Preliminary Treatment Flashcards
Purpose of Preliminary Treatment
- remove harmful debris
- remove inorganics
- remove grease and oil
- freshen septic wastewater
- flow measurement
- emergency storage
Negative consequences of not screening or removing grit
Damage to pumps, plug pipes, take up space in tanks
Flow measuring device types
- constant differential
- Weirs and flumes
- Velocity meters
- Differential producers
- Displacement units
What is a rotameter?
Flow measuring device where a float is placed in the flow line and pressure differences above and below the float cause it to move. Instantaneous flow rate is read on calibrated scale attached to the tube.
How do head area flow measuring devices work?
A mechanical barrier or constriction in the flow causes a predictable rise in water level upstream. This measurement (head) can be used to calculate flow.
Examples of head area flow meters
Weirs and flumes
How do velocity meters work?
They measure velocity of water moving through a defined area, which allows for calculation of flow
Examples of velocity meters
- propeller
- magnetic
- shuntflow
How do differential producer flow meters work?
A mechanical constriction inside a pipe causes increased flow velocity. Increased velocity results in lower pressure at the restriction. The pressure difference can be used to calculate flow rate.
How do displacement unit flow measuring devices work?
Water or gas fills a tank or chamber of known dimensions. Rate of fill is measured and used to calculate flow rate
Difference between a rack and a bar screen
Spacing of a rack is 3-4 inches, and spacing of a bar screen is 3/8 to 2 inches. Bar screens are usually mechanically cleaned whereas racks are usually manually cleaned.
How much screen material will a mechanically cleaned bar screen produce?
Depends on influent, but a range of 0.5-12 cubic feet per million gallons
Advantages of comminutors over screens
Communitors reduce odor, flies, and the need for screening disposal
Disadvantages of comminutors
Shredded materials tend to reweave together downstream and wood and plastic is often rejected and requires manual removal
What has replaced mercury seals in comminutors and why?
Mechanical seals; mercury is toxic
