P(AM-DAC-BA) was synthesized through copolymerization of acrylamide (AM), acryloyloxyethyl trimethyl ammonium chloride (DAC), and butylacrylate (BA) under ultraviolet (UV) initiation using response surface methodology (RSM). The influences of light intensity, illumination time, and photoinitiator concentration on the intrinsic viscosity
In recent years, the mount of sewage treated by municipal wastewater treatment plant increases rapidly, resulting in a large amount of excess activated sludge generated, whose moisture content is up to 99.5%, and the sludge containing organic matter and noxious substances will harm the environment and must be handled properly [
Currently sludge conditioners used by municipal wastewater treatment plant are cationic polyacrylamides (CPAMs) [
Response surface methodology (RSM) is used to optimize the testing program or the relationship between indicators and factors in mathematical model, using multiple regression equation to fit the second factor and the functional relationship between indicators, through the regression equation analysis to find the optimal process parameters [
In order to enhance the dewaterability of the waste-activated sludge, P(AM-DAC-BA) in this study was synthesized through UV-initiated polymerization with AM, DAC, and BA as monomers.
To obtain an optimum synthetic condition, the effect of photoinitiator concentration, incident light intensity, and illumination time on intrinsic viscosity was investigated, and using these investigated data, the response surface methodology (RSM) was fitted to analyze their mutual effect and attain a better condition for preparation of P(AM-DAC-BA). The P(AM-DAC-BA) used in sludge dewatering showed that the dewaterability of waste activated sludge was affected by flocculant dosage and pH significantly.
The acrylamide (AM) (Chongqing Lanjie Tap Water Company, Chongqing, China), butyl acrylate (BA) (Tianjin Guangfu Fine Chemical Research Institute, Tianjin, China), acryloyloxye-thyltrimethyl ammonium chloride (DAC, 80% in water) (Guangchuangjing IMP. EXP. Co., Ltd., Shanghai, China), and the photoinitiator 2,2′-azobis(2-methylpropionamide) dihydrochloride (V-50) (Ruihong biological technology, Shanghai, China) were purchased. All the reagents used in this experiment were without further purification. Commercially available cationic polyacrylamide (CPAM) was sourced from Shandong Polymeibio-Chemicals Co., Ltd., China.
Some instruments, such as UV-A ultraviolet ray intensity meter (Beijing Normal University Photoelectric Instrument Factory, China), TU-1901 double beam ultraviolet and visible spectrophotometer (Beijing Purkinje General Instrument Co., Ltd., China), 550Series II infrared spectrometer (BRUKER 4 Company, Switzerland), were used in this study.
A predetermined mass ratio of AM, DAC, and BA was added into the quartz jar, then adding a certain amount of deionized water, surfactant, and urea. The reagent was mixed and stirred, and then the reaction solution was bubbled with nitrogen gas for 30 min at room temperature, and, subsequently, the predetermined photoinitiator dosage was added. The reaction vessel was exposed to radiation for 120 min with a 500 W high pressure mercury lamp at ambient temperature. After radiation, the product was purified with acetone and ethanol and then was dried and made into powder. Based on the results of the optimized design by response surface method (RSM), the ranges of synthesis condition were listed in Table
The intrinsic viscosity of CPAM was determined by one point method; the detailed method and operating steps were shown in “the determination method of intrinsic viscosity of polyacrylamide” (GB-T12005.1-1989).
Experimental sludge was collected from sludge storage tank of Chongqing Baihan wastewater treatment plant; the water content of which was 98.56%, pH value was 7.56–7.62, and the sludge with fine particles was brown and odor. The density of sludge was 1.02 g/mL.
In sludge dewatering experiments, the P(AM-DAC-BA) was added into a 250 mL beaker with 200 mL sludge included. The pH of the sludge was adjusted with 0.1 mol/L NaOH and 0.1 mol/L HCl. Subsequently, it was rapidly mixed by stirring at 120 rpm for 30 s, followed by a 10 min quiescent settling time. Then, the supernatant was extracted from the beaker 1 cm below the solution surface by a syringe for turbidity measurement. Sludge samples were filtered through a vacuum suction filter machine at the pressure of 0.05 MPa. The dry solid content (DS) was evaluated by the following equation:
In order to optimize synthetic condition, establish the model for UV-induced polymerization of P(AM-DAC-BA) and investigate the UV effect on
Experimental ranges and levels of the independent test variables.
Variables | Ranges and levels | ||
---|---|---|---|
−1 | 0 | +1 | |
Incident light intensity ( |
700 | 1200 | 1700 |
Illumination time (min) ( |
40 | 80 | 120 |
Photoinitiator concentration (wt.‰) ( |
0.25 | 0.50 | 0.75 |
3-Factor Box-Behnken design and the value of response function (
Run order | Coded variables | Intrinsic viscosity |
|||
---|---|---|---|---|---|
|
|
|
Actual value | Predicted value | |
1 | 0 | 0 | 0 | 1.85 | 1.86 |
2 | +1 | 0 | 1 | 1.93 | 1.86 |
3 | −1 | 0 | 1 | 1.45 | 1.35 |
4 | 0 | −1 | 1 | 1.36 | 1.42 |
5 | +1 | +1 | 0 | 2.03 | 2.00 |
6 | −1 | +1 | 0 | 1.64 | 1.63 |
7 | +1 | 0 | −1 | 0.81 | 0.90 |
8 | 0 | 0 | 0 | 1.85 | 1.86 |
9 | 0 | −1 | −1 | 0.71 | 0.61 |
10 | −1 | 0 | −1 | 0.90 | 0.96 |
11 | 0 | +1 | 1 | 1.95 | 2.04 |
12 | −1 | −1 | 0 | 1.00 | 1.02 |
13 | 0 | 0 | 0 | 1.80 | 1.86 |
14 | 0 | +1 | −1 | 1.57 | 1.86 |
15 | 0 | 0 | 0 | 1.85 | 1.86 |
16 | +1 | −1 | 0 | 1.09 | 1.09 |
17 | 0 | 0 | 0 | 1.95 | 1.86 |
On the basis of response surface methodology, Box-Behnken experimental designs were selected for optimizing and modeling of the preparation of P(AM-DAC-BA), with
The adequacy of the models was investigated using the analysis of variance (ANOVA) and the results were shown in Table
ANOVA results for response parameters.
Source | Sum of squares | Degree of freedom | Mean square |
|
|
---|---|---|---|---|---|
Regression | 3.18 | 9 | 0.35 | 37.33 | <0.0001 |
|
0.095 | 1 | 0.095 | 9.99 | 0.0159 |
|
1.15 | 1 | 1.15 | 121.12 | <0.0001 |
|
0.91 | 1 | 0.91 | 96.17 | <0.0001 |
|
0.022 | 1 | 0.022 | 2.37 | 0.1672 |
|
0.081 | 1 | 0.081 | 8.57 | 0.0221 |
|
0.018 | 1 | 0.018 | 1.92 | 0.2080 |
|
0.31 | 1 | 0.31 | 33.00 | 0.0007 |
|
0.092 | 1 | 0.092 | 9.67 | 0.0171 |
|
0.42 | 1 | 0.42 | 44.09 | 0.0003 |
Residual | 0.066 | 7 | 0.009475 | ||
Lack of fit | 0.054 | 3 | 0.018 | 6.04 | 0.0575 |
Pure error | 0.012 | 4 | 0.003 | ||
| |||||
Total |
|
|
|||
|
0.9796 | ||||
|
0.9534 |
In order to investigate the various factors and their mutual effect on the intrinsic viscosity of P(AM-DAC-BA), the response surface plots were used to display the response as a function of two factors by keeping the third factor constant, and two-dimensional response surface contours were plotted to investigate the mutual effect of operational variables. The response variables were shown in Figure
Mutual effect of photoinitiator concentration, incident light intensity, and illumination time on intrinsic viscosity
Figures
In order to obtain the maximum value of the intrinsic viscosity, the response optimizer in the Design Expert V8.0.6 software was used to search for an optimal synthetic condition. The best condition for the intrinsic viscosity of P(AM-DAC-BA) in theory was that photoinitiator concentration was 0.60‰, incident light intensity 1491.67
Measured and calculated values for confirmation experiments.
Run | Photoinitiator concentration (wt.‰) |
Incident light intensity ( |
Illumination time (min) | [ |
|
---|---|---|---|---|---|
Measured | Calculated | ||||
18 | 0.66 | 1422.2 | 102.2 |
|
2.10 |
19 | 0.51 | 1407.5 | 101.7 |
|
2.10 |
20 | 0.62 | 1285.8 | 88.3 |
|
2.00 |
Figure
FTIR spectrum of P(AM-DMDAAC-BA).
The effect of dosage on the sludge dewatering performance is shown in Figure
Effect of dosage on sludge dewatering performance.
When the dosage was low, the flocculant could not neutralize negatively charged particles, and the sludge flocs were small and friable, not sufficient to improve sludge dewatering performance. So the supernatant turbidity was high and DS was low. When the dosage was too high, the flocculant caused flocculation system positively charged, increasing repulsion between flocs and leading to the restabilization of the flocs [
Figure
Effect of pH on sludge dewatering performance.
Under the strong acid and alkaline condition, the sludge dewatering performance was deteriorated resulting in a small floc and a small volume of supernatant. At low pH, H+ on the surface of colloidal sludge increased the electrostatic repulsion between sludge particles, leading to the increase of specific resistance of sludge [
In addition, comparison of dewatering performance between P(AM-DAC-BA) and CPAM was shown in Table
Comparison of dewatering performance between P(AM-DAC-BA) and CPAM.
DS (%) | pH | |||
---|---|---|---|---|
Flocculant | Dosage (mg/L) | 4 | 7 | 11 |
P(AM-DAC-BA) | 40 | 16.5 | 28.4 | 12.5 |
CPAM | 13.2 | 26.5 | 8.1 |
Flocculant, P(AM-DAC-BA), was synthesized in this study through UV-initiated polymerization using AM, DAC, and AM as monomers. The best synthetic conditions that were optimized by fitting RSM model while setting the photoinitiator concentration from 0.025 to 0.075 wt%, incident light intensity from 700.0 to 1700.0
The authors declare that there is no conflict of interests regarding the publication of this paper.
This research was supported by the National Natural Science Foundation of China (Project nos. 21177164 and 51078366).