WATER TREATMENT: SEDIMENTATION

In potable water treatment, the term "sedimentation" describes a physical process whereby particles settle to the bottom of a tank or basin. This is largely a function of gravity, the density of the water and the particles density, and the velocity of the water passing through the tank or basin. Sedimentation is usually found in treatment facilities as Pre-sedimentation Basins, where sand, grit, and other particles are removed prior to beginning subsequent treatment process units, and in Sedimentation following Coagulation/Flocculation. [We will dedicate most of this discussion to the latter application.]

In Pre-sedimentation basins, local raw water conditions dictate the length of detention time for the water passing through (low times for heavy particles, and higher detention times for lower density particle removal), and whether the tank is automatically cleaned with mechanical equipment (high loadings) or is manually cleaned (lower loadings). 

THE PROCESS: The process is utilized to reduce suspended matter and turbidities to a level where the filtration process can economically and practically handle the filter loadings and reductions required. Upstream of the sedimentation basin we usually have rapid mix basins for mixing a coagulant into the raw water, followed by flocculation basins to flocculate the coagulated particles into larger, easier to settle floc. (Please see the chapter on coagulation/ flocculation).

The types of sedimentation tanks are circular, rectangular (includes square), double deck, tube settlers, and solids contact (precipitators, reactors, contra-flow).

There are four zones to a sedimentation basin: Inlet Zone, Settling Zone, Sludge Zone, and the Outlet Zone. Design the tank inlet zone for gentle entry and distribution of treatment process water into the settling (sedimentation) zone for an undisturbed, quiet pass-through of the tank, greater than or equal to three (3) hour detention time for clarification; sludge (solids) zone for sludge (solids) storage and concentration; outlet area (zone) to skim off the clarified water with no short-circuiting over weirs into collecting launders. 'Short-circuiting' refers to water that moves quickly through the tank like in a current, without properly dispersing and allowing the settling of the particles.

At the top left we show a partially filled rectangular sedimentation basin. At the bottom of the photo you can see the chains and flights that are used to thicken and move the settled sludge to the sludge hopper, where sludge pumps will pump it away for further treatment and disposal. Some water treatment sedimentation basins do NOT have such sludge removal equipment, and rely on operators to take the basin out of service, and hose the sludge down the drain for disposal. Rectangular basins usually have average side-wall depths of 10 to 18 feet, and usually have length to width ratios of 3:1 to 5:1. There appears to be a trend over the past few years to create deeper rectangular and circular clarifiers by a couple feet or so. It appears this is to allow for better separation between the sludge zone and the clarifier zones, so that currents in the clarifier zone do not adversely impact and scour the sludge in the bottom of the tank. This in turn allows for slightly deeper sludge zones in some cases.

The photo at the left shows the flights (the fiberglass or wooden planks used to thicken and scrape the solids to the sump for removal). At the top and bottom of each of the flights is a pair of (red colored) wearing shoes (which are bolted to the flight), whose purpose is to reduce the friction of the entire flight dragging on the basin floor and the return rail, and also to allow for the wearing friction surface to be easily be replaced, instead of the whole flight. The wear shoes are about $8 each (four shoes to a flight), and the flights are about $250 each (depends on size, quantity purchased, etc.) It is obviously much less costly to replace the shoes than the entire flight.

The settling particulate matter is affected by the particle size, shape, density, electrical charge; number of particles; water temp.; sedimentation tank physical characteristics (shape, conditions such as wind and density currents). Smooth particles settle faster than irregular shapes; dense particles settle to the bottom faster than "fluffy light" ones; most are negatively charged; colder water is more dense than warm water, which slows particle settling.

"Hindered settling" is a term used in sedimentation to describe settling of large numbers of particles that are settling as "a mass," often interfering with each others settling, rather than settling "unhindered" as individual, discrete particles. This phenomenon may also manifest itself when all of the settled particles come together at the bottom of the tank prior to being incorporated into the sludge zone (may also be called the compression zone).  While this condition appears to be more prevalent in wastewater treatment secondary sedimentation, it is worthy to note in potable water treatment where large amounts of flocculated algae, etc., may be involved.

The image at the left illustrates tube settlers, which is a matrix of plates that allow solids to settle on them as the water rises up from below the tube settlers and flows out the top leaving the solids behind. These tube settlers or plates are placed in a clarifier as a method of increasing the amount of water that can be treated. (Just imagine a clarifier with it filled with bundles of these plates or tubes, and you have the concept!) They have been used in shallow clarifiers to increase solids capture, and reduce construction costs.

Circular clarifiers (picture to the left) are limited in size by the weight of the center rotating mechanism that collects the sludge, and the great amount of torque that is created at the ends of the rotating arms. They are usually less than 120 feet in diameter as a result. Typical average depths are between 12 and 20 feet. Most new designs have center flocculating wells, in which the incoming coagulated and flocculated water is gently flocculated one last time before passing under a baffle into the clarifier zone.

DESIGN Horizontal clarifier parameters:
Ave Hydraulic Loading using lime coag/floc: 1,400-2,100 gal/day/sqft
Ave Hydraulic Loading using alum coag/floc: 600-1000 gal/day/sqft

Detention Time: Greater than or equal to 3 hours

OPERATION:
Monitor influent and effluent turbidity, pH, alkalinity, temperature. Increasing turbidity requires an increase in the coagulant chemical(s) dosage, and/or decrease flow rate through tank. Must run jar tests, adjust slurry (sludge) removal rates. Lowering water temperatures increase the water density which increases resistance to settling particles. Must run jar tests.

INTERACTIONS:
Solids carry-over from the sedimentation tank causes greater loading to the down stream filtration process which results in shorter filter runs; may clog filters, cause break through, and even higher filter effluent turbidities affecting disinfection process. Must remove solids (sludge) routinely, as septic (anaerobic) conditions may result; solids scouring; loss of detention time in the basin.

SOLIDS CONTACT CLARIFIERS:
"Special creatures" employing the coagulation, flocculation, and sedimentation process in one tank. Less expense to construct, requires more skill and insight to operate. Most designs allow for operation in either the sludge blanket depth or in a sludge recirc. mode. The sludge blanket mode uses a deep blanket of settled sludge, through which the coag/floc incoming water must slowly pass through. The new flocculated particles attach to the sludge particles in the blanket. In the sludge recirculation mode the incoming coag/floc raw water is mixed with a ratio of existing sludge that has already settled, with the goal of creating larger floc which will then capture more of the new, incoming smaller particles, and settle everything in the clarification zone. Process control is with emphasis on chemical dosage, solids control with slurry (sludge) level, solids recirculation rate from sedimentation portion into flocculation well. Slurry pool with higher blanket level helps to buffer impact of changing pH, alkalinity, dissolved oxygen, algal activity, and temperature. Worst problem is instability during rapid changes in process water flow rates, temperature, and turbidity levels. Increasing turbidities- try to anticipate and increase coagulant dosage to meet increasing amounts.

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