Laboratory-scale evaluation of different aspects related to Ceratium hirundinella removal during simulation of a conventional water treatment plant which includes sedimentation
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The freshwater dinoflagellate species, Ceratium hirundinella (C. hirundinella) possesses unique characteristics, such as a thecal-plate cell covering of cellulose, spines and flagella. Unlike most other algae and cyanobacteria, C. hirundinella cells are relatively large in size (up to 450 μm in length and 50 μm in width). These unique characteristics (e.g. cell covering and flagella) and adaptations (e.g. spines) give the dinoflagellate cells the ability to reduce their sinking rate from the euphotic zone and to migrate easily through the water column. When source water contains high concentrations of C. hirundinella cells, water treatment problems and poor aesthetic water quality can be expected. These water treatment problems may include 1) the disruption of coagulation and flocculation, 2) clogging of sand filters and 3) taste and odour problems when cells penetrate into the final water. In Chapter 9 of this study, a list of operational guidelines (including alert levels) and recommendations to assist managers and operators of plants when C. hirundinella cells are causing water treatment problems. During events of high C. hirundinella concentrations in source water, managers and operators of conventional water treatment plants need strategies to optimize coagulants and unit processes. Thus when source water contains motile nuisance algae, such as C. hirundinella, in moderate or abundant quantities, it is advisable to conduct jar stirring test experiments using both turbidity and total photosynthetic pigment (or chlorophyll-a) analyses as indicators of appropriate coagulant choice and dosages. The aims of this study are summarized as follows: * To optimize coagulants and conventional water treatment processes by implementing relevant algal removal strategies and indicators during jar stirring test experiments, * To investigate the changes in surface charge (known as zeta potential) on C. hirundinella cells before and after adding coagulants as part of the treatment processes, * To investigate the physical and chemical impacts on the morphology of C. hirundinella cells after coagulation, flocculation and sedimentation, * To identify organic compounds that may be responsible for taste and odour problems associated with C. hirundinella, * To investigate the efficiency of pre-chlorination on the removal C. hirundinella cells when dosing various coagulants, and * Give recommendations and operational guidelines relevant for a conventional water treatment plant to improve C. hirundinella removal A combined water treatment system (Phipps and Bird Model), consisting of a six paddle jar test apparatus and six sand filter columns, was used to simulate conventional processes (coagulation, flocculation, sedimentation and rapid sand filtration). Source water samples containing relatively high C. hirundinella concentrations (> 500 cell/mℓ) were collected from Benoni Lake (26º10’50.40’’S; 28º17’50.11’’ E) in plastic containers and stored as a homogenous sample in a 200 litre container under laboratory conditions (± 22 °C). Samples were collected from the source water as well as after sedimentation (from the supernatant or sludge) to determine turbidity, total photosynthetic pigment analyses (chlorophyll) and for phytoplankton analyses. Flocs (containing C. hirundinella cells) were collected from the sludge or sediment for scanning electron microscopy investigations and to perform zeta potential analyses. Concentrated C. hirundinella samples were frozen at -80 °C according to the proposed sampling protocol for organic compound analyses. Results obtained from this study proved that using the relevant indicators to determine the appropriate coagulant dosages during jar stirring tests may generally improve the removal of problem-causing algae, such as C. hirundinella cells. Improved algal removal efficiencies will subsequently ensure final water with good aesthetic quality. The surface charge (zeta potential) on C. hirundinella cells can be used to evaluate the best coagulation conditions within an operating window of -10 mV to +3 mV when dosing various coagulants. Scanning electron microscopy investigations revealed major damaging effects to C. hirundinella cells when dosing high Ca(OH)2 concentrations. However, when dosing lower Ca(OH)2 concentrations, in combination with organic polymer, better C. hirundinella cell removal efficiencies with less damaging effects to cells was observed. This study also indicated that the pre-chlorination, without causing cell lyses, can be applied to render the highly motile cells immobile which will subsequently assist the coagulation unit process. The aesthetic quality (e.g. tastes and odours) of drinking water may be influenced when C. hirundinella cells release organic material into the water as a result of cell lyses. Organic compounds, such as fatty acids and dicarboxylic acids can lead to taste and odour problems which associate with the presence of C. hirundinella. Organic compounds also serve as precursors for the formation of harmful chlorine by-products formed during chlorination.