BOD & COD in Effluent of Pulp and Paper Mills
By Sara Mortenson (revised by Megan Webster and Stefanie Vagianos)
Background
The main objective of a pulp mill is to generate fibers to be made into paper. In a chemical pulp mill, this is done by degrading and dissolving lignin away, leaving the fibers. The main kinds of pulping processes are Kraft, Sulfite, Chemimechanical, and Groundwood.
The Five Standard Manufacturing processes
Preparing raw material
The preparation of the raw material may include log washing, bark removal and/or chipping. The steps necessary depend on the form in which the mill receives the raw materials. Virgin wood is the most widely used of raw materials, however, secondary fibers are being used in increasing amounts.
Pulping
Bleaching
The pulp obtained from any of the pulping processes is usually brown and will, therefore, require bleaching. The bleaching can be done with numerous processes, and it is done so that the bleached pulp has an appropriate brightness reading. The brightness scale is out of 100; the original pulp will have a brightness of 25 after Kraft pulping and 50-55 after sulfite pulping. The processes used for bleaching can involve any combination or sequence of acid treatment, chlorination, chlorine dioxide addition, alkaline extraction, hypochlorite addition, hydrosulfite addition, oxygen addition, peroxide addition, peracetic acid addition, and water soak.
Papermaking
The papermaking process usually does not vary, no matter what pulp was used, or what quality of paper is necessary. A layer of fiber from a suspension of pulp is placed on a fine screen, the "wire," and drained. The layer is then removed, pressed, and dried. There are two different types of machines used for papermaking. The cylinder machine utilizes cylinders which rotate within the suspension and the fourdrinier utilizes an endless wire belt.
Converting
The converting process is a high energy process where the products from papermaking are cut, formed, and/or packaged. The converting usually occurs at separate plants.
The Kraft Process
In a typical pulp mill, pulp is generated by the Kraft process. Wood chips are cooked in a solution of sodium hydroxide (NaOH) and sodium sulfide (Na2S). The cook time of the process is approximately 2 to 4 hours, depending on the mill. The cooking temperature is between 170 to 180 degrees Celsius. The pH of the cooking liquor is 13+. In the cooking process the alkaline causes the breakdown of lignin molecules which are soluble in the cooking liquor.
The cooking stage of pulp processing can either be performed in a continuous digester or a batch digester. After the cooking process, pulp is screened to remove large unwanted particles, cleaned to remove residual lignin, and sometimes bleached. Finally, after almost all of the lignin is removed, the pulp is ready to be brought to the paper machines. The paper machines are able to produce many different types of paper, with varying thicknesses and printing characteristics.
Environmental Effects
In the process of pulping wood and making paper, many undesirable effects are generate. One important issue facing all mills is the generation of food sources for microorganisms which deplete oxygen levels in the rivers. The oxygen depletion (biological oxygen demand: BOD) occurs if the materials are not removed before the mill effluent reaches the river. Mill effluent must be treated to reduce BOD and other environmental threats.