40 Questions
What is the primary mechanism of filtration in a rapid sand filter?
Straining
What is the main advantage of slow sand filters?
They are relatively small and compact
What is the purpose of the gravel layer in a slow sand filter?
To support the sand layer
What is the filtration rate of a slow sand filter?
0.1-0.3 m3 per hour
What is the main difference between slow sand filters and rapid sand filters?
The filtration rate
What is the common application of slow sand filters?
Groundwater treatment
What is the role of absorption in the filtration process?
It is not very important in the filtration process
How is a rapid sand filter cleaned?
Through backwashing
What is the purpose of sedimentation in water treatment?
To allow suspended particles to settle out of the water
Which zone in the sedimentation basin provides a calm area for suspended particles to settle?
Settling zone
What happens to the clarified water after sedimentation?
It drains out of the top of the sedimentation tank
What is the purpose of pre-chlorination and dechlorination in water treatment?
To kill algae and remaining bacteria from sedimentation
What is the purpose of filtration in water treatment?
To remove suspended particles from water
What happens to the flocs in the sedimentation tank?
They settle out of the water and are scraped and vacuumed off the bottom
What is the function of the inlet zone in the sedimentation basin?
To provide a smooth transition from the flocculation zone
What is the function of the sludge zone in the sedimentation basin?
To store the sludge before it is removed for additional treatment or disposal
What is the primary mechanism of particle removal in rapid sand filters?
Adsorption and some straining
What is the purpose of the underdrain in a rapid sand filter?
To collect the filtered water
What is the purpose of backwashing in rapid sand filters?
To remove trapped particles
How do rapid sand filters typically operate during filtration?
The influent flows down through the sand and support gravel
What is the primary reason rapid sand filters operate more quickly than slow sand filters?
The influent water is already relatively clear
What is used to assess when a rapid sand filter needs backwashing?
Length of the filter run, turbidity of the effluent water, and head loss within the filter
What is the purpose of the backwash pump in rapid sand filters?
To force treated water from the system back up through the filter bed
What is closed during backwashing in rapid sand filters?
The influent valve
What is the approximate diameter of a grain of fine sand in a filtration system?
0.1mm
Why would a filter made of fine sand not be able to produce safe drinking water?
Because it would not be able to remove particles smaller than 0.1mm
What is the mechanism of filtration that involves particles becoming attracted to and sticking to the sand particles?
Adsorption
What is the term for the process of passing water through a filter with smaller pores than the particles to be removed?
Straining
What is the minimum size of particles that can be removed by a filter with a fine sand layer?
0.1mm
What is the term for the gathering of gas, liquid, or dissolved solids onto the surface of another material?
Adsorption
What is the mechanism of filtration that involves the breakdown of particles by biological processes?
Biological action
What are the four mechanisms of filtration?
Straining, adsorption, biological action, and absorption
What is the purpose of collecting dirty backwash water in wash troughs?
To allow it to settle in a tank, pond, or basin
What is the typical range of available chlorine in Sodium hypochlorite solution?
10-14%
What is the purpose of maintaining a certain amount of residual chlorine in the water?
To protect against future recontamination
What is a limitation of chlorine as a disinfectant?
It is not effective against protozoans that form cysts in water
What is a potential drawback of using chlorine as a disinfectant?
It reacts with natural organic compounds in the water to form potentially harmful disinfection by-products
What is the typical percentage of available chlorine in Calcium hypochlorite granules?
30-40%
What is the minimum amount of residual chlorine recommended in the water after half an hour contact time?
0.2-0.5 mg/L
What is the common name for Calcium hypochlorite granules?
Bleaching powder
Study Notes
Mechanisms of Filtration
- Absorption: soaking up of one substance into the body of another substance, but not very important in the filtration process
- Types of filters: slow sand filtration and rapid sand filtration
- Slow sand filtration: filtration rate of 0.1-0.3 m3 per hour, pros include reliability, small size, and low maintenance, but requires large land area and manual cleaning
- Rapid sand filtration: filtration rate of 4-20 m3 per hour, pros include high filtration rate, but requires chemical pretreatment and backwashing
- Filter media: sand and anthracite coal
- Filtration mechanism: biological action, straining, adsorption, and absorption
Sedimentation
- Sedimentation: accomplished by decreasing the velocity of the water to a point below which particles will no longer remain in suspension
- Sedimentation basin:
- Inlet zone: provides a smooth transition from the flocculation zone and distributes the flow uniformly
- Settling zone: the largest portion of the basin, provides a calm area for suspended particles to settle
- Sludge zone: provides a storage area for the sludge before it is removed for additional treatment or disposal
- Outlet zone: provides a smooth transition from the sedimentation zone to the outlet from the tank
Filtration Process
- Purpose: to remove suspended particles from water by passing it through filters
- Filters: consist of filters with varying sizes of pores, often made up of sand, gravel, and charcoal
- Mechanisms of filtration:
- Straining: passing water through a filter with smaller pores than the particles to be removed
- Adsorption: particles becoming attracted to and "sticking" to the sand particles
- Biological action: breakdown of particles in water by biological processes
- Absorption: soaking up of one substance into the body of another substance
Rapid Sand Filter
- Mechanism of particle removal: primarily adsorption and some straining, no biological action
- Filter media: sand and anthracite coal
- Operation: similar to slow sand filter, but influent water is already relatively clear due to coagulation/flocculation and sedimentation
- Cleaning: through backwashing, which reverses the flow of water through the filter, cleaning out trapped particles
Disinfection
- Purpose: to kill remaining bacteria and other microorganisms
- Methods: chlorination, usually using liquefied chlorine gas, sodium hypochlorite solution, or calcium hypochlorite granules
- Objective: to add enough chlorine to leave 0.2-0.5 mg/L residual chlorine after half an hour contact time
- Drawbacks: chlorine has limited effectiveness against protozoans, and can react with natural organic compounds to form potentially harmful disinfection byproducts (DBPs)
This quiz covers the different mechanisms of filtration, including biological action and absorption, and how they are used in water treatment processes.
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