#Co-first authors: Rongwei Xiong and Xiufang Gao are co-first authors of the article
The safe and efficient disposal and utilization of sludge are major issues to be solved in solid waste treatment and environmental protection due to the complex characteristics of sludge and the low rate of innocuous treatments. Composting is a process of decomposing organic matter and transformed low-molecular organic acids into high-molecular humus substances under the action of microorganisms. Although land-use after composting has become an important direction for sludge treatment, heavy metal pollution is still the bottleneck problem restricting land use of sludge compost. Adding zeolite, hydroxyapatite, and other conditioning agents to the composting process affects the concentration or form of some heavy metals and effectively reduces the environmental risk. Lime and phosphorus modifiers change heavy metal speciation in samples, playing a role in decreasing biological availability and mobility. In this study, the effects of sludge composting treatment and conditioning agents on the concentrations and forms of heavy metals are reviewed. This review will provide a theoretical basis to treat heavy metals in sludge composting and lay the foundation for the land utilization and waste recycling of sludge.
A number of environmental problems are occurring with the increase in the urban population [
Sludge treatment includes sanitary landfill, incineration, constructed wetland, composting and other methods. Sanitary landfill method has the advantages of simplicity, easy operation, low cost and strong adaptability. Although the most common method is sanitary landfilling in China, the future of sanitary landfilling is doubtful. The stability problem resulting from the poor physical nature of sludge and limited available landfill sites in large cities such as Shanghai and Shenzhen have become great barriers for landfill disposal [
Sludge must pass through a series of treatments to ensure the safety of the products after composting [
Proper composting effectively reduces heavy metal concentrations in sludge [
Heavy metals are metals with significant biological toxicity, such as mercury, cadmium (Cd), Cr, lead (Pb), and arsenic (As). Heavy metals also refer to the toxic heavy metals, such as copper (Cu), zinc (Zn), tin, and Ni. Applications of heavy metal compost may pollute the soil [
Treatment | Sludge source | Heavy metal | Changes of heavy metals | Ref. |
---|---|---|---|---|
The aerobically digested and dewatered sludges | The Min–Shen MSTP in Northern Taiwan | Cu | Cu mainly exist in the Organic matter/sulfide-bound fraction in the composting process | [ |
Zn | Zn from stable components [Organic matter/sulfide-bound fraction and Residual fraction] significantly shifted to flow components [Exchangeable fraction and Carbonate bound fraction, Fe/Mn oxide-bound fraction] | |||
Pb | Pb increased from 26% to 36% in the mobile fractions after compost | |||
Dehydrated digested sewage sludge | Beixiaohe wastewater treatment plant, Beijing | Cr | Residue fractions of Cr is mainly converted from carbonate binding, Fe-Mn oxide binding and organic binding. The binding rate of Cr and oxidized parts decreased from 32.8% to 25.9% | [ |
Ni | Organic matter-bound Ni was a major contributor to the residual fraction. The binding rate of Ni and oxidized parts decreased from 60.7% to 42.9% | |||
Sewage sludge and compost samples containing municipal sludge | Podlasie province (Poland) | Cu | Cu was bound to organic matter, the Cu content in dehydrated sludge and mature compost is up to 76.50% and 75.10%, respectively | [ |
Zn | The binding rate of Zn with reducible fraction increased from 42.80% to 59.10% in compost | |||
Cr | The binding rate of Cr and oxidized parts decreased from 41.6% to 32.0% | |||
Ni | The binding rate of Ni and oxidized parts decreased from 38.0% to 32.4% | |||
Sewage sludge | A sewage treatment plant in Pomerania, northern Poland | Cu | The compost was stable after maturation and the mobility of heavy metals was reduced | [ |
Zn | ||||
Cr | ||||
Cd | ||||
Sewage sludge | Beixiaohe |
Zn | The contents of residue fraction for Zn was decreased | [ |
Cd | ||||
Pb | ||||
Cr | Increase | |||
Ni |
Method name | Treatment | Sludge source | Heavy metal | Changes of heavy metals | Ref. |
---|---|---|---|---|---|
Phosphate amendments | Dewatered fresh sewage sludge (DFSS) | A municipal wastewater treatment plant in Shanghai | Cu | Mobility was reduced by 18.8% | [ |
Zn | Mobility was reduced by 1.7% | ||||
Cd | Mobility was reduced by 24.2% | ||||
Pb | With maximum passivation capacity, mobility increased by 1.8% | ||||
Hydroxy- |
Sewage sludge | Qinhuangdao City, China | Cd | Adding 1.5% hydroxyapatite, the increment of exchangeable Cd decreased by 38.3% and the increment of residual Cd increased by 37.7% | [ |
Phanerochaete chrysosporium | Sewage |
River sediment | Cu | The decreased percentage of Cu is 25.29% | [ |
Zn | The decreased percentage of Zn is 14.67% | ||||
Cd | The decreased percentage of Cd is 31.08% | ||||
Pb | The decreased percentage of Pb is 13.84% | ||||
Clinoptilolite | Dewatered anaerobically stabilized primary sewage sludge (DASPSS) | Psittalia’s wastewater treatment plant (Rock Island in Saronic |
Cu | After maturation, Cu is associated with residual and organic components (54% and 32%, respectively) | [ |
Cr | 47.74% and 32.75% of Cr is associated with the organic fraction and the residual fraction, respectively. All heavy metals are reduced | ||||
Pb | 75% of the Pb was associated with the residual fraction and 15.58% with the reducible fraction | ||||
Ni | Before composting, Ni was associated with reducible content (36%) and residual content (23%). At high temperatures, 52% of Ni is associated with residual fraction | ||||
Lime | Dewatered anaerobically digested sewage sludge | Tai Po sewage treatment plant in Hong Kong | Cu | At the beginning of the compost, Cu mainly exists in its residual form, and after the compost, Cu mainly exists in the oxidizable form. The content of all metals decreases with the increase of lime correction rate | [ |
Zn | The residual Zn in the composting process is mainly converted into oxidizable Zn | ||||
Pb | After compost, Pb is mainly in residual form | ||||
Ni | At first, Ni is mainly in reducible form, but after composting, it is mainly in residual form | ||||
Synthetic zeolites | Sewage |
NA | Zn | Adding 0.5% and 1.0% zeolite can significantly reduce unstable Zn within 90 days | [ |
Natural zeolite clinoptilolite | Dewatered and anaerobically stabilized primary sewage sludge |
Psittalia wastewater |
Cu | 25% w/w of zeolite takes up to 27% of Cu | [ |
Zn | 25% w/w of zeolite takes up to 40% of Zn | ||||
Cr | 25% w/w of zeolite takes up to 14% of Cr | ||||
Pb | 25% w/w of zeolite takes up to 55% of Pb | ||||
Ni | 25% w/w of zeolite takes up to 60% of Ni | ||||
Clinoptilolite | Dewatered anaerobically stabilized primary sewage |
The Psittalia |
Cu | 25%–30% clinoptilolite take up to 28%–45% of Cu | [ |
Zn | 25%–30% clinoptilolite take up to 40%–46% of Zn | ||||
Cr | 25%–30% clinoptilolite take up to 10%–15% of Cr | ||||
Cd | 25%–30% clinoptilolite take up to 100% of Cd. | ||||
Pb | 25%–30% clinoptilolite take up to 50%–55% of Pb. | ||||
Ni | 25%–30% clinoptilolite take up to 50%–55% of Ni | ||||
Biochar | The dewatered fresh sewage sludge | An urban sewage treatment plant in Tianjin, China | Cu | The content of effective Cu decreased | [ |
Zn | The content of effective Zn increased | ||||
Cr | The content of effective Cr decreased | ||||
Cd | The content of effective Cd increased | ||||
Pb | The content of effective Pb decreased the most | ||||
Ni | The content of effective Ni decreased | ||||
Modified biochar | Dewatered fresh sewage sludge (DFSS) | The wastewater treatment plant at |
Cu | It increases slightly in the biological oxidation stage and then decreases gradually | [ |
Zn | |||||
Pb | It starts to increase throughout maturity and then gradually decreases | ||||
Ni | |||||
Sodium sulfide and lime |
Dewatered aerobic sewage sludge | The Quyang sewage treatment plant in Shanghai | Cu | Organic matter and sulfide combined with Cu are mainly converted into residual parts | [ |
Zn | After composting, the Zn residual part dominated the compost | ||||
Ni | Carbonate binding state, iron and manganese oxide binding state, organic matter and Ni sulfide are mainly converted into residual forms of Ni | ||||
Earthworms | Dewatered sludge | NA | Cu | The concentration of heavy metals decreased after earthworms absorbed heavy metals and removed earthworms from compost | [ |
Zn | |||||
Cd | |||||
Pb | |||||
Ni | |||||
Earthworms | Final SS produced by activated sludge process | Sewage treatment plant in Kuala Lumpur | Cu | The concentration of heavy metals decreased | [ |
Zn | |||||
Cr | |||||
Cd | |||||
Pb |
By analyzing
Soil Cu content far exceeds the carrying capacity of the soil environment with the mining of Cu, discharge of waste from smelters, the long-term and large use of Cu-containing fungicides, and the composting of municipal sludge. Global Cu content typically ranges from 2 to 100 mg/kg. Different soils have different Cu capacities, and different regions have different effects. There is no specific value. High Cu content is harmful to animals, plants, and soil microorganisms. Damage to the cytoplasmic membrane of the root system hinders root elongation, lateral roots become shorter, new leaves lose their green color, and old leaves to die. In animals, organ damage and metabolic disturbances lead to physical discomfort. Cu controls the growth of microorganisms, resulting in changes in quantity and population structure and seriously threatens stability of the ecosystem and human security. Therefore, soil Cu pollution is an urgent problem [
Nomeda et al. [
In contrast, zeolite has a pore-forming silica lattice, with uniform pore size and large holes on the inner surface of the crystal allowing for strong adsorption of heavy metal ions. About 25%–30% clinoptilolite is used during the composting process, and 28%–45% of the Cu is absorbed by zeolite [
When Wong et al. [
Some authors have found that adding 6% biochar has the best passivation effect on Cu, and that Cu content decreases significantly after composting [
In addition, Wang et al. [
The main sources of Zn pollution include Zn mining, smelting, machinery manufacturing, Zn plating, instrumentation, organic synthesis, papermaking, and other industrial emissions, automobile tire wear and coal burning dust, soot containing Zn and compounds, and Zn in industrial wastewater with a Zn hydroxyl complex. Moreover, the scope of Zn pollution is wide, as Zn pollutes the air, water, and soil. However, soil pollution is different from air pollution. Once formed, harmful substances that accumulate in the soil migrate into the water, air, and plants, and eventually enter the human body, causing long-term effects. Therefore, we must reduce heavy metal content in compost to reach a standard value before applying sludge compost.
In Nomeda et al. [
In addition, Zorpas et al. [
Wong et al. [
Awasthi et al. [
In particular, Wang et al. added phosphate modifiers to compost and the mobility of Zn was reduced by 1.7% [
Cd is one of the most toxic elements in the food chain, and is a limiting factor in the land use of sludge. Cd is a common mobile metal in sludge. It is not a necessary trace element in the natural environment, and is very harmful. Bożym et al. reported that high Cd concentrations reduce or completely inhibit the activities of microorganisms under aerobic or anaerobic conditions. Cd often pollutes soil; however, the mobility of Cd is greatly reduced after composting [
The soluble and exchangeable heavy metals are effective in plants, while the residual heavy metals belong to the ineffective part. Under certain conditions, other forms may be released slowly and in small amounts to supplement the effective state. The uptake of Cd by plants does not depend on the total amount of Cd in soil, but on the availability and existing forms of Cd. Therefore, experts have used a variety of conditioning agents to observe the results of various morphological changes. Hydroxyapatite (HAP) is a good modifier. HAP improves the environment by reducing the absorption of Cd by turfgrass. Liu et al. [
The activity and bioavailability of Cd in sludge compost is high. Chen et al. [
Pb is a relatively stable metal that usually exists as a bivalent insoluble compound in the soil environment, and water-soluble Pb content is very low. Pb is a nonessential element in plants. It affects seed germination when absorbed by plants and is toxic to the human body. However, sludge composting effectively reduces Pb content. Liu et al. [
The biological availability of Pb is related to organic matter, the structure and cation exchange capacity of the soil, and to the form and distribution of Pb in the soil. Nomeda et al. investigated the distribution of Pb during sludge composting using sludge from Taiwan (Pb: 1,200 mg/kg), whose concentration is far lower than that of the European Union sludge directive. Pb is closely related to stable components (organic matter/sulfide component and residual component). The mobility and bioavailability of Pb increases during composting. The effect of the granular metal form on the environment is closely related to migratory behavior during sludge composting. The Pb content in the remaining part of the sludge is higher, reaching 70%. More than 93% of the Cu is distributed in the stable components of sewage sludge and recycled compost, while the Cu content in sawdust is only 56%. The distribution of Pb is similar to that of Cu. Moreover, the fluidity of Pb in raw materials is high. Pb mainly exists in a residual state in sewage sludge, from 32% to 17%. The transformation of Pb to more mobile (bioavailable) components occurs during the last stage of composting, so the proportion of Pb in stable components is higher. This finding indicates that heavy metals, such as Pb, are not easily released from these sludge and compost mixtures [
Wong et al. [
Pb forms a stable complex with humus in sludge soil, and the Pb concentration in soil is positively correlated with humus content. The mobility and availability of Pb in soil depend on pH, eh, organic matter content, texture, available phosphorus, and amorphous iron manganese oxides. However, adding conditioning agents also has an effect on the migration and transformation of Pb, such as adding phosphate. Phosphate reduces activation of Pb, and the toxicity of phosphate in compost does not increase, and it even promotes plant growth. Adding phosphate ensures an increase in the temperature and degradation of organic matter during sludge composting. Wang et al. [
Biochar and microorganisms are increasingly being used during sludge composting. Liu et al. [
Cr is widely found in the earth’s crust. Cr minerals naturally occur in the form of oxides, hydroxides, sulfides, and silicates. Trace Cr promotes the production of some crops (such as wheat, peas, and cucumber), but a high concentration of Cr produces serious toxic effects on plants, and symptoms, such as dwarfism, leaf coil, and root brown, as well as short and stunted plants. Cr (III) is necessary for humans and is easily absorbed and accumulates in the human body, but it can be toxic. Cr enters the food chain through the soil-plant system and threatens human health.
The form of a heavy metal plays a very important role in its toxicity. Zheng et al. [
Zorpas et al. [
Clinoptilolite effectively adsorbs heavy metals. Zorpas et al. [
Nickel is often dissolved in water in the form of halides, nitrates, sulfates, and some inorganic and organic complexes. Excessive consumption (250 mg of soluble Ni per day) of Ni leads to poisoning. The Ni in the environment is the most important source of Ni. Preventing Ni from entering the environment is a human health concern.
The Ni concentration in sludge increased by 30.4% after the aerobic composting experiment performed by Zheng et al. [
Ignatowicz used the BCR method to measure heavy metal content and the formation of heavy metal-organic complexes, and the binding degree was very high. Most heavy metals have a binding degree of 30%–40%, so mobility of the heavy metals is greatly reduced. The three stages of chemical analysis of the extract revealed high Ni content, which stabilized at 35.30% of the total content in water sludge after mixing with wood chips. The Ni content in treated compost should not exceed the limit of biological utilization [
Zorpas et al. [
Wong et al. [
Wang et al. [
In summary, the removal mechanism and quantitative relationship between different metals can be explained by different methods. The clinoptilolite molecular sieve has the characteristics of ion exchange, adsorption separation, catalysis, and stability. Clinoptilolite removes Cd, Cu, Zn, Ni, Pb, and Cr by ion-exchange adsorption. Among them, ion exchange has a significant effect on the formation of the carbonate state, and clinoptilolite reduces the bioavailability of all heavy metals through adsorption.
Lime reduces the formation of metallic organics during composting of lime sludge by neutralizing the organic acids released, and converts some of the organic Cd, organic Cu, organic Zn, organic Ni, organic Pb, and organic Cr into residues and reduces the fluidity of all heavy metals.
Lime-modified biochar compost is superior to lime compost. Biochar itself contains many properties that are conducive to the passivation of heavy metals and this type of compost inhibits their activation. The content of surface oxygen-containing functional groups is beneficial to the passivation of heavy metals. Lime-modified biochar compost has high concentrations of humic and fulvic acids, which effectively reduce heavy metal contents. Adding biochar and microbial modifiers reduces the effectiveness of Cu, Cd, Ni, Zn, Cr, and Pb.
In addition, earthworm composting removes heavy metals from the compost through bioaccumulation. The main mechanism of the resistance to heavy metal pollution by earthworms is as follows: the lipid antioxidase system reduces oxidative stress, detoxifies chelated metals, activates lysosomes and the cytoplasm, and inhibits heavy metal activity. Thus, the concentrations of Cu, Zn, Cd, Pb, Ni, and Cr decrease.
Phosphate transfers heavy metals from the bioavailable stage to the stable stage. Phosphate treatment reduces the fluidity of Cd, Cu, Zn, and other heavy metals in compost. Adding phosphate greatly improves the fixation effect of heavy metals while reducing their activation. The mobility of a series of heavy metals represented by Cu and Zn decreases after adding phosphate modifiers, which may be due to the formation of CuFeS2 and Zn Cu (P2O7) crystals during composting.
Finally,
Based on the detailed interpretation of each metal, the main methods to remove heavy metals from sludge are discussed in detail. Then, the characteristics of the heavy metals in sludge compost are briefly described. The characteristics of these heavy metals are a major factor affecting removal efficiency.
Pb2+ is the most common and active form of the two valence states of Pb, including the zero state and the divalent state. Pb2+ easily combines with inorganic ions, humic acids, or amino acids to form poorly soluble compounds.
Chromium (VI) is the most toxic and fluid material in sludge compost and is found mainly in the form of chromate and dichromate. Cr (VI) is reduced to Cr (III) with weak toxicity and mobility in the environment by organic matter, S2−, and Fe2−. The leaching concentration of Cr (VI) increases with pH.
Environmental pH has a strong effect on Cd activity, and the Cd2+ activity is higher in soil under acidic conditions (pH 4.5–5.5). Under a high soil pH, Cd2+ precipitates with hydroxide and carbonate ions. Cd2+ also forms a precipitate with phosphate, dichromate, and S ions.
Cu has three valence states, such as 0, +1, and +2, with the strongest toxicity in the bivalent state. Cu2+ activity is highly dependent on pH, which decreases with the increase in soil pH. Cu2+ activity is regulated by the adsorption of carbonate, phosphate, and clay minerals.
In the environment, Zn2+ combines with hydroxides, carbonates, sulfates, phosphates, and other anions to form precipitates, and also combines with organic acids to form complexes. Under reducing conditions, Zn and Fe/Mn and other hydrated oxides form co-precipitates.
The characteristics of these heavy metals are a major factor affecting removal efficiency. For example, biochar can be added to sludge compost to affect removal efficiency. Biochar for passivation of heavy metals in sludge, the removal of heavy metals in the compost similarities: biochar of nitrogen and phosphorus, potassium, calcium, magnesium, such as inorganic nutrients can be extracted, they are all with increase of pH value method to suppress the activation of heavy metals. The efficiency of removing heavy metals is affected by the sensitivity of each metal to pH, and oxygen-containing functional groups on the surface favor passivation of heavy metals.
And specific involves the change of each heavy metal quantity how much and is not the same, Principal component analysis (PCA) was used to analyze the effectiveness of heavy metals before and after composting. The contribution rates of principal component PC1–PC3 were 47.01%, 24.75%, and 20.57%, respectively. PC1 was the main factor affecting the availability of heavy metals in biochar compost. To further analyze the contribution of heavy metals to the three PCs, a factor loading analysis was performed on the three PCs. Pb had the largest contribution to PC1, Cr had the largest contribution to PC2, and As had the largest contribution to PC3. These results show that the different treatments have different passivation effects on the heavy metals, among which the Pb passivation effect was the best, followed by As and Cr.
Natural clinoptilolite, such as oblique zeolite, absorb and remove metals by ion exchange. Clinoptilolite uses ion exchange to remove cations from the aqueous solution and solid phases, which is how clinoptilolite removes heavy metals from water or sludge.
The removal effect of different heavy metals by clinoptilolite, such as Zn2+, Pb2+, Cd2+ and Ni2+ is also different. The selectivity of Pb, Cu, Cd, Zn, and Cr is higher than that of Ni. Thus, the clinoptilolite selective sequence is Pb > Cu > Cd > Zn > Cr > Ni. Clinoptilolite easily absorbs almost all metals bound to the exchangeable and carbonate components. Under these conditions, clinoptilolite occupies all metals in the exchangeable and carbonate components in the order of Cu > Cr > Ni > Pb > Zn.
The metal content in clinoptilolite increases with an increase in particle size. Surface dust blocks some of the pores in the clinoptilolite structure, resulting in slower ion exchange kinetics for smaller particles than for larger particles during Pb2+/Na+ ion exchange in water systems. In addition, structural damage to the small particles due to the grinding process affects the removal of heavy metals. The treatment effect also differs due to the different nature of some of the heavy metals. For example, the particle size of clinoptilolite has a more obvious effect on the absorption of Ni, and clinoptilolite is a relatively poor remover of Ni2+ in an aqueous solution, which is due to the high stability of the aqueous complex. More Zn2+ was removed, most Cu2+ was removed, and less Ni2+ was removed under the same conditions. The stable macrocomplexes of these metals may be too large to easily enter the zeolite pores.
Another common factor that prevents adsorption by clinoptilolite is ion size. If the ion is larger than the pore, the substance is excluded. The size of the hydration ions of the metals generated the following selective sequence: Pb > Ni > Cu > Cd > Zn > Cr.
Finally, earthworms remove heavy metals. The main mechanism by which earthworms remove heavy metal pollution is to reduce oxidative stress using the lipid antioxidase system, chelation, and activation of lysosomes and plasmids to inhibit heavy metal activity.
The variations in the concentrations of different metals can be attributed to the enrichment of heavy metals by earthworms. The concentrations of compounds in organisms are mainly based on absorption, elimination, and biotransformation. Eliminating unnecessary excess metal-to-metal bioaccumulation in sludge is a very important treatment.
The initial absorption of base metals, such as Cu and Zn, is rapid, and subsequent equilibrium is achieved after several days of exposure, indicating physiological control and possible excretion of these elements by earthworms. External metals, such as Cd, are excreted slowly or are not excreted. The same biological concentration of Cu in earthworms at all sampling sites resulted in a decrease of the bioaccumulation factor (BAF) in contaminated soil, which may be the result of this mechanism. A low metal concentration in soil corresponds to a high BAF. The excretion of some Zn by earthworms helps regulate the concentration of metal ions, and BAF is low in most contaminated soils, even at very high Zn concentrations. Eisenia fetida earthworms accumulated some Ni in this study, but only a small amount. The BAFs of the five heavy metals in E. fetida were ranked as Cd > Zn > Cu > Ni > Pb.
The compost structure, ventilation, nutrient balance, water content [
pH is an important parameter affecting the composting process.
High pH values can inhibit microbial growth [
The water content of sludge compost should be controlled between 40% and 60%, and the temperature should be about 55°C. A turning frequency of more than once per week effectively promotes the efficiency of composting [
The concentrations of some heavy metals in sludge composting decrease due to the solubility of water [
Leached metal contents decrease with an increase in temperature, and accumulate significantly in the solid phase. The metal form effectively migrates to a minimally toxic or non-toxic state, which activates pathogens and microbial activities [
Ventilation is another important condition. Adequate ventilation and optimum humidity, as well as the removal of excess water from metabolic activities and the supply of adequate oxygen are important [
Good composting products can be obtained with high microbial activity, fast composting efficiency, and a short composting cycle when the pH value is 6–8, water content is 40%–60%, the temperature is controlled at 55°C, C/N is 20–35: 1, the C/P is 75–150: 1, and oxygen and organic content is 20%–80%. Among these factors, the pH value, water content, and temperature affect the removal of heavy metals from sludge. As the main factor determining heavy metal leaching, and an increase in pH reduces soluble heavy metal concentrations; thus, reducing bioavailability. The solubility of water is used to reduce the concentration of some heavy metals. The increase in temperature leads to increased metal leaching and the metal migrates to a state of minimal toxicity or non-toxicity. To sum up, various sludge compost parameters play a role in reducing heavy metal toxicity. Appropriate parameters combined with the use of conditioning agents can reduce the dosage; thus, reducing the environmental pollution.
Research shows that the toxicity of heavy metals decreases gradually after sludge composting. Due to the rapid development of the sewage treatment industry and the progress in sludge treatment technology, Cu, Zn, Cd, and other heavy metal contents in sludge are reduced. Therefore, after the heavy metals in the sludge compost are passivated, the harm to the soil will also be reduced, but there are still some problems that need to be resolved.
The development of new multifunctional conditioning agents will be a trend of future research because of environmental risk and an unstable repair effect. The remediation of heavy metals during sludge composting is affected by many factors. Notably, a single conditioning agent only has a significant impact on a small number of heavy metals in the sludge. However, the use of a variety of conditioning agents, such as lime and clay minerals, will have a great influence on the repair effect. Therefore, it is important to develop multi-functional conditioning agents for various heavy metals to reduce the cost and improve the practical effect.
This review systematically analyzes and summarizes the effects of conditioning agents on the concentration and morphology of heavy metals during sludge composting. Adding conditioning agents to the composting process greatly reduces the harmful effects of heavy metals during sludge composting. Swelling agents, such as calcium oxide, zeolite, and biochar, are good loose fertilizers that decrease the concentration of heavy metals in sludge and passivate zeolite to reduce the mobility of heavy metals. biochar treatment promotes the passivation of heavy metals, particularly Pb. Zeolite can effectively significantly reduce the unstable Zn. treatment with earthworms not only optimizes the soil but reduces the concentrations of heavy metals, such as Cu, Cd, Cr, Pb, and Zn. The removal rate of Cu, Zn, Cd, Cr and Ni could reach 27%, 40%–46%, 31.08%, 10%–15% and 50%–55% by adding conditioning agents.
Most experiments now use a single conditioning agent, but different conditioning agents have different abilities to remove heavy metals, so they can be used in combination or new conditioning agents can be developed to remove many kinds of heavy metals, but also not cancel the original advantages. This is one direction for the future.