Investigation of the Effect of Using Activated Sludge Treated with Acetic Acid on Physical and Strength Properties of Recycled Pulp

• This study aimed to scrutinize chemical treatment of Mazandaran Wood and Paper mill activated sludge with 3 % acetic acid for the manufacture of environmentally eco-friendly bio composite. In this study, the paper pulp required was obtained from the waste newsprint paper. The activated sludge was treated in a beaker for 3 min in 3 % acetic acid and then in a water bath at 75 and 100 °C for 45 and 90 min. After that, the treated activated sludge was mixed and re ﬁ ned with waste newsprint paper pulp at ratios of 0, 5, 10, 15 and 20 %. The test specimens were prepared according to TAPPI standard and the physical properties (water absorption and porosity) and mechanical (tensile strength, tear strength, ring crush test, burst strength and folding strength) were measured. The results showed that the activated sludge treatment caused the increase in tear strength, ring crush test, burst strength and porosity as well as the decrease in water absorption, tensile strength and folding strength. FT-IR spectrum of the treated samples showed that the treatment with acetic acid strengthened and activated the functional groups on the surface of activated sludge ﬁ bers. The activated sludge added at a level of 5 % at 100 °C for 90 min as well as at a level of 20 % at 75 °C for 45 min was more suitable than other treatments.

Ključne riječi: aktivni mulj; octena kiselina; novinski papir; ekološki prihvatljiv biokompozit; otpornost 1 INTRODUCTION 1. UVOD Today, population growth and rising living standards, paper consumption and production, as well as production of other industrial and agricultural products have risen and as a result, the amount and variety of solid waste produced has increased. Therefore, environmental pollution from agricultural and industrial waste is a global concern and safe management of hazardous waste will be one of the greatest challenges facing humanity. Therefore, fi nding a suitable social, technical, economic and environmental solution to have a greener and cleaner environment is a global concern. Several countries are trying to solve this problem by recycling waste (Lertsutthiwong et al., 2008;Pappu et al., 2007). Mazandaran Wood and Paper Industries Complex annually produce 75000 tons, 52000 tons, 38000 tons of fl uting paper, newsprint and printing and writing paper, respectively. In addition, solid waste, paper mill sludge, waste from the pulp and paper process or paper recycling due to the large amounts produced leads to a lot of pressure on the environment at the paper mills. Some paper mills burn sludge to decrease the produced sludge volume, although the space required for this operation is rare and expensive. The paper mill sludge also contains 45-55 % moisture and 20 % solids, which causes problems for transport. The landfi ll of paper mill waste, in addition to not being economical, causes environmental issues and groundwater pollution. On the other hand, a large area of land is required for the landfi ll of fi bers (Horace et al., 1999;Smook, 1995). On average, sludge production (organic and inorganic) in the wood and paper mill of Mazandaran is 1700 tons per month. In addition, sludge dry percentage is about 25 % (along with the moisture), so the dry sludge produced per month and day are about 425 and 4 tons, respectively. Semi-dry activated sludge contains organic and inorganic substances such as sodium, sulfur, extractives, fats, waxes, sterols, and fi ber fi nes, and calcium carbonate (due to fi ller) along with the tree bark. Moreover, active sludge contains hazardous chemical compounds that can decrease the amount of harmful chemical compounds by chemical treatment (Son et al., 2001) Ghasht Roodkhani et al. (2010) reported that the values of tensile, tear, burs, folding strength, and density of the mixed papers compared to the control sample decreased, while the values of bulk, thickness, brightness, and opacity increased. Nazeri et al. (2008) showed that the increase in CMP (Chemical Mechanical Pulp) fi ne amount leads to increase the air resistance and opacity and also decrease roughness, so variations in CMP fi nes have no effect on brightness. Tudarvari et al. (2016) examined the effect of fi ber length variations of different OCC (Old Corrugated Container) pulp-recycled papers on strength properties. The results showed that OCC pulp produced from different paper sources have different fi ber length distribution that has a signifi cant impact on the fl uting paper strength properties. The OCC pulp with higher amounts of long fi ber and lower amounts of fi nes has good strength properties (tensile strength, burst strength, and tear strength) for producing fl uting paper. Girones et al. (2010) stated that increasing the amount of sludge in composites will result in a composite with lower tensile strength. Therefore, in composites that are exposed to tensile force, the paper mill sludge acts as a fi ller, and when the material is subjected to bending force, the sludge acts somewhat like a reinforcement. Ismail and Bakar (2004) reported that increasing the amount of paper sludge in composites increases water absorption and decreases tensile strength. Ismail and Bakar (2005) reported that the ester treatment and acetylation of the sludge of the paper mill in the manufacture of composites improves the tensile strength and Young's modulus and also reduces water absorption.
The aim of this research is to use activated sludge as an additive with different ratios and also to use modifi ed activated sludge with acetic acid to improve the physical-mechanical properties of paper produced from recycled fi bers and activated sludge.

Sample preparation 2.1. Priprema uzoraka
In this study, 10 kg of waste newspaper with dimensions of 5 cm × 5 cm was prepared by hand. The newspaper pieces were kept in water for 24 hours and then converted into a pulp in a disintegrator (British Pulp Evaluation Apparatus manufactured by Mavis Engineering Ltd.London N.1. England) at 300 rpm for 30 min. In addition, the waste dry sludge was prepared from Mazandaran wood and paper mill located in the Sari's Pahne Kola and transferred to the laboratory, and then about 2 kg was soaked in water for 24 hours. After that, both materials were mixed by the disintegrator at around 300 rpm. Then, the newspaper paste was dewatered with 40-mesh and sludge with 200-mesh. The waste newspaper paste and activated sludge were calculated and mixed in the amount required for the experiment (handsheet) according to To improve the mixing conditions with newspaper, paste and activated sludge, sludge samples with the ratios listed in Table 1 were placed separately in beakers containing 3 % acetic acid for 30 min. Then, they were treated in a Bain Marie bath (45, 90 min and 75, 100 °C). Test specimens were prepared and pressed according to TAPPI T205 sp-12 (2015) standard with a base weight of 120 g/m 2 and then dried. Finally, the strength and water absorption properties of handsheets were measured.

Attenuated total refl ection (ATR) spectroscopy 2.4. Pojačana infracrvena spektroskopija (ATR)
ATR spectroscopy was used to determine the status of the active groups present in the handheld samples made from untreated and acetic acid-treated paper paste and sludge. For this purpose, the JASCO FTIR-4700 device made in Japan (Japan Spectroscopic CO., LTD. Hachioji city Tokio Japan) was used.

Statistical analysis 2.5. Statistička analiza
In this study, the effect of activated sludge on physical and strength properties of recycled pulp obtained by mixing activated sludge and newspaper wastepaper was statistically evaluated. Statistical analysis was performed using SPSS software (IBM Corporation, v.23, Armonk, NY, USA) in the form of one-way analysis of variance. Finally, the means were grouped using Duncan's test at a confi dence level of 5 %.

REZULTATI I RASPRAVA
The results of statistical analysis showed that there was a signifi cant difference in the physical and mechanical properties of the handsheets from activated sludge treated with acetic acid (at different temperatures and times) and untreated at the 5 % level (Table 2).

Usporedba FT-IR spektara uzoraka aktivnog mulja
FT-IR spectra of control, untreated and acetic acid-treated sludge are presented in Figure 1. The parts of the spectra with the wavenumber between 2700 and 3700 cm -1 and 2100 and 2500 cm -1 are shown separately.

Tear strength 3.2. Čvrstoća na kidanje
The average thickness of the handsheets was about 300 μm (0.3 mm). The results of analysis of variance showed that there was a signifi cant difference in the strength properties of the paper samples (p < 0.05). The results showed that by increasing activated sludge, the tear strength in the paper made decreased. Since the  fi ber length, the linear density of fi bers and fi nes in activated sludge are lower than in the waste newsprint paper components, the addition of untreated activated sludge to the samples decreased the tear strength.
Comparing the spectrum of untreated activated sludge and control, the attenuation of the peak related to hydroxyl groups is visible, which indicates less resistance of paper made with untreated activated sludge. Significant changes after modifi cation in different bands are attributed to different components, which include cellulose, hemicellulose, and lignin. The decrease in spectrum intensity at 3300 cm -1 wavelength due to treatment can be attributed to hydroxyl (OH) groups (Sabazoodkhiz et al., 2017). The hydroxyl tensile region is of particular importance for explaining hydro-  ) also reported that, compared to control treatment paper, handmade papers fi lled with extracted calcium carbonate reduced tear resistance by approximately 19 %. The activated sludge treated with 3 % acetic acid signifi cantly increased the tear strength. The acetic acid treatment can indicate the improvement of fi ber surface and better paper formation, and provides a more suitable substrate for the physical and mechanical properties of paper. The most suitable treatment results, with the highest tear strength, were achieved with the samples treated with acetic acid at 75 ° C for 45 min. The results showed that by adding the treated activated-sludge (acetic acid) to the untreated samples with a ratio of 5 to 20 %, the tear strength initially de-creased and then showed a signifi cant increase of up to 20 %. Moreover, a mixing ratio of 5 and 20 % of the treated activated-sludge (with acetic acid) was always more tear resistant than the control sample. And also, by adding 5 % (at 100 °C for 90 min) and 20 % (at 75 °C for 45 min) of the treated activated sludge, the most suitable tear strength was observed. Additionally, the absorption peak intensity of 1791 cm -1 in the treated samples was less than 1544 cm -1 , which indicates the presence of a reaction between acidic groups in acetic acid and hydroxyl groups in wood paste, which creates a high-intensity ester functional group. (Neeru et al., 2013) (Figure 1).

Tensile strength 3.3. Vlačna čvrstoća
The results showed that the increasing activated sludge increased the tensile strength of the resulting paper. Due to the low fi ber lengths and fi nes in activated sludge and also higher specifi c surface, the addition of untreated activated sludge to the specimens increased the specifi c surface and tensile strength of the test specimens. The tensile strength index of the samples can be seen in Figure 3. Tensile strength is one of the most important strengths of the paper that protects it against tensile stresses. Paper weight, fi ber roughness, and fi ber width are the factors affecting tensile strength. Tensile strength increases with increasing fi ber length, increasing refi ning intensity and press pressure, and performing surface sizing on paper (Scott, 2007). The results showed that the tensile strength index increased by adding untreated activated sludge to the recycled fi ber pulp. It appears that the reason of this increase can be attributed to the high specifi c surface area of the activated sludge (Nazeri et al., 2008). Primary and secondary materials improve the  (Ismail and Bakar, 2005). The removal of the corresponding peak in the adsorption region of 1248.34 cm -1 in activated sludge can be due to the fl exural vibrations of phenolic hydroxyl. Additionally, the removal of peaks in the adsorption zones of 1237.03 cm -1 is closely related to the C=O, C=C, and C=O bonds of the carbonyl units, and the adsorption region of 1412.12 cm -1 is related to aromatic C-H and C-H associated with type II alcohols; C=O is also treated in activated sludge and attenuates or removes carbonyl groups (Figure 1) (Mehmood et al., 2019).

Otpornost na tlačnu silu
The results showed that, by increasing activated sludge, the ring crush of the resulting samples decreased. Since the lengths and fi nes of fi bers in activated sludge are shorter than in waste newsprint paper components, the addition of untreated activated sludge to the specimens causes the short fi bers and some components of fi ne sludge to be placed in the Z direction of the treated specimens, which will decrease the ring crush. Activated-sludge treated with 3 % acetic acid decreased in some treatments and slightly increased in ring crush test. Among the treatments, the best treatment and the highest ring crush test in the acetic-acid treated samples were at 100 °C for 90 min (Figure 4). The results showed that the addition of 5 to 20 % activated-sludge treated with acetic acid to the untreated samples resulted in a signifi cant increase in the ring crush. In addition, the mixing of 15 % and 20 % of activated sludge treated with acetic acid consistently showed the highest increase in ring crush than the control sample. The data analysis illustrated that there is a signifi cant difference between the mean ring crush tests of treatments at the 1 % level.

Burst strength 3.5. Čvrstoća na pucanje
The results showed that, by increasing the activated sludge, the resistance to bursting of the made samples decreased. This can be due to the presence of more fi llers between the fi bers, such as calcium carbonate (polymorphism of calcite), and the small particle size of calcium carbonate fi llers in untreated activated sludge, which reduces the level of bonds. In contrast, the addition of activated sludge and weakening of the bond between the fi bers reduces the mechanical strength (Akbari et al., 2017). Additionally, it seems that the short length of fi bers in activated sludge and the use of recycled fi bers can be another reason to reduce the mechanical properties (Miri et al., 2016). Additionally, except in samples treated with 3 % acetic acid activated-sludge at 100 °C for 90 min, the resistance to bursting decreased slightly in other treatments ( Figure 5). The results showed that by adding 5 to 20 % of acetic-acid treated activated-sludge to the untreated samples, the resistance to bursting of the samples increased markedly up to 20 %. The burst strength of the sample from the mixing of 15 to 20 % showed a signifi cant increase as well. The best tensile strength in  Amani, Vaysi, Najafi , Eshagh Ebadi: Investigation of the Effect of Using Activated... erties (water absorption) of the made paper samples (p < 0.05). The results showed that, by increasing the activated sludge, the water absorption percentage of the samples made increased. Since the fi nes and specifi c surface of the activated sludge are higher than those of waste newsprint paper components, the addition of untreated activated-sludge to the samples increased the water absorption of the samples ( Figure 6). Therefore, hydrophilic structures in activated sludge as well as moisture absorption by recycled newspaper fi bers can be a reason to reduce the strength properties and in-samples treated with acetic acid was at 100 °C for 90 and 45 min. Hemicellulose and lignin are affected by the treatments, which reduces the amount of hydroxyl groups and has a signifi cant effect on lignin (Mehmood et al., 2019). Aliphatic is seen in the absorption range of 3312 cm -1 (Figure 1).

Upijanje vode (Cobb 60)
The results of analysis of variance showed that there was a signifi cant difference in the physical prop- crease water absorption. The chemical treatment of activated sludge with 3 % acetic acid showed a significant increase in water absorption of the samples. Moreover, among the treatments, the highest water absorption was observed in samples treated with acetic acid at 100 °C for 45 min. The results showed that, by adding 5 to 20 % of the activated sludge treated with acetic-acid to the untreated samples, the water absorption of the samples was fi rstly decreased and then signifi cantly increased up to 20 %. The water absorption in the treated samples with mixing ratios was always lower than the control samples. The lowest and the best water absorption was observed in samples treated with acetic acid at 100 °C for 90 min. Additionally, the region of the 2300 cm -1 peak belonged to the carbonyl groups, which indicates hemicellulose leaching (Pandey et al., 2012) ( Figure 1). Ismail and Bakar (2004) reported that increasing the amount of paper sludge in composites increased the Young's modulus and water absorption.

ZAKLJUČAK
The main purpose of this study was to improve the physical and mechanical properties of paper made from activated sludge and recycled fi bers by acetic acid treatment.
The addition of the untreated activated-sludge of the paper mill to the waste newsprint paper decreased some mechanical resistances up to 20 % and increased tensile strength up to 27 %; it also increased the physical properties such as the water absorption up to 55 % and porosity of the specimens up to 4 %.
The addition of the activated-sludge treated with 3 % acetic acid to the waste newsprint paper increased the maximum mechanical strength up to 35 % and de-creased the water absorption by up to 50 % and the tensile strength up to 20 %. In addition, a signifi cant increase in tear strength and ring crush test, as well as a decrease in water absorption, are important features for the specimens made.
The best mixing ratio for the specimens made included activated-sludge treated with 3 % acetic acid in a mixing ratio of 5 and 20 % with the waste newspaper at 100 and 75 °C for 90 and 45 min, respectively, and especially 75 °C for 45 min.