1.2 Overview of Research
Turbidity is a water quality parameter correlated with the concentration of suspended colloidal particles. Turbidity measurement is based on light scattering caused by suspended or colloidal material present in that liquid. The amount of light scattered by a known standard (typically Formazin) provides the scale for measurement of turbidity in Nephelometric Turbidity Units (NTUs).
Colloidal particles are of particular interest in water treatment because they correlate with the presence of pathogenic organisms, interfere with disinfection, and negatively impact drinking water quality. Colloidal particles (0.001-1.0 μm) are difficult to remove by gravity sedimentation because of their low settling velocities.
Naturally occurring colloids typically have a negative surface charge and electrostatic repulsion acts to hinder particle aggregation. Use of a coagulant such as alum (Al 2(SO 4 )3*14(H 2 0)) is commonly employed in water treatment to neutralize the negative colloid surface charge. Alum dissolves and forms positively charged species such as Al 3+, Al(OH) 2+ and Al(OH) 2+ that could adsorb to the surface of colloids. Precipitation of Al(OH) 3(s) also occurs on colloid surfaces. The solid surface charge of Al(OH) 3(s) is positively charged at circumneutral pH .
Aluminum can also form polymer species in water. In solution, Al 2 to Al 6 polymers can form fairly rapidly, however longer polymers that aid bridging between floc particles can take days to form utilizing alum. Different forms of polyaluminum chloride (PACl) can assist the formation of longer chain polymers facilitating bridging between particles in much shorter time.
A rapid mix reactor is used to blend raw water and coagulant. There are two goals in rapid mix. The first is large scale turbulent mixing that can be accomplished by a flow expansion. The second goal in a rapid mix reactor is to achieve a high energy dissipation rate (~ 0.5 to 1 W/kg) to provide small scale turbulent mixing so that molecular diffusion can finish the mixing process in a few seconds.
Charge neutralization of the colloidal suspension allows particle aggregation and floc formation. A flocculator with controlled energy dissipation rate and residence time is used to promote floc particle growth. Particle size is correlated with the terminal settling velocity of a particle. After flocculation the resulting larger, flocculated, particles can be separated by gravity sedimentation (discussed in greater detail below).
After sedimentation, the clarified low-turbidity effluent is disinfected with an oxidant such as calcium hypochlorite. The goal of disinfection is to kill or inactivate pathogens present in the water. Chlorine disinfection is non-site specific and will act to oxidize any organic with which it comes into contact. Therefore chlorine disinfection is less effective at higher turbidities. Because there is no filtration in the AguaClara water treatment process sequence, sedimentation is a key unit process for removal of suspended and colloidal particles to minimize their interference with disinfection.
This thesis focuses on use of upflow floc blanket clarification technology as a tool for producing high quality, low turbidity water. Performance is measured as the removal of turbidity (correlating to concentration of colloidal particles). The operational parameters that control floc blanket performance will be characterized and optimized from the point of view of the operator. Optimization of parameters such as coagulant dosing, and energy dissipation rate in the flocculator, and upflow velocity and floc blanket height in the clarifier, can provide better design guidelines to be utilized in the creation of AguaClara sedimentation tanks.