Wastewater effects on Phaeodactylum tricornutum (Bohlin): Setting up a classification system

•Wastewater is a complex environmental matrix to assess and manage.•Phaeodactylum tricornutum marine diatoms show both toxicity and stimulation effects.•A wastewater effect score can organise effect data and help in management actions. Phytoplankton is highly productive in marine coastal ecosystems that generally present high levels of anthropogenic pressures. Microalgae presents very high surface-to-volume ratio and may respond promptly to contaminants showing either an increase in growth or inhibition effects. Thus phytoplankton organisms provide information on the potential impacts of contaminants on the supported marine-coastal food webs. The diatom Phaeodactylum tricornutum Bohlin is commonly used in toxicity testing according to the ISO 10253:2006 standardised protocol, but a comprehensive inventory of tested substances has not been compiled yet. The aim of this study is to establish a wastewater effect score based on P. tricornutum exposed to domestic, municipal and industrial wastewaters from activated sludge sequencing batch reactor and ultra-filtration membrane biological reactors. Wastewater samples produced stimulation and inhibition effects identified by biostimulation unit (BU50) and toxicity unit (TU50) both at 50% effect, respectively. Within the stimulation scenario, toxicity was low if 0