Publication: Pulp & Paper Magazine
Issue: March 1, 1996
Author: D.G. Gudlauski

Whitewater System Closure Means Managing Microbiological Buildup

Extensive system monitoring and a specific program for handling microbiological growth are necessary when mills "close the loop"

Editor's Note: This article is the second in a six-part series exploring the technological, regulatory, and market issues that surround the pulp and paper industry's efforts to close up its effluent streams. The series, authored by various experts from Betz PaperChem, examines the impact effluent closure will have on North America's pulp and paper operations, including the pulp mill, deinking facilities, the paper machine, the chemical recovery island, and waste treatment systems. The first article (Pulp & Paper, Feb. 1996, p. 55) provided an overview of various effluent closure issues. This article examines paper machine whitewater closure and associated microbiological growth.

 

Closing up a pulp and paper mill's whitewater system without attending to the potential consequences may result in reduced production and quality. Closure produces a new set of challenging conditions in which water quality, materials management, and deposition control all become extremely important.

With increased emphasis on overall deposition control, a mill will need to re-evaluate its approach for microbiological control. Understanding what problems could occur and why they might occur will help a mill maintain control before problems appear.

Whitewater closure has occurred predominantly in uncoated linerboard and corrugating medium grade mills operating at acid to neutral papermaking conditions. As more printing/writing grade mills begin to close up their water systems, the importance of water quality, materials management, and deposition control is elevated since quality and production in these grades are usually more sensitive to deposition and microbiological growth.

This article discusses issues relating to microbiological growth in unbleached paper and paperboard grade, and printing/writing grade papermaking processes, with suggested strategies for addressing these issues.

CHANGING CONDITIONS. Increased whitewater closure changes the environment and growth conditions for microorganisms in the papermaking process. Changes such as increased temperature, lower dissolved oxygen content, longer residence time, and increased total solids create conditions for increased incubation and for different inoculation points of micro-organisms that result in quality, runability, and safety issues. Microbiological growth can manifest itself as either slime deposition, formation of volatile gases, or spoilage-producing acids that affect the bonding of organic compounds, leading to material degradation, such as additives and woodfibers).

Quality of paper or paperboard is affected by sheet defects from microbiological deposition, odor complaints for paperboard from volatile fatty acid (VFA) production, and reduction in sheet strength from fiber spoilage. Runability of the papermaking process is affected by some of the following:

Breaks due to microbiological deposition

Downtime to wash up or boil out deposition

Sewering of fiber or additives due to degradation

Reduced production capacity due to screen plugging

Downtime for repairs due to deposit corrosion.

Breaks and the corresponding downtime are the most common runability problems. Safety becomes an issue when anaerobic bacteria produce toxic or explosive gases, such as hydrogen sulfide (H2S), hydrogen (H2), and methane (CH4) in stock or water chests. Slime deposition can make catwalks and machine surfaces slippery, causing lost time accidents. Neighboring communities may complain of odor due to VFAs or hydrogen sulfide being liberated from the sheet and/or process.

System chemistry and biology is affected in several ways that make these issues a real problem. As fresh water is substituted with recycled and filtered white water, the microbiological characteristics of a system shift. Fresh water is a primary source for filamentous bacteria and other fresh water microorganisms. Fresh water is also usually treated by chlorination or use of another powerful oxidizer, thereby keeping the microbiological contamination level under control.

Use of recycled water on the paper machine can reinoculate the system or allow deposition in areas where it has not been seen before. Equipment where misting showers are used could be new sites for microbiological growth. This deposition can slough off, causing sheet defects or breaks. It can also cause catwalk surfaces to be slippery. Other areas where recycled whitewater is used can elevate the contamination level in such places as savealls and points of heavy consistency regulation.

BACTERIAL SHIFTS. Localized changes in temperature, pH, solids level, and dissolved oxygen levels can cause a shift in bacterial populations. In open systems, aerobic bacteria flourish, usually leading to slime deposition. In closed systems, anaerobic bacteria levels increase, causing a shift of problems inside the machine areas from slime deposition to odor, under-deposit corrosion, toxic gases, and fiber degradation.

As temperature increases, the solubility of oxygen in water decreases, which promotes increased anaerobic bacterial growth and the formation of spores. Oxygen is a component in the respiratory process, as well as other biological functions. It serves as an oxidizing agent for organic compounds taken into the cell, generating energy, water, and carbon dioxide.

As the total dissolved solids, chemical oxygen demand, and temperature in the system increase, the oxygen in the system is depleted. Continuous recirculation of whitewater with low levels of additional freshwater cause dissolve oxygen levels to decrease from 8 mg/l down to 2 to 4 mg/l and lower (Figure 1). This creates an environment loss suitable for those bacteria that utilize oxygen.

Temperatures higher than 120F simply promote a shift change from mesophillic to thermophillic bacteria populations. Most slime-forming bacteria are present in the mesophillic temperature range. While temperatures higher than 140F inhibit aerobic populations, they do not affect spore forming bacteria.

The information in Figure 1 shows that both aerobic and anaerobic populations decrease with temperature above 130F. However, the microbiological population does shift to a higher percentage of anaerobes. As temperature increases, there is a buildup of dissolved organic materials, increasing the nutrient content of water streams.

When environmental conditions become conducive for growth, the spores begin metabolizing. Metabolic rates can increase with temperature, allowing further proliferation of microbiological levels. For the most part, as systems close, actual slime formation ceases due to higher temperatures and reduced oxygen.

Two anaerobic species most prevalent in papermaking systems are Desulfovibrio spp. and Clostridium spp. Either species may be found in soil, water, broke, additives, recycled fiber, and sludge. The metabolic byproducts of these anaerobic bacteria are volatile fatty acids and hydrogen sulfide gases. Both have strong, distinct odors that can permeate the air, papermaking systems, and the finished product.

Bacteria produce these organic acids via two metabolic pathways-carbohydrate fermentation and non-carbohydrate protein fermentation. During anaerobic fermentation, most of the energy within the substrate molecule is not used. These bacteria convert a carbohydrate substrate to pyruvic acid, which can be metabolized to VFAs. Anaerobic bacteria are not capable of breaking these end products down further, resulting in a buildup of acid.

Some bacteria grow fermentatively even in aerobic conditions and produce organic acids that decrease the pH. Facultative anaerobic bacteria can switch metabolic processes from fermentation to repiration depending on the environment. These are the bacteria usually responsible for producing high concentrations of VFAs in the process water and sludge. It is also possible to find these bacteria in starches, coatings, and fillers, where areas of stagnation can occur. Table 1 provides some examples of volatile fatty acid fermentation products that most frequently cause odor-related quality problems.

Clostridium sporogenes is a spore-forming bacterium found in soil, water, broke, additives, recycled fiber, and sludge. In its vegetative state, C. sporogenes is a unicellular, rod-shaped bacteria that looks just like other bacterial species in the system. Its thick spore coat can resist temperature extremes, desiccation, radiation, and exposure to many toxic chemicals. In this state, C. sporogenes can survive the paper machine dryer sections and show up in the finished sheet. It will make its way back into the system through the broke.

DISSOLVED SOLIDS INCREASE. An increase in dissolved solids will have cumulative affects. It leads to poor shower water quality and eventual plugging. Deposition sloughs off the machine and shower surfaces and results in defects and breaks.

A higher level of solids puts a greater load on the retention program. If fewer fines are retained, they gather at sites on the machine, providing initial deposition and eventual microbiological growth. This can lead to under-deposit corrosion. As bacteria metabolize, they secrete exopolysaccharides (slime) and acidic byproducts. These acids attack metal surfaces, resulting in pitting, etching, and promotion of stress fracture corrosion.

Higher solids also increase the demand for microbiological-inhibiting products, such as proprietary biocides and oxidizers. Treatment costs increase, and deposition control is more difficult. Increased loading and incubation can also cause fiber spoilage in areas such as high density fiber storage.

Overall, as incubation time for the water system increases, deposition can be seen in more locations, and micro-organism levels may increase. As a result, the papermaking system may need to adapt or equilibrate to running at higher microbiological levels.

TREATMENT STRATEGIES. The impact of these changed process conditions can be minimized through effective monitoring and control strategies. A mill should be able to identify all of the contamination sources. It is necessary to understand the system, water flows, current micro-organism types, and control performance, as well as what process areas could be microbiologically sensitive. A mill should be able to anticipate potential levels of contamination from water reuse, what the process changes will be, and a way to remove excess solids.

Monitoring the process includes conducting water analysis, measuring microbiologic population shifts through microbiological plating, checking suspect areas for VFA or other toxic gas levels, and measuring program performance for runability and quality impact as a result of contamination or control.

Fines retention can be maximized mechanically through purging dissolved solids with reverse osmosis units, filtering, and crystallization. Fines retention can be handled chemically through effective retention programs. Better fines and colloidal particle retention, along with effective mechanical removal and chemical treatment, will minimize organic and inorganic deposition that fiber and microbiological deposits can adhere to.

Methods to prevent or control buildup of volatile gas include aeration by agitation or air lines, chest ventilation, biocide treatment, and pH control. Aeration will increase the dissolved oxygen in white- water to a level anaerobic bacteria cannot tolerate. Other than good agitation, aeration is not recommended for stock chests because this will promote the growth of aerobic bacteria.

Microbiological contamination should be chemically treated at key points of incubation and inoculation, such as water sources, closed stagnant loops, high density storage, broke, or recycled fiber chests. Clean machines should be maintained through effective and frequent wash ups or boil outs.

 

D.G. Gudlauski is product specialist, Betz PaperChem, Jacksonville, Fla.

 

 

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