Design Factors(1)


Surface overflow rate (SOR)
depends on solids characteristics.

At peak hourly flow: SORp < 1500 gpd/sf;
At average flow: SORA < 700 ~ 1000 gpd/sf. 

Detention time
1.5 - 2.5 hours based on average flow, QA,d.

Inlet structure

    Dissipate the inlet velocity; 
    Distribute flow equally both horizontally & vertically; 
    Eliminate corner pockets and dead ends; 
    Maintain velocity of inlet channel> 1.0 fps; 
    Baffle of full-width, 6" below the water surface and12" below the inlet orifice bottom. 

Figure 8. Inlet Structure

Outlet structure
    Uniform withdrawal and avoid short circuiting; 
    Adjustable weir for leveling; 
    V-notch or flat weir under free fall flow; 
    Weir rate 10,000 gpd/ft - 15,000 gpd/ft. 

Figure 9. Outlet Structure

Scum collection and handling
Tilting scum trough extends the full width, close to outlet and stop the loss of scum into effluent; Blade scraper manually or mechanically scrap scum. 
Scum transport and handling
    Use glass lined pipes and keep lines warm > 15oC; 
    Keep mixing and maintain warm temperature in tank during pumping; 
    Standby pump and cleanout, flushing connections. 
Sludge collection and removal
    Direction of sludge removal is from outlet to hopper; 
    Conveyor chain and flights or travelling bridge; 
    Collector linear speed = 2 fpm - 3 fpm; 
    Cross screw collector rotates at 10 rpm. 

Figure 10

Sludge hopper

    Minimum side wall slope, 1.7:1 (vertical:horizontal); 
    Hopper bottom width, 2 ft; 
    Extra depth are not acceptable to avoid septic. 
Sludge withdrawal
    Non-clog Centrifugal pumping can be used; 
    Sludge line > 6"; 
    Shortest and straight suction pipes; 
    Continuous or interval withdrawal; 
    Flushing water connected to suction & discharge lines. 


(1). The Committee of the Great Lakes--Upper Mississippi River Board of State Sanitary Engineers (Ten States), Recommended Standards for Sewage Water, Health Education Service, Albany, New York, 1978.

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