Wastewater Treatment

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Activated Sludge Process will encompass
* Design of aeration basins. Relevant design parameters are * BOD load from the upstream process
* Mixed Liquor Suspended Solids Concentration (MLSS – mg/l) – typically 2,000 to 4,000 * Sludge Age 5 to 25 days
* Sludge settleabiltiy (SVI ml/g) typically 100-120 * Process Oxygen Demand (POD)
* Generally fine bubble diffusers used in modern plant design – actual oxygen transfer efficiency dependent of depth of water column * Blowers capacity – takes account of concentration of oxygen in air, α and ß factors for aeration basin * Dissolved Oxygen in aeration basin usually maintained in the range of 0.5 to 2 mg/l

* Activated Sludge Process - continued
* Sludge Production must be estimated from relevant inputs including incoming BOD/SS loads, sludge age etc * This should take account separately of increased sludge arising from chemical precipitation of phosphorus – ferric chloride * Influent phosphorus load may be calculated at rate of 3 g/PE * Ferric chloride dosed at rate of c 1.5 moles of Fe3+ per mole of Phosphorus in wastewater * Chemical sludge production from weight of precipitate * Returned Activated Sludge (RAS) – pumping generally provided for at up to 1.5 DWF- here that is equal to half the FFT (as 3 DWF). DS content of RAS from final settlement tank design

* Activated Sludge Process - continued
* Final Settlement Tanks - Important Design Parameters
* Almost invariably in circular tanks. Mixed liquor introduced to centre of tank and flows radially outward to overflow weirs * Surface Loading rate (m3/m2/hr) at FFT. Note RAS flows are not included in surface loading calcs. * Weir loading rate

* Side wall depth – typically >2.0m
* Floor Slope
* Sludge withdrawal – half bridge scrapers and central sludge hopper

In some plants where separate grit-removal facilities are not used ahead of primary sedimentation tank, or where the grit-removal facilities are not sufficient to handle peak flows and peak grit loads, it may be essential to remove the grit before further processing of the sludge. Where further thickening of the primary sludge is desired, a practical consideration is Sludge Degritting. The most effective method of Degritting sludge is through the application of centrifugal forces in a flowing system to achieve separation of the grit particles from the organic sludge. Such separation is achieved through the use of cyclone degritters, which has no moving parts. The sludge is applied tangential to a cylindrical feed section, thus imparting a centrifugal force. The heavier particles move to the outside of the cylinder section and are discharged through a cylindrical feed section. The organic sludge is discharged through a separate outlet. The efficiency of the cyclone degritters is affected by pressure and by the concentration of the organics in the sludge. To obtain effective grit separation, the sludge may be relatively dilute 1 to 2 percent. As the sludge concentration increases, the particle size that can be removed decreases.

Sludge is generated in primary, secondary and advanced wastewater-treatment process. Primary sludge consists of settleable solids carried in the raw wastewater. Secondary sludge consists of biological solids as well as additional settleable solids. Sludge produced in the advanced wastewater may consist of biological and chemical solid. Sludge is blended to produce a uniform mixture to downstream operations and process. Uniform mixtures are most important in short-detention-time systems, such as sludge dewatering, heat treatment, and incineration.

Gravity thickening is the simplest and least expensive process for consolidating waste sludge. Thickening is the practice of...
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