NipImpressions publicly released the following article earlier this month (Aug. 18, 2018) on its webpage based on research concluded earlier in the decade by the author and thereupon reflected on as far as her ideas and scientific concluding opinion.

Mention that you work in the pulp and paper industry and the response is often: "Papermills stink." However, odors have decreased dramatically as the industry spent millions of dollars to reduce emissions. As a result, community complaints have diminished because of these actions.

But this does not mean that odor issues have completely disappeared. Changes in the pulp and paper processes have contributed to organoleptic (taste and odor) complaints in finished products. Water system closure, neutral to alkaline papermaking, recycled fiber, latex coatings, and giant storage towers can all contribute to odor in the finished products. Even the specific type of pulping process can contribute to off taste and odor issues, especially when these are combined with certain storage conditions. 

• Paper that emits a sulfur fishy odor when the paper goes through a copy machine
• Board used for chocolate packaging imparts grassy/green taste and odor
• Paperboard with a musty or dirt-like aroma
• Silk ties and lingerie that stink after storage in a recycle paperboard box
• Cup holder sleeves for coffee or lattes that emit an odor as the sleeve warms
• Tissue that smells "bad" when used
• Tissue and towel with a vomit aroma when damp.

Microbes can be the source of numerous foul odors. These include the odors of rotten eggs from anaerobic sulfate reducing bacteria, rancid butter or "vomit" aromas from volatile fatty acids (VFA) produced by several types of anaerobes, as well as musty, odors produced by certain fungi and actinomycetes.

Although it is tempting to assume that microbes are the source of all odors, microbial spoilage is only part of the problem. There are even cases where the products used to control microbial activity (biocides) were the source of objectionable odors. In other cases, a recycle mill can carried over contaminants from the original paper and from materials stored or spilled on that paper. A sweet, inexpensive perfume, mixed with a limonene based cleaning chemical added to acetic, butyric and propionic acids from spoiled stock presents a very unique bouquet. 

The pulping process can contribute malodors. Autoxidation of unsaturated wood resins causes the formation of hexanal, butanal, petanal, etc. Hexanal is usually described as the most abundant volatile present. The organoleptic property it imparts can be described as grassy or green and is a particular problem for both dairy and chocolate packaging. Use of TMP pulp is a risk factor for these odors. Storage of TMP made from spruce and pine is known to cause formation of odorous aldehydes. Odor precursor formation appears to be affected by the type of bleaching. Research cited suggested that dithionite reduces the formation of the aldehydes compared to peroxide bleaching.

 

Determining the origin of an off odor(s) can be difficult, frustrating, and expensive. While not as sensitive as a dog’s nose, the human nose is an excellent odor detector. Malodors can be caused by the combination of more than one material. Either material alone might not be objectionable but combination results in a malodor. To further complicate the issue, many odor problems are transitory. If the odor was caused by something temporarily used or dumped into the system, it may be impossible to track down the cause when rejected paper or board is returned to the mill. In the meantime, the mill has spent considerable money on testing and still doesn’t know the original cause of the odor. 

Complaints may emerge weeks or even months after the paper was manufactured. Shipment in rail cars during the summer will intensify the odor. Any source of heat will help volatilize odors. This can include means such as paper warmed in a copy machine or the press room, food packaged in paperboard used in a microwave, or protective sleeve for a coffee cup that gradually warms.

When complaints surface, the end user is displeased, as in turn is the converter. The account may be in peril. Even worse, the mill doesn’t know when or if the off-odor will reoccur. Subsequent complaints could lose customers to another paper supplier or even plastics. To complicate matters, by the time the mill starts to do diagnostic problem analysis, the conditions that created the odor may be long gone. There is a tendency to go through the mill changing several variables at once, and/or making assumptions about the origin that may not be correct. Proper discovery is vital in odor situations.

The simplest problem solving often starts by having someone with a "good nose" to walk through the mill to look for obvious areas of spoilage or foul odors. Biological odors are the easiest to detect this manner. People who work at the mill on a daily basis may suffer olfactory fatigue and not be able to smell them. Because of this, an outside expert is often needed to help in the investigation.

An odor consultant will require a description of the odors, when they occurred, what additives were used, the source of pulp, pulp species, and the pulping process. They will examine dwell time in pulp in storage chests and broke towers and the configurations of the chests. The type of biocides used throughout the process will be noted. The consultant will help the mill set up an odor description form and work with them to derive a problem-solving chart.

Odor characterization may require a variety of methods for analysis. Most methods start with containing the paper product in a closed jar and incubating it at a controlled temperature and humidity. The samples may be directly evaluated for off-odors by a formal screening panel.

In most mills the initial evaluation is done informally. People within the mill smell the samples and attempt to describe the odor. Odor volatilization may be enhanced by friction or other means of warming and/or the closed jar testing. If the problem is recurrent a panel of people with "good noses" may be formed to evaluate samples. If the company has a formal complaint process in place, customers may be asked to fill out forms by checking off odor descriptions that most closely describe what they smell.

Analytical detection and quantification of the odor is done by a variety of methods. For paper or paperboard products and fluids, mass spectrometry, gas chromatography (GC), infrared (GC-MS-IR) techniques to analyze odors causing food tainting originating in adhesives, varnishes, VFA, etc. are frequently utilized.5 These techniques require special equipment and a trained analyst. Sometimes simple wet lab tests can be used for specific substances. For example, HACH has a test method for volatile acids in fluids that provides a "total" acid value.6 It is useful for monitoring total VFA levels and trends.

The use of methods such as GC in the attempt to match the odor in the finished paper or board or the odor complaint with materials within the mill seems both simple and logical. With microbial spoilage and VFA formation, this can be straightforward. With other odors it can be more complicated. Sometimes innocuous materials used at very low quantities may give the same signal as the odor agent. It is not uncommon for a mill to ban the wrong chemical from a mill site and not solve the underlying problem. Since many odor problems are intermittent in nature, a "cure" may have nothing to do with resolving the original odor. Papermaking is not a controlled scientific experiment. These are dynamic systems and multiple changes occur at once.

Starch systems are one of the first places to explore with any odor complaints. This is because it is an excellent nutrient and many starch systems are inadequately treated to prevent biological growth. Spoiled starch emits a variety odors including yeasty and alcohol odors associated with fermentations associated with bread and beer making as well as VFA, hydrogen sulfide and even putrid spoiled protein odors if the starch had a high protein content. Problems with starch can be intermittent due to the quality of housekeeping, the quality of starch entering the mill, the type of starch, and even with the biocide compatibility. Starch is added directly into the wet end, as a coating, or indirectly via coated or sized broke. Wet end starch that is not retained in the sheet increases the likelihood that foul odors will occur in the process waters.

Spoiled protein is among the foulest odors encountered. The human brain has been programmed to avoid these putrefied odors for safety reasons going back in our primitive past. In addition to the putrid odors, hydrogen sulfide is often observed.

Acid odors and hydrogen sulfide are often encountered. Biocide incompatibility or the overuse of the wrong biocide can also cause a problem with the finished paper products. Knowledge of the original biocide used to preserve the clay or filler at the original manufacturing site can prevent problems with overuse or incompatibility.

The final coatings can contain any mixture of the individual odors in the individual additives. In addition, most coating systems have recirculation lines that cause recontamination of fresh stock and lines and filters where deposits may form. These deposits allow foul smelling bacteria to thrive.

Fiber chests are prone to anaerobic activity. This is due to the design of the chests, which is often plug flow with minimal circulation. Low oxygenation enhances anaerobic conditions. If the chests are held long times the likelihood for potential odor problems will exponentially increase. Broke systems in particular are a problem since they often contain coatings that contain high levels of nutrients. Recycle deinked stock favors anaerobic growth.

Oversized water systems and clarifiers with long dwell times or contaminated with a heavy sludge layer allow anaerobes to thrive. This allows growth conditions that favor production of VFA and hydrogen sulfide. Geosmin (trans-1, 10-dimethyl-trans-9-decalol) [sesquiterpenoid compounds, unsaturated rings of carbon, oxygen and hydrogen] and 2-methylisoborneol (MIB) produce musty odors.

In summer months, algae from surface water produce musty geosmin aromas. These odors are typically detected when the board is subjected to heating, either during conversion, or by end-users. Coffee cup sleeves or tissue are typical examples. The complaints often come weeks to months after the board has been shipped. One mill lost 19 days production due to geosmin. Most people are able to detect geosmin and MIB at very low levels (4 nanograms per liter for geosmin, 9 nanograms per liter for MIB). Formed as metabolic by-products by actinomycetes ("mold-like" filamentous bacteria) and cyanobacteria (blue-green algae), geosmin and MIB are released into the water or the paper product upon cell death. These organisms are not picked up on routine plate counts. Just adding biocide to kill the geosmin/MIB organisms after they form will not eliminate this particular odor problem. The odors are already present and further released on cell death.

Waste sludge and wastewater reused in the process both present problems. Today the cost of landfill in some areas has prompted certain mills to reuse aerobic stabilized sludge as filler. Since it is loaded with microbial cells it is prone to odor problems. Biologically stabilized wastewater used as "fresh" water has more problems with contaminants. These factors will influence odor in the final product.

 

Preventing the problem requires discovering what led to the odors in the first place. If the problem has a non-biological origin, replacing the offending additive may help reduce the problem. These are not always chemical additives. Unsaturated lipids from the pulp mill will take special attention. These may require additional pulp washing, absorption treatment or special oxidation treatment to reduce taste and odor issues. For certain grades, banning certain wood species or pulp types might be critical to avoid odor problems. An odor consultant will be able to help the mill identify areas of concern that match the odor complaint. 

If the odor has a biological basis, a microbiologist can work with the mill to reduce the environmental factors favoring the growth of the offending microbes. Understanding microbial metabolism can help avoid problems and decrease the time it takes to resolve complaints. For instance, engineers at one mill site added a huge new high-density tower to add storage capacity and improve pulp mill efficiency. From a biological perspective, this is a giant anaerobic fermentor and odor generation system. The mill cannot tear out the tower, so a chemical means is needed to control problem. While a biocide program can help minimize the issue, the approach should be coupled with techniques that minimize channeling and the build up of aged stock as well as feeding techniques that guarantee enough biocide is fed when the chest levels are high to reduce VFA formation.

In many cases, selecting the correct biocide program with additions in the correct places will prevent the problem. The biocide selection for a particular use may have been done incorrectly. People attempt to solve a problem by adding the wrong biocide or add it a poor addition point, because they don’t fully understand the chemistry of the system, the retention time of the chest or types of microbes that are present. Oxidation-reduction potential and pH must be matched to the appropriate conditions for the particular type of mill. While oxidants can neutralize many types of odors and control many microbes, they are not a simple cure all for all types of mills. The problem is much more complex.

One extremely closed recycled mill attempted to reduce odor problems that were traced to a clarifier. They added multiple totes of inexpensive sodium hypochlorite, assuming it to be the least expensive option. The odor problem decreased minimally, but not sufficiently to avoid complaints. The high organic loading and very reductive environment created an enormous halogen demand that could not be met with an oxidant program. By both changing to a biocide program compatible with the redox of the machine and altering environmental factors to reduce conditions favoring anaerobes were eliminated.