1. Wastewater Treatment

2. Municipal WW Management Systems Sources of Wastewater Processing at the Source Wastewater Collection Transmission and Pumping Treatment Reuse/Disposal

3. Sources & Types of WW u Domestic Wastewater –from residences, commercial & institutional –flows ~70-90% of water supplied u Industrial Wastewater –highly dependent on industry u Infliltration/Inflow –enters through leaks, foundation drains, etc. u Stormwater –for combined sewers - largely in older cities

4. Typical WW Characteristics

5. 1972: Federal Water Pollution Control Act u PL 92-500 subsequently amended and now called the Clean Water Act –established water quality goals “fishable & swimmable” and timetable –established National Pollution Discharge Elimination System (NPDES) –construction grants for WW treatment u required secondary treatment (30/30) –30 mg/L BOD 5 –30 mg/L TSS

6. Conventional WW Treatment Biological Process Preliminary Treatment Secondary SedimentationSludge Disinfection Primary SedimentationSludge

7. Preliminary Treatment u To remove materials that will interfere with subsequent treatment –Coarse Screening (bar racks) –Medium Screening –Comminution –Flow measuring –Pumping –Grit removal –Pre-aeration

8. Bar Racks u Metal bars spaced a few cm apart across water flow u mechanical or manually cleaned u size of unit set by approach velocity –0.6-1.0 m/s for mechanically cleaned –0.3-0.7 m/s for manually cleaned u see Fig 10.4 and example 10.2 on pg. 311

9. Grit Removal u Grit chambers intended to remove sand, cinders, gravel that may enter system by cracks in pipes, inflow etc. u Grit can cause excess wear in pipes and pumps u small sedimentation tanks; designed with the help of Stoke’s Law u no organics removal

10. Grit Chamber

11. Primary Sedimentation u Purpose: to remove suspended solids (smaller than grit, and less harmful) u Typical efficiency –67% TSS removal –33% BOD removal u Design parameters –overflow rate –weir loading rate –detention time Primary effluent is largely composed of soluble and colloidal organics which can be converted to settleable microbial solids and CO 2 by biological treatment

12. Primary Sedimentation –Primary Treatment –Removes ~50% of suspended solids

13. Primary Sed. Tank

14. Primary Clarifier: Center Feed

15. Secondary Treatment u Generally includes some biological process plus secondary clarification u Required under PL92-500 u Converts soluble and colloidal organic materials to biomass and CO 2

16. Biological Treatment u Suspended Growth –Activated Sludge »Conventional, Extended Aeration, Contact Stabilization –Aerated lagoons –Aerobic digestion u Attached Growth –Trickling Filters –Rotating Biological Contactors

17. Suspended Growth Systems Sludge Secondary Sedimentation Return Activated Sludge Waste Activated Sludge Aeration TankAir

18. Attached Growth: Trickling Filters Rocks or Plastic Media Underdrain Rotary Distributor With rocks, depth is limited to 2-3 m because of oxygen needs Air

19. Rotating Biological Contactors Drum diameters are typically 10-12 ft.Drum diameters are typically 10-12 ft. Rotation speed: ~1.5 rpmRotation speed: ~1.5 rpm May be in several stagesMay be in several stages No flow recycleNo flow recycle Requires pilotingRequires piloting

20. Secondary Clarifier

21. Sludge Disposal u Thickening –gravity, flotation u Digestion –aerobic, anaerobic u Mechanical Dewatering –Vacuum filtration, centrifugation, pressure filtr. u Disposal –land application, burial, incineration

22. Anaerobic Digestion Sludge held without aeration for 10-90 daysSludge held without aeration for 10-90 days Process can be accelerated by heating to 35-40 o CProcess can be accelerated by heating to 35-40 o C These are called High Rate Digestors (10-20 days)These are called High Rate Digestors (10-20 days) AdvantagesAdvantages low solids productionlow solids production useable methane gas produceduseable methane gas produced DisadvantagesDisadvantages high capital costshigh capital costs susceptibility to shocks and overloadssusceptibility to shocks and overloads

23. Sludge Dewatering u Sludge drying beds –historically the most common –sand bed, 15-30 days, evaporation & seepage u Vacuum Filtration –cylindrical rotating drum covered with fabric –submerged with applied vacuum u Continuous belt filter presses (follows) u Plate pressure filters –vertical plates mounted on a frame

24. Belt Filter Press (Komline-Sanderson)