Assessment of mobility and bioavailability of mercury compounds in sewage sludge and composts

Understanding of mercury and methylmercury transformation in sludge composting by metagenomic analysis

Municipal sewage especially the produced sewage sludge is a significant source releasing mercury (Hg) to the environment. However, the Hg speciation especially methylmercury (MeHg) transformation in sewage sludge treatment process remains poorly understood. This study investigated the transformation of Hg speciation especially MeHg in sludge composting. The distribution of Hg transformation related gene pairs hgcAB and merAB, and their putative microbial hosts were comprehensively analyzed. Both Hg (from 3.16±0.22 mg/kg to 3.20±0.19 mg/kg) and MeHg content (from 4.77±0.64 ng/g to 4.36±0.37 ng/g) were not obviously changed before and after composting, but about 19.69% of Hg and 27.36% of MeHg were lost according to mass balance calculation. The metagenomic analysis further revealed that anaerobes (Desulfobacterota and Euryarchaeota) were the mainly putative Hg methylators especially carrying high abundance of hgcA gene in the initial periods of composting. Among the 151 reconstructed metagenome-assembled genomes (MAGs), only 4 hgcA gene carriers (Myxococcota, Firmicutes, Cyclobacteriaceae, and Methanothermobacter) and 16 merB gene carriers were identified. But almost all of the MAGs carried hgcB gene and merA gene. The merA gene was widely distributed in genomes, which indicated the widespread functionality of microbes for reducing Hg(II) to Hg(0). The hgcA carrying microbes tends to present the similar metabolic pathways including methanogenesis and sulfur metabolism. Besides, both the irregular distribution of hgcA in various species (including Actinobacteria, Archaea, Bacteroidetes, Desulfobacterota, Euryarchaeota, and Nitrospirae, etc.) and opposite evolution trends between hgcA gene abundance and its host genome abundance can be an indication of horizontal gene transfer or gene deletions of hgcA during composting. Our findings thus revealed that sludge composting is not only a hotspot for Hg speciation transformation, but also a potential hotspot for MeHg transformation.

Evaluation of the Use of Sewage Sludge Biochar as a Soil Amendment—A Review

In recent decades, minimization and recycling/reuse policies were introduced to reduce the quantities of generated waste and for alternative waste recovery. Organic wastes represent 46% of total global solid waste. Possible uses of organic wastes include using it as fertilizer and amendment for soil, for energy recovery and for the production of chemical substances. Sewage sludge disposal and reuse are identified as future problems concerning waste. The total amount of sludge generated in the entire world has increased dramatically, and this tendency is expected to increase significantly in the years to come. In most developed countries, special attention is given to sewage sludge treatment in order to improve the quality and safety of using it on the ground surface. Sewage sludge pyrolysis is considered an acceptable method, from an economic and ecological perspective, for the beneficial reuse of sewage sludge. This method has many advantages because, during the pyrolysis process, the sludge volume is reduced by 80%, pathogenic agents and hazardous compounds from sewage sludge are eliminated, metals are immobilized in solid residue and organic and inorganic fractions are immobilized in a stabilized form of pyrolytic residues (biochar). The biochar generated by sewage sludge pyrolysis does not contain pathogenic agents and is rich in carbon and nutrients.

Migration and Removal of Labile Cadmium Contaminants in Paddy Soils by Electrokinetic Remediation without Changing Soil pH

Electrokinetic remediation (EKR) is a viable, advanced cleaning strategy that can permanently reduce the toxicity of soil contaminants. However, EKR is prone to causing changes in soil pH. The negative impacts must be minimized if field-scale application is to be realized. In this study, EKR with polarity reversal was used to avoid soil pH polarization and to clean up cadmium (Cd)-contaminated paddy soils. Results showed that Cd desorbed from oxidizable and residual fractions to labile and easily available parts. Soil moisture content above 0.35 g g⁻¹ was conductive to achieving the desirable Cd-migration rate. The exchangeable Cd phase eventually migrated from both ends of that soil compartment towards the intermediate. Moreover, the addition of citric acid at the concentration of 0.1 mol L⁻¹ was an effective enhancement strategy. The methodology enriched Cd contaminants to specific sites. The technology can be used for electrokinetic-assisted phytoremediation during the rice growing period. Hyperaccumulator is planted in the intermediate area to remove the Cd contaminants. On the other hand, Cd removal is achieved in the region close to the electrodes. The present study provides a theoretical basis for in situ remediation. It has a wider significance for field-scale application.

Mercury emission profile for the torrefaction of sewage sludge at a full-scale plant and application of polymer sorbent

We evaluated mercury (Hg) behavior in a full-scale sewage sludge torrefaction plant with a capacity of 150 wet tons/day, which operates under a nitrogen atmosphere at a temperature range of 250-350 °C. Thermodynamic calculations and monitoring results show that elemental Hg (Hg⁰) was the dominant species in both the pyrolysis gas during the torrefaction stage and in the flue gas from downstream air pollution control devices. A wet scrubber (WS) effectively removed oxidized Hg from the flue gas and moved Hg to wastewater, and an electrostatic precipitator (ESP) removed significant particulate-bound Hg but showed a limited capacity for overall Hg removal. Hg bound to total suspended solids had a much higher concentration than that of dissolved Hg in wastewater. Total suspended solid removal from wastewater is therefore recommended to reduce Hg discharge. Existing air pollution control devices, which consist of a cyclone, WS, and ESP, are not sufficient for Hg removal due to the poor Hg⁰ removal performance of the WS and ESP; a further Hg⁰ removal unit is necessary. A commercial packed tower with sorbent polymer catalyst composite material was effective in removing Hg (83.3%) during sludge torrefaction.

Application of Technological Processes to Create a Unitary Model for Energy Recovery from Municipal Waste

The subject matter of this paper is the functioning of a highly effective waste management system. Assumptions of the Energy Recovery Waste Processing (ERWP) model, being a universal solution for towns and regions irrespective of their population, are presented here. The result of simulations illustrating the energetic potential of municipal waste stored and processed in biological and physicochemical processes are also presented. Calculations were performed for the municipality of Koszalin (Poland), with a population of 106,000. Mixed household and commercial waste, organic waste, waste from selective collection and sewage sludge from a municipal wastewater treatment plant were considered in the waste mass balance. Empirical equations and unit coefficients describing the energetic efficiency of particular processes originating from the author’s own research work as well as from the results available from the scientific literature were used in the calculations. The developed ERWP model is based on the functioning of four objects constituting a comprehensive technical infrastructure, i.e., biological stabilisation in air condition (BSAC), mechanical treatment plant (MTP), cogeneration system plant (CSP) and gas production plant (GPP) where two independent modules operate, namely, dry/wet methane fermentation (DMF and WMF). Each day, this system generates highly energetic refuse-derived fuel (RDF) for combustion in amounts of 82.2 t for CSP and 127.3 t for GPP, generating 5519 m3 of gas/d. The value of the energy contained in such generated gas and in waste making up an alternative fuel is 1027.4 GJ, which is equivalent to 285.4 MWh. It should be noted that the creation of a waste management system based on the ERWP model assumptions fulfills the criteria of energetic recycling and allows for recovery of energy in the form of gas and heat equivalent to 79,917.6 MWh/a, i.e., 754 kWh/inhabitant/a.

Investigations on Biogas Recovery from Anaerobic Digestion of Raw Sludge and Its Mixture with Agri-Food Wastes: Application to the Largest Industrial Estate in Oman

This work is intended to evaluate the technical, environmental, and economic feasibility of converting the sludge produced at an industrial estate’s wastewater treatment plant (WWTP) in Oman into energy through anaerobic digestion (AD). In this study, three different scenarios were analyzed. They concerned the digestion of the total amount of the produced sludge alone (240 m3 day−1) (scenario 1), and its co-digestion with wet agri-food wastes (AFW) at rates of two tonnes day−1 (scenario 2) and ten tonnes day−1 (scenario 3). Based on the analyses of sludge samples, an intensive literature review regarding sludge and AFW Physico-chemical and energetic characteristics and the use of the cost–benefit analysis (CBA) approach, it was found that, for the overall duration of the project (20 years), the AD of the sludge alone (scenario 1) permitted the production of 43.9 GWh of electricity, the reduction of greenhouse gas (GHG) emissions (more than 37,000 tonnes equivalent CO2 (TCO2)) and exhibited positive net present value (NPV: $393,483) and an internal return rate (IRR) of 19.4%. Co-digesting sludge with AFW significantly increased all of these key performance indicators. For instance, scenario 3 results in the recovery of electrical energy of 82.2 GWh and avoids the emission of 70,602 tCO2. Moreover, a higher NPV and IRR of $851,876 and 21.8%, respectively, and a payback period (PBP) of only seven years were calculated. The sensitivity analysis revealed that a decrease in total expenses by 15% results in a significant increase of the NPV and the IRR to $1,418,704 and 33.9%, respectively, for scenario 3. Considering a pessimistic assumption (an increase of the total expenses by 15%), all studied scenarios remain attractive. For instance, for scenario 3, the NPV, IRR, and PBP were evaluated to $285,047, 13.5%, and 9 years, respectively. Therefore, the co-digestion of sludge with agri-food wastes for energy recovery purposes could be considered a promising, eco-friendly, and economically viable approach in the Omani industrial estates.

Fate of mercury and methylmercury in full-scale sludge anaerobic digestion combined with thermal hydrolysis

Methylmercury (MeHg) is one of the highly toxic and bio-accumulated forms of mercury. Its presence in wastewater treatment processes has been evidenced in recent studies. Considering its enrichment in sewage sludge and the ecological risk associated with its land application, this study investigated the fate of mercury and MeHg in full-scale anaerobic digestion combined with Cambi thermal hydrolysis based on one-year sampling. Results showed that the advanced anaerobic digestion could increase the total mercury (THg) content from 4.35 ± 0.43 mg/kg in raw sludge to 6.37 ± 1.05 mg/kg in digested sludge, and the MeHg content decreased from 1.61 to 8.94 ng/g in raw sludge to 0.21-2.03 ng/g after anaerobic digestion. The demethylation of MeHg was dominant in both thermal hydrolysis and anaerobic digestion; it was mostly derived from the physico-chemical impacts such as chemical decomposition in thermal hydrolysis and precipitation in anaerobic digestion. Although the reported microbial methylators, such as Methanosarcina and Clostridia, were dominant in anaerobic digestion, the relative abundances of hgcA and merA were relatively low and did not correlate with the MeHg profiles. Thus, microbial methylation or demethylation seems negligible in terms of MeHg transformation.

Passivation of lead and cadmium and increase of the nutrient content during sewage sludge composting by phosphate amendments

As an efficient and cost-effective biological treatment method for sewage sludge, composting has been widely used worldwide. To passivate heavy metals and enhance the nutrient content in compost, in the present study, phosphate rock, calcium magnesium phosphate, and monopotassium phosphate were added to the composting substrate. According to the Community Bureau of Reference sequential extraction procedure, phosphate rock and monopotassium phosphate amendments exhibit a good passivation effect on Cd and Pb. The X-ray diffraction patterns proved the formation of Pb₃(PO₄)₂ and Cd₅(PO₄)₂SiO₄ crystals, and X-ray absorption near-edge structure spectroscopy illustrated the change in P speciation after phosphate amendment. Furthermore, phosphate amendment increased the contents of total P and available P, and it reduced the loss of N during sewage sludge composting. The germination index showed that the target phosphate amendments in sewage sludge compost had no negative effects on seed germination, and this method has great potential to be used as a soil amendment.

Hazards assessment of the intake of trace metals by common mallow (Malva parviflora K.) growing in polluted soils

Human risks increase by consuming plants growing in trace/heavy metals contaminated soil irrigated with polluted water. The present study aims to assess the ability of common mallow to accumulate trace/heavy metals from polluted soils at South Greater Cairo, Egypt; and their hazardous effects on consumer's health. Five quadrats were used to collect soil and plant samples from three sites of un-polluted and polluted fields for chemical analysis and measurement of growth variables, as well as for assessing the daily intake of metals (DIM) and hazard quotient (HQ). Irrigation water analysis showed elevated concentrations of the investigated metals and mineral salts in the polluted area compared with the un-polluted one. Plant samples showed reduction in their growth parameters; as well as pigments and nutrient content in the metal-contaminated soil. In addition, Pb, Cd, Cu, Ni, Fe, Mn, Zn, and Co concentrations in the shoots and roots of plants grown in polluted fields were higher compared with plants grown in un-polluted site. The bioaccumulation and translocation factors of most investigated metals indicated the great ability of common mallow to accumulate such metals, which would increase the human intake of metals in their daily diets compared to their reference values (R_fD). The hazard quotient (HQ) of Pb, Cr, Fe, Mn and Zn for children and Pb, Cd, Fe and Mn for adults was >1, indicating health hazards for the consumers of common mallow cultivated in the polluted area.

Conventional and novel techniques for the determination of Hg uptake by lettuce in amended agricultural peri-urban soils

Peri-urban agriculture provides environmental benefits to the nearby urban areas. However, domestic and industrial infrastructures can be sources of pollution that can affect agricultural production. In this work, the diffusive gradient in thin film (DGT) technique was used to assess the bioavailability of mercury (Hg) in organic-amended agricultural soils, and uptake by lettuce. Two different amendments were studied individually in three different sets using a wood-based biochar at two rates (3% and 6%, w/w), and compost at one rate (30% w/w). The effect of the amendments on Hg bioavailability, mobility and uptake was investigated by means of both DGT analyses and accumulation of Hg by lettuce. DGT manufactured in-house devices with polyacrylamide gel using both open and restricted diffusive layers (ODL and RDL, respectively) were used to determine organic and inorganic Hg labile species in soils, respectively. The Hg concentration in lettuce leaves and roots were analyzed and compared with DGT measurements to predict the uptake of Hg from the different organic-amended soils and the non-amended soils. Results show that the application of biochar reduces the bioavailability of Hg in soil and, in consequence, the Hg uptake by lettuce. Inorganic Hg species were predominant in all the different sets of the experiment (62-97%), although the addition of the different amendments reduced the free ionic species in soil.

Regression models for monitoring trace metal accumulations by Faba sativa Bernh. plants grown in soils amended with different rates of sewage sludge

The present study was conducted using a pot experiment to develop regression models for the prediction of trace metal concentrations in faba bean (Faba sativa Bernh.) plants cultivated in soils amended with different rates of sewage sludge to monitor possible human health risks. The trace metal concentrations in the different tissues of faba bean showed that most of the investigated trace metals were accumulated in the plant roots rather than in the other tissues. Meanwhile, the fruits accumulated the lowest concentration of most trace metals. The trace metal concentrations of the faba bean plants had a significant positive correlation with the organic matter content and a significant negative correlation with the soil pH. Transfer of trace metals from the soil to faba bean roots indicated that Al, Cu, Pb and Zn had a transfer factor that exceeded one, whereas the TF of the investigated trace metals from the roots to the fruits did not exceed one. The daily intake rate of the investigated trace metals did not exceed one in both adults and children. On the other side, the hazard quotient of trace metals from consuming faba bean fruits had values <1 for most investigated trace metals except Al and Mn in adults and in children. It is worth mentioning that the predicted trace metal concentrations via the established regression models and measured values from the validation data set were not significantly different (P > 0.05). Therefore, these developed models will be useful for prediction of trace metals uptake by faba bean grown in soil amended with sewage sludge so possible human risks can be identified.

Prediction models for evaluating the heavy metal uptake by spinach (Spinacia oleracea L.) from soil amended with sewage sludge

The risk evaluation of polluted soil requires the application of precise models to predict the heavy metal uptake by plants so possible human risks can be identified. Therefore, the present work was conducted to develop regression models for predicting the concentrations of heavy metals in spinach plants from their concentration in the soil by using the organic matter content and soil pH as co-factors. The soil improved with sewage sludge was slightly alkaline and had a relatively high organic matter content. Similar to the soil analysis, Fe had the highest median concentration, while Cd had the lowest concentration in the roots and leaves. Heavy metals accumulated in the roots and leaves in the order Fe > Mn > Zn > Cu > Cr > Ni > Co > Pb > Cd. The bio-concentration factor of the investigated heavy metals, from soil to roots, did not exceed one. The spinach was recognized by a translocation factor <1.0 for all of the heavy metals except Zn. Plant heavy metal concentrations were positively correlated with the soil organic matter content and negatively correlated with soil pH. The leaf Cr, Fe and Zn and the root Cr, Fe, Pb and Zn concentrations were positively correlated with the respective soil heavy metals. In addition, a linear correlation was found between the bio-concentration factor of heavy metals and soil pH and organic matter content. Regression models with high model efficiency and coefficients of determination and low mean normalized average errors, which indicate the efficiency of the models, were produced for predicting the plant heavy metal contents by using the soil pH and organic matter content as co-factors.

Nitrogen loss reduction by adding KH2PO4-K2HPO4 buffer solution during composting of sewage sludge

Nitrogen loss through gaseous emission, mainly ammonia emission, was an inevitable problem during sewage sludge composting. In this study, MgSO₄ + K₃PO₄ (Run A), K₂SO₄ + KH₂PO₄-K₂HPO₄ (Run B) and MgSO₄ + KH₂PO₄-K₂HPO₄ (Run C) were mixed with mixtures before composting, aiming at researching the effects of buffer solution on reducing nitrogen loss during composting. Ammonia loss of Run C was reduced by 53.8% and 45.5%, and nitrogen loss of Run C was decreased by 61.2% and 67.1%, compared to that of Run A and Run B, respectively. Besides, organic matter degradation of Run C was 36.8%. Among the three amended treatments, nitrogen loss in Run C was effectively reduced and organic matter degradation was slightly improved. The addition of MgSO₄ and KH₂PO₄-K₂HPO₄ was confirmed to be effective to maintain a desired pH range for struvite precipitation as well as to reserve more ammonia in the compost to promote the formation of struvite.