The deep chests and long dwell times in papermaking systems provide ideal conditions for the growth of anaerobic and facultative bacteria that cause spoilage. These bacteria can cause problems even in the absence of visible deposits on machine surfaces. Spoilage by these organisms causes odour in the finished paper and, at times, community complaints from people residing near the paper mill.
Anaerobic Growth and Associated Problems: Bacteria that require oxygen for metabolism are known as obligate aerobes. In contrast, obligate anaerobes do not utilize oxygen and may not survive in its presence. Another group, facultative anaerobes, has the ability to utilize oxygen if it is available, but they are also capable of growing in its absence. Problems caused by obligate and facultative anaerobes in papermaking systems are typically related to their metabolic end-products. End-products of anaerobic respiration can include hydrogen sulfide (H2S) and methane (CH4). End-products of anaerobic fermentation can include volatile fatty acids (VFA) like propionic acid, acetic acid, butyric acid, and valeric acid, as well as CO2 and H2. These VFAs can cause odour issues even at low concentrations.
Bacteria that are capable of causing odour problems have been found in all types of mills; however, mills exhibiting the following operational factors are more at risk:
High degree of closure
Low dissolved oxygen in fluids
Poorly agitated/ventilated chests
Recycling Water and Raw Material: Many mills recycle large volumes of water as a result of environmental and economic pressures. Water system closure or reuse can increase microbial problems. In particular, a mill water loop closure results in increased water temperature accompanied by a decrease in the dissolved oxygen (DO) level in the water. Lower concentrations of DO provide conditions that favour the growth of anaerobic bacteria and increased numbers of anaerobic bacteria relative to aerobic bacteria have been observed in such systems. In addition, the concentration of soluble chemicals in the mill water increases. These chemicals often serve as microbial nutrients increasing the potential for spoilage-related problems.
In extremely closed systems, water reuse may include returning water from a primary clarifier. Often these clarifiers are large and have low turnover rates. This leads to the development of anaerobic conditions and subsequent problems such as the accumulation of VFA. Returning water from a later stage of the waste treatment process (e.g., stabilization lagoon) may lessen the impact of anaerobic bacteria. However, this can create other problems such as increasing the introduction of wastewater organisms, including filamentous bacteria, into the process.
There is also increased pressure to utilize recycled raw materials. In the past, recycled fiber was used primarily in boxboard mills. Now it can be found in almost all grades of paper. The use of pre-consumer recycled fiber can make adequate microbial control even more difficult, especially if the material contains coating or starch. The presence of these nutrients increases the potential for spoilage-related issues. Post-consumer recycled fibers are generally subjected to moist and dirty conditions and are often stored outdoors, uncovered, and exposed to the elements. This can increase the bacterial loading of the system, particularly with fungi. Recycled furnish can be contaminated with a higher loading of microorganisms than is typical of virgin pulp.
Control Methods: Good operational practices along with right biocide chemistry program coupled with intelligent automation will be the most cost effective way to prevent VFA formation. These operational practises include:
Preserve stock chests during extended stoppage (> 5 hours)
Keep all chests agitated
Install air sparging lines into problematic chest
Anaerobic and facultative bacteria grow in stagnant, untreated areas of the mill. Therefore, microbial control is critical, even if there are no apparent problems on the paper machine itself. For example, preservation of storage chests is required to prevent anaerobic bacteria from growing and producing foul smelling compounds that can lead to customer complaints or even result in non-saleable product. Although a seemingly complicated problem, odour problems and microbial growth can be effectively controlled with the right monitoring and control system in place. HABER’s microbio control program is helping many paper mills effectively fight these problems today.