The effect of wastewater composition and operating conditions in activated sludge (AS) microbiological and morphological characteristics was studied in three AS wastewater treatment plants (WWTPs): (a) a high organic load slaughterhouse AS WWTP, operating at complete solids retention, monitored from its start-up and for 425 days; (b) a seasonally operational, low nitrogen load fruit canning industry AS WWTP, operating at complete solids retention, monitored from its start-up and until the end of the season (87 days); (c) a municipal AS WWTP, treating wastewater from a semi-combined sewer system, monitored during the transitions from dry to rainy and again to dry periods of operation. The sludge microbiological and morphological characteristics were correlated to nutrients' availability, solids retention time, hydraulic retention time, dissolved oxygen, mixed liquor suspended solids (MLVSS), organic load (F/M) and substrate utilization rate. The AS WWTPs' operation was distinguished in periods based on biomass growth phase, characterized by different biological and morphological characteristics and on operational conditions. An anoxic/aerobic selector minimizes the readily biodegradable compounds in influent, inhibiting filamentous growth. Plant performance controlling is presented in a logic flowchart in which operational parameters are linked to microbial manipulation, resulting in a useful tool for researchers and engineers.

Industrial wastewaters are recognized as a valuable resource, however, their disposal without proper treatment can result in environmental deterioration. The associated environmental/operational cost of wastewater treatment necessitates upgrade of applied processes towards the goals of sustainability and mitigation of climate change. The implementation of cyanobacteria-based processes can contribute to these goals via resources recovery, production of high-value products, carbon fixation and green-energy production. The present study evaluates the cyanobacterium Synechococcus elongatus PCC 7942 (S7942) as a biological component for novel and sustainable alternatives to typical biological nutrient removal processes. Valuable results regarding cultivation temperature boundaries, applied disinfection techniques and analytical methods, as well as regarding relations between parameters expressing S7942 biomass concentration are presented. The results show that at typical industrial wastewater temperatures, S7942 efficiently grew and removed nitrates from treated snack-industry's wastewater. Moreover, in cultures with treated and relatively saline dairy wastewater, its growth rate slightly decreased, but nevertheless nitrates removal rate remained efficiently high. A comparison between typical denitrification processes and the proposed nutrient removal process indicated that a S7942-based system may constitute an alternative or a supplementary to denitrification process. Thus, Synechococcus elongatus PCC 7942 proved to be a potent candidate towards sustainable industrial wastewater treatment applications.

Semi-Micro-Kjeldahl method is a TKN analysis method included in Standard Methods for the Examination of Water & Wastewater, but no data on its trueness and precision are available. The method itself has inherent limitations and principal interferences, negative or positive, provoked from high nitrate concentrations. In this work, Semi-Micro-Kjeldahl method performance and nitrate interference were studied. Trueness, precision under repeatability conditions, intermediate precision, limit of quantification, recovery and measurement uncertainty, were calculated with satisfactory results. The Lower Laboratory Limit of Quantification was determined at 2 mg L⁻¹ TKN concentration; lower than 4 mg L⁻¹ proposed by standard methods. Nitrogen digestion recovery (nicotinic acid spiking solution) and nitrogen loss (ammonium chloride spiking solution), ranged from 92% to 98% and from 92% to 104% respectively and were in the general acceptable limits (90%–110%) and in the nutrients acceptable limits (80%–120%). The results of the inter-laboratory measurements confirmed the assumptions taken and verified the calculations of measurement uncertainty. Nitrate concentration limit over which negative and positive interference is significant was quantified at 400 mg L⁻¹. Consequently, as this study demonstrates, the Semi-Micro-Kjeldahl method is applicable for the majority of water and wastewater samples.