Piloting DAF for Full-Scale Implementation to Mitigate Impacts of Changing Source Water Quality
Abstract
In the last few decades, water treatment facilities in Atlantic Canada have experienced
changes in their source waters, specifically elevated natural organic matter (NOM) levels. This is
widely referred to as brownification and has been reported in surfaces waters across the Northern
Hemisphere. Lake recovery is noted as one factor behind brownification. Lake recovery is
characterized by increasing acid neutralization capacity, alkalinity, and pH, which often
correspond to increases in NOM concentrations. Climate change is also expected to promulgate
the impacts posed by lake recovery with higher temperatures and increased frequency of runoff
events. The conditions associated with lake recovery are also favourable for harmful algal
blooms to occur which can produce toxins that are harmful to humans, and are difficult to
remove through conventional water treatment processes.
Treatment plants with source waters experiencing brownification are noticing significant
historical increases in chemical dosing for coagulation processes, leading to increased
operational costs and waste residuals. Dissolved air floatation (DAF) has been noted as being an
effective clarification process for removing low density contaminants such as NOM and algae.
DAF poses great advantages as it can reduce chemical dosing as well as reduce flocculation time
and tank volumes. This work evaluates the operations of a DAF pilot plant in Nova Scotia for the
intended purpose of informing design decisions for full scale implementation. Parameters that
were investigated included coagulant dose, coagulant type, and flocculation time. Results suggest
that these factors may influence removal mechanisms and that consideration must be taken to
effectively target contaminants of concern. The project was operated throughout various seasons
which provided data depicting seasonal effects on the process. Data has shown DAF clarification
to provide promising results in terms of turbidity and natural organic matter removal. Longer
flocculation times may prove to be more robust with DAF but shorter flocculation times provide
adequate results and are far more economically friendly.