Physical Wastewater Treatment Processes Flashcards
1
Q
List the WWTP process flow diagram
A
Screening, grit removal, flow equalisation, primary sedimentation, biological treatment, secondary sedimentation, discharge
2
Q
What can be added as a ‘cleaning’ step to a WWTP?
A
Tertiary treatment
3
Q
Name 5 different screening devices
A
Bar screen (mechanical or hand) Drum screen Disposable bag Comminuter Grinder
4
Q
Name 2 types of screening
A
Coarse, fine
5
Q
Where does the product removed in screening end up?
A
Landfill/incineration
6
Q
When would you use mechanical screening?
A
For larger applications such as cities
7
Q
Why is flow equalisation necessary?
A
- To eliminate/minimise problems in the process caused by fluctuations in incoming flow rates and concentrations
- Optimise the time required for treatment in secondary and tertiary processes
- Lower the strength of the wastewater by diluting it with water already in the equalisation basin
8
Q
What would cause changes in incoming flow rate?
A
- Day vs. night production processes
- Different manufacturing processes carried out at different times
- Storms/high rain fall
9
Q
Why are equalisation basins well-mixed?
A
- Prevent solid deposition
- Important in concentration equalisation
- Provides aeration which reduces odour (and kick starts decomposition)
10
Q
Draw a flowchart for inline equalisation
A
Drawing 10
11
Q
Draw a flowchart for offline equalisation
A
Drawing 11
12
Q
What is the difference in use between inline and offline equalisation tanks?
A
Offline equalisation is probably a retrofit to an old style plant and is only used when surges are experiences.
13
Q
List 4 advantages of flow equalisation
A
- Improves performance of downstream operations
- Reduces the operating and capital cost of downstream processes
- Biological treatment is enhanced
- Minimises surface area required in settler and filter
14
Q
List 3 disadvantages of flow equalisation
A
- Large land area may be required
- Additional capital and operating costs may be required
- May cause odour problems - particularly important if near residential areas
15
Q
Name the 3 different types of oil in wastewater, describe their features and how they can be separated
A
- Free oil
>40 mm droplets separated through buoyancy forces in gravity separators - Emulsified oil
<20 mm, stable emulsions, gravity separators are used but inefficient - Dissolved oil
true solution in water
16
Q
What are the two classes of devices used for the physical separation of oils?
A
Gravity separators
Air flotation devices
17
Q
When is air flotation typically used and what is its principle of operation?
A
- When gravity separation is not adequate
- Fine gas bubbles attach to the oil droplets and increase their buoyancy, causing them to float toward the free surface of the liquid
18
Q
What law does air flotation work on?
A
Stokes law - the physical relationship that governs the settling solid particles in a liquid - it is the drag force exerted on spherical objects in a fluid
19
Q
Why do particles rise in air flotation?
A
Air molecules combine with the oil to form ‘life preservers’ - mass having a specific gravity which is less than the liquid, which makes solids that would eventually settle rise to the top instead
20
Q
Name and describe the 3 types of bubble generation
A
- Dispersed air flotation - mechanically dispersing air injected under rotating impellers, or sparged by diffusers
- Vacuum flotation - air is dispersed into the water to achieve saturation conditions and then a vacuum applied to create microbubbles
- Dissolved air flotation (DAF) - air is dissolved in pressurised wastewater (in a retention tank) and then fed into a flotation unit by passing through a reducing valve to release microbubbles above a diffuser
21
Q
What are the advantages of flotation over sedimentation?
A
- More efficient removal of ‘light particles’ that settle slowly
- Shorter time (good for treating larger volumes and keeping efficiencies high)
- Decreased footprint
22
Q
What are the disadvantages of flotation over sedimentation?
A
- Increased service and maintenance (as it is a more complex equipment set up)
- Cost
- Energy (creating microbubbles
23
Q
How is the choice between flotation and sedimentation decided?
A
Flotation is generally used when we know the waste water has a high amount of particles that will settle slowly
24
Q
Describe the working principle of a DAF unit
A
Water is fairly coagulated on entrance.
Flocculation step - add a chemical flocculant to overcome the charges of the particles to stick the together and form flocs - this happens during rapid mixing.
Microbubbles attach to the flocs and float to the surface. Water at the bottom is fairly clarified.
(97-98% efficient)
25
Define sedimentation
Separation from water by gravitational settling of suspended particles that are heavier than water
26
Describe the principle of sedimentation
The velocity of the water is lowered below the suspension velocity and the suspended particles settle due to gravity, they are then removed as sludge, and floating solids are removed as scum.
27
What does Thames water think you can do with the floating oils from sedimentation?
Turn it into biofuels
28
How is a sedimentation basin designed?
Large rectangular/circular tank designed to hold the water for a long enough time for the suspended solids to settle out. Longer retention times give better performance but tanks get bigger and more expensive
29
What are the 4 zones in a sedimentation basin?
1. inlet
2. settling
3. sludge
4. outlet
30
Draw a rectangular sedimentation basin
Drawing 12
31
Draw a circular sedimentation basin
Drawing 13
32
Draw a DAF tank
Drawing 14
33
What are the 3 different sedimentation stages throughout the WWTP what type category are they, and what is their purpose?
1. Grit removal - Type 1 (remove grit that has subsiding velocities or specific gravities substantially greater than those of the organic putrescible solids in the water
2. Primary clarifier - type 2 (remove readily settlable solids and floating material)
3. Secondary clarifier - type 3 (to remove sludge generated from biological treatment)
34
Describe Type 1 settling
The particles are discrete and do not influence each other - it is easier to predict the settling the time here.
Particles act independently so you can model how long it will take for them to drop based on their size and density.
35
Describe Type 2 settling
Flocculant settling - different flow patterns for each particle. May get some particle-particle interactions where they collide and stick together to form flocs which may make them settle more quickly. Particle-particle forces may also prevent further consolidation.
36
Describe Type 3 settling
Almost impossible to model and is hindered as the particles all interact with each other
37
What assumptions are made in Type 1 settling?
Settlement characteristics are based on Stoke's law.
1. Laminar flow
2. Spherical particles
3. Homogeneous material
4. Smooth surfaces
5. Particles do not interact with each other
38
What assumptions are made in Type 2 settling?
Particles can flocculate as they settle, so density increases with time and velocity increases with time to Stoke's equation can't be used.
Lab tests with settling column must be used to develop design data.
39
Draw a graph to show the difference in settling time between type 1 and type 2 sedimentation
Drawing 15
40
What assumptions are made in Type 3 settling?
Intermediate concentration particles are close to each other and interparticle forces hinder settling of neighbouring particles.
The particles remain in fixed positions relative to each other which forms a blanket and the mass of particles settles as a zone.
41
What are the 3 types of settling in type 3 sedimentation? Draw a graph to show how these influence over time
Zone settling, transition settling and compression settling
| Drawing 16
42
List the following in order of time taken to settle: granular, flocculent, dispersed
Granular
Flocculent
Disperse (if it settles at all)
43
List 6 characteristics of grit
1. predominantly inert
2. composition variable (silt, sand, gravel, metal, glass, seeds etc.)
3. moisture content 13-65%, VOC 1-56%
4. retained on a no. 100 sieve (>0.15mm)
5. load is typically 0.005-0.05 m3/1000m3 of sewage
6. organic content 10-30%
44
What 4 things will affect how much grit is in the rain water?
1. weather
2. road maintenance
3. nature and amount of any industrial waste
4. tanker discharges to the sewerage system
45
List 3 purposes of grit removal
1. protect mechanical equipment from abrasion and abnormal wear
2. reduce conduit clogging caused by deposition of grit particles in pipes and channels
3. prevent loading the treatment plant with inert matter that might interefere iwth the operation of treatment units such as siltation of AD and aeration tanks
46
List 4 types of grit removal processes
1. Constant velocity grit channels (horizontal flow - square/rectangular configuration)
2. Detritor (most common)
3. Aerated grit channels
4. Vortex grit trap
47
What velocity and efficiency do detritors run to?
0.3 m/s to ensure grit is not disturbed once it has settled, 85-95% removal
48
Where is grit disposed?
Landfill (most common), burial, as a drying bed or track/path top dressing, landscaping, pathways (end use is dependent upon the economics and the quality of the grit)
49
What are the two streams exiting from primary sedimentation?
1. Pollutant - removed and retained as sludge
| 2. Wastewater - greater volume and sent for further treatment (primary tank effluent)
50
What typical removal efficiencies can you expect from primary sedimentation?
```
50-80% of suspended material
30-50% of the BOD/COD
around 10% of the total bacteria
retains grease and scum
balances the hydraulic and organic loading to the downstream biological processes
```
51
What is DWF?
Dry weather flow - the sewage flow from a premises without the addition of any surface water. (The average daily sewage flow entering a WWTP or sewer measured 7 days without rain (excluding local or bank holidays etc) and during which on the preceding 7 days, rainfall did not exceed 25 mm on any day. how much flow you are going to have based on local weather conditions
52
What is maximum DWF units?
6DWF, usually flow to full treatment is 3DWF.
| Equalisation/storm tanks can be used for higher than this but not often.
53
What are typical settling velocities for clay, primary organic waste, aluminium flocs, activated sludge, grit in mm/s?
clay 0.07, primary organic waste 0.42, aluminium flocs 0.83, activated sludge 2, grit 20
54
What is the surface overflow rate?
volume of sewage (m3) in 24 hours/surface area of tank in m2
This is the most important design factor as the tank must be deep enough for the particles to settle, but also to have flow such that settled solids are not disturbed
(avg flow = 30-50 m3/m2/d, peak flow = 80-120 m3/m2/d)
55
What are typical retention times for clay, primary organic waste, aluminium flocs, activated sludge, grit
clay 11.9, primary organic waste 1.98, aluminium flocs 1, activated sludge 0.42, grit 0.042
56
Weir overflow rate
volume of sewage (in m3) in 24 hours/ total weir length of tank in m
(the amount of water leaving the settling tanks, usually 120-450 m3/m/d)
57
What is weir overflow rate?
volume of sewage (in m3) in 24 hours/ total weir length of tank in m
(the amount of water leaving the settling tanks, usually 120-450 m3/m/d)
58
When is weir overflow rate too fast/too slow?
<100 m3/m/d it may not be self cleaning and >500 m3/m/d there may excessive sludge and scum loss
59
Name the 3 types of sedimentatioin tanks
Rectangular, horizontal flow
Circular, radial flow
Hopper bottomed, upward flow
60
Draw a rectangular sedimentation tank and list its features
Drawing 17
Flow from one end of the tank to the other, bottom slope towards the inlet, 14-15 m/h, length to breadth ratio 3-5:1, avg depth 2-3 m, scraper speed 5mm/s to prevent sludge disturbance
61
Draw a circular sedimentation tank and list its features
Drawing 18
Central feed pipe discharges just below the top water level, the overflow weir is around the perimeter, D=10-30m, side walls 1.5-2m, floor slope 7.5-22.5 degrees, sludge hopper to hold 1 days sludge, scraper 25 mm/s (slightly faster than rectangular as the incline is greater)
62
Draw an upward flor sedimentation tank and list its features
Drawing 19
Used for small applications, inlet just below the top water level, but above max sludge level, a sludge blanket forms across the tank and acts as a filter - particles form flocs and sink
Upward flow 1.2-1.8 m/hr (weight of particles reduces their speed until almost stationary
Hopper slope >60 deg
