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Wastewater sludge is used as an alternative fuel due to its high organic content and calorific value. However, influent characteristics and operational practices of wastewater treatment plants (WWTPs) can increase the sulfur content of sludge, devaluing it as a fuel. Thus, we investigated the biochemical mechanisms that elevate the sulfur content of sludge in a full-scale industrial WWTP receiving wastewater of the textile dyeing industry and a domestic WWTP by monitoring the sulfate, sulfur, and iron contents and the biochemical transformation of sulfate to sulfur in the wastewater and sludge treatment streams. A batch sulfate reduction rate test and microbial 16S rRNA and dsrB gene sequencing analyses were applied to assess the potential and activity of sulfate-reducing bacteria and their effect on sulfur deposition. This study indicated that the primary clarifier and anaerobic digester prominently reduced sulfate concentration through biochemical sulfate reduction and iron–sulfur complexation under anaerobic conditions, from 1247 mg/L in the influent to 6.2~59.8 mg/L in the industrial WWTP and from 46.7 mg/L to 0~0.8 mg/L in the domestic WWTPs. The anaerobic sludge, adapted in the high sulfate concentration of the industrial WWTP, exhibited a two times higher specific sulfate reduction rate (0.13 mg SO42−/gVSS/h) and sulfur content (3.14% DS) than the domestic WWTP sludge. Gene sequencing analysis of the population structure of common microbes and sulfate-reducing bacteria indicated the diversity of microorganisms involved in biochemical sulfate reduction in the sulfur cycle, supporting the data revealed by chemical analysis and batch tests. [ABSTRACT FROM AUTHOR]

The seasonal variation in concentration of transparent exopolymer particles (TEPs), particulate organic carbon (POC) and particulate organic nitrogen (PON) were investigated together with floc size and the concentration of suspended particulate matter (SPM) along the cross‐shore gradient, from the high turbid nearshore toward the low‐turbid offshore waters in the Southern Bight of the North Sea. Our data demonstrate that biophysical flocculation cannot be explained by these heterogeneous parameters, but requires a distinction between a more reactive labile ("fresh") and a less reactive refractory ("mineral‐associated") fraction. Based on all data, we separated the labile and mineral‐associated POC, PON, and TEP using a semi‐empirical model approach. The model's estimates of fresh and mineral‐associated organic matter (OM) show that great parts of the POC, PON, and TEP are associated with suspended minerals, which are present in the water column throughout the year, whereas the occurrence of fresh TEP, POC, and PON is restricted to spring and summer months. In spite of a constantly high abundance of total TEP throughout the entire year, it is its fresh fraction that promotes the formation of larger and faster sinking biomineral flocs, thereby contributing to reducing the SPM concentration in the water column over spring and summer. Our results show that the different components of the SPM, such as minerals, extracellular OM and living organisms, form an integrated dynamic system with direct interactions and feedback controls. Plain Language Summary: Particles suspended in coastal waters occur as loose aggregates of tiny mineral and organic particles, also known as flocs. Their mass concentration is higher in winter than in summer, but their sizes are smaller in winter. The seasonal cycle of phytoplankton activity drives this phenomenon. In spring, phytoplankton blooms and starts to produce fresh and sticky organic matter. This glue binds particles together after collisions and promotes increasingly larger and yet stable flocs. Analytical lab methods cannot distinguish between the freshly produced sticky and the older inactive organic material that is stored in the sediments and entrained into the water by erosion and resuspension. Therefore, previous studies were not able to detect the relation between the occurrence of the sticky material and the floc‐sizes in coastal waters. However, our innovative model approach allows a separation into both fractions. We observe a clear increase in floc sizes during spring and summer when fresh sticky organic material is available. As larger particles sink faster, they are removed from the water column, which allows a higher light penetration. In this way, phytoplankton is exposed to changing conditions, an illustration of how direct interactions and feedback loops occur in an integrated dynamic system. [ABSTRACT FROM AUTHOR]