Exploring the role of potassium ferrate and straw fiber in enhancing the strength of cement-based solidified municipal sludge

According to data released by the Ministry of Housing, Urban and Rural Construction, China's wet sludge production (80% moisture content) reached 66.65 million tons/year in 2020 (Ministry of Housing and Urban-Rural Development of the People's Republic of China 2020). At present, 80% of the sludge in China is not stabilized or disposed of safely and properly and represents an ongoing environmental hazard. How to safely and economically dispose of sludge has become a priority in the field of wastewater treatment. Common methods of sludge disposal include land use, sanitary landfill, incineration, and utilization by the construction industry. Sanitary landfills have the particular advantages of simplicity, large capacity, and high efficiency and are considered a practical way to dispose of sludge in China. However, the average dewatered sludge still has a moisture content of between 75 and 85%, with corresponding poor geotechnical properties. The strength and moisture content of untreated sludge do not meet landfilling standards. Sludge solidification can overcome these problems and, as such, has become a commonly used pretreatment method in sludge landfills.

Sludge solidification involves the formation of hydration products with a certain strength and viscosity by adding cementing substances to the sludge, thereby encapsulating the loose sludge, reducing the sludge moisture content, improving the mechanical properties of the sludge, and reducing the amount of heavy metal leaching (Li et al. 2018a). At present, the commonly used cementitious materials are mainly inorganic materials, such as cement, lime, fly ash (Chen et al. 2019; Chen et al. 2020a; Abdoul Fatah et al. 2021). Based on economics, technical feasibility, and pollutant stabilization, cement-based solidification remains the most widely used approach (Lang et al. 2020b). However, cement hydration reactions and cementation processes are easily interfered with by the presence of organic matter (Alqedra et al. 2011; Kang et al. 2017), resulting in poor overall curing. Municipal sludge contains a lot of organic matter, such as proteins, fats, and polysaccharides, which greatly weakens the cement solidification effect (Chen et al. 2020b). A great deal of research has been done to try to solve this problem. Katsioti et al. (2008) used bentonite as an additive to improve the performance of cement-based solidification, exploiting its high adsorption for organic matter, but the strength did not increase effectively. Zhen et al. (2011, 2012) used various additives to counteract the influence of organic matter, such as calcium sulfate and a new aluminate 12CaO·7Al₂O₃, and found that the strength of pure cement solidified sludge was significantly improved compared with that without any additives. Subsequently, researchers began to consider oxidation cracking of the sludge organic structure to improve the curing effect. Sun et al. (2016) used potassium permanganate as an additive and found an increase of about 18% in strength when the proportion of potassium permanganate went from 0% to 2%. Li et al. (2019a) added a small amount of potassium persulfate to the cement base and found that when the potassium persulfate content was 1%, the compressive strength of the solidified sludge with potassium persulfate added in simulated actual and ideal environments increased by 1.6 and 3.0 times, respectively, over that without potassium persulfate. These studies show that oxidation cracking of the sludge organic structure is a valid approach to improving the strength of cement-based solidified sludge. Potassium ferrate (PF), as a safe, non-toxic, and highly oxidizing oxidant, has been widely studied for use in sludge dewatering, sludge reduction, and dredged sludge solidification (Li et al. 2018b, 2019b; Wang et al. 2019; Wu et al. 2020). However, the structure and composition of the organic matter in dredged sludge differs markedly from that of sewage sludge, and, thus, these studies offer little guidance for the research on sludge solidification. The application of PF in sludge solidification has yet to be studied.

However, the use of PF alone as an auxiliary additive to enhance the cement curing effect is limited, and effective, inexpensive, and readily-available skeletal materials need to be considered. Commonly used skeletal materials for sludge solidification include bentonite, fly ash, and GGBS (Katsioti et al. 2008; Lang et al. 2020a; Abdoul Fatah et al. 2021), all of which exploit the active components of inorganic clay and its dilution effect on sludge organic matter (Liang et al. 2016). However, to date, the improvements in strength are not very satisfactory, and the process of strength development is very slow, which is not conducive to the timely disposal of sludge solidified in landfills. As an alternative skeletal material, biomass waste straw fiber, which itself has certain tensile and flexural properties, can play a reinforcing role in sludge solidification, greatly enhancing the strength and toughness of the solidified body. Considering the green, renewable, low-cost, and porous characteristics of straw, applying straw fibers to sludge solidification has many attractions. At present, straw fiber is mostly used in the research of blocks and concrete (Hu et al. 2009; Liu et al. 2012). Except for Zhu et al. (2016), there has been little research to date on the application of straw fiber in sludge solidification, and the role of straw fiber in sludge solidification is not clear.

To reduce the influence of organic matter on cement hydration in the sludge solidification process and minimize the amount of cement used to achieve the desired solidified effect, in this paper, PF and straw fibers were used in combination to enhance the strength of cement-based solidified sludge. Single-factor tests, orthogonal tests, and linear weighted optimization were used to obtain the suitable ratio of the raw materials that meet practical requirements, and then the mechanism was explored by analysis of the composition and microstructure. The results provide theoretical support and data reference for the application of the process in sludge solidification.

The dewatered sludge used in this study was obtained from a domestic wastewater plant in Jiujiang City, Jiangxi Province, China. After the sludge was retrieved, it was stored in a closed container at 4 °C. However, due to the long test period, the moisture content would still change to some extent. Its physicochemical properties are given in Table 1. The chemical composition of the sludge was determined by X-ray fluorescence (XRF), and the results are shown in Table 2.

Using the unconfined compressive strength of solidified sludge cured for 7 days as the evaluation index, the effective dosage interval of cement, PF and straw fiber, and the influence rule on the compressive strength were investigated sequentially. Firstly, the effect of cement dosage on strength was investigated. The wet sludge mass was used as the base, and the dosage ratios were set as 0%, 5%, 10%, 15%, 20%, 30%, and 40% to obtain a suitable value of cement admixture. Then, the effects of PF and straw fiber on strength were investigated sequentially based on appropriate cement dosage, with the dosage ratios being set to be 0%, 3%, 5%, 10%, 15%, and 20%. A suitable cement admixture is defined as one that provides a significant strength enhancement of the solidified sludge but does not necessarily reach the required strength of 50 kPa for disposal in a landfill. Its main purpose was to ensure a sufficiently solidified sludge to avoid subsequent tests failing due to a poor solidification effect preventing the strength from being measured, while reducing the amount of cement as much as possible.

Based on the results of the single-factor test, the effective dosage intervals of cement, PF and straw fiber were determined. These intervals were divided equally into four levels for a three-factor, and four-level orthogonal optimization test, as shown in Table 5. Once again, the strength of solidified sludge cured for 7 days was used as the evaluation index.

All samples were prepared according to the above design dosages, and three parallel specimens were set up in each group with the wet sludge mass as the base. First, the dewatered sludge was mixed evenly with a mixer for 3 min, and then the PF for the oxidation pretreatment was added, and this mixture was rapidly stirred (285 ± 10 rpm) for 30 min. When the pretreatment was complete, the cement or straw fiber was added, followed by 5 min of slow stirring (140 ± 5 rpm) and 5 min of rapid stirring (285 ± 10 rpm). After stirring, the solidified sludge slurry was rapidly transferred into molds 39.1 mm in diameter and 80 mm in height. After 24 h, the specimens were demolded and cured for a given time in the curing chamber (20 ± 2 °C, relative humidity ≥ 90%).

The unconfined compressive strength (UCS) was determined as per the China National Standard GB/T 50123–2019 using a YYW-2 strain-controlled unconfined compressive strength testing machine (Nanjing Ningxi Soil Instrument Co., Ltd). The moisture content of the test was set according to the sludge test method for municipal wastewater treatment plants (China National Standard CJ/T 221–2005).

XRD analysis was used for mineral testing. The test samples were the raw sludge, the cement, the single mixed cement sample, and the optimized mixed sample. After the samples had been tested for their strength, they were soaked in anhydrous ethanol to terminate the hydration reaction and then dried in an oven at 45 °C for 24 h. After drying, the samples were ground with a mortar and pestle and sieved through a 200-mesh sieve. These were then scanned with a Rigaku SmartLab SE X-ray diffractometer with a Cu Kα source from 5° to 85°, a scan speed of 10°/min, a tube voltage of 40 kV, and a current of 40 mA.

SEM analysis was used to study the microstructure. The test samples were the raw sludge, the single mixed cement sample, and the optimized mixed sample. Once the samples had been tested for their strength, they were soaked in anhydrous ethanol to terminate the hydration reaction, cut into 10 mm × 10 mm × 30 mm strips, and put into the freeze dryer at about −45 °C for 8 h, after which they were vacuum dried for 48 h. After drying, the end of each sample was broken off, and the broken fresh cross-section was sputter-coated by a thin layer of gold to remove the charging effect. Finally, the sample morphology was imaged using a TESCAN MIRA LMS scanning electron microscope.

OPC is a commonly used solidification cementitious material. Solidification proceeds mainly through the hydration reaction with the water in the sludge to produce some gel-type substances and some crystal substances, such as C-S-H (amorphous gel hydrated calcium silicate), CH (calcium hydroxide), and Aft (ettringite). The sludge is wrapped, glued, and filled with these hydration products to enhance the strength and stability of the sludge solidified body.

As the redox potential shows, the oxidation of PF is significantly higher than that of potassium permanganate (⁠⁠, acidic conditions; ⁠, neutral and weakly alkaline conditions; ⁠, alkaline conditions). It acts on the sludge in two main ways. First, it degrades and destroys the organic matter in the sludge and destroys microbial cells and extracellular polymer (EPS) structure through its strong oxidation, which leads to a large amount of bound water being released. Second, the sludge is agglomerated by the high flocculation ability of the reductive product, iron hydroxide colloid (Zhang et al. 2012; Wang et al. 2019).

The lack of activity when the dosage of PF was less than 5% was due to the low effective content of PF, which is only 10% of the fishpond disinfectant powder. When this threshold was exceeded, PF came into play and broke down the EPS structure in the sludge by oxidation (Wu et al. 2015), releasing a large amount of bound water and consuming some organic matter, allowing the cement hydration reaction to proceed adequately. The excess free water in it is also easily consumed by evaporation, with reduced moisture content and increased strength. When PF was reduced, the iron hydroxide formed also had an excellent cohesive effect, which could agglomerate the sludge and increase the density and strength of the solidified body (Wang et al. 2019). In addition, the iron hydroxide itself flocculated and precipitated, so when the moisture content decreased, it helped to increase the strength.

The SF surface is porous, strong water absorption, and the SF itself has a certain tensile capacity. During the sludge solidification process, it does not participate in the reaction but can play the role of the skeleton, anchoring the solidifying sludge and filling the pores.

Through the analysis of the characteristics of the SF, it was found that SF had high water absorption and high adsorption and could absorb some of the organic matter in the sludge, thereby reducing the interference of organic matter with the hydration reaction of the cement. Katsioti et al. (2008) and Zhao et al. (2021) also achieved the same effect by using bentonite and functionalized polypropylene fibers to improve the strength of cement through the adsorption of organic matter and heavy metals, respectively, to improve the hydration reaction. The hydrophilicity of the SF can also greatly reduce the moisture content of the solidified body and increase its strength. The SF also have very good tensile properties, forming an intricate skeleton support system, and anchoring the solidifying sludge and cement, effectively enhancing the strength of the solidified sludge (Wang et al. 2015).

The single-factor experiments revealed the effective dosage range of each curing material and its influence on the strength of the solidified body. To further explore the optimal ratio of the raw materials, the orthogonal test method was adopted in this paper. The UCS after curing for 7 days was used as the evaluation index, and the orthogonal test was designed with the 16-run, three-factor four-level orthogonal table L₁₆(4³), and the corresponding factor levels are shown in Table 5. Using this approach, the results in Table 6 were obtained.

Table 6 shows that the composite strength effect of OPC, PF, and SF was ideal, and the primary and secondary relationship of each influencing factor can be obtained through the orthogonal table and is reflected by the R-value range. The greater the R-value, the greater the intensity affected by this factor. It was evident from the table that the influence on the UCS followed the order: PF > SF > OPC. The k-value represents the average strength of each factor dosage, and, as can be seen, it fluctuated with the amount of OPC dosage and increased with the dosage of SF and PF. This means that, when all factors were compounded together, the effect of SF and PF on strength was positive, but the role of OPC is ambiguous, with too much harming the strength, the amount of water in the sludge available to help OPC to complete the hydration reaction decreases, i.e., the water-cement ratio becomes too small. As a result, hydration products cannot fully form, resulting in reduced strength (Wang et al. 2022).

SPSS software (IBM SPSS Statistics, USA) was used to carry out an analysis of variance (ANOVA) of the orthogonal test results, and the results are shown in Table 7. In general, both the R-value range and the variance reflect the magnitude of the effect of the factors, and the results are the same. But in total, the R-value range error is greater, because it cannot distinguish between data fluctuations caused by the change of factor condition and data fluctuations caused by experimental error, nor can it give an accurate quantitative estimate of the significance or other aspects of the influencing factor. Therefore, ANOVA is often performed simultaneously with the orthogonal test to ensure the accuracy of the results (Ke et al. 2019). Table 7 shows that the variance of the influencing factors followed the order: SF > PF > OPC. Unlike the range results, here SF had the greatest effect on strength, followed by PF and finally OPC. Sig (PF) and Sig (SF) were both < 0.05, making them both significant factors, while Sig (OPC) was > 0.05, making it not a significant factor (Jian et al. 2014).

In summary, SF had the greatest effect on the UCS of the solidified sludge cured for 7 days, followed by PF and finally OPC. Both PF and SF had significant effects on the UCS, while OPC had less of an effect. Therefore, for optimum performance, the greater the PF and SF dosage, the better the effect on the UCS, while the OPC dosage is only required to ensure a proper cementing effect. However, in practice, many other factors, such as the cost of the curing agent, the volume increase of the solidified body, and the moisture content required for landfills, also have to be taken into account. The cost of PF is high, with the cost of the 10% effective PF aquaculture disinfection powder used in this experiment being up to US $672 per ton. The cost of the OPC and SF were much lower at US $54 and US $84 per ton, respectively. Thus, once PF was added, the cost increased rapidly. In addition, compared to the density of OPC and PF (1.5 g/mL and 1.09 g/mL, respectively), the density of the SF was very small (0.2 g/mL), resulting in a very large volume increase as more and more SF was added, which not only increases treatment costs but would also take up a lot of landfill space. Therefore, the orthogonal test results need to be further optimized.

The 16 runs of the orthogonal test were used as 16 curing agent proportioning schemes and optimized for four additional indices (UCS, moisture content, volume increase ratio, and cost), as shown in Table 8.

Standardization refers to the process of transforming each evaluation index into a dimensionless, non-differentiated index, thus, facilitating subsequent comprehensive evaluation and ranking.

Since normalization may result in zero values, which may cause some of the data processing to be meaningless, the normalized data need to be scaled and panned as a whole – i.e., ⁠. But in order not to destroy the shape of the original data and retain as much of the original data as possible, the value of α must be as small as possible; that is, α is the smallest value closest to ⁠. This paper takes α = 0.0001. The results in Table 9 were obtained after standardization and translation.

Entropy weighting is an objective weighting method that determines the weight of an indicator based on the amount of information it contains. This approach can effectively use the index data, exclude the influence of subjective factors, and obtain the objective weight of each evaluation index.

As can be seen from Table 13, the cost and volume increase ratio (VIR) have the highest weights, followed by the UCS and moisture content. This shows that a single consideration of the solidification effect is not appropriate and should be evaluated in combination with the practical requirements.

As Table 14 shows, the top three solutions in order of CI are program 1, program 6, and program 5. Combined with the requirements of sanitary landfill mud quality (unconfined compressive strength > 50 kPa and moisture content < 60%), it can be seen that the moisture content of program 1 does not meet the requirements, so its data are invalid. Further, while the cost of program 5 is lower, the strength and capacity increase ratio are not ideal, and the CI is lower than that of program 6. Therefore, the best program in practical terms is program 6: 20% OPC, 10% PF, and 5% SF. This program has an unconfined compressive strength of 126.87 kPa, moisture content of 55.85%, a VIR of 1.24, and a curing material cost of $82/ton.

To prove the accuracy of the optimized scheme, a validation test was conducted to obtain a UCS of 121.96 kPa and a moisture content of 56.19%. Compared with the previous program 6, the strength decreased by 3.87%, and the water content increased by 0.61% with a small relative error, indicating that the optimized program produces accurate and useful results, and the optimized solution meets the requirements of landfill material quality.

PF and SF were used to improve the OPC solidification of treated sewage sludge. The main conclusions can be summarized as follows:

• 1.

  Single-factor tests were conducted to obtain a minimum active dosage of OPC, PF, and SF, which was 10%, 5%, and 5%, respectively. Below these values, there was little solidification of the sludge, while above these values, the UCS of the solidified body increases with the dosage.

• 2.

  An orthogonal test with three factors (OPC, PF, and SF) and four levels showed that PF and SF had a significant effect on the UCS and were positively correlated with it, with SF having the greater effect of the two, while OPC did not have a significant effect on the UCS. Therefore, the ideal optimal condition is that the higher the PF and SF dosage, the better, while the OPC dosage can be kept at the minimum level required to promote solidification.

• 3.

  Combined with the actual landfill requirements, four additional indicators were introduced, the UCS, water content, VIR, curing agent cost, and linear weighting used to further optimize the final suitable ratio of additives. The best performance was found to be a 20% OPC, 10% PF, and 5% SF mixture, which had a 126.87 kPa unconfined compressive strength and 55.85% moisture content.

• 4.

  XRD and SEM analysis showed that PF and SF were able to promote cement hydration, forming more hydration products, and the material obtained by mixing in the three additives in the optimized ratio was much denser than the original sludge and the solidified sludge produced by mixing it 20% OPC alone.

This work was supported by the National Natural Science Foundation of China (22066012), Key research and development program of science and technology department of Jiangxi province, China (20181BBG70043), Natural Science Foundation of Gansu Province, China (22JR5RA254), Open Research Fund of Jiangxi Key Laboratory of Industrial Ecological Simulation and Environmental Health in Yangtze River Basin, China (JJ2021002), 2021 Innovative Training Program for college students of Jiangxi province, China (118432021079) and 2021 Innovative Training Program for college students of Jiujiang University, Jiangxi province, China (X202111843200). Wenzhou Science and Technology Project (S20220010).

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.