Nutrient transport and retention in tropical rivers receiving wastewater treatment plant discharges

Author(s)

Nícolas Reinaldo Finkler

• Document type: Doctoral Thesis
• Press: São Carlos.
• Institution: Universidade de São Paulo (USP). Escola de Engenharia de São Carlos (EESC/SBD)
• Defense date: 2022-05-06

Examining board members

Davi Gasparini Fernandes Cunha; Solange Filoso; Björn Gücker; Leila Jorge Patrizzi; Teresa Cristina Tarle Pissarra

• Advisor: Davi Gasparini Fernandes Cunha

Abstract

Wastewater treatment plants (WWTP) have eliminated many of the problems related to the presence of sewage in natural watercourses. Nevertheless, WWTP can be a relevant source of nutrients depending on the technology used in the treatment. Effluents contribute to the supply of nitrogen (N) and phosphorus (P) to receiving aquatic ecosystems, creating short- and long-term implications for the functioning of these environments. This study aimed to evaluate the influence of WWTP discharges on the self-purifying capacity of receiving tropical rivers to retain dissolved forms of N and P. Uptake lengths (SW and SW-net) and velocities (Vf and Vf-net) of soluble reactive phosphorus (SRP), nitrate (NO3-N), and ammonium (NH4-N) were estimated for three receiving rivers (with treatment technologies such as UASB reactors, stabilization ponds and activated sludge) and two reference streams (which do not receive effluents). Both SW (for reference streams) and SW-net (for receiving rivers) represent the average distance traveled by a nutrient molecule until its removal from the water column (sensu nutrient spiraling). Vf (reference streams) and Vf-net (receiving rivers) represent the efficiency that nutrients are removed, indicating the biological demand. The metrics were quantified by analyzing the longitudinal patterns of concentrations downstream of the nutrient addition (the WWTP outfall in receiving rivers, and a pump with a concentrated solution in reference streams). These estimates were associated with hydrodynamic and physicochemical variables, and metabolic rates (ecosystem respiration, ER and gross and net primary production, GPP, and NPP, respectively). In total, five field sampling were carried out between March 2019 and July 2021. The results indicated that the nutrient uptake lengths in receiving rivers (mean ± standard deviation of SW-net of 4.2 ± 1.6, 5.0 ± 2.9 and 3.3 ± 1.8 km for NH4-N, NO3-N, and SRP, respectively) was longer than in reference streams (0.1 ± 0.07, 0.3 ± 0.2 and 0.2 ± 0.1 km SW for NH4-N, NO3-N, and SRP, respectively), suggesting greater potential for nutrient export in reaches downstream of the WWTP. There were no significant differences for NO3-N and SRP uptake velocities between receiving rivers and reference streams, whereas higher values were observed for NH4-N in the reference streams. There was a predominance of heterotrophy (ER > GPP) for all watercourses. In the receiving streams (mean NPP = -11.7 ± 10.9 g O2/m².day) were observed greater heterotrophic conditions than in reference streams (mean NPP = -6.9 ± 5.3 g O2/m².day), which indicates greater consumption of dissolved oxygen and organic matter in the former. The reference streams showed higher retention than receiving rivers, suggesting lower nutrient export from these reaches. The NH-N concentrations discharged by the WWTP were related to retention of the three forms of nutrients along the receiving rivers. In addition, it was observed the influence of both ambient conditions upstream of the WWTP, and the effluents discharge with relation to the receiving rivers discharge on nutrient retention estimates. Some potential retention mechanisms suggested by the results indicated that nitrification and adsorption may have been predominant; however, they are not able to permanently remove the nutrients discharged by the WWTP, increasing potentially the N and P export to downstream ecosystems. As in other studies, the receiving rivers were not effective sinks of nutrients, but nutrient transformers. It expects that the results generated in the study can subsidize, for example, the definition of new criteria for effluent discharges, and eventual technological adaptations in the WWTP in order to maximize the self-purifying capacity of the receiving rivers. (AU)

• FAPESP's process: 18/13171-1 - Point-source pollution effects on nutrient retention in tropical streams: experimental manipulation of stoichiometry rate of nitrogen and phosphorus
• Grantee: Nícolas Reinaldo Finkler
• Support Opportunities: Scholarships in Brazil - Doctorate