1. Tertiary Filtration

2. Granular Media Rapid Sand Filtration (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

3. Figure 11-2

4. III/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

5. III- 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  Ca OH-):
Ca2+ + CO32-  CaCO3
Mg OH-  Mg(OH)2
3 Ca PO43-  Ca3(PO4)2
5 Ca PO43- + OH-  Ca5(PO4)3OH (hydroxyapatite) 
Table 6-5

6. Figure 6-13

7. Table 6-5

8. III- Filtration with Chemical Addition – Continued
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)

9. Table 6-4

10. Table 6-3