Physical water/wastewater treatment processes

Transcription

1 Physical water/wastewater treatment processes

2 Tentative schedule (I) Week 1: Introduction Week 2: Overview of water/wastewater treatment processes Week 3: Major contaminants (Chemicals and pathogens) Week 4: Preliminary treatment (Screen) Week 5: Preliminary treatment (Grit Chamber) Week 6: Sedimentation 1 Week 7: Sedimentation 2 Week 8: Mid-term

3 Sedimentation

4 Classification of settling Based on the concentration of the particles and the ability of the particles to interact Type 1: discrete, nonflocculent particles in a dilute suspension Type 2: flocculent particles in a dilute suspension Type 3: intermediate concentration of particles, considerable interparticle forces Type 4: high concentation of particles, significant interparticle forces (compression)

5 Type III settling Also called zone or hindered settling. The settling of an intermediate concentration of particles in which the particles are so close together that interparticle forces hinder the settling of neighboring particles The mass of particles settle as a zone. Example: the settling that occurs in the intermediate depths in a final clarifier for the activated sludge process

6 Type IV settling Also called compression settling The settling of particles that are of such a high concentration that the particles touch each other and settling can occur only by compression of the compacting mass. Example: the compression settling the occurs in the lower depths of a final clarifier for the activated sludge process

7 Wastewater treatment plant (typical municipal wastewater)

8 Settling of a concentrated suspension

9 Designing of final clarifiers for the activated sludge processes Based on solids flux concept The rate of solids thickening per unit area in plan view (kg/h-m 2 ) G s = C t V t Where G s = solid flux by gravity C t = solid concentration V t = hindered settling velocity G b = C t V b Where G b = bulk flux V b = bulk velocity Total solid flux (G t ) for gravity setting and bulk movement G t = G s + G b = C t V t + C t V b

10 Designing of final clarifiers for the activated sludge processes

11 Designing of final clarifiers for the activated sludge processes V b = Q u /A Where V b = bulk velocity Q u = flowrate of the underflow A = plan area of the tank M t = Q 0 C 0 = Q u C u Where Mt = mass rate of solids settling Q 0 = influent flowrate of the tank C 0 = influent solids concentration C u = underflow conc. A L = M t /G L = Q 0 C 0 /G L A L = limiting cross-sectional area Where G L = limiting flux

12 Designing of final clarifiers for the activated sludge processes Finally, V b = Q u /A = M t /C u A = G L /C u Q u = M t /C u Mt/A = G L

13 Designing of final clarifiers for the activated sludge processes

14 Example 1: Final clarifier Batch settling tests have been performed using an acclimated activated sludge to give the data in Table 9.6 The design mixed liquor flow to the final clarifier is 160 L/s, the MLSS is 2500 mg/l, and the underflow concentration is 12,000 mg/l. Determine the diameter of the final clarifier.

15 Example 1. Analysis

16 Actual sedimentation basins (circular) Inlets: center or on the periphery Center: < 9.14 m in diameter: downward flow Center: > 9.14 m in diameter: upward flow Outlet: weir The depth of a circular clarifier: the depth at the side of the tank (swd)

17 Actual sedimentation basins (circular)

18 Actual sedimentation basins (circular)

19 Actual sedimentation basins (Circular)

20 Actual sedimentation basins (circular)

21 Theoretical vs. actual retention time Actual retention time is affected by Dead spaces in the basins Eddy currents Wind currents Thermal currents If there are dead space, Mean t/theoretical t < 1 If short circuiting is occuring, Mean t/median t < 1

22 Settling basin and tracer studies

23 Main design criteria Overflow rate (design settling velocity) Detention time Depth

24 Sedimentation in water treatment plants (I) Two types: Plain sedimentation and sedimentation for chemically coagulated water Plain sedimentation Water with high turbidity (due to silt) Very long detention time (30 days) and extremely large volume Sedimentation for chemically coagulated water Determining factors: the characteristics of the water, the coagulant used, and the degree of flocculation The main design parameters (settling velocities, the required overflow rates, and detention times) should be determined only by experimental settling tests

25 Batch settling test (procedure) Samples are removed at periodic time intervals and the suspended solids concentration are determined The percent removal is calculated for each sample and plotted on a graph as a number versus time and depth of the collection Interpolation are made between the plotted points and the curves of equal percent removal are drawn

26 Sedimentation in water treatment plants (II) Water coagulated with alum Produce light flocs Overflow rate: 20.4 to 32.6 m 3 /d-m 2 Weir or orifice channel loading: 149 to 224 m 3 /d-m 2 Detention time: 2 to 8 hours (4 to 6 hours common) Water coagulated with iron salts Produce dense flocs Overflow rate: 28.6 to 40.8 m 3 /d-m 2 Weir or orifice channel loading:199 to 273 m 3 /d-m 2 Detention time: 2 to 8 hours (4 to 6 hours common)

27 Sedimentation in water treatment plants (III) Water after softening Overflow rate: 28.6 to 61.2 m 3 /d-m 2 Weir or orifice channel loading:273 to 323 m 3 /d-m 2 Detention time: 4 to 8 hours

28 Example 2: Clarifier for water treatment A rectangular clarification basin is to be designed for a rapid sand filtration plant. The flow is 30,000 m 3 /day, the overflow rate or surface loading is 24.4 m3/d-m 2, and the detention time is 6 h. Two sludge scraper mechanisms for square tanks are to be used in tandem to give a rectangular tank with a length to width ratio of 2:1. Determine the dimensions of the basin.

29 Sedimentation in wastewater treatment plants Primary sedimentation: To remove settleable solids from raw wastewaters Secondary settling procedure To remove the MLSS (activated sludge) To remove any growths that may slough off the filter (trickling filters) Sedimentation for chemically coagulated water To remove flocculated suspended solids (advanced or tertiary wastewater treatment plants)

30 Wastewater treatment plant (typical municipal wastewater)

31 Water treatment plant (typical surface water)

32 Batch settling test (procedure) Samples are removed at periodic time intervals and the suspended solids concentration are determined The percent removal is calculated for each sample and plotted on a graph as a number versus time and depth of the collection Interpolation are made between the plotted points and the curves of equal percent removal are drawn

33 Designing of final clarifiers for the activated sludge processes Finally, V b = Q u /A = M t /C u A = G L /C u Q u = M t /C u Mt/A = G L

34 Recommended criteria for primary clarifier (I)

35 Recommended criteria for primary clarifier (II) Detention time: 45 min to 2 hr Multiple tanks should be used when the flow > 3.8MLD Peak weir loading < 248 m 3 /d-m (< flow of 3.8 MLD) < 373 m 3 /d-m (> flow of 3.8 MLD) BOD 5 removal Correlated to detention time and overflow rate

36 BOD 5 removal vs. overflow rate

37 BOD 5 removal vs. retention time

38 Example 3: Primary clarifier A primary clarifier for a municipal wastewater treatment plant is to be designed for an average flow of 7570 m 3 /d. The regulatory agency criteria for primary clarifiers are as follows: peak overflow rate = 89.6 m 3 /d-m 2, average overflow rate = 36.7 m 3 /d-m 2, minimum side water depth = 3.0 m, and peak weir loading = 389 m 3 /d-m. The ratio of the peak hourly flow to the average hourly flow is Determine: 1. The diameter of the clarifier 2. The peak weir loading if peripheral weirs are used. Is it acceptable?

39 Recommended criteria for secondary clarifier

40 Suggested depth for final clarifiers

41 Recommended criteria for final clarifier Detention time: 1.0 to 2.5 hr Multiple tanks should be used when the flow > 3.8MLD Peak weir loading < 248 m 3 /d-m 2 (< flow of 3.8 MLD) < 373 m 3 /d-m 2 (> flow of 3.8 MLD)

42 Designing of final clarifiers for the activated sludge processes

43 Example 4: Final clarifier A final clarifier is to be designed for an activated sludge treatment plant serving a municipality. The state s regulatory agency criteria for final clarifiers used for activated sludge are as follows: peak overflow rate = 57.0 m 3 /d-m 2, average overflow rate = 24.4 m 3 /d-m 2, peak solids loading = 244 kg/d-m 2, peak weir loading = 373 m 3 /d-m, and depth = 3.35 to 4.57 m. The flow to the reactor basin prior to junction with the recycle line = 11,360 m 3 /day. The maximum recycled sludge flow is 100% of the influent flow and is constant throughout the day. The MLSS = 3,000 mg/l, and the ratio of the peak hourly influent flow to the average hourly flow is Determine 1. The diameter of the clarifier 2. The depth of the clarifier 3. The peak weir loading if peripheral weirs are used. Is it acceptable?

44 Inclined-settling devices To increase overflow rates (3-6 times) To increase the capacity of existing clarifier Can be used both circular and rectangular clarifier

45 Inclined-tube settler

46 Inclined-tube settler on a circular clarifier

47 Inclined-tube settler on a rectangular clarifier

48 Inlet and outlet hydraulics (a rectangular tank) (I) Inlet (an orifice flume) The discharge from the most distant orifice from the influent pipe be at least 95 % of the that from the most closest orifice Q = 0.6 A 2gh Where Q = discharge from an orifice (m 3 /s) A = orifice area (m 2 ) h = head loss (m) g = gravitational acceleration

49 Inlet and outlet for a rectangular tank

50 Inlet details

51 Outlet details

52 Inlet and outlet hydraulics (a rectangular tank) (II) Outlet: suppresed weir or V-notched weir Suppressed weir Q = 1.84LH 3/2 Where Q = discharge (m3/s) L = weir length (m) H = head (m) V-notched weir Q = 1.40H 5/2

53 Inlet and outlet hydraulics (a rectangular tank) (III) H 0 = (d 2 + 2Q n2 LQ 2 gb 2 d b 2 r 4/3 d m ) Where H 0 = upstream water depth (m) d = downstream water depth (m) Q = total discharge (m 3 /s) b = channel width (m) n = Manning s friction factor L = channel length (m) r = mean hydaulic radius (m) d m = mean depth (m)

54 Inlet and outlet hydraulics (a rectangular tank) (IV) d m = H 0 1/3(H 0 d) r = d m b/2 d m +b The minimum value of d = critical depth (y c ) = (q 2 /g) 1/3 where q = the discharge per unit width of channel

55 Example 5 A rectangular setting tank basin is m wide by m long and has a flow of m3/s. The inlet flume is an orifice channel with eight orifices that are 216 mm in diameter, each with an area of m2. The difference in the elevations of the water surface at the influent pipe (that is, the center of the flume) and at the last orifice in the flume is m/ This head loss is due to the friction and form loss in the flume. The effluent weir plate consists of 90 C V-notch weirs spaced at 203 mm centers. The effuent channel is m in length and extends across the downstream end of the basin, which is 21.33, and m upstream along each side. The effluent channel is rectangular in cross section and is in width. There is a 102 mm freefall between the crests of the V-notch weirs and the maximum water depth. The Manning friction coefficient is 0.032, and there is a freefall at the effluent box. Determine, 1. The ratio of the flow from the last influent orifice to the flow from the influent orifice nearest to the influent pipe 2. the head loss on the V-notch weirs