Microbial performance and community resilience : toxicological assessment of biological nitrification

  • Carla Carolina Lopez Smitter

Student thesis: Doctoral Thesis

Abstract

Microbial communities play a vital role in the biogeochemical nitrogen cycle, supporting the transformation of nitrogen compounds in different environmental habitats and engineered facilities such as wastewater treatment plants (WWTPs). Traditional biological nitrification is a biochemical process carried out by two autotrophic microbial clades of nitrifiers: ammonia-oxidising bacteria (AOB) and nitrite-oxidising bacteria (NOB). Together they perform the two-step ammonia oxidation to nitrate via nitrite. Although complete oxidation of ammonia can be performed by a single organism (Comammox), AOB/NOB guilds represent an essential microbial population for the removal of inorganic nitrogen compounds in conventional WWTPs. These important microbes are sensitive to the toxic action of many pollutants and operational changes, and their loss of activity can lead to nitrification failure in WWTP. As a result, many nutrients can be disposed into the environment with detrimental effects on the ecosystem and human health.Over the years, there has been a growing concern about pharmaceuticals and personal care products (PPCPs) in wastewater streams. They comprise a vast number of chemicals, including antibiotics and commercial products. Some PPCPs have toxic potential for many organisms, including nitrifying bacteria. The main aim of this PhD was to assess the short term effect of common PPCPs on the metabolic activity of enriched nitrifying cultures.The first stage of the PhD research comprised the cultivation of AOB-NOB populations in batch reactors under selective growth conditions to obtain biomass for the toxicity bioassays. Two types of samples were used as sources of nitrifiers, WWTP sludge for no salt adapted nitrifiers species and coastal marine sediments targeting halotolerant AOB-NOB communities. Subsequently, the toxicity experiments were carried out in short-batch tests to assess the individual toxic effect of selected PPCPs on nitrifiers.The experimental findings presented in the thesis demonstrated that some of the tested PPCPs can negatively affect the metabolism of nitrifying populations in short-term exposures. In the WWTP cultures, the antimicrobial agent triclosan exhibited the most significant nitrification inhibition, reducing the overall oxidation process by 50 % (EC50) with the lowest concentration of 89.1 μg L−1. Within the antibiotics tested, colistin produced the highest nitrification inhibition, with an EC50 of 1 mg L−1, with a more pronounced effect on AOB species compared to NOB strains. Other common antibiotics, ampicillin and ofloxacin, produced inhibition with EC50 values of 23.7 mg L−1 and 12.7 mg L−1, respectively. Moderate inhibition was observed with the insect repellent ingredient DEET, causing less than 40% nitrification inhibition at 10 mg L−1. On the other hand, the stimulant caffeine had no critical inhibitory effects on the activity of AOB/NOB species. Additionally, the acute toxicity bioassays performed with the salt-tolerant nitrifying cultures showed that triclosan did not affect the oxidation activity up to the maximum concentration tested (0.1 mg L-1) after 24 h of toxic exposure. Overall, the toxicant (PPCPs) concentrations causing nitrification inhibition presented in this thesis were considerably higher than relevant levels reported in environmental samples and WWTP streams. However, excessive human consumption of triclosan may lead to higher concentration in the sewage closer to the toxicity threshold found in the WWTP samples, posing a risk to nitrifying bacteria performance in biological wastewater treatments.The research presented here highlights the importance of investigating pollutants' toxic effects on nitrifying populations. The experimental threshold responses can contribute to the AOB/NOB inhibition data, supporting mitigation strategies to maintain a stable nitrification process in WWTPs. In addition, this study captured practical recommendations associated with the lessons learned during the preparation of toxicity assays with enriched nitrifying cultures. This information can help future researchers develop high-quality testing protocols to continue understanding the toxic effect of pollutants in AOB/NOB communities.
Date of Award29 Sept 2022
Original languageEnglish
Awarding Institution
  • University Of Strathclyde
SponsorsUniversity of Strathclyde
SupervisorCharles Knapp (Supervisor) & Vernon Phoenix (Supervisor)

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