WASTEWATER TREATMENT: SEDIMENTATION
PROCESS: Basically, anything that floats is removed and called scum. Anything that sinks is removed and called sludge. This may be accomplished in rectangular (includes square) or circular tanks (basins), double deck, and solids contact (precipitators, reactors, contra-flow). Rectangular tanks generally have endless chains that have wooden, fiberglass, or plastic flights attached to them to scrape the sludge to a hopper at one end for removal. Circular tanks have rakes on a rotating arm that scrapes the sludge into a hopper to be pumped out for further treatment. The scum is usually removed by sprays that push it on the surface of the water to a skimmer, or by scrapers to a scum box, etc. for removal and treatment.

The picture at the left is of a partially filled circular clarifier which shows the center column inlet and baffling, and the sludge collection mechanism is partially submerged.

They may be used early in the treatment where they settle out solids from the raw wastewater (Primary Sedimentation) or later in the treatment train process after dissolved organics have been converted into microorganisms (Secondary Sedimentation). In the case of the secondary sedimentation use, they are used to allow the microbes to settle from the water being treated.

There are four zones to a sedimentation basin, (regardless of shape): Inlet Zone, Settling Zone, Sludge Zone, and the Outlet Zone.

Again, regardless of the shape of the tank we design the tank inlet zone for gentle entry and distribution of treatment process water which then flows into the settling (sedimentation) zone for undisturbed settling between 1 to 3 hours detention time for clarification as the water passes through the tank. The sludge (solids) zone for sludge (solids) storage and concentration is at the bottom of the tank; and then the outlet area (zone) to skim off the clarified water with no short-circuiting through the tank and over the weirs into the 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.

Note: 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 faster than "fluffy light" ones; colder water is more dense than warm water, which slows particle settling.

At the 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.

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, 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 to a flight), and the flights are about $250 each (depends on size, quantity purchased, etc.)

"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). This condition may occur in activated sludge treatment processes that are operating at high Mixed Liquor Suspended Solids (MLSS) concentrations.

DESIGN: PRIMARY CLARIFIER EFFICIENCIES:
Settleable Solids 90-99%
BOD 25-35%
Suspended Solids 40-60% (K.Kerri 25-50%)
Total Solids 10-15%
Bacteria 25-75%
PRIMARY CLARIFIER PARAMETERS:
Detention Time 1 - 3 hours
Ave Hydraulic Loading 300-1200 gal/day/sqft
Peak Hydraulic Loading rate 2000-3000 gal/day/sqft
Weir Overflow Rate 10000-20000 gal/day/ft

INTERACTIONS:
(Click picture to enlarge.) This process is quite inexpensive to run, but if it fails, can cause serious upsets! Insure that you are removing the correct amount of sludge and scum in your Primary Clarifiers...too much washes out the digesters; too little and you overload the scrapers, cause odors, cause the sludge to go septic and gassify, and if the unit fails, and you have to bypass your only clarifier, this may cause all of the scum and sludge to go to your secondary treatment process. By maintaining a 1-3 foot blanket of sludge in the bottom of your clarifier, providing proper preventative maintenance, in the greasing/oil changes; inspection of the wearing shoes (if chain and flight); and inspection of the drive chains/etc. you should be all right. The sludge in your Primary Sedimentation Basins should be 4 to 8% dry solids. It is a good policy to take your clarifiers (basins) off line once a year to inspect and replace worn components. This is best accomplished in dry weather, obviously! If the unit fails, drain the unit using the sludge pumps, the drain valve to the headworks, or portable pumps (even rented ones), etc. It is best to direct the sludge to the digester(s) as concentrated as possible, without overloading it. Some may be directed to the headworks or to the entry to the other tank(s) to disperse the loading. From the other tanks, then set up a sludge pumping scheme to pump your concentrated sludge to the digester. You do not want to overload one tank, so share the load among what you have remaining. Odors will be heavy, especially in summer, so place your hoses to wash down the walls, floor, chains etc. as they become visible in the failed tank.. When the unit is at the point of no longer flowing solids to the sump, hose what you can, and then place blocks of wood under the flights to facilitate hosing the solids. If the unit will be off line for some time, spray the units chains with a light oil or mixture of diesel fuel and used oil to keep the chains and gears from rusting and/or freezing up solid. Make your repairs and then place back in service; or if equipped with wooden flights, fill with plant process (W3) water to a level to submerge the wooden flights to prevent warping or cracking. Routinely operate the flights and chains of tanks that have been removed from service, so that they do not rust solid.

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