Evidence-Based Framework to Manage Cyanobacteria and Cyanotoxins in Water and Sludge from Drinking Water Treatment Plants

Abstract

Freshwater bodies and, consequently, drinking water treatment plants (DWTPs) sources are increasingly facing toxic cyanobacterial blooms. Even though conventional treatment processes including coagulation, flocculation, sedimentation, and filtration can control cyanobacteria and cell-bound cyanotoxins, these processes may encounter challenges such as inefficient removal of dissolved metabolites and cyanobacterial cell breakthrough. Furthermore, conventional treatment processes may lead to the accumulation of cyanobacteria cells and cyanotoxins in sludge. Pre-oxidation can enhance coagulation efficiency as it provides the first barrier against cyanobacteria and cyanotoxins and it decreases cell accumulation in DWTP sludge. This critical review aims to: (i) evaluate the state of the science of cyanobacteria and cyanotoxin management throughout DWTPs, as well as their associated sludge, and (ii) develop a decision framework to manage cyanobacteria and cyanotoxins in DWTPs and sludge. The review identified that lab-cultured-based pre-oxidation studies may not represent the real bloom pre-oxidation efficacy. Moreover, the application of a common exposure unit CT (residual concentration × contact time) provides a proper understanding of cyanobacteria pre-oxidation efficiency. Recently, reported challenges on cyanobacterial survival and growth in sludge alongside the cell lysis and cyanotoxin release raised health and technical concerns with regards to sludge storage and sludge supernatant recycling to the head of DWTPs. According to the review, oxidation has not been identified as a feasible option to handle cyanobacterial-laden sludge due to low cell and cyanotoxin removal efficacy. Based on the reviewed literature, a decision framework is proposed to manage cyanobacteria and cyanotoxins and their associated sludge in DWTPs.

Practical Framework for Evaluation and Improvement of Drinking Water Treatment Robustness in Preparation for Extreme-Weather-Related Adverse Water Quality Events

“…One metric that quantifies robustness is the turbidity robustness index (TRI), − which assesses the robustness of clarification and filtration processes in full-scale DWTPs. − These studies, however, have been limited to individual treatment steps rather than an entire DWTP, have only focused on historical data, without considering possible turbidity extremes, and were limited to relatively short time frames. Limited research studies have developed climate adaptation frameworks for entire DWTPs but were generally not able to quantitatively assess full-scale DWTPs. , In the gray literature, there are frameworks for improving the resilience of DWTPs during extreme weather events, which are only qualitative in nature and are targeted more toward restarting a DWTP after failure than on continuing operations to avoid a failure, − as is the focus of robustness.…”

“…Limited research studies have developed climate adaptation frameworks for entire DWTPs but were generally not able to quantitatively assess full-scale DWTPs. 21 , 22 In the gray literature, there are frameworks for improving the resilience of DWTPs during extreme weather events, which are only qualitative in nature and are targeted more toward restarting a DWTP after failure than on continuing operations to avoid a failure, 23 − 25 as is the focus of robustness.…”

Microcystins in Water: Detection, Microbial Degradation Strategies, and Mechanisms

“…It has been demonstrated that traditional drinking water treatments (such as coagulation, flocculation, and filtration) can efficiently remove cyanotoxins from cyanobacterial cells and intracellular cyanobacteria, but additional treatment methods are required for extracellular toxins. Several processes have been proposed for natural reductions in microcystin levels, including dilution, adsorption, thermal decomposition aided by pH, photolysis, and biological degradation [ 20 ]. However, some processes have limitations in terms of their effectiveness, including limited light penetration for direct photolysis and weak sediment adsorption.…”