Sequential ‘acid leaching–anion exchange–aerobic composting’ process for recycle of municipal sludge

Nowadays, municipal sludge treatment and disposal has become a prominent problem (Xing et al. 2011; Wang et al. 2017). In order to avoid secondary pollution, the sludge must be harmlessly treated and recycled (Wang et al. 2013a; Ko 2014; Wang 2017). Since sludge has high organic matter content and good fertilizer content, it can be used as a soil amendment and agricultural fertilizer. However, municipal sludge often contains high concentration of heavy metals, which limits the agricultural purposes of sludge (Kominko et al. 2019). To meet the agricultural standard, heavy metal removal is the key for sludge fertilization (Zhang et al. 2017).

At present, the heavy metals in sludge are usually passivated rather than removed directly. However, through the aerobic composting process, the chemical form of heavy metals in sludge may change and pollute the environment. Another main method is to use acid to oxidize heavy metals and change their state to dissolve them from sludge (Tyagi et al. 1991). For the acid removal process, the removal rate of some heavy metals by inorganic acids can reach more than 95% (Marchioretto et al. 2002). Nevertheless, the inorganic acids method is not applicable because of the large demand for acid. Meanwhile, the instrument is also easily corroded by strong acid (Igarashi et al. 2020). In contrast, the removal of heavy metals from sludge by organic acid is more promising, with a moderate pH value of 3–4 (Gunarathne et al. 2019). In addition, organic acids are degradable under aerobic or anaerobic conditions and can be used for land use after composting, so this method has been widely used. The release of heavy metals was investigated using three organic acids (acetic, malic, and citric). The high rate of metals release by organic acids was explained through ligand-promoted mechanisms that enhance the release of metal ions from the sludge (Gunarathne et al. 2019). The acid oxidation of heavy metals only changes the state of heavy metals and dissolves them from sludge, but the subsequent acid wastewater treatment process has been seldom investigated. Meanwhile, it is also very important to examine the feasibility of the subsequent recovery of heavy metals after leaching and the reuse of sludge.

In this study, we carried out a study on a novel sequential process: ‘acid leaching–anion exchange–aerobic composting’ for Cr recovery and recycle of municipal sludge. This study mainly included three steps (Figure 1): (1) removal of Cr from sludge by citric acid leaching; then the obtained leachate was used to recover Cr, and acid treated sludge was used for aerobic composting; (2) Cr removal and recovery from leachate: removal of Cr from leachate by ion exchange resin, regeneration of ion exchange resin by two-step method; recovery of Cr by Al replacement; (3) aerobic compost and maturity assessment of acid treated sludge. The systematic research could provide a reference for municipal sludge recycling.

Concentrated sludge from a belt filter press of a municipal sewage treatment plant (Shenzhen, China) was used in this study. Total nitrogen (TN), total phosphorus (TP) and heavy metals (total As, Hg, Cr, Pb and Cd) in sludge were determined by the method described in the study by (Kominko et al. (2019). The analysis of heavy metal existing forms was carried out by continuous extraction method (Naoum et al. 2001).

The test columns with inner diameter of 4 cm and length of 20 cm were used to load sludge. The column bottom was firstly fixed with nylon cloth, quartz sand (1.5 cm thick) was added, and then another layer of nylon cloth was laid on top. Afterwards, 90 g dry sludge (10 cm high) was loaded, and then a layer of nylon cloth with quartz sand (1.5 cm thick) was laid above it. After loading with sludge, the test column was fixed and soaked in deionized water until the sludge was wetted. The leaching dose was maintained at 500 mL/kg or 1,000 mL/kg, and the leaching speed was 0.5 mL/min. The content of total heavy metals in the leachate was determined. All the experiments were repeated three times.

Cr was separated from leachate by ion exchange resin; the process was divided into a static test and dynamic test.

• (1)

  Static test: 10 mL of leachate after citric acid leaching was mixed with 10 mL anion or cation exchange resin, and kept for 2 hours. The content of total Cr was determined after filtration by qualitative filter paper.

• (2)

  Dynamic test: 1 L of the leaching effluent was passed through 50 mL anion or cation exchange resin column at the flow rate of 5 mL/min. The content of total Cr in effluent was determined.

After enriching with heavy metals, a two-step method was used to regenerate ion exchange resin. In the first step, 8% sodium hydroxide solution was used as regenerant. In the second step, a mixed solution of ethanol:sulfuric acid:water = 1:6.9:25 (weight ratio) was used as regenerant. Static and dynamic operation modes were also used in the regeneration process. The static method was to immerse ion exchange resins in the regeneration liquid. The dynamic method was that the regeneration liquid up-flowed slowly with a flow rate of 0.5 mL/min from the bottom to top of the resin column.

G is the mass of ethanol (g), M is the content of Cr⁶⁺ in wastewater (g/L), V is the volume of exchange wastewater (L), M′ is the content of Cr⁶⁺ in regenerated solution (g/L), V′ is the volume of regenerated liquid (L) after regeneration with sodium hydroxide solution, and 0.66 is the conversion coefficient.

In order to speed up the reaction, the oxide film on the surface of an Al sheet was removed by sandpaper, and then the Al sheet was cut into pieces (2 × 2 mm²) and put into the heavy metal eluent for replacement reaction. The displaced solution was sampled and analyzed every 8 hours, and the replacement reaction lasted for 3 days.

The acid treated sludge was used as a composting matrix, sawdust and mushroom residue were used as composting fillers, and microbial fortifier CK-21 (DANO, Japan) was added into the composting fillers. The wet weight ratio of sludge:sawdust:mushroom residue was 35:5:7, C/N was 23.9, and the initial moisture content was 62.4%. The room temperature was 23–25 °C. A wooden board was laid on the bottom of the stack. Above the board, a layer of dry leaves about 15 cm thick was used as the stack bottom. All materials were evenly mixed and stacked. Each stack was about 1.0 m long, 0.5 m wide and 0.5 m high. The stack was covered with plastic cloth to keep warm and moist. The composting time was 15 days.

The evaluation of compost maturity can be divided into four categories: physical index, chemical index, biological index and spectral analysis method. In this study, five indexes – temperature, moisture content, pH, seed germination index (GI), and hygienic index – were selected to evaluate compost maturity and stability.

This method has been used by the Italian government as a criterion for evaluating the maturity of organic waste and manure compost. The specific steps were as follows:

• a.

  After adjusting the moisture content of the compost sample to about 60%, the extract was obtained by pressure filtration.

• b.

  Seven seeds of water celery were placed in the extract solution and cultured in darkness at 27 °C for 24 hours. Fifteen repeats were set up.

• c.

  The germination and root growth of seeds were measured and expressed in percentage form. The GI is the percentage of the growth of the germination part and the root part.

Determining the quantity and composition of sewage sludge is a major component of designing sludge treatment units and their handling and disposal facilities (Najafzadeh & Zeinolabedini 2018; Zeinolabedini & Najafzadeh 2019). Municipal sludge contains a variety of plant nutrients, which is a necessary condition for agricultural utilization of sludge. Generally speaking, the higher the nutrient content of sludge, the more suitable it is for agricultural utilization (Kirchmann et al. 2017). In this study, TN and TP of the sludge were 22 g/kg and 12.9 g/kg, respectively. Compared with the statistical data of farm manure, the TN and TP of the sludge were higher than that of most farm manure (Khem et al. 2018), which indicated that the municipal sludge can be used for agricultural purposes.

In the raw sludge, the content of total Cr, As, Cd, Hg and Pb was 1,213, 7.0, 0.47, 94.4, 1.99 mg/kg, respectively. It can be seen that only the content of total Cr exceeded the agricultural sludge standard. For the heavy metal Cr in agricultural sludge, the limit in EU countries is no more than 1,000 mg/kg (86/278/EEC) (European Commission 1986). Germany has raised the standard on this basis, with the limit of 900 mg/kg (AbfKlaerV1992) (Germany 1992). The standards are somewhat stricter in China, requiring that the content of Cr in agricultural sludge cannot exceed 600 mg/kg (GB18918-2002 2002). Among the standards of heavy metals for agricultural sludge, there was no limit to As. The limit of Hg, Pb and Cd was 5, 300 and 5 mg/kg, respectively (86/278/EEC; AbfKlaerV1992; GB18918-2002), while the measured values of Hg, Pb and Cd were 0.47, 94.4 and 1.99 mg/kg, respectively. The values of Hg, Pb and Cd were far below the standards, so it was needless to focus on them. Therefore, Cr was the only heavy metal in municipal sludge that exceeded the standard. And it was necessary to reduce concentration of Cr to below 600 mg/kg in sludge.

Citric acid can oxidize insoluble Cr³⁺ in sludge to more soluble Cr⁶⁺ and dissolve it from sludge (Neale et al. 1997). The sludge samples were first leached at a flow rate of 0.5 mL/min with the citric acid concentrations of 0.05, 0.1, 0.2, 0.4 and 0.8 mol/L. The removal rate of Cr is shown in Figure 2(a).

As shown in Figure 2(a), the removal rate of Cr increased with the increase of leaching dose and concentration. When the concentration of citric acid was 0.4 mol/L, the removal rate reached the maximum of 61.4%. The total residual Cr in sludge was 493.3 mg/kg.

Then, in order to obtain more accurate citric acid concentration, 0.24, 0.28, 0.32, 0.36 and 0.4 mol/L citric acid was used for the above treatment processes. The results are shown in Figure 2(b). It can be seen that the removal rate of Cr with 0.36 mol/L citric acid could reach 57.43%, and the total residual Cr in sludge was 544.04 mg/kg, which also met the standards of GB18918-2002, Germany and other EU countries.

At a certain range of citric acid dosing (0.24–0.4 mol/L), increasing the concentration of acid for leaching can improve the removal rate of Cr. In order to save cost, 1,000 mL of 0.36 mol/L citric acid per kg of sludge was used for Cr leaching. The obtained leachate and acid treated sludge were used for the following study. And, for further study, models will be used, such as longitudinal dispersion coefficients, to obtain a more accurate Cr removal rate and to design more efficient reactors for leaching (Saberi-Movahed et al. 2020).

Ion exchange tests were carried out on leachate obtained in section ‘Removal of Cr from sludge by citric acid leaching’. Both static and dynamic modes were operated for ion exchange. The results are shown in Figure 3. The analysis showed that anion exchange resin had a better removal rate on Cr in leachate with both static and dynamic exchange. Of these, the dynamic anion exchange resin achieved the optimum removal rate, which was over 95%.

The results of regeneration are shown in Figure 4. When the regenerated liquid volume was 50, 100 and 150 mL respectively, the dynamic elution rate was higher than the static elution rate. It can also be seen that when the regenerated liquid volume was 100 mL, the elution rate was obviously higher than that when the regenerated liquid volume was 50 mL, but if the regenerated liquid volume continued to increase, the elution rate did not increase significantly. Based on the above findings, the up-flow dynamic regeneration was selected and the volume of regenerated liquid was set as 100 mL. Therefore, when the regeneration solution:resin volume was 2:1, the up-flow dynamic elution rate can reach 86%.

Cr in the eluent can be replaced by zero-valent aluminum (Al) in acidic conditions at room temperature. The crushed Al sheets were directly added to the eluent of the elution process. Finally, the Cr containing mixture and chemical coagulants such as polyaluminum chloride can be obtained.

The Cr replacement rate under different replacement time is shown in Figure 5. With the replacement time increasing, the replacement rate increased gradually. When the replacement time was up to 72 hours, the replacement rate was 63.3%. On the one hand, it was related to the activity of metals. On the other hand, because of the low concentration of heavy metals in the regeneration solution, it was easy to be disturbed by other metal ions (such as high valence Al ions). If the dynamic replacement method and soft computing models are used to design the flow rate, higher replacement rate will be obtained (Najafzadeh & Zeinolabedini 2019).

The change of temperature during the composting process is an important parameter for evaluating composting maturity. It is generally believed that if the temperature of the compost body is maintained for over 3 days at 55 °C, the pathogenic bacteria in the compost can be killed and the requirements of the compost hygiene index and compost maturity can be fulfilled (de Bertoldi 1998; de Bertoldi & Civilini 2006).

Figure 6(a) shows the composting temperature curve. After 1 day of composting, the temperature of the compost body rose rapidly to above 55 °C and was maintained for 4 days, accompanied by microbial metabolism and mass reproduction. After a high temperature period, the easily degradable organic matter has been basically exhausted, the microbial activity cannot produce enough heat, and the temperature of the compost body gradually decreased, approaching the environment temperature after 12 days. The change of temperature in the composting process generally includes three stages: heating (25–50 °C), high temperature persistence (above 50 °C) and cooling (35–50 °C).

Figure 6(b) shows the change curve of compost moisture content. Increasing composting temperature promoted water evaporation. The moisture content of compost decreased from 62.4% to 36.3%, with a decrease rate of 41.8%. The reduction was obvious and the sludge reduction effect was significant.

Figure 6(c) shows the change curve of compost pH. In the early stage of composting, production of organic acids by microbial decomposition of organic matter was greater than production of ammonia, and the pH decreased slowly to 6.56 on the fourth day. Then, organic matter decomposed by the strong action of microorganisms produced a large amount of NH₃. The pH increased gradually, and reached 7.24 at the 11th day, then decreased gradually. At the end of composting, pH was 7.03. Many researchers have proposed that the value of pH can be used as an index to evaluate the maturity of compost: in the initial stage of composting or fermentation, the value of pH is weak acid to neutral, generally 6.5–7.5; in the mature compost, it is generally weak alkaline, and the value of pH is about 8–9 (Jeong & Kim 2001; Liu et al. 2007). However, the pH value is also affected by the raw materials of compost. Thus, it can only be used as a reference index for composting maturity.

Figure 6(d) shows the variation of seed GI during composting. Generally speaking, when the seed GI of sludge is more than 50%, it indicates that the compost product has reached maturity (Xu et al. 2012; Wang et al. 2013b; Meng et al. 2018). In this experiment, the seed GI of sludge compost firstly decreased and then increased, and reached the lowest point on the fifth day. It was concluded that sludge composting at high temperature would produce a certain amount of NH₄⁺-N, organic acids, aldehydes and other substances, which might inhibit seed germination (Wang et al. 2018; Wang et al. 2019). With the decrease of composting temperature, the concentration of NH₄⁺-N and organic matter decreased, and GI increased gradually. GI increased to 68.3% on the 12th day, and remained above 50%. This indicated that sludge compost reached maturity after 12 days.

Salmonella and Streptococcus entericus are commonly used as indicators of safety for compost products. Health standards vary slightly in different countries and regions. Hsu and Lo suggested that 1 g compost contains less than 1 cell of Salmonella and 0.1–0.25 viral plaques, indicating that compost is stable and its application will not cause harm to human health and the surrounding environment (Hsu & Lo 2001). The Agricultural Standard of Grade A Sludge stipulated by the United States Environmental Protection Agency (US EPA) requires fecal coliform concentration <1,000 MPN/g, Salmonella concentration <3 MPN/4 g and Ascaris eggs <1 cell/4 g.

The hygienic indexes of compost products were tested. The results showed that the sterilizing rate of pathogenic bacteria in the composting process was 100%. All the hygienic indexes of the composting products could meet the requirements of GB18918-2002 in China and grade A sludge agricultural standard stipulated by the US EPA, and could be used as manure.

After composting was matured, the speciation of heavy metal Cr in the sample was analyzed and compared (Table 1).

Studies showed that the order of stability of Cr in these states was as follows: residual state > organic matter binding state > carbonate binding state > iron-manganese oxide binding state > ion exchangeable state (Presley et al. 1980; Singh et al. 1999; Amir et al. 2005). As can be seen from Table 1, the exchangeable, carbonate binding and organic binding states in the sludge increased, while the proportion of residual states decreased, indicating that most of the Cr removed by citric acid existed in a stable form. After the treatment of citric acid leaching, the Cr in the sludge became unstable. However, after the aerobic composting process, Cr mainly existed in the residual state (50.8%) and organic-binding state (32.0%). The results showed that the stability of Cr was enhanced by aerobic composting. For the other heavy metals, citric acid increased the removal rate of Cd and Pb in sludge (Ma et al. 2020). In the sewage sludge compost under acid precipitation, the mobility of Cr, Cu and Pb decreased by 51–56% due to their retention by particulate organic matter, while the leaching of As, Cd and Ni was increased, as occurred in complexes (Qi et al. 2020). The increase of heavy metal stability demonstrated the feasibility of acid treated sludge compost for agricultural purpose.

A novel sequential citric acid leaching–anion exchange–aerobic composting process for Cr recovery and recycle of municipal sludge was conducted in this study. After citric acid leaching, the residual total Cr content in sludge was 544.04 mg/kg, which met the requirements of agricultural sludge. Meanwhile, the acid for leaching sludge can be recycled after anion exchange. And the anion exchange resin can be also reused after regeneration. The replacement rate of Cr in eluent can reach 63.3%. The results of the aerobic composting process for acid treated sludge showed that the compost products can be safe for agriculture use.

This work was supported by the National Natural Science Foundation of China (No. 51878215), Natural Science Foundation of Guangdong Province, China (2018A030313185) and Shenzhen Science and Technology Innovation Project (KJYY20171011144235970).

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