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Stoichiometry of coagulation revisited
J Y Shin1, R F Spinette, C R O'Melia
1Geography and Environmental Engineering, The Johns Hopkins University, Baltimore, Maryland 21218, USA.
Environmental Science & Technology
|May 29, 2008
Summary
Natural organic matter (NOM) significantly influences coagulant (alum) doses for potable water treatment, primarily driven by dissolved organic carbon (DOC) concentrations. Low NOM can decrease alum doses by aiding particle precipitation.
Area of Science:
- Water treatment
- Environmental chemistry
- Colloid science
Background:
- Coagulant dose optimization is crucial for effective potable water treatment.
- Natural organic matter (NOM) and colloidal particles are key water contaminants affecting treatment.
- Understanding contaminant interactions with coagulants like alum is essential.
Purpose of the Study:
- To investigate the roles of NOM and silica particles in determining alum doses for potable water.
- To assess the impact of varying NOM and silica concentrations at different pH levels (6 and 7).
- To determine the primary driver of coagulant demand in raw water.
Main Methods:
- Laboratory jar tests were conducted to simulate water treatment processes.
- Concentrations of NOM and colloidal silica were systematically varied.
- Turbidity and dissolved organic carbon (DOC) were measured to assess treatment effectiveness.
Main Results:
- Coagulant requirements were predominantly dictated by DOC concentration at both pH conditions.
- Low NOM concentrations significantly reduced effective alum doses in low-silica waters.
- Strong stoichiometric relationships existed between DOC and coagulant demand.
- Silica particles only contributed to coagulant demand at very high concentrations.
Conclusions:
- Dissolved organic carbon (DOC) is the main factor controlling alum dose in potable water treatment.
- NOM plays a complex role, potentially reducing coagulant demand under specific conditions.
- Alum dose optimization must consider the interplay between NOM, silica, and water chemistry.
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