Water closure can often lead to a number of problems at mills. But solutions are on hand to help papermakers overcome these obstacles

By Pierre Berard

Filling in the holes after closing the loop

Shutting yourself off from the outside world is usually viewed as a negative thing. But this is not always the case. In the paper industry, the last 20 years have seen European paper mills become increasingly closed off in an attempt to reduce water consumption. Along with effluent treatment, system closure is viewed as the best solution to reduce emissions to surface water. This behavior is likely to increase as the European Directive on Integrated Pollution Prevention and Control (IPPC) encourages more mills to 'close the loop'.

But several problems are often associated with water closure, including water and product odors, corrosion, machine runnability and poorer additive performance. Help may be at hand for papermakers though. As engineers gain a clearer understanding of wet end chemistry, it becomes easier to identify these problems, while the use of increasingly sophisticated simulation programs can offer possible solutions.

Water forms an essential part of the papermaking process, but it acts as a carrier rather than a significant component of the final product. As a result, most of it is recycled. The average discharge from western European paper mills is currently less than 20 m3/ton, or less than 10 m3/ton for packaging grades from recycled fibers. But the push toward zero liquid effluent in papermaking is hampered by a number of practical problems. One is the increase of dissolved inorganic salts and organic materials in the process water, while another is the cost of efficiently cleaning the water in order to avoid concentration buildup.

According to a survey carried out by the pulp and paper division of the supplier, Hercules, effluent discharge limits were cited by 91% of respondents as the major driving force behind paper mill closures. Next came government regulations (52%) and community perception (48%). Wastewater treatment costs, fresh water costs, raw material/energy savings and public relations were also mentioned as important factors by more than 33% of respondents. In the same survey, the main problems associated with water closure were identified as wet end deposition (58%), mill odor (55%), foam and corrosion (47% each), product odor (45%), decreased machine runnability (38%), drainage loss (33%) and retention loss (32%).

The last part of the survey covered the most successful technologies used to overcome the problems associated with water closure. Odor/volatile fatty acids (VFA) control was the most frequently mentioned (25%), followed by drainage/retention (22%), wet end deposition control and general microbiological control (13% each). In the ranking, these areas were followed by clothing treatment and foam control at 12% each.

Headbox hassles

Table 1 gives examples of headbox water analysis from four mills producing recycled fluting and liner grades. Mill A is a 100% closed European mill without any process water treatment. Mill B is an almost 100% closed European mill without any process water treatment (0.3 m3 of effluent per ton of paper). Mill C is a 100% closed European mill with a two-step biological treatment and a softening unit, while Mill D is a 100% closed North American mill without any process water treatments.

Closing the system dramatically increases the concentration of different materials in the water loops. These include ions such as bicarbonates, sulfate and calcium, as well as organic colloidal and dissolved materials such as lignin, starch, hemicellulose and extractives. The high (or very high) concentration levels of the following components are at the root of problems associated with water closure:

• corrosion from chloride and sulfate
• total organic carbon (TOC) and sulfate odors
• deposition from dissolved and colloidal
• substances (DCS)
• scaling from calcium and carbonates
• (possibly sulfate as well)
• retention/drainage from conductivity, DCS and cationic demand
• foam from conductivity and DCS.

Temperature and pH also play a major role in wet end chemistry and its associated problems. The pH value is directly influenced by the addition of acids (mainly sulfuric) or acidic materials such as alum. In the absence of acid or alum, pH generally falls to between 6 and 6.8 due to the presence of volatile fatty acids created by microbiological activity on the organic DCS. Temperatures can increase to 40-55°C with water closure due to the reduction of heat losses. This temperature modifies the microbiological population, influences the tackiness of some deposits and also increases scaling due to calcium carbonate.

Simulated solution

Many companies have developed sophisticated tools to predict the impact of water closure on the concentration of both dissolved and suspended materials in different water loops. The most advanced include dynamic simulation which take into account transition periods after a sudden event and wet end chemistry calculations based on more or less simple equilibrium equations between several components. Some of them can also calculate temperature or corrosion risks.

Similar software has been used for several years to optimize different chemical additions and equipment operations, including machine retention, save-all or dissolved air flotation unit (DAF) efficiency and biological reactor performance. While this tool is not suitable for evaluating mechanical solutions, it has proven very valuable as a monitoring and troubleshooting device. The software simulates mass and flow balances and calculates the enrichment factor of materials with different substantivity to the fibers. When changing machine retention, save-all efficiency or clarification performance, the software can predict the consistency of dissolved or suspended solids at several points in the system. The system can also be used to model screening, recovery and clarification performance, as well as allowing a mill to optimize the re-use of the different flows.

Figure 1 - Hydraulic Balance for an 18 ton/br Machine without Effluent Recycling

Take one case where an 18 ton/hr machine produces lightweight coated (LWC) base sheet from thermomechanical pulp (TMP) and bleached chemical pulp. The PM consumes 23 m3/ton of fresh water. Half of the filtrates from the save-all are recycled, but there is no recycling from the wastewater treatment plant. Figure 1 shows the hydraulic balance as well as the enrichment factor calculated for an additive without any substantivity to the fibers. After some modifications to the effluent plant, it became possible to recycle up to 40% of the effluent. At this maximum recycling rate, fresh water usage was reduced to 14 m3/ton and the machine's new hydraulic balance is shown in Figure 2.

The impact of the substantivity to the fibers of a given material or chemical is particularly important when predicting their build-up in the system as a function of system closure. When the considered component presents some affinity for the fibers, its enrichment factor will be considerably reduced. For example, if this system is 100% closed, the enrichment factor of an additive without any substantivity to the fibers (and any degradation in the effluent plant) will reach 66.6%. But it will only reach 24.6% at 5% substantivity and 14.8% at 10% substantivity.

Another model calculates the scaling tendency resulting from existing or calculated operating conditions. It uses an equation to express the relationship of pH, calcium, total alkalinity, dissolved solids and temperature to the solubility of calcium carbonate in water. The Langelier (Saturation) index is the difference between the actual pH of the water and the pH at which water with a given calcium content and alkalinity is in equilibrium with calcium carbonate. This index is a qualitative indication of the tendency of calcium carbonate to deposit (if it is positive), or to dissolve (if it is negative). It gives some indication of the severity of the potential scaling problems caused by calcium carbonate. Between +0.5 and +2.0, the use of an appropriate scale inhibitor will prevent scale formation. If the Langelier Index exceeds +2.0, it becomes increasingly difficult to control calcium carbonate scale1.

Slimy business

Another piece of software that has been used on more than 20 new machines in Europe is aimed at optimizing the injection cycles of slime control agents. In this application, the simulation tool distributes a given amount of chemicals in the most efficient way by achieving a defined kill threshold value during the largest exposure time. It also allows simulation of an accidental spill and calculation of product concentration in the effluent.

Though this specialized software is not compatible with the more recent water closure software, it still takes into account the fresh water consumption level. By running two successive simulations at different fresh water usage rates, it is possible to appreciate the impact of the water closure on the concentration of a non-substantive additive at different points of the papermaking system, while keeping the additive usage constant. Another possible way to use the software is to keep the concentration at a constant value and then calculate the new required amount of the additive when the fresh water usage decreases.

Using the 18 ton/hr machine, a given amount of a microbiocide is added to the machine chest by a variable-speed pump. The speed is adjusted to keep a constant dose of 20 ppm in the white water pit during injection, with a fresh water usage of 23 m3/ton. When this fresh water flow is reduced to 14.3 m3/ton with the effluent recycling, the amount of product in the white water pit reaches 21.6 ppm. This might not appear particularly impressive, but the difference becomes much more important when analyzing the residual level of the product in the pit two hours after the addition is stopped. In the first case, without recycling, the level drops to 4.3 ppm. With recycling, the level reaches 8.8 ppm. Of course, these values depend on the volume of the various chests in the system.

Figure 2 - Hydraulic Balance for an 18 ton/hr Machien with 40% Effluent Recycling

Acid diet

The changes caused by water closure include increased temperature, lower dissolved oxygen content and increased dissolved materials. These conditions favor the anaerobic production of both volatile fatty acids (VFAs) and toxic or explosive gases such as hydrogen sulfide, hydrogen and methane. Customers or neighboring communities may complain of odor due to VFAs or hydrogen sulfide being liberated from the sheet and/or process.

The impact of these changed process conditions can be minimized through effective monitoring and control strategies though. Monitoring the process includes conducting water analysis, measuring microbiological population shifts through appropriate techniques, checking suspect areas for VFA or other toxic gas levels and measuring program performance for runnability and quality impact as a result of microbiological contamination2.

Prevention or control of volatile gas includes careful design of water loops. Here, a trend to minimize water volume to reduce retention time is emerging, along with aeration of water systems, pH control, optimum fines retention and specific biocidal and non-biocidal treatments. In cases of VFA production, Hercules has developed a patented technology that allows the mill to monitor and control the feed of chemicals based on the level of generated acids.

In one case, a 100% closed packaging mill suffered from a severe hydrogen sulfide generation problem, which led the local community to complain. A series of measurements with two different analyzers indicated that hydrogen sulfide content in the air varied from 2-3 ppm at the clarifier, 5-20 ppm at a large buffer tank and from 30-40 ppm after the anaerobic biological reactor. After extensive analysis, a chemical addition treatment was implemented to limit the development of anaerobic bacteria prior to the biological plant and in the sludge tank.

Safety deposit

Closing the system will also dramatically increase the concentration of different materials in the water loops. For example, one 100% deinked newsprint mill experienced severe deposition problems following several rebuilds to increase production at constant total water usage. This led to downtime and reduced paper quality. When the entire process was evaluated, a number of problems were identified including excessive starch usage, poor retention, unbalanced wet end chemistry, stickies from raw materials and microbiological activity.

Reduction of water usage made the situation worse, due to increasing levels of dissolved and suspended matter in the white water. At this point, the mill decided to implement a total colloids management program that included a dual system retention program, (combining controlled colloids coagulation with fillers and fines flocculation), optimized starch usage, tailored wet end deposition control program and a microbicide addition in the back system.

This program significantly reduced the total breaks by 29% and breaks in the press section by 46%. Production also went up by 5% and holes were reduced by 33%.

Figure 3 - Suspended Solids vs COD

Cutting the amount of effluent will also increase the dissolved materials contained in the headbox furnish. Generally, the pH levels tend to decrease when mills use a low amount of fresh water. Meanwhile, high conductivity in the headbox furnish can also reduce ionic attraction forces between the retention/drainage aid and the fines or fillers. Another very important factor is the impact of conductivity on polymer conformation. While a polymer is considered as fully extended under low conductivity conditions, it will ball up under high salt conditions. This conductivity increase will then limit the opportunities for the polymer to interact with furnish components.

Dissolved organic materials such as lignin, starch and latex also interfere with the cationic retention/drainage aids by neutralizing their active sites. They act as a barrier around the surface of the furnish components and prevent efficient interaction between themselves and the polymers. Recent developments in polymer synthesis have allowed the production of new flocculant structures, with the emphasis on charge density and distribution along the backbone.

Another interesting example comes from a mill that produces packaging grades from 100% recycled fiber. This mill is almost closed and has no biological treatment on the recycled process water. The effluent rate varies by about 0.5 m3/ton. In this case, the suspended solids content in the filtrate from the save-all and the (filtered) COD of these filtrates have been monitored (Figure 3). It can be seen that at a given retention treatment on the machine, the suspended solids content increases with the COD. On day 63, the retention program was modified using a flocculant with a different structure. The filtrate quality, as measured by suspended solids content, comes back to standard values. Although in this example COD depends more on effluent discharge volume than colloidal retention, the suspended solids values show that Polymer 1 was not able to provide a good retention under high COD conditions, while Polymer 2 retained its efficiency.

This case illustrates the impact of the content of dissolved materials on the retention program efficiency and the possibility of solving a problem by using different polymer structures. Both colloids and fines retentions are critical to ensure proper machine runnability. Good retention of colloidal materials can increase the efficiency of chemical additives such as size, dyes and starch by limiting the build-up of detrimental substances. It can also reduce interference between anionic trash and flocculants and potentially lower foaming tendency of the suspension, as well as reducing organic load in the effluent streams.

Good fines retention can also offer furnish savings by limiting loss to effluents. Other advantages include improved machine runnability, lower white water consistency and reduced foaming problems.

Second time round

In another example, a mill producing LWC from TMP and bleached chemical pulp consumes around 40 m3 of water for each ton of paper sold. This figure includes the TMP plant and non-process water used for boiler and cooling. The mill decided to implement an ambitious program to reduce its use of fresh water. The most important saving was achieved through the re-use of large amounts of the effluent. The recycling rate could reach 60%, although most of the time it fluctuated between 40% and 52%.

High recycling rates at the mill led to two main problems - an increase in the microbiological load in the so-called fresh water and a decrease in retention performance. While it was not difficult to solve the microbiological problem through a redesign of the control program, the retention problem required a lot of investigation. The main difficulty came from the frequent changes both in production grades (including important changes in coating formulation) and in the recycling rate itself. To make things worse, the quality of the TMP effluent varied dramatically. For example, COD fluctuated between 1,500 mg/l and 4,500 mg/l, depending on the wood species, the wood aging and the bleaching process.

As a better understanding was developed of the quality and variability of effluents from the various parts of the mill, it was possible to partially stabilize the quality of the final effluent. As a result, it became apparent that retention on the machines was driven by two main parameters - conductivity and cationic demand in the short loop. Most of the time these two measurements varied in the same direction, but not always. In-depth product performance evaluation, coupled with new developments in retention aid manufacturing, led to the introduction of a flocculant that was more tolerant of high conductivity in the system. At the same time, the swing in cationic demand could be played down through the use of an appropriate coagulant.

Closing note

It is clear that water closure significantly affects the chemistry of a paper machine. Fortunately, even simple computer programs can reliably predict the effect of closure if base data such as fiber substantivity are known. The basic results from this kind of computer program can then be checked in real-life paper machine systems. The increase in concentration of several contaminants may affect the performance of many process chemicals, but with the tools that are available today, alternative treatments can be designed before the actual problem occurs.

Pierre Berard is an applications development manager at Hercules Pulp and Paper Division

References

1. Betz Handbook of Industrial Water Conditioning, 9th Edition, Chapter 25
2. D Gudlauski, "White water system closure means managing microbiological build-up", Pulp and Paper Magazine, March 1996

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