Pulp & Paper: A Look at Wastewater Treatment Trends and Technologies
There are many industrial processes in use across the nation and around the world that discharge large amounts of wastewater effluent into local waterways and ground soil.
By Art Haddaway
There are many industrial processes in use across the nation and around the world that discharge large amounts of wastewater effluent into local waterways and ground soil. Generated from a variety of manufacturing processes, these discharges often serve as a primary source of pollution for aquatic life, public health and the environment.
It has been estimated that industrial applications account for more than half the volume of all U.S. water pollution, including the most harmful impurities, with roughly 370,000 manufacturing facilities consuming vast amounts of freshwater to eliminate wastes. As such, treatment and discharge of wastewater poses an immense challenge.
Industries that consume large quantities of water include everything from textiles and garments to petroleum refining to automotive production, with agriculture and power-generation responsible for an estimated 90 percent of direct water withdrawals. Another industrial sector -- pulp and paper -- is especially vulnerable to pollution risks given its heavy reliance on water.
|Microorganisms in a sample of activated sludge. These organisms feed on organic matter and reduce the biological oxygen demand of the effluent.|
"The pulp and paper industry is water-intensive because water is needed in just about all the processes that are used," said Dr. Bill Thacker, senior research engineer with the National Council for Air and Stream Improvement (NCASI). Such processes include digesting wood chips to form pulp, washing and bleaching the material to achieve whiteness, and generating steam as a means to dry the paper -- and a substantial volume of effluent is created as a result. He explained that while many mills strive to recycle this water, there is also a considerable amount of contaminant-laden residual that must be either treated onsite or sent to a municipal treatment plant offsite before being released into the environment.
Market insight research published by Frost & Sullivan has indicated that about 85 percent of the water the pulp and paper industry consumes is only used for processing, ultimately generating large volumes of contaminated wastewater. The Food and Agriculture Organization (FAO) of the United Nations Secretariat estimated that the total effluent discharged annually from these mills is about 40,000 million cubic meters, assuming an average of 200 cubic meters of effluent per ton of pulp and paper.
"The main water use depends very much on the pulping process that is used to pulp the wood," said Phil Riebel, president and chief operating officer of Two Sides, an independent, non-profit organization created to promote the responsible production and use of print and paper. "You can have groundwood pulping, mechanical pulping and chemical (kraft) pulping," he explained. "A kraft pulp mill will usually be the highest user of water just because the process requires it to dissolve chemicals, and there's a lot of water and steam involved."
However, while the discharge of contaminated effluent is certainly a growing concern, the pulp and paper industry is also making great strides to reduce the quantity of wastewater it expends along with the amount of toxins contained in the effluent. When it comes to properly treating these discharges, there are a number of effective trends and technologies helping the pulp and paper industry reduce harmful pollutants and bacteria in its wastewater.
Although several common types of impurities are found in paper industry wastewater effluent, many are site-specific and vary according to location, amount and condition of equipment used, and type of permits and regulations enforced. Further, industrial waste consists of both organic and inorganic substances that regularly require primary and secondary -- and even tertiary -- treatment methods.
"Relative to decision making, what the paper industry does with wastewater treatment is not, I believe, unique compared to other industries," said Thacker. "The objective is to provide treatment sufficient to reliably meet permit limits and/or to allow for in-mill recycling while minimizing costs."
For many years, the majority of pulp and paper mills have used the process of clarification -- a common primary treatment method -- to reduce and remove total suspended solids (TSS) and other inevitable particulate matter such as bark, dirt and wood fibers generated from the paper-making process. In addition to removing these solids, this method also reduces the amount of biological oxygen demand (BOD) levels in the wastewater.
As noted in FAO's report, "Before external treatment, the effluents may be estimated to contain a total of 40 to 50 million tons of suspended solids (per year). This very large figure serves to underline the importance of external effluent treatment. Such treatment can substantially reduce the amount of suspended solids which would otherwise pollute the waters in the vicinity of pulp and paper mills."
Pulp and paper effluent has also regularly undergone secondary biological treatment to remove organic matter such as residual TSS and BOD and address chemical oxygen demand (COD) and adsorbable organically-bound halogens (AOX) as well as other carbon-based bacteria. Processes including activated sludge and aeration within basins or lagoons are commonly used here and utilize microbes and nutrients such as nitrogen and phosphorus to further treat for these organic substances.
|Aerated stabilization basins utilize microbes and nutrients such as nitrogen and phosphorus to further reduce the presence of organic substances.|
One wastewater treatment trend gaining traction is anaerobic digestion. While this process has high capital installment costs, it doesn't require oxygen (like aerobic treatment does), ultimately resulting in significant energy savings over time. Anaerobic systems also generate less sludge for the amount of BOD removed compared to the high quantities created from aerobic treatment. Likewise, this approach can break down the elements in the solids and turn them into biogas, which can be used to harness energy onsite, greatly reducing sludge disposal costs.
According to Thacker, roughly 300 pulp and paper mills out of 1,300 worldwide have installed anaerobic treatment technologies. He added that about 85 percent of these systems were commissioned in just the last 15 years. "Anaerobic bacteria can be very sensitive to wastewater conditions and over time, the designs of the anaerobic reactors have improved to where they're much more efficient, more economical and more reliable," he said.
Attention has also been given to advanced or tertiary treatment, where fine particles are more precisely extracted, oxygen-demanding substances are better stabilized and residual nutrients used in secondary treatment are thoroughly withdrawn. Likewise, approaches such as adsorption, UV disinfection, coagulation/precipitation, membrane filtration, dechlorination, and chemical oxidation are being incorporated at pulp and paper mills to support this process.
|Primary clarifiers help reduce and remove total suspended solids and other particulate matter such as bark, dirt and wood fibers generated from the pulp and paper-making process.|
Tertiary treatment is beneficial in that facilities have the capability of reaching the desired level of pollution control and even potentially reusing the water for a variety of on- and off-site applications such as plant cooling and agricultural irrigation. "Some mills have had to adopt tertiary treatment," said Riebel. "It depends on the river or waterway they're discharging in. If the government is still not satisfied with the environmental impacts, they may impose tertiary or an additional form of treatment."
Newer technological applications such as enzymes, ozonation treatment, closed-loop systems, fixed bed biofilm processing, and nitrogen fixation are also being realized for effective sustainability. Much consideration is also being given to sludge management given that it represents a large portion of wastewater effluent treatment costs.
"We've made some huge strides in improving the quality of our waterways because pulp and paper mills have installed very efficient treatment plants and are improving their effluent," said Riebel. "In terms of nature and the environment, it's a very positive step."