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!)
Vortex grit chambers
- consists of a cylindrical tank in which the flow enters tangentially creating a vortex flow pattern
- centrifugal and gravitational forces cause the grit to separate
Oil and grease trap
- low solubility reduced their rate of microbial degradation
- hydrophobic = sticks together, create a film which blocks oxygen transfer to the water
> anaerobic phenomena can occur causing corrosion, crust blocks mechanical parts - reduce surface tension = make oxygen transfer more difficult (which microorganisms need)
- Separation by density difference = stoke’s law
- Oil particles bump into slanted plates and form larger oil droplets which glide upwards, floating layer can be mechanically removed
- Design = similar to primary settler - rising velocity should be higher than surface loading
Primary Settlers
- large amounts of suspended solids can be effectively removed
- gravity is major player
- design process involves selecting settling velocity, vo and design so all particles with this settling velocity or higher will be removed
- Xr = vp/vo where Xr is the fraction of particles with settling vp that are removed
Coagulation
- Small suspended, colloidal particles with a diameter ranging from 0.01 to 1 µm can be removed
- due to the addition of chemicals (Fe (III), Al (III) salts) the electrostatic repulsion between the colloidal suspended particles is (significantly)
reduced - the colloidal material becomes unstable
> small particles will start to agglomerate and
in a short period of time can grow to flocs of
about 1 µm
Flocculation
- from this floc diameter on, the particles will
grow further by gently stirring the liquid such
that the now larger and heavier flocs can start to
settle - The growth of the flocs can be further enhanced by the addition of, e.g., poly-electrolytes
- The agglomerated particles can be removed by
settling, sand filtration, flotation or membrane
filtration
Flotation
- The separation of suspended material (or a
dispersed non miscible fluid) from a liquid is based on a difference in density - Two types of flotation can be distinguished:
> natural flotation: through which the real lower
density of the to be separated material is
exploited (e.g., oil from water)
> flotation by bubble adsorption: which makes use of macro gas bubbles (Induced Air Flotation) or micro gas bubbles (Diffused Air Flotation)
