Organic micropollutants in sewage sludge: influence of thermal and ultrasound hydrolysis processes prior to anaerobic stabilization

Advances in ultrasound-assisted synthesis of photocatalysts and sonophotocatalytic processes: A review

Water pollution and the global energy crisis are two significant challenges that the world is facing today. Ultrasound-assisted synthesis offers a simple, versatile, and green synthetic tool for nanostructured materials that are often unavailable by traditional synthesis. Furthermore, the integration of ultrasound and photocatalysis has recently received considerable interest due to its potential for environmental remediation as a low-cost, efficient, and environmentally friendly technique. The underlying principles and mechanisms of sonophotocatalysis, including enhanced mass transfer, improved catalyst-pollutant interaction, and reactive species production have been discussed. Various organic pollutants as dyes, pharmaceuticals, pesticides, and emerging organic pollutants are targeted based on their improved sonophotocatalytic degradation efficiency. Additionally, the important factors affecting sonophotocatalytic processes and the advantages and challenges associated with these processes are discussed. Overall, this review provides a comprehensive understanding of sono-assisted synthesis and photocatalytic degradation of organic pollutants and prospects for progress in this field.

Occurrence and emission of polycyclic aromatic hydrocarbons from water treatment plant sludge in Taiwan

The concentrations level and distribution of 16 US EPA polycyclic aromatic hydrocarbon (PAHs) from the water treatment plant (WTP), sewage treatment plant (STP), and industrial water treatment plant (ITP) sludge in Taiwan were determined and then assessed the sources, and potential toxicity (carcinogenic polycyclic aromatic hydrocarbons [CPAHs] and toxic BaP equivalent [TEQ]). Results indicated that the total concentrations of PAHs ranged between 58 and 16,436 μg/kg dw. Among the 17 samples, the 2-4 ring of total PAHs were the predominant compound in three kinds of treatment plant (> 60%). Especially, ITP1 owns 95.8% of 2-4 ring of total PAHs and ITP3 owns 54% of five- and six-ring of total PAHs. The molecular indices and principal component analysis (PCA) were used to determine the source contributions, with the results showing that the contributions of combustion/grass, coal or wood combustion and combustion/ liquid (oil) fossil fuel combustion. A PAH toxicity indicated by TEQ was 2.5-506 μg TEQ/g dw. Although, the results indicated that these were not recommended for land applications, but analyses are beneficial to develop effective management strategies for controlling PAH discharge in treatment plants and establishing strategies for its reuse in managing pollutants.

Occurrence, effects, and biodegradation of plastic additives in sludge anaerobic digestion: A review

The retention of microplastics, a complex blend of polymers and plastic additives (PAs), in municipal sludge has been reported. The inevitable release of PAs from microplastics might affect the subsequent biological disposal of sludge, and their final fate are of great public concern. Therefore, this review describes the current knowledge in the occurrence of PAs in sludge and significant advances in their effects on sludge anaerobic digestion (AD) and their biodegradation performance. Specifically, the compositions and contents of plasticizers, stabilizers, and flame retardants in sludge worldwide are systematically summarized. The discrepant impacts of PAs on hydrolysis, acidification, and methanogenesis processes are analyzed and compared, with corresponding trends deduced. Furthermore, the biodegradation performances of PAs during sludge AD are also discussed. For most of the PAs detected in sludge, available data for their fate and effects on AD is yet limited. Moreover, the potential role of AD microbes in the release of PAs from microplastics was still unknown. Especially, the potential effects of PAs released from biodegradable microplastics on sludge AD and their fate should be of concern. The obtained knowledge would update our understanding of the risk assessment and control of PAs in sludge AD. Recommendations for future investigation are made.

Developing the OECD 106 fate testing protocol for active pharmaceuticals in soil

The ability to determine accurately the fate of APIs in soil is essential for rigorous risk assessment associated with wastewater reuse or biosolid recycling to land, particularly in lower income countries where water and fertiliser is scarce. Four APIs (naproxen, ofloxacin, propranolol and nevirapine) with wide ranging functionality were used as examples in the development of the OECD 106 soil partitioning and/or degradation study, with naproxen used to illustrate applying the full methodology. The data showed key methodological criteria require careful consideration and testing to generate accurate and consistent results. Only glass fibre membranes were suitable for all APIs, without unduly adsorbing APIs to their surface, thus effectively restricting the minimum practical pore size to 0.7 µm. Polypropylene plastic centrifuge tubes were shown to be suitable, with careful determination of recoveries. Direct injection liquid chromatography-mass spectrometry could reliably resolve all 4 APIs down to less than µg L^-1 in soil solutions, although allowance for matrix effects via standard additions was required in some cases. Greatest analytical challenges were found for the highest molecular weight API with the greatest affinity for sorption to surfaces (ofloxacin). Key variables that can impact on partitioning such as solution pH and dissolved organic carbon concentrations were shown to vary within tests over time and should be accounted for.

Effect of pyrolysis temperature on removal of organic pollutants present in anaerobically stabilized sewage sludge

Sewage sludge was excluded from the list of component materials for the production of EU fertilizing products and it was banned as feedstock to produce pyrolysis & gasification materials in European Commission's technical proposals for selected new fertilizing materials under the Regulation 2019/1009 (STRUBIAS report). This exclusion of pyrolysis as a viable way to treat sewage sludge was mainly due to the lack of data on the fate of organic pollutants at pyrolysis conditions. In this work, we are addressing this knowledge gap. We studied slow pyrolysis as a potential process to efficiently treat organic pollutants present in stabilized sewage sludge. Sewage sludge was pyrolyzed in a quartz fixed bed reactor at temperatures of 400-800 °C for 2 h and the sludge and resulting sludge-chars were analyzed for the presence of four groups of organic pollutants, namely (i) polychlorinated biphenyls (PCBs), (ii) polycyclic aromatic hydrocarbons (PAHs), (iii) pharmaceuticals, and (iv) endocrine-disrupting and hormonal compounds. Pyrolysis at ≥ 400 °C effectively removed pharmaceuticals (group iii) to below detection limits, whereas pyrolysis at temperatures higher than 600 °C was required to remove more than 99.8% of the compounds from groups i, ii and iv. Based on these findings, we propose, that high temperature (>600 °C) slow pyrolysis can satisfactory remove organic pollutants from the resulting sludge-char, which could be safely applied as soil improver.

Occurrence, fate, and persistence of emerging micropollutants in sewage sludge treatment

Sludge generated at sewage treatment plants is of environmental concern due to the voluminous production and the presence of a high concentration of emerging contaminants (ECs). This review discusses the fate of ECs in sewage sludge treatment with an emphasis on fundamental mechanisms driving the degradation of compounds based on chemical properties of the contaminant and process operating conditions. The removal of ECs in sewage sludge through various treatment processes of sludge stabilization, such as anaerobic digestion (AD), composting, and pre-treatment methods (thermal, sonication, and oxidation) followed by AD, are discussed. Several transformation mechanisms and remediation strategies for the removal of ECs in sludge are summarized. The study concludes that pH, sludge type, and the types of functional groups are the key factors affecting the sorption of ECs to sludge. During conventional waste stabilization processes such as composting, the degradation of ECs depends on the type of feedstock (TOC, N, P, C/N, C/P) and the initial concentration of the contaminant. In AD, the degree of degradation depends on the hydrophilicity of the compound. The estrogenicity of the sludge may sometimes increase due to the conversion to estrogenic compounds. The pre-treatment techniques can increase the partitioning of ECs in the soluble fraction resulting in enhanced biodegradation up to 10-60%. However, the formation of by-products and loss of OH· to scavenging under high organic content during advanced oxidation processes can make the process uneconomical and require further research.

Anaerobic digestibility of resin acids in primary sludge: Effect of ozone pretreatment

Resin acids in pulp and paper mills wastewater are potentially partitioned in the solids in post-primary clarification due to higher hydrophobicity with log K_ow ∼1.74-5.80. They are known to adversely affect anaerobic digestion (AD) process, although the effect has not been quantified deterministically in control studies. The objective of the present work was to determine the effect of untreated and ozonated spiked resin acids on AD of primary sludge. Batch adsorption tests were conducted to determine the solid-liquid partition coefficient (K_d) of resin acids on the primary sludge. Higher K_d was obtained at pH 4; however, it was decreased by 78-98% at pH 8. Thereafter, batch AD of model resin acids in primary sludge using food to microorganism ratio (S₀/X) of 0.5gtCOD/gVSSindicated only 15-20% removal of resin acids in the liquid phase anaerobically. While, ozonation in pure water using 0.74-1.48 mg O₃/mg tCOD showed >90% reduction of the test resin acids, an ozone dose of 0.52 mg O₃/mg tCOD reduced 50-70% spiked resin acids' load to the digester. However, no further removal of resin acids occurred during AD over 30 days. About 42% reduction in methane production compared to the control digestor occurred in the presence of 150 mg/L of resin acids. When treated with 0.52 mg O₃/mg tCOD, methane production improved and was comparable to the control digestor, indicating that resin acids may not be detrimental to AD at a concentration range of 45-75 mg/L.