Texas Basic Waste Water Operations Ch#07 Sedimentation

Sedimentation separates suspended solids from water by gravity.

Baffles slow the wastewater and disperse the flow evenly through the tank. The baffle in the center of a circular clarifier is called a stilling well. Inlet baffle design should prevent short circuiting. A skimmer at the surface of the water will direct scum to a scum box. A grease ring will keep the floating material from going over the weir with the clarified effluent. Sludge rakes move the sludge toward disposal at the bottom of the clarifier.

Primary clarifiers or primary settling tanks settle primary (raw) sludge and are placed after preliminary units and ahead of secondary treatment. This reduces the organic load on secondary treatment. Primary clarifier effluent is light or dark gray. Secondary clarifiers, or final clarifiers, settle sludge that has been biologically treated under aerobic conditions such as aeration or other means of oxidation. The effluent should be clear.

Short-Circuiting not short-cutting. Short-circuiting is the uneven flow of water through a tank. The flow travel time is less than the flow-through time. Short-circuiting affects detention time and velocity, resulting in reduced solid and BOD removal. Factors causing a hydraulic short circuit include uneven weirs, inadequate baffles, and wave action and currents. Solid Weight - Heavy suspended solids settle quickly and carry solids too light to settle (colloidal) downward. |!!!Water Temperature - Water density depends on temperature. As water warms, it becomes less dense and solids settle more rapidly in warm water. If the wastewater is fresh, settling is usually better in the summer than winter.!!!| Detention Time - Detention time is the time, usually in hours, that water is held in a tank. About half of the suspended solids in wastewater settle in 30 minutes. In two hours, about 70% of the solids settle. Usually, clarifier detention time ranges from 1 to 2 hours, but may be 4 hours or longer. | Velocity - If the flow is too fast, the detention time is reduced and light particles will not settle. Tank design requires a flow of 1.0 ft/min or 0.02 ft/sec in horizontal tanks, less in up-flow tanks, and 0.1 to 0.3 ft/sec in circular tanks. | Surface Loading - Surface loading or overflow rate is the number of gallons leaving the clarifier per day per square foot of clarifier surface (gal/ft2 /day). | Wastewater Condition - Septic wastewater is slower to settle because smaller particle size and gas bubbles cause solids to float.

Inlet zone - The inflow distributes evenly over the tank. Settling zone - Settling occurs uniformly; short detention time causes light solids to go over the weir with the effluent. Sludge zone - The settled solids collect below the settling zone. Outlet zone - Clarified effluent discharges.

Short-circuiting is the uneven flow of water through a tank. The flow travel time is less than the flow-through time. Short-circuiting affects detention time and velocity, resulting in reduced solid and BOD removal. Factors causing a hydraulic short circuit include uneven weirs, inadequate baffles, and wave action and currents.

Primary clarifiers should remove 35% of the BOD and 90–95% of the settleable solids.

Primary sludge is about 94–96% water and 4–6% solids. Secondary sludge contains more water (about 99%) than primary sludge and is more difficult to settle.

Maintenance includes daily inspection of skimmer (if the skimmer fails, the sludge rake also fails), lubrication of the rake drive (or any other maintenance), and daily weir cleaning. NOTE: while this is the referenced passage from the text book it hardly explains why so i would like to clarify that the skimmer is mounted on the outside end of the rake so if it stops moving then it will be because the rake stopped moving.

Look for floating sludge or excessive gas bubbles, which indicate return sludge pump or sludge rake failure, or poor pumping schedule.

Sludge removal is usually intermittent. Usually, sludge from primary clarifiers goes to a digester. Raw sludge settles slowly and must be collected gently to prevent resuspension of the solids. Withdraw sludge slowly or water will break through the sludge layer, diluting the sludge. Maintain a sludge blanket to compact the sludge before pumping it to the digester. NOTE: For future classes remember that "sludge is removed slowly when adequately thickened".

Usual blanket depth in a secondary clarifier is no more than 1/4 of the sidewall depth. Blanket depth varies as conditions change.

Imhoff tanks are two-story units with three compartments. The upper compartment is the settling tank. The lower compartment is the digester. The settling compartment provides detention time of about two hours. Gas vents alongside the settling tank are the third compartment. A trapped slot in the bottom of the settling compartment permits settled solids to pass into the digester, but prevents septic solids from rising into the settling compartment. Gas bubbles and septic solids are deflected into the gas vents by the sloping bottom of the settling tank.

Imhoff tanks are two-story units with three compartments. The upper compartment is the settling tank. The lower compartment is the digester. The settling compartment provides detention time of about two hours. Gas vents alongside the settling tank are the third compartment. A trapped slot in the bottom of the settling compartment permits settled solids to pass into the digester, but prevents septic solids from rising into the settling compartment. Gas bubbles and septic solids are deflected into the gas vents by the sloping bottom of the settling tank.

Imhoff tanks are two-story units with three compartments. The upper compartment is the settling tank. The lower compartment is the digester. The settling compartment provides detention time of about two hours. Gas vents alongside the settling tank are the third compartment. A trapped slot in the bottom of the settling compartment permits settled solids to pass into the digester, but prevents septic solids from rising into the settling compartment. Gas bubbles and septic solids are deflected into the gas vents by the sloping bottom of the settling tank.

Imhoff tanks are two-story units with three compartments. The upper compartment is the settling tank. The lower compartment is the digester. The settling compartment provides detention time of about two hours. Gas vents alongside the settling tank are the third compartment. A trapped slot in the bottom of the settling compartment permits settled solids to pass into the digester, but prevents septic solids from rising into the settling compartment. Gas bubbles and septic solids are deflected into the gas vents by the sloping bottom of the settling tank.

Imhoff tanks are two-story units with three compartments. The upper compartment is the settling tank. The lower compartment is the digester. The settling compartment provides detention time of about two hours. Gas vents alongside the settling tank are the third compartment. A trapped slot in the bottom of the settling compartment permits settled solids to pass into the digester, but prevents septic solids from rising into the settling compartment. Gas bubbles and septic solids are deflected into the gas vents by the sloping bottom of the settling tank. NOTE: referenced five times in a row it might be that this is somewhat important

To clean, drag a chain through the slot in a saw-like motion. Chaining prevents solids from collecting in the settling chamber, becoming septic, and rising and flowing over the weir. NOTE: This is referring to the trap slot at the bottom of an imhoff digester settling chamber.

When sludge is within 18 inches of the slot, withdraw sludge. Digested sludge is dark in color with a tarry odor. Do not withdraw more than 50% of the sludge at once.

Foaming is indicated when gas vents boil over into the sedimentation compartment. Foaming may be caused by industrial waste, drawing too much sludge, restricted gas vents, or low pH. Foaming may be prevented by wasting small amounts of sludge, controlling industrial waste, keeping sludge pH near neutral with lime, and removing scum on gas vents.

Foaming is indicated when gas vents boil over into the sedimentation compartment. Foaming may be caused by industrial waste, drawing too much sludge, restricted gas vents, or low pH. Foaming may be prevented by wasting small amounts of sludge, controlling industrial waste, keeping sludge pH near neutral with lime, and removing scum on gas vents.