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A novel acrylamide-free flocculant and its application for sludge dewatering
w a t e r r e s e a r c h 5 7 ( 2 0 1 4 ) 3 0 4 e3 1 2
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A novel acrylamide-free flocculant and its application for sludge dewatering Lianghua Lu, Zhida Pan*, Nan Hao, Wenqing Peng Water and Process Chemistry Lab, Global Research Center of General Electric, Shanghai 201203, China
article info
abstract
Article history:
In the present research, copolymers of methyl acrylate (MA) with anionic or cationic
Received 20 December 2013
monomers were synthesized via emulsion polymerization, and used as sludge dewatering
Received in revised form
aids in wastewater treatment. The copolymerization of different stoichiometry of two
11 March 2014
monomers afforded a variety of water soluble copolymers with charge densities ranging
Accepted 17 March 2014
from 40% to 80%, which align with the charge density of current flocculant products. These
Available online 1 April 2014
copolymers resemble current commercial products, but provide a greener solution by eliminating acrylamide monomer, which is a suspected carcinogen. High molecular weight
Keywords:
copolymers were achieved by applying powder-like synthesis process with intrinsic vis-
Flocculant
cosity of final products as high as 12.98 dl/g for anionic flocculant and 10.74 dl/g for cationic
Acrylamide-free
flocculant. The copolymers of methyl acrylate and [2-(Acryloyloxy)ethyl]trimethylammo-
Poly(MA-co-AA)
nium chloride (AETAC) with 55% charge density exhibited comparable performance in clay
Poly (MA-co-AETAC)
settling test, real water jar test, and sludge dewatering, when compared to AM-based
Ultra-high molecular weight poly-
commercial product in the real wastewater treatment application. ª 2014 Elsevier Ltd. All rights reserved.
mer Sludge dewatering
1.
Introduction
Sludge treatment, with high cost in terms of manpower, energy and unit processes, remains as a major challenge in wastewater treatment. In addition to the economic aspects, wastewater treatment plants are also facing the pressure of sludge handling in a sustainable way due to the environmental debate according to the current laws and regulations. One feasible strategy is to reduce the sludge volume since moisture content represents 60e85% weight of the sludge matrix. Apart from the free water could be readily removed by
gravity separation, bound waterdinvolving the interaction between water and solid surfacedrequires more energy to be released from the sludge (Yu et al., 2008). Chemical flocculation and dewatering are considered efficient to break down the gelatinous structure of sludge and release the bound water. Flocculation is a widely applied industrial process for solideliquid separation during wastewater treatment and manufacturing (Letterman et al., 1999). Colloidal particles in nature normally carry charges on their surface and provide certain potential energy barrier, which lead to the stabilization of suspension in water. There are many mechanism
Abbreviations: AM, acrylamide; AA, acrylic acid; MA, methyl acrylate; AETAC, [2-(Acryloyloxy)ethyl]trimethylammonium chloride; PAM, polyacrylamide. * Corresponding author. 1800 Cailun Rd, Shanghai 201203, China. Tel.: þ86 21 38771055; fax: þ86 21 50806339. E-mail address: [email protected] (Z. Pan). http://dx.doi.org/10.1016/j.watres.2014.03.047 0043-1354/ª 2014 Elsevier Ltd. All rights reserved.
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studies on flocculation and coagulation reported in literature in the past, charge neutralization and polymer bridging effect are well recognized as the most important understandings in this area (Schwoyer, 1981). The destabilization of colloids is induced through charge neutralization, colloid collision and agglomeration, while the flocs of large size are formed by fixing the destabilized particles to the long polymeric chains. So a high molecular weight (typically >106) is critical to the performance of flocculant. Currently most commercially available flocculants are AM based, this interesting fact has last for decades and is not just accidental. Acrylamide has the highest Kp/kt constant in all known monomers, which means it is the most reactive monomer for free radical polymerization, no others could build up super high molecular weight so easily like AM. Secondly acrylamide shows extremely good water solubility, by 2150 g/L at 30 C. Thirdly, it is one of the most cost-effective monomers known so far. Due to its importance in water treatment and oil drilling as well as other many applications, polyacrylamide (PAM) is among the chemicals with the largest production volume (Kurenkov et al., 2002; Karmakar and Chakraborty, 2006). However, with the identification as a suspected carcinogen and high neural toxicity, concerns for acrylamide have been increasingly raised on their environmental and health risks through the wide application of PAM in water treatment (Smith and Oehme, 1991). For example, EPA has regulations on residual acrylamide concentration in PAM e 0.05 percent by weight, and also the PAM dosage (1 ppm) to avoid the possible accumulation of acrylamide in the raw water (EPA 822-R-04005, 2004). EU has set the limitation of acrylamide in the grouting, any grouting products containing 0.1wt% acrylamide or greater are not allowed to use in construction from November 2012 (Official Journal of the European Union L101/ 13, 15.4.2011). From technical point of view, a reliable control on monomer residue is more challenging than high molecular weight build-up in industrial manufacturing, especially for the powder type products. Since acrylamide has a relatively high boiling point of 125 C/25 mmHg, it is not feasible to remove the monomer either by reduced vacuum or solvent washing.
Currently the residual monomer is diminished by adding sufficient amount of radical initiator before quenching in emulsion polymerization. However it is technically difficult to eliminate monomer residue in a bulk solid state efficiently. In most PAM application, the reportable threshold of acrylamide residue on the MSDS is set to 0.1% (1000 ppm) by weight of product, and 0.05% (500 ppm) or even less in some specific areas like drinking water. It remains as the biggest technical barrier for many small PAM manufacturers to well control the monomer residue. On the other hand, why it is so challenging to develop non-AM based flocculant in the past several decades? The answer is that there is no second polymer found to have comparable performance with similar cost. One commercial available non-AM base flocculant is Poly(acrylic acid) (PAA in sodium salt format). From the comparison on clay settling rate between PAM and PAA flocculants tested in our lab (in Fig.1), it seems that Poly(acrylic acid) shows significant performance gap compared to the polyacrylamide flocculant, which might be one of the reasons to limit its application in wastewater treatment although it is more environmentally friendly. Methyl acrylate (MA) is an inexpensive commodity chemical and widely used to make synthetic fiber, paint and pharmaceutical intermediates (Bhanu et al., 2002; An et al., 2008; DiLuccio et al., 1989). It shows certain water solubility of 6 g/ 100 ml at 20 C, and is readily miscible in water and some organic compounds. Methyl acrylate is very reactive for free radical polymerization, which provides great possibility to build up high molecular weight. But most importantly, although methyl acrylate shows certain toxicity toward aquatic creatures, it is not classified as a carcinogen compared to acrylamide, seen in Table 1 (Sigma-aldrich; OSHA), and it is technically feasible to control the monomer residue to a low level as we found later, which bring extra benefits for the new polymer. The copolymerization of methyl acrylate and acrylic acid is no doubt already reported somewhere in the pool of old literature (Matsui and Paul, 2002; Goretta and Otremba, 1980) but rarely used in the flocculation application since most of the copolymers reported were water insoluble. It was found only in one patent in 1970’s that the copolymer was mentioned to be used as flocculant with limited performance (Ide et al., 1974). In that patent, both methyl acrylate and acrylic acid (30:70 by weight) were dissolved (or dispersed) in water, and the copolymerization was initiated at 30 C by
Table 1 e Toxicity comparison of acrylamide and methyl acrylate.
Fig. 1 e Clay settling performance comparison between PAM and PAA.
Acrylamide
Methyl acrylate
LD50 (Oral, Rat) EC50 (Water Flea) TLV (ACGIH) PEL (OSHA)
124 mg/kg 160 mg/L, 48 h 0.03 mg/m3 TWA 0.3 mg/m3 TWA
IARC
Group 2A (possibly carcinogen)
277 mg/kg 2.2 mg/L, 48 h 2 ppm TWA 10 ppm, 35 mg/m3 TWA Group 3 (not carcinogen)
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Table 2 e Intrinsic viscosity of Poly-(MA-co-AA) flocculants with varied charge densities. Flocculant type
Charge density (mol%)
Intrinsic viscosity {h} dl/g
PAM flocculant Poly(MA-co-AA)-1 Poly(MA-co-AA)-2 Poly(MA-co-AA)-3 Poly(MA-co-AA)-4
30% 40% 50% 60% 70%
13.74 9.27 11.98 11.64 12.98
adding ammonium persulfate. The polymer was precipitated from water gradually and collected by decanting the supernatant. From the clay settling performance described in the patent, the highest settling rate was 32.1 cm/min (5.4 mm/s). Unfortunately, there is no polyacrylamide data for comparison. However if we take the clay settling rate of PAM at around 20 mm/s in Fig. 1 as reference, no doubt the gap is huge. It is believed that precipitation polymerization is difficult to achieve super high molecular weight above 10 million, which might be the most plausible reason behind the gap. In the present study, a unique copolymerization process was applied to prepare acrylamide-free flocculants with ultrahigh molecular weight with tunable negative or positive charges. A novel MA-based copolymer as cationic flocculant was reported for the first time. Given that the new flocculants have very close cost advantages compared with PAM, the flocculation performance was also evaluated by clay settling testing, which reveals comparable sedimentation rate to PAM based flocculants for the first time of non-AM chemistry. The residual monomer level was also investigated in details with process optimization in order to meet the real application requirement in a green way. To further evaluate the novel synthetic non-AM flocculants in real wastewater treatment, the synthesized cationic flocculants were applied for sludge dewatering in various types of wastewater.
2.
Material and methods
2.1.
Materials
Methyl acrylate (99%, contains <100 ppm monomethyl ether hydroquinone as inhibitor), acrylic acid (99%, anhydrous,
contains 180e200 ppm MEHQ as inhibitor), [2-(Acryloyloxy) ethyl]-trimethylammonium chloride (AETEC) (80% solution in water, contains 800 ppm monomethyl ether hydroquinone as inhibitor), a,a0 -Azodiisobutyramidine dihydrochloride (V50), N,N,N0 ,N00 ,N00 -Pentamethyldiethylenetriamine (PMDETA) were purchased from SigmaeAldrich and used without further purification. Sodium persulfate (SPS), sodium dodecyl sulfate (SDS), Tween 20 and azodiisobutyronitrile (AIBN) were obtained from SinoPharm Chemical Reagent Co. Ltd. Prior to usage, AIBN was recrystallized in ethanol and stored in 20 C fridge. Cationic clay (clay responsive to the cationic polymer in synthetic water) and anionic clay (clay responsive to the anionic polymer in synthetic water) were sourced from BASF. Sludge used in application includes: biological sludge obtained from CTC (GE China Technology Center, CN) municipal water treatment station, polymer sludge from US manufacturers, and wastewater samples from typical petrochemical wastewater treatment facilities.
2.2.
General method for copolymerization
All the synthesis experiments of anionic flocculants were conducted in a 100 mL round bottom flask using septum/ syringe technique. For making a 60 mol.% charged anionic polymer as an example, 50 g water and 10 g acrylic acid were charged into the flask. 5.55 g sodium hydroxide in 10 g water solution was used to neutralize the acrylic acid till pH ¼ 7 under ice-water bath. Under stirring, 8 g methyl acrylate and 1 g tween-20 were added to the mixture sequentially. 30 mg sodium persulfate (SPS) and 30 mg 2,20 azobis[2-methylpropionamidine] dihydrochloride (V50) in 4 g water were injected into the flask. The reactant mixture was degassed by bubbling argon for 20 min under mild magnetic stirring. The copolymerization was initiated by adding 0.1 ml N,N,N0 ,N00 ,N00 -Pentamethyldiethylenetriamine (PMDETA) as the reductant. The ice-water bath was then removed and the temperature was allowed to rise slowly by itself and recorded by a thermocouple. Typically the viscosity will increase rapidly in 10e30 min and the movement of stirring bar will be “frozen”. Once the temperature reached its peak value (50e70 C), a 50 C water bath was placed to drive the polymerization to complete for 1 h. The final product was obtained as a soft rubber-like material depending on its solid content.
Fig. 2 e Clay settling performance and turbidity of MA-based anionic flocculants. a) Clay settling performance of the poly(MA-co-AA) flocculants with different charge densities; b) Turbidity of the supernatant after flocculation treatment with poly(MA-co-AA) flocculants.
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The synthesis of cationic flocculants was carried out in a 100 ml of four-neck round bottom flask equipped with a mechanical overhead stirrer, a condenser with a bleed-off bubbler line on top and a thermocouple. For making a 55 mol.% charged cationic polymer as an example, 3.87 g methyl acrylate, 13.3 g AETAC of 80 wt.% solution in water, 54 g water and 1 g tween-20 were charged to the flask and stirred for 15 min to emulsify the solution. Then 12 mg sodium persulfate and 27.2 mg Vaz 50 were added into the vial and argon was purged for 20 min afterward at 2 C. After adding 0.1 ml of the reductant N,N,N0 ,N00 ,N00 -Pentamethyldiethylenetriamine (PMDETA), the solution was slowly warmed up by removal of ice bath to initiate polymerization reaction. The reaction was monitored by temperature change and heated to 50 C for 2 h after the temperature dropped from the highest point, which finally resulted in a soft pudding-like immobilized material.
2.3.
Characterization
1
H NMR spectra of flocculants were recorded on a Bruker Advanced 400 MHz NMR spectrometer, in a solvent of D2O or MeOD. Viscosities of synthesized flocculants were determined using either by Brookfield viscometer DV-II þ Pro with UltraLow spindle 0# or by standard Ubbelohde viscometer protocol. Due to the extremely high viscosity of copolymer products, 1000 ppm of polymers in deionized water was used for measurement.
2.4.
Flocculation experiment
2.4.1. Flocculation procedure 2.4.1.1. Anionic clay preparation. 16,625 ml of deionized water was added to a 5-gallon plastic pail with a steadily mixing of the water at sufficient speed to maintain a slight vortex. 875.00 g anionic clay (clay responsive to the anionic polymer) from BASF was weighted into a 1000-ml plastic beaker by pouring in a steady stream to the side of the vortex. After mixing for 60-min, pH was adjusted to 9.0 0.2 with 1N NaOH via pH controller. Slurry was sampled during stirring and measured for total solids concentration on a Mettler Toledo Moisture Analyzer. Additional clay or water was added until the solid concentration reaches 4.9e5.1wt% range.
Fig. 3 e Performance evaluation of the methyl acrylatebased cationic flocculants with comparison to acrylamidebased flocculant by clay settling test.
2.4.1.2. Cationic clay preparation. 350.0 g of cationic clay (clay responsive to the cationic polymer) was added into 17,150 ml deionized water in a 5-gallon plastic pail, by pouring in a steady stream to the side of the vortex. After 60-minutes mixing, the total solid concentration was adjusted to 1.9e2.1% with similar method described for anionic clay. For each flocculation test, it is recommended to always use freshly made clay slurry. 2.4.1.3. Clay settling test. Clay settling test was conducted in a 250 mL of cylinder equipped with stopper. After adding 2.5 mL of 10% NaCl solution, the cylinder was filled with 2% clay slurry to 250 ml mark, and the clay slurry was thoroughly mixed by inverting the cylinder with stopper 2 times up and down. Polymer solution was then dosed into the cylinder at a desired amount and the mixture was mixed again by inverting cylinder up and down 3 times (5 times for anionic clay testing). The time intervals were recorded when solid/liquid interface settled from 210-mL mark to 150-mL mark after putting cylinder on bench. The settling rate was obtained as dividing 60mm by the settling time. 2.4.2.
Sludge dewatering procedure
After assembling a Buchner funnel with filter cloth and a rubber gasket to secure the cloth in place, the device was placed above the graduated cylinder so any filtrate will go into the cylinder. The flocculants were made down at 0.5% solid loading initially, with a further dilution to 0.1%. 200 mL of sludge was weighted into one of the beakers, while polymer solution was added to the other beaker with water adjustment to a total volume of 250 ml, including sludge and polymer. With two beakers containing sludge and polymer solution, a mixture by transferring the sludge between the two beakers was conducted until good mixing was observed (10 times). The filtrate volume collected in 30 and 60 s were recorded after the sample was poured into the top of the Buchner funnel. The sludge wetness, stability under pressure (by squeezing the
Table 3 e MA-based cationic flocculants synthesis with different charge densities.
Entry
Charge densityb
PMA-co-AETAC-1a PMA-co-AETAC-2 PMA-co-AETAC-3 PMA-co-AETAC-4 PMA-co-AETAC-5c
40% 50% (51%) 60% (58%) 70% (66%) 80%
a b c
Initiator (mol%) Vaz 50
Redox
Intrinsic viscosity {h} dl/g
0 0.05 0.05 0.05 0
0.05 0.05 0.05 0.05 0.05
3.31 5.24 6.89 7.97 6.99
15% surfactant used for efficient copolymerization. Charge density in bracket were determined by 1H NMR. No surfactant used.
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Table 4 e Different batches of the copolymer with 55% charge density.
Table 5 e Conditions used for monomer residue reduce and the product properties.
Entry
Entry
PAM-co-AETAC Poly(MA-co-AETAC)-6 Poly(MA-co-AETAC)-7 a
Solid loading
Charge densitya
Intrinsic viscosity {h} dl/g
e 20% 20%
55% 55% (54%) 55% (55%)
6.64 10.74 10.93
Charge density in bracket was determined by 1H NMR.
1 2 3 4
Condition
Redox (0.05 mol%) Redox (0.05 mol%) þ V50 (0.5 mol%) Redox (0.05 mol%) þ AIBN (1 mol%) Redox (0.05 mol%) þ drying
Charge Viscosity MA density @1000 residue ppm (ppm) 54% e
655 cP 608 cP
56%
488.5 cP
54%
e
769 179 99 7
filtration cake), filtrate turbidity, and the cake releasing were also used as accessorial parameters to evaluate the flocculants performance in sludge dewatering.
3.
Results and discussion
3.1.
AM-free anionic flocculants Poly(MA-co-AA)
To synthesize new AM-free flocculants, efforts have been devoted to material screening to achieve polymers with high molecule weight, suitable charge density and good copolymerization capability. The new chemistry of methyl acrylate was found very successful and efficient to build up high molecular weight polymers together with sodium acrylate with the SPS/amine redox initiator system, if by all means the number of radical species is well controlled during the polymerization. After the first successful synthesis of the high MW flocculant poly(MA-co-AA), the optimization of charge density and polymerization process was implemented. Due to the ultrahigh molecular weight, intrinsic viscosity was used to evaluate the relative molecular weight with the viscosity of PAM flocculant as reference (Table 2). It is notable that the new synthetic poly-(MA-co-AA) flocculants exhibit very nice solubility although poly(methyl acrylate) is water insoluble as known, which could be explained with the well distribution of MA in a more soluble sodium acrylate environment of the copolymer. Even with 60% of MA in the copolymer, the flocculant is still dispersible as a turbid solution within the pH range 5.3e8.5.
Fig. 4 e Clay settling performance comparison of Poly(MAco-AETAC) flocculants against AM-based flocculant.
With the synthetic poly(MA-co-AA) flocculants, the clay settling test was used to evaluate the flocculant performance. By reducing the content of sodium acrylate to 60%, the performance at high dosage range of 5e10 ppm were obviously improved, and poly(MA-co-AA)-3 can catch up with PAMbased flocculant in the whole range studied, as shown in Fig. 2. The highest clay settling rate of new poly(MA-co-AA) flocculant reached 16.5 mm/s at 5 ppm dosage. This indicates that the poly(MA-co-AA) using the novel polymerization method has achieved ultra-high molecular weight, which is potentially very attractive in the flocculation application to replace AM-based flocculants. Furthermore, the residue turbidity was checked as a secondary factor of performance evaluation. With the treatment of the MA-based anionic flocculants, the turbidity of the clay solution dropped significantly from over 1000 NTU to around 10 NTU with 2 ppm dosage. With the comparison to the PAM based flocculant, it could be seen clearly that the new poly(MA-co-AA) flocculants exhibited comparable or even better turbidity than PAM flocculant after the flocculation treatment (Fig. 2).
3.2.
AM-free cantionic flocculants Poly(MA-co-AETAC)
Given the property of the sludge in real application, cationic flocculants were applied widely for biological sludge treatment e one of the most challenge sludge for dewatering (Chen, 1998; Wang et al., 2012). Based on the methyl acrylate chemistry we developed, a series of cationic AM-free
Fig. 5 e Clay settling Performance evaluation of the condition optimization for monomer residue.
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Table 6 e Jar test of sludge from the secondary clarifier of a polymer plant. Poly(AM-co-AETAC)
Poly(MA-co-AETAC)
Dosage (ppm)
Bed height (mL)
Turbidity (NTU)
Dosage (ppm)
Bed height (mL)
Turbidity (NTU)
0 2.35 4.7 9.4 14 18.8 28
380 360 240 200 175 (large floc) 170 (bean-like floc) <150 (over-dose)
1396 5.42 2.20 2.32 2.70 3.27 4.50
0 2 4 8 12 16 24
380 350 240 200 175 (large floc) 155 (bean-like floc) <150 (over-dose)
1396 5.77 2.10 1.98 2.18 2.35 2.76
Total initial volume of sludge is 400 mL.
flocculants were synthesized with varied charge densities. The final charge densities were identified by 1H NMR spectra, which are very close to the monomer feeding ratios. This indicated that the copolymerization of the two monomers in this emulsion polymerization system is sufficient. The performance of the copolymers with varied charge densities was evaluated based on the standard clay settling test (Fig. 3). For cationic clay used in the test, copolymers with moderate charge density (50e70%) showed better performance. The reason for the relative poor performance of the 40% and 80% charged flocculants probably could be the low molecular weight of copolymer with 40% charge (indicated by the low intrinsic viscosity) and too high charge density of copolymer (80% charge), where the flocs bearing excessive positive charges can repel to each other. Compared to the commercial AM-based cationic flocculant with 55% charge density, the methyl acrylate polymer incorporated AETAC reached similar performance. Since the charge density of flocculant will influence the performance, it is unfair to compare the flocculants with different charge densities in the same type of sludge. After further identification, poly(AM-coAETAC) flocculants with 55% charge density, the most widely used cationic flocculant for biological sludge, was selected as the benchmark for the following work. Thus, the subsequent investigation was focus on the optimization of the synthesis of 55% charged copolymer and its potential applications. Table 3 With the developed synthesis process of MA-based cationic flocculants, experiments to synthesize the copolymer with 55% cationic charge were conducted with very good repeatability (Table 4). From the intrinsic viscosity, it is
indicated that the poly(MA-co-AETAC) with 55% positive charge achieved an ultra-high molecular weight with good water solubility. Performance evaluation of copolymer Poly(MA-co-AETAC) with 55% charge density revealed comparable or even better performance against AM-based cationic flocculant at various dosage (Fig. 4).
3.2.1.
Process optimization to minimize monomer residue
From the results above, it could be seen that poly(MA-coAETEC) copolymer with 55% charge density exhibited very competitive results to the commercial PAM based cationic flocculants in matter of performance, which could potentially serve as a green alternative of AM-based flocculants. Inspired by these results, we investigated more on the manufacture possibility of the new MA-based flocculants. Given the aquatic toxicity of methyl acrylate (OECD to rainbow trout is 3.4 mg/L), the monomer residue is one important concern need to be addressed during the synthesis process development, in terms of the ecological effects to environment. A base line was established by analyzing the monomer residue in current process, which is around 769 ppm by GCeMS measurement. To push the monomer residue to a relative environmentalfriendly level, efforts have been devoted to optimize the synthesis procedure with the initial approach to minimize the monomer residue by monomer burn-up. Different additive initiators (V50 and AIBN) were applied to aid the monomer burn-up after the polymerization (Table 5). Besides monomer residue level, the charge density and viscosity were also tested to evaluate the effect of process change. Although the burn-up with AIBN could reduce the methyl acrylate residue level from 769 ppm to 99 ppm, the viscosity results as well as clay settling
Fig. 6 e Water releasing amount from the CTC municipal biological sludge after the treatment with flocculants.
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rate indicated that the copolymerization initiated by redox system and AIBN only gave low MW copolymer (Fig. 5). Considering the extremely high viscosity of the copolymer synthesized by current process, dry powder might be a good product form for the future manufacturing. So, co-removal of methyl acrylate with water evaporation during the dry powder process is interesting to know. A resembled process was mimicked in the lab by drying the sliced copolymer in the vacuum oven directly after the polymerization. The GCeMS measurement of the dry powder suggested there was only 7 ppm monomer left in the final product, which should have minimized impact to the environment. The performance of the dried flocculants after make down was evaluated as shown in Fig. 5, which exhibited comparable results to AMbased flocculant. These encouraging results show that dry powder process can be a very promising approach for the large scale manufacture with satisfied monomer residue level (Table 6).
3.3.
Real water test
To elucidate the application of new developed flocculants in sludge dewatering, real water tests were carried out. Buchner funnel dewatering testing was used to evaluate the Poly(MAco-AETAC) flocculants performance on sludge dewatering, which were benchmarked against the commercial polyacrylamide product (PAM-co-AETAC) with 55% charge density. Preliminary evaluation results were achieved on the biological
Fig. 7 e Sludge dewatering test of Poly(MA-co-AETAC) and PAM-co-AETAC on polymer plant sludge.
sludge from municipal wastewater treatment with a solid loading of 2.8% and flocculant dosages at 50 ppm and 75 ppm respectively. The water releasing amount in 30 s and 60 s were recorded to give a side-by-side comparison of dewatering ability. As shown in Fig. 6, it could be clearly seen that new flocculant Poly(MA-co-AETAC) exhibited comparable dewatering capability at 50 ppm and better performance at 75 ppm in the comparison to commercial polyacrylamide product PAM-co-AETAC. In terms of effluent turbidity and sludge cake shape, copolymer Poly(MA-co-AETAC) showed remarkable advantage to commercial polyacrylamide product at 50 ppm and comparable results at 75 ppm (Supporting information S4). Another sludge dewatering application of Poly(MA-coAETAC) on biological sludge from an US polymer plant are summarized below. In the Buchner funnel dewatering test, MA-based new chemistry showed very close water releasing capability and better effluent turbidity (Fig. 7). It was also observed that the sludge could be easily released from the belt after treatment if >28 ppm poly(MA-co-AETAC) was dosed, but slightly sticking with 18 ppm polymer addition. This might be due to small floc size formed at low dosage of flocculant (see Fig. 8). Further investigation of the application of Poly(MA-coAETAC) was carried with sludge obtained from the secondary clarifier of a 2nd US polymer plant, the original water turbidity is 2552 NTU and pH measured as 7.36. Jar test was used for this evaluation with 2 min agitation at 100 rpm followed by another 5 min agitation at 35 rpm. After 5 min settling, the floc bed height was measured as well as the
Fig. 8 e Jar test of sludge from the secondary clarifier of a polymer plant.
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Fig. 9 e Sludge dewatering of Poly(MA-co-AETAC) and polyacrylamide-based flocculant on refinery wastewater. turbidity of the upper layer water, as shown in support Figure S5. An operation window from 4 ppm to 12 ppm was established, with comparable bed height of the floc and turbidity of the water. To further evaluate the performance of new AM-free flocculants on real biological sludge dewatering, an on-site application of Poly(MA-co-AETAC) in refinery biological sludge was performed at a typical Chinese petroleum refinery plant. The sample was obtained from sludge stabilization tank, which contains sludge from second clarify after gravity dewatering with a solid loading of 2.24%. Followed by the standard sludge dewatering evaluation procedure, a side-by-side comparison of the copolymer Poly(MA-co-AETAC) to AM-based commercial product Poly(AM-co-AETAC) was conducted with the results shown in Fig. 9. All these tests suggest that poly(MA-co-AETAC) may be applied for sludge dewatering, although the sludge resistance to high shear levels in many mechanical dewatering processes remains to be evaluated.
4.
Conclusion
A novel type of MA-based flocculant was developed via emulsion polymerization to achieve an ultra-high molecular weight. The new flocculants demonstrate good solubility even with high amount of methyl acrylate in the anionic or cationic copolymers. Flocculation testing proves that the MA-based copolymer flocculants exhibit comparable results to the commercial PAM flocculants with even better supernatant turbidity in some cases. Validated on various sludge from different wastewater resources, the new developed Ploy(MAco-AETAC) shows comparable performance to the commercial polyacrylamide based products in the sludge dewatering. It is believed that high molecular weight contributes to the super performance of new water chemicals. The new chemistry is potentially a competitive alternative of current polyacrylamide product lines and could be beneficial to customer under the tightened environmental regulation.
Acknowledgements This research was supported by funding from Global Research Center of General Electric Company. We thank Dr. Lisa Spagnola for her kind help on the sludge providing and instructions on sludge dewatering test.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.watres.2014.03.047.
references
An, Q.F., Qian, J.W., Zhao, Q., Gao, C.J., 2008. Polyacrylonitrileblock-poly(methyl acrylate) membranes 2: swelling behavior and pervaporation performance for separating benzene/ cyclohexane. J. Membr. Sci. 313 (1e2), 60e67. Bhanu, V.A., Rangarajan, P., Wiles, K., Bortner, M., Sankarpandian, M., Godshall, D., Glass, T.E., Banthia, A.K., Yang, J., Wilkes, G., Baird, D., McGrath, J.E., 2002. Synthesis and characterization of acrylonitrile methyl acrylate statistical copolymers as melt processable carbon fiber precursors. Polymer 43 (18), 4841e4850. Chen, H.L.T., 1998. Applications of structured cationic polyelectrolytes in wastewater treatment. In: Amjad, Z. (Ed.), Water Soluble Polymers: Solution Properties and Applications. Springer, pp. 193e206. Chapter 16. DiLuccio, R.C., Hussain, M.A., CoffinBeach, D., Torosian, G., Shefter, E., Hurwitz, A.R., 1989. Polyvinyl alcohol-methyl acrylate copolymers as a sustained-release oral delivery system. Pharm. Res. 6 (10), 844e847. EPA, Office of Water, US, EPA 822-R-04e005, 2004. Edition of the Drinking Water Standards and Health Advisories. Goretta, L.A., Otremba, R.R., 1980. Continuous Process for the Preparation of an Acrylic acid-methyl Acrylate Copolymer in a Tubular Reactor. US patent US4196272A1. Ide, F., Kotake, Y., Korenaga, S., 1974. US 3850898. Karmakar, G.P., Chakraborty, C., 2006. Improved oil recovery using polymeric gelants: a review. Indian J. Chem. Technol. 13, 162e167. Kurenkov, V.F., Hartan, H.G., Lobanov, F.I., 2002. Application of polyacrylamide flocculants for water treatment. Chem. Comput. Simulations. Butlerov Commun. 3 (11), 31e40. Letterman, R.D., Amirtharajah, A., O’Melia, C.R., 1999. Coagulation and Flocculation in Water Quality and Treatment, fifth ed. McGraw-Hill Inc., NY. Chapter 6. Matsui, S., Paul, D.R., 2002. Pervaporation separation of aromatic/aliphatic hydrocarbons by crosslinked poly(methyl acrylate-co-acrylic acid) membranes. J. Memb. Sci. 195 (2), 229e245. Official Journal of the European Union L101/13, published on 15.4, 2011. Commission Regulation (EU) No366/2011. OSHA: https://www.osha.gov/dts/chemicalsampling/data Schwoyer, L.K. (Ed.), 1981. Polyelectrolytes for Water and Wastewater Treatment. CRC Press. Sigma-Aldrich: http://www.sigmaaldrich.com/MSDS
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Smith, E.A., Oehme, F., 1991. Acrylamide and polyacrylamide: a review of production, use, environmental fate and neurotoxicity. Rev. Environ. Health 9 (4), 215e228. Wang, J., Chen, C., Gao, Q., Lia, T., Zhu, F., 2012. Relationship between the characteristics of cationic polyacrylamide and
sewage sludge dewatering performance in a full-scale plant. Procedia Environ. Sci. 16, 409e417. Yu, G.H., He, P.J., Shao, L.M., He, P.P., 2008. Stratification structure of sludge flocs with implications to dewaterability. Environ. Sci. Technol. 42 (21), 7944e7949.
Available online at www.sciencedirect.com
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A novel acrylamide-free flocculant and its application for sludge dewatering Lianghua Lu, Zhida Pan*, Nan Hao, Wenqing Peng Water and Process Chemistry Lab, Global Research Center of General Electric, Shanghai 201203, China
article info
abstract
Article history:
In the present research, copolymers of methyl acrylate (MA) with anionic or cationic
Received 20 December 2013
monomers were synthesized via emulsion polymerization, and used as sludge dewatering
Received in revised form
aids in wastewater treatment. The copolymerization of different stoichiometry of two
11 March 2014
monomers afforded a variety of water soluble copolymers with charge densities ranging
Accepted 17 March 2014
from 40% to 80%, which align with the charge density of current flocculant products. These
Available online 1 April 2014
copolymers resemble current commercial products, but provide a greener solution by eliminating acrylamide monomer, which is a suspected carcinogen. High molecular weight
Keywords:
copolymers were achieved by applying powder-like synthesis process with intrinsic vis-
Flocculant
cosity of final products as high as 12.98 dl/g for anionic flocculant and 10.74 dl/g for cationic
Acrylamide-free
flocculant. The copolymers of methyl acrylate and [2-(Acryloyloxy)ethyl]trimethylammo-
Poly(MA-co-AA)
nium chloride (AETAC) with 55% charge density exhibited comparable performance in clay
Poly (MA-co-AETAC)
settling test, real water jar test, and sludge dewatering, when compared to AM-based
Ultra-high molecular weight poly-
commercial product in the real wastewater treatment application. ª 2014 Elsevier Ltd. All rights reserved.
mer Sludge dewatering
1.
Introduction
Sludge treatment, with high cost in terms of manpower, energy and unit processes, remains as a major challenge in wastewater treatment. In addition to the economic aspects, wastewater treatment plants are also facing the pressure of sludge handling in a sustainable way due to the environmental debate according to the current laws and regulations. One feasible strategy is to reduce the sludge volume since moisture content represents 60e85% weight of the sludge matrix. Apart from the free water could be readily removed by
gravity separation, bound waterdinvolving the interaction between water and solid surfacedrequires more energy to be released from the sludge (Yu et al., 2008). Chemical flocculation and dewatering are considered efficient to break down the gelatinous structure of sludge and release the bound water. Flocculation is a widely applied industrial process for solideliquid separation during wastewater treatment and manufacturing (Letterman et al., 1999). Colloidal particles in nature normally carry charges on their surface and provide certain potential energy barrier, which lead to the stabilization of suspension in water. There are many mechanism
Abbreviations: AM, acrylamide; AA, acrylic acid; MA, methyl acrylate; AETAC, [2-(Acryloyloxy)ethyl]trimethylammonium chloride; PAM, polyacrylamide. * Corresponding author. 1800 Cailun Rd, Shanghai 201203, China. Tel.: þ86 21 38771055; fax: þ86 21 50806339. E-mail address: [email protected] (Z. Pan). http://dx.doi.org/10.1016/j.watres.2014.03.047 0043-1354/ª 2014 Elsevier Ltd. All rights reserved.
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studies on flocculation and coagulation reported in literature in the past, charge neutralization and polymer bridging effect are well recognized as the most important understandings in this area (Schwoyer, 1981). The destabilization of colloids is induced through charge neutralization, colloid collision and agglomeration, while the flocs of large size are formed by fixing the destabilized particles to the long polymeric chains. So a high molecular weight (typically >106) is critical to the performance of flocculant. Currently most commercially available flocculants are AM based, this interesting fact has last for decades and is not just accidental. Acrylamide has the highest Kp/kt constant in all known monomers, which means it is the most reactive monomer for free radical polymerization, no others could build up super high molecular weight so easily like AM. Secondly acrylamide shows extremely good water solubility, by 2150 g/L at 30 C. Thirdly, it is one of the most cost-effective monomers known so far. Due to its importance in water treatment and oil drilling as well as other many applications, polyacrylamide (PAM) is among the chemicals with the largest production volume (Kurenkov et al., 2002; Karmakar and Chakraborty, 2006). However, with the identification as a suspected carcinogen and high neural toxicity, concerns for acrylamide have been increasingly raised on their environmental and health risks through the wide application of PAM in water treatment (Smith and Oehme, 1991). For example, EPA has regulations on residual acrylamide concentration in PAM e 0.05 percent by weight, and also the PAM dosage (1 ppm) to avoid the possible accumulation of acrylamide in the raw water (EPA 822-R-04005, 2004). EU has set the limitation of acrylamide in the grouting, any grouting products containing 0.1wt% acrylamide or greater are not allowed to use in construction from November 2012 (Official Journal of the European Union L101/ 13, 15.4.2011). From technical point of view, a reliable control on monomer residue is more challenging than high molecular weight build-up in industrial manufacturing, especially for the powder type products. Since acrylamide has a relatively high boiling point of 125 C/25 mmHg, it is not feasible to remove the monomer either by reduced vacuum or solvent washing.
Currently the residual monomer is diminished by adding sufficient amount of radical initiator before quenching in emulsion polymerization. However it is technically difficult to eliminate monomer residue in a bulk solid state efficiently. In most PAM application, the reportable threshold of acrylamide residue on the MSDS is set to 0.1% (1000 ppm) by weight of product, and 0.05% (500 ppm) or even less in some specific areas like drinking water. It remains as the biggest technical barrier for many small PAM manufacturers to well control the monomer residue. On the other hand, why it is so challenging to develop non-AM based flocculant in the past several decades? The answer is that there is no second polymer found to have comparable performance with similar cost. One commercial available non-AM base flocculant is Poly(acrylic acid) (PAA in sodium salt format). From the comparison on clay settling rate between PAM and PAA flocculants tested in our lab (in Fig.1), it seems that Poly(acrylic acid) shows significant performance gap compared to the polyacrylamide flocculant, which might be one of the reasons to limit its application in wastewater treatment although it is more environmentally friendly. Methyl acrylate (MA) is an inexpensive commodity chemical and widely used to make synthetic fiber, paint and pharmaceutical intermediates (Bhanu et al., 2002; An et al., 2008; DiLuccio et al., 1989). It shows certain water solubility of 6 g/ 100 ml at 20 C, and is readily miscible in water and some organic compounds. Methyl acrylate is very reactive for free radical polymerization, which provides great possibility to build up high molecular weight. But most importantly, although methyl acrylate shows certain toxicity toward aquatic creatures, it is not classified as a carcinogen compared to acrylamide, seen in Table 1 (Sigma-aldrich; OSHA), and it is technically feasible to control the monomer residue to a low level as we found later, which bring extra benefits for the new polymer. The copolymerization of methyl acrylate and acrylic acid is no doubt already reported somewhere in the pool of old literature (Matsui and Paul, 2002; Goretta and Otremba, 1980) but rarely used in the flocculation application since most of the copolymers reported were water insoluble. It was found only in one patent in 1970’s that the copolymer was mentioned to be used as flocculant with limited performance (Ide et al., 1974). In that patent, both methyl acrylate and acrylic acid (30:70 by weight) were dissolved (or dispersed) in water, and the copolymerization was initiated at 30 C by
Table 1 e Toxicity comparison of acrylamide and methyl acrylate.
Fig. 1 e Clay settling performance comparison between PAM and PAA.
Acrylamide
Methyl acrylate
LD50 (Oral, Rat) EC50 (Water Flea) TLV (ACGIH) PEL (OSHA)
124 mg/kg 160 mg/L, 48 h 0.03 mg/m3 TWA 0.3 mg/m3 TWA
IARC
Group 2A (possibly carcinogen)
277 mg/kg 2.2 mg/L, 48 h 2 ppm TWA 10 ppm, 35 mg/m3 TWA Group 3 (not carcinogen)
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Table 2 e Intrinsic viscosity of Poly-(MA-co-AA) flocculants with varied charge densities. Flocculant type
Charge density (mol%)
Intrinsic viscosity {h} dl/g
PAM flocculant Poly(MA-co-AA)-1 Poly(MA-co-AA)-2 Poly(MA-co-AA)-3 Poly(MA-co-AA)-4
30% 40% 50% 60% 70%
13.74 9.27 11.98 11.64 12.98
adding ammonium persulfate. The polymer was precipitated from water gradually and collected by decanting the supernatant. From the clay settling performance described in the patent, the highest settling rate was 32.1 cm/min (5.4 mm/s). Unfortunately, there is no polyacrylamide data for comparison. However if we take the clay settling rate of PAM at around 20 mm/s in Fig. 1 as reference, no doubt the gap is huge. It is believed that precipitation polymerization is difficult to achieve super high molecular weight above 10 million, which might be the most plausible reason behind the gap. In the present study, a unique copolymerization process was applied to prepare acrylamide-free flocculants with ultrahigh molecular weight with tunable negative or positive charges. A novel MA-based copolymer as cationic flocculant was reported for the first time. Given that the new flocculants have very close cost advantages compared with PAM, the flocculation performance was also evaluated by clay settling testing, which reveals comparable sedimentation rate to PAM based flocculants for the first time of non-AM chemistry. The residual monomer level was also investigated in details with process optimization in order to meet the real application requirement in a green way. To further evaluate the novel synthetic non-AM flocculants in real wastewater treatment, the synthesized cationic flocculants were applied for sludge dewatering in various types of wastewater.
2.
Material and methods
2.1.
Materials
Methyl acrylate (99%, contains <100 ppm monomethyl ether hydroquinone as inhibitor), acrylic acid (99%, anhydrous,
contains 180e200 ppm MEHQ as inhibitor), [2-(Acryloyloxy) ethyl]-trimethylammonium chloride (AETEC) (80% solution in water, contains 800 ppm monomethyl ether hydroquinone as inhibitor), a,a0 -Azodiisobutyramidine dihydrochloride (V50), N,N,N0 ,N00 ,N00 -Pentamethyldiethylenetriamine (PMDETA) were purchased from SigmaeAldrich and used without further purification. Sodium persulfate (SPS), sodium dodecyl sulfate (SDS), Tween 20 and azodiisobutyronitrile (AIBN) were obtained from SinoPharm Chemical Reagent Co. Ltd. Prior to usage, AIBN was recrystallized in ethanol and stored in 20 C fridge. Cationic clay (clay responsive to the cationic polymer in synthetic water) and anionic clay (clay responsive to the anionic polymer in synthetic water) were sourced from BASF. Sludge used in application includes: biological sludge obtained from CTC (GE China Technology Center, CN) municipal water treatment station, polymer sludge from US manufacturers, and wastewater samples from typical petrochemical wastewater treatment facilities.
2.2.
General method for copolymerization
All the synthesis experiments of anionic flocculants were conducted in a 100 mL round bottom flask using septum/ syringe technique. For making a 60 mol.% charged anionic polymer as an example, 50 g water and 10 g acrylic acid were charged into the flask. 5.55 g sodium hydroxide in 10 g water solution was used to neutralize the acrylic acid till pH ¼ 7 under ice-water bath. Under stirring, 8 g methyl acrylate and 1 g tween-20 were added to the mixture sequentially. 30 mg sodium persulfate (SPS) and 30 mg 2,20 azobis[2-methylpropionamidine] dihydrochloride (V50) in 4 g water were injected into the flask. The reactant mixture was degassed by bubbling argon for 20 min under mild magnetic stirring. The copolymerization was initiated by adding 0.1 ml N,N,N0 ,N00 ,N00 -Pentamethyldiethylenetriamine (PMDETA) as the reductant. The ice-water bath was then removed and the temperature was allowed to rise slowly by itself and recorded by a thermocouple. Typically the viscosity will increase rapidly in 10e30 min and the movement of stirring bar will be “frozen”. Once the temperature reached its peak value (50e70 C), a 50 C water bath was placed to drive the polymerization to complete for 1 h. The final product was obtained as a soft rubber-like material depending on its solid content.
Fig. 2 e Clay settling performance and turbidity of MA-based anionic flocculants. a) Clay settling performance of the poly(MA-co-AA) flocculants with different charge densities; b) Turbidity of the supernatant after flocculation treatment with poly(MA-co-AA) flocculants.
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The synthesis of cationic flocculants was carried out in a 100 ml of four-neck round bottom flask equipped with a mechanical overhead stirrer, a condenser with a bleed-off bubbler line on top and a thermocouple. For making a 55 mol.% charged cationic polymer as an example, 3.87 g methyl acrylate, 13.3 g AETAC of 80 wt.% solution in water, 54 g water and 1 g tween-20 were charged to the flask and stirred for 15 min to emulsify the solution. Then 12 mg sodium persulfate and 27.2 mg Vaz 50 were added into the vial and argon was purged for 20 min afterward at 2 C. After adding 0.1 ml of the reductant N,N,N0 ,N00 ,N00 -Pentamethyldiethylenetriamine (PMDETA), the solution was slowly warmed up by removal of ice bath to initiate polymerization reaction. The reaction was monitored by temperature change and heated to 50 C for 2 h after the temperature dropped from the highest point, which finally resulted in a soft pudding-like immobilized material.
2.3.
Characterization
1
H NMR spectra of flocculants were recorded on a Bruker Advanced 400 MHz NMR spectrometer, in a solvent of D2O or MeOD. Viscosities of synthesized flocculants were determined using either by Brookfield viscometer DV-II þ Pro with UltraLow spindle 0# or by standard Ubbelohde viscometer protocol. Due to the extremely high viscosity of copolymer products, 1000 ppm of polymers in deionized water was used for measurement.
2.4.
Flocculation experiment
2.4.1. Flocculation procedure 2.4.1.1. Anionic clay preparation. 16,625 ml of deionized water was added to a 5-gallon plastic pail with a steadily mixing of the water at sufficient speed to maintain a slight vortex. 875.00 g anionic clay (clay responsive to the anionic polymer) from BASF was weighted into a 1000-ml plastic beaker by pouring in a steady stream to the side of the vortex. After mixing for 60-min, pH was adjusted to 9.0 0.2 with 1N NaOH via pH controller. Slurry was sampled during stirring and measured for total solids concentration on a Mettler Toledo Moisture Analyzer. Additional clay or water was added until the solid concentration reaches 4.9e5.1wt% range.
Fig. 3 e Performance evaluation of the methyl acrylatebased cationic flocculants with comparison to acrylamidebased flocculant by clay settling test.
2.4.1.2. Cationic clay preparation. 350.0 g of cationic clay (clay responsive to the cationic polymer) was added into 17,150 ml deionized water in a 5-gallon plastic pail, by pouring in a steady stream to the side of the vortex. After 60-minutes mixing, the total solid concentration was adjusted to 1.9e2.1% with similar method described for anionic clay. For each flocculation test, it is recommended to always use freshly made clay slurry. 2.4.1.3. Clay settling test. Clay settling test was conducted in a 250 mL of cylinder equipped with stopper. After adding 2.5 mL of 10% NaCl solution, the cylinder was filled with 2% clay slurry to 250 ml mark, and the clay slurry was thoroughly mixed by inverting the cylinder with stopper 2 times up and down. Polymer solution was then dosed into the cylinder at a desired amount and the mixture was mixed again by inverting cylinder up and down 3 times (5 times for anionic clay testing). The time intervals were recorded when solid/liquid interface settled from 210-mL mark to 150-mL mark after putting cylinder on bench. The settling rate was obtained as dividing 60mm by the settling time. 2.4.2.
Sludge dewatering procedure
After assembling a Buchner funnel with filter cloth and a rubber gasket to secure the cloth in place, the device was placed above the graduated cylinder so any filtrate will go into the cylinder. The flocculants were made down at 0.5% solid loading initially, with a further dilution to 0.1%. 200 mL of sludge was weighted into one of the beakers, while polymer solution was added to the other beaker with water adjustment to a total volume of 250 ml, including sludge and polymer. With two beakers containing sludge and polymer solution, a mixture by transferring the sludge between the two beakers was conducted until good mixing was observed (10 times). The filtrate volume collected in 30 and 60 s were recorded after the sample was poured into the top of the Buchner funnel. The sludge wetness, stability under pressure (by squeezing the
Table 3 e MA-based cationic flocculants synthesis with different charge densities.
Entry
Charge densityb
PMA-co-AETAC-1a PMA-co-AETAC-2 PMA-co-AETAC-3 PMA-co-AETAC-4 PMA-co-AETAC-5c
40% 50% (51%) 60% (58%) 70% (66%) 80%
a b c
Initiator (mol%) Vaz 50
Redox
Intrinsic viscosity {h} dl/g
0 0.05 0.05 0.05 0
0.05 0.05 0.05 0.05 0.05
3.31 5.24 6.89 7.97 6.99
15% surfactant used for efficient copolymerization. Charge density in bracket were determined by 1H NMR. No surfactant used.
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Table 4 e Different batches of the copolymer with 55% charge density.
Table 5 e Conditions used for monomer residue reduce and the product properties.
Entry
Entry
PAM-co-AETAC Poly(MA-co-AETAC)-6 Poly(MA-co-AETAC)-7 a
Solid loading
Charge densitya
Intrinsic viscosity {h} dl/g
e 20% 20%
55% 55% (54%) 55% (55%)
6.64 10.74 10.93
Charge density in bracket was determined by 1H NMR.
1 2 3 4
Condition
Redox (0.05 mol%) Redox (0.05 mol%) þ V50 (0.5 mol%) Redox (0.05 mol%) þ AIBN (1 mol%) Redox (0.05 mol%) þ drying
Charge Viscosity MA density @1000 residue ppm (ppm) 54% e
655 cP 608 cP
56%
488.5 cP
54%
e
769 179 99 7
filtration cake), filtrate turbidity, and the cake releasing were also used as accessorial parameters to evaluate the flocculants performance in sludge dewatering.
3.
Results and discussion
3.1.
AM-free anionic flocculants Poly(MA-co-AA)
To synthesize new AM-free flocculants, efforts have been devoted to material screening to achieve polymers with high molecule weight, suitable charge density and good copolymerization capability. The new chemistry of methyl acrylate was found very successful and efficient to build up high molecular weight polymers together with sodium acrylate with the SPS/amine redox initiator system, if by all means the number of radical species is well controlled during the polymerization. After the first successful synthesis of the high MW flocculant poly(MA-co-AA), the optimization of charge density and polymerization process was implemented. Due to the ultrahigh molecular weight, intrinsic viscosity was used to evaluate the relative molecular weight with the viscosity of PAM flocculant as reference (Table 2). It is notable that the new synthetic poly-(MA-co-AA) flocculants exhibit very nice solubility although poly(methyl acrylate) is water insoluble as known, which could be explained with the well distribution of MA in a more soluble sodium acrylate environment of the copolymer. Even with 60% of MA in the copolymer, the flocculant is still dispersible as a turbid solution within the pH range 5.3e8.5.
Fig. 4 e Clay settling performance comparison of Poly(MAco-AETAC) flocculants against AM-based flocculant.
With the synthetic poly(MA-co-AA) flocculants, the clay settling test was used to evaluate the flocculant performance. By reducing the content of sodium acrylate to 60%, the performance at high dosage range of 5e10 ppm were obviously improved, and poly(MA-co-AA)-3 can catch up with PAMbased flocculant in the whole range studied, as shown in Fig. 2. The highest clay settling rate of new poly(MA-co-AA) flocculant reached 16.5 mm/s at 5 ppm dosage. This indicates that the poly(MA-co-AA) using the novel polymerization method has achieved ultra-high molecular weight, which is potentially very attractive in the flocculation application to replace AM-based flocculants. Furthermore, the residue turbidity was checked as a secondary factor of performance evaluation. With the treatment of the MA-based anionic flocculants, the turbidity of the clay solution dropped significantly from over 1000 NTU to around 10 NTU with 2 ppm dosage. With the comparison to the PAM based flocculant, it could be seen clearly that the new poly(MA-co-AA) flocculants exhibited comparable or even better turbidity than PAM flocculant after the flocculation treatment (Fig. 2).
3.2.
AM-free cantionic flocculants Poly(MA-co-AETAC)
Given the property of the sludge in real application, cationic flocculants were applied widely for biological sludge treatment e one of the most challenge sludge for dewatering (Chen, 1998; Wang et al., 2012). Based on the methyl acrylate chemistry we developed, a series of cationic AM-free
Fig. 5 e Clay settling Performance evaluation of the condition optimization for monomer residue.
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Table 6 e Jar test of sludge from the secondary clarifier of a polymer plant. Poly(AM-co-AETAC)
Poly(MA-co-AETAC)
Dosage (ppm)
Bed height (mL)
Turbidity (NTU)
Dosage (ppm)
Bed height (mL)
Turbidity (NTU)
0 2.35 4.7 9.4 14 18.8 28
380 360 240 200 175 (large floc) 170 (bean-like floc) <150 (over-dose)
1396 5.42 2.20 2.32 2.70 3.27 4.50
0 2 4 8 12 16 24
380 350 240 200 175 (large floc) 155 (bean-like floc) <150 (over-dose)
1396 5.77 2.10 1.98 2.18 2.35 2.76
Total initial volume of sludge is 400 mL.
flocculants were synthesized with varied charge densities. The final charge densities were identified by 1H NMR spectra, which are very close to the monomer feeding ratios. This indicated that the copolymerization of the two monomers in this emulsion polymerization system is sufficient. The performance of the copolymers with varied charge densities was evaluated based on the standard clay settling test (Fig. 3). For cationic clay used in the test, copolymers with moderate charge density (50e70%) showed better performance. The reason for the relative poor performance of the 40% and 80% charged flocculants probably could be the low molecular weight of copolymer with 40% charge (indicated by the low intrinsic viscosity) and too high charge density of copolymer (80% charge), where the flocs bearing excessive positive charges can repel to each other. Compared to the commercial AM-based cationic flocculant with 55% charge density, the methyl acrylate polymer incorporated AETAC reached similar performance. Since the charge density of flocculant will influence the performance, it is unfair to compare the flocculants with different charge densities in the same type of sludge. After further identification, poly(AM-coAETAC) flocculants with 55% charge density, the most widely used cationic flocculant for biological sludge, was selected as the benchmark for the following work. Thus, the subsequent investigation was focus on the optimization of the synthesis of 55% charged copolymer and its potential applications. Table 3 With the developed synthesis process of MA-based cationic flocculants, experiments to synthesize the copolymer with 55% cationic charge were conducted with very good repeatability (Table 4). From the intrinsic viscosity, it is
indicated that the poly(MA-co-AETAC) with 55% positive charge achieved an ultra-high molecular weight with good water solubility. Performance evaluation of copolymer Poly(MA-co-AETAC) with 55% charge density revealed comparable or even better performance against AM-based cationic flocculant at various dosage (Fig. 4).
3.2.1.
Process optimization to minimize monomer residue
From the results above, it could be seen that poly(MA-coAETEC) copolymer with 55% charge density exhibited very competitive results to the commercial PAM based cationic flocculants in matter of performance, which could potentially serve as a green alternative of AM-based flocculants. Inspired by these results, we investigated more on the manufacture possibility of the new MA-based flocculants. Given the aquatic toxicity of methyl acrylate (OECD to rainbow trout is 3.4 mg/L), the monomer residue is one important concern need to be addressed during the synthesis process development, in terms of the ecological effects to environment. A base line was established by analyzing the monomer residue in current process, which is around 769 ppm by GCeMS measurement. To push the monomer residue to a relative environmentalfriendly level, efforts have been devoted to optimize the synthesis procedure with the initial approach to minimize the monomer residue by monomer burn-up. Different additive initiators (V50 and AIBN) were applied to aid the monomer burn-up after the polymerization (Table 5). Besides monomer residue level, the charge density and viscosity were also tested to evaluate the effect of process change. Although the burn-up with AIBN could reduce the methyl acrylate residue level from 769 ppm to 99 ppm, the viscosity results as well as clay settling
Fig. 6 e Water releasing amount from the CTC municipal biological sludge after the treatment with flocculants.
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rate indicated that the copolymerization initiated by redox system and AIBN only gave low MW copolymer (Fig. 5). Considering the extremely high viscosity of the copolymer synthesized by current process, dry powder might be a good product form for the future manufacturing. So, co-removal of methyl acrylate with water evaporation during the dry powder process is interesting to know. A resembled process was mimicked in the lab by drying the sliced copolymer in the vacuum oven directly after the polymerization. The GCeMS measurement of the dry powder suggested there was only 7 ppm monomer left in the final product, which should have minimized impact to the environment. The performance of the dried flocculants after make down was evaluated as shown in Fig. 5, which exhibited comparable results to AMbased flocculant. These encouraging results show that dry powder process can be a very promising approach for the large scale manufacture with satisfied monomer residue level (Table 6).
3.3.
Real water test
To elucidate the application of new developed flocculants in sludge dewatering, real water tests were carried out. Buchner funnel dewatering testing was used to evaluate the Poly(MAco-AETAC) flocculants performance on sludge dewatering, which were benchmarked against the commercial polyacrylamide product (PAM-co-AETAC) with 55% charge density. Preliminary evaluation results were achieved on the biological
Fig. 7 e Sludge dewatering test of Poly(MA-co-AETAC) and PAM-co-AETAC on polymer plant sludge.
sludge from municipal wastewater treatment with a solid loading of 2.8% and flocculant dosages at 50 ppm and 75 ppm respectively. The water releasing amount in 30 s and 60 s were recorded to give a side-by-side comparison of dewatering ability. As shown in Fig. 6, it could be clearly seen that new flocculant Poly(MA-co-AETAC) exhibited comparable dewatering capability at 50 ppm and better performance at 75 ppm in the comparison to commercial polyacrylamide product PAM-co-AETAC. In terms of effluent turbidity and sludge cake shape, copolymer Poly(MA-co-AETAC) showed remarkable advantage to commercial polyacrylamide product at 50 ppm and comparable results at 75 ppm (Supporting information S4). Another sludge dewatering application of Poly(MA-coAETAC) on biological sludge from an US polymer plant are summarized below. In the Buchner funnel dewatering test, MA-based new chemistry showed very close water releasing capability and better effluent turbidity (Fig. 7). It was also observed that the sludge could be easily released from the belt after treatment if >28 ppm poly(MA-co-AETAC) was dosed, but slightly sticking with 18 ppm polymer addition. This might be due to small floc size formed at low dosage of flocculant (see Fig. 8). Further investigation of the application of Poly(MA-coAETAC) was carried with sludge obtained from the secondary clarifier of a 2nd US polymer plant, the original water turbidity is 2552 NTU and pH measured as 7.36. Jar test was used for this evaluation with 2 min agitation at 100 rpm followed by another 5 min agitation at 35 rpm. After 5 min settling, the floc bed height was measured as well as the
Fig. 8 e Jar test of sludge from the secondary clarifier of a polymer plant.
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Fig. 9 e Sludge dewatering of Poly(MA-co-AETAC) and polyacrylamide-based flocculant on refinery wastewater. turbidity of the upper layer water, as shown in support Figure S5. An operation window from 4 ppm to 12 ppm was established, with comparable bed height of the floc and turbidity of the water. To further evaluate the performance of new AM-free flocculants on real biological sludge dewatering, an on-site application of Poly(MA-co-AETAC) in refinery biological sludge was performed at a typical Chinese petroleum refinery plant. The sample was obtained from sludge stabilization tank, which contains sludge from second clarify after gravity dewatering with a solid loading of 2.24%. Followed by the standard sludge dewatering evaluation procedure, a side-by-side comparison of the copolymer Poly(MA-co-AETAC) to AM-based commercial product Poly(AM-co-AETAC) was conducted with the results shown in Fig. 9. All these tests suggest that poly(MA-co-AETAC) may be applied for sludge dewatering, although the sludge resistance to high shear levels in many mechanical dewatering processes remains to be evaluated.
4.
Conclusion
A novel type of MA-based flocculant was developed via emulsion polymerization to achieve an ultra-high molecular weight. The new flocculants demonstrate good solubility even with high amount of methyl acrylate in the anionic or cationic copolymers. Flocculation testing proves that the MA-based copolymer flocculants exhibit comparable results to the commercial PAM flocculants with even better supernatant turbidity in some cases. Validated on various sludge from different wastewater resources, the new developed Ploy(MAco-AETAC) shows comparable performance to the commercial polyacrylamide based products in the sludge dewatering. It is believed that high molecular weight contributes to the super performance of new water chemicals. The new chemistry is potentially a competitive alternative of current polyacrylamide product lines and could be beneficial to customer under the tightened environmental regulation.
Acknowledgements This research was supported by funding from Global Research Center of General Electric Company. We thank Dr. Lisa Spagnola for her kind help on the sludge providing and instructions on sludge dewatering test.
Appendix A. Supplementary data Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.watres.2014.03.047.
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