A Novel Approach for Rapid Dewatering of Water-Based Ink Wastewater Sludge under Low Temperature and Its Mechanism

Abstract

Enhanced dewatering remains a major challenge in sludge disposal. In this paper, a thermal solidification treatment under low temperatures was first and successfully employed to improve the dewaterability of sludge from water-based ink wastewater. A total of 95.1% of dewatering ratio, 36.6% of moisture content, and 91.9% of sludge volume reduction could be acquired at 75 °C and 30 min setting time by the reported approach. The DSC results indicated that a large amount of bound water trapped in sludge could be released into the bulk solution to become free water after thermal solidification treatment. The bound water content of sludge can be reduced from 3.40 to 0.20 g g⁻¹ dry solid. The reported thermal solidification treatment is a thermophysical process that does not involve the solubilization of solid substrates and does not cause an increase in the COD value of dehydrated water. The obtained sludge after cooling was a clot of compacted cake with a uniform coralline-like structure. The softening and shrinking of acrylic resin within the sludge during heating contributed to the formation of compacted cake and thus enhanced the dewaterability. This study offers an economical and efficient treatment of sludge for the water-based ink printing industry.

1. Introduction

Printing inks are extensively used in publication, packaging, textile printing, and others. In spite of the soaring popularity of e-books and strict regulatory policies over the last decades, the global printing ink market is growing at about 6–7% per year, and more than 4 million tons of ink are consumed annually [1]. In an era marked by growing environmental awareness and sustainability concerns, solvent-based inks are rapidly losing popularity, and water-based inks use water as a carrier, reducing the emission of volatile organic compounds (VOCs). In 2024, the estimated consumption of water-based inks accounted for 25–30% of the overall printing ink market share in China. At the same time, water-based ink wastewater discharge is steadily increasing. Wastewater obtained after cleaning/washing of the laboratory and industrial equipment is characteristically high in both chroma and organic content and is usually difficult to treat biologically. Thus, it will bring severe pollution to ecosystems if not treated satisfactorily [2].

Many challenges remain in finding inexpensive and effective treatment processes for water-based ink wastewater, but there has been little research on the treatment or recycling of this kind of wastewater and its sludge, according to the authors’ knowledge. The reported approaches are mainly about pretreatment, including coagulation or flocculation [3,4], acid precipitation–electrocatalysis [5], chemical oxidation–coagulation [6], adsorption [7], and microfiltration [8], in combination with the biochemical process. Among them, flocculation is a relatively simple and efficient method for removing a substantial portion of the organic content and is especially very good for the removal of color and turbidity. However, flocculation pretreatment of the highly concentrated water-based ink wastewater generally results in a pasty sludge with a suspended solid content of up to 5–9% [3]. Thus, traditional solid–liquid separation methods like air flotation and precipitation are not suitable for sludge separation from pretreated water-based ink wastewater. Actually, the sludge is dewatered directly by mechanical pressure and centrifuge force in many enterprises, in which the dewatering speed and efficiency are very low while the energy cost is high. The obtained sludge still has a large volume and high moisture content (more than 80% after hours of mechanical dewatering). It is known that sludge dewatering is the most important and difficult step in the wastewater treatment process, and it is used to reduce the final sludge volume in order to minimize the cost of sludge transportation and disposal. For the treatment of water-based ink wastewater slurry, figuring out how to improve dewaterability becomes the key issue.

The present paper aims to report a simple and effective approach to improving the dewaterability of the coagulation sludge from water-based ink wastewater in order to minimize the volume of sludge. Traditionally, sludge conditioning and modification are the commonly used methods to enhance activated sludge dewaterability, including chemical conditioning [9,10], freeze–thaw conditioning [11], hydrothermal treatment [12], and electrolysis [13]. The solid content of dewatered sludge would increase after chemical conditioning, while freeze–thawing pretreatment requires special equipment or severe cold regions to reach a low temperature of less than −20 °C. Compared with chemical conditioning and freeze–thaw conditioning, the hydrothermal method is more frequently applied. Hydrothermal treatment is a thermochemical convention process, which is achieved by applying temperatures of 160–220 °C under saturated pressure for several hours [14]. Solid–liquid separation properties of sludge can be clearly improved because hydrothermal treatment at high pressure and temperature leads to the disruption of microorganism cells in sludge and the release of the bound water from the sludge [12,14]. However, the application of this method causes an increase in the soluble chemical oxygen demand (COD) value of the sludge-hydrated water due to the hydrolysis of organic matter. Additionally, high-temperature/high-pressure processing is very energy-consuming. The above-mentioned methods are mainly performed on the treatment of activated sludge, less on the dewatering of industrial sludge, especially for industrial coagulated sludge.

Water-based ink is mainly composed of pigments, polymers (resin binders), solvents, and additives. The unused components are discharged as wastewater that is high in color, as well as the hydrophilic resin binder. The water-soluble macromolecular resins, such as widely used acrylic resin, are probably the main reason leading to the formation of pasty sludge, which is very difficult to dewater. The authors also found that polymer chain conformations of water-soluble resins would be significantly changed by low-temperature heating under acidic conditions. Therefore, a thermal solidification treatment under low temperatures was proposed and employed for the dewatering of water-based ink wastewater sludge by the authors based on the characteristics of contaminants in sludge. Such a process does not need additional coagulants or severe operating conditions. From the literature reviews, no other reports were found on the dewatering of water-based ink wastewater sludge using this method.

The aim of this work was to evaluate the effectiveness of low-temperature thermal solidification on the dewaterability of water-based ink wastewater sludge and to explore the experimental conditions for the most interesting results. The mechanism of sludge dewatering by this low-temperature solidification was also investigated.

2. Materials and Methods

2.1. Sludge Source

The sample used in this study was undiluted water-based printing ink wastewater, which was collected from a wastewater treatment plant of a plastic printing company in Qingdao, China. The main characteristics of the sample were as follows: pH 7.18, water content 94.4%, COD 129,100 mg L⁻¹, suspended solids (SS) 1860 g L⁻¹, total solids (TS) 62,180 mg L⁻¹, NH₃–N 962 mg L⁻¹, and chroma 28,000. The sample was stored in a refrigerator at 4 °C until used.

2.2. Sludge Treatment and Dewatering Procedure

A total of 98.0 wt% H₂SO₄ was added drop to drop to adjust the solution pH to 3.0, and the process proceeded with rapid mixing of the wastewater sample at 300 rpm for 2 min, slow mixing at 100 rpm for 15 min, and then maintaining standstill for 30 min. The obtained sample was named raw sludge.

A total of 100 mL of raw sludge was added to a 300 mL stainless steel heating tube. The seal cover of the tube was tightened, and the heating tube was placed in a HIF-2A infrared heater. The sample was heated to a specified temperature and maintained at this temperature for a given period of time. Then, the heating tube was removed from the infrared heater, and the sample was cooled to room temperature. For the thermal solidification samples (thermal solidification sludge), the dewatered sludge cake was removed, and the dehydrated water was gathered. For the thermal unsolidification samples, filtration dewatering was conducted through a quantitative filter paper (medium speed) under a 0.05 MPa vacuum until the vacuum was destroyed, then the dewatered sludge cake and filtrate were collected. Only a few samples (sludge cake) were drying in the air. All other samples were dried to a constant weight in an oven at 103–105 °C.

All of the measurements were carried out at least three times to maintain a standard deviation of less than 5%.

2.3. Sludge Dewaterability

The dewatering ratio of treated sludge D was calculated by

$$D={\displaystyle \frac{M_0-M_2}{M_0-M_1}}\times 100\%$$

(1)

where M₀ is the mass of raw sludge, M₂ is the mass of dewatered sludge, and M₁ is the dry weights of treated sludge and is recorded after drying to constant weight in a forced-draft oven at 103–105 °C.

The volume of dewatered sludge V_T was determined using the depletion method and given by

$$V_{\mathrm{T}}=V_0-V$$

(2)

where V₀ is the total volume of dewatered sludge and dehydrated water, and V is the volume of dehydrated water.

The moisture content of treated sludge M was calculated by

$$M={\displaystyle \frac{m-m_1}{m}}\times 100\%$$

(3)

where m is the mass of treated sludge, and m₁ is the dry weight of treated sludge and is recorded after drying to a constant weight in a forced-draft oven at 103–105 °C.

2.4. Sludge Characterization

2.4.1. Differential Scanning Calorimetry Analysis

Differential scanning calorimetry (DSC) was performed to measure the amount of free water in sludge cake [15]. This method is based on the assumption that bound water will not freeze below −20 °C. The enthalpy absorbed during the freezing is, therefore, proportional to the free water amount. The amount of bound water can be subsequently obtained from the total amount of water and the measured free water amount. A Netzsch STA409 DSC (Netzsch, Selb, Germany) apparatus equipped with a scanning calorimeter cell was employed to record the thermograms of samples. The temperature of the sample first was decreased at a rate of 10 °C min⁻¹ to −60 °C and then was raised back to room temperature at the same rate. The amount of free water W_F was calculated as follows:

$$W_{\mathrm{F}}={\displaystyle \frac{\mathsf{\Delta }H}{\mathsf{\Delta }{H}_0}}$$

(4)

where ΔH is the heat absorbed during the melting process and ΔH₀ is the melting enthalpy of free water (ΔH₀ = 334.7 J g⁻¹). The total amount of water in sludge W_T was determined by drying to a constant weight in a forced-draft oven at 103–105 °C. Thus, the amount of bound water in sludge W_B was calculated by subtracting the amount of free water W_F from the total amount of water W_T:

$$W_{\mathrm{B}}=W_{\mathrm{T}}-W_{\mathrm{F}}=W_{\mathrm{T}}-{\displaystyle \frac{\mathsf{\Delta }H}{\mathsf{\Delta }{H}_0}}$$

(5)

2.4.2. SEM Observation

Both the raw sludge and dewatered sludge were dried in the air and then sputter-coated with gold prior to SEM observation. A SEM image of the sludge was obtained using a Hitachi TM-3000 Scanning Electron Microscope (Hitachi, Tokyo, Japan) operated at 15 kV.

2.4.3. Specific Surface Area and Pore Size Distribution

Specific surface area and pore size distribution were measured using physical adsorption isotherms of N₂ by a Beishide 3H-2000PS2 surface area porosity analyzer (Beishide Instrument Technology, Beijing, China).

2.4.4. XPS Test

The X-ray photoelectron spectroscopy (XPS) of sludge was determined by a Kratos XSAM-800 (Kratos Analytical Ltd., Manchester, UK) spectrometer with an AlKα (hv = 1486.6 eV) radiation X-ray source.

2.5. Wastewater Analysis

COD was determined using the closed reflux colorimetric method according to the standard methods [16].

3. Results and Discussion

3.1. Dewatering Efficiency

The appearances of the final sludge after thermal treatment at different temperatures for 30 min are shown in Figure 1. The sludge dewaterability was significantly improved after low-temperature thermal treatment. When thermally treated below 40 °C, the final sludge was flocculent, and the flocs were dispersed and irregularly shaped, and almost no change was observed in comparison with raw sludge (Figure 1a). When the temperature was elevated to 60 °C, the particles aggregated and formed large flocs, resulting in good settleability (Figure 1b). However, the formed flocs were composed mostly of loosely clumped sludge, which was easily broken into fragments when placed under vigorous shaking. When the temperature was increased to 80 °C, the obtained sludge was a clot of compacted cake, and the residue solution was clear and transparent in appearance (Figure 1c). Large flocs often denote good settleability and dewaterability. Obviously, some factors that related closely with excellent dewaterbility were activated under a higher temperature of 80 °C. The detailed mechanism of the above interesting phenomena will be revealed in the following text after more investigation.

3.2. Effect of Temperature and Time on Sludge Dewatering

The dewatering ratio of sludge and volume of dewatered sludge under various temperatures were investigated in this paper, and the results are shown in Figure 2a (the setting time was 60 min). As the temperature increased from 45 to 65 °C, the dewatering ratio of sludge increased from 70.5% to 93.1%, and the final sludge volume decreased from 33.5 to 10.5 mL. These improvements are primarily associated with the thermal treatment, which can promote the suspended particles bumping into larger particle reunion below 70 °C so that the free water and part of the bound water are released from the sludge. When the temperature was further increased from 70 to 80 °C, the dewatering ratio of sludge stayed in the range of 94.9–95.1%, while the final sludge volume remained around 8.1 mL, and sludge volume reduction was 91.9%. The explanation is that at the temperature of 70 °C, which may be reaching the softening point of acrylic resins, the polymer in the sludge softened and shrank quickly; as a result, the sludge dewatered and solidified automatically. In this study, we named the thermal solidification treatment in order to distinguish it from the traditional hydrothermal treatment. The suggested thermal solidification temperature should not be lower than 70 °C.

Figure 2b shows that thermal treatment at 70 °C with increasing time resulted in a significant increase in the dewatering ratio of sludge from 89.6% to 95.1% and an obvious decrease in the final sludge volume from 13.8 to 8.1 mL. It indicated that the polymer chains need a certain period of time to accomplish the thermal shrink process after reaching their softening point. It can also be seen that the dewatering ratio of sludge and the final sludge volume were not further improved when the thermal setting time was extended from 30 min to 60 min, indicating that no more sludge volume reduction was seen after the polymer chains shrank to an extent. Therefore, the suggested setting time was no less than 30 min.

3.3. Characteristics of Sludge

The appearances of raw sludge and thermally solidified sludge were also greatly different under SEM. As shown in Figure 3b, the sludge particles adhered to each other to form a uniform coralline-like compact structure after thermal solidification, and a much larger number of pores in the interior of sludge were observed in comparison with the raw sludge (the SEM image of raw sludge is shown in Figure 3a). The cooling sludge had greater hardness and could not be broken easily. Definitely, solidification and shrinkage of sludge, as well as bigger pores between sludge particles, will all be propitious to the release of water from sludge.

The DSC thermograms of sludge are shown in Figure 4a,b. Large amounts of heat were released during the range of −18 to −27 °C. When the temperature was below −30 °C, no obvious exothermic peak was observed. The heat released by the sample during cooling is proportional to the free water amount. Therefore, the bound water content of sludge can be calculated. Results showed that the bound water content of sludge after thermal solidification treatment at 75 °C for 30 min was reduced to below 0.20 g g⁻¹ dry solid (Figure 4b), while the bound content of raw sludge was 3.40 g g⁻¹ dry solid (Figure 4a). It indicated that the inner interactions and microstructure of sludge were greatly changed during thermal solidification treatment. This can also be confirmed by the variation in specific surface area and pore size distribution of sludge (Figure 4c–f). In comparison with raw sludge, the specific surface area of the sample after thermal solidification treatment was decreased from 10.34 to 2.47 m² g⁻¹, while the mean pore size was decreased from 20.46 nm to 11.14 nm. Probably, shrinkage of sludge caused by thermal solidification treatment resulted in a reduction in specific surface and pore size.

The oxygen-containing functional groups exert a great influence on the surface properties of sludge particles. Oxygen functionalities on the sludge surface have been categorized as hydroxyl (–OH), carboxyl (–COOH), carbonyl (C=O), and ether (–O–) [17,18]. The first three groups (hydroxyl, carboxyl, and carbonyl) have strong hydrophilicity and enhance the water-absorbing capacity of the sludge. In this study, XPS was used to obtain information about the surface characteristics of sludge particles. The O 1s/C 1s peak area ratio (see

Table 1. Relative contents of functional group in C 1s from XPS spectra.

Sludge Sample	O/C	C 1s (%Amount)
		Peak 1 284.7–284.8 eV	Peak 2 286.3 eV	Peak 3 288.9–289.1 eV	Peak 2 + Peak 3
		C–C and C–H	C–O and C–OH	O=C–O	Hydrophilic Groups
Raw sludge	0.244	84.0	11.2	4.8	16.0
Thermal solidified sludge	0.231	88.5	7.8	3.7	11.5

) decreased when the sludge was treated by the thermal solidification method at 75 °C for 30 min. This may be explained by the fact that parts of carboxyl groups of acrylic resins were enclosed within the sludge after thermal solidification treatment.

For the sake of a better understanding of the surface chemistry change during the thermal solidification treatment, XPS Peak 4.1 was employed to deconvolve the high-resolution C 1s spectra in this study. As shown in Figure 5, the high-resolution C 1s spectra of sludge samples were resolved into three individual component peaks: C–C/C–H (peak 1, 284.7–284.8 eV), C–O/C–OH (peak 2, 286.3 eV), and O=C–O (peak 3, 288.9–289.1 eV). Among these groups, C–C and C–H (peak 1) are hydrophobic groups, while hydroxyl (peak 2) and carboxyl groups (peaks 3) are hydrophilic groups. Table 1 summarizes the results of the C1 s peaks. C–C and C–H were the predominant carbon chemical bonds for raw sludge and accounted for the 84.0% peak area ratio at the C 1s peak, reflecting that its relative amount was significantly higher than that of the carbon bonded to oxygen-containing functions (16.0%). Therefore, it could be identified that the primary carbonic compounds in the sludge sample were aliphatic compounds. For the sample after thermal solidification treatment, the relative amount of C–C and C–H (peak 1) was increased to 88.5%, and the content of hydrophilic groups was decreased to 11.5%. The fitting data in the C 1s peak can only give quantitative information on the relative contents of oxygen-containing functional groups, but we can conclude that the hydrophobicity of the sludge after thermal solidification treatment was decreased.

Characteristics of sludge cake after thermal solidification treatment are summarized in

Table 2. Characteristics of sludge cake after thermal solidification (volume of raw sludge was 100 mL).

Parameter	Value
Wet weight of sludge cake (g)	9.318
Dry weight of sludge cake (g)	5.911
The moisture content (%)	36.6
The amount of free water (g/g dry solid)	0.393
The free water proportion (%)	68.2
The bound water content (g/g dry solid)	0.183
The bound water proportion (%)	31.8

. The moisture content of the sludge after thermal solidification treatment could be reduced to 36.6%, indicating an enhanced dewatering of sludge. The remaining free water content accounted for 68.2% of the total moisture in the treated sludge. This part of water was mainly present in the internal pores of treated sludge. Since no mechanical force was applied in the thermal solidification process, the contraction and condensation of the sludge were totally dependent on the shrinkage of the softened polymer chains. Such shrinkage is very limited, and it cannot extrude all the water contained in the internal pores of sludge. Meanwhile, the existence of internal water would hinder further contraction of the sludge. However, the remaining 68.2% of free water in the internal pores of sludge can be easily removed through a certain mechanical process.

The dry weight and volume of sludge cake from thermal solidification of 100 mL raw sludge were 5.93 g and 8.1 mL, respectively. The calculated bulk density of this sludge cake was about 0.730 g cm⁻³, indicating that the formed structure of the sludge cake was not a completely dense structure but still filled with a certain number of pores, which was in agreement with the SEM observation (Figure 3b).

The bound water content was reduced to merely 0.183 g g⁻¹ dry sludge due to the changes in the surface characteristics of sludge after thermal solidification. The hydrophilic groups of polymer chains were enclosed within the sludge, and the sludge shrank by this cohesion under the action of heat, which reduced the specific surface area of the sludge and the interaction between water and hydrophilic groups on the surface of the sludge. With the increase in hydrophobicity of sludge particles, the majority of bound water in the raw sludge was turned into free water.

3.4. Characteristics of Dehydrated Water

The COD of dehydrated water at different temperatures was investigated and shown in Figure 6. The COD of the supernatant obtained by centrifugation of raw sludge was 4553.5 mg L⁻¹ at room temperature. When the temperature varied from 45 to 95 °C, the values of COD were within the range of 4500–4600 mg L^−1, and there was no significant change compared with the sample without thermal solidification treatment. The experimental results indicated that the significant changes in morphology and characteristics of sludge during the thermal treatment process were not caused by hydrolysis of organic matter or solubilization of solid substrates, which was very different from hydrothermal treatment of activated sludge. For the activated sludge, it is known that the lysis of cell walls during hydrothermal treatment releases solutions due to the disruption of sludge microorganism cells [19], which results in the hydrolysis of organic matter and reduction in viscosity. The water bonded into flocs was set free and transformed into bulk water due to the improvement of the characteristics of sludge particles. Hydrolysis of organic matter and solubilization of solid substrates during hydrothermal treatment will inevitably lead to increased pollutants in the solutions, thus giving the residue solutions a high COD [20]. In contrast, thermal solidification treatment in this paper is a thermophysical process due to the low temperature and almost unchanged COD value of dehydrated water.

3.5. Dewatering Mechanism and Model

The increase in the dewatering ratio and reduction in sludge volume resulting from thermal solidification treatment is supposed to improve the sludge dewatering by changing the inner interactions and microstructure of flocs. Based on the properties and microstructure of flocs, the sludge dewaterability and the floc’s inner interaction are schematically described in Figure 7. During thermal solidification treatment, sludge dewatering processes can be divided into two steps according to the temperature:

(1)

When the temperature increased to 45 °C, the particles aggregated and formed large flocs, resulting in better dewaterability. This is mainly because the hydrogen bonding between hydrophilic groups in the sludge (such as carboxyl and hydroxyl) and water molecules was destroyed due to temperature increases, which reduced the water-absorbing capacity of the sludge flocs. As the temperature continued increasing (below 70 °C), the thermal motion of water molecules increased, and part of the bound water within the sludge was transformed into free water and could be easily removed. Accompanied by the reunion of particles, the volume of sludge decreased, and a large, loosely clumped sludge was obtained.

(2)

As the temperature was elevated above the softening point of acrylic resins (70 °C), the polymer chains were curled up and folded rather than being stretched out. The softening and shrinking of the polymer chains can connect or wrap the suspended solids in the sludge by means of van der Waals forces, hydrogen bonding, or intermolecular cohesion, leaving more chances for the floccules to contact and aggregate. During this process, a large number of hydrophilic carboxyl groups of polymer chains might be enclosed within the sludge or form internal hydrogen bonding, which reduces the interaction with water molecules and increases the hydrophobicity of the sludge particles. Therefore, the bound water content of sludge with carboxyl groups was released. At the same time, the sludge particles were packed closely together, thus reducing the water-holding capacity and volume of sludge; a large amount of surface water was also released. Less water was absorbed on the sludge surfaces and held by the sludge particles, which released more water molecules. The released free water can diffuse from the inner portions of the flocs to the solution through the internal pore within the sludge due to the shrinkage of acrylic chains and the extrusion of sludge particles. Finally, the particles adhered to each other to form a clot of compacted cake with a large number of pores within the interior and high hardness, thereby improving the dewaterability of sludge.

4. Conclusions

A novel thermal solidification treatment under low temperatures by using contaminants within the sludge was proposed with the purpose of enhancing the dewaterability of water-based ink wastewater sludge. After thermal solidification treatment at 75 °C for about 30 min, the dewatering ratio of sludge could reach up to 95.1%, and the moisture content could be reduced to 36.6%. It is a mainly thermophysical process. It changed the floccule’s structure and destroyed the interactions between the water and the sludge, thus resulting in a large amount of bound water being transformed into free water and released from the sludge. The softening and shrinking of acrylic resin within the sludge during heating contributed to the compacted cake and thus enhanced the dewaterability. The bound water content can be reduced to 0.183 g g⁻¹ dry solid, and the remaining water was mainly free water with a content of more than 68%.

It should be pointed out that this method is only suitable for the dewatering of water-based ink sludge. This may be due to the fact that only water-based inks contain this acrylic resin, which can undergo softening and shrinking during heating. Therefore, in the following work, we will try to synthesize acrylic resin with these properties and use this method for the dewatering of other industrial sludge.