Comparing in situ particle monitoring to microscopic counts of plankton in a drinking water reservoir

Temporal and spatial characteristics of flocculated suspended solids in a deep reservoir: an in situ observation in the Biliuhe Reservoir

The amount of total suspended solids (TSS) is the most visible indicator for evaluating water quality in reservoirs. Previous investigations paid more attention to TSS of the surface layer in reservoirs, while suspended particles are prone to settle, resuspend, and aggregate at the bottom of reservoir. There may be different patterns of the TSS in different depths. This study is to assess the TSS concentration by weight analysis, find the evidence of the existence of flocculated suspended particles by in situ underwater imaging analysis, and discuss the impact of the flocculation process of suspended solids on water quality in deep reservoirs. Although the TSS concentration is lower than other reservoirs with the same trophic level, many flocs were found at the bottom of the deep-water area (> 15 m) in the Biliuhe Reservoir according to the recordings of the in situ underwater camera. The further comprehensive analysis demonstrates that the fine particle in flood season and resuspension is the main source of suspended flocs at the bottom of the reservoir. While the slow settling velocity results in the flocculation of fine suspended particles and long-term residence in the bottom layer of the reservoir. TSS has a significant correlation with iron and total phosphorus. Resuspension, flocculation, and settling impact on the transport of suspended sediment and associated contaminants. The evidence from this study suggests that the impact of flocs on water quality should be further discussed to ensure water supply safety.

Monitoring and removal of cyanobacterial toxins from drinking water by algal-activated carbon

Microcystins (MCs) are the most potent toxins that can be produced by cyanobacteria in drinking water supplies. This study investigated the abundance of toxin-producing algae in 11 drinking water treatment plants (DWTPs). A total of 26 different algal taxa were identified in treated water, from which 12% were blue green, 29% were green, and 59% were diatoms. MC levels maintained strong positive correlations with number of cyanophycean cells in raw and treated water of different DWTPs. Furthermore, the efficiency of various algal-based adsorbent columns used for the removal of these toxins was evaluated. The MCs was adsorbed in the following order: mixed algal-activated carbon (AAC) ≥ individual AAC > mixed algal powder > individual algal powder. The results showed that the AAC had the highest efficient columns capable of removing 100% dissolved MCs from drinking water samples, thereby offering an economically feasible technology for efficient removal and recovery of MCs in DWTPs.

Phytoplankton appearance in particle size spectra - deriving conversion functions between microscopic and particle counter measurements

Analysis of aquatic field samples by particle counters are a widespread method but the representation of phytoplankton abundance and of size classes in which phytoplankton appears in the resulting size spectra is not well studied. To address this gap, two freshwater phytoplankton species were analysed in a particle counter and using a microscope: the colony forming Asterionella formosa (Bacillariophyceae) and the single-celled Microcystis aeruginosa (Cyanobacteria). Field samples, growth experiments and model approaches were used to study the image of phytoplankton derived by two different commonly used and standardized counting methods. In our results, the colony forming A. formosa had to be considered in units of colonies because the counting device enumerated only 23% of the single cells but 85% of the colonies that were determined under the microscope. Furthermore, the size class representation in the particle counter of both taxa appeared in much smaller ranges than expected from microscopic size measurements. Model simulations of movements and rotations of phytoplankton in the measuring device can explain half of the size shift. We deduce that about 86% of the cell areas of both studied species are transparent from two approaches. First, areas derived from simulations of rotated phytoplankton colonies equal the measured particle spectra of the laboratory cultures when the shadow areas are reduced to 14%. Secondly, field counts of A. formosa can be integrated into particle size spectra of the total particulate material when the same reduction factor is applied. For the considered optical counting device, field samples of A. formosa can be detected in particle size spectra when colony sizes as well as transparency of the cells and reduction of cell sizes by rotations are taken into account.