Tertiary FiltrationTertiary Filtration

Granular Media Rapid Sand Filtration Granular Media Rapid Sand Filtration (RSF)(RSF)

RSF w/o Chemicals: Removal of Residual SS (Biological Floc)

RSF w/ Coagulant (Al, Fe), w/ or w/o Polymer, or Lime;

Enhanced Solids Removal, P (PO43-) Precipitation

WW RSF (Larger, More Variable Size, and Higher Concentrations of Particles in II-Effl.)

vs. RSF in D.W. (Settled-Water Turbidity: 1 –10 NTU) WW Solids: Residual Biosolids, Chemical Flocs and Precipitates III-Filtration Serves as “Pretreatment” for Other AWT Processes General Goal: SS 5 mg/L; Turbidity 5 NTU; P 1 mg/L Performance (NTU): Figure 11-2

Figure 11-2

III/AWT RSF Design and OperationIII/AWT RSF Design and Operation

WW RSF Generally Similar to D.W. RSF Except:Shorter Run Times (e.g., 24 hrs, Daily Backwash) and Greater Surface Straining

Media: Single (Sand or Anthracite) vs. Multi-Media (Sand, Anthracite, and/or Garnet); Shallow- vs. Deep-Bed; Coarser Media Possible

Filtration Rate: 2 – >10 gpm/ft2; Constant vs. Declining Rate Biofiltration: Nitrification Unlikely Because of Low DO; De-Nitrification if Designed Accordingly (Low Filtration Rate, Supplemental

C); Increased Headloss by Biological Activity; Control by “Pre”-Chlorination Activated Sludge Bioflocs Stronger and more Filterable than Trickling Filter

Solids Biological Flocs are Stronger than Chemical Flocs and Use of Coarser Media is

Possible; Chemical Floc Properties Improved by Polymers Performance of “Plain” Filtration Following Activated Sludge:

SS = 1 – 5 mg/L

III- Filtration with Chemical AdditionIII- Filtration with Chemical Addition

Chemicals Added: Metal Coagulants (Al(III), Fe(III)), Organic Polymers, Lime Metal Coagulants:

Al3+ (or Fe3+) + 3 OH- Al(OH)3 (or Fe(OH)3)In AWT, Generally Operate in Sweep Floc Region

Al3+ (or Fe3+) + PO43- AlPO4 (or FePO4); Figure 6-13

Organic Polymers:Primary Coagulants (Cationic Polymers) vs. Coagulant (Filter) Aids

Lime (Ca(OH)2 Ca2+ + 2 OH-):

Ca2+ + CO32- CaCO3

Mg2+ + 2 OH- Mg(OH)23 Ca2+ + 2 PO4

3- Ca3(PO4)25 Ca2+ + 3 PO4

3- + OH- Ca5(PO4)3OH (hydroxyapatite) Table 6-5

Figure 6-13

Table 6-5

III- Filtration with Chemical Addition – III- Filtration with Chemical Addition – ContinuedContinued

P Forms: Particulate vs. Dissolved; Table 6-4

Org-P, polyphosphates (e.g., P2O74-, hydrolyzes to orthophosphate),

orthophosphate (PO43-) ; pH Effects (part of H3PO4 system); Table 6-3

Point of Application for P Removal:Before I-Sedimentation (High Doses)Before Biological Process (Inert Solids)Before II-Sedimentation (In Lieu of III-Filter, High Doses), Before III-Filtration (Lower Doses; In-Line, Static Mixer, Rapid Mixer or Flocculator)

Clarification Before Filtration if High Doses Performance: Up to 95 % P Removal; Turbidity 1 NTU Other Benefits of Chemical Clarification:

High-pH Disinfection by Lime; Physical Removal of Pathogens;

Chemical Precipitation of Metals (e.g., Zn(OH)2, or Adsorption onto Al(OH)3 Floc)

Table 6-4

Table 6-3