Paper production wastewater treatment procedure
Features of paper production wastewater
Paper production wastewater is remarkably detrimental, amongst which bla...
Paper production wastewater treatment procedure
Features of paper production wastewater
Paper production wastewater is remarkably detrimental, amongst which black water presents the greatest peril, and the pollutants it comprises represent over 90% of the total pollution discharges of the paper sector. Since black water is alkaline, dark, pungent, and foam abundant, and necessitates a considerable volume of dissolved oxygen in water, it gravely contaminates the water source, inflicting damage on the environment and human health. The most significant environmental pollution instigated by intermediate water is the chlorine-bearing wastewater generated during the bleaching operation, such as chlorination bleaching wastewater, hypochlorite bleaching wastewater, etc. Additionally, bleaching wastewater includes highly toxic carcinogenic substances like dioxins, posing severe threats to the ecological environment and human health.
The principal pollutants encompassed therein are as follows
(1) Suspended solids: encompassing sedimentable suspended solids and non sedimentable suspended solids, primarily fibers and fiber fines (i.e. fragmented fiber particles and miscellaneous cells).
(2) Biodegradable organic compounds: including low molecular weight hemicellulose, methanol, acetic acid, formic acid, sugars, etc.
(3) Difficult to biodegrade organic matter: predominantly derived from lignin and macromolecular carbohydrates contained in fiber raw materials.
(4) Toxic substances: Rosin acid and unsaturated fatty acids contained in black liquor.
(5) Acid alkali toxins: The pH value of alkaline pulping wastewater is 9-10, and the pH value of acidic pulping wastewater is 1.2-2.0.
(6) Chromaticity: The residual lignin in pulp wastewater is highly colored.
Composition of paper production wastewater
Pulp and paper wastewater is primarily bifurcated into three types: black liquor, intermediate wastewater, and white water.
Black liquor: Plant fibers are steamed with alkaline agents containing NaOH or NaOH+sodium sulfate to solubilize lignin, and the resultant cooking solution is termed "black liquor" (alkaline cooking yields black liquor, acid cooking produces red liquor, and the vast majority of cooking employs alkaline cooking). Black liquor encompasses lignin, pentose sugar, and total alkali, rendering it a high concentration and recalcitrant wastewater.
Intermediate wastewater: The wastewater generated during the washing, screening, and bleaching processes of alkali boiled pulp, with a COD load of approximately 310kg per ton of pulp. BOD/COD ranges between 0.20-0.35, indicating poor biodegradability. The pollutants are predominantly soluble COD such as lignin, cellulose, and organic acids, and the most severe pollution is the chlorine-bearing wastewater produced by bleaching.
White water: It possesses a substantial quantity of water and primarily comprises fine fibers, fillers, coatings, and dissolved wood components. It is predominantly insoluble in COD and possesses poor biodegradability. The added preservatives are toxic.
3 Paper production wastewater treatment methodologies
Black liquor and intermediate wastewater: alkali recovery, acid precipitation method, LB-1 alkali precipitation method, membrane separation method, flocculation sedimentation, biofilm method, anaerobic biological treatment, mesh microfiltration, air flotation, advanced oxidation.
White water: filtration, air flotation, sedimentation, screening.
The concentration of SS and COD in waste paper papermaking wastewater is relatively high, and COD is composed of two parts: insoluble COD and soluble COD. Non soluble COD typically accounts for the majority of the total COD composition. When SS is eliminated from the wastewater, the vast majority of insoluble COD is also eliminated. Therefore, the primary challenge to be addressed in the treatment of waste paper and papermaking wastewater is the elimination of SS and COD. There are primarily the following methods:
(1) Air flotation or sedimentation method. By employing air flotation or sedimentation methods and adding coagulants, the majority of SS can be eliminated, while the majority of insoluble COD and some dissolved COD and BOD5 can be eliminated.
(2) Conjoining physical and biochemical treatments. For large and medium-sized paper making enterprises with low wastewater discharge and high COD content, it is challenging to attain the national first level discharge standards through single-stage air flotation or sedimentation physicochemical methods, as soluble COD and BOD5 primarily need to be effectively eliminated through biochemical methods. Generally, physicochemical and biochemical treatment methods are employed.
(3) Sludge disposal and comprehensive utilization. The loss of pulp during the papermaking process is inevitable. There are
Tel
0371-12345678
Email
