Enhancement of sedimentation and coagulation with static magnetic field

The static magnetic field can be an alternative method for wastewater treatment. It has been proved that this physical factor, accelerates the biochemical processes, catalyzes advanced oxidation, intensifies anaerobic and aerobic processes or reduces swelling of activated sludge. There are also reports proving the positive impact of the static magnetic field on the coagulation and sedimentation, as well as the conditioning and dewatering of sludge. In order to be applied in larger scale the published results should be verified and confirmed. In the studies, the enhancement of sedimentation by the static magnetic field was observed. The best sedimentation was noted in the experiment, where magnetizers were placed on activated sludge bioreactor and secondary settling tank. No effect of the static magnetic field on coagulation with the utilization of PIX 113 was observed. However, the static magnetic field enhanced coagulation with the utilization of PAX-XL9. The results suggest that increased sedimentation of colloids and activated sludge, can in practice mean a reduction in the size of the necessary equipment for sedimentation with an unchanged efficiency of the process.


Introduction
Due to constantly increasing volume of produced wastewater, changes of their composition and properties, a steady evolution and development in the methods used for wastewater treatment can be observed, nevertheless effective and economically viable methods are still sought [1].An alternative to the methods used so far, can be an innovative technique based on the use of physical factor, that is a static magnetic field (SMF) [2].The SMF causes selective ionization, changes in electrical conductance of solutions.Due to this the molecules are polarized and receive and appropriate charge.
It has been proved that this physical factor, accelerates the biochemical processes, catalyzes advanced oxidation, intensifies anaerobic and aerobic processes or reduces swelling of activated sludge [1,3].There are also reports proving the positive impact of the SMF on the coagulation and sedimentation [4], as well as the conditioning and dewatering of sludge [1,5].The results, however, require verification and confirmation.
These examples confirm that the use of the SMF might become an alternative and prospective method which allows to improve wastewater and sludge treatment.Economic considerations prompt to use magnetizers as assistive technology systems in water purification, wastewater and sludge treatment [1,6].Magnetic fluids activators (MFA) are characterized by simple design.The greatest advantage is the lack of need to supply any form of energy and that the intensity of the magnetic field induction is not weakened over time [7].For these reason such devices can be operated efficiently over many years.Moreover, magnetizers can be used in system without the need of adaptation period [1].
The aim of the study was to evaluate sedimentation and coagulation efficiency in the bioreactors and in the secondary settling tanks.

Materials and methods
The studies were carried out on a laboratory scale.The wastewater used in experiments was a model of dairy wastewater obtained by dissolving skimmed milk powder in tap water.The inoculum was activated sludge from municipal wastewater treatment plant in Olsztyn.The wastewater was characterized by chemical oxygen demand (COD) of 4181 ± 890 mg O2/dm 3 , pH of 4.8 ± 0.2 and temperature 20.2 ± 1.2°C.
The studies were divided into two stages.In stage 1, the influence of the SMF on the sludge sedimentation in the secondary settling tank was investigated.The experiments were carried out in two activated sludge bioreactors, one with magnetizers placed on tank (series 1) and second one without magnetizers placed on tank (series 2) (Fig. 1).Raw wastewater were pumped to bioreactors from retention tank.The mixture of the activated sludge and the effluent was directed to the two secondary radial settling tanks, one with magnetizers placed (variant 1) and second one without magnetizers placed (variant 2).The technological parameters of the bioreactors and secondary settling tanks are shown in Table 1.The sedimentation was describe by the volume of activated sludge after 30 minutes in 1 dm 3 of wastewater (cm 3 /dm 3 ) and the sludge volume index (SVI30) (cm 3 /g TSS).The magnetizers were placed directly on the circumferences of the bioreactor and secondary settling tank.The magnetizers were built of two parts forming a ring.Permanently magnetized ceramic frits in the MFA emitted the SMF.In order to increase the direct impact of SMF, a set consisting of two rings were placed on bioreactors.The parameters of the MFA were width of the ring of 65 mm, height of the individual ceramic magnet of 45 mm, weight of the individual ring of 1.25 kg, and nominal diameter range of 90-110 mm.The maximal induction of SMF was 6 000 Gauss (0.6 T).
In the stage 2, the effect of the SMF on coagulation was observed.The coagulation was carried out in the chambers.To the chamber the inorganic coagulants were dosed.After dosing of coagulant the rapid stirring, slow stirring (flocculation) and sedimentation take place.Basic technological parameters of the chambers are presented in Table 2.The two series of experiments were done.In the series 1, the source of SMF was placed on coagulation chamber.In the series 2, the coagulation chamber was operated without the magnets (control) (Fig 2 .).The chambers were equipped with stirrer with adjustable speed.Each series was divided into two variants to test different type of the coagulant.Moreover, four different doses of the coagulants were investigated.The criterion for separating variant was type of used inorganic coagulant.In the variant 1, the ferric coagulant name PIX 113 was tested.In the variant 2, the aluminum coagulant name PAX -XL9 was tested.For both coagulants four doses were applied: 0.05 g/dm 3 , 0.1 g/dm 3 , 0.2 g/dm 3 , 0.4 g/dm 3 .The characteristics of the used coagulants is presented in Table 3.
Table 3.Chemical compositions of the coagulants.

PIX 113 PAX-XL9
Total Fe 11.8 ± 0.4% Al2O3 8.5 ± 0.4% Fe 2+ 0.4 ± 0.3% Al2 4.5 ±0.2 pH < 1 Chlorides 10.0 ± 1.0% pH 3.0 ± 0.6 % In the 2 stage, the effect of the SMF on coagulation was provided by the use of permanent neodymium magnets.Magnets were directed up with the positive side, opposite to the charges of the most natural colloids (Fig. 2).The characteristics of the used neodymium magnets is shown in Table 4.
In order to assess the effectiveness of coagulation supported by the SMF, changes in concentration of the COD in 15 minute intervals were recorded.The standard dichromate method was used to determine the COD.During this method high temperature mineralizer HT200S and spectrophotometer UV-VIS DR 5000 were used.Statistical analysis of the results was done with STATISTICA 10.0 PL.The hypothesis on the distribution of each analyzed variable was verified with the Shapiro-Wilk W test.The significance of differences between variables was stated with the use of one-way analysis of variance (ANOVA).The homogeneity of variance in groups was checked with the Levene's test.The significance of differences between analyzed variables was determined with the RIR Tukey test.Differences were found significant at α = 0.05.

Results and discussion
In the stage 1 the concentration of activated sludge in both series was about 3000 mg TSS/dm 3 .Similar values were observed by Hattori et al. (2001).The authors suggest that the magnetic field with induction of 0.08 T seems to not affect the growth of activated sludge [7].However, Xu and Sun (2008) observed that magnetic field with induction of 0.024 T and 0.06 T improved the quantity of mixed bacteria in medium.The authors also noted that propagation rate of mixed bacteria was the biggest at the SMF induction of 0.06 T. From the presented results it appears that the enhancement of bacteria growth can be observed with magnetic induction lower then 0.1 T. Whereas higher densities of magnetic field does not affect the growth of bacteria [5].
In the studies, the enhancement of sedimentation by the SMF was observed.The best sedimentation was noted in variant 1 of series 1, where magnetizers were used on activated sludge bioreactor and secondary settling tank.The volume of the sludge after 30 minutes of sedimentation was 440 ± 15 cm 3 /dm 3 and SVI30 was 145.25 ± 4.95 cm 3 /g (Fig. 3, 4).The application of magnetizers only on the activated sludge bioreactor decreased the sedimentation properties of the sludge.The volume of the sludge after 30 minutes of sedimentation and the SVI30 increased of 200 cm 3 /dm 3 and 70 cm 3 /g, respectively.The sedimentation of the activated sludge was decreased in the series 2, when the activated sludge bioreactor was operated without influence of the SMF.The volume of the sludge after 30 minutes of sedimentation was 920 ± 12.5 cm 3 /dm 3 and 960 ± 9.5 cm 3 /dm 3 in the variant 3 and 4, respectively.Similar tendency was observed in case of the SVI30.Hattori et al. (2001) also used SMF to enhance to sludge sedimentation [7].The authors applied the SMF with induction of 0.08 T. In the experiment, their used the wastewater circulation system in which the activated sludge flow through magnetic field.The SVI30 observed by Hattori et al. (2001) was lower then in presented experiments and it was about 90 cm 3 /g.The enhancement of sedimentation was also observed by Gokon et al. (2002).The authors have shown, that despite the higher concentration of the activated sludge, the smaller volume was occupied by the sludge after sedimentation in reactor with SMF in comparison to the control reactor [8].Similar result were also observed for aerobic granular sludge.The magnetic field with induction of 48 mT, significantly decreased SVI from 77.32 cm 3 /g to 20.93 cm 3 /g [9].
The impact of SMF on the rate of COD removal was only found in the case of the highest dose of coagulant PAX-XL9.The rate of the COD removal in the control was 73 ± 9.26 mgO2/dm 3 •min, while in the chamber with magnets it increased to 87 ± 7.23 mgO2/dm 3 •min.(Fig. 8) In the case of the coagulant PIX 113, the rate of COD removal was 74±7.85 mgO2/dm 3 •min in the control, and 79 ± 11.77 mgO2/dm 3 •min in the chamber with magnets (Fig. 7) .

Fig. 3 .
Fig. 3.The volume of the sludge after 30 minutes of sedimentation.

Fig. 5 .
Fig. 5.The COD values depending on the dose of the coagulant PIX 113.

Fig. 6 .
Fig. 6.The COD values depending on the dose of the coagulant PAX-XL9.

Table 1 .
Technological parameters of the bioreactors and the secondary settling tanks.

Table 2 .
Technological parameters of coagulation chambers.

Table 4 .
Characteristics of neodymium magnets.