Texas Waste Water Treatment Ch#05 Preliminary & Primary Treatment

Grit in raw wastewater usually consists of sand, silt, gravel, and cinders. Grit removal units usually come after the bar screen and before the grease removal unit. It is desirable, where possible, to locate grit removal facilities ahead of raw wastewater pumps. Grit should be removed to prevent wear on pump impellers, stoppages, and filling up space in treatment units.

If the velocity is too high, the grit will be carried over into the primary clarifier; if the velocity is too low, some suspended organic solids may settle out with the grit and become septic in the chamber.

Pre-aeration prior to treatment helps to freshen the waste water, scour gases, and remove grease.

A grit removal unit of any type, no matter how well designed or well operated, will allow some organic matter, called detritus, to cling to and settle out with the grit. This organic matter could either be returned to the plant for normal appropriate processing or contribute to the total mass the plant must pay to dispose of.

Flow equalization refers to the sequestration of a portion of the influent waste water in a plant's equalization basin for later treatment so that the treatment process sees a more constant flow or to divert shock loads. Sending a more uniform flow through the treatment plant, is an important part of preliminary treatment. With uniform flow, the treatment system can be operated more efficiently and will produce a higher quality effluent. Sequestration of, for example, toxic industrial waste would prevent those toxins from negatively affecting the microorganisms integral to the treatment process.

A common way to remove grease is to pass the wastewater through a tank or chamber with a detention period from 5 to 15 minutes. Diffused air is introduced near the bottom of the tank. Grease particles cling to the air bubbles and rise to the surface where they can be skimmed. The amount of air required is between 0.5 and 1.0 ft.3 /gal. of wastewater. About 2 ppm of chlorine, fed as a gas with the air, aids in grease removal.

Primary settling should remove 90%–95% of the settleable solids, 50%–70% of the suspended solids, 35% of the BOD, and, 20%–30% of the total organic matter. Primary treatment is the removal of settleable solids from the raw waste water, and consists of a sedimentation basin or tank for the purpose of allowing gravity and density differences to separate settleable solids from the liquid. The main purpose of a sedimentation tank is to allow scum to rise and solids to settle under quiescent (low-velocity) conditions. Raw wastewater contains suspended material that may settle to the bottom when the flow velocity is greatly reduced. Velocities in sedimentation tanks are reduced to between 0.01 and 0.03 ft./sec. to allow the settleable solids to settle.

As the temperature of the water in the waste water increases it becomes less dense and less able to suspend solids, as flow velocity decreases it allows more of the solids to settle out instead of being carried along on the current, and larger and heavier solids settle faster while solids that have trapped gas bubbles from decomposition in transit will settle more slowly.

while this IS a question in module 5 review the following sentence does not appear until module 11. "Primary sludge has a fecal odor and is from 94% to 96% moisture." Module 5 has only this to say "The dry weight solids content (of primary sludge) is 4% to 6%..."

The Imhoff cone (imhoff cone = conical container) test can determine settleable suspended solids. A sample is collected in a 1-liter Imhoff cone at both the inlet and outlet ends of the clarifier. The samples are allowed to settle for 2 hours, after which the settled solids in each sample are measured. This test may also be used to determine the consistency of the sludge being pumped as waste or return activated sludge. the book does NOT address how this test may determine the depth of the sludge in the clarifier but the answer reflects that given by TEEX that may appear on a licensing test. Per the text book it should be the efficiency of the clarifier and possibly an estimation of your pumping schedule..

Temperature is a convenient measure of average molecular kinetic energy which describes how quickly the molecules are moving and how much they are bouncing around in the fluid. as temperature increases the molecules bounce around like teens in a mosh pit creating more average personal space for each molecule. since there are relatively less molecules per unit volume the fluid as a whole has become less dense.

Short circuit - Uneven flow through a tank encountering less resistance than designed causing the waste water to leave the clarifier before the solids have settled out..

Sludge withdrawal rate should be slow and frequent. too slow and it will cause problems in the clarifier but too fast can cause the sludge to be thin and send too much water to the digester.

Sludge withdrawal rate should be slow and frequent. too slow and it will cause problems in the clarifier but too fast can cause the sludge to be thin and send too much water to the digester.

A trapped slot in the bottom of the upper/settling compartment (Clarifier/sedimentation tank) permits the settled solids to pass through into the sludge compartment (digester). This trapped slot is designed to prevent gases resulting from the digestion process from rising into the settling compartment and agitating the settled sludge. The sloping bottom of the upper/settling compartment deflects gas-laden solids and causes them to rise into a third compartment, which is the scum chamber and gas vent.