Wastewater Treatment
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
DIFFERENT PROCESS OF SLUDGE
* 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
DIFFERENT PROCESS OF SLUDGE