Primary Wastewater Treatment -- Types of Sedimentation Clarifiers

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Circular, rectangular, plate (also called lamella), drag, and oil/water separators are the basic types of sedimentation clarifiers. The parameters previously discussed in general for sedimentation pertain to these units.

Ill. 8 Center-feed clarifier. (Image from Envirotech.)

Circular Clarifiers

Circular center-feed clarifiers are the most common sedimentation configuration. Ill. 8 illustrates the side inlet, bridge supported clarifier mechanism used for basins smaller than approximately 50 ft (15 m) in diameter. For basins 50 to 200 ft (15 to 61 m) in diameter, the clarifier mechanism is supported by a centrally located column, with the inlet pipe being beneath the floor and rising up through the column to the feed well. The center-feed clarifier has four distinct sections, each with its own function.

The inlet section or feed well of the center-feed clarifier, provides a smooth transition from the high influent velocity to the low uniform velocity required in the settling zone. The feed well must be carefully designed to provide the necessary water velocity reduction to prevent turbulence and short-circuiting in the settling zone, with resulting solids carryover. The quiescent settling zone must be large enough to meet the overflow rate and depth requirements for discrete and flocculant settling.

The outlet zone provides a transition from the low velocity of the settling zone to the relatively higher outlet velocity, which is typically limited to values less than 14 gpm per linear foot (10.4 m3/h per linear meter) of outlet weir plate. Treated water is typically discharged over v-notched weir plates, which serve two purposes. The v-notched weirs provide equal removal of the water from 360° of the periphery of the circular clarifier. The v-notches also maintain the water surface elevation in the clarifier close to constant for varying flow rates, which allows floating material to be skimmed from the surface.

The fourth section, the sludge zone, must effectively collect and compact the solids removed in the settling zone, and remove this sludge from the clarifier without disturbing the sedimentation zone above.

The floor is normally sloped at 1 on 12 to the center of the unit, where sludge is collected in a hopper for removal (blowdown). Mechanically driven sludge rakes or plows move the sludge down the slopping floor to the sludge hopper. The rakes are commonly driven from a drive unit located at the center of the tank. Another less frequent configuration uses a traction drive mounted on the top of the basin wall, which drives a rotating walkway that rotates the rakes and skimmer.

The peripheral-feed (rim-feed) clarifier ( Ill. 9) attempts to use the entire volume of the circular clarifier basin for sedimentation. The influent trough width decreases from the influent location to a point 180° away, and has orifices in the floor spaced to distribute the flow equally around the tank periphery. Water enters the lower section at the periphery at extremely low velocity, providing immediate sedimentation of large particles. The velocity accelerates toward the center, and then decreases as the flow is reversed and redirected to the overflow weirs on the effluent trough, located inboard of the influent launder. This type of clarifier is sensitive to temperature changes and hydraulic load fluctuations, since the flow pattern depends entirely on inlet hydraulics.

Rectangular Clarifiers

Ill. 9 Peripheral-feed clarifier. (Envirex, a Rexnord Company.)

Ill. 10 Rectangular clarifier.

Ill. 11 Rectangular clarifiers with common wall construction and traveling bridge collectors. (Walker Process Division, Chicago Bridge & Iron Company.)

Ill. 12 Traveling bridge mechanism.

Ill. 13 Circular sludge collector with rakes designed for corner cleaning. (Image from FMC Corporation.)

The rectangular basin is somewhat like a section taken through a center feed clarifier, with the inlet at one end and the outlet at the other.

A typical rectangular basin has a length to width ratio of approximately 4:1. Flow through rectangular clarifiers enters at one end, passes through an inlet baffle arrangement, and traverses the length of the tank to the effluent weirs and troughs. Sludge removal is normally accomplished by a dual-purpose flight system (Ill. 10). The rake mechanism consists of two, parallel endless conveyor chains running the length of the basin, with rake flights (cross pieces) extending across the width of the tank attached between the chains. The settled solids are moved by the flights to the sludge hopper, typically located at the influent end of the basin. The flights on their return travel to the effluent end, serve as skimmers to remove floating material. The flight system moves very slowly to avoid turbulence, which could interfere with the settling process. The rotating scum trough removes floating material. The trough may be either manually operated or automatically rotated to remove the floatable material that accumulates at the lip of the skimmer. The chain and flight mechanism design, limits the width of this type of rectangular clarifier to about 20 ft (6.1 m). Common walls can be used between multiple units to reduce construction costs (Ill. 11).

A single sludge scraper supported by a traveling bridge is used to move the sludge in wide rectangular basins. The bridge moves on rails mounted on the tank walls (Ill. 12). The bridge may be towed by cables or powered by a traction drive mounted on the wall. A skimmer is also supported from the bridge to move floating material to the scum trough that may be located at either the influent or the effluent end. The bridge and skimmer move material only in one direction and are retracted for the return trip to the opposite end.

The traveling bridge is generally provided with a programmable controller for the traveling cycles of the bridge. For example, the bridge may scrape the sludge in the first third of the basin two or three times (where a large quantity of solids settle), and then scrape the total basin length, then returning to the opposite side, and resting for a period before starting the cycle again. The best traveling cycle time is dependent on the type of wastewater and the solids characteristics.

Settled solids may be collected in a single hopper, multiple hoppers, or in a transverse trench which has a hopper at one end for either the chain and flight, or traveling bridge sludge removal mechanism. The transverse trench may be equipped with a chain and flight collector or a screw conveyor to move the sludge to the hopper.

Circular sludge rake mechanisms are designed with pantograph extensions, so that the mechanism can rake the corners of a square basin. Two of these mechanisms are then used to remove sludge in rectangular basins (Ill. 13).

Ill. 14 Parkson Lamella clarifier. (Parkson Corporation.)

Ill. 15 Plate clarifier floor area compared to a conventional rectangular clarifier.

Parallel Plate and Tube Clarifiers

Prefabricated clarifiers that incorporate parallel plates or tubes on a slope, are commonly used for smaller plant flow rates (< about 1000 gpm [227 m3/h]). Ill. 14 illustrates the Parkson Lamella clarifier that incorporates a flash mix tank and a flocculation tank. The flow is introduced by means of a feed duct from the flocculation tank to the clarifier feed box, which is a bottomless channel between the plate sections. The flow is then directed downward to the individual side entry plate slots. The feed is distributed across the width of the plates and then flows upward under laminar flow conditions.

The solids settle on the plate surfaces, while the clarified water exits from the top of the plates through orifice holes. These holes are placed immediately above each plate and are sized to induce a calculated pres sure drop to ensure that the influent flow is evenly distributed among the plates. The solids slide down the plate surfaces into the sludge hopper, from where the sludge is removed. Flocculation is induced as the solids roll down the inclined surface.

The plates are spaced 2 to 4 in (51 to 102 mm) apart and inclined 45 to 60° from horizontal, depending on the application and manufacturer. The total effective settling area is based on the horizontal projected area of each plate (Ill. 15). The total projected area is used to calculate the rise rate. The design rise rate for plate clarifiers is the same as for conventional clarifiers. The only advantage that a plate clarifier has over a conventional clarifier, is that a large horizontal settling area can be provided in a small area of land or footprint. Ill. 15 illustrates the area used for a plate clarifier as compared to the same area provided by a conventional rectangular clarifier. Some manufacturers use sloped parallel tubes instead of parallel plates, in which case the tube modules are referred to as tube settlers. The sedimentation principles for tube settlers are the same as for the parallel plate configuration.

Parallel plate sedimentation is based on both discrete and flocculant particle settling, and the overflow rate for the total plate projection area is determined by settling tests, the same as for conventional type clarifiers (Ill. 16). The parallel plate arrangement does not provide the necessary area and depth for hindered or compression settling. Its application is therefore limited to dilute suspensions having a maximum suspended solids concentration of approximately 500 to 1000 mg/L, depending on the application.

The plate surfaces may have to be periodically cleaned, if the solids have a sticky nature and adhere to the plates. The accumulation of solids on the plate surfaces results in solids carryover, due to increased water velocity.

Modules or packs of sloped parallel plates or tube settlers are used to increase the settling area of an existing conventional rectangular or circular clarifier (Ill. 16). The additional settling area increases the clarifier influent capacity, while the overflow rate either is reduced or remains constant. The modules are installed in the sedimentation zone of the clarifier and do not project into the clarifier's sludge thickening zone.

Ill. 16 Plastic tube modules installed in an existing conventional clarifier. (Img: Neptune Microfloc, Inc.)

Ill. 17 An illustration of an Actiflo clarifier.

Actiflo Clarifier

The Actiflo clarifier (Ill. 17) uses microsand as ballast to add weight to the floc for fast settling. The sand is added to a tank, where coagulant and wastewater are added. The coagulation process occurs on the sand surface, and as the particles increase in size, their weight and settling rate are much higher than without the sand. The clarifier section of the unit is typically a lamella design with inclined plates.

Since the rate of fall for the particle is much higher, the velocity of water (rise rate) can be much higher and still achieve excellent water clarity. These clarifiers are equipped with hydrocyclones to separate the microsand from the floc, so the sand can be reused. These clarifiers are often used for wastewater treatment, where high flows are desired and physical space is limited. Typical rise rates range from 10 to 20 gpm/ft2 (24 to 49 m3/[h · m2]), and these rise rates are calculated based on the number of plates in the clarifier.

DensaDeg Clarifier

The DensaDeg clarifier (Ill. 18) uses extensive mixing and solids contact to build heavy compact floc. There is a coagulation tank with fast mixing followed by a flocculation tank with relatively turbulent mixing. This flocculation tank circulates the water, which then flows up through a center cylinder, and then back down around the outside of the cylinder. Settled floc is returned to this tank to add solids and to increase solids contact. With the extensive mixing, the floc breaks down and reforms into more compact heavier floc. This process is quite compatible with lime softening and wastewater treatment applications.

The unit is somewhat more chemical demanding than the Actiflo unit, but it does not need microsand or hydrocyclones for operation. Typical rise rates range from 10 to 20 gpm/ft2 (24 to 49 m3/[h m2]).

Ill. 18 An illustration of a DensaDeg clarifier.

Ill. 19 Drag tank. (FMC Corporation.)

Drag Separator

The rectangular drag tank is a modification of the conventional rectangular clarifier constructed in either a concrete or steel basin.

Ill. 19 illustrates a concrete drag tank. The tank is designed for the removal of dense, anhydrous, gritty solids, such as granulated slag from a foundry cupola or hot strip steel mill scale. Water drains from the solids as they are dragged from the vessel by the flights moving up the beach. The movement of the flights up the beach breaks down fragile solids; hence, the type of sol ids in the wastewater restricts drag tank applications. The detention time in the tank is usually short, limiting flocculation even when chemicals are used and resulting in poor effluent clarity.

Chemical flocculation ahead of the tank improves effluent clarity.

The flights can be arranged to skim the tank surface to remove floating material such as oil.

Oil/Water Separators

An oil/water separator is used to separate free oil from wastewaters in refineries, chemical process plants, steel mills, and other industries as primary treatment. These units do not separate soluble impurities, nor do they break oil-in-water emulsions without chemical treatment.

Settleable solids settle to the bottom of the basin or to a sludge collection pit. The types of oil/water separators include:

• American Petroleum Institute separator

• Corrugated Plate Interceptor

• Tilted Plate Interceptor (TPI) or Parallel Plate Interceptor

• Circular oil/water separator

Generally, the free oil/water separator is the first unit in cleaning up an oily wastewater before further treatment or reuse. In many cases, an equalization pond is located upstream from the separator. The pond equalizes the wastewater flow and characteristics to the separator and through the wastewater treatment system, to provide as constant as possible hydraulic and organic loadings on the separator and other processes. Wide variations in the flow to a separator affect the free oil capture efficiency, since the capture is directly related to the hydraulic loading. After the separator, the free oil ideally has been lowered to a level such that the remaining emulsified oil can be removed by dissolved air flotation or similar device, to protect the following biological treatment process or meet oil discharge requirements.

Ill. 20 API separator.

American Petroleum Institute Separator

The American Petroleum Institute (API) separator is a rectangular clarifier with chain and flight oil skimmers (Ill. 20), designed in accordance with procedures and specifications developed by the American Petroleum Institute. API has determined that the design of wastewater separators should be based on the rate of rise of oil globules having diameters of 0.15 mm. This globule size, although somewhat arbitrary, has been adopted for design purposes. Both laboratory experiments and a study of existing plant data indicate that satisfactory free oil removal is achieved, when the 0.15 mm diameter particle is used.

The removal of a given oil globule is a function of the overflow rate (also called the rise rate) in an ideal separator, with no short circuiting or turbulence. The overflow rate is the influent flow rate divided by the separator's surface area, and has the dimensions of velocity. Any oil globule with a rate of rise equal to or greater than the overflow rate is removed in the separator. This means that any globule having a rate of rise equal to or greater than the water depth divided by the retention time will reach the surface, even though it starts from the bottom of the basin.

The design of an API oil/water separator is based on four relationships:

1. The rise rate of 0.15 mm diameter oil globules is directly related to the difference in the specific gravity of the water and oil, and inversely to the absolute viscosity of the waste water at the design temperature.

2. The minimum horizontal area is based on the oil globule rise rate, corrected by the API factors for turbulence and short circuiting in the separator.

3. A horizontal water flow not to exceed 15 times the oil globule rise rate or 3 ft/min (0.91 m/min).

4. A minimum water depth to separator width ratio of 0.3. The depth of the separator is to be 3 ft (0.91 m) minimum to 8 ft (2.4 m) maximum. The separator width limits are 6 ft (1.8 m) minimum to 20 ft (6.1 m) maximum. Chain and flight scrapers are limited to a maximum width of 20 ft (6.1 m).

The API standards further recommend, that a minimum of two parallel channels be provided, so that one is available for use when it becomes necessary for the other to be taken out of service for repair or cleaning.

A manually rotatable, slotted pipe skimmer is typically used for removal of the oil. Some installations use a rotating drum skimmer that can be either fixed or floating. The drum skimmer picks up a thin film of oil as it rotates, which is scraped off and drained into a collection sump. The drum can be made of carbon steel, stainless steel, aluminum, or plastic depending on the manufacturer. Optimum rotational speed is 0.5 to 1.5 ft/s (0.15 to 0.46 m/s) with a drum submergence of 0.5 in (13 mm) or greater.

Corrugated Plate Interceptor

The Corrugated Plate Interceptor (CPI) (Ill. 21) contains corrugated plates that are 0.75 to 2 in (19 to 51 mm) apart, and at an angle of approximately 45° from vertical. Free oil globules rise only until contacting a corrugated plate, coalesce into larger drops with other globules, rise to the top of the plate pack, and finally to the water surface where they are skimmed off. The CPI maintains laminar flow conditions, while decreasing the distance that oil globules must rise to be collected, with an overall reduction of space taken by the oil/ water separator. Settleable solids settle to the surface of the plates and drop down to the sludge pit.

The capacity of a CPI is based on the same free oil globule rise rate and hydraulic overflow principles as the API separator. In this case, the horizontal projected area of the corrugated plates, are used for the horizontal area in the API procedure.

Ill. 21 Corrugated Plate Interceptor.

Ill. 22 Circular oil/water separator.

Parallel Plate Interceptor

The parallel plate interceptor (PPI) is similar to a plate clarifier. It is not extensively used in the refining or petrochemical industry, since the corrugated plates in the CPI are more efficient in coalescing free oil. However, PPI are used in other industries. The PPI operates on the same principles as the CPI.

Circular Oil/Water Separator

The design of circular oil/water separators follows the arrangement of conventional circular clarifiers with a central influent entrance, a discharge on the periphery, and includes sludge rakes and oil skimming. The sizing of the unit is the same as the API separator, since the circular arrangement provides a horizontal wastewater flow, free oil globules rising, and suspended matter settling. A diagram of a circular oil/water separator is shown in Ill. 22. The larger clarifier handles the oily wastewater, while the smaller is a primary clarifier for wastewater that does not contain any oil.

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