Achilles heel of environmental risk from recycling of sludge to soil as amendment: A summary in recent ten years (2007-2016)

Effect and potential mechanisms of sludge-derived chromium, nickel, and lead on soil nitrification: Implications for sustainable land utilization of digested sludge

Increasing occurrence of heavy metals (HMs) in sewage sludge threatens its widespread land utilization in China due to its potential impact on nutrient cycling in soil, requiring a better understanding of HM-induced impacts on nitrification. Herein, lab-scale experiments were conducted over 185-day, evaluating the effect of sludge-derived chromium (Cr3+), nickel (Ni2+), and lead (Pb2+) on soil nitrification at different concentrations. Quantitative polymerase chain reaction and linear regression results revealed an inhibitory sequence of gene abundance by HMs' labile fraction: ammonia-oxidizing bacteria (AOB)-ammonia monooxygenase (amoA)> nitrite oxidoreductase subunit alpha (nxrA)> nitrite oxidoreductase subunit beta (nxrB). The toxicity of HMs' incremental labile fraction decreased in the order of Ni2+>Cr3+>Pb2+, with respective threshold values of 5.01, 24.03 and 38.42 mg·kg-1. Furthermore, extending incubation time reduced HMs inhibition on ammonia oxidation, mainly related to their fraction bound to carbonate minerals. Random Forest analysis, variation partitioning analysis, and Mantel test indicated that soil physicochemical properties primarily affected nitrification genes, especially in the test of Cr3+ on AOB-amoA, nxrA, nxrB, Ni2+ for complete ammonia-oxidizing bacteria-amoA, and Pb2+ for nxrA and nxrB. These findings underline the importance of labile HMs fractions and soil physicochemical properties to nitrification, guiding the establishment of HM control standards for sludge utilization.

Occurrence and distribution of microplastics in long-term biosolid-applied rehabilitation land: An overlooked pathway for microplastic entry into terrestrial ecosystems in Australia

Wastewater treatment plants (WWTPs) efficiently eliminate over 98% of microplastics (MPs) from wastewater discharge, subsequently accumulating them in sludge. This sludge is frequently employed as fertilizer in agricultural practices or land rehabilitation. While there is significant research on biosolid application in agriculture, the discussion regarding its application in rehabilitating industrial zones and MPs contamination is limited. The current study investigates the abundance, distribution, and composition of MPs in rehabilitation land with long-term biosolid-application in Australia. Three minesite fields (designated 1-3), each with distinct biosolid application histories since 2011, 2012, and 2017, and a control field without any biosolid application history, were chosen for this study. The abundances of MPs in biosolid-applied fields 1-3 (6.04 ± 1.92 x 102 MP kg-1; 4.94 ± 0.73 x 102 MP kg-1; 2.48 ± 0.70 x 102 MP kg-1) were considerably higher compared to non-biosolid-applied field (0.70 ± 0.63 x 102 MP kg -1). This indicates that the application of biosolids significantly contributes to the presence of MPs in the soil. Moreover, the results suggest that with each successive application, the abundance of MPs increases. The abundance and size of MPs in both biosolid and non-biosolid soils decreased as the soil depth increased. Microbeads were dominant in soils where biosolids were applied (up to 61.9%), while fibres were dominant in non-biosolid soils (accounting for 85.7%). The distribution of plastic polymer types varied among fields and soil depths. Most MPs were microbeads of polyamide (PA), fragments of polyethylene (PE), foam of polystyrene (PS), and fibres of rayon. This research presents evidence that the extended utilization of biosolids results in elevated MP pollution in minesite rehabilitation land, highlighting a frequently overlooked origin of MP contamination in terrestrial settings. Additional evaluations needed to understand ecological risks of MPs in soil ecosystems affected by biosolid application.

Biochar alleviating heavy metals phytotoxicity in sludge-amended soil varies with plant adaptability

Recycling sewage sludge (SS) to soil potentially causes soil heavy metal (HM) pollution and plant phytotoxicity. Biochar plays an important role in alleviating HM phytotoxicity, and responses vary with the feedstocks and usage of biochar. However, the effect of plant adaptability on biochar-mediated alleviation is poorly understood. Here, SS-derived biochar (SB) and rice straw-derived biochar (RB) applied at rates of 1.5% and 3% (W/W, SB1.5, SB3, RB1.5, and RB3) were used to improve the properties of soil amended with SS at 50% (W/W). Alleviation of phytotoxicity by biochar was further analyzed with SS-sensitive plant Monstera deliciosa and SS-resistant plant Ruellia simplex. Results revealed that both SB and RB significantly decreased the soil's bulk density and increased water retention. They also changed soil organic matter content and HMs fractionation. The addition of SB or RB alleviated the SS phytotoxicity, and they significantly promoted the growth and the root morphology and physiological index of M. deliciosa. But for R. simplex, these significant changes only synchronously occurred in SB3 treatment. The alleviation in M. deliciosa was more prominent and more closely connected with soil property changes than in R. simplex. Also, more soil property predictors were observed to play an important role in M. deliciosa growth than in R. simplex growth. These results indicated that biochar alleviating HMs phytotoxicity in SS-amended soil is associated with the changes of soil property. Moreover, the alleviation varies more prominently with plant adaptability than with biochar feedstocks and usage.

Evaluation of GLDA-acid on sludge treatment effect and seed germination analysis

Mixtures of N, N-bis(carboxymethyl)-L-glutamic acid tetrasodium salt (GLDA) with citric acid (CA), glutamic acid (GLU), and aspartic acid (ASP) at the optimal proportion of 1:1, 1:2, and 2:1, respectively. They were employed for heavy metal removal from the sludge. The removal rate of common heavy metals (Cu, Pb, Zn, Ni, Cr, and Cd) and the retention degree of nutrients (total nitrogen, total phosphorus, available-N, Olsen-P, and organic matter) in the treated sludge were analyzed. Fuzzy comprehensive evaluation of the sludge was performed using MATLAB to determine the agricultural grade of the sludge. The sludge after GLDA-acid treatment was mixed with soil at different proportions, and Chinese cabbage, cucumber, and wheat were cultured. SPSS was used for survival analysis to analyze the feasibility of the sludge agriculture. The results showed that the optimal ratio of GLDA-CA and GLDA-GLU was 1:2 and that of GLDA-ASP was 1:1. After GLDA-acid treatment, the sludge was classified as Grade A agricultural sludge based on MATLAB fuzzy comprehensive evaluation and analysis. When the amount of sludge added was 20%, the growth of Chinese cabbage, cucumber, and wheat was promoted. Survival analysis further proved that the amount of sludge only affected the median germination time. Without considering the economic benefits, GLDA-acid can be preferred for sludge treatment, which can not only effectively remove heavy metals in sludge, but also have a small impact on agricultural use.

Feasibility of Biochar Derived from Sewage Sludge to Promote Sustainable Agriculture and Mitigate GHG Emissions-A Review

Sewage sludge (SS) has been connected to a variety of global environmental problems. Assessing the risk of various disposal techniques can be quite useful in recommending appropriate management. The preparation of sewage sludge biochar (SSB) and its impacts on soil characteristics, plant health, nutrient leaching, and greenhouse gas emissions (GHGs) are critically reviewed in this study. Comparing the features of SSB obtained at various pyrolysis temperatures revealed changes in its elemental content. Lower hydrogen/carbon ratios in SSB generated at higher pyrolysis temperatures point to the existence of more aromatic carbon molecules. Additionally, the preparation of SSB has an increased ash content, a lower yield, and a higher surface area as a result of the rise in pyrolysis temperature. The worldwide potential of SS output and CO₂-equivalent emissions in 2050 were predicted as factors of global population and common disposal management in order to create a futuristic strategy and cope with the quantity of abundant global SS. According to estimations, the worldwide SS output and associated CO₂-eq emissions were around 115 million tons dry solid (Mt DS) and 14,139 teragrams (Tg), respectively, in 2020. This quantity will rise to about 138 Mt DS sewage sludge and 16985 Tg CO₂-eq emissions in 2050, a 20% increase. In this regard, developing and populous countries may support economic growth by utilizing low-cost methods for producing biochar and employing it in local agriculture. To completely comprehend the benefits and drawbacks of SSB as a soil supplement, further study on long-term field applications of SSB is required.

Bacterial agents affected bacterial community structure to mitigate greenhouse gas emissions during sewage sludge composting

The present work studied the influence of bacterial agents (B1, B2) and bamboo biochar (BB) on greenhouse gas emissions and bacterial community during the sewage sludge composting. Results showed that compared with CK, the total methane emissions ofC, B1, B1C, B2, and B2C treatments declined by 16.4%, 25.2%, 45.4%, 7.8%, and 44.4%, respectively. The total N₂O emissions ofC and B1C treatments declined by 5.1% and 3.7% while B1, B2, and B2C treatments increased the total N₂O emissions by 6.7%, 21.6%, and 10.4%, respectively. These results illustrated that the addition of BB is conducive for reducing greenhouse gas emissions while different bacterial agents have various effects. According to pearson correlation analysis, N₂O emissions and Acidimicrobiia, Alphaproteobacteria, Gammaproteobacteria, and Tepidiformia have strong negative correlation while positive correlation with Bacilli and Clostridia. Methane emissions have a strong negative correlation with Actinobacteria. CO₂ emissions have a strong positive correlation with Bacilli.

Leaching characteristic of potentially toxic metals of artificial soil made from municipal sludge compost

Landscaping of municipal sludge is a good choice to solve the sludge disposal problem, and EDTA treatment can effectively promote the uptake of heavy metals (HMs) by plants, but the heavy metal leaching process and its main control factors are still poorly understood during the sludge landscaping disposal. In this study, the migration behavior of HMs in artificial soil made from municipal sludge compost (MSC) were investigated using soil column experiments. After six leaching events for a total of one year's rainfall, the average reduction percentage of total phosphorus, total nitrogen, organic matter in the MSC artificial soil were 13.4%, 10.1%, and 7.8%, respectively, while those of copper, lead, zinc, cadmium, nickel were 12.9%, 8.37%, 11.5%, 5.94%, and 10.7%, respectively. Treating ethylenediaminetetraacetic acid (EDTA) to the MSC artificial soil further enhanced the leach index of HMs to different degrees. HM concentration in leachate were increased with peak times postponed. Though EDTA treatment increased HM concentrations in subsoil, it did not change their water washing efficiency. The retention of HMs in subsoil was related to properties of local soil and its interaction with leachate. The potential ecological risk was of quite strong risk category in the MSC artificial soil and rapidly decreased from moderate to slight risk in subsoil. Cadmium was the main contribution accounting for 46%-93% of ecological risk. For landscaping applications, the composition of MSC artificial soil and local soil, as well as the capacity of the surrounding water, needs to be considered.

Effects of humic acid on sludge performance, antibiotics resistance genes propagation and functional genes expression during Cu(II)-containing wastewater treatment via metagenomics analysis

The humic acid (HA) function on the sludge performance, antibiotics resistance genes (ARGs) propagation and functional genes expression during Cu(II)-containing wastewater treatment was comprehensively investigated via metagenomics analysis. Results showed that the pollutants removal was significantly inhibited after long-term exposure of 5 mg/L Cu(II), while the inhibitory effects were moderately alleviated after addition of 10 mg/L HA. The extracellular polymeric substances (EPS) production with Cu(II) acclimation was higher than the sludge with Cu(II) and HA acclimation. The microbial community was significantly affected by the HA addition, while the relative abundance of dominant ARGs had no distinct differences with or without HA addition under Cu(II) stress. The functional genes were largely implemented for microbial metabolism, while no significant differences were found with HA addition under Cu(II) stress. Thus, the HA function for ARGs propagation and functional genes expression needed to be further research under Cu(II) stress in wastewater treatment.

Optimal Solutions for the Use of Sewage Sludge on Agricultural Lands

The use of sewage sludge in agriculture decreases the pressure on landfills. In Romania, massive investments have been made in wastewater treatment stations, which have resulted in the accumulation of important quantities of sewage sludge. The presence of these sewage sludges coincides with large areas of degraded agricultural land. The aim of the present article is to identify the best technological combinations meant to solve these problems simultaneously. Adapting the quality and parameters of the sludge to the specificity of the land solves the possible compatibility problems, thus reducing the impact on the environment. The physico-chemical characteristics of the fermented sludge were monitored and optimal solutions for their treatment were suggested so as to allow that the sludge could be used in agriculture according to the characteristics of the soils. The content of heavy metals in the sewage sludge was closely monitored because the use of sewage sludge as a fertilizer does not allow for any increases in the concentrations of these in soils. The article identifies those agricultural areas which are suitable for the use of sludge, as well as ways of correcting some parameters (e.g., pH), which allow the improvement of soil quality and obtained higher agricultural production.

Review of Sewage Sludge as a Soil Amendment in Relation to Current International Guidelines: A Heavy Metal Perspective

Overexploitation of resources makes the reutilization of waste a focal topic of modern society, and the question of the kind of wastes that can be used is continuously raised. Sewage sludge (SS) is derived from the wastewater treatment plants, considered important underused biomass, and can be used as a biofertilizer when properly stabilized due to the high content of inorganic matter, nitrate, and phosphorus. However, a wide range of pollutants can be present in these biosolids, limiting or prohibiting their use as biofertilizer, depending on the type and origin of industrial waste and household products. Long-term applications of these biosolids could substantially increase the concentration of contaminants, causing detrimental effects on the environment and induce hyperaccumulation or phytotoxicity in the produced crops. In this work, some critical parameters for soils and SS agronomic use, such as organic matter, nitrogen, phosphorous, and potassium (NPK), and heavy metals concentration have been reviewed. Several cases of food crop production and the accumulation of heavy metals after SS application are also discussed. SS production, usage, and legislation in EU are assessed to determine the possibility of sustainable management of this bioresource. Additionally, the World Health Organization (WHO) and Food and Agriculture Organization (FAO) guidelines are addressed. The opportunity to produce bioenergy crops, employing sewage sludge to enhance degraded land, is also considered, due to energy security. Although there are numerous advantages of sewage sludge, proper screening for heavy metals in all the variants (biosolids, soil, food products) is a must. SS application requires appropriate strict guidelines with appropriate regulatory oversight to control contamination of agricultural soils.

Diversified effects of co-planting landscape plants on heavy metals pollution remediation in urban soil amended with sewage sludge

Recycling sewage sludge (SS) as a soil amendment potentially causes soil heavy metals (HMs) contamination. This study investigated the potential roles of landscape plants co-planting in SS-amended soil remediation. Three landscape trees Mangifera persiciforma, Bischofia javanica, and Neolamarckia cadamba (NC), and three ground cover plants Dianella ensifolia, Syngonium podophyllum, and Schefflera odorata (SO) were selected for the tree-ground cover co-planting. Species in different co-planting treatments exhibited diversified effects on the growth, root morphology, HMs uptake, and HMs accumulation. Five plant characteristics including total root length, total surface of roots (diameter <2 mm), specific root length, shoot dry weight and root dry weight played crucial roles in plant HMs uptake. Structural equation modeling analysis revealed that different co-planting treatments drive species to develop an active, passive, or avoidance strategy to accumulate HMs, resulting in a diversity of HMs removal efficiency. Co-planting of NC with SO promoted NC and SO HMs accumulation and resulted in the greatest HMs contents decline (48.0% for Cd, 24.9% for Cu, 33.8% for Zn, and 27.2% for Ni) and the lowest potential ecological risk. Co-planting of landscape tree and ground cover plants with an active strategy can be a potential candidate for HMs phytoremediation of SS-amended soil.

Micro- and mesoplastics release from the Indonesian municipal solid waste landfill leachate to the aquatic environment: Case study in Galuga Landfill Area, Indonesia

Small-sized plastic debris (micro- and mesoplastics) are emerging pollutants and widely detected in aquatic environments. However, micro- and mesoplastics pollution research with regard to landfills is limited. In this study, the occurrence, characteristics, and possible release of micro- and mesoplastic waste from the Galuga landfill leachate to the aquatic environment were studied. Micro- and mesoplastics were identified in all surface water samples from leachate influent and effluent of Galuga landfills. The average daily release to the aquatic environment was estimated at 80,640 ± 604.80 microplastic and 618,240 ± 1905.45 mesoplastic particles, respectively. The amount of microplastic increased three-fold and nine-fold for mesoplastics after input from the leachate drain. Micro- and mesoplastic main chemical compositions were polyethylene, polypropylene, polystyrene, polyethylene terephthalate, polyester, and cellophane. This study implied that the leachate may cause micro and mesoplastic contamination to the aquatic environment. The results raised the knowledge of small-sized plastic debris in aquatic environments.

Source, occurrence, migration and potential environmental risk of microplastics in sewage sludge and during sludge amendment to soil

Microplastics (MPs) are an emerging global pollutant. MPs research is mainly concentrated on water, with limited research on MPs in sewage sludge. MPs from various sources are collected into sewage and most of the MPs are trapped in the sludge during the sewage treatment process. Sludge is not only a sink of MPs, but also a source. Soil amendment with sludge provides nutrients into the soil, but it can also import substantial MPs into the soil, which has certain environmental risks. Therefore, we focused on the MPs in sludge and sludge-amended soil and conducted a literature review to summarize the sources, physical properties and fate of the MPs in sludge, as well as their separation, identification and statistical methods. MPs can accumulate in the soil, influence the properties of the soil, and also migrate, which might result in the pollution of deep soils and groundwater. In addition, the adsorption by MPs of heavy metals, organic pollutants, antibiotics and antibiotic resistance genes cannot be ignored as sewage sludge generally contains substantial concentrations of these pollutants. They can be adsorbed by the MPs and transferred into the soil with sludge amendment of soil. The combination and interaction of MPs with its adsorbed pollutants might increase environmental risk, further leading to possibility of them being uptaken by plants. The specific long-term risks to the environment caused by MPs in soil with sludge amendment require further exploration and investigation.

Salinity derived from sludge compost amendment is a crucial influencing factor of qualitative performance of sports-field turf

Treated sewage sludge (or biosolid) is a suitable alternative substrate for use in turf production, but the potential of this resource remains to be explored in higher value-added field. In this study, the negative effects of sludge compost and biochar-amended sludge compost amendment on the qualitative performance of sports-field turf, including vegetation stolon index, grade for turf, and sports performance index, were investigated. It was found that sludge compost and biochar-amended sludge compost amendment induced a significant increase in organic matter, total nitrogen, total potassium, and electrical conductivity. In addition, biochar-amended sludge compost also resulted in a significant increase in turf chlorophyll content and the rate of turf weed emergence. Interestingly, electrical conductivity explained 55.4% of the qualitative changes, and electrical conductivity and pH together explained 68% of the qualitative changes. In summary, salinity was the main factor responsible for the negative effect of sludge compost amendment on qualitative performance of sports-field turf.

Removal of heavy metals in sludge via joint EDTA-acid treatment: Effects on seed germination

Heavy metals (Cu, Pb, Zn, Ni, Cr, and Cd) were removed from sludge via joint treatment with ethylene diamine tetraacetic acid (EDTA) and three organic acids (citric acid, glutamic acid, or aspartic acid) at optimal EDTA-acid concentration ratios of 1:1, 1:2 and 2:1, respectively. Heavy metal removal rates and post-treatment nutrient retention in sludge was then analyzed. The effects of different proportions sludge and soil mixes on the germination of Chinese white cabbage (Brassica campestris L. ssp. Chinensis Makino) seeds was then studied, and the mechanism by which nutrient content in the soil/sludge mixture affects seed germination was explored. The results indicated that the removal rate of the heavy metals decreased in the order of Zn > Ni > Cd > Pb > Cu > Cr, when EDTA was used in conjunction with citric acid and glutamic acid. In contrast, when EDTA was combined with aspartic acid, the removal rate of the heavy metals decreased in the order of Ni > Zn > Cd > Cu > Pb > Cr. Regarding the effect of heavy metal removal and sludge nutrient retention, EDTA-citric acid and EDTA-aspartic acid treatment had optimum results at a 1:1 ratio, while EDTA-glutamic acid treatment was optimum at a 1:2 ratio. At an optimum sludge to soil ratio of 1:4, the germination and root elongation inhibition rate of Chinese white cabbage seeds could be promoted, and the sludge could meet standard agricultural requirements. SPSS correlation analysis demonstrated that the seed germination index and nutrient content in sludge/soil mixtures were significantly correlated, demonstrating the feasibility of sludge for agricultural purposes.

Soil influences on uptake and transfer of pharmaceuticals from sewage sludge amended soils to spinach

Sewage sludge from wastewater treatment plants, which may contain various contaminants including pharmaceuticals, is often used as a soil amendment. These contaminants may subsequently be taken up by plants. In the present study we examined uptake of select pharmaceuticals from sewage sludge applied to soils by spinach plants. Seven soils were amended with sewage sludge from two wastewater treatment plants (A and B). Concentrations of compounds in plant tissues (roots and leaves) of spinach planted 45 days in these soils under greenhouse conditions were evaluated after harvest. The largest bioaccumulation in the roots and leaves was observed for sertraline (bioaccumulation factors (BAF) of 3.3-37.9 and 1-13.4, respectively), tramadol (1.3-10.0 and 4.8-30.0), and carbamazepine (2.2-17.2 and 6.1-48.8) and its metabolite carbamazepine 10,11-epoxide (not-quantified to 7.3 and 9.3-96.7). Elevated bioaccumulation in spinach roots was also identified for telmisartan (3.0-20.3) and miconazole (4.3-15.1), and leaves for metoprolol acid (not-quantified to 24.3). BAF values resulting from application of sludge B were similar to or moderately higher than BAFs from sludge A. The BAF values of carbamazepine and carbamazepine 10,11-epoxide in all tissues were negatively correlated with soil cation exchange capacity (CEC). This negative correlation between BAF and CEC was also observed for tramadol (A-roots and B-leaves), citalopram (B-roots), and telmisartan (B-roots) or between BAF and clay content for metoprolol acid (A-leaves and B-roots), tramadol (B-roots and A-leaves) and venlafaxine (B-roots). However, in the case of some other compounds (i.e. sertraline, amitriptyline, mirtazapine, metoprolol), uptake and the subsequent translocation and transformation from 3 soils of a higher pH and base cation saturation (Stagnic Chernozem Siltic, Haplic Chernozem and Greyic Phaeozem) significantly differed from 4 soils with a lower pH and base cation saturation (Haplic Luvisol, Haplic Cambisol, Dystric Cambisol and Arenosol Epieutric). Such observations proved strong compound dependent influences of soil conditions on various compounds bioaccumulations in plants and necessity of studying these processes always in diverse soils.

Assessment of the methodologies used in microbiological control of sewage sludge

Sewage sludge usually contains potentially polluting substances such as heavy metals, organic pollutants and various organisms including bacteria, protozoa, helminths, viruses and algae, some of which may be pathogenic. Certain of these pathogens could be transferred to the soil if the sludge is used on agricultural or land recovery applications. For its application on agricultural land, sewage sludge must comply with the limits established in the legislation, which in Europe does not include quality standards regarding microbiological parameters. Nevertheless, the presence of pathogens could limit its agricultural use, as it could pose a risk to human, animal and environmental health. This study compares 4 different methodologies used in microbiological analysis in order to identify the most efficient and reliable method on determining bacteria in sewage sludge. Escherichia coli and Enterococcus faecium are used as bacterial indicators. The results obtained in this work indicate that results obtained with three different plate count methods cannot be comparable with those obtained with the MPN method. The membrane filtration method is recommended for its high precision and sensitivity, both in low and high bacterial loads. It is also concluded that it would be necessary to establish the quality standard in concordance with the method used.

Humic substances developed during organic waste composting: Formation mechanisms, structural properties, and agronomic functions

Aerobic composting is a typical biochemical process of stabilization and harmlessness of organic wastes during which organic matter degrades, and then aggregates, to produce humic substances (HSs). HSs are a core product of-and a crucial indicator of-the maturation of compost that can be used in soil amendments. The formation of HSs is affected by the characteristics of the raw materials involved, the presence of compost additives, microbial activity, temperature, pH, the C/N ratio, moisture content, oxygen content and particle size, all of which can interact with each other. The formation of HSs is therefore complex. Moreover, it is difficult to identify definitive structures of humic acids (HAs) and fulvic acids (FAs), which are the two major components of HSs. However, HSs represent the same functional groups and structural arrangements, which helps to predict their structures. Functional groups represented by phenol and carboxylic acid groups of HAs and FAs can provide various agronomic functions, such as plant growth enhancement, water and nutrient retention, and disease suppression capacity. Overall, HSs can act as a soil amendment, fertilizer, and plant growth regulator. These functions of HSs enhance the reuse potential of organic waste compost products; however, this requires scientific control of various composting parameters and appropriate application of final products.

Hydroxyapatite reduces potential Cadmium risk by amendment of sludge compost to turf-grass grown soil in a consecutive two-year study

Recycling of sludge compost to soil as conditioner is generally regarded as the best means of disposal. However, concerns regarding heavy metal residues and sludge toxicity have recently received increasing public attention. Cadmium (Cd) is a mobile metal commonly found in sludge; therefore, the risk posed by Cd contaminated sludge should be carefully assessed. In this report, the effects of addition of hydroxyapatite (HAP) with sludge compost amendment on potential Cd risk were investigated. The results of consecutive two years showed that exchangeable Cd content in treatment of sludge compost with 1.5% HAP decreased by 6.0% compared with single sludge compost treatment, and residual Cd increased by 7.6%. Compared with single sludge compost, the incremental rate of exchangeable Cd dropped by 38.3% and the reductive rate of residual Cd increased by 37.7% in response to 1.5% HAP addition, indicating that HAP played a role of decreasing Cd phytoavailability. The HAP reduced the amount of Cd uptaken by turf-grass in both root and leaf. Moreover, HAP remarkably improved the quality of turf grass grown in amended soil, including leaf greenness, green maintainable period and root strength. However, HAP did not attenuate the downward mobility of Cd. Taken these together, these findings indicated that HAP can be used as a potential candidate to control surface Cd risk of sludge compost amended soil rather than that from leachate.

Effects of 4-chlorophenol toxicity on sludge performance and microbial community in sequencing batch reactors

In this study, the effects of 4-chlorophenol (4-CP) influent concentrations ranging from 10 to 100 mg L^-1 on sludge toxicity, enzymatic activity and microbial community, along with their correlations, were investigated in a sequencing batch bioreactor (SBR), which was defined as the acclimated SBR. Another SBR was set as a control group that did not receive the influent 4-CP. The results showed that the sludge toxicity increased as the influent 4-CP increased, exhibiting a positive correlation with 4-CP loads. The enzymatic activity was stimulated after long-term acclimation with 4-CP and was positively related to the 4-CP loads and sludge toxicity. During the stable operational stages of the acclimated SBR, the microbial diversity first increased and then decreased as the 4-CP loads increased, while the similarity of the microbial community between the acclimated and control SBRs decreased with increasing 4-CP loads. The aim of this study is to provide theoretical support for reducing sludge toxicity in industrial wastewater treatment.

Integrated application of sewage sludge, earthworms and Jatropha curcas on abandoned rare-earth mine land soil

Improving soil fertility is a critical component of abandoned rare-earth mine land (ARL) revegetation. To study the effects of sewage sludge (SS), earthworms, and Jatropha curcas in ARL revegetation, SS (40% in mass ratio) and earthworms (0, 40, 60, and 80 individual adult Eisenia fetida kg^-1) were applied to abandoned rare-earth mine land soil (ARLS) and then J. curcas was grown in a potting experiment. The organic carbon, nutrients (N, P, K) and heavy metals (HMs; Cd, Cu, Zn) contents in ARLS and the biomass and nutrients uptake in J. curcas were significantly increased by SS amendment. Application of 80 individual E. fetida kg^-1 significantly increased availability of P and K in SS-amended ARLS relative to other treatments. Earthworms increased the height, ground diameter and biomass of J. curcas, but the promotion of biomass became weaker as earthworm density increased. After J. curcas harvest, the contents of Cd, Cu and Zn in SS-amended ARLS were reduced by 15%, 23%, and 19%, respectively. With the joint application of J. curcas and earthworms, a much larger decrease in contents of Cd (34-40%), Cu (31-44%) and Zn (24-29%) in SS-amended ARLS were observed, and the HMs ecological risks were reduced from "moderate potential" to "low potential". Moreover, J. curcas and earthworms together exerted more reduction in the exchangeable fraction HMs in SS-amended ARLS than J. curcas alone. Our results suggest that the integrated application of SS, earthworms and J. curcas is an effective approach for ARL revegetation.

Effects of ambient temperature and aeration frequency on emissions of ammonia and greenhouse gases from a sewage sludge aerobic composting plant

This study analyzed emissions characteristics of NH₃ and greenhouse gases (i.e. N₂O, CH₄, and CO₂) from a municipal sewage sludge aerobic composting plant. Samples were collected during different seasons in which ambient temperatures and aeration frequencies varied. Results revealed (1) the maximum gas emissions occurred during the mesophilic phase for N₂O (22%-56%) and CH₄ (65%-95%), and in the thermophilic phase for NH₃ (84%-86%) and CO₂ (65%-74%); (2) raising ambient temperatures promoted emissions of NH₃ and greenhouse gases, while improved aeration frequency increased NH₃ but decreased greenhouse gas emissions; (3) CO₂ and N₂O were found to be the key greenhouse gases emitted during aerobic composting according to assessment of the CO₂ equivalent. The results obtained from this study suggest that adjusting ambient temperature to -3 to 5 °C and aeration frequency in composting workshops can be useful approaches for the reduction of NH₃ and greenhouse gas emissions from municipal sewage sludge composting plants.

Wet flue gas desulfurization wastewater treatment with reclaimed water treatment plant sludge: a case study

To upgrade a wet flue gas desulfurization (FGD) wastewater treatment process in a typical thermal power plant (TPP) in Hunan province, China, a new concept for reusing polyaluminum chloride (PAC)-based water treatment plant sludge (WTPS) as a coagulant is proposed. Results show that, for an optimal WTPS dosage of 1,000 mg/L, the corresponding removal capacities for suspended solids (SS) and chemical oxygen demand (COD) from the practical FGD wastewater were 58.3% and 40.3%, respectively. Through an advanced treatment with a dosage of 50 mg/L of PAC, pH of 9, and stirring of 150 r/min for 60 s and 50 r/min for 15 min, the total removal efficiencies of SS, COD, and total cadmium (Cd) from the FGD wastewater were 93.7%, 88.8%, and 84.6%, respectively. Therefore, a new modification process (that involves mixing with WTPS - slag cleaner - neutralization - coagulation - sedimentation) was proposed. The proposed process is economically superior, and the average cost for the FGD wastewater treatment was only 1.08 USD/t. This could provide a cost-effective alternative process for upgrading FGD wastewater treatment facilities of TPPs.

Effects of landscape plant species and concentration of sewage sludge compost on plant growth, nutrient uptake, and heavy metal removal

Landscape plants have great potentials in heavy metals (HMs) removal as sewage sludge compost (SSC) is increasingly used in urban forestry. We hypothesize that woody plants might perform better in HMs phytoremediation because they have greater biomass and deeper roots than herbaceous plants. We tested the differences in growth responses and HMs phytoremediation among several herbaceous and woody species growing under different SSC concentrations through pot experiments. The mixing percentage of SSC with soil at 0%, 15%, 30%, 60, and 100% were used as growth substrate for three woody (Ficus altissima Bl., Neolamarckia cadamba (Roxb.) Bosser, and Bischofia javanica Bl.) and two herbaceous (Alocasia macrorrhiza (L.) G. Don and Dianella ensifolia (L.) DC) plants. Results showed that the biomass, relative growth rate, and nutrient uptake for all plants increased significantly at each SSC concentration compared to the control; woody plants had higher biomass and nutrient use efficiency than herbaceous plants. All plants growing in SSC-amended soils accumulated appreciable amounts of HMs and reduced the contents of HMs present in the substrates. The woody plants were generally more effective than herbaceous plants in potentials of HMs phytoextraction, but A. macrorrhiza showed higher bioconcentration and translocation of Cu and Zn and D. ensifolia had higher bioconcentration and translocation of Cd than woody plants. The optimal application concentrations were 30% or less for woody plants and 15% for herbaceous plants for plant growth and ecological risk control, respectively. Intercropping suitable woody and herbaceous landscape plants in urban forestry might have promising potentials to minimize the ecological risks in the phytoremediation of SSC.

Microplastics in sewage sludge from the wastewater treatment plants in China

Sludge disposal such as land application is suspected as a significant source of microplastic (MP) pollution in the environment. To examine such a hypothesis, the present study was conducted to investigate the occurrence of MPs in sludge by analyzing 79 sewage sludge samples collected from 28 wastewater treatment plants (WWTPs) in 11 Chinese provinces. MP concentrations in the sludge samples ranged from 1.60-56.4 × 10³ particles per kilogram of dry sludge, with an average of 22.7 ± 12.1 × 10³ particles per kilogram of dry sludge. Thereinto, the sludge-based MP contents were greater in eastern China than in western China and varied during different months. Their colors and types were mainly white (59.6%) and fibers (63%), respectively. Microscope Fourier Transform infrared spectroscopy revealed that most of MPs belonged to polyolefin, acrylic fibers, polyethylene and polyamide. Some WWTP parameters, such as servicing area, proportion of industrial wastewater, secondary treatment and sludge dewatering may have affected MP concentrations in sludge. Based on the total sludge production in China, the average amount of sludge-based MPs entering into natural environmental was estimated to be 1.56 × 10¹⁴ particles per year. The findings confirmed that sewage sludge discharge is an important source of MP pollution in the environment. Further evaluation of the associated environmental hazards with MPs is deemed necessary.

Optimization of lactic acid fermentation for pathogen inactivation in fecal sludge

The efficiency of lactic acid fermentation (LAF) as a pretreatment for human feces was investigated in laboratory-scale experiments that lasted for 3 weeks. The sanitization effect of LAF on fecal sludge (FS) was conducted in triplicate. This study used three materials, namely, lactobacillus of lactic acid bacteria, fermented cassava flour, and fermented rice flour, which were known to enhance the production of lactic acid. Each material was mixed in three different reactors at equal ratio with raw FS (i.e., 1:1 v/w, w/w, and w/w). The pH decline rate, lactic acid production rate, and fecal coliform suppression degree were monitored over the period of the treatment process as parameters to evaluate the efficiency of various LAF for pathogen inactivation in FS. Results showed that only fermented rice flour was able to completely inactivate the indicator organism (fecal coliform) at the end of fermentation. Final plate counts of 8.6 × 10⁸ CFU/100 mL, 2.4 × 10⁸ CFU/100 mL, and zero (0) were achieved from lactobacillus, fermented cassava flour, and fermented rice flour treatment processes, respectively. The final pH from the reactors that contained lactobacillus and FS, cassava flour and FS, and fermented rice flour and FS were 5.5, 8, and 3.9, respectively. This study revealed that not all LAF materials can effectively suppress pathogens in FS. The results serve as the foundation in developing an effective, cheap, and easy to use LAF on FS pretreatment for pathogen inactivation.

Locally produced lactic acid bacteria for pathogen inactivation and odor control in fecal sludge

Providing safe fecal sludge (FS) sanitation has remained an important goal of global communities because of the high risks imposed on human health of the exposure to un-sanitized FS. This study used lactic acid fermentation as a pre-treatment technology to evaluate the sanitization effect of lactic acid bacteria (LAB) on FS. A combination of fermented rice flour and brown sugar was used as the medium to prepare LAB, and fecal coliforms were used as the indicator organisms. The addition of a LAB suspension grown in fermented rice flour and brown sugar to FS was studied to evaluate the survival of fecal coliforms. The pH decreased during ongoing lactic acid fermentation after the addition of the LAB suspension. The results revealed that fecal coliforms in reactors containing 1:1 and 2:1 w/w of FS and LAB suspension decreased to half of the initial concentration within seven days of the treatment process in comparison with that of the control reactor. Viable plate counts of 0.6 × 10⁸, 0.9 × 10⁸, and 2.4 × 10⁸ CFU/100 mL were recorded from reactors 1:1, 2:1, and the control, respectively. The total elimination of the fecal coliforms below the detection limit (<3 log 10 CFU/100 mL) was observed in both reactors after 15-17 days, whereas the number of fecal coliforms remained at 2.3 × 10⁸ CFU/100 mL in the control reactor. The fecal coliforms were eliminated because of the acidification caused by the LAB during the incubation time. The final pH in the treatment reactors 1:1 and 2:1 was 3.7 and 3.9. While the final pH in the control reactor was 7.91. The results revealed that the bacterial pathogens in FS can be completely eliminated through a low-cost technique and a simple lactic acid fermentation process.

Application of microwave-pretreated cephalosporin mycelial dreg (CMD) as soil amendment: Temporal changes in chemical and fluorescent parameters of soil organic matter

Land application of treated cephalosporin mycelial dreg (CMD) as a soil amendment is an alternative to its disposal in landfills and incineration because it has environmental and agronomic benefits. This study validated the efficacy of using the dewatered, microwave-pretreated CMD as a soil amendment. Pot experiments were conducted to assess the temporal changes in soil organic matter (SOM) profiles via chemical and fluorescent parameters. During the ageing period, the CMD-treated soil experienced a sudden rise in soil pH and soil electrical conductivity, along with a rapid decline in soil organic carbon and soil organic nitrogen content. The specific Ex/Em peak related to protein-like substances gradually disappeared, while those related to humic acid-like substances continued to increase thereafter. Fluorescence regional integration (FRI) results showed an ascended P_V,n/P_III,n index (1.94) and significant correlations with chemical data (M²=0.2875, r=0.8441, P<0.001, 999 permutations for Procrustes analysis). Taken together, despite the temporal changes in chemical and fluorescent data after soil conditioning, the increased content of SOM containing humic acid-like substances was observed at the end of the incubation period compared with control soil samples, indicating that the microwave-pretreated CMD might be applied as a soil amendment.

Municipal sewage sludge compost promotes Mangifera persiciforma tree growth with no risk of heavy metal contamination of soil

Application of sewage sludge compost (SSC) as a fertilizer on landscaping provides a potential way for the effective disposal of sludge. However, the response of landscape trees to SSC application and the impacts of heavy metals from SSC on soil are poorly understood. We conducted a pot experiment to investigate the effects of SSC addition on Mangifera persiciforma growth and quantified its uptake of heavy metals from SSC by setting five treatments with mass ratios of SSC to lateritic soil as 0%:100% (CK), 15%:85% (S15), 30%:70% (S30), 60%:40% (S60), and 100%:0% (S100). As expected, the fertility and heavy metal concentrations (Cu, Zn, Pb and Cd) in substrate significantly increased with SSC addition. The best performance in terms of plant height, ground diameter, biomass and N, P, K uptake were found in S30, implying a reasonable amount of SSC could benefit the growth of M. persiciforma. The concentrations of Cu, Pb and Cd in S30 were insignificantly different from CK after harvest, indicating that M. persiciforma reduced the risk of heavy metal contamination of soil arising from SSC application. This study suggests that a reasonable rate of SSC addition can enhance M. persiciforma growth without causing the contamination of landscaping soil by heavy metals.