13-05-2011, 11:35 AM
[attachment=13722]
Primary Treatment of Wastewater
Equalizatio
Smooth out fluctuations in flow rate
Results in more consistent treatment
Flow Measurement
Flow rate information needed for efficient operation, chemical addition, etc
Equalization
Objective
Decrease fluctuations in flow rate, to provide more consistent treatment
Accomplished by storing excess wastewater during high flow periods
Excess wastewater is released during low flow periods
Pumping
Sometimes needed to lift the water to a higher elevation than the discharge point of the main trunk sewer line.
After pumping, the plant is designed to operate under gravity flow to the point of discharge at the receiving stream.
A Screw Pump
Flow Measurement
Objective
Measure flow rate to facilitate plant operation
Several operations need flow rate data for good operation
Chlorination
pH adjustment
Also required for NPDES reports
Design of Influent Channel
Objective
Design a combination of circular sewer and rectangular channels to deliver wastewater to the head works of the treatment plant
Apply open channel flow hydraulics – applying Manning’s equation considering:
Minimum velocity to reduce solids deposition in the channel
Hydraulic grade, slope of channel invert to provide scour of solids
Channel dimensions that match or transition the influent circular sewer with a rectangular channel
Primary Treatment
- Designed to remove settleable solids and
reduce the organic load (BOD) on the secondary units.
Primary treatment includes
- Bar screen
- Comminutor
- Grit chamber
- Primary clarifier
Bar Screen Vendor-Provided Equipment
Purpose: to remove large objects (sticks, cans, etc) which may cause flow obstructions.
Depending on the size of the plant, bar screens are either hand or mechanically cleaned.
Hand cleaned: used primarily at small plants.
Figure (a) Manually cleaned bar rack (from Peavy, Rowe, and Tchobanoglous, 1985, p. 218)
Bar Screen Mechanically Cleaned
More frequently used because labor and overflowing are greatly reduced.
A by-pass channel with a hand cleaned bar screen must also be provided. A second mechanically cleaned bar screen can also be provided.
The purpose of the by-pass channel is to provide treatment in case of a mechanical failure.
Screens are either front or back cleaned.
Bar Screen
Mechanical Bar Screen General Design Criteria
Bar Width: 1/4 to 5/8 in
Spacing: 5/8 to 3 in
Depth: 1 to 1.5 inches
Slope: 30 – 45o from the vertical.
(from Peavy, Rowe, and Tchobanoglous, 1985, p. 219)
Mechanical Bar Screen General Design Criteria
Approach velocity – 1.25 fps @ minimum flow (as determined by the Manning Eqn.), the purpose in controlling the approach velocity is to prevent deposition of grit in the channel.
Velocity through the screen - < 3 fps, to prevent excessive headloss and to prevent forcing of screenings through the openings.
Quantities of screenings – 0.5-5 ft3/ MG, average 2 ft3/MG
Mechanical Bar Screen General Design Criteria
Disposal of screenings – landfill or incineration
Density: 80% moisture (60 pcf) right off the screen, dry (12 pcf)
hL = 0.5 – 2.5 ft (max)
hL=(Vs2-vc2)/(2g * 0.7)
Vs= velocity through the bars
vc= approach velocity in the upstream channel
Comminutors Vendor-Provided Equipment
Purpose: to chop solids between 1/4 - 3/8 inch to prevent pumps from being clogged.
Comminutors are installed directly into the influent channel.
Since comminutors come in a standard size, it is not unusual to select the comminutor first, then size the channel.
Comminutors should be provided with a
by-pass channel and a hand cleaned bar screen.
(from Peavy, Rowe, and Tchobanoglous, 1985, p. 220)
Grit Chambers
Purpose: to remove inorganic material referred to as grit. Grit includes sand, eggshells, bone chips, coffee grounds, etc.
Grit is removed to prevent abrasion of pumps and to reduce deposits in pipe lines, channels, and digesters.
Grit Chamber General Design Criteria
Specific gravity of grit: 2.65
Diameter of grit: 0.22 mm
Settling velocity: 0.075 fps
Equivalent overflow rate: 48,400 gpd/ft2
Grit Chamber General Design Criteria
Quantity of girt: 1/3 to 24 ft3/MG
Ave = 4 ft3 /MG
Disposal of grit: land fill or incineration (Grit must be washed before disposal)
Grit chamber storage:
Small plant: provide storage below the design invert depending on the quantity and frequency of removal.
Large plant: continuous removal, the conveyor hopper is designed based on the size of the equipment.
Grit Chambers Types
Square Clarifier (Detritus Tank)
Aerated Tanks
Grit Chamber Square Clarifier (Detritus Tank)
Detritus tanks are designed so that the horizontal velocity is 1.0 fps at maximum flow. This means that at low flow, the velocity is less than 1.0 fps, and therefore, organic material will accumulate.
Organics are removed by counter current washing as the grit moves up an incline for disposal.
(from Tchobanoglous and Burton, 1991, p. 456)
Grit Chamber Square Clarifier (Detritus Tank)
Basic Design Criteria
Vs = 0.075 fps @ Average Flow
td < 1 min
Overflow rate: 48,400 gpd/ft2
Vh: 0.75-1.25 fps (keeps organics in suspension)
Grit Chamber Aerated Grit Chamber
Upon discovering that grit accumulated in the bottom of activated sludge aeration basins, it has became common practice to use aerated grit chambers.
Aeration also provides pretreatment of the waste by removing odors and inducing flocculation of the organic material making primary clarification more effective.
(from Tchobanoglous and Burton, 1991, p. 461)
Aerated Grit Chamber Benefits of Pre-aeration
By providing preaeration, primary treatment is improved through:
Grit removal
Flocculation
Odor Control
Grease Separation
Design the detention time and aeration rate to control all four
Aerated Grit Chamber General Design Criteria
Rate of aeration: 5 cfm/ft length (provide for variable rates of aeration which is adjusted according to the flow and efficiency of grit removal).
Width to Depth Ratio: a critical factor in providing an effective spiral-rolling action in the grit chamber
W
= 1 – 2.2 : 1Depth = 10 – 15 ft (starting point: set depth first)
Length:Width Ratio = 3:1, final dimensions are adjusted so that the detention time is 3-10 minutes.
Primary Clarifier
Purpose: to remove settleable organics and floating scum (grease and oils).
Efficiencies:
Suspended solids 50 – 65%
BOD 30 – 35%
Primary clarifiers are either circular or rectangular. They are very similar to sedimentation basins used in water treatment except that scum removal is always provided in addition to sludge collection.
A Circular Primary Sedimentation Tank
An Empty Primary Clarifier
An Operating Primary Clarifier
Oil Skimmer in a Primary Clarifier
Primary Clarifiers Design Criteria
Type II Settling Clarifier: during settling organic solids come in contact with each other and aggregate increasing the particle size and settling rate. Aggregation increases with time, therefore detention time is important.
Td: 90 – 150 min at average flow (Avg 2 hr)
Overflow rate: 600 – 1,200 gpd/ft2
Weir loading rate: 10,000 – 15,000 gpd/ft.
Aerial View Housatonic Wastewater Plant, Milford, CT (Avg. Flow Rate = 8 MGD)
Aerial View of Blue Plains Wastewater Treatment Plant, Washington D.C. (avg. flow rate = 309 Million gals/day)
Sludge Quantities
Quantity of sludge collected in the primary clarifier depends on:
Specific gravity of the dry solids
% moisture
Efficiency of settling
The following relationship is used to determine the specific gravity of the sludge (mixture of solids and ter):
S = Sp. Gr. of sludge
Ss = Sp. Gr. of dry solids
Sw = Sp. Gr. of water (1.0)
Ps = % solids (sludge)
Pw = % water (sludge)
Sludge Quantities
The volume of sludge can be determined from the following relationship:
S = specific gravity of sludge
V = sludge volume, gals
Ws = dry weight of solids, lb
= specific weight of water (62.4 lb / ft3)
(from Peavy, Rowe, and Tchobanoglous, 1985, p. 228)
(from Peavy, Rowe, and Tchobanoglous, 1985, p. 228)
Primary Treatment Efficiency
BOD = 30 – 35% SS = 50 – 65%
Wastewater entering secondary treatment
Strong: BOD = 260–280 mg/L
SS= 120–175 mg/L
Medium: BOD= 145–155 mg/L
SS= 80–110 mg/L
Weak: BOD= 70–80 mg/L
SS= 35–50 mg/L
Forms of BOD: (a) Colloidal
(b) Soluble/Dissolved
