Impact of urban wastewater discharges in the water quality of Ria Formosa: a case study of Faro-Olhão urban wastewater treatment plant

A Ria Formosa é uma lagoa costeira situada na costa sul de Portugal com elevada importância social e económica para a região que representa o maior produtor nacional de amêijoa Ruditapes decussatus (ca. 90%). Este sistema está sob diversas pressões antrópicas, incluindo descargas de águas residuais urbanas que comprometem a qualidade da água. Este estudo, estando integrado no projeto CONPRAR, visa avaliar o impacto da recente ETAR Faro-Olhão (antiga Faro Nascente) na qualidade da água, especialmente na proximidade de viveiros de bivalves, utilizando uma abordagem química e bacteriológica. As ETAR Faro Nascente e de Olhão foram desativadas em outubro e foi implementada no mesmo local de Faro Nascente a nova ETAR de Faro-Olhão que trata os afluentes das duas ETAR então desativadas e parte de S. Brás de Alportel aumentando o efluente diário da ETAR (~ 14 000 m3 /dia). Esta usa um novo tratamento terciário (NEREDA). A amostragem de água foi realizada mensalmente ao longo do gradiente espacial da ETAR (2000 m) para duas secções da área de estudo no canal de descarga da ETAR (Esteiro da Garganta). Os dados obtidos mostram uma melhoria da qualidade da água ao longo do gradiente de dispersão do efluente para as variáveis químicas e bacteriológicas. Foram avaliados três períodos, setembro-outubro 2018, antes da desativação das duas ETAR de Faro Nascente e Olhão Poente, novembro de 2018 - abril de 2019, 6 meses após a implementação da nova ETAR Faro-Olhão e maio de 2019. Observou-se que o período novembro de 2018 - abril de 2019, foi quando se registou o maior impacto da nova ETAR e quando ocorreu a maior variabilidade associada à não estabilização do tratamento da mesma, a qual foi atingida em maio de 2019. Espacialmente, pelo índice trófico TRIX a maior influência da descarga foi identificada até 750 m para ambas as seções da área de estudo, zona onde não ocorre cultivo de bivalves. A maré desempenhou um papel importante na melhoria da qualidade da água, por promover uma elevada renovação da água, particularmente em preia mar de maré viva. A situação de maré morta e em baixa-mar revelou-se a pior a nível de qualidade de água, devido ao maior tempo de residência e menor renovação de água. Conclui-se assim que face à elevada taxa de troca com o oceano adjacente o impacto da ETAR é diminuído quando comparado com outros sistemas onde o renovação e circulação são mais restritas.Ria Formosa is a coastal lagoon located in the south coast of Portugal, with high social and economic importance to the region representing the largest national producer of the clam Ruditapes decussatus (ca. 90%). This system is under several anthropogenic pressures including urban wastewater discharges which compromise its water quality. This study, being integrated in the project CONPRAR, aims to assess the impact of the recent Faro-Olhão WWTP (former Faro Nascente) upon water quality, especially in the vicinity of shellfish beds using a chemical and bacteriological approach. On an initial phase, Faro Nascente and Olhão Poente were decommissioned, and their effluents are treated at the new Faro-Olhão WWTP implemented at the same place of the former Faro Nascente. It uses a new biological treatment (NEREDA) for the population served from Faro, Olhão and a part of S. Brás de Alportel increasing the daily outflow of the WWTP (~14 000 m3/day). Water sampling was conducted monthly along the spatial gradient of the WWTP (2000 m) for two sections of the study area in the discharge channel of the WWTP (Esteiro da Garganta). Data shows an improvement of water quality further from the stations closest to the discharge for both chemical and bacteriological variables. Three different periods studied: September-October 2018, corresponding to the period before the deactivation of both WWTP’s; November 2018 – April 2019, six months after the implementation of the new Faro-Olhão WWTP and May 2019. It was noticed that the period November 2018 – April 2019 was associated to the greatest impact of the WWTP, when values were highest and most variable, also associated to an non stable biological treatment, which, stability was achieved in May 2019. Spatially, the impact of the discharge was noticeable until 750 m for both sections of the study area (TRIX) where bivalve farming does not occur. The tide had an important role in the improvement of water quality by promoting a great water renewal, particularly during flood at spring tides. Low water (neap tides) was therefore associated to the worst-case scenario due to higher residence times and less water renewal. In sum, due to the high rate of water exchange with the adjacent ocean, the impact of the WWTP was decreased when compared to other coastal systems where water renewal and circulation are more restricted.Este trabalho foi realizado no âmbito do projeto CONPRAR, financiado pelo programa operacional MAR2020 (MAR-01.04.02-FEAMP-0003) para apoio do Fundo Europeu dos Assuntos Marítimos e das Pescas em Portugal

Impacts of decommissioning and upgrading urban wastewater treatment plants on the water quality in a shellfish farming coastal lagoon (Ria Formosa, South Portugal)

Ria Formosa is a productive coastal lagoon, located on the south coast of Portugal, and represents the largest national producer of shellfish bivalves (ca. 90% production). This ecosystem is subjected to various anthropogenic pressures, including the discharge of urban wastewater treatment plants (UWWTP), which impacts the lagoon water quality. This study aimed to assess the impact of alterations in the functioning of two UWWTP on the water quality of Ria Formosa, based on chemical variables, phytoplankton composition ( including potential harmful species) and faecal contamination. During the period September 2018 - October 2019, water sampling was conducted along dominant longitudinal gradients of the effluent dispersion from the discharge point (1-2 km), for two sites: a decommissioned (OP) and a modified (FO) UWWTP. After modification, the later started receiving a higher influent volume (ca. 40%), under an innovative technology system (biological treatment in aerobic granular sludge). Based on chemical water quality variables, phytoplankton and indicators of faecal contamination, a significant improvement along the longitudinal gradient from the discharge point was observed after OP decommissioning. This improvement was fast, being detected two months after decommissioning, positively affecting areas used as shellfish farming grounds. However, distribution patterns of bacteriological indicators and regular shellfish harvesting interdictions suggested an alternative source of faecal contamination after OP decommissioning. At FO, both chemical variables and bacteriological indicators of faecal contamination revealed a slower improvement, only six-months after the UWWTP alteration. Before that, increased and highly variable ammonium, chlorophyll a concentration, phytoplankton abundances and Escherichia coli densities, revealed an unstable phase. Overall, a lower water quality at FO in respect to OP reflected not only a higher effluent volume but also more restricted water circulation for the former.[PO Mar2020 (MAR-01.04.02-FEAMP-0003)]info:eu-repo/semantics/publishedVersio