, namely that it is being operated as a thermophilic process at around 55°C. The use of membranes for biomass separation removes one of the potential risks associated with thermophilic operation in conventional activated sludge plants, namely that of poor biomass settlability.

Figure 1 - MBR Treatment at VHP's Mill in the Netherlands

The work that led to this installation was part of a larger program of work conducted by TNO in the Netherlands on techniques to facilitate closure of mill water systems. The work on MBR processes also involved pilot scale trials at two other paper mills in the tissue and linerboard sectors. The VHP mill produces about 5,000 tonnes/yr of bank note and other security papers using cotton as raw material. The bleaching process generates a wastewater with a moderate COD level (average 3.5 g/l) and high temperature (75-85°C) and pH (11-12). Fresh water use is also high (100 m³/tonne paper) as is the energy use for heating the water for bleaching (about 10 m³/tonne) at about 25 GJ/tonne pulp. The treatment concept using MBR aimed to produce a warm water capable of replacing most of the cold fresh water used for bleaching, which would also reduce some of the energy input (Figure 1). The pilot trials showed that treatment efficiency was optimal at about 55°C, but declined both above and below this temperature. The full-scale plant has a bioreactor volume of 250 m³ to treat a COD load of just over 1 tonne/day. Unlike the MBR at the French mill, the membrane unit is located outside the bioreactor and has a surface area of 82.5 m² for an influent flow of about 290 m³/day. The net flux on the membranes is similar to that at the French mill. Since its startup in November 2000, the plant performance has exceeded that of the pilot unit producing an effluent with COD 450-500 mg/l and enabling 80-85 percent of the treated effluent to be recycled.

Electrocution treatment

The consumption of electrical power in wastewater treatment can be significant, particularly for the aeration stage in the activated sludge process and for general pumping between treatment stages at mills with high specific wastewater flows. Usually, this is about as close as electricity actually gets to the wastewater itself, but the application of electrochemical processes to wastewater treatment is beginning to get off the ground. Two electrochemical treatment processes have been operating at two small UK mills over the last 12 months. The larger of the two is being applied to the wastewater from the Charles Turner tissue mill near Bolton, which has recently installed a new tissue machine and associated deinking plant. The mill used to have a conventional activated sludge plant following primary sedimentation, but this had to be dismantled to make way for the new paper machine. Instead of simply replacing the old plant with a new one, the mill decided to go for a relatively new and, at least on this scale, untested process using a very different technology from Axonics.

After initial experimentation with electrochemical treatment of the raw wastewater, which worked but involved high chemical costs, the treatment sequence is now as shown in Figure 2. The first electrochemical cell uses iron electrodes and generates various free radicals that effect the oxidation of the dissolved organics. Although this reaction is very fast (less than one second), a short retention time of 20 minutes is provided after this stage and before the second electrochemical cell, which generates aluminum hydroxide from the aluminum electrodes for coagulation of residual suspended solids. The final flotation stage is effected through the hydrogen gas generated in the second cell. Carry-over of plastics from the deinking plant has caused some problems with cell operation, but these have been resolved through better upstream treatment and cell re-design. The incoming COD has been lowered from 250-350 mg/l down to as low as 40 mg/l with very effective removal of any color from residual dyes. A key to the operation is the control of the power consumption in relation to wastewater COD and conductivity, but this is typically no more than 100 kWhr/day. Both sets of electrodes do, of course, dissolve away over time and have to be replaced about every three months. The operation of the plant is not yet fully optimized and some aspects of the operation cannot be divulged due to commercial confidentiality.

Figure 2 - The Treatment Process at a Tissue Mill

The other application of electrochemical treatment is at the Chartham mill of Arjo Wiggins Fine Papers, which has been evaluating a similar approach developed by Free Radical Technology (FRT). This process also generates free radicals in the water being treated by means of one or more electrochemical cells and, as in any such technique, this may activate other ions in the system such as the oxidation of chloride to hypochlorite and oxygen to ozone. The process has so far been applied to deal with a problem that is not uncommon at many mills, namely odors from sludge handling. The mill has a fairly conventional treatment process involving primary sedimentation followed by biological filtration with the surplus sludge being screw pressed. The press filtrate is returned to the primary treatment stage, but often in an anaerobic state which adversely affected process efficiency. Chemical treatment by chlorination was unable to rectify the problem. Electrochemical treatment was followed by retention for about 40 minutes in a contact tank. Development of anaerobic conditions in the filtrate was prevented, odors eliminated and a substantial (over 1,000 times) reduction in its microbial content achieved. Consistent efficiency has been obtained at an applied voltage of 5-6 volts with a current of 40 amps, ie power consumption of about 5 kWhr/day.

One other technique involving the passage of an electric current through the wastewater is electrodialysis, which utilizes charged membranes to separate anions and cations into a neutral concentrate. The research group at TNO has again been looking into this technique for removal of salts (chloride, sulphate) from closed system wastewater. Results were promising, but there is no full-scale application yet. It looks as though mill specialists in wastewater treatment, having become sort of experts in aquatic microbiology, will now have to start brushing up their electrochemical know-how.

Leslie Webb directs the activities of Envirocell and can be contacted on telephone/fax +44 1372 276599, by email leswebb@envirocell.co.uk or on the web www.envirocell.co.uk