Primary Treatment Flashcards
Purpose of Clarifiers
To remove readily settleable or floatable solids
Purpose of influent baffles
Provides for even flow, distribution, flocculation, and energy dissipation
Purpose of effluent weirs
Provides even flow distribution out of the tank. Need to be level to prevent formation of currents in the tank
Purpose of skimming device
To remove floating material
Purpose of internal baffles
Negates the effect of density currents/short circuits
Short circuits
Different velocities in any part of a tank that can effect settling and solids removal
Detention time for clarifiers
2-3 hours
How does temperature impact settle rate?
Lower temperature increases the density of water which increases settling time. 50% faster at 80 degrees F than 38F
Horizontal velocities in a clarifier for settling
1-2 feet per minute
Weir loading rate
A measure of flow over a defined amount of weir. Higher weir loading can result in poor settling. 10000-20000 gpd/ft
Optimal Surface loading rate in a primary clarifier
Ranges from 300-1200 gpd/ft2. Lower rate leads to longer detention times.
Percent solids of sludge removed from primary clarifier
4-6% is typical
What role do chemical additions play in a clarifier?
Act as coagulent for improved settling and can precipitate phosphorous
Primary Treatment settleable solids removal (%)
90-99%
Primary Treatment TSS removal (%)
40-60%
Primary Treatment TS removal (%)
10-15%
Primary Treatment BOD removal (%)
20-50%
Solids loading rate (secondary clarifiers)
12-30lbs/day/square foot
Sludge gasification
Soluble and suspended organic matter undergoes anaerobic decomposition releasing gas. Can lead to popups.
Bulking
Clouds of billowing sludge when it does not settle properly. Often cause by filamentous bacteria in activated slude.
Clarifier efficiencies are affected by what factors?
Type of solids, time in the collection system, detention time, condition of equipment, sidestreams such as recycle flows
What are the two most common methods of flow equalization?
In-line or side-line equalization
In-line flow equalization
Blends all incoming flows into the holding tank and discharges at the average flow rate
Side-line equalization
Diverts flows that are higher than average rate or concentration for release when flows are below average
