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Design of Sedimentation Basins

by Duc Ngo and Jessica Nichols


Introduction

Sedimentation is a solid-liquid separation that utilizes gravity to remove suspended solids used in water and wastewater treatment. These suspended solids, or particles, are important to remove from water for several reasons. Some of these reasons include: visibility impairment (aesthetic & safety reasons), disease transmission (bugs attach to particles that can then be ingested), and lastly because toxic materials can either exist as particles or can absorb to the particles.


Circular clarifier at wastewater treatment plant.



There are four general types or classes of particle settling that are based on the concentration of the particles to interact. These criteria directly influence the design and construction of sedimentation.

The four types of settling are: discrete, flocculant, hindred, and compression. Each of these occur in both water and wastewater treatment. Discrete settling requires the lowest suspended solids concentration and its analysis is the simplest. In discrete settling, individual particles settle independently and do not interfere with the settling of other particles present. One application of discrete settling is grit chambers. A second type of settling is flocculant. This is where particle concentrations are high enough that agglomeration occurs. This increases average particle mass causing particle to sink faster.

Flocculation settling is used in primary clarifiers and the upper zones of secondary clarifiers. The third type of particle settling is called hindered. In hindered, or zone settling, particle concentration is sufficient enough so that particle interfere with the settling or other particles and then they sink together. Hindred settling is mainly used in secondary clarifiers. The last type of settling is compression. Compression settling involves the highest concentration of suspended solids and occurs in the lower reaches of clarifiers. The particles settle by compressing the mass of the particles below. Compression occurs not only in the lower zones of secondary clarifiers but also in sludge thickening tanks.


General Sedimentation Information:

Actual sedimentation basins are rectangular, square, or circular in plan area. A single-rectangular basin will cost more than a circular basin of the same size; however, if numerous tanks are required, the rectangular units can be constructed with common walls and be the most economical.


Rectangular tanks

Inlets for Rectangular Tanks:

In an ideal rectangular tank, water enters through the flocculation basin which is usually adjacent to the tank. The flocculation basin is the same width as the settling tank but is usually not as deep. The two basins are separated by a wood baffle fence or a concrete wall with numerous ports. The inlet water enters uniformly across the rectangular basin by means of the inlet zone. The inlet zone does not extend down to the full depth of the settling tank but extends down to the depth of the flocculator. If the rectangular tank does not adjoin the flocculator, the inlet water is distributed uniformly across the basin by a flume with ports into the tank. In this case, a baffle in front of the flume will disperse the water downward to give a deep inlet zone.

Outlets for Rectangular Tanks:

One type of outlet for a rectangular tank is a weir which spills into the effluent flume that extends across the entire width of the basin. If, however, the water is chemically coagulated, a weir should be avoided because the turbulence will break up much of the fine floc and result in poor filter performance. For chemically treated waters it is best to have an orifice flume across the basin width. An orifice flume does not have a high degree of turbulence and will not break up fine floc.


Circular Tanks

Inlets for Circular Tanks:

In circular tanks, the flow enters either the center of the tank (center feed) or the side of the tank (side feed). If the tank is less than 30 ft (9.14 m) in diameter, the inlet pipe will enter through the wall and discharge into the baffle well. Then, the flow enters in a downward direction. If the tank is greater than about 30 ft (9.14 m), the inlet pipe will run underneath the tank and discharge vertically in the center of the center of the baffle well. The depth of a circular clarifier is considered to be the depth at the side of the tank, and is referred to as the side water depth (swd). This depth is used for determining tank volume and detention time.

Outlets for Circular Tanks:

The outlets for most circular tanks consist of a weir channel around the periphery giving a uniform flow removal. The center-feed circular clarifiers that are used in wastewater treatment have both mechanical sludge rakes and surface skimming. Circular tanks used in water treatment are similar to those in wastewater treatment except that surface skimmers are not required. The bottom of a circular tank slopes to the center at a slope of 1:12, thus it forms a flat inverted cone. In design, the cone is not considered in the design volume, which is taken as being the plan area times the depth of the water at the sides of the tank. The sludge is usually collected in a hopper near the center of the tank.

Inlet for Periphery-feed Circular Tanks:

As flow enters a periphery (or side) feed tank, it is deflected so that it moves around the periphery in an orifice channel. From the channel the flow discharges through the orifice into the clarifier. Sometimes, instead of an orifice channel there is simply a skirt surrounding the inside of the tank and the liquid flows out underneath the skirt into the tank. Peripheral entry does not give as uniform a flow as the center feed tanks.

Outlet for Periphery-feed Circular Tanks:

The outlet of a side feed tank consists of a weir channel in the center of the basin.


Detention Time

Actual settling basins are affected by the dead spaces in the basins, eddy currents, wind currents and thermal currents. In the ideal settling basin all of the fluid elements pass through the basin at equal time to the theoretical detention time, t, which is equal to V/Q. Actual basins, however, have most of the fluids passing at a time shorter than the theoretical detention time. Dead spaces and eddy currents have rotational flow and do very little sedimentation since the inflow and outflow from these spaces is very small. As a result, the net volume available for settling is reduced and the mean flow-through time for the fluid element is decreased. Also, wind and thermal currents create flows that pass directly from the inlet to the outlet of the basin, which decreases the mean flow-through time. The magnitude of the effects of the dead spaces, thermal currents, etc. and the hydraulic characteristics of a basin may be measured using tracer studies. A slug of tracer is added to the influent and the tracer concentration is observed at the outlet.

If there are dead spaces, the following relationship occurs:
     Mean t / Theoretical t < 1

If there are no dead spaces, the relationship is:
     Mean t / Theoretical t = 1

If short circuiting is occurring, the time relationship is:
     Median t / Mean t < 1

If there is no short circuiting:
     Mean t = Median t

If the basin is unstable, the time-concentration plot cannot be reproduced in a series of tracer tests. Consequently, erratic basin performance can be expected. Rectangular basins approach the ideal time-concentration plot more closely than circular tanks, but between the two circular types, peripheral feed is more ideal and has better performance than center feed.


Water Treatment

In water treatment, the main applications of sedimentation are

  1. plain settling of surface waters prior to treatment
  2. settling of coagulated and flocculated waters prior to treatment
  3. settling of treated waters in an iron or manganese removal plan
  4. .



These photographs are sedimentation basins for a water treatment plant. The floc have several hours to travel across the length of the basin.



In water treatment, sedimentation of both untreated waters (plain sedimentation) and chemically coagulated waters is practiced. If a water has a high turbidity due to silt, plain sedimentation may be used to reduce the turbidity. Plain sedimentation is frequently used for waters having consistent turbidities greater than 1000 mg/L. When plain sedimentation is used, the detention time may be as much as 30 days, and as a result of the extremely large volume, these basins are usually earthen and are constructed using dikes. In most cases, a water to be settled has been coagulated by the addition of chemicals such as those employed in rapid sand filtration plants and lime-soda softening plants.


The settling characteristics of the floc or precipitate depend upon the characteristics of the water, the coagulant used, and the degree of flocculation. The only method to determine accurately the settling velocities and the required overflow rates and detention times is to perform experimental settling tests.


Detention Parameters for Various Coagulants
Type of Treatment Overflow Rate Detention Time Channel Loadings
  (gal/day-ft2) (hr) (gal/day-ft)
Alum coagulation 500-800 2-8 12,000-18,000
Iron coagualtion 700-1000 2-8 16,000-22,000
Lime-soda caogulation 700-1500 4-8 22,000-26,000

Wastewater Treatment

In wastewater treatment, the main application of sedimentation are :

  1. sand and silt removal
  2. suspended solids removal in primary clarifiers
  3. biological floc removal in activated sludge final clarifiers.

In conventional wastewater treatment plants, primary sedimentation is used to remove as much settleable solids as possible from raw wastewaters. Secondary settling in activated sludge plants is employed to remove the MLSS and in trickling filter plants to remove any growths that may slough off the filters. As a result good secondary settling produces a high quality effluent low in suspended solids. In advanced or tertiary wastewater treatment plants, sedimentation is used for coagulated wastewaters to remove flocculated suspended solids and/or other chemical precipitates.

Recommended criteria for primary clarifiers treating municipal wastewaters are:

Wastewater Design Parameters
Type of Treatment Depth Overflow Rate
Average Peak
(ft) (gal/day-ft2) (gal/day-ft2)
Primary settling followed by secondary treatment 10-12 800-1200 2000-3000
Primary Settling with Waste Activated Sludge 12-15 600-800 1200-1500


These photographs are clarifiers for a waste water treatment plant. Two of the figures are shown without any raw water in them. The top arm rotates to skim floating debris from the surface.



This photograph (below) shows the clarifier with raw water.

The detention times based on the average daily flows are usually from about 45 min to 2 hr., however, the depths and overflow rates listed in the above table should control in design. Multiple tanks should be used when the flow exceeds 1.0 MGD. For plants having capacity less than 1.0 MGD peak weir loadings should not exceed 20,000 gal/day-ft. For plants having a capacity greater than 1.0 MGD peak loadings should not exceed 30,000 gal/day-ft. A surface skimmer and baffle are necessary for primary clarifiers to remove scum from the water surface.

Some regulatory agencies specify the minimum depth required for wastewater clarifiers. Other agencies specify the minimum detention time and the minimum overflow rate or surface loading rate, and the engineer must determine the minimum depth. The overflow rate or surface loading rate must be converted to a settling rate, and the depth is equal to the settling rate times the detention time.




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Send comments or suggestions to:
Student Authors: Duc Ngo dngo@vt.edu,  and Jessica Nichols
Faculty Advisor: Daniel Gallagher, dang@vt.edu
Copyright © 1996 Daniel Gallagher
Last Modified: 02/24/1998