4.3 PULP AND PAPER MILL WASTEWATER EXPERIMENTS
5.2.5 Pollution load experiment with 50 times and 100 times diluted pulp and paper
Figure 19 shows that 50 times diluted pulp and paper mill wastewater needed less polymer titrant for the charge neutralization after one hour of magnetic treatment than after two, three- or fourth-hour treatment. So, in this case, only one-hour treatment is enough to achieve the zero-charge state.
Figure19: Zero charge state of the 50 times diluted pulp and paper wastewater sample during 4 hours of MWT with superfloc C-577 used as a polymer titrant, the untreated graph is without MWT
Figure 20 shows that 100 times diluted pulp and paper wastewater, untreated, and one, two- and three-hour treated samples needed the same amount of polymer titrant to achieve zero charge state.
Figure 20: Zero charge state of the 100 times diluted pulp and paper wastewater sample during 4 hours of MWT with superfloc C-577 used as a polymer titrant, the untreated graph is without MWT
Untreated 1 hour 2 hour 3 hour 4 hour
-400
Untreated 1 hour 2 hour 3 hour 4 hour
6 DISCUSSION
MWT is widely used for reducing household water hardness levels. According to several studies (Higashitani et al., 1993; Barrett and Parsons, 1998; Coey and Cass, 2000; Kobe et al., 2001;
Kobe et al., 2002; Botello-Zubiate, 2004; Knez and Pohar, 2005; Coey, 2012) MWT reduces the calcite formation of CaCO3 in the water. Calcite builds up the hard scale on the surface.
MWT turns the calcite into aragonite. Aragonite forms a slurry on the surface, which can be wash away by the water stream from the surface. According to many reports (Baker and Judd, 1996; Parsons et al., 1997a; Barrett and Parsons, 1998; Myśliwiec, Szcześ and Chibowski, 2016) MWT can influence the zeta potential and particle charge of the CaCO3. An experiment done by Parsons et al. (1997a) shows that the average reduction of zeta potential is 16 % for calcium carbonate in a solution after the magnetic treatment. Gehr et al. (1995) showed that the suspension of calcium sulfate’s particles reduces 25% of surface charge, and also the level of suspended and dissolved solids changed in their experiments. Higashitani et al. (1992. 1993) proposed that the reduction of zeta potential of the particles of the solution leads to the particles being unstable which results in rapid aggregation and sedimentation.
Particle charge is an important parameter for estimating the effects of MWT. PCD03 device needs to be cleaned after every experiment so that no particle or solution of the previous experiment can alter the result of the present experiment. According to the operation instruction (BTG Mütek, 2003) the cleaning solution prepared for the PCD03 device needs to be prepared with NaBr or KCl and acetone and water. NaBr or KCl salts produce ions (positive and negative charge) in the water. That is why the effect of KCl on MWT was studied. All results with four different salt concentrations along with the control experiment showed that the particle charge vs. pH curve reached the zero charge state exactly at the same place. It was thus proved that the cleaning solution which was used before every experiment does not alter the particle charge results of the other experiments and could be safely used.
Turbidity is one of the parameters by which the presence of particles in the water can be determined. Generally, it is done before and after any kind of water treatment to check how much colloidal and dissolved solids are reduced by the water treatment process. Wang et al.
(1997) showed in their experiment that under magnetic treatment the turbidity of calcium carbonate solution is increased during 0 to 5 minutes and then it starts to decrease due to the settlement of the particles, while without MWT turbidity remains the same in all time span. The effect of MWT on the turbidity of the TiO2 solution experiment was done to check if the MWT
creates any impact on turbidity. According to the results, the turbidity of the untreated samples was slightly less than the turbidity of treated samples. All treated samples had turbidity of above 35 NTU while untreated samples’ turbidity is 32 NTU. The results are in line with those of Wang et al. (1997). In the conducted experiment, sample particles or flocs did not have time to settle due to the experimental setup, also flock size and characteristics were not measured. The same experiment was done with the pulp and paper mill wastewater sample. In that experiment, it was tried to find out the correlation between CODCr and turbidity and MWT. In figure 15 the experiment was done without MWT but the result for the turbidity was the same as in figure 14 with MWT, while CODCr data was fluctuating like in figure 14 experiment. It is thus not clear that MWT has any effect on turbidity and CODCr. One possible explanation can be that due to the experimental limitation of the Bauer device we cannot move the flocks from the Bauer device. That is why the concentration of the particles is the same in the sample before and after treatment.
Many studies (Baker and Judd, 1996; Parsons et al., 1997a; Barrett and Parsons, 1998;
Myśliwiec, Szcześ and Chibowski, 2016) have shown that MWT can influence the zeta potential and particle charge of the CaCO3. Colloidal particles in water have a net negative or positive surface charge that is why they generally repel each other. By adding a coagulant this surface charge can be reduced (Leiviskä, 2009). So, the main goal to add coagulants in wastewater is to reduce the surface charge of the particles. The idea of MWT is also the same, to reduce the surface charge.
The effect of MWT on TiO2 particles experimented, results shown in figure 12 and the control experiment were shown in figure 13. In figure 13, it can be seen that without MWT particle charges are the same in three samples but in figure 12 it can be seen that the untreated sample’s particle charge is near 150mV and during the first hour of treatment sample’s particle charge increased near 250mV. The charges started to drop and during the 8th-hour charge is negative.
So, it can be assumed that the sample’s particles gained zero charge state between the 7th and 9th hour. The reduction of particle charge result is supported by several earlier experiments (Baker and Judd, 1996; Parsons et al., 1997a; Barrett and Parsons, 1998; Myśliwiec, Szcześ and Chibowski, 2016).
Pulp and paper wastewater are thick and viscous substance which contains lots of organic material, as shown by the very high CODCr result obtained in this study. According to Haq and Raj (2019) final wastewater of pulp and paper has strong color and it contains high
concentrations of BOD, COD, AOX (absorbable organic halides), SS (suspended solids), TDS (Total dissolved solids), phenolics, and plant materials such as lignin, tannin and resin acids.
Several coagulants such as PIX-322, PAX-60, PAX-XL 100, and Superfloc C-577 were tested in this study to find out the optimum coagulant and polymer titrant for this type of wastewater.
PIX-322 and PAX-60 could not coagulate the dissolved solids, while PAX-XL 100 could but it took a high amount of PAX-XL 100 (400µl to see some floc in 100 ml sample) coagulant to produce floc and suspended solids. Among those coagulants, Superfloc C-577 showed the best results. It produced floc and flocs settled in a short time. 200µl of Superfloc C-577 for a 100 ml sample was needed, which is half of the needed amount of PAX-XL 100. The optimum doses of the coagulant are usually close to that required to neutralize the surface charge carried by the particles and thus it can be used in determining the coagulant dosage (Beulker and Jekel, 1993).
Baker and Judd (1996) conducted an experiment that was reproduced by Tombacz et al. (1991) experiment. In the experiment (Baker and Judd, 1996), they compared static and dynamic magnetic treatment methods. The dynamic magnetic treatment method consisted of flowing the recirculating solution through the magnetic field. The static magnetic treatment system was continuous exposure to the same magnetic field. In the dynamic treatment method, the particle gained zero charges during the first and second hour of MWT. In this thesis, the control experiment of pulp and paper wastewater (figure 17) showed that the polymer titrant was needed the same amount for different hour samples. All the samples were without MWT, so the particles had not gained or lost any charges because of the treatment and behaved in the same way in the polymer titration. On the other hand, in figure 16 samples were treated with MWT at different times. Particle charges of different MWT treatments were neutralized with different polymer doses compared to each other and untreated samples meaning that MWT had changed their charges. The lowest polymer consumption in 1-hour MWT treatment shows that the particles had lost most of their negative charges during the first-hour MWT treatment. The result is similar to Baker and Judd (1996) and Tombacz et al. (1991) experiment results. The TiO2 experiment result shown in figure12 also showed a similar result.
In the TiO2 experiments, it was seen that the TiO2 particles gained zero charge state between the 7th and 9th hour (figure 12-13). The same experiment was done to find out if the pulp and paper mill wastewater sample could also reach zero charge state without the help of coagulant.
However, the pulp and paper samples did not show that kind of result. But it was seen that the pulp and paper wastewater sample gained isoelectric point during the first hour which supports (Baker and Judd (1996) and Tombacz et al. (1991) experiment results.
Parsons et al. (1997a) experimented and showed that the reduction of zeta potential of calcium carbonate solution is dependent on the levels of calcium in the solution. Similarly, in this study, the sample containing more particles gained a zero charge state more quickly than the samples containing fewer particles. Figures 19 and 20 showing that MWT is more effective as the particle content is high.
7 CONCLUSIONS
MWT to reduce hardness in the household water system has been used for decades. There is almost no scientific work done to implement it in the wastewater system. Two types of wastewater were used in this experiment, one is a homogenous TiO2 solution and the other one is heterogenous pulp and paper mill wastewater. Homogenous TiO2 solution achieved zero charge state with the MWT process and without any influence of coagulant while heterogenous pulp and paper mill wastewater did not achieve zero charges state though it was in the MWT process for a longer time. However, in both cases, they showed that change of particle charge through MWT mostly happens during the first hour. The results also showed that the density of particles or pollution load influences the process of MWT. In the case of pulp and paper wastewater, it also took lesser coagulant to achieve zero charges state comparing to the without MWT process. From all of the above experiments, it can be said that MWT can be a good option in the case of pulp and paper wastewater treatment to reduce coagulant dose and thus expenses.
Further experiments are still needed to evaluate the potential of the use of MWT in wastewater treatment.
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