An Overview of Waste Water Treatment Systems

As a young engineer working at Mead Paper in Kingsport, Tennessee, I was assigned shift duty during the startup of a new waste treatment facility. This consisted only of primary and secondary clarifiers, with a Gorator™ pump used to grind up lumps of debris in the sludge. We had a "hot line" from the paper mill to provide us advance notice if a system was being dumped. When that happened, one of my duties was to open quickly a large manual gate valve to accommodate the increased flow into the primary clarifier. It took nearly all my strength to open that large valve! Fortunately, the project came on-line without much incident, so my time involved at this spanned only about a month.

Compared to the complex waste water treatment systems used today in many mills, the above arrangement is very simple, and would not be adequate to meet today's stricter regulations. In most cases, tertiary clarifiers and other technologies have been required. This is not surprising, considering that the pulp and paper industry uses large volumes of process water, resulting in significant amounts of waste water and residual sludge. In fact, about 85% of the water used is considered process water. Not only have discharge regulations become increasing tighter, these are reviewed every year by the EPA.

Some of these regulations include standards for total solids, suspended solids, chemical oxygen demand, biological oxygen demand, color, absorbable organic halides (AOX), and chlorinated organic compounds. One waste water treatment method that has grown significantly is the use of anaerobic digestion in waste water treatment, even though the initial capital costs are relatively high. It is reported that about 25% of mills world-wide use these systems. The benefits include not having to use oxygen and the generation of less sludge. However, anaerobic bacteria can be sensitive to changes in waste water conditions. In addition, these systems may be in conflict with more recent EPA air emission control recommendations, which seem to favor an aerobic approach with mechanical aeration.

The pulp and paper industry has done an admirable job of reducing waste water contaminants over the last half century. It is reported that water volume alone has been reduced by more than 50%. It is further estimated that there has been a decrease of about 75% in total suspended solids, and that biological oxygen demand (BOD) has been reduced by over 85%. This has been achieved not only by using nutrients and aeration within lagoons, but also by increased recycling of process water. In fact, some European mills have been able to clean and reuse up to 95% of their waste water on site, while meeting local standards. But impurities in mill waste water are site specific, and some paper grades, such as tissue, demand very high water quality standards, resulting in the use of more fresh water. Impurities tend to concentrate in process water that is recycled, and can cause serious product quality problems if not controlled. In spite of this concern, there are some recycle loops that are easily closed, such as excess paper machine white water being reused as dilution water in pulping, stock preparation, or shower water. Treated paper mill white water is also recycled to the pulp mill bleach plant for use in shower water and pulp dilution.

The increased use of more advanced waste water treatment technologies and recycling has also been driven by environmental regulations imposed on the industry. Since installing complex tertiary processes can involve significant capital investment, mills have increasingly explored recycling due to a higher economic incentive. But some of these approaches, which include using membranes for micro-filtration, ultra-filtration, and nano-filtration, have not been economically feasible for all mills. As a further complication, if there is residual chlorine in the waste water, it can be detrimental to membrane life, resulting in the need to consider replacing chlorine with ozone in the bleaching process. Color can also be a concern in recycled water, and it is removed using lime treatment, fungal treatment techniques, enzymes, chemical oxidation, or ozone.

In spite of being both capital and energy intensive, the use of advanced waste water treatment systems is projected to grow about 9% per year. This is related to the requirement to more fully protect water resources. Let's briefly review a few of the methods in use today.

• Membrane filtration. These methods originated in desalination of sea water, but have evolved into waste water reuse. There are a number of classes employed, to include reverse osmosis, nano-filtration, and ultra-filtration (uses water soluble polymeric chemicals for metal removal). Membranes are often made from polymer materials, but ceramic and metal oxide membranes are available.
• Ozonation. Ozone acts rapidly both as a disinfectant and for color removal. Ozone is often used in combination with ultra-violet light. An advantage of ozone is that it can be generated on-site.
• Polymer induced flocculation. This involves the use of polymers, such as polyacrylamide.
• There are numerous other methods, including filtration assisted crystallization, evaporation, electro dialysis (a membrane process), and newer high-speed filtration devices.

One cannot complete a discussion of waste water treatment without including a reference to the sludge generated in the mills. Sludge is the largest volume waste stream generated in the industry. The sources include boiler ash, lime mud, wood processing residuals, and settlement of papermaking fibers/fillers in treatment clarifiers. Often, sludge must be activated to reduce discharge BOD of the waste water. Sludge represents a large portion of the waste water treatment cost, and mostly ends up being transported to local landfills. There are promising technologies that convert sludge into fertilizer and bio-gas. Keep in mind that there is more potential for beneficial use of sludge from mills that use alkaline paper-making processes.

Fortunately, the large volume wastes produced by the pulp and paper industry are not considered at this time to be hazardous under RCRA (Resources Conservation and Recovery Act). But it is possible that some substances in spills might generate hazardous waste and fall under TCLP (Toxicity Characteristics Leaching Procedures) used to determine whether there are hazardous elements present in waste discharge). Mills must always be cognizant of this possibility in their individual operations, and take appropriate action if these problems arise.

Robert Moore is a retired chemical engineer, and is an experienced technical and fictional writer. His past work experience spanned the chemical, paper and equipment manufacturing industries, including holding management positions at Voith Paper, Scapa plc, and The Mead Paper Corporation. He is also the author of humorous short stories about life in southwest Virginia, circa 1940-1960.

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