Keratinolytic and Keratinophilic Fungi in Sewage Sludge: Factors Influencing their Occurrence

Abstract

The present study was conducted to determine the qualitative and quantitative composition of keratinolytic and keratinophilic fungi in sewage sludge. In particular, the aim was to determine the influence of sewage and sludge treatment technologies, as well as physico-chemical and microbiological factors on fungal composition in the sludge environment. Twenty-one sewage sludges from 16 wastewater treatment plants in Upper Silesia, Poland were examined for mycological, bacteriological, parasitological and physico-chemical properties. The wastewater treatment plants used different sewage and sludge treatment technologies. Each sludge was the ‘final’ product of sewage and sludge treatment processes at a given wastewater treatment plant. Keratinolytic and keratinophilic fungi were examined in sludges using the hair baiting method, with four incubation temperatures (23, 29, 33, and 37°C). Actidione-resistant fungal strains were identified. Keratinophilic fungi occurred abundantly in the sludges examined. The fungal composition reflected both sewage and sludge treatment technologies and the influence of ‘combinations’ of physico-chemical and microbiological factors characteristic for a given urban agglomeration, wastewater treatment plant or group of wastewater treatment plants. The most 132important factors affecting the fungal composition in sewage sludge were as follows: temperature, pH, ammonium nitrogen, proteolytic activity, organic carbon and total nitrogen, C:N ratio, total sulfur, C:S ratio, available phosphorus and particle size distribution. The study confirmed the division of the fungi examined into keratinolytic and keratinophilic. Keratinolytic fungi were able to decompose keratin, while keratinophilic fungi utilized simple and easy degradable components of keratinous remnants and the products of keratin decomposition. Keratinolytic fungi could eliminate keratinophilic fungi from hair. Keratinolytic fungi preferred neutral and alkaline sludges, while keratinophilic fungi occurred more frequently in acidic sludges. Quantitiative relationships between Trichophyton terrestre, with its teleomorph Arthroderma quadrifidum and Chrysosporium keratinophilum reflected ammonium nitrogen concentration and pH, C:N ratio, as well as colloidal loam and water contents in sewage sludge. The composition of keratinolytic fungi can be an indicator of the sludge organic matter stabilization process. This conclusion addresses both sewage and sludge treatment technologies and microbiological organic matter transformations. Due to the correlation with fecal coliform quantities, the composition of keratinolytic fungi can also be useful as a rough indicator of the sludge hygienization process. The relationships can be used in both wastewater treatment plants and soil reclamation practice. From the mycological point of view, opportunistic fungi, especially Microsporum gypseum and Pseudallescheria boydii, pose the major health risk in sewage sludge. In the light of the available data, pathogenic fungi should be regarded as an important element of public health risk posed by sewage sludge, especially when applied to land. The need for future studies are described below.