Effects of sewage sludge pretreatment methods on its use in agricultural applications

Prevalence of diverse antimicrobial resistance genes and bacteria in sewage treatment plant-derived sludge environment

Antimicrobial resistance (AMR) contamination in the environment is one of the most significant worldwide threats of the 21st century. Since sludge is heavily exposed to diverse contaminants, including pharmaceuticals, the inhabitant bacterial population is expected to exhibit resistance to antimicrobial agents. In this study, sewage treatment plant (STP) sludge samples were analyzed to assess the antibiotic-resistant bacterial population, abundance of AMR genes (ermF, qnrS, Sul1, blaGES, blaCTX-M, and blaNDM), and mobile genetic elements (intl1 and IS26). Out of 16, six bacterial isolates exhibited resistance to 13 antibiotics with a high multiple antibiotic resistance index (MARI) (0.93) and high metal tolerance. Quantitative polymerase chain reaction showed the abundance of target genes ranging from 6.6 × 103 to 6.5 × 108 copies g-1 sludge. The overall outcome reveals that STP sludge comprised varied multidrug-resistant bacterial populations. It will give insights into the functions of heavy metals and biofilm development in the selection and spread of AMR genes and the associated bacteria. Therefore, the application of sludge needs proper screening for AMR and metal contamination prior to its countless applications. This study will contribute immensely to the risk analysis of STP effluents on environmental health, including control of AMR transmission.

Comparison and Three-Dimensional Fluorescence Spectrum Analysis of Activated Sludge Treatment with Fenton and UV-Fenton

This study investigated the effects of single Fenton and Fenton and UV combined processes on the cracking degree of anaerobic sludge under the same conditions. The optimal experimental conditions were obtained by repeated determination of Fe2+ dosage, H2O2 dosage and reaction time, so as to achieve the maximum cracking of sludge. In addition, this study applied three-dimensional fluorescence spectrum analysis technology to analyze the organic matter leached from the treated sludge, and different regions of the three-dimensional fluorescence spectra were analyzed and compared for each treatment condition. Repeated experiments showed that the optimal conditions for Fenton are a pH of 3, reaction time of 40 min, 1.4 g/L of Fe2+ and 9 g/L of H2O2. The Fenton process cracking yielded a protein concentration of 0.66 mg/L and sCOD of 5489 mg/L, and the UV-Fenton pretreatment yielded a protein concentration of 0.74 mg/L and sCOD of 5856 mg/L. The sludge particle size was reduced from the original 54.52 mm to 40.30 mm and 36.37 mm, respectively. In addition to these parameters, it was also demonstrated that the Fenton process has a strong cracking effect on sludge by indicators such as the SEM and sludge water content and that UV irradiation can play a role in assisting and helping sludge cracking.

Pollution Status and Associated Risk Assessment of Heavy Metals in Sewage Sludge in the Yangtze River Delta, China

The risk assessment of heavy metals (HMs) in sewage sludge (SS) is essential before land application. Six HMs in nineteen SS collected in the Yangtze River Delta were analyzed to assess risks to environment, ecosystem, and human health. HMs concentrations were ranked in the order of Zn > Cu > Cr > Ni > Pb > Cd, with Cu, Zn, and Ni in a total of 16% of samples exceeding the legal standard. Zn showed greatest extractability according to EDTA-extractable concentrations. HMs in 16% of SS samples posed heavy contamination to the environment with Zn as the major pollutant. HMs in 26% of samples posed ecological risk to the ecosystem and Cd was the highest risky HM. The probabilistic health risk assessment revealed that HMs posed carcinogenic risks to all populations, but non-carcinogenic risks only to children. This work will provide fundamental information for land application of SS in this area.

Effect of ultrasound-assisted EDTA and citric acid washing on heavy metal removal, residual heavy metal mobility, and sewage sludge quality

We investigated the effects of ultrasound-assisted ethylenediaminetetraacetic acid (EDTA) and citric acid (CA) washing on heavy metal (HM) removal, residual HM mobility, and sewage sludge quality. EDTA and CA washing of sewage sludge successfully reduced the total concentration of HMs after one round of washing, but the mobility of residual HMs increased significantly. The eluate had a high concentration of HMs and nutrients (nitrogen, phosphorus, potassium, and total organic carbon), although the nutritional content of the sludge remained high. The three-phase ratio of the sludge after six rounds of washing by CA was closest to the ideal three-phase ratio, and the degree of influence on the physical structure of the soil after a land application was reduced, according to the fluctuation of generalized soil structure index (GSSI) and soil three-phase structure distance (STPSD) values. The results indicate that CA as an environmental-friendly washing agent can be the superior choice for sludge HM washing; single washing of sewage sludge may increase the mobility of residual HMs, so multiple washings should be considered for land application of sludge.

Heavy metal toxicity in poultry: a comprehensive review

Arsenic (As), lead (Pb), cadmium (Cd), and mercury (Hg) have been recognized as most toxic heavy metals that are continuously released into the environment, both from natural sources and from anthropogenic production of fertilizers, industrial activities, and waste disposal. Therefore, As, Cd, Hg, and Pb are found in increasing concentrations in bodies of water, fodder, feed, and in the tissues of livestock, including poultry, in the surroundings of industrial areas, leading to metabolic, structural, and functional abnormalities in various organs in all animals. In poultry, bioaccumulation of As, Pb, Cd, and Hg occurs in many organs (mainly in the kidneys, liver, reproductive organs, and lungs) as a result of continuous exposure to heavy metals. Consumption of Cd lowers the efficiency of feed conversion, egg production, and growth in poultry. Chronic exposure to As, Pb, Cd, and Hg at low doses can change the microscopic structure of tissues (mainly in the brain, liver, kidneys, and reproductive organs) as a result of the increased content of these heavy metals in these tissues. Histopathological changes occurring in the kidneys, liver, and reproductive organs are reflected in their negative impact on enzyme activity and serum biochemical parameters. Metal toxicity is determined by route of exposure, length of exposure, and absorbed dosage, whether chronic and acute. This review presents a discussion of bioaccumulation of As, Cd, Pb, and Hg in poultry and the associated histopathological changes and toxic concentrations in different tissues.

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.

Thermophilic-mesophilic biodegradation: An optimized dual-stage biodegradation technique for expeditious stabilization of sewage sludge

The present study investigated the stabilization of fresh sewage sludge through a dual-stage biodegradation process; rotary drum composting in series with vermicomposting. After thermophilic exposure in a rotary drum composter, the partially degraded feedstock was separated into S1 without vermiculture, S2 and S3 with Eudrilus eugeniae and Eisenia fetida vermi-monocultures, respectively. The S3-derived vermicompost exhibited an 80% and 88% reduction in CO₂ and ammonium-nitrogen evolution rates, respectively, demonstrating the expedient stabilization of sludge. The robust, more than 85% seed germination index supported S2 and S3 derived vermicompost viability. A significant decrease in heavy metals was evinced with S2 and S3-derived vermicompost; the S1-derived end product exhibited higher Zn, Cr, and Pb levels in the absence of vermicomposting. Furthermore, soil amended with 20% vermicompost from S3 displayed 50% more plant growth than S1. Thus, the optimized thermophilic-mesophilic dual-biodegradation technique stabilizes sewage sludge quickly, has a lot of potential in sludge management facilities around the world, and produces a marketable end product.