Wastewater Treatment

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

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

Typical WW Characteristics

1972: Federal Water Pollution Control Act u PL 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

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

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

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 – m/s for mechanically cleaned – m/s for manually cleaned u see Fig 10.4 and example 10.2 on pg. 311

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

Grit Chamber

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

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

Primary Sed. Tank

Primary Clarifier: Center Feed

Secondary Treatment u Generally includes some biological process plus secondary clarification u Required under PL u Converts soluble and colloidal organic materials to biomass and CO 2

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

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

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

Rotating Biological Contactors Drum diameters are typically ft.Drum diameters are typically 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

Secondary Clarifier

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

Anaerobic Digestion Sludge held without aeration for daysSludge held without aeration for days Process can be accelerated by heating to o CProcess can be accelerated by heating to 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

Sludge Dewatering u Sludge drying beds –historically the most common –sand bed, 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

Belt Filter Press (Komline-Sanderson)