Cationic proteins for enhancing biosludge dewaterability: A comparative assessment of surface and conditioning characteristics of synthetic polymers, surfactants and proteins

Separation and Purification Technology 191 (2018) 200–207

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Cationic proteins for enhancing biosludge dewaterability: A comparative assessment of surface and conditioning characteristics of synthetic polymers, surfactants and proteins Sofia Bonilla, D. Grant Allen

MARK

⁎

Department of Chemical Engineering and Applied Chemistry at the University of Toronto, 200 College St., Toronto, Ontario M5S 3E5, Canada

A R T I C L E I N F O

A B S T R A C T

Keywords: Sludge Proteins Surfactants Dewaterability Cationic

Synthetic organic polymers are commonly used to facilitate challenging solid-liquid separations such as biosludge dewatering. However, there is interest in reducing the use of polymers due to their toxicity and synthetic sourcing. Surfactants and proteins have shown potential to enhance sludge dewaterability but little is known about the properties and/or mechanism(s) that promote this enhancement. In this study, synthetic polymers, surfactants and proteins were investigated to evaluate whether surface properties such as charge, surfactant activity and hydrophobicity, play a role in how these conditioners affect biosludge dewatering. Capillary suction time (CST), dry solids content, filtrate rate and filtrate solids content were used to assess dewaterability. Results show that surface charge determines the potential of conditioners. The effect of charge was greater for surfactants and proteins than for polymers. In contrast with previous reports, surfactant activity negatively affected the dewaterability of biosludge. Cationic conditioners, regardless of the group improved biosludge dewaterability. However, the dose of cationic proteins is still high compared to currently used synthetic polymers (e.g. protamine is 0.1 g/g TSS vs. synthetic polymer 0.03 g/g TSS). Our results suggest that there is potential for using proteins to improve biosludge dewaterability but a further reduction in protein dose and/or an increase in the protein’s efficiency as a conditioner is needed.

1. Introduction Biosludge dewatering is a challenge in wastewater treatment plants. Biosludge, also known as waste activated sludge, is a colloidal suspension of microbial aggregates with high moisture content (> 98%) and a gel-like matrix of extracellular polymeric substances that hinders the removal of water, making biosludge particularly difficult to dewater [18,11,20]. Several pretreatment and conditioning strategies are used to improve biosludge dewaterability. Chemicals that improve biosludge dewaterability, also known as conditioners, are widely employed in wastewater treatment plants. Synthetic, water-soluble polymers are the most commonly used. Polymers are effective at low doses but there are some disadvantages associated with their use as conditioners. They represent a major portion of the overall cost of the treatment, are petroleum-derived, dosesensitive and can be toxic to aquatic systems [9,4]. Moreover, high moisture content in the cake after dewatering has been associated with the hydration of high molecular weight polymers [9,2]. Cationic polymers are preferred for negatively-charged colloidal suspensions, such as biosludge. The cationic charge reduces the ⁎

Corresponding author. E-mail address: [email protected] (D.G. Allen).

http://dx.doi.org/10.1016/j.seppur.2017.08.048 Received 5 January 2017; Received in revised form 10 July 2017; Accepted 16 August 2017 Available online 09 September 2017 1383-5866/ © 2017 Elsevier B.V. All rights reserved.

repulsion between polymer molecules and biosludge particles which destabilizes the suspension and facilitates bridging of particles [12]. Bridging leads to large, strong flocs and is the main mechanism by which polymers improve biosludge dewaterability [17,4]. It has been reported that polymers that carry the same charge as the suspension can also lead to flocculation with bridging as the sole mechanism [30] and it is acknowledged that while charge neutralization aids particle bridging, it is not a requirement. In addition to synthetic polymers, surfactants have also been proposed as potential conditioners of biosludge. Their use has been extensively reported for enhancing liquid-solid separations in the mineral industry. A review of studies in fine particle suspensions was prepared by Besra et al. [1]. Surfactant addition is thought to complement polymer conditioning when the end-goal is to reduce the moisture content in cakes after mechanical dewatering [25]. Reducing the surface tension of the suspension facilitates movement of water through cake pores [24]. Dual conditioning (i.e., surfactant-polymer) has been reported on various suspensions and improvements were found regardless of the iconicity (i.e. charge) of the surfactants studied [8,14,3]. However, when surfactants have been used on biosludge, and as a

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single conditioner step (in the absence of polymer), only cationic surfactants have shown improvements on biosludge dewaterability [29,25,28,27]. The effect of surfactant activity on biosludge dewaterability has yet to be explored. Proteins have shown potential as a ‘greener’ alternative to enhance liquid-solid separations. Proteins can improve the dewaterability of biosludge and promote the solid-liquid separation of kaolin suspensions. Given the abundance of proteins in renewable materials and organic waste, it is conceivable that proteins could be a feasible alternative to chemical conditioners in the near future. However, a lack of understanding of the mechanisms and the key properties that affect the potential of proteins as conditioners hinders the development of protein-based conditioners and treatments. Previous studies of lysozyme on biosludge and kaolin suspensions suggest that charge neutralization is the main mechanism for such enhancement [6,5]. However, proteins have also been reported to have surfactant activity [21]. Thus, it is currently unknown if the protein’s cationic surface charge and/or its surfactant activity is responsible for the improvement of biosludge dewatering properties. The aim of this study was to evaluate the effect of various conditioners representing the three chemical groups previously discussed, i.e., polymers, surfactants, and proteins, on biosludge dewaterability, to get a better understanding of their effect on dewatering properties. Surface charge, surface tension and contact angles of conditioners were evaluated to investigate the effect of surface properties on their potential to improve dewatering.

Table 1 Conditioners used in this study to compare their surface properties and effect on biosludge dewaterability.

2. Materials and methods

2.2.3. Proteins Cationic proteins (active and inactive lysozyme, and protamine) were selected to investigate their surface properties and their effect on dewaterability. In addition to cationic proteins, bovine albumin serum (BSA) was added as a control since it does not carry a net cationic charge at the close-to-neutral pH values of biosludge (Table 1). Active and inactive stock solutions of lysozyme (50 g/L) were prepared as previously described in [5]. Stock solutions of protamine (20 g/L) and BSA (65 g/L) were prepared in deionized water and mixed using a vortex until dissolved. Proteins were added to biosludge and samples were mixed three times (by inversion) and left for 60 min before CST measurements. Dewaterability assessment was conducted as described in Section 2.4.

Conditioners

Supplier

Chargea

Polymers Zetag 8165 (Polyacrylamide)

BASF

Zetag 8185 (Polyacrylamide) Organopol 5400 (Polyacrylamide) AF 9645 (Polyacrylamide)

BASF BASF AXCHEM

Cationic high) Cationic Cationic Cationic

Surfactants Triton X-100 Sodium dodecyl sulfate (SDS) Cetyltrimethylammonium bromide (CTAB)

Sigma Sigma Sigma

Non-ionic Anionic Cationic

Proteins Lysozyme Protamine Bovine Albumin Serum (BSA)

Bioshop Sigma Sigma

pIb–10.7 pIb–12.5 pIb–4.8

a b

(Medium(high) (low) (high)

Information provided by vendor. pI : Isoelectric point.

A stock solution of 8 g/L was prepared for each of the surfactants in deionized water. In each of the experiments, the surfactants were added, mixed three times by inversion and left for 60 min before CST measurements. Dewaterability assessment was conducted as described in Section 2.4.

2.1. Biosludge Biosludge from a secondary clarifier was obtained from a Canadian pulp and paper mill which produces a variety of pulp, paper and specialty products using sulfite pulping and mechanical pulping (bleached chemi-thermomechanical pulp- BCTMP). Biosludge is the by-product of the aeration stage in the wastewater treatment plant treating mill effluents. Samples were kept at 4 °C in the laboratory prior to the experiments and for a maximum of three weeks. All the experiments were carried out with the same batch of biosludge which had a total suspended solids (TSS) content of 12.4 ( ± 0.3) g/L and volatile suspended solids (VSS) content of 10.5 ( ± 0.3) g/L and pH 6.9.

2.3. Surface properties analyses

2.2. Conditioners

2.3.1. Surface charge Surface charge measurements were conducted with colloidal titration using the principles reported by Kawamura et al. [16]. In a 50 ml beaker, 5 ml of conditioner sample, 2 ml of poly (diallyldimethylammonium chloride) solution (3% w/w) and 2 drops of 0.1% (w/v) toluidine blue were added and gently mixed. The mixture was then back-titrated by adding potassium salt of polyvinyl sulfate (PVSK) (0.0025 N) until the neutral endpoint, indicated by a change of color from blue to purple, was maintained for at least 10 s. The milliequivalent charge of the samples was then compared to that of pure water to find out the surface charge of conditioners.

2.2.1. Synthetic organic polymers Different cationic polymers were used to evaluate their surface properties and compare their effect on dewaterability with surfactants and proteins. Polymers represent the benchmark as conditioners for improving biosludge dewaterability since they are used in virtually all wastewater treatment plants. A stock solution (0.5% w/v) of each polymer was prepared a day in advance of the experiment with pure deionized water. Polymers were added to water while vortexing to facilitate dispersion. The suspension was further mixed for 1 h and allowed to sit undisturbed until the next day when the experiments were conducted. Four polymers with different characteristics were used in this study (Table 1). Dewaterability assessment was conducted as described in Section 2.4.

2.3.2. Surface tension Surface tension of the conditioner and the conditioned sludge were measured with a Sigma 700 tensiometer (KSV Instruments, Helsinki, Finland) using the Wilhelmy plate method. Measurements were carried out at 22 ( ± 2) °C, using a stabilization time of 10 min. Before every experiment, and under the same conditions, the surface tension of deionized water was measured to confirm a value of 72 ( ± 1) mN/m for deionized water and ensure the accuracy of the instrument. Samples were measured at least 5 times and a maximum variability within replicates of ± 2 mN/m was observed.

2.2.2. Surfactants To test the effect of surfactants on the dewaterability of biosludge and investigate the effect of surfactant activity on the potential of conditioners, three surfactants with different ionicity were selected. Triton X-100, CTAB and SDS represent non-ionic, cationic and anionic surfactants, respectively, and have been previously studied for enhancing biosludge dewaterability [8,14,3]. See Table 1 for more information on the surfactants used in this study. 201

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Fig. 1. Effect of different doses of proteins on biosludge dewaterability. (a) active lysozyme; (b) inactive lysozyme; (c) protamine; (d) bovine serum albumin (BSA). Dewaterability was assessed by normalized capillary suction time (CST) (left axis), solids content (%) in the cake after pressing and solids content in the filtrate after gravity thickening (i.e. crown press) (right axis). Note different range in X-axis (i.e. lower doses) for c and d. Error bars represent standard deviation of replicates.

2.4. Dewaterability assessment

2.3.3. Contact angle measurements Contact angles of the conditioners were measured on glass and on biosludge to investigate if their wetting properties affected their potential for enhancing biosludge dewaterability.

2.4.1. Capillary suction time (CST) Capillary Suction Time (CST) was used to evaluate the effect of the conditioners on the dewaterability of biosludge. The instrument consists of two electrodes: once the water reaches the first electrode after travelling through a filter paper from the sludge reservoir, a timer counts the seconds until the water reaches the second electrode where the timer stops. The time required for water to travel from the first to the second electrode is the CST. A lower CST implies better dewaterability. As a baseline, the CST of pure water was 5.4 ( ± 0.2) s. A Type 304 M Laboratory CST Meter (Triton Electronics Ltd.) was used and tests were performed in at least triplicates at 22 °C ± 2 °C. In falcon tubes, biosludge was conditioned with at least five different doses for each of the conditioners studied. All CST measurements were done in triplicate. CST results in this work are presented as normalized CST by dividing the obtained CST values in s and divide them by the initial TSS concentration of sludge as previously reported in Guang-Hui et al. [13].

2.3.3.1. Contact angle on biosludge. The biosludge surface was prepared as described by Liao et al. [19]. Using a goniometer, a drop of conditioner was placed onto the glass surface while a video recorded. Drop images were later extracted from the video. At least 5 drops were used to find the contact angle of the conditioners on biosludge. Hence, 10 angle values (2 sides per drop) were used for each of the samples analyzed. The maximum standard deviation observed was ± 4°. Except for the polymeric flocculants, all the conditioners studied were absorbed relatively quickly (< 10 s) into the biosludge surface. Thus, all the measurements were taken from images taken after 5 s of contact when there was no observable absorption. 2.3.3.2. Contact angle on glass. As a result of the challenges associated with using biosludge as a surface for contact angle measurements (e.g. surface roughness, variability and absorption of conditioners), the contact angle of conditioners was also measured on glass to compare the results obtained on biosludge surfaces. Glass microscope slides were cleaned with 70% ethanol, rinsed with deionized water and oven- dried before the experiments. Unlike biosludge surfaces, conditioners were not absorbed into glass, thus, angles were measured when no further expansion of the drop was observed and evaporation was not significant. At least 5 drops were used to find the contact angle of glass for each of the conditioners. Hence, 10 angle values were used for each of the samples analyzed.

2.4.2. Crown press To compare the CST results and obtain a dewaterability assessment more applicable to industrial practice, a bench-scale belt press (Crown® press from Phipps & Bird Inc.) was used. Samples of 200 mL of biosludge and the respective conditioner were first transferred to the gravity thickening component of the equipment for 10 min. During gravity thickening, the filtration rate was measured and after 10 min, the filtrate was collected and analyzed for total solids content. The resulting cake was transferred to the belt press area where a pressure schedule of 120, 150 and 200 lbs (6.3, 7.9, 10.5 psi, respectively) was 202

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used for all samples. Each pressure was sustained for 10 s followed by a fast release. The total solids content in the cake was measured to assess the effect of the conditioners on mechanical dewatering of biosludge. Crown press experiments were prepared in duplicates and the total solids for each replicate was measured in triplicate. Where an optimum dose was evident from the CST data, the optimum, and the doses before and after were used for further dewaterability assessment with the crown press. In total, three doses were tested per conditioner. When a clear optimum from the CST data could not be observed, three consecutive doses that had shown a significant effect on CST were selected. The control for these experiments was a sample of biosludge with the same volume of deionized water added instead of conditioner. 3. Results and discussion 3.1. Effect of conditioners on CST, cake and filtrate solids content From the four proteins tested, only the proteins with cationic charge (active lysozyme, inactive lysozyme and protamine) resulted in improved dewatering in both assessment methods, i.e. cake solids after mechanical dewatering and CST (Fig. 1). Active and inactive lysozyme showed similar trends as conditioners as has been previously reported [6]. Active lysozyme increased the cake solids from 8.1( ± 0.7) to 13.9 ( ± 0.3)% while inactive lysozyme increased it to 12.2 ( ± 0.7)% with a dose of 0.3 g/g TSS. Protamine increased the cake solids after mechanical pressing to 11.2 ( ± 0.6)% with a dose of 0.1 g/g TSS and even with half of that dose (i.e. 0.05 g/g TSS), solids were significantly increased to 10.8 ( ± 0.3)%. On the other hand, as shown in Fig. 1d, BSA had detrimental effect on the dewaterability of biosludge indicated by the three measures of dewaterability (i.e. increased CST, increased filtrate solids and decrease cake solids content). These results confirm the potential of cationic proteins as conditioners for enhancing biosludge dewaterability. Only the surfactant with cationic charge, CTAB, improved biosludge dewaterability. Triton X-100 (non-ionic) and SDS (anionic) had a negative effect on CST and dry solids (Fig. 2). The optimum dose for CTAB was 0.35 g/g TSS, at which the maximum CST reduction was observed. CTAB increased the cake solids content of biosludge after mechanical pressing from 8.1 ( ± 0.7)% to 14.8 ( ± 0.7)% with a dose of 0.5 g/g TSS. At this dose, CST data showed an overdose effect; however, this overdose was not observed in cake solids data (Fig. 2a). Even at low doses, anionic and non-ionic surfactants (SDS and Triton X-100, respectively) had a detrimental effect on dewaterability. Considering both, their effect on dewaterability and the required dose, polymers remain the best conditioners. With a significantly lower dose than other conditioners, polymers were able to reduce CST to water-like values (∼6 s) (i.e. lowest possible CST) (Fig. 3 and supplementary material). Three of the four polymers studied (Zetag 8165, Zetag 8185 and AF9645) had an optimum dose of 0.03 g/g TSS. After this dose, a sharp increase was observed in CST, indicative of an overdose. This overdose was also observed in the dry solids content after mechanical pressing. Organopol’s performance showed clear differences when compared to the other polymers in this study. For this polymer, a lower optimum dose (0.005 g/g TSS) was observed but the effect on CST was smaller. These difference could be due to the low cationic charge of organopol. Cationic proteins (lysozyme active, lysozyme inactive and protamine) and the cationic surfactant (CTAB) showed significant improvements but higher doses were needed and the improvements observed on dewaterability were not as large as with polymer conditioning (Fig. 4). Unlike proteins and polymers, all surfactants resulted in increased filtrate solids with increasing doses (Fig. 2). Results suggest that surfactants disrupt biosludge flocs, resulting in smaller particles that can pass through the gravity filter which increases the solids content in the filtrate. The detrimental effect of surfactants on biosludge dewatering

Fig. 2. Effect of different doses of surfactants on biosludge dewaterability. (a) CTAB; (b) Triton X-100; (c) SDS. Dewaterability was assessed by normalized capillary suction time (CST) (left axis), solids content (%) in the cake after pressing solids content in the filtrate after gravity thickening (i.e. crown press) (right axis). Note different range in X-axis, 10fold higher for CTAB vs Triton X-100 or SDS. Error bars show standard deviation of triplicates.

properties is not surprising. Surfactants are widely used to disrupt cell membranes [15] and breakage of particles has been previously reported to be detrimental to the dewatering properties of biosludge [10,22,27]. Therefore, even though surfactants have shown to be a promising conditioning treatment for inorganic suspensions, they may result in worsening dewatering properties for biosludge and potentially other biological suspensions. 203

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Fig. 3. Effect of different doses of polymers on biosludge dewaterability. (a) Zetag 8165; (b) AF9645; (c) Organopol; (d) Zetag 8185. Dewaterability was assessed by normalized capillary suction time (CST) (left axis) and solids content (%) in the cake after pressing and in the filtrate solids after gravity thickening (i.e. crown press) (right axis). Increased solids content and reduced capillary suction time are indicative of improved dewatering properties. Error bars represent standard deviation of triplicates. Note different range in X-axis.

have a significant impact on the effectiveness of the conditioning treatments with the various synthetic polymers, proteins and surfactants used in this study. However, these variables were not explored as it was out of the scope of this study. Results from mechanical dewatering assessment (i.e. Crown press) are consistent with CST. As shown in Fig. 5, high CST values tend to result in high cake solids and vice versa. A strong linear correlation was observed between dry solids content and CST data for the eleven 11) conditioners studied at their optimum dose. The limitations of CST are

Regardless of the conditioner group, cationic conditioners resulted in reduced CST (i.e. better dewaterability) while anionic conditioners increased CST (i.e. worse dewaterability). Polymers, surfactants and proteins can enhance the dewaterability of biosludge and their effect appears to be mainly affected by their cationic charge. In Fig. 4, conditioners used in this study are organized by their effect on dewaterability and only cationic conditioners showed dewaterability improvements (i.e. reduced the CST of biosludge). It is important to note that conditions such as pH, temperature and ionic strength would likely

Fig. 4. Effect of conditioners on normalized capillary suction time (CST) at their optimum dose. Bar graph represents the normalized CST (left axis), the corresponding dose (i.e.optimum) is presented as orange diamonds (right axis). Error bars show standard deviation of CST triplicates. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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Fig. 5. Correlation of normalized capillary suction time (CST) and dry solids content data for the three groups of conditioners at their optimum dose. Error bars represent standard deviation of triplicates.

Fig. 6. Effect of surface charge on the effect of conditioner on normalized CST of biosludge at their optimal dose. Trend line equations, r^2 and p values are shown for three cases: all conditioners, proteins and surfactants, and polymers.

well-known and have been discussed elsewhere [26,7,23]. Nonetheless, our results demonstrate that CST can be used to infer trends in the effect of conditioners on dewaterability. Determining trends is particularly important for screening potential future conditioners. From the eleven conditioners studied, there were two conditioners that showed slight inconsistencies between dry solids content and CST data around the “optimum dose” i.e. active lysozyme and CTAB. In the case of active lysozyme, while CST appeared to remain relatively constant with doses 0.1–0.3 g/g TSS, the dry solids content increased from 10.4 to 13.9% in the same dosage range (Fig. 1a). In the case of CTAB, CST data showed a clear overdose at the highest dose (0.5 g/g TSS) while crown press data showed an increase in dry solids content from 12.2 ( ± 0.2) to 14.8 ( ± 0.7)% with increasing doses from 0.35 to 0.5 g/g TSS suggesting that an optimum had not been reached (Fig. 2a). Capillary suction time is affected by all solids present in biosludge, while the crown press is separated in two stages, gravity thickening (i.e. filtrate solids) and belt pressing (i.e. cake solids). In the crown press, small particles solids are removed prior to mechanical dewatering and so they do not affect cake solids content. It is recommended that when gravity thickening and mechanical pressing are separated in two-stages, as in the case of the crown press, both measurements are taken into account to assess the effect of conditioners. Furthermore, filtrate solids are of great importance to wastewater treatment efficiencies as these solids would be recycled back to the aeration tank, increasing the organic load of the plant.

While only the cationic surfactant CTAB increased the cake solids content of biosludge after mechanical dewatering (extent), the rate of filtration was positively affected by CTAB and Triton X-100 (non-ionic). There was no improvement on the filtration rate after SDS conditioning. CTAB and Triton X-100 increased the filtrations rates and the final amount of water removed during gravity thickening (Fig. 6c). The effect of Triton X-100 on biosludge dewaterability is the opposite of what has been reported previously in the literature. Surfactants were proposed to improve cake solids content but do not have a significant effect on filtration rate[3]. 3.3. Effect of surface charge, surfactant activity and wettability on conditioning of biosludge The results consistently indicate that more cationic surface charge in the conditioners improves biosludge dewaterability. Surface charge (charge equivalents/g TSS) and CST data show a strong negative correlation (Fig. 7). If all the conditioners are considered as a group, the correlation between surface charge and CST is moderate (r = 0.69, p < 0.0001). However, if the conditioners are separated into two groups: polymers and, surfactants and proteins, the correlation in each data set is stronger, r = 0.93, p < 0.001 and r = 0.91, p < 0.0001, respectively. Thus, two different relationships appear to describe the effect of surface charge on biosludge dewaterability for the three groups conditioners studied. This suggests that the effect of surface charge, although significant for all the conditioners, is stronger for surfactants and proteins than for polymers. This is in agreement with the mechanism of polymers where bridging plays a major role and is only promoted by charge neutralization [17]. Charge neutralization is possibly the main mechanism of surfactants and proteins for improving biosludge dewaterability as their effect is greatly affected by their surface charge. Contrary to what has been proposed in the literature, increased surfactant activity (i.e. reducing the surface tension) does not improve the effect of the conditioners on the dewaterability of biosludge. Increasing surfactant activity of the conditioners leads to poor dewaterability (Fig. 7a). If surface tension of the conditioners increased, CST decreased (i.e. better dewaterability). This linear correlation was significant (p < 0.05). The trend observed for the effect of surface tension on biosludge dewaterability could be attributed to floc breakage as a result of surfactant activity on biosludge flocs. Cell lysis and worsening dewaterability after addition of SDS has been previously observed [27]. This suggests that surfactants may not be good conditioners for biosludge since smaller particles are generally not desirable in liquid-solid separation processes.

3.2. Effect of conditioners on filtration rate during gravity thickening Conditioning of biosludge with polymers resulted in improved gravity filtration rates. AF9645, Zetag 8165 and 8185 showed overlapping filtration curves (Fig. 6a) where gravity thickening was virtually complete after 1 min with 138 ( ± 5–16) mL filtered. On the other hand, for the control (biosludge with water instead of conditioner), only 54 ± 4 mL were filtered after 1 min. Organopol, at a dose of 0.01g/g TSS improve the filtration rate (63 ± 4 mL) compared to the control, but in agreement with other dewatering assessment methods (i.e. cake solids % and CST) it showed a limited effect when compared to the other polymers studied. More water is removed during the thickening step with polymer conditioning. Cationic proteins slightly improved the filtration rate of biosludge during gravity thickening. Protamine and active lysozyme showed better filtration rates than the control (Fig. 6b). After 2 min, both proteins showed an increase in filtrate volume with a maximum of 112 ( ± 1) and 113 ( ± 2) mL for active lysozyme and protamine respectively, while the control had only filtered 95 ( ± 10) mL after 4 min. Inactive lysozyme and BSA showed no improvement on the filtrate rate. 205

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Fig. 7. (a) Effect of surface tension of conditioners and biosludge (conditioned) on the dewaterability of biosludge; (b) effect of wettability (contact angle) on the dewaterability of biosludge as measured with Normalized CST.

caused by smaller particles. Only the cationic surfactant, CTAB, showed improvements in biosludge dewaterability, thus confirming the importance of charge when evaluating the potential of conditioners. Larger particles produced as a result of the cationic charges in the conditioner can also increase the radius of the capillary, improving the dewatering process by reducing de pressure differential, Δp.

Table 2 Pearson coefficients (r2) and significance (p-value) for assessing the strength of the correlation between surface tension and contact angle, and the effect of conditioners on dewaterability (i.e. Normalized CST). Correlations were evaluated for all the conditioners as one group and separately per group of conditioner. Pearson correlation r2

p-value

Surface tension All conditioners Polymers Surfactants Proteins

0.5 0.6 0.6 0.2

< 0.01 0.2 0.6 0.7

Contact angle All conditioners Polymers Surfactants Proteins

0.8 0.1 0.9 0.3

< 0.01 0.6 0.2 0.3

4. Conclusions All the cationic chemicals in this study improved the dewaterability of biosludge. Within each group of conditioners, increased cationic charge resulted in better conditioning performance. While charge plays a major role in the efficacy of all conditioners, this effect is greater for proteins and surfactants. Polymers were significantly better as conditioners because they improve dewaterability and increased dry solids content with low dosages. Protamine was the best conditioner from the group of proteins and surfactants but the dose required was three times more than the doses of most polymers. Nonetheless, cationic proteins have potential as conditioners of biosludge and as flocculants. Proteins are abundant in nature, can be extracted from wastes and are biodegradable which can be a potential advantage over polymers. Finding cationic proteins and their sources can improve the feasibility of using them in industrial processes. Proteins are an alternative when synthetic polymers are not desirable, e.g. food processing. Since surface charge determines the potential of proteins as conditioners, it is proposed as the first cut-off to screen proteins for enhancing biosludge dewaterability. Surfactant activity or the wettability of conditioners were not found to be consistent indicators of conditioning performance. Increased surfactant activity can lead to smaller particles that have a detrimental effect on biosludge dewaterability.

Neither surfactant activity nor wettability appears to be a good property for screening potential biosludge conditioners. As shown in Table 2, linear correlation and significance values are weak. The correlations observed in Fig. 7 appear to be more the result of intrinsic properties for each group of conditioner rather than surfactant activity or wettability determining the effect of conditioners on biosludge dewaterability. Surface charge determines the potential of proteins and surfactants for conditioning biosludge. Lysozyme and protamine, both cationic proteins, resulted in improved biosludge dewaterability. Surface charge can be used to screen proteins and likely other biopolymers to assess their potential as conditioners of biosludge. Surfactants, although used to enhance separation of inorganic suspensions, appear to break flocs in biosludge which results in poor dewatering characteristics. While the effect of surface charge and particle size on the dewaterability of biosludge has been widely studied and is fairly well understood, the effect of surfactant activity is not well understood. Our results suggest that surfactants may lead to opposite effects on biosludge dewaterability. According to the Laplace equation, for water to flow through the cake and filter in a dewatering process, a pressure differential (Δp) needs to be surpassed. The relationship between Δp and the surface tension at the water-air interface shows that reducing the surface tension is one way of reducing Δp and achieving a higher solids content in the cake. This supports the rationale behind the use of surfactants to improve dewatering processes. However, surfactants also appear to break sludge particles, possibly reducing the radius of the capillary, which leads to an increase in Δp. Our results show that surfactants did not improve cake solids. In fact, surfactant activity resulted in poor dewatering properties likely due to filter and cake blinding

Acknowledgements This work was part of the research program on “Increasing Energy and Chemical Recovery Efficiency in the Kraft Process”, jointly supported by the Natural Sciences and Engineering Research Council of Canada (NSERC) (CRDPJ 428559-11) and a consortium of the following companies: Andritz, AV Nackawic, Babcock & Wilcox, Boise, Carter Holt Harvey, Celulose Nipo-Brasileira, Clyde-Bergemann, DMI Peace River Pulp, Eldorado, ERCO Worldwide, Fibria, FP Innovations, International Paper, Irving Pulp & Paper, Kiln Flame Systems, Klabin, MeadWestvaco, StoraEnso Research, Suzano, Tembec, Tolko Industries and Valmet. Special thanks to Silvia Zarate, Aurelio Stammitti and Prof. Edgar Acosta for their advice on surface tension and contact angle measurements. 206

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Appendix A. Supplementary material [14]

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.seppur.2017.08.04.

[15] [16]

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