BOD & COD in Effluent of Pulp and Paper Mills
By Sara Mortenson (revised by Megan Webster and Stefanie Vagianos)
Comparing the advantages and disadvantages of the chemical, mechanical and semichemical processes, we find that the optimal process is the CTMP. the only visible drawbacks are the highly concentrated characteristics of the effluent produced. Weighing this versus the yield of pulp, strength, quality, and water and energy saving capabilities, and especially the modern abilities of interprocess treatment and biological treatment to treat pulp and paper wastewater, the CTMP is the best solution.
With chemical processes falling close behind and knowing that the majority of the pulping in the United States is done by the Kraft process, we cannot ignore them. If managed correctly, both processes can be effective in reducing the amounts of waste they produce and the resources they use both interprocess and post-process treatment. Several suggestions for treatment are listed below:
1. Oxygen bleaching instead of chlorine bleaching. This includes the ability to burn the spent bleach liquor in a conventional Kraft recovery furnace to recover the alkali.
2. Recycling certain wastestreams to be used in the bleaching process. For example, use the "white water" from the paper machine of effluent from one bleaching stage as washwater for the preceding one. Other possibilities include elimination or improvement of showers and washing at the final stages in bleaching (by using high pressure, low volume systems, it would cut water use from 640,000 gal/day to 42,000 gal/day). Note that without reusing water a reported 27,000 gallons of fresh water were used per ton of pulp (1972) and chemicals, fiber, and heat are lost. (See tables)
3. Development of new biological treatments (anaerobic and aerobic).
4. Pretreatment of individual wastestreams.
5. Ozone bleaching instead of chlorine bleaching.
Referring to an article entitled, "The 2020 Mill-Utility Requirements for Market Kraft Pulp", it is estimated that environmental issues such as recycling of post consumer waste, sludge incineration, chlorine-free bleaching, energy efficiency, and zero liquid effluent will have produced many demands on the pulp and paper industry by the year 2020. About 23% of recycled fiber will be used, and will be mandatory. Oxygen delignification followed by ozone bleaching and several other steps will allow an increase in the amount of water recycling in the process. Chemicals will be recovered by liquor evaporation and lime reburning. Pulp yield will increase 75%. The amount of effluent will decrease from 22,300/ton pulp to 11,700/ton pulp, due to increased use of recycled fiber, reuse of process waters, and evaporation.