Water Treatment Flashcards
What are the 4 basic operations for water treatment
Coagulation/flocculation
Sedimentation
Filtration
Disinfection
What does aeration do?
Increases oxygen content and reduces carbon dioxide content
Can cause colour change if oxides present
Reduces concentration of taste and odour causing substances
Removes volatile synthetic organic chemicals (SOCs) considered hazardous to public health
How does aeration work?
Scrubbing action caused by turbulence from water and air mixing together. Fe and Mn fall out of solution
Methods of aeration
Mechanical aerators: employ motor-driven impellers alone or in combination with air injection devices. They are also installed at water reservoirs to control taste and odor.
Pressure aerators: Typically used for oxidising iron and manganese. There are two basic types of pressure aerators in which either compressed air is injected directly in pressurised pipeline or water is sprayed into the top of a closed tank while the tank is continuously supplied with compressed air.
What is coagulation?
Coagulation refers to a reduction in the forces which tend to keep suspended particles apart.
Coagulation is the process of inducing contacts between a chemical (coagulant) and colloidal particle to cause a reaction – The reaction product is called a microfloc.
What is flocculation?
Flocculation refers to the joining together of small particles into large settleable and filterable particles.
Inducing the growth of microflocs to form flocs.
What are colloidal particles?
Suspended minerals, clay, silt, organic debris and microscopic organisms ranging in size from about 0.001 to 1.0 microns
Settling rate of colloidal particles is quite low – removal by sedimentation is not practicable, unlike larger ones like sand/silt.
Chemical coagulation and flocculation required to aggregate the smaller particles to form larger particles to settle out in sedimentation.
What are the most commonly used coagulants?
Aluminum sulphate (filter alum) and Iron salts
Why does allum have a pH range of 5-8 as a coagulant?
At pH of 5-8 allum is least soluble. This is desirable as we want the particles to precipitate out and form flocs.
What is sedimentation?
the gravitational settling of suspended particles that are heavier than the surrounding fluid.
What is the sludge layer?
Suspended particles retained at the tank floor
Heavier particles with ρparticles > ρfluid settle under gravitational forces
What is the scum layer?
Suspended particles retained at the water surface
Flotation: Lighter particles with ρparticles < ρfluid tend to move vertically upward
What is Plain sedimentation?
The use of a pre-sedimentation basin for grit (small loose stone or sand particles) removal.
For wastewater treatment, why is sedimentation used?
Grit (or sand and silt) removal.
Suspended solids removal in primary clarifier.
Biological sludge removal in activated sludge.
Humus removal in trickling filter final clarifiers.
What is filtration?
A process used for separating suspended or colloidal impurities from a liquid by passing the liquid through a porous medium.
Polishing step to remove small flocs or precipitant particles not removed after sedimentation of coagulated or softened water.
Should filtration be relied upon for health protection?
No
Under certain conditions, filtration may serve as the primary turbidity removal process – e.g. direct filtration of raw water.
Although filtration removes many pathogenic microorganisms (e.g. Giardia & Cryptosporidium), it should not be relied upon for complete health protection.
Describe how filtration is commonly carried out
The most commonly used system involves passing water through a stationary bed of granular medium (filter).
Solids in the water are retained by the filter medium. Several modes in granular filtration medium (Upflow, biflow, pressure & vacuum).
The most common in practice is gravity filtration in a downward mode – the weight of the water above the filter column provides the driving force.
Key parameters that affect filtration
Grain size ditribution
Porosity
Grain density
Effective size parameter
What is slow sand filtration?
These microorganisms assist the filtration process by removing contaminants, though water temperatures below 10°C may slow them. Sand that hosts these organisms is said to be “ripened” and is preferable to clean or new sand. Depending on water content and temperature, it may take several weeks or months to ripen sand. The process eventually clogs the sand bed and slows flow rates to the point that it must be unclogged, typically by reversing the flow, or “backwashing”
Most suspended particles and organic materials are decomposed on theschmutzdeckelayer. Particles in raw water go through the sand media filter. The restrained particles become food for the bacteria on theschmutzdeckelayer.
