Primary Treatment Flashcards
Why is primary treatment important
- to reduce part of the waste water loading
- to reduce the risk of hampering or damaging the subsequent processes and process equipment
- raw sewage often includes hosuehold waste liquids, surface and storm water which may contain contaminants such as soil particles, heavy metals, organic compounds, animal waste, oil and grease which need to be removed before entering the biodegradation part
What are the primary treatment processes
- Archimedes screw
- Screening
- Sedimentation
Archimedes’ Screw/Screw Pump
- a machine historically used for transferring water from a low lying body of water into irrigation ditches
- as the bottom end of the tube turns, it scoops up a volume of water
- the amount of water will slide up in the spiral tube as the shaft is turned until it finally pours out from the top of the tube
What are two examples of screening methods
Sieve, Bar Rack
In the screening process, what is the order of screening
- First a coarse screen with clear openings from 6 to 150mm to remove coarse material then fine screened sieves to remove suspended materials with openings < 6mm
Bar Racks
- vertical bars slanted in water with a breadth of 0.5 to 4m
- for a flat bar rack scraping via a chain rake and for a curved bar rack scraping occurs via a rotating rake
- it is possible for a combination with a cutting mechanism
- the most important design parameter is head loss
head loss is inversely proportional to the area between the bars as velocity is dependent on area
What is the design strategy for bar racks
- select bar type and configuration
- select an optimal velocity
- calculate surface area between the bars
- calculated head loss (10 -20 cm is acceptable)
- otherwise enlarge d, select other bar type or reduce velocity
What are the types of sieve screens
- sieve bend
- rotating drum
- vibrating sieve
What type of material needs to be used for seives
materials with holes in it such as wedge wire material
What does sedimentation need to remove and what does it allow
grit, organic material in primary settlers, biological floc removal in secondary settler, chemical floc removal, solid concentration in sludge thickener to produce clarified effluent and to produce sludge with a high solids concentration
What does the settling of particles depend on
- the concentration of the particles
- the flocculation characteristics of the particles
What are the four types of sedimentation
- Type 1: unhindered settling of granular particles
- Type 2: unhindered settling of flocculant particles
- Type 3: hindered settling of granular and/or flocculant particles
- Type 4: thickening
Type 1
- unhindered settling of granular particles
- particles in suspension at rather low concentrations are able to settle without interaction
- settling velocity = a constant of granular (e.g. sand)
Type 2
- unhindered settling of flocculant particles
- particles tend to agglomerate when they bounce in the fluid and will start settling faster with depth
- phenomenon in primary settling tanks
Type 3
- hindered settling
- at higher concentration settling with interactions
- formation of a clear interface between settling mass and the clarified liquid above
- no hindrance of underlying particle layers
- hindered settling of flocculant particles at lower concentrations
Type 4
- thickening
- concentration of particles so high that interaction with underlying layers occurs
- compression and de-watering
What is the use of grit chambers and where are they located
-designed to remove grit (dp > 0.15mm) which are heavy solid materials with v > or dens > than organic biodegradable solids in wastewater such as sand, gravel and cinders
- located after the pumps and after the bar screens and before the primary settler (which remove the heavy organic solids)
Why are sand and gravel important to remove in grit chambers
- sand is hard (inorganic) silicon component with erosive and abrasive characteristics
- sand and gravel are not biodegradable and can only accumulate in the system and/or block pipes and equipment
therefore:
- grit chambers protect moving mechanical equipment from abrasion and accompanying abnormal wear
- reduce the formation of heavy deposits in pipelines, channels and conduits
- reduce the frequency of digester cleaning caused by excessive accumulation of grit
What are the types of grit chambers
- horizontal flow type (rectangular/square configuration)
- aerated type
- vortex type
Horizontal flow grit chamber
- the flow passes through the chamber in a horizontal direction
- the straight line velocity of flow is controlled by the dimensions of the unit, the influent distribution gate and a weir at the effluent end
- for an efficiently working grit chamber, the surface loading must not exceed the settling velocity of the particles
- surface loading = limit of settling velocity of removed particles
Scouring velocity
- horizontal velocity should be less than scouring velocity
- scouring velocity is the velocity at which tangential force applied to the particles is exactly equal to the frictional force between the particles and the bottom
- it is the velocity at which the particles are brought in suspension again
Horizontal flow grit chamber design considerations
- determine the surface loading based on the desire performance, specify the flow rate and the particle types and sizes for which the grit chamber has to be effective
- given the surface loading and the settling velocity, L x B can be fixed (L x B = Q/vo)
- determine the horizontal velocity vh which should be less than vs (equation or target values 0.3 m/s) (vh/vo = L/D)
- select L, B or D and calculate the corresponding values for the remaining parameters
- add 20 to 30 cm to the depth D to allow accumulation of the sand
Types of Horizontal Grit Chambers
- Channel grit chamber
> a long channel with a rectangular weir
> most often the loading of wastewater is not constant: divide total flow over a series of channels operating in parallel)
> length to width ratio is often selected to range between 10:1 and 15:1
> cross section should be parabolic
> centrifugal pumps are used to remove sand - Square or Dorr grit chamber
> 1m of depth
> scraper
> integrated grit washing
Aerated grit chambers
consists of a spiral flow aeration tank where the spiral velocity is induced and controlled by the tank dimensions and quantity of air supplied to the unit (expensive!)
