Primary Wastewater Treatment -- Flotation Clarification

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The dissolved air flotation (DAF) clarification process is used to separate suspended solids or liquors from water that are lighter than or only slightly heavier than water. These wastes include oils, fats, greases, fibers, biosolids (thickening and secondary clarification in biological treatment plants), algae, and metal hydroxides to name a few. DAF is therefore used in many industries, such as oil refining, petrochemical, chemical, steel, meat and poultry packing, vegetable processing, pulp and paper, railroad terminals, and the prepared food industry. In these industries, oily waste may coat solid particles, giving them a tendency to float rather than settle. An API separator may precede the DAF system for the removal of free oil in many applications.

The flotation process consists of attaching fine gas bubbles (10 to 100 µm) to suspended solids or oily material, which reduces the specific gravity of the solids. The fine bubbles are produced by dissolving gas into the water at elevated pressure, followed by the subsequent reduction back to atmospheric pressure. The dissolved gas (in excess of atmospheric saturation) is released as fine bubbles, when the pressure is reduced. Air is commonly used as the gas, but other gases such as nitrogen are used, depending on the application.

Nitrogen is used in applications where there is a possibility of explosion, such as refineries.

Ill. 23 Dissolved air flotation system.

The Dissolved Air Flotation System

A schematic flow diagram of a typical DAF unit is illustrated in Ill. 23. Part of the effluent is saturated with air at the elevated pressure in the air saturation tank and recycled to the influent. The backpressure valve (also referred to as the pressure relief valve) at the indicated pressure release point in the flotation cell, reduces the pres sure on the recycle flow where it mixes with the influent. The back pressure valve is located external to the flotation cell in other designs.

In this case, the influent is mixed with the recycle flow after the pressure relief valve outside of the flotation cell. The excess dissolved air then comes out of solution, since the flow is now at atmospheric pressure, and the small gas bubbles attach to the particles in the wastewater and rise to the surface of the cell. The floated material is skimmed off the cell for further handling or recovery. The clarified effluent is discharged, reclaimed, or receives further treatment, depending on discharge requirements. The flotation cell can be either circular or rectangular. The circular unit is more economical for larger flows.

An organic flocculant or coagulant plus a flocculant are used in many instances to improve the solids capture or break oil emulsions. As in gravity settling, the coagulant can be an inorganic, such as an iron or aluminum salt, organic polymer, or a blend of inorganic and organic. Anionic, cationic, or nonionic organic flocculants may be used depending on the wastewater. Cationic flocculants are most commonly used, since the small air bubbles have a slightly negative charge. A bench DAF column test (Ill. 24) can be used to simulate the clarification process and evaluate various chemical programs. The typical addition points for the chemicals are indicated in Ill. 23.

Ill. 24 Bench DAF test. (Infilco Degremont Inc.)

Types of Dissolved Air Flotation Systems

The three different types of DAF systems are referred to as total pressurization, partial pressurization, and recycle pressurization. In total pressurization, the entire waste stream is pressurized and saturated with air ( Ill. 25a). The material to be separated must be able to withstand the shearing forces in the pressure pump, air saturation tank, and backpressure valve, or the floc must be capable of quickly reforming after the pressure is released. In addition, the material in the wastewater may plug the air saturation tank and backpressure valve, depending on the quantity and size of material.

This method is not used for oily or greasy wastes, as they can be mechanically emulsified.

Only a portion of the influent is pressurized in partial pressurization ( Ill. 25b). This method reduces pumping costs but has the same performance problems as indicated for total pressurization.

In recycle pressurization, ( Ill. 25c) a side stream (recycled effluent) of clarified water is pressurized, saturated with air, and then mixed with the chemically pretreated waste stream. The pretreatment may be in a separate flocculation tank or in-line, depending on the application. This method is preferred when the wastewater must be pretreated with chemicals, to eliminate shearing of the floc and eliminate plugging of the pressurization system. This is the preferred method to produce a low suspended solids effluent, even though the recycle flow increases the size of the flotation cell. Most systems today use recycle pressurization.

Ill. 25 Types of DAF systems.

Operating Parameters

The hydraulic loading, expressed as gpm/ft2 (m3/[h m2]) of flotation area, typically controls the capacity of a DAF cell for clarification applications. The solids loading or flux, expressed as lb/[h ft2] (kg/[h · m2]), may be the limiting factor, when the total suspended solids are greater then about 1000 mg/L. The flotation effect is established by the recycle ratio and the air/solids ratio.

Hydraulic Loading Rate

The hydraulic loading rate (also referred to as rise rate or overflow rate) is one of the important considerations, since the degree of clarification efficiency is a function of the hydraulic loading. The loading rate is normally in the range of 1.5 to 2 gpm/ft2 (3.7 to 4.9 m3/[h m2]) including recycle. The higher loading is for "easy to float" material such as oil and grease, while the lower number is for "slow floating" material such as biosolids and algae. Generally, a rise rate greater than 2 gpm/ft2(4.9 m3/[h m2]) results in higher effluent suspended solids.

An exception is DAF systems floating fiber, such as white water clarification in paper mills, which are designed for 2.5 gpm/ft2(6.1 m3/[h · m2 ). These loading rates typically capture 80 to 85% of the suspended solids without chemical treatment, and 90 to 95% plus with chemical treatment. Chemical treatment is necessary to resolve oil and fat emulsions to obtain removal of these emulsified materials.

The hydraulic loading rate can be calculated using Eq. (22.5):

HLR = () / A

where HLR = hydraulic loading rate, gpm/ft2(m3/[h m2])

Fi= influent flow, gpm (m3/h)

Fr= recycle flow, gpm (m3/h)

A = net flotation area inside flotation baffles, ft2 (m2 ) Some DAF cells have plate packs like parallel plate clarifiers to reduce the flotation cell floor space. The flotation cell hydraulic loading rate in this case is on the horizontal projected area of the plates, just like with plate clarifiers. The hydraulic loading rates previously discussed, are also used on the projected horizontal area in this case.

Care must be taken to ensure that the loading rate is calculated on the floor space area and not the projected plate area.

Solids Loading Rate

The capacity for DAF clarification applications is typically controlled by the hydraulic loading rate. The solids loading rate determines the DAF capacity for clarification, when the influent solids exceed approximately 1000 mg/L and for biological solids thickening. Solids loading is calculated by dividing the influent total suspended solids by the net flotation area. The solids in the recycle flow are customarily ignored in this calculation. The actual solids loading rate for clarification applications needs to be periodically checked using the following:

U.S. Units :

SLR = ()( ) / 2000 A

where SLR = solids loading rate, lb/[h · ft2 ]

Fi = influent flow rate, gpm

TSS = total suspended solids, mg/L

A = net flotation area inside flotation baffles, ft2

The 2000 factor = (694 gpm/mgd)(24 h/d)/8.33 lb/gal.

Metric Units

where SLR = solids loading rate, kg/[h m2]

Fi= influent flow rate, m3/h

TSS = total suspended solids, mg/L

A = net flotation area inside flotation baffles, m2

The 1000 factor = (106 mg/kg)(1 m3/1000 L).

Typical solids loading rates are 1.5 lb/[h ft2] (7.3 kg/[h m2]) without chemicals and 2 lb/[h·ft2] (9.8 kg/[h m2]) with chemical treatment. A polymer can be used to increase the solids loading rate to achieve the same solids capture or improve the solids capture efficiency at lower loading rate.

Air/Solids Ratio

The performance of a flotation system depends upon having sufficient air bubbles present to make contact with the suspended solids and float them. An insufficient quantity of air results in only partial flotation and poor clarification. On the other hand, excessive amounts of air can cause turbulence and high surface velocity, which can lead to short-circuiting and high effluent suspended solids. The performance of a flotation unit in terms of effluent quality and float solids concentration is related to an air/solids ratio, defined as pound (kilo gram) air per pound (kilogram) dry suspended solids. The ratio is calculated from operating data using Eq. (22.8):

where A/S = air/solids ratio, lb/lb (kg/kg)

Air = air solubility at operating conditions, mL/L

ERT = saturation tank (pressurization tank) efficiency, decimal

Fr= recycle flow rate, gpm (m3/h)

TSS = total influent suspended solids, mg/L

F = influent flow rate, gpm (m3/h)

1.3 = approximates the density of air at ambient temperature, mg/cm3

The air saturation tank dissolving efficiency (ERT) is in the range of 0.5 to 0.8 for clean water depending on the manufacturer.

Ill. 26 Suggested recycle rates for DAF clarification process based on waste TSS.

Recycle Ratio

In DAF clarification, the term recycle, expressed in percent of total influent flow, has been historically used instead of the air/solids ratio, even though the air/solids ratio is critical to the clarification process. Normally, the necessary air/solids ratio in clarification processes is considerably higher that what is used in DAF thickening. The main consideration in clarification is to provide a sufficient quantity of air bubbles to find and contact the suspended solids.

Statistically, more air bubbles are needed to contact the consider ably lower influent suspended solids concentration in clarification of dilute suspensions, as opposed to thickening applications. This low influent suspended solids concentration is the reason for the higher air/solids ratio.

Ill. 26 gives a general picture of the recycle rate requirement, based on the total suspended solids in the wastewater. This figure was compiled from a number of DAF units treating a variety of wastewaters including oil, fiber, fat, grease, latex, and others in a diversity of industries. The area between the operational range lines gives a choice of two recycle rates; the lower recycle rate is selected for an "easy to float" material, and the higher rate for a "slow floating material". A recycle rate of 30 to 50% is generally used, if the influent total suspended solids are unknown, depending on how hard or easy the material is to float.

Generally, wastewaters containing oils, fats, greases, fibers, and other lighter than water solids are classified as easy to float. Types of slow floating materials are biosolids, metal hydroxides, algae, and granular media solids.

Ill. 27 Typical pressurization system.

Pressurization Systems

Ill. 27 is a diagram of a typical pressurization system for a dissolved air flotation system. The operating pressure of the system just upstream from the pressure relief (backpressure) valve is generally 60 to 70 psig (414 to 483 kPag), although there are a few manufactures that design their systems for 40 to 45 psig (276 to 310 kPag). The system has to be operated at the manufacture's design operating pres sure, as any changes in pressure result in changes in recycle flow, hydraulic loading, saturation tank retention time, air dissolving efficiency, and quantity of air dissolved.

The internals of the saturation tank are designed to break the recycle water into small droplets, to maximize the water surface area and thus the transfer rate of the gas into the liquid. The internals vary depending on the manufacturer, and can be spray nozzles (Ill. 28), packing similar to a high-rate cooling tower, spray jet with a splash plate, and various baffle configurations. All of these designs require a gas atmosphere above the liquid level in the tank.

The liquid level in the saturation tank is controlled in one of two ways. The first, referred to as the bleed-off system, continuously feeds air to the tank. A level control valve (Ill. 29) opens with decreasing water surface, to vent air that has not gone into solution, and returns the water surface to its design elevation. The second is called the on/off feed air system in that the air to the tank is turned on/off in response to the rising/falling of the water surface in the tank.

Losing the water level surface control by flooding the tank with air or water, due to too much or too little air addition, or failure of the automatic level control system, are common causes for failure of the flotation process. Flooding the tank with air results in poor dissolving efficiency and the presence of "geyser" eruptions on the surface of the flotation cell, due to the free air being introduced into the cell. Flooding the tank with water results in the destruction of the water droplet production in the tank and the loss of air dissolving efficiency.

Ill. 28 DAF system without air saturation tank.

Ill. 29 Circular flotation cell.

Some DAF designs do not use an air saturation tank (Ill. 28). Instead, a centrifugal pump that is specially designed to handle air in-water solutions is used. The pump is capable of handling up to 35% entrained air without air binding, and produces air bubbles down to 30 µm. The design in Ill. 28 uses a slanted tube to vent excess air and several backpressure valves to release the system pressure. This type of system is used in easier to float applications, such as poultry packing wastewater.

A milky white water in the flotation cell indicates that the pressurization system is functioning properly. High dissolved solids or antifoams in the wastewater can adversely affect air solubility, reducing the amount of small air bubbles available for flotation.

Flotation Cell

The circular flotation cell or tank is similar to a circular clarifier having surface skimmers and sludge rakes or plows (Ill. 29). The number of skimmers provided is based on the anticipated float quantity. The influent is introduced at about mid-depth in a horizontal direction to prevent the flow from boiling in the center of the tank.

A deep flotation baffle is provided around the periphery, so that the clarified water has to flow downward from the inlet to the effluent weirs, while the floating material rises.

Ill. 30 Shallow tank flotation cell.

Another type of circular flotation cell is called the shallow tank configuration (Ill. 30), since the side water depth of the tank is only about 2.5 ft (0.76 m), while the more conventional circular cell (Ill. 29) has a tank side water depth of 6 to 8 ft (1.8 to 2.4 m). The influent feed containing the recycle flow is introduced into the tank from a rotating, radial manifold. The rotational velocity of the manifold is opposite to, and set equal to, the influent water velocity.

The influent is thus introduced into the tank at essentially zero velocity. This in essence static condition allows efficient flotation in the shallow tank, leaving the clarified water in the lower stratum of the tank, from where it is removed by the effluent extraction pipes attached to the rotating carriage. The rotating scoop attached to the carriage lifts the floated solids from the surface and transports them to the center of the unit, from where they are discharged from the tank. This type of unit is commonly used in the pulp and paper industry.

A rectangular flotation cell may be equipped with a chain and flight scraper for the floated material, and a second chain and flight scraper for the settled sludge (Ill. 31). Depending on the application, only a scraper for the float may be provided, in which case the floor of the basin may be sloped to hoppers. A reciprocating float scraper mechanism in place of the chain and flight type is used in some designs. The float is scraped to a sump at the feed end of the basin. An adjustable effluent overflow weir is provided, following a deep float containment baffle.

Ill. 31 Rectangular flotation cell.

Ill. 32 Nozzle-type induced air flotation. (Wemco Division, Envirotech Corporation.)

Float Removal Control

Most DAF units are equipped with adjustable overflow effluent weirs that determine the operating water level in the unit. Consequently, the weirs control how deep or shallow the skimmer blades dip into the floated solids. The water level rises as the overflow weir is raised, permitting the skimmer blades to dip more deeply into the floated material. This results in a greater percent of the floated material being removed with each pass of the skimmer. This in turn normally results in a cleaner effluent but a wetter (lower percent solids) float. Lowering the weir has the opposite effect. The solids get dryer (higher per cent solids), but the effluent quality may deteriorate.

Float skimmer blades remove the floated material from the top of the flotation unit. The skimmers may be driven by variable speed motors, which operate continuously, or by constant speed motors, operating intermittently on a timer. In either case, the mechanism needs to be adjusted to remove the float at the rate dictated by the treatment goals.

Generally, the slower (run less frequently) the skimmers move, the drier the float removed, but deterioration of the effluent water quality may result. Increasing the speed of the skimmers (run more frequently), result in more total solids being removed, which may improve the effluent quality at the expense of a wetter float.

Plant Flows

Minimum, maximum, and average flows must be considered and provisions made to cope with surges. A DAF clarifier is hydraulically capable of processing a higher flow rate with a constant recycle rate for brief or infrequent surges in plant flow, but the effluent quality may suffer. An equalization basin ahead of the DAF system is best for eliminating large, regular variations in wastewater, so that the system can be operated at as constant a flow as possible. Generally, reducing overflow rates below 1 gpm/ft2 (2.4 m3/[h m2]) does not measurably improve suspended solids capture.

Induced Air Flotation

Mechanically entraining air and dispersing it through the liquid as fine bubbles, in contrast to the release of dissolved gas from solution, is also used to achieve flotation. A rectangular tank is divided into four flotation cells. Each cell is equipped with either a motor-driven aerating rotor mechanism, or an aspirating nozzle (Ill. 32 illustrates the nozzle arrangement.). The specially designed rotor mechanism or nozzle draws air into the cell and disperses it throughout the cell. Oil globules and fine suspended solids attach to the bubbles as they rise to the surface, from where they are removed by skimmer paddles.

The induced air flotation (IAF) system is primarily used to remove oil from wastewaters with or without the addition of chemicals. It cannot handle wastewaters that contain heavy, settleable solids that will not float, since the unit does not have any capabilities for removing settled sludge.

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