CEE 371 Water and Wastewater Systems

Transcription

1 Updated: 9 December 2009 CEE 371 Water and Wastewater ystems Print version Lecture #32 Wastewater Treatment: Biological Principles and Biological Filtration Reading: Chapter 11, pp David Reckhow CEE 371 L#32 1 Biological WW Treatment Attached Growth Processes (Biological Filtration) olid upport Trickling Filters Biological Towers RBCs Membrane Bioreactors uspended Growth Processes (Biological Aeration) Activated ludge Review Microbiological Fundamentals and Metabolism from CEE 370 David Reckhow CEE 371 L#32 2 Lecture #32 Dave Reckhow 1

2 Question Bacterial growth is usually described by A. linear model B. Monod model C. Casagrande model D. treeter-phelps model E. Cannot be described David Reckhow CEE 371 L#32 3 Biological Filtration H&H, Fig 11-18, pp.381 Biomass: attached growth Bacterial community With protozoa, fungi, rotifers and higher forms Metabolism Dissolved organics diffuse in Oxygen enters from voids Physiologic state Aerobic mostly Anaerobic near surface facultative David Reckhow CEE 371 L#32 4 Lecture #32 Dave Reckhow 2

3 Biological Filtration II Layout Trickling filters: media is crushed rock Biological Tower: plastic media Rotating Biological Contactors: large discs Principles Bacteria attached to surfaces of supporting media David Reckhow CEE 371 L#32 5 Trickling Filters WW pumped up a vertical riser to a rotary distributorib Flow out of nozzles drives distributor arms WW then trickles through media Underdrains remove effluent and introduce air Bed depths range from 5-7 ft Always used after primary settling Drawbacks Efficiency may be marginal in cold climates Odors & filter flies (Psychoda) H&H, Fig 11-9, pp.368 David Reckhow CEE 371 L#32 6 Lecture #32 Dave Reckhow 3

4 Trickling Filters David Reckhow CEE 370 L#32 7 Trickling Filters Performance Most BOD removed in upper 2-3 feet Biomass in bottom 3 feet are starved Excess biomass sloughs off Media may become plugged if organic loading is high and hydraulic loading low Recirculation of some treated WW A second pass to remove more BOD Increase hydraulic loading for better flushing > 10 MG/acre/d for BOD loadings > 25 lb/1000 ft 3 /d David Reckhow CEE 371 L#32 8 Lecture #32 Dave Reckhow 4

5 H&H, Table 11-3, pp.383 Trickling Filter Design Criteria Where ettled BOD = wastewater BOD remaining after primary sedimentation Volume of media = bulk volume of stone in filter Q = wastewater flow not including recycle Q R = recirculation flow A = surface area of the filter David Reckhow CEE 371 L#32 9 ingle tage Flow chematic One of many possible designs H&H, Fig 11-20, pp.384 David Reckhow CEE 371 L#32 10 Lecture #32 Dave Reckhow 5

6 Two tage Design Helps to achieve 30 mg/l BOD Especially with higher influent BOD and cold temperatures Intermediate settling may be omitted Both filters are typically of identical design H&H, Fig 11-21, pp.384 David Reckhow CEE 371 L#32 11 H&H: Empirical Model From NRC data First tage TF econd tage TF Equation 11-8 H&H, Fig 11-22, pp.385 H&H, Fig 11-23, pp.385 David Reckhow CEE 371 L#32 12 Lecture #32 Dave Reckhow 6

7 H&H Model (cont.) Temperature adjustment H&H, Fig 11-24, pp.385 Overall Efficiency 35 E = 100% 100% E 2 E % BOD removal in 1 o Where E 1 and E 2 are the individual TF efficiencies From Figures and Adjusted for the particular temperature using Figure David Reckhow CEE 371 L#32 13 Alternative TF Model The NRC formula for a single stage Trickling Filter is: 100 E20 = 0.5 W VF Temperature (T in o C) effects may be adjusted with: E T = E ( T 20) The BOD effluent concentration ( effl ) is then: 100 ET effl = Pr im 100 Where: E 20 = BOD removal at 20 o C in percent prim = BOD concentration in primary effluent W = BOD load applied in lb/day W=Q prim or Qs o V= volume of filter media in ft 3 R = recirculation ratio (Q R /Q) F = recirculation factor 1+ R F = ( R) 2 David Reckhow CEE 371 L#32 14 Lecture #32 Dave Reckhow 7

8 Biological Towers Use of plastic media instead of stone Higher specific surface area (30-40 ft 2 /ft 3 ) for more biomass Better liquid distribution Light weight, yet strong enough for heights of 20 ft H&H, Fig 11-26, pp.388 David Reckhow CEE 371 L#32 15 Flow schematic for biological tower As with TF, use with primary settling Also use recycle 50 lb/1000 ft 3 /d or greater BOD loading 1 gpm/ft 2 or greater hydraulic loading H&H, Fig 11-27, pp.389 David Reckhow CEE 371 L#32 16 Lecture #32 Dave Reckhow 8

9 Biological Tower model I First order soluble BOD removal e kt As D Q = e / o Where e = soluble BOD in BT effluent o = soluble BOD in BT influent (note typo in book) k T = rate coefficient at T o C ( ) ( T k 20) T = k A s = specific surface area of media (area/volume) D = depth of media Q= hydraulic loading n= empirical constant, normally 0.5 David Reckhow CEE 371 L#32 17 n Flow MB Q+Q R, s o Primary Effluent Q, s P Biological Tower Mix Q R, s e Recirculation Flow Q, s P econdary Effluent Mass balance around mixing chamber ( Q + QR ) o = Q p + QRe QR Q p + QRe p Q e o = o = QR ( Q + QR ) ( 1+ Q ) + David Reckhow CEE 371 L#32 18 Lecture #32 Dave Reckhow 9

10 Biological Tower model II BOD as diluted by recirculation p + Re o = 1 + R Where 0 = soluble BOD in influent after dilution from recirculated flow P = soluble BOD in primary effluent before dilution with recirculated flow R = recirculation ratio (=Q R /Q) David Reckhow CEE 371 L#32 19 Biological Tower model III Combining We get e p = e A sd kt [ Q ( 1+ R) ] k As D n T QP ( 1+ R) ( 1+ R) Re P n [ ] n e kt As D / Q = e For the ratio of secondary effluent BOD ( e ) to the primary effluent, P (prior to recycle) o p + Re o = 1 + R David Reckhow CEE 371 L#32 20 Lecture #32 Dave Reckhow 10

11 RBCs Marquette, MI David Reckhow CEE 370 L#32 21 End To next lecture David Reckhow CEE 371 L#32 22 Lecture #32 Dave Reckhow 11