Study on Demulsification Pre-Treatment of Emulsified Wastewater

Abstract

In this paper, the flocculation effect of single flocculants and compound flocculants on emulsified wastewater under different conditions was studied. The effect of flocculant type, dose, settling time and composite ratio of flocculant on the treatment of emulsified wastewater were investigated through single-factor condition tests, and then the optimal conditions of flocculants for treating emulsified wastewater were obtained. The results showed that the single inorganic flocculant could break the stable state of the emulsion when the pH was adjusted to 7 and formed tiny flocs, but it took a long time to settle down. The single organic flocculant had no flocculation effect. The inorganic–organic composite flocculants could effectively solve the problem of emulsion breaking and flocculation, and could form large flocs and alum flower formations at the same time, with mud and water effectively separated. The best conditions for the composite flocculants were as follows: under the condition of room temperature and pH 7, the dose of PFC was 2250 mg/L and APAM was 5 mg, the homogeneous stirring time was 10 min, and the settling time was 0.5 h. The removal rate of COD and turbidity reached 84.75% and 99.86%. The study could provide a new treatment idea and method for the pretreatment of stabilized high-turbidity wastewater.

1. Introduction

Emulsified wastewater mainly comes from petroleum, chemical, iron and steel, coking, gas generation, machinery and other industrial enterprises, as well as from the railroad transportation industry, textiles and light industry [1,2,3,4,5,6,7]. With the rapid development of industry, the discharge of emulsified wastewater has been increasing. According to relevant statistics, the world’s emulsified oily wastewater flowing into lakes and oceans every year ranges from 0.5 to 10 million tons and has become one of the main causes of the pollution of water bodies. Nowadays, many countries in the world have formulated regulations and restrictions on the oil concentration of discharged wastewater, and our country stipulates that the maximum permissible concentration of discharged oil-containing wastewater is 10 mg/L, which makes it more difficult to deal with and greatly aggravates the task of breaking the emulsion [1,2,3,4,5,6,7,8,9,10,11].

The difficulty of pretreatment of emulsion wastewater at home and abroad lies in the emulsion breaking [1,7,8,9,10,12,13] and then flocculation pretreatment [14,15,16,17,18,19]. Common emulsion breaking methods are the acidification emulsion breaking method [20], microbial method [21], and ultrasonic [12] and other emulsion breaking methods [1,9,10,22], while flocculation methods mainly include PAM coagulation [14,15,17], air flotation [23,24], magnetic separation [22,25], extraction [26], deep oxidation [20] and other flocculation methods [23,24,27,28].

In this study, the use of inorganic and organic flocculants for pretreatment of stabilized emulsions, through a single flocculant and combined flocculants study, found that the use of inorganic flocculants in the flocculation at the same time to achieve the purpose of breaking the emulsion, and then further flocculation with organic flocculants, would quickly achieve the separation of mud and water. This study provided another feasible solution for the pretreatment of emulsion.

2. Materials and Methods

2.1. Reagents

The inorganic flocculants Polymerized Aluminum Chloride (PAC), Polymerized Ferric Chloride (PFC), and Polymerized Aluminum Ferric Silicate (PSAF) were analytically pure. The organic flocculants Anionic Polyacrylamide (APAM, 18 million molecular weight), Cationic Polyacrylamide (CPAM, 18 million molecular weight), and Nonionic Polyacrylamide (NPAM, 18 million molecular weight) were industrial grade. The main experimental reagents needed are listed in

Table 1. Experimental reagents and chemicals.

Chemicals	Specifications	Purity	Manufacturer
PAC	Industrial Grade	30%	China National Pharmaceutical Group Corporation Chemical Reagent Company Limited, Shanghai, China
PFC	Industrial Grade	30%	China National Pharmaceutical Group Corporation Chemical Reagent Company Limited, Shanghai, China
PSAF	Industrial Grade	30%	China National Pharmaceutical Group Corporation Chemical Reagent Company Limited, Shanghai, China
PAM	Industrial Grade	99%	China National Pharmaceutical Group Corporation Chemical Reagent Company Limited, Shanghai, China
COD Testing Reagent	HR 20~1500 mg/L	-	HACH, Nanjing, China
NaOH	Analytical Grade	99.9%	Chengdu Kelong Chemical Reagent Factory, Chengdu, China
Distilled Water	Grade One	-	Laboratory Preparation, Nanjing, China

.

The emulsified wastewater came from Yangzhou Jinzhu Resin Co., Ltd., Yangzhou, China which mainly produces cation exchange resins. The raw materials used in the production process mainly include initiator, pore maker, styrene, divinylbenzene, acrylonitrile, paraffin, concentrated sulfuric acid, concentrated alkali, etc. The resulting wastewater forms an emulsion. The particle size of oil beads in the emulsion tested by the company is generally between 0.1 µm and 10 µm. Due to the addition of various surfactant components in the emulsion, the interfacial tension of oil and water is reduced, so that the emulsified oil can exist stably in the emulsion for a long time.

Before the experiment, the emulsion was placed for 2 weeks without any change. At the same time, the water quality index of the emulsion was tested, and the results are listed in

Table 2. Water quality indicators for emulsified wastewater.

Water Quality Indicators	Numerical Value
COD (mg/L)	7060
NH4+-N (mg/L)	300
Turbidity (NTU)	1780

.

2.2. Equipment

The main equipment required for this experiment is listed in

Table 3. Experimental equipment.

Experimental Name	Model	Manufacturer
Analytical Balance	EX2242H	OHAUS, Nanjing, China
Laboratory Ultra-pure Water System	Plus-E3 TS	Nanjing Pudi Technology Development, Nanjing, China
Electric Stirrer	JJ-3 Six-Position Electric Stirrer	Jiangsu Jinyi Instrument Technology, Changzhou, China
Digestion Apparatus	DRB200	HACH, Nanjing, China
Portable Visible Spectrophotometer	DRB1900-05C	Shanghai Surl Instrument, Shanghai, China
pH Meter	Leici PHS-3C	Shanghai Yidian Scientific Instrument, Shanghai, China
Nephelometer	2100Q	HACH, Nanjing, China
Pipette	TOPAID800230	Zhejiang Top Medical Equipment, Zhejiang, China
Pipette	NK6010329	NuoKe Biology, Shanghai, China

below.

2.3. Methods

COD determination method: First, configure the analysis of water samples. Take COD prefabricated tube reagent bottle. Use the first bottle as blank. Remove 2 mL of pure water into the prefabricated tube. Hold the lid tightly and shake well. For the second bottle, remove 2 mL of water sample to be measured into the prefabricated tube, hold the lid tightly and shake well. The water sample is then dissolved at 150 °C for 2 h. After digestion, the water sample was cooled to room temperature. Finally, the water sample was measured.

Turbidity measurement method: 1. Boot preheating. 2. Calibrate the instrument; the instrument will automatically collect the turbidity data for several different concentrations of the solution and use it to establish the calibration curve. 3. Measure the water sample, put the water sample to be measured into the water sample collection container, and put the container into the instrument, then select the “measurement” function on the instrument interface, operate according to the instructions on the screen, and finally the instrument will automatically measure and display the turbidity value on the screen.

A total of 200 mL of emulsified wastewater was placed in a 250 mL beaker. The beaker was placed on a JJ-3 type six-link electric stirrer and stirred at a speed of 300 r/min for 10 min under different conditions with different amounts of reagents added. After a certain settling time, the supernatant was taken to measure its COD and turbidity. The COD removal efficiency was used to represent the effectiveness of coagulation. COD was determined using the rapid digestion spectrophotometric method, and turbidity was measured using the scattering turbidity measurement method.

The dose, settling time, and coagulant composite ratio were the main factors affecting the emulsified wastewater coagulation treatment efficiency. Single-factor condition experiments were conducted to determine the influence of each operating condition on the treatment effect of single and composite coagulants for emulsified wastewater, in order to obtain the optimal conditions for treating oily wastewater using different coagulants.

3. Results and Analysis

3.1. Study of Single Flocculant PAC

3.1.1. Determination of Dose

A 200 mL sample of emulsified wastewater was placed in a 250 mL beaker. Different amounts of PAC with a concentration of 15% were added separately. Under normal temperature conditions, the pH was adjusted to around 7, and the mixture was placed in a JJ-3 six-link electric stirrer with a uniform speed of 300 r/min. The reaction time was 10 min. After the reaction was completed, the mixture was removed and left to settle on a stable table for 2 h. After 2 h, the supernatant was taken 2 cm below the liquid surface to measure its COD and turbidity. The COD values and COD removal efficiency in emulsified wastewater under different PAC doses are shown in

Table 4. COD value and COD removal efficiency at different PAC doses.

Dose (mg/L)	COD Value (mg/L)	COD Removal Efficiency (%)
375	1930	72.66
750	1810	74.36
1125	1760	75.07
1500	1690	76.06
1875	1600	77.34
2250	1510	78.61
2625	1790	74.65
3000	1980	71.95
3375	2320	67.14

and Figure 1. The turbidity values and their removal rates are shown in

Table 5. Turbidity value and turbidity removal efficiency at different PAC doses.

Dose (mg/L)	Turbidity Value (NTU)	Turbidity Removal Efficiency (%)
375	169	90.51
750	36.4	97.96
1125	13.26	99.26
1500	7.28	99.59
1875	6.28	99.65
2250	2.72	99.85
2625	6.12	99.66
3000	9.31	99.48
3375	10.75	99.40

and Figure 2.

From Figure 1, it can be observed that with the increase in PAC dose, the COD value initially decreased and then increased. The lowest value was reached at a dose of 2250 mg/L, where the COD value was 1510 mg/L. It can be observed that the COD removal efficiency initially increased and then decreased with the increase in PAC dose. The peak COD removal efficiency was achieved at a PAC dose of 2250 mg/L, reaching 78.61%. However, further addition of PAC led to a gradual decrease in COD removal efficiency.

From Figure 2, it can be observed that the turbidity values generally decreased with the increase in PAC dose. The lowest value was reached at a dose of 2250 mg/L, where the turbidity was 2.72 NTU. Further addition of PAC led to an increase in turbidity. The turbidity removal efficiency increased with the increase in PAC dose. The peak turbidity removal efficiency of 99.85% was achieved at a PAC dose of 2250 mg/L. However, with further increases in the PAC dose, the turbidity removal efficiency gradually decreased.

3.1.2. Determination of Settling Time

From Figure 3, it can be seen that the settling ratio of PAC continued to increase within 2 h, but there was no significant change in the settling ratio after 2 h. Therefore, the optimal settling time was 2 h.

Experimental studies have shown that after the addition of PAC, by adjusting its pH value, due to the production of aluminum hydroxide microprecipitation, the stable state of the emulsion was broken, thus forming tiny flocs.

3.2. Study of Single Flocculant PFC

3.2.1. Determination of Dose

A 200 mL sample of emulsified wastewater was transferred to a 250 mL beaker, and different amounts of PFC with a concentration of 15% were added. The pH was adjusted to around 7 under ambient conditions, and the mixture was placed in a six-station electric stirrer with a uniform speed of 300 r/min. The reaction time was set to 10 min. After the reaction, the sample was removed and left to settle on a stable surface for 2 h. After 2 h, the supernatant was taken from 2 cm below the liquid surface to measure its COD and turbidity, and the COD and turbidity removal efficiency were calculated. The effect of different PFC doses on COD and their removal rates after treating emulsified wastewater are shown in

Table 6. COD value and COD removal efficiency at different PFC doses.

Dose (mg/L)	COD Value (mg/L)	COD Removal Efficiency (%)
375	2100	70.25
750	2070	70.68
1125	1950	72.38
1500	1710	75.78
1875	1290	81.73
2250	840	88.10
2625	1480	79.04
3000	1860	73.65
3375	2370	66.43

and Figure 4. The impact of different PFC doses on turbidity and their removal rates after treating emulsified wastewater are presented in

Table 7. Turbidity value and turbidity removal efficiency at different PFC doses.

Dose (mg/L)	Turbidity Value (NTU)	Turbidity Removal Efficiency (%)
375	101	94.33
750	37.38	97.9
1125	23.18	98.7
1500	19.6	98.9
1875	11.05	99.38
2250	9.2	99.48
2625	11.2	99.37
3000	11.72	99.34
3375	13.03	99.27

and Figure 5.

From Figure 4, it can be observed that with the increase in PFC dose, the COD value initially decreased and then increased. The lowest value was reached at a dose of 2250 mg/L, where the COD value was 840 mg/L. From Figure 4, it also can be observed that with the continuous increase in PFC dose, the removal rate of COD in the solution initially increased and then decreased. When the dose of PFC was 2250 mg/L, the maximum removal rate of COD in the solution reached 88.10%. However, when the dose exceeded 2250 mg/L, the removal rate of COD gradually decreased. Therefore, the optimal dose of coagulant PFC was 2250 mg/L.

From Figure 5, it can be observed that with the increase in PFC dose, the turbidity value generally decreased. The lowest value was reached at a dose of 2250 mg/L, where the turbidity was 9.2 NTU. However, further addition of PFC led to an upward trend in turbidity. From Figure 5, it can be observed that with the increase in PFC dose, the turbidity removal efficiency showed an increasing trend. The peak turbidity removal efficiency was reached at a dose of 2250 mg/L, where it was 99.48%. However, further addition of PFC led to a decrease in turbidity removal efficiency.

3.2.2. Determination of Settling Time

From Figure 6, it can be observed that the settling ratio of PFC continuously increased within 2 h. However, there was no significant change in the settling ratio after 2 h. Therefore, the optimal settling time for PFC was determined to be 2 h. The best efficacy for breaking and coagulating was observed when the dose of polymeric ferric chloride was 2250 mg/L, with a COD removal efficiency of 88.10% and a turbidity removal efficiency of 99.48%, and a settling time of 2 h.

Experimental studies have shown that by adjusting the pH of PFC after its addition, tiny flocs are formed due to microprecipitation of iron hydroxide, which destabilizes the emulsion.

3.3. Study of Single Flocculant PSAF

3.3.1. Determination of Dose

A 200 mL sample of emulsified wastewater was transferred to a 250 mL beaker, and different amounts of PSAF with a concentration of 30% were added. The pH was adjusted to around 7 under room temperature conditions. The mixture was then placed in a six-unit electric stirrer with a uniform speed of 300 r/min. The reaction time was 10 min. After the reaction, the mixture was removed and placed on a stable tabletop for 2 h. After 2 h, a sample of the upper clear liquid was taken from 2 cm below the liquid surface for measuring its COD and turbidity. The COD values and removal rates under different PSAF doses are as shown in

Table 8. COD value and COD removal efficiency at different PSAF doses.

Dose (mg/L)	COD Value (mg/L)	COD Removal Efficiency (%)
750	2110	70.11
1500	2080	70.54
2250	1960	72.24
3000	1740	75.35
3750	1560	77.90
4500	1520	78.47
5250	2070	70.68
6000	2310	67.28
6750	2520	64.31

and Figure 6, while the turbidity effects and removal rates after treating the emulsified wastewater are shown in

Table 9. Turbidity value and turbidity removal efficiency at different PSAF doses.

Dose (mg/L)	Turbidity Value (NTU)	Turbidity Removal Efficiency (%)
750	84.2	95.27
1500	9.2	99.48
2250	6.84	99.62
3000	6.12	99.66
3750	5.35	99.7
4500	4.28	99.76
5250	10.6	99.4
6000	12.31	99.31
6750	14.81	99.17

and Figure 7.

From Figure 7, it can be observed that the COD values exhibited a trend of initially decreasing and then increasing with the increase in PSAF dose. The lowest value was reached at a dose of 4500 mg/L, where the COD value was 1520 mg/L. From Figure 7, it can be observed that the COD removal efficiency initially increased and then decreased with the increase in PSAF dose. The peak COD removal efficiency was achieved at a PSAF dose of 4500 mg/L, reaching 78.47%. Further addition of PSAF resulted in a decline in the COD removal efficiency.

From Figure 8, it can be observed that the turbidity values generally decreased with the increase in PSAF dose. The turbidity reached its lowest value of 4.28 NTU at a dose of 4500 mg/L. However, with continued addition of PSAF, the turbidity showed an increasing trend. From Figure 8, it can be observed that with the increasing dose of PSAF, the turbidity removal efficiency of the solution showed an overall upward trend. The highest turbidity removal efficiency of the solution was achieved when the dose of PSAF was 4500 mg/L, reaching 99.76%. However, when the dose exceeded 4500 mg/L, the turbidity removal efficiency decreased.

3.3.2. Determination of Settling Time

From Figure 9, it can be observed that PSAF flocculant took a long time to flocculate the emulsion, with sedimentation occurring only after 30 min and only a small amount of supernatant in the upper layer after 2 h. Precipitation hardly changed with the increase in time. Therefore, the optimal settling time for the flocculation was 2 h. When the dose of PSAF was 4500 mg/L, the best coagulation and deflocculation effect was achieved, with a COD removal efficiency of 78.47% and a turbidity removal efficiency of 99.76%. The corresponding optimal settling time was 2 h.

Experimental studies showed that the addition of PSAF led to the formation of tiny flocs by adjusting its pH, which destabilized the emulsion due to the colloidal effect of polysilicic acid.

In this study, the flocculation effect of emulsion was investigated using three inorganic flocculants, PAC, PFC and PSAF, which were effective for turbidity removal no matter how much dose was applied. However, in addition to turbidity, another important factor affecting the biochemical performance of the back-end effluent before it entered the biochemical tanks was the influent COD. When the influent COD was high, the residence time of the back-end biochemical treatment was longer, or larger biochemical tanks were needed. Because the COD concentration of the raw water was very high, the lowest COD should be used in the water in order to achieve optimal treatment. Therefore, when determining the optimum dose of chemicals, the lowest COD was also fully considered for determination. If the actual wastewater treatment could be considered from an economic point of view, the dose of flocculant could be reduced on the condition that the back-end treatment could meet the standard. For wastewater treatment to meet the discharge standards, in addition to considering the economic efficiency, one also needs to consider the relationship between the actual operation of the residence time and the amount of sewage discharged, as this aspect mainly affects the treatment efficiency and infrastructure costs. The results showed that the three inorganic flocculants could effectively break the stable emulsion system, but the settling time was too long, and the supernatant was reduced, so it could not be applied in the front-end treatment process of the emulsion alone.

3.4. Study of Single Flocculant APAM

A 200 mL sample of emulsified wastewater was placed in a 250 mL beaker, and varying amounts of APAM with a concentration of 0.2% were added. The doses of APAM were 1 mg/L, 5 mg/L, 10 mg/L, and 15 mg/L from left to right according to Figure 10. The pH was adjusted to around 7 under normal temperature conditions, and the solution was placed in a six-linked electric stirrer with a uniform speed of 300 r/min. The reaction time was set to 10 min. After the reaction was completed, the solution was removed and allowed to settle on a stable tabletop for 2 h. However, regardless of the varying doses of APAM, no flocs were observed, and there was no change in the COD and turbidity of the solution before and after coagulation.

From Figure 10, it can be observed that when using APAM for destabilizing emulsified wastewater, no significant effect was observed regardless of the dose increase.

The results showed that although APAM had a long molecular chain, which could be utilized to bridge between the particles to form a floc with large particles, due to the stability of the emulsion, the bridging reaction could not be triggered.

3.5. Study of Single Flocculant CPAM

A 200 mL sample of emulsified wastewater was taken and placed in a 250 mL beaker. Different amounts of CPAM with a concentration of 0.3% were added. The doses of CPAM were 1 mg/L, 5 mg/L, 10 mg/L, and 15 mg/L from left to right according to Figure 11. The pH was adjusted to around 7 under normal conditions, and the mixture was placed in a six-link electric stirrer with a uniform speed of 300 r/min. The reaction time was set to 10 min. After the reaction, the mixture was taken out and left to settle on a stable table for 2 h. However, regardless of the change in dose of CPAM, no flocs were observed, and there was no change in COD and turbidity of the solution before and after flocculation.

From Figure 11, it can be observed that when using CPAM for emulsion wastewater treatment, regardless of the dose, there was no apparent effect.

The results showed that although CPAM had a long molecular chain, which could be utilized to bridge between the particles to form a floc with large particles, due to the stability of the emulsion, the bridging reaction could be triggered, so CPAM did not play a flocculating role even though the turbidity of the emulsion was very high.

3.6. Study of Single Flocculant NPAM

A 200 mL sample of emulsified wastewater was taken and placed in a 250 mL beaker. Different amounts of 0.5% concentration NPAM were added. The doses of NPAM were 1 mg/L, 5 mg/L, 10 mg/L, and 15 mg/L from left to right according to Figure 12. Under normal temperature conditions, the pH was adjusted to around 7, and the mixture was placed in a six-connected electric stirrer with a uniform speed of 300 r/min. The reaction time was set to 10 min. After the reaction was completed, the solution was removed and allowed to settle on a stable tabletop for 2 h. However, regardless of the variation in the dose of NPAM, no flocs appeared, and there was no change in the COD and turbidity of the solution before and after flocculation.

As shown in Figure 12, there was no observable effect regardless of the dose of NPAM used for emulsion wastewater treatment.

The flocculation principle of NPAM relies on its high molecular weight and the bending of polypeptide chains to achieve flocculation of particles. It was difficult to flocculate stable emulsions of less than 10 µm by osmosis, so NPAM alone had no flocculation effect on such emulsions.

The results of the above three single-factor studies showed that the organic PAM flocculants had no flocculation effect on emulsified oil wastewater.

3.7. Study of PFC and APAM Composite

To synthesize the above experimental results for single-factor flocculants, in order to achieve better flocculation and sedimentation, the best inorganic flocculant, PFC, was used in the composite flocculant experiments, combining different organic flocculants for the tests. Since there was no change in the amount of wastewater in the experiment, the dose of inorganic flocculant was chosen as 2250 mg/L of PFC with a concentration of 15% for every 250 mL of emulsified wastewater in the single-factor experiment.

3.7.1. Determination of Dose

A 200 mL sample of emulsified wastewater was taken and placed into a 250 mL beaker. Initially, 2250 mg/L of PFC was added, followed by the adjustment of the pH to around 7. Subsequently, different amounts of APAM were added. Under normal temperature conditions, the mixture was placed in a six-station electric stirrer rotating at a uniform speed of 300 rpm. The reaction time was set for 10 min. Once the reaction was completed, the mixture was removed and allowed to stand quietly on a stable table for 2 h. After the 2 h period, a sample of the upper clear liquid 2 cm below the liquid surface was taken to measure its COD value and turbidity. Then, the COD removal efficiency and turbidity removal efficiency were calculated. The effects of different doses of APAM on the COD removal efficiency and turbidity after treating the emulsified wastewater are shown in

Table 10. COD value and COD removal efficiency at different APAM doses.

Dose (mg/L)	COD Value (mg/L)	COD Removal Efficiency (%)
1	1586	77.54
2	1407	80.07
3	1260	82.15
4	1136	83.91
5	1077	84.75
10	1062	84.96

and Figure 13. The effects on turbidity and turbidity removal efficiency after treating the emulsified wastewater are shown in

Table 11. Turbidity value and turbidity removal efficiency at different APAM doses.

Dose (mg/L)	Turbidity Value (NTU)	Turbidity Removal Efficiency (%)
1	32.86	98.15
2	9.17	99.48
3	5.23	99.71
4	3.12	99.82
5	2.56	99.86
10	2.39	99.87

and Figure 14.

From Figure 13, it can be observed that when the dose of inorganic flocculant PFC was constant, the COD value decreased as the dose of organic flocculant APAM increased. When the dose of PFC was 2250 mg/L and the dose of APAM was 5 mg/L, the COD value was 1077 mg/L. However, when the dose of inorganic flocculant PFC was unchanged and the dose of organic flocculant APAM was increased to 10 mg/L, the COD value decreased only to 1062 mg/L. From Figure 13, it can be observed that with the dose of PFC constant, as the dose of APAM increased, the COD removal efficiency gradually increased. When the dose of inorganic flocculant PFC was unchanged and the dose of organic flocculant APAM was increased from 5 mg/L to 10 mg/L, the change in COD removal efficiency was not significant.

From Figure 14, it can be observed that when the dose of inorganic flocculant PFC was constant, as the dose of APAM increased, the turbidity values generally decreased. When the dose of APAM was 5 mg/L, the turbidity was 2.56 NTU. However, when the dose was increased to 10 mg/L, the turbidity only decreased to 2.39 NTU, and the change was not significant. From Figure 14, it can be observed that as the dose of APAM increased, the solution turbidity removal rate showed an upward trend. When the dose was 5 mg/L, the rate of turbidity removal reached 99.86%. However, when the dose was increased to 10 mg/L, the removal rate only increased by 0.01%.

3.7.2. Determination of Settling Time

From Figure 15, it can be observed that the settling ratio of the composite coagulant of PFC and APAM increased continuously within 30 min during the emulsion breaking process. After 30 min, there was a slight increase in the settling ratio, but the improvement was not significant. Therefore, the optimal settling time was determined to be 30 min.

When the amount of inorganic flocculant was 2250 mg/L, the pH was adjusted to neutral, and the organic flocculant dose was 5 mg/L, the COD removal efficiency of the emulsion could reach 84.75%, the turbidity removal efficiency could reach 99.86%, and the settling time was greatly shortened.

3.8. Study of PFC and CPAM Composite

3.8.1. Determination of Dose

A 200 mL sample of emulsified wastewater was taken and placed in a 250 mL beaker. Initially, 2250 mg/L of PFC was added, the pH was then adjusted to around 7, and finally, different amounts of CPAM were added. Under normal temperature conditions, the mixture was placed in a six-station electric stirrer rotating at a uniform speed of 300 rpm. The reaction time was set for 10 min. After the reaction was completed, the mixture was removed and allowed to stand quietly on a stable table for 2 h. After the 2 h period, a sample of the upper clear liquid 2 cm below the liquid surface was taken to measure its COD value and turbidity. Subsequently, the COD removal efficiency and turbidity removal efficiency were calculated. The effects of different doses of APAM on the COD removal efficiency and turbidity after treating the emulsified wastewater are shown in

Table 12. COD value and COD removal efficiency at different CPAM doses.

Dose (mg/L)	COD Value (mg/L)	COD Removal Efficiency (%)
1.5	2540	65.30
3	2260	67.99
4.5	2100	70.25
6	1970	72.10
7.5	1730	75.50
15	1720	75.64

and Figure 16. The effects on turbidity and turbidity removal efficiency after treating the emulsified wastewater are shown in

Table 13. Turbidity value and turbidity removal efficiency at different CPAM doses.

Dose (mg/L)	Turbidity Value (NTU)	Turbidity Removal Efficiency (%)
1.5	20.90	98.83
3	6.15	99.65
4.5	4.95	99.72
6	3.26	99.82
7.5	2.65	99.85
15	2.51	99.86

and Figure 17.

As shown in Figure 16, with the increase in the dose of CPAM, the COD value gradually decreased. When the dose was 7.5 mg/L, the COD value was 1730 mg/L. As the dose increased to 15 mg/L, the COD value only decreased to 1720 mg/L. From Figure 16, it can be observed that the COD removal efficiency gradually increased with the increase in the dose of CPAM. When the dose was 7.5 mg/L, the COD removal efficiency reached 75.50%. As the dose increased to 15 mg/L, the COD removal efficiency slightly increased to 75.64%, with insignificant improvement.

From Figure 17, it can be observed that the turbidity decreased with the increase in the dose of CPAM. When the dose was 7.5 mg/L, the turbidity was 2.65 NTU. As the dose increased to 15 mg/L, the turbidity only decreased to 2.51 NTU, with minimal reduction in effectiveness. From Figure 17, it can be observed that the turbidity removal efficiency increased with the increase in the dose of CPAM. When the dose was 7.5 mg/L, the turbidity removal efficiency reached 99.85%. As the dose increased to 15 mg/L, although there was a slight increase, the turbidity removal efficiency only rose by 0.01%.

3.8.2. Determination of Settling Time

From Figure 18, it can be observed that the settling ratio of the composite coagulant of PFC and APAM increased continuously within 1 h during the emulsion breaking process. After 1 h, there was a slight increase in the settling ratio, but the improvement was not significant. Therefore, the optimal settling time was 1 h.

From the COD degradation rate, turbidity degradation rate and economic benefits of comprehensive consideration, when the PFC dose was 2250 mg/L and CPAM dose was 7.5 mg/L, the COD removal efficiency was 75.50%, the turbidity removal efficiency was 99.85%, and the composite flocculant dose was conducive to the formation of large flocs.

However, compared to the combination of inorganic flocculant PFC and organic flocculant APAM, the COD removal efficiency was reduced, while the settling time was increased. Taken together, the combination of PFC and APAM was superior to the combination of PFC and CPAM in emulsified wastewater treatment.

3.9. Study of PFC and NPAM Composite

3.9.1. Determination of Dose

Two-hundred-milliliter samples of emulsified wastewater were taken and placed into 250 mL beakers. Initially, 2250 mg/L of PFC was added, followed by adjusting the pH to around 7, and finally, different amounts of NPAM were added. Under normal temperature conditions, the mixtures were placed in a six-station electric stirrer rotating at a uniform speed of 300 rpm. The reaction time was set for 10 min. After the reactions were completed, the beakers were removed and allowed to stand quietly on a stable table for 2 h. After the 2 h period, samples of the upper clear liquid 2 cm below the liquid surface were taken from each beaker to measure their COD value and turbidity. Then, the COD removal efficiency and turbidity removal efficiency were calculated for each sample. The effects of different doses of APAM on the COD removal efficiency and turbidity after treating the emulsified wastewater are shown in

Table 14. COD value and COD removal efficiency at different NPAM doses.

Dose (mg/L)	COD Value (mg/L)	COD Removal Efficiency (%)
2.5	2340	66.86
5	2090	70.40
7.5	1720	75.64
10	1560	77.90
12.5	1350	80.88
25	1320	81.30

and Figure 19. The effects on turbidity and turbidity removal efficiency after treating the emulsified wastewater are shown in

Table 15. Turbidity value and turbidity removal efficiency at different NPAM doses.

Dose (mg/L)	Turbidity Value (NTU)	Turbidity Removal Efficiency (%)
2.5	47.52	97.33
5	36.59	97.94
7.5	27.84	98.44
10	22.18	98.75
12.5	19.62	98.90
25	18.23	98.98

and Figure 20.

From Figure 19, it can be observed that the COD values gradually decreased with the increasing dose of NPAM. At a dose of 12.5 mg/L, the COD value was 1350 mg/L, and with a further increase in the dose to 25 mg/L, the COD value decreased to only 1320 mg/L. Figure 19 shows that as the dose of NPAM increased, the COD removal efficiency of the solution steadily increased. At a dose of 12.5 mg/L, the COD removal efficiency reached 80.88%. However, with an increase in the dose to 25 mg/L, the COD removal efficiency only increased slightly, to 81.30%, an insignificant improvement.

Figure 20 illustrates that as the dose of NPAM increased, the turbidity values exhibited an overall decreasing trend. At a dose of 12.5 mg/L, the turbidity was measured at 19.62 NTU. However, with an increase in the dose to 25 mg/L, the turbidity decreased only slightly, to 18.23 NTU, indicating minimal reduction in turbidity. Figure 20 indicates that with an increase in the dose of NPAM, the turbidity removal efficiency of the solution showed an upward trend. At a dose of 12.5 mg/L, the turbidity removal efficiency reached 98.90%. However, with an increase in the dose to 25 mg/L, the turbidity removal efficiency showed a slight increase, rising by only 0.08%.

3.9.2. Determination of Settling Time

From Figure 21, it can be observed that the settling ratio of the composite coagulant of PFC and NPAM increased continuously within 1 h during the emulsion breaking process. After 1 h, there was a slight increase in the settling ratio, but the improvement was not significant. Therefore, the optimal settling time was 1 h.

From the COD degradation rate, turbidity degradation rate and economic benefits of comprehensive consideration, when the PFC dose was 2250 mg/L, the CPAM dose was 12.5 mg/L, the COD removal efficiency was 80.88%, and the turbidity removal efficiency was 98.90%, the composite flocculant dose was conducive to the formation of large flocs.

Summarizing the results of the three composite flocculant experiments, regardless of the COD removal efficiency, turbidity removal efficiency or settling time, PFC and APAM were the most effective.

From

Table 16. Statistics on the flocculation effect of different flocculants on emulsions.

Flocculant	Optimal Dose (mg/L)	COD Removal Efficiency (%)	Turbidity Removal Efficiency (%)	Solid/Liquid Ratio	Sediment Size	Settling Time (h)
PAC	2250	78.61	99.85	4:1	small	2
PFC	2250	88.10	99.48	1:1	small	2
PSAF	4500	78.47	99.76	5:1	small	2
APAM	—	—	—	—	—	—
CPAM	—	—	—	—	—	—
NPAM	—	—	—	—	—	—
PFC + APAM	2250 + 5	84.75	99.86	3:4	large	0.5
PFC + CPAM	2250 + 7.5	75.50	99.85	4:5	large	0.5
PFC + NPAM	2250 + 12.5	80.88	98.90	1:1	large	0.5

, it can be seen that inorganic flocculants could form tiny flocs, because the stable emulsion was broken with the process of adjusting pH 7. However, it could not be applied in actual wastewater treatment projects due to the excessively long settling time. Organic flocculants could not form flocs because the long-chain PAM could not flocculate the stable emulsion through the bridging effect. Inorganic–organic combination flocculants could form large flocs, because the inorganic flocculants first broke the stabilized emulsion and formed small particle precipitates, and then the organic flocculants could flocculate effectively. So, the combined flocculants could handle stable emulsions very effectively.

4. Conclusions

In this article, three single inorganic flocculants, three single organic flocculants, and three inorganic–organic combination flocculants were investigated. The experimental results showed that the flocculation pretreatment effect of the organic–inorganic combination flocculants was better than that of the single inorganic flocculants, while the single organic flocculants had no flocculation effect on the emulsified oil stabilization system.

For single inorganic flocculants, all three flocculants had a certain demulsification effect. For the same emulsified wastewater sample, as the dose gradually increased, the removal rate of COD by the three flocculants showed a trend of first increasing to a peak and then decreasing. The best of the three single flocculants was PFC, followed by PAC, and finally PSAF. Under the condition of room temperature and pH 7, with a PFC dose of 3 mL, constant stirring time of 10 min, and settling time of 1 h, the best removal effects for COD and turbidity were achieved. The COD value was reduced to 840 mg/L, and the turbidity was reduced to 9.2 NTU. However, the inorganic flocculant supernatant is less, the precipitation time was longer, the precipitation amount was larger, and the supernatant was less, indicating that the inorganic flocculant could not be effectively applied to sewage treatment.

However, the three types of organic coagulants had no coagulation effect on the emulsion, which also indicated that although the turbidity of the emulsion was very high, the suspension solid particle size was very small and could not bridge between particles to form flocs, hence no coagulation effect.

When inorganic–organic flocculants were combined, it was found that the removal rates for turbidity and COD were better, the combination of flocculants could form an effective alum flower, and the sedimentation efficiency was greatly improved. Inorganic–organic combination flocculants could effectively pretreat emulsions. The main reason might be that inorganic flocculants generate inorganic salt precipitation when adjusting pH, thus breaking the stable state of the emulsion. Then, the added organic flocculants bridge the micro-precipitation through long chain structure to form large flocs.

This study was based on an actual pretreatment process for production wastewater, especially emulsified wastewater, a special type of wastewater, and an effective method was proposed. It provided a set of new technical methods for the treatment of this type of wastewater. Through this set of methods, it could not only significantly reduce the pollutant content in the wastewater, but also improve the efficiency of the subsequent treatment process. The application of this technology provides an innovative solution for the purification and treatment of emulsified wastewater, which will help promote the development of environmental protection technology, and at the same time contribute to the sustainable development of the related industries with new technological strength.