Effect of bioenzyme on deinking mill effluent treatment by dissolved air flotation, Solutions!, Online Exclusives, October 2004

APPLICATION: This study focuses on improving the water recycling efficiency in pulp and paper mills by combining novel physico-chemical treatment along with existing technology to clarify white water.

INTRODUCTION
Reduction of fresh water consumption is of strategic importance for many pulp and paper mills in North America. An effective way to reduce fresh water and chemical additive consumption in a deinking mill and in papermaking processes would be the progressive recirculation of white water by selectively reusing them in different phases in the mill. It is now well known that Dissolved Air Flotation (DAF) is an efficient way to clarify white water in a mill environment and recirculate it back to the system [1]. One of the drawbacks of the existing DAF technology is the very poor removal efficiency of dissolved solids, color and anionic components. Therefore, the recirculation of DAF-clarified white water leads to the progressive accumulation of dissolved solids and anionic components in the process water and their eventual deposition in the paper machine and other operating units. The elimination of these contaminants from white water in an integrated mill is a subject of research and industrial interest. Any novel process or additive that can eliminate dissolved solid or anionic components from white water will have a beneficial effect on achieving progressive mill closure in a DIP or papermaking mill.

Until now ultra-filtration, membrane technology and reverse osmosis have been used to remove high molecular weight dissolved organic components from a mill effluent stream [2]. All these processes are capital intensive and found only limited applications. There is high demand for low capital-intensive chemical or biochemical separation processes that can effectively eliminate anionic components and dissolved solids with a higher tolerance of process variations. The present study has been focused to improve the efficiency of the DAF process by investigating various chemical and biochemical-assisted separations of dissolved solid, color and suspended solid components. One of the common additives used in a DAF process is a suitable combination of colloidal nano-silica and a cationic polymer, such as polyacrylamide [1,3]. Although bentonite microparticles are still in use, nano-silica particles are finding increasingly more applications because of their high surface area and very low dosage.

Sodium aluminosilicate or zeolite is known to be an effective metal chelating agent. This unique characteristic is advantageous for removing ions of salts from wastewater. Deinking mill wastewater contains a significant amount of dissolved calcium as well as other form of Ca-salts. White water also contains a large amount of heavy metals and anions. We have demonstrated that selective Na-aluminosilicates can be easily separated from a pulp suspension by a flotation process [4]. In recent years, attention has been drawn to the use of biochemicals to remove dissolved components including color from wastewater. In addition, a number of processes have been developed which are directed at the removal of specific contaminants, for example: enzymes from an atypical strain of Bacillus stearothermophilus have been used to degrade algal cell walls [5]; a combination of bacteria and enzymes have been used to improve the water quality of standing bodies of water [6]; cellulases have been used to digest wood/paper compositions [7]. In a patent, Xanthomonas maltophilia and Bacillus thuringiensis have been used to degrade polar organic solvents [8]. In yet another patent, yeast has been used to digest carbohydrate-containing wastewater [9]. Combinations of amylase, lipase and/or proteases have been also used to digest colloidal material such as starch, grease, fat and protein [10]. However, each of these compositions is directed at only a specific contaminant and they do not address the variety of contaminants, which are usually found in white water and polluted water. Therefore, it is desirable to provide a composition and method for the digestion and separation of a variety of pollutants by modifying the DAF process, which has already gained a widespread acceptability in pulp and paper mills.

The present work is directed at a composition for use in the DAF process to treat white water. The composition comprises a biopolymer or bioenzyme, chemical flocculants and coagulants and/or a nano-particle. Emphasis has been given to the removal of dissolved organic components and color from white water by extending the retention time after the treatment process from a few minutes to 12 hours.

MATERIALS & METHODOLOGY
White water was collected from a local deinking mill. Two white water samples were obtained from the alkaline loop (Figure 1) and kept refrigerated at 3°C. This deinking mill is involved in recycling ONP and OMG and uses a flotation deinking process to separate the ink from the fibre. The sampling date was November, 2000 and analysis was done without unnecessary time delay.

Chemicals were added to a 750 mL wastewater sample and the sample was thoroughly stirred after each addition. The sample was placed in the DAF apparatus and air was forced up through the sample in the column. The air pressure was given in three equal spurts over a span of one minute. Retention time was initially one hour and then the clarified wastewater was collected through the side valve. Standard C.P.P.A. tests were performed on the collected sample. A total of seven DAF runs were initially executed.

From these results, the best two DAF trials were used for further study. A new bioenzyme was received from LPM Technologies Inc. and DAF runs were performed using this bioenzyme and the above best two chemical sequences. Initially, a bioenzyme only control was done at 500 ppm. Secondly, DAF runs were performed at three different bioenzyme concentrations with pH and temperature control. Finally, the best two chemical sequences from the previous DAF runs were integrated with the bioenzyme at a trial concentration of 10 ppm and with pH and temperature control.

Upon observation of the DAF process and the results, it was determined that increasing the retention time in the column may lead to a substantial improvement in all tests. The bioenzyme DAF runs with chemical addition were repeated with pH, temperature and time control and tests were done at 1, 4, 8, and 12 hours to observe the effects of increased retention time. Only the minimum amount of clarified wastewater needed to perform the tests at a particular time interval was removed from the side valve, while the rest of the clarified wastewater was left in the column to allow the run to continue.

Solution Preparation

All chemicals were mixed with distilled water to make 1% or 5% solutions when necessary. The Chitosan, a biopolymer, and three different colloidal silica solutions with known particle size were used: Silica-C1 700 m2/g, aqueous colloidal dispersion of silica; Silica-C2 900 m2/g, aqueous colloidal dispersion of silica; and Silica-C3, amorphous silica in aqueous colloidal solution. Chitosan solutions were made up with 50% distilled water and 50% of a 2% acetic acid solution. When temperature and pH control was needed, the temperature and pH of the wastewater sample was taken pre-addition of any chemicals and before it was placed in the column. A final temperature and pH of the clarified white water sample removed was also taken. The pH of the bioenzyme DAF runs was maintained at 6.0 +/- 0.2. The pH was adjusted using sulphuric acid. The bioenzyme DAF solutions were heated to approximately 50?C. All other DAF runs were performed at room temperature.

Standard Tests

1. Determination of Solids Content of Pulp and Paper Mill Effluents (CPPA Standards): Total Suspended Solids, Fixed Suspended Solid (Ash), Total Dissolved Solids, Fixed Dissolved Solids (Ash), Volatile Suspended Solids, Volatile Dissolved Solids, and Total Solids were determined according to CPPA standards.



2. Hydrogen Ion Concentration (pH), Temperature and Conductivity were measured.



3. The turbidity, transmittance (colour) and absorbance were measured in NTU, %T, and A respectively. This was done using a HACH 2100AN Turbidimeter.

RESULTS

Effect of biopolymer and colloidal silica

Table 1 shows the composition of the white water received from the deinking mill. The two samples that were received from two different alkaline tanks showed good consistency in their composition. The above white water samples were used throughout our study to modify the clarification process using DAF. Table 2 demonstrates the results of several DAF experiments performed in the laboratory using our 1.5 L lab-DAF system. In this study, Chitosan was used as a flocculation enhancer and dissolved solid removal agent. The results were compared with the conventional chemical flocculating system, which is given as composition 1 in Table 2. We have selected composition 1 as a control system for this study because this chemical combination of 40 ppm polyamine, 6 ppm polyacrylamide, and 500 ppm propyltrimethyl ammonium chloride (PAC) gave the overall best results among 30 alternatives tested in our labs using 10 different chemicals. Composition 1 gave the highest transmittance for the sample received, but did not lower the dissolved solids and colour significantly. The use of a biopolymer in composition 2 with the following combination of 10 ppm polyacrylamide, 200 ppm PAC, 10 ppm chitosan and 20 ppm polyamine did not show any improvement over our control system. Surprisingly, we have found that a high dosage of chitosan above 200 ppm helped to reduce the dissolved solids by about 15% (results are not shown).

Table 1. Results of white water samples received.

Avg. = Average, T = Temperature, Cond. = Conductivity, Abs. = Absorbance, Trans. = Transmittance, Turb. = Turbidity, TSS = Total Suspended Solids, FSS = Fixed Suspended Solids, TDS = Total Dissolved Solids, FDS = Fixed Dissolved Solids, VSS = Volatile Suspended Solids, VDS = Volatile Dissolved Solids, TS = Total Solids.

Table 2. Results of DAF-treated samples with chemical and/or biopolymer treatment.

Trial Chemical Compositions: 1: 40 ppm polyamine, 6 ppm polyacrylamide, 500 ppm PAC; 2: 200 ppm PAC, 20 ppm polyamine, 10 ppm Chitosan 151906B, 10 ppm polyacrylamide; 3: 100 ppm PAC, 300 ppm Aluminum Chlorohydrate, 20 ppm Polyamine, 10 ppm Chitosan, 10 ppm Polyacrylamide, 200 ppm CBV 901 (Zeolite); 4: 300 ppm PAC, 200 ppm BMA 780, 40 ppm polyamine, 6 ppm polyacrylamide; 5: 300 ppm PAC, 200 ppm BMA 890, 40 ppm polyamine, 6 ppm polyacrylamide; 6: 300 ppm PAC, 200 ppm BMA 090, 40 ppm polyamine, 6 ppm polyacrylamide; 7: 300 ppm PAC, 200 ppm zeolite, 40 ppm polyamine, 6 ppm polyacrylamide.

Table 3. Results of bioenzyme DAF-treated samples without chemical addition.

Trial Chemical Compositions: 1: 500 ppm bioenzyme only; 2: 100 ppm bioenzyme only; 3: 10 ppm bioenzyme only.

Although this indicates the potential of this biopolymer to react with dissolved components, the reaction is probably not strong enough to have a significant impact when using a low dosage. It is recognized that the high cost of chitosan might be one of the critical factors to prevent widespread commercial use of this biopolymer in white water clarification.

In compositions 3 and 7 in Table 2, it is observed that the addition of high surface area solid zeolite particles did not improve the clarification efficiency; instead, it reduced the transmittance of the white water significantly compared to our control system. This result is in contrast to our initial presumption, which states that high surface area zeolite particles would absorb some of the colour components from the water system. One of the many possible reasons for this unexpected result is a poor separation of fine zeolite particles by DAF. This was evident from the results of a filtration process carried out with the clarified water. An accumulation of zeolite particles on the filter paper was evident. Subsequently, we have carried out several trials with colloidal silica in compositions 4, 5 and 6 in Table 2. Only little improvement has been achieved with respect to dissolved solid removal over our control system. Results from Table 2 suggest that under the specified conditions, they are not effective in removing dissolved solid and colour from the white water.

Effect of bioenzyme

The results for the bioenzyme-assisted DAF runs without chemical addition are given in Table 3 and with chemical addition in Table 4. Bioenzyme as a sole additive in the dissolved air flotation process does not have any beneficial effect on dissolved solid and color removal (as indicated by the transmittance values) and it is true for all three bioenzyme concentrations in Table 3. Further, we have combined this bioenzyme with the best two chemical-assisted DAF runs with a trial concentration of 10 ppm of the given enzyme. Results in Table 4 demonstrate that this bioenzyme can remove dissolved solid from the white water when it is suitably combined with other flocculating and coagulating agents. The effectiveness of the bioenzyme is further improved if the surface area of the colloidal silica particles is increased as evident from the lower value of dissolved solids. Silica-C2 has a higher surface area than Silica-C1. Although the bioenzyme used in this study was effective to reduce dissolved solid by about 20%, higher removal efficiency is desired to offset the cost of bioenzyme. In order to explore the full potential of the bioenzyme used in this study a modification of the DAF process has been proposed.

Modification of DAF with an additional retention tank

We propose a longer retention time of the clarified water from the DAF after the chemical treatment. It is presumed that the degradation of dissolved organic components in presence of bioenzyme is kinetically controlled. In order to verify the above hypothesis, two reference DAF runs were repeated to observe the kinetic effects. We have carried out a time dependent study on the effect of bioenzyme and white water and the results are presented in Table 5. ‘1 hour’ in Table 5 signifies that the water has basically been analyzed immediately after the completion of the DAF process, i.e., allowing no further reaction time between chemicals and water. Results show gradual improvement of water quality with time in all respects. All measured components including dissolved solid and suspended solid were progressively removed with time. Typical water quality parameters and their time dependence are graphically presented in Figures 2-4.

Table 4. Results of bioenzyme DAF-treated samples with chemical addition.

Trial Chemical Compositions: 1: 300 ppm PAC, 200 ppm Silica C-2, 40 ppm polyamine, 6 ppm polyacrylamide, 10 ppm bioenzyme; 2: 300 ppm PAC, 200 ppm Silica C-1, 40 ppm polyamine, 6 ppm polyacrylamide,10 ppm bioenzyme.

Table 5. Results of bioenzyme DAF-treated samples with chemical addition and time control.

Trial Chemical Compositions: 1: 10 ppm bioenzyme only; 2: 300 ppm PAC, 200 ppm Silica C-2, 40 ppm polyamine, 6 ppm polyacrylamide, 10 ppm bioenzyme; 3: 300 ppm PAC, 200 ppm Silica C-1, 40 ppm polyamine, 6 ppm polyacrylamide, 10 ppm bioenzyme. Note: 1 hour = no retention time after DAF process

Figure 2. Absorbance results for bioenzyme DAF-treated samples at 1, 4, 8, and 12 hours. (i) 10 ppm bioenzyme control; (ii) Case 1: 300 ppm PAC, 200 ppm Silica C-2, 6 ppm polyamine, 40 ppm polyacrylamide, 10 ppm bioenzyme; (iii) Case 2: 300 ppm PAC, 200 ppm Silica C-1, 6 ppm polyamine, 40 ppm polyacrylamide, 10 ppm bioenzyme.

Figure 3. Turbidity results for bioenzyme DAF-treated samples at 1, 4, 8, and 12 hours. (i) 10 ppm bioenzyme control; (ii) Case 1: 300 ppm PAC, 200 ppm Silica C-2, 6 ppm polyamine, 40 ppm polyacrylamide, 10 ppm bioenzyme; (iii) Case 2: 300 ppm PAC, 200 ppm Silica C-1, 6 ppm polyamine, 40 ppm polyacrylamide, 10 ppm bioenzyme.

Figure 4. Total dissolved solid results for bioenzyme DAF-treated samples at 1, 4, 8, and 12 hours. (i) 10ppm bioenzyme control; (ii) 300 ppm PAC, 200 ppm Silica C-2, 6 ppm polyamine, 40 ppm polyacrylamide, 10 ppm bioenzyme; (iii) 300 ppm PAC, 200 ppm Silica C-1, 6 ppm polyamine, 40 ppm polyacrylamide, 10 ppm bioenzyme.

From the results of the bioenzyme DAF runs, it was observed that the chemical sequence which gave the lowest total dissolved solids at 1 hour retention was 300 ppm PAC, 200 ppm Silica-C2, 40 ppm polyamine, and 6 ppm polyacrylamide. At 12 hour retention, the lowest total dissolved solids was given by 300 ppm PAC, 200 ppm Silica-C1, 40 ppm polyamine, 6 ppm polyacrylamide and 10 ppm bioenzyme. The chemical sequence for the lowest volatile dissolved solids was found to be 300 ppm PAC, 200 ppm Silica C-1, 40 ppm polyamine, 6 ppm polyacrylamide and 10 ppm bioenzyme for the full 12 hours. Absorbance, transmittance and turbidity measurements for the bioenzyme runs were still relatively high over the 12 hours in comparison to the DAF run of PAC, polyamine and polyacrylamide alone. This will be considered for future bioenzyme DAF runs. As well, the combination of 300 ppm PAC, 200 ppm zeolite CBV 100, 40 ppm polyamine, and 6 ppm polyacrylamide will be considered, as its total dissolved and volatile solids results were also relatively low.

DISCUSSION

Chitosan, which is a cationic biopolymer with relative high charge density, showed excellent flocculation efficiency even with a low dose. Apparently this biopolymer does not undergo any bio-chemical reaction with the low and high molecular dissolved solids present in the white water system within the time interval of DAF operation as evident from the very high residual total dissolved solid (TDS) after the clarification process. It is also important to note that none of the tested compositions given in Table 2 had a positive influence on the dissolved solid and conductivity of the water used. This observation strongly suggests that removal of dissolved solid and anionic components from white water is not dependent upon the flocculation efficiency of a chemical or biochemical. Poor performance of porous zeolite with respect to TDS removal also fails to explain an anticipated positive influence of adsorption properties of zeolite on the removal of high molecular weight dissolved solids.

Interesting insight has been obtained when bioenzyme was used as a cofactor in treatment with chemical compositions for the DAF process. The lowest value of the dissolved solid has been achieved by combining conventional chemicals with the bioenzyme investigated. A synergistic effect of this novel cofactor in the white water clarification process was apparent. Although the reason for this synergism effect, if any, is not the subject of this study, it is possible that a separate removal mechanism exists for the conventional chemicals and the bioenzyme. Poor removal efficiency of bioenzyme alone and the synergistic effect in combination with conventional chemicals strongly suggests a possible biochemical reaction between the enzyme and the dissolved solid. This hypothesis is further supported by the fact that the removal efficiency with bioenzyme as a cofactor is a kinetically controlled process. Results from Table 5, after a 12-hour reaction interval, provide possible evidence of bioenzyme reaction with dissolved solid. This reaction between enzyme and dissolved solid is further supported by the time dependent results of transmittance, turbidity and absorbance. Progressive increase in the transmittance value and decrease in the turbidity and absorbance values with the system containing bioenzyme alone demonstrate the color and colloidal component removal functions of the bioenzyme studied. Another important observation is that the cofactor did not remove fixed dissolved solid (low volatile dissolved compounds) but almost all of the dissolved solid removal was in fact volatile dissolved solid. This strongly suggests that the bioenzyme investigated in this report is reactive towards highly volatile dissolved compounds.

LITERATURE

1. Guss, D. B., “The synergy of flotation and filtration in wastepaper treatment and recycling,” Proceedings of the 1994 Recycling Symposium, Boston, p. 407.
2. Webb, L. J., Pulp Paper Intl. 39(6): 43(1997).
3. Bourassa, C., “Étude de système de traitement DAF d’une eau usée de procédé de désencrage,” Masters degree thesis, Université du Québec, Trois Rivières, Québec March, 2000.
4. Rebarbar, E. S., Tappi J. 81(5): 97(1998).
5. Liu, Chi-Li, U. S. pat. 5,139,945 (Aug. 18, 1992).
6. Runyon, Larry, U. S. pat. 5,227,067 (July 13, 1993).
7. Fugua, C. R., Thomas, R. L. and C. H. Gooding, U. S. pat. 5,326,477 (July 5, 1994).
8. Middleditch, B. S. and P. S. K. Lee, U. S. pat. 5,369,031 (November 29, 1994).
9. Chigusa, K. and Matsumaru, M., U. S. pat. 5,075,008 (December 24, 1991).
10. Wong, J. M. and Lowe, T. J., U. S. pat. 5,882,059 (November 21, 1989).

About the authors:
Ansuya Mehta and Yonghao Ni are with the Dr. Jack McKenzie Limerick Pulp and Paper Research Centre, University of New Brunswick; Mohini Sain,is with the Forestry and Pulp & Paper Research Centre—Chemical Engineering & Applied Chemistry, University of Toronto; and Daniel Morneau is with LPM Technologies Inc., Québec, Canada. You can reach Mohini Sain by email at m.sain@utoronto.ca.