Heavy Metals In Sewage Sludge Research Articles

Microbiologically induced calcite precipitation for in situ stabilization of heavy metals contributes to land application of sewage sludge

Microbiologically induced calcite precipitation (MICP) has shed new light on solving the problem of in situ stabilization of heavy metals (HMs) in sewage sludge before land disposal. In this study, we examined whether MICP treatment can be integrated into a sewage sludge anaerobic digestion-land application process. Our results showed that MICP treatment not only prevented the transfer of ionic-state Cd from the sludge to the supernatant (98.46 % immobilization efficiency) but also reduced the soluble exchangeable Pb and Cd fractions by up to 100 % and 48.54 % and increased the residual fractions by 22.54 % and 81.77 %, respectively. In addition, the analysis of the stability of HMs in MICP-treated sludge revealed maximum reductions of 100 % and 89.56 % for TCLP-extractable Pb and Cd, respectively. Three-dimensional fluorescence, scanning electron microscopy-energy-dispersive X-ray spectroscopy, X-ray diffraction, and Fourier-transform infrared spectroscopy analyses confirmed the excellent performance of the ureolytic bacteria Sporosarcina ureilytica ML-2 in the sludge system. High-throughput sequencing showed that the relative abundance of Sporosarcina sp. reached 53.18 % in MICP-treated sludge, and the urease metabolism functional genes unit increased by a maximum of 239.3 %. The MICP technology may be a feasible method for permanently stabilizing HMs in sewage sludge before land disposal.

Agent-assisted electrokinetic treatment of sewage sludge: Heavy metal removal effectiveness and nutrient content characteristics

Sewage sludge (SS) is rich in nutrient elements such as phosphorus (P), nitrogen (N), and potassium (K), and therefore a candidate material for use in agriculture. But high content of heavy metals (HMs) can be a major obstacle to its further utilization. Therefore, an appropriate HM removal technology is required before its land application. In this study, an innovative biodegradable agent (citric acid, FeCl3, ammonium hydroxide, tetrasodium iminodisuccinate (IDS), and tea saponin) assisted electrokinetic treatment (EK) was performed to investigate the HM removal efficiency (RHMs) and nutrient transportation. Citric acid, IDS, and FeCl3-assisted EK showed a preferable average RHMs (Rave) reduction of 52.74−59.23%, with low energy consumption. After treatment, the content of Hg (0.51 mg kg−1), Ni (13.23 mg kg−1), and Pb (26.45 mg kg−1) elements met the criteria of national risk control standard, in all cases. Following the treatment, most HMs in SS had a reduced potential to be absorbed by plants or be leached into water systems. Risk assessment indicated that the Geoaccumulation index (Igeo) value of HMs has decreased by 0.28−2.40, and the risk of Pb (Igeo=−0.74) reduced to unpolluted potential. Meanwhile, no excessive nutrient loss in SS occurred as a result of the treatment, on the contrary, there was a slight increase in P content (18.17 mg g−1). These results indicate that agent-assisted EK treatment could be an environmentally-friendly method for RHMs and nutrient element recovery from SS, opening new opportunities for sustainable SS recycling and its inclusion into circular economy concepts.

A Comparison of Conventional and Ultrasound-Assisted BCR Sequential Extraction Methods for the Fractionation of Heavy Metals in Sewage Sludge of Different Characteristics.

The purpose of this study was to determine the heavy metal (HM: Cd, Cr, Cu, Ni, Pb, Zn, and Hg) content in particular chemical fractions (forms) of sewage sludge with different characteristics (primary and dewatered sludge) using conventional (CSE) and ultrasound-assisted (USE) BCR sequential extraction methods (Community Bureau of Reference, now the Standards, Measurements and Testing Programme). The concentrations of HMs were determined using inductively coupled plasma optical spectrometry (ICP-OES). Only mercury was assayed with cold vapor atomic absorption spectrometry (CVAAS). Ultrasound treatment was conducted in the ultrasonic bath (Sonic 5, Polsonic). The optimal sonication time (30 min) was determined using ERM-CC144 (Joint Research Center; JCR) certified reference material. The conducted experiment revealed that the use of ultrasound waves shortened the extraction time to 4 h and 30 min (Stages I to III). The recoveries (RM) of heavy metals ranged from 62.8% to 130.2% (CSE) and from 79.8% to 135.7% (USE) for primary sludge, and from 87.2% to 113.2% (CSE) and from 87.8% to 112.0% (USE) for dewatered sludge. The only exception was Hg in dewatered sludge. The conducted research revealed minor differences in the concentrations and fractionation patterns for Cd, Ni, and Zn extracted from sludge samples by the tested methods. However, it was confirmed that the above findings do not significantly affect the results of a potential ecological risk assessment (with minor exceptions for Cd and Zn in the primary sludge), which is extremely essential for the natural use of sludge, and especially dewatered sludge (the final sludge). The shorter extraction time and lower energy consumption prove that ultrasound-assisted extraction is a fast and simple method for HM fractionation, and that it provides an alternative to the conventional procedure. Therefore, it can be considered a “green method” for the assessment of the bioavailability and mobility of heavy metals in solid samples.

Enhancement of heavy metal immobilization in sewage sludge biochar by combining alkaline hydrothermal treatment and pyrolysis

Heavy metals, including Cd, As, Ni, Cu, Pb, Cr, Zn, and Mn in sewage sludge (SS) are among the greatest impediments to SS application because of their potential toxicity to living organisms and latent environmental pollution risks. In this study, hydrothermal treatment (HT)/alkaline HT combined with pyrolysis was used to treat SS for heavy metal immobilization. The heavy metal characteristics, including total, fraction, and leaching concentrations and potential ecological risks, were comprehensively investigated to ensure the safe application of biochar. The results showed that the heavy metals mainly remained in the hydrochar and biochar. Additionally, both HT and alkaline HT promoted the residual fractions of Cr, Zn, Ni, and Cu in biochar. Furthermore, alkaline HT-aided pyrolysis was more effective for immobilizing Zn and Ni with their residual fractions increasing by 100% and 60% respectively, compared with direct sludge pyrolysis. Moreover, the ecological risk of heavy metals in SS significantly decreased from 57.27 to 30.05 after alkaline HT-combined pyrolysis treatment, contributing to the low potential ecological risk of SS-based biochar. This study provides a promising strategy for converting SS to biochar with improved heavy metal immobilization.

Speciation Evolution of Phosphorus and Sulfur Derived from Sewage Sludge Biochar in Soil: Ageing Effects.

Phosphorus (P) and sulfur (S) are usually involved simultaneously in the immobilization of heavy metals in sewage sludge during pyrolysis, and thus their speciation in sewage sludge-derived biochar (SSB) profoundly affects the recycling of the nutrients and the environmental risks of sewage sludge. Here, we investigated the speciation evolution of P and S in SSB induced by ageing processes in soil using X-ray absorption near edge structure spectroscopy. Results showed that Ca-bound compounds like hydroxyapatite dominated the P forms, while over 60% of S existed as reduced inorganic sulfides in the SSB. The stable Ca-associated P species in SSB tended to be transformed gradually into relatively soluble species during ageing in soil. The speciation composition of S in SSB remained almost unaffected when aged in pot soils, whereas about 33.6% of reduced sulfides were transformed into oxidized species after 1-year ageing in field soils. SSB significantly increased the proportion of sulfides and the contents of available P and S in the amended soil but showed relatively weak effects on the speciation distribution of P in the soil because of their similar compositions. These findings provide insights into biogeochemistry of nutrients and behaviors of heavy metals in SSB after its application to the soil environments.

Solar Drying as an Eco-Friendly Technology for Sewage Sludge Stabilization: Assessment of Micropollutant Behavior, Pathogen Removal, and Agronomic Value

Sewage sludge (SS) is a biosolid that includes nutrients, organic matter, and a mixture of micropollutants and pathogens. Regarding its final disposal, several criteria should be met to avoid the dissemination of the included micropollutants in the environment. Hence, an adequate treatment prior to SS disposal is highly required. Solar drying is being acknowledged as a sustainable process of SS treatment, yet it is still unclear to what extent this technique is efficient. This review aimed to assess the impact of solar drying on the composition of SS from environmental and agronomic standpoints. Herein, we present the state-of-the-art with regard to solar drying efficiency in terms of water content reduction, DM increase, agronomic parameters evolvement, and micropollutant stabilization including pathogens, heavy metals (HMs), and organic micropollutants. The reviewed literature is mostly focused on two drying cycles: summer and winter, thus addressing the extreme conditions met within a year with respect to temperature. Under different climatic conditions, more than 80% of dry matter is reached during summer. In winter, the efficiency decreases to an average of 50% of DM. Negatively correlated to DM content, pathogen concentration in SS significantly decreased, while DM increased. Thus, more efficiency in terms of pathogen abatement is reported in summer than in winter (e.g., 96% against 60% during summer and winter, respectively, under semi-arid climate). The high reliance of solar drying efficiency on weather has been deduced in terms of DM content increase and pathogen removal. Where climatic conditions are not favorable for solar drying, hybrid design and liming are the highly recommended methods to remove pathogens from SS. A few studies on the fate of HMs in SS during solar drying concluded that solar drying does not involve any removal mechanisms. Changes in HM speciation in solar-dried sludge were reported highlighting a decrease in their mobility. As for organic micropollutants (PAHs and antibiotics), only their occurrence in SS is reported in the literature, and their behavior during the solar drying process is still not addressed. This review allowed concluding the following: 1) solar drying is a sustainable, relevant process for SS handing in terms of volume reduction and pathogen removal, particularly in semi-arid regions; 2) solar drying does not lower the SS agronomic value and does not remove HMs, but under semi-arid climate, it changes HM speciation and reduces their mobility. The gap in research regarding organic micropollutant and heavy metal behavior during SS solar drying has been emphasized as a way forward for research within this topic. Hence, more research is required to help stakeholders decide on the feasibility of an agricultural disposal of solar-dried sludge.

Risk Assessment and Potential Analysis of the Agricultural Use of Sewage Sludge in Central Shanxi Province.

The agricultural use of sewage sludge has become an economic disposal method used worldwide. However, heavy metals contained in sewage sludge have become the crucial limiting factors for this way of disposal. This study showed that regulatory limit values are not enough to determine whether sewage sludge is suitable for agricultural use; risk assessment and potential analysis should be applied. Correlation analysis and hierarchical cluster analysis (HCA) should also be performed to identify heavy metals’ sources and show their influence on sewage sludge utilization. Samples were collected from 13 wastewater treatment plants (WWTPs) located in central Shanxi Province. Results indicated that the mean contents of heavy metals in sewage sludge were all less than the limit threshold of China. According to the monomial potential ecological risk coefficient (), the agricultural use of sewage sludge had low ecological risks for all heavy metals, except for Hg and Cd. Based on the potential ecological risk index (RI), only three stations had moderate risk, other nine stations all had higher potential risk. The mean potentials by all heavy metals were all beyond 10 years, which is the limit of the maximum application time specified by China. Combining all heavy metals, only one station’s potential was less than 10 years. Although the contents of heavy metals were all within the threshold values, large quantities of sewage sludge are not suitable for agricultural use. Coal-related industries, which were the main sources of Hg and Cd, greatly affected the agricultural use of sewage sludge.

Co-pyrolysis of sewage sludge and Ca(H2PO4)2: heavy metal stabilization, mechanism, and toxic leaching

The presence of unstable heavy metals in sewage sludge (SS) restricts its resource utilization. In this study, Ca(H2PO4)2 and SS were co-pyrolyzed to produce biochar, which contained relatively stable heavy metals. X-ray diffraction spectroscopy, Fourier transform infrared spectroscopy, and inductively coupled plasma atomic emission techniques were used to analyze the physical and chemical properties and heavy metal content of the biochar. The results indicated that co-pyrolysis of SS with Ca(H2PO4)2 resulted in the production of more stable heavy metals in the SS. The optimal co-pyrolysis conditions were a blended ratio of 15% Ca(H2PO4)2, 650 °C final temperature, 15 °C min−1, and 60 min retention time. The potential stabilization mechanisms of heavy metals were as follows: (1) organic decomposition and moisture (sourced from Ca(H2PO4)2 decomposition) evaporation resulted in greater biochar surface porosity; (2) phosphorous substances were complexed with heavy metals to form metal phosphates; and (3) the mixture reactions among inorganic substances, pyrolysis products of organics, and heavy metals resulted in the formation of highly aromatic metallic compounds. Additionally, the potential environmental risks posed by the heavy metals decreased from 65.73 (in SS) to 4.39 (in biochar derived from co-pyrolysis of SS and 15% of Ca(H2PO4)2). This study reports on a good approach for the disposal of SS and the reduction of its environmental risk.

Pozzolanic reactivity of aluminum-rich sewage sludge ash: Influence of calcination process and effect of calcination products on cement hydration

The application of aluminum-based flocculant in wastewater treatment results in a large amount of aluminum-rich sewage sludge. This work investigated the influence of calcination process on physicochemical characteristics and pozzolanic activity of aluminum (Al)-rich sludge ash and studied the effect of sludge ash on cement hydration. The results showed that higher calcination temperature from 600 ℃ to 900 ℃ increased the amorphous content in sludge ash. The pozzolanic activity of sludge ash calcined at 800 ℃ and 900 ℃ was confirmed by Frattini test. In view of strength activity index of blended mortar and energy conservation, the optimal calcination condition of sewage sludge ash was calcined at 800 ℃ with air-cooling. The addition of sludge ash promoted the transformation of ettringite to monosulfate phase in cement paste. However, the high Al concentration dissolved from S6 and S7 ash inhibited significantly the cement hydration and resulted in low compressive strength values of the blended mortars. The pozzolanic reaction of S8 and S9 ash produced more hydration heat and additional Al-bearing products such as katoite and monosulfate which contributed to the strength development of mortars. Furthermore, the heavy metals in sewage sludge can be immobilized in ash structure during calcination process and the structure of hydration products, which ensures the environmental security of sludge ash utilization in construction materials.

Assessment of Treatment Effect of Heavy Metal Pollution from Sewage Sludge in Wastewater Treatment Plant Discharge in China’s Nanjing MV Industrial Park

To explore methods for a comprehensive assessment of the treatment effect of heavy metal pollution in the sewage sludge from China’s industrial parks, we studied the wastewater treatment plant of the Nanjing MV Industrial Park as an example. Eight common heavy metals in sewage sludge – Zinc (Zn), Copper (Cu), Lead (Pb), Mercury (Hg), Chromium (Cr), Nickel (Ni), Arsenic (As), Cadmium (Cd) were studied. The treatment effect of these containments was comprehensively assessed using the absolute niche fitness model, the relative niche model and the spatial niche fitness model. All three models showed that Pb > Cu > Ni > Hg > Cd > Cr > Zn > As in the samples. However, they produced – different numerical values – the absolute niche suitability model < the spatial niche suitability model < and the relative niche suitability model. Therefore, we concluded that special attention should be paid to the carcinogenic risk of As and Cr heavy metals to the person exposed to the sewage sludge.

Combining impregnation and co-pyrolysis to reduce the environmental risk of biochar derived from sewage sludge

Sewage sludge derived biochar has great potential for agricultural application, whereas the risk of heavy metals in sewage sludge is a key challenge for utilization. This study investigated the synergetic effect of co-pyrolysis and ZnCl2 impregnation treatment on the surface characteristics and potential ecological risk of heavy metals in sewage sludge derived biochar. It was concluded that ZnCl2 impregnation led to an increase in biochar yield, O and S content, but decrease the ash content and pH. Additionally, the thermal stability of ZnCl2-added biochar was decreased. Semi-quantitive specific functional groups analysis suggested that ZnCl2 impregnation had a negative effect on the content of CO and C-O, while promoted the formation of aromatic CC. And the functional group of CO was retained in biochar by co-pyrolysis with Camellia oleifera shell due to its high content of potassium. For heavy metal passivation, ZnCl2 impregnation decreased total content of heavy metals by chlorination, while the oxidizable fraction (F3) of heavy metals showed an increasing tendency. Results of potential ecological risk assessment indicated that combining ZnCl2 impregnation and co-pyrolysis had great potential to reduce the ecological risk of heavy metals in sewage sludge derived biochar.

Comparison of the Possibilities of Environmental Usage of Sewage Sludge from Treatment Plants Operating with MBR and SBR Technology.

Sewage sludge from sewage treatment plants has soil-forming and fertilising properties. However, sewage sludge cannot always be used in nature, including agriculture. One of the main reasons is the concentration of heavy metals. Sludge from wastewater treatment plants operating in MBR (membrane biological reactor) and SBR (sequential batch reactor) systems was analysed. Studies comparing the risk analysis of the natural use of sludge from MBR and SBR treatment plants were performed for the first time, due to the fact that more and more MBR plants, which are a BAT technology, are being developed in Poland, displacing the classical SBR plants. MBR technology uses a combination of activated sludge and filtration with microfiltration membranes. Wastewater treated in these reactors meets the highest quality standards, both in terms of physicochemical and microbiological aspects. This paper presents studies on the mobility of heavy metals in sewage sludge carried out using the BCR sequential extraction method. Geo-accumulation index (GAI), potential environmental risk index (ER), risk assessment code (RAC), and environmental risk determinant (ERD) were calculated. Heavy metals dominated the stable fractions in all cases. Furthermore, an increased content of copper and cadmium was observed in the MBR sludge. This fact is favourable in view of the efforts to eliminate heavy metals in the environment.

Sorption of Heavy Metals by Sewage Sludge and Its Mixtures with Soil from Wastewater Treatment Plants Operating in MBR and INR Technology.

Sewage sludge is a very complex system, with solids and water. It is generated as waste from wastewater treatment. Sewage sludge is used to fertilize agricultural and forest areas and to rehabilitate devastated areas. It is a good organic fertilizer because it contains significant amounts of nutrients beneficial for plant development and humus-forming substances. The composition of sludge from municipal wastewater treatment plants is similar to soil organic matter, therefore it can be used to improve the physicochemical properties of soil, increasing its sorption capacity. Research material was collected in the Swietokrzyskie and Mazowieckie Voivodships. Sewage sludge was collected from the wastewater treatment plants in Sitkowka Nowiny (Sitkowka) and Kunow, as well as high-quality agricultural soil from Opatowiec and sandy-clay soil from Jastrzebie. Research was carried out on the sorption of heavy metals (Cd, Cr, Cu, Pb, Ni, Zn) by mixtures of sewage sludge with soil. The calculations were made for the concentrations of heavy metals in sewage sludge, soil, and sewage sludge–soil mixtures. The geoaccumulation index (Igeo) and the risk assessment code (RAC) were calculated. Increased sorption capacity was demonstrated in samples with a predominance of sewage sludge. It was shown that heavy metals from sewage sludge, after mixing with soil, changed their form from immobile to mobile.

Risk Analysis of Heavy Metal Accumulation from Sewage Sludge of Selected Wastewater Treatment Plants in Poland

Sewage sludge (SS) from wastewater treatment plants (WWTPs) has important soil-forming and fertilizing properties. However, it may not always be used for this purpose. One of the main reasons why SS cannot be used for natural purposes is its heavy metal (HM) content. SS from the wastewater treatment plant in Poland was subjected to an analysis of the potential anthropogenic hazard of HMs, especially in terms of their mobility and accumulation in soil. Calculations were made for the concentrations of HMs in SS from the analyzed wastewater treatment plants and in arable soil from measurement points in places of its potential use. The geoaccumulation index (GAI), potential environmental risk index (PERI), risk assessment code (RAC) and environmental risk determinant (ERD) were calculated. Then the values of the indicators were compared with the mobility of HMs, which was the highest risk of soil contamination. It was shown that a high level of potential risk and geoaccumulation indicators did not necessarily disqualify the use of SS, provided that HMs were in immovable fractions.

Removal behavior and chemical speciation distributions of heavy metals in sewage sludge during bioleaching and combined bioleaching/Fenton-like processes

The removal and chemical speciation changes of heavy metals in the sewage sludge during the single bioleaching and combined bioleaching/Fenton-like processes were compared in this study. The improvement in the dewaterability of the treated sludge was also investigated. The single bioleaching led to a removal of Zn, Cu, Cd, Cr, Mn, Ni, As and Pb of 67.28%, 50.78%, 64.86%, 6.32%, 56.15%, 49.83%, 20.78% and 10.52% in 10 days, respectively. The chemical speciation analysis showed that the solubilization of heavy metals in mobile forms (exchangeable/acid soluble and reducible forms) and oxidizable form was the main reason for their removal. Subsequent Fenton-like treatment was carried out at different bioleaching stages when the bioleached sludge dropped to certain pH values (4.5, 4.0 and 3.0), by adding H2O2 at different dosages. The highest removal ratio of Zn, Cu, Cd, Cr, Mn and Ni could reach 75.53%, 52.17%, 71.91%, 11.63%, 66.29% and 65.19% after combined bioleaching/Fenton-like process, respectively, with appropriate pH and H2O2 dosages in less than 6 days. The solubilization efficiencies of these heavy metals in mobile forms were further improved by Fenton-like treatment. The removal efficiencies of As and Pb decreased due to their transformation into insoluble forms (mostly residual fraction) after Fenton treatment. The capillary suction times (CST) of the raw sludge (98.7 s) decreased by 79.43% after bioleaching and 87.44% after combined process, respectively.

Extent of bioleaching and bioavailability reduction of potentially toxic heavy metals from sewage sludge through pH-controlled fermentation

Utilization of anaerobically stabilized sewage sludge on arable lands serve as a renewable alternative to chemical fertilizers as it enables recycling of valuable nutrients to food chain. However, probable presence of heavy metals in sewage sludge restricts the use of stabilized sludge on lands. In this study, a novel approach based on pH-controlled fermentation and anaerobic metal bioleaching was developed to reduce ecotoxicity potential of fermented sludge prior to its land application. Sewage sludge was subjected to pH-controlled fermentation process at acidic, neutral, and alkaline pH levels with the aim of increasing metal solubilization and decreasing bioavailable metal fractions through anaerobic bioleaching. Alkaline reactor performed the best among all reactors and resulted in 3-fold higher hydrolysis (34%) and 6-fold higher acidification (19%) efficiencies along with 43-fold (in average) higher metal solubilization than that of neutral pH reactor. As a result of alkaline fermentation, 32–57% of the metals remained as bioavailable and 34–59% of the metals were encapsulated as non-bioavailable within solid fraction of fermented sludge (biosolid), whereas 8–12% of total metal was solubilized into fermentation liquor. Our results reveal that anaerobic bioleaching through alkaline fermentation enables biosolid production with less metal content and low bioavailability, facilitating its utilization for agricultural purposes.

Heavy metal(loid)s in sewage sludge in China: concentrations and spatial-temporal variations.

Sewage sludge (SS) production in China has increased rapidly, accompanying the fast expansion of its sewage treatment capacity. Heavy metals (HMs) in SS have been a great concern, hampering the utilization and disposal of SS. In this study, heavy metal (HM) contents in SS from throughout China were determined. The median values of HMs in SS decreased in the order Zn > Cu > Cr > Pb > Ni > As > Hg > Cd. The general attainment rates of HMs in SS are satisfying (> 90%). Combining the present data with those obtained from references, spatial distributions and temporal trends of HMs in SS were analyzed. Depending on the specific HM element, the spatial variation trend might be decreasing trends from south to north and from east to west of China. The element-specific hot spots of SS with relatively high HM contents were identified. Analysis of the historical data in different time intervals reviewed obvious decreasing trends in HM contents of SS in China, indicating the well implementation of more and more stringent environmental regulations.

Stabilization of heavy metals in sewage sludge by attapulgite

ABSTRACT The mobility and bioavailability of heavy metals in sewage sludge are the major risks to utilize for agricultural application. In this study, the chemical speciation of Cu, Ni, Cr, and Zn in the Lanzhou municipal sewage sludge were investigated with the addition of a natural attapulgite. The influences of attapulgite amendment in sewage sludge on heavy metals stabilization were evaluated by investigating the leaching procedure and sequential chemical extraction experiments. The sequential extraction procedure described by European Community Bureau of Reference was used in sludge to determine the distribution of heavy metal species. The addition of attapulgite accelerated more reducible speciation of all metals transformed into residual speciation evidently. It reduced the leaching content of metals significantly and decreased the ecotoxicity accounted for the germination index values climbed rapidly with increased attapulgite addition. Implications: This research developed a method to stabilize heavy metals in municipal sewage sludge with clay. The activated attapulgite improved the treatment of sewage sludge containing heavy metals, and reduced the environmental risk of heavy metals.

Risk Assessment of Soil Contamination with Heavy Metals from Municipal Sewage Sludge

Sewage sludge (SS) is a by-product of processes conducted during the treatment of wastewater. It can be used in many different ways. One of them is the use of SS in agriculture as an organic fertiliser, but the main criterion for such use is the heavy metals (HMs) content. Knowledge of the total content of HMs in SS does not translate into the danger it may pose. The toxicity of metals is largely dependent on their mobility. The mobility of SS from three different wastewater treatment plants (WWTP) of the Świętokrzyskie Voivodeship, which were characterised by an increased zinc content, was examined in this study. The aim of the study was to prove whether the high level of zinc in SS actually disqualifies the possibility of its natural use. Calculations were made for five environmental hazard indicators: the geoaccumulation index of heavy metals in soil (Igeo), potential environmental risk indicator (PERI), risk assessment code (RAC), environmental risk factor (ERF), and the authors’ own environmental risk determinant (ERD) indicator. The obtained results show how important mobility analysis is when assessing the possibility of natural use of SS.

Effect of biochar addition on sludge aerobic composting and greenbelt utilization

Biochar (B) addition is considered to be an effective optimization method for accelerating the composting process and improving the quality of the final compost. Many studies have focused on the use of B in sludge compost as additives. However, the optimal level of B in the composting process and the effect of greenbelts utilization after composting have not been explained yet. In this study, autocontrol composting reactors were constructed and the effects of different B concentrations on nutrient transformation and morphological changes of heavy metals in sewage sludge–corn cob (C) aerobic composting were carried out. The feasibility of lawn grass ( Festuca arundinacea ) planting in soil composed of garden soil and different concentrations of composting sludge (40%, 60%, and 80%) was analyzed. The results revealed that a longer high temperature period can be maintained when the ratio of sludge:C:B is 10:6:1 (v/v). The detection result of TN, TP and TK indicated that B plays a positive role in nutrient retention in sludge. The addition of B has a positive effect on the passivation of Cu, Zn, Pb, Ni, and Cr in sludge. After composting, the residual fraction of heavy metals increased and exchangeable fraction decreased. The bacterial community diversity and richness showed dynamic changes during composting process. The enriched microbial groups of different treatments were analyzed. The linear discriminant analysis effect size (LEFSe) results showed that enriched microbial groups were totally different between different treatments and more bacterial clades were enriched at treatment S2. The heavy metal concentrations in the leached water were safe for plant and groundwater. The best growth conditions for F. arundinacea was the addition of composting sludge with a ratio of sludge:C:B of 10:6:1 (v/v) under an addition rate of 40%. • 10:6:1 (v/v) (Sludge: corn cob: biochar) is the best composting treatment. • Biochar has a positive effect on the passivation of heavy metals in sludge. • Bacterial community diversity and richness dynamically change during composting. • 40% of composting sludge addition is the best growth condition for F. arundinacea .