New regulations governing effluent impact are forcing mills to scrutinize the performance of their wastewater treatment plants

by Leslie Webb

Wastewater treatment: regulations, bugs and beds

Fiber, energy and water are the three indispensable materials needed to make paper, and the management of each is critical to the industry's move to long-term sustainability. Of the three, water management in a sustainable fashion should be the easiest task as the resource is renewable without any input from man, thanks to the natural hydrological cycle. The sustainability issues for the pulp and paper industry in this area therefore tend to be local rather than global in nature and include:

• avoiding depletion of ground water levels
• ensuring that wastewater discharges are of adequate quality
• re-using process waters as intensively as practicable, but not at the expense of increased usage of other non-renewable resources, eg energy for more pumping.

Considerate Clusters

Albeit with some trepidation, the most eagerly-awaited regulations concerning discharges to water have been the US Cluster Rules, particularly that part dealing with discharges from bleached chemical pulp mills. When phase one of the rules was finally published in April this year, a perceptible sigh of relief could be heard from the industry as they got pretty much what they had been arguing for over the previous four years.

This legislation is not aimed at improving controls over parameters such as total suspended solids (TSS) and biological oxygen demand (in fact, these limits were left unchanged), but at introducing limits for a host of new parameters centered on the use of chlorine bleaching compounds, eg AOX, adsorbable organo-halogen compounds and a range of chlorinated organics.

In setting the AOX and other limits, the USA's Environmental Protection Agency (EPA) decided that the use of modified cooking or oxygen delignification along with 100% chlorine dioxide substitution does not represent "Best Available Technology" (BAT) for existing bleached kraft mills, but that it does for new mills.


Figure 1 - Potential Impacts from Wastewater Discharges

Straightforward elemental chlorine-free (ECF) bleaching is therefore equivalent to BAT for existing mills. The standards are set as both daily maxima and monthly averages, the latter being the more stringent of the two (Table 1). For bleached sulfite mills producing paper-grade pulp, the BAT and NSPS (new source performance standards) limits are the same, both requiring totally chlorine free (TCF) operation.

An interesting feature of the rules is that mills can volunteer to meet one of three advanced-technology tiers (achieving more stringent AOX standards), for which they are rewarded with a longer-duration permit, reduced monitoring requirements and inspection frequency, and participation in some form of public recognition program.

By contrast, interest in the use of AOX for regulation in Europe peaked in the late 1980s to early 1990s and is no longer used as an indicator of real environmental effects at the current levels of discharge (generally < 0.5 kg AOX/ton pulp at ECF mills). Over the last few years, the potential acute and chronic toxic effects of wastewaters from ECF and TCF mills have been widely studied in North America and Scandinavia.

Due to the degree of treatment provided, the differences between raw wastewater chemistries and, in particular, between the biology of the receiving waters, it is important to be careful about generalizing conclusions between apparently similar mills.

Due to the presence of wood-derived substances (eg resin acids and plant sterols at all mills), residual bleaching chemicals (eg chlorate at ECF mills) and bleaching byproducts (eg chlorinated organics at all mills using chlorine compounds), toxic effects can be seen at any mill that has no wastewater treatment - depending on dilution, etc.

At mills with biological wastewater treatment, it is still possible to see effects on fish populations, but these are usually chronic (eg on reproductive functions) rather than acute.

Pollutant transfer

The Cluster Rules are not just about discharges to water, as they also introduce controls on emissions of hazardous air pollutants (HAPs) for the first time. For kraft mills, the most important HAP is methanol, which is generated in pulping and accumulates mainly in the condensates from evaporation of black liquor. The conventional approach to methanol control is steam stripping with the methanol being burnt in the recovery boiler/lime kiln or in a dedicated incinerator.

The alternative is to treat the condensate in an existing biological treatment plant. But there would need to be substantial spare capacity in view of the high loads from this source (10 kg methanol/ton pulp corresponding to 15 kg COD/ton). As steam stripping is very expensive in terms of both capital and operating costs - $10 million and $5 million/yr respectively - alternative approaches are being sought. For now, anaerobic biological treatment in a dedicated reactor looks like a top contender to achieve the BAT limits.


Figure 2 - Activated Sludge Plant Designs

Anaerobic treatment has been used for some years at a few sulfite mills to treat sulfite evaporator condensates, but their chemistry (no methanol and lots of acetic acid) is completely different to that of kraft condensates. Pilot-scale work on anaerobic treatment of kraft condensates (published at this year's Tappi Environment Conference) has been carried out by Boise Cascade at three of its kraft mills. It is using what is now the traditional approach to anaerobic treatment, namely an upflow anaerobic sludge blanket (UASB) reactor and a newer high rate version, Paque's internal circulation (IC) reactor.

The breakdown of methanol by anaerobic bacteria is relatively simple compared to breakdown of a carbohydrate-rich waste stream as no hydrolysis and acidification steps are required, the reaction being simply:

4CH3OH --> 3CH4 + CO2 + 2H2O

This work has confirmed the ease of methanol removal (98-99%) by anaerobic treatment at high reactor loadings, although overall efficiencies were somewhat lower at around 85% BOD (biological oxygen demand) and COD removal. At one mill, the addition of E-stage bleach filtrate maintained efficiency during hardwood, but not softwood, pulping. At another mill, the addition of pulping liquor impaired efficiency, emphasizing the need for control of potential contamination from other streams.

The capital cost for anaerobic treatment at each mill (production rates 700-950 tons pulp/day) is estimated at $5 million, much lower than for steam stripping. Plus there are effectively zero operating costs due to the value of the methane generated.

Not just BOD removal

The traditional role of biological treatment is the removal of BOD that would otherwise consume oxygen in the receiving water. The above application of anaerobic bio-treatment is an example of the increasing number of cases where this is not necessarily the prime motive. The application of anaerobic/aerobic treatment at Zulpich Papier's mill (see PPI, 1997, June, p.43-46) is another example. In this case BOD removal is required, but the removal of sulfate (in the anaerobic stage) and calcium (in the aerobic stage) are valuable side benefits at this fully-closed mill.

Whereas aerobic bio-treatment processes are effectively blind to the presence of sulfate, anaerobic treatment processes are not and sulfate is usually considered to be a nuisance as it is inevitably converted to sulfide. However, as demonstrated at Zulpich, this can be an advantage if there is a need to remove sulfate, which could be the case for mills discharging to a municipal treatment works or where, as at Zulpich, the treated effluent is recycled. The presence of sulfate is not a problem when wastewater is discharged externally provided that the receiving water does not itself become anaerobic.

As part of a substantial research program into improving the quality of its effluent for internal recycling, Shotton Paper in the UK has also investigated the use of anaerobic treatment for sulfate (and BOD) removal (see Environmental Technology, 1998, 19, p163-171). The primary wastewater has a relatively low chemical oxygen demand (COD) to sulfate ratio at 3.4:1, the disadvantage of which is usually a depression of the methane yield during anaerobic treatment.

This was confirmed in the pilot-scale work at Shotton, where the methane yield was only 60% of the theoretical value in the absence of sulfate. Nevertheless, the treatment performance in the UASB stage was around 65% COD at retention times of some five hours with approximately 95% sulfate removal. Residual sulfide in the UASB effluent (70-100 mg/l) was effectively removed in an aerobic sulfide-oxidizing reactor, but a substantial proportion was oxidized back to sulfate rather than the desired end-product of elemental sulfur.

Anaerobic treatment has a low requirement for nitrogen (N) and phosphorous (P) nutrients. This is an advantage at most mills, due to their virtual absence in the raw wastewater, but a disadvantage at the few mills with an excess of nutrients. The removal of N and P compounds in all bio-treatment processes is simply by incorporation in the biomass. So although high biomass production is undesirable from a disposal viewpoint, it is useful for excess nutrient removal.

Aerobic bio-treatment is an excellent biomass producer, but the actual amount depends on process loadings (notably the biomass or sludge age) and the extent of grazing by higher micro-organisms. Although only carried out on a small scale, research in Sweden has shown that it is possible to optimize growth of the latter so that biomass production is reduced to half the normal level.

N and P nutrients that remain in mill wastewaters may cause eutrophication, and nitrogen in the form of ammonia can be toxic, notably in slow-moving waters like lakes. Fortunately N and P compounds are scarce in most mill wastewaters, but pulping wastewaters contain enough natural, wood-derived phosphorous, although usually not enough nitrogen, for bio-treatment. An exception to the latter is the wastewater from sulfite pulp mills using an ammonium base. Fortunately, there is a very effective way of removing the excess ammonia in a harmless form. This is the nitrification/denitrification system as employed by the Port Alice mill of Western Pulp in Canada.

In the mill's activated sludge plant, the ammonia is converted to nitrate biologically by operating at a low loading (sludge age 10-20 days). After settlement, the nitrate-containing activated sludge is recycled normally. But instead of immediately aerating it with the raw wastewater, an anoxic zone is provided where the nitrate oxygen is used by the bacteria instead of dissolved oxygen. In sewage treatment where there is always an excess of nitrogen, this system is widely used as not only is the ammonia transformed to harmless nitrogen gas, but it also reduces the energy input for aeration. The anoxic zone can act as a so-called selector against the growth of undesirable filamentous bacteria.

At the Canadian mill, the activated sludge treatment uses an anoxic zone accounting for 20% of the total treatment volume and removes some 97% of the incoming ammonia.

The best selector

There are few activated sludge plants at pulp and paper mills that have not experienced what is the Achilles heel of this treatment system - namely the problem of sludge bulking. This occurs when the active microbial suspension is unable to consolidate adequately for return to the aeration tank and eventually suspended solids start to overflow with the treated wastewater. These solids have a much higher BOD than mill suspended solids and are therefore more troublesome in receiving waters.

Although not the only cause, bulking is largely due to the excessive growth of long, stringy filamentous bacteria that prevent good packing on settlement. Selectors are supposed to function by providing conditions whereby the microbial "goodies" (the floc-forming bacteria) are able to grow faster than the "baddies" (the filaments). There are three main types of selector:

• aerobic selectors which are aerated
• anoxic selectors which are not aerated, but stirred with nitrate present
• anaerobic selectors which are not aerated, but stirred with no nitrate present.

A recent survey at North American mills with activated sludge plants has shown that the predominant cause of bulking appears to be septicity in the incoming wastewater, which generates sulfides and organic acids used by filamentous bacteria. Septicity can be caused by hold-up in sewers and in primary settlement clarifiers.

Other problems related to septic conditions in primary settlement tanks have become more prevalent in recent years and this fits in with the observed big increase in septicity as the apparent cause for bulking. A corollary of this observation is that mills having flotation as the primary clarification stage should be less troubled by bulking, but there is no clear data to support this.

One recent development in activated sludge technology is the sequencing batch reactor (SBR) system, where the BOD profile in the reactor is similar to that in plug flow aeration tanks (Figure 2). SBR systems, plug-flow aeration lanes and completely-mixed tanks with selectors all impose a high initial loading on the biomass and a steep BOD gradient in space and/or time, which works against filaments. The Canadian industry still seems to be setting the pace in the adoption of SBR plants with at least four mills in the Abitibi-Consolidated group having chosen this route for complying with the tougher BOD standards introduced in the 1990s.

Given the headaches caused by bulking in activated sludge plants, it is perhaps surprising that other aerobic bio-treatment processes are not more widely used. Fixed film bio-reactors of various possible types have never proven that popular in the pulp and paper industry, but recent innovations in the technology are beginning to change that position.

In traditional fixed film systems such as percolating or trickling filters, oxygen is provided by a natural draught of air passing through the structure, but this limits the design possibilities to open filters with relatively low volumetric loadings. This has been overcome in numerous designs by what are known as biological or submerged aerated filters. These are particularly suited to the treatment of dilute wastewaters (BOD <100 mg/l) which are not ideally suited to activated sludge treatment.


The Kaldnes moving bed bioreactors Norske Skog's NSSC pulp and fluting paper mill at Sande in Norway

One design of reactor is the moving bed process developed by Kaldnes Miljoteknologi (part of Anglian Water's Purac subsidiary) in Norway. This process uses so-called carrier elements comprising small (7x9 mm) polythene pieces, which provide a surface area of about 330 m per m of reactor volume for the bacteria to attach themselves to. The reactor is aerated from bottom diffusers. Depending on the TSS quality of treated effluent required, there is sometimes no secondary clarifier provided as most of the biomass is attached to the carrier.

So far, six plants have been installed at pulp and paper mills in Europe, North America and Australia. Two of the installations managed to minimize installation costs by utilizing existing tanks.

Depending on the BOD quality required, loadings up to 35 kg COD/m.day are possible. This compares with loadings of no more than 5-10% of this level in normal activated sludge treatment. However, due to the cost of the carrier elements, high loadings are necessary in order to be competitive against conventional activated sludge treatment. Performance data for some of the plants is as follows:

• at the Sande NSSC/fluting mill in Norway, 70-80% soluble COD and 95% Microtox removals with a sludge production of about 0.33 kg/kg COD removed
• at the Stora Grycksbo fine paper mill in Sweden, 75% soluble COD and 92% solu- ble BOD removals
• at the Appura recycled tissue mill in Germany, over 90% COD removal

   

• at the Ivex recycled packaging mill in the USA, 70% COD and 93% BOD removals.

While the US industry (or at least a large part of it) now knows what it has to do (and spend) to comply with the Cluster Rules, the European industry awaits its own destiny in the guise of the directive on Integrated Pollution Prevention and Control (IPPC).

IPPC has a much broader focus than the Cluster Rules (for example, it covers prevention of accidents and site energy efficiency) and should be able to deal with all trade-offs, not just those between air and water emissions. An IPPC bureau was set up in 1997 at one of the EU's research centers in Sevilla, Spain. It has established a technical working group for the pulp and paper sector and is in the process of producing what is called a BAT Reference (BREF) document, which will provide guidance to member states. This should be available before the end of 1998.