Profile of polycyclic aromatic hydrocarbons in digested sewage sludge

One of the methods of sludge disposal in cities is its use as an amendment in adjoining agricultural soil. Contaminants in sludge can, however, nullify the benefits. Polycyclic aromatic hydrocarbons (PAHs) are a class of persistent organic pollutants, if present, can pose health risks by virtue of their carcinogenic potential and persistence in soil. The present study deals with profile analysis of PAHs in digested sewage sludge of a megacity, Delhi. Mean concentration of the sum of 16 PAHs in the sludge from 5 sewage treatment plants (STPs) was 20.67 ± 4.14 mg/kg, dry weight (dw). Carcinogenic PAHs represented ∼47% of the total PAHs. Benzo[g,h,i]perylene recorded the highest mean concentration followed by dibenzo[a,h]anthracene. Low molecular weight PAHs and high molecular weight PAHs contributed 19% and 81% to total PAHs content respectively. Principal component analysis identified the major sources of PAHs in sludge as coal and natural gas combustion. Wood combustion, oil and oil derivatives are the other identified sources. The observed PAH levels in sludge from selected STPs of Delhi were found to violate the limit values proposed by the European Union and the US legislation for application in agriculture.

Occurrence of microplastics in municipal sewage treatment plants: a review

Municipal sewage treatment plants (STPs) are thought to be important point sources of microplastics in freshwater systems and many peer-reviewed articles have been published on this issue since mid-2010s. In this review, we summarize existing literature on the occurrence of microplastics in STPs and experimental methods used for isolation and identification of microplastics. The number concentrations of microplastics in STP influents were 15.1-640 L-1 , whereas those in the STP effluents were highly variable and ranged from not detectable to 65 L-1 . For most of cases, conventional STPs are removing microplastics very effectively. Fragments and fibers are dominant shapes of microplastics. Thermoplastics (polyethylene and polypropylene) and polyester are the predominant materials recovered. Although further research is needed, size distribution of microplastics in STPs is likely to follow a power law, implying that different studies using different size cutoffs may be compared after establishing a power law relationship.