Primary Wastewater Treatment -- Introduction/Clarification by Sedimentation

Introduction

Industrial wastewater treatment systems take many forms, depending on the constituents in the wastewater and the goals of the treatment (discharge requirements). Primary treatment, as the name implies, precedes any additional treatment such as biological processes (secondary treatment). The primary treatment may be the only treatment, when the final discharge is to a publicly owned treatment works (POTW), or the treated water meets recycle requirements, such as blast furnace scrubber water in the steel industry. Primary treatment may be preceded by bar screens, grit (sand) removal, equalization basin, and pH correction to protect subsequent equipments and processes.

Wastewaters contain many different types of organic and inorganic solids, both dissolved and suspended depending on the wastewater sources. Physical-chemical processes are used to remove specific dissolved matter in the wastewater, such as heavy metals and emulsified oils, and to oxidize or precipitate toxic chemicals. These processes may include sedimentation or dissolved air flotation.

Clarification by Sedimentation

Clarification of wastewater through the process of sedimentation is the separation of suspended solids by gravitational settling. The sedimentation process is used in primary settling basins, removal of chemically treated solids, and solids concentration. Sedimentation basins perform the two-fold function of producing both a clarified water product, and a concentrated slurry (sludge) product. Two distinct forms of sedimentation vessels are in common use. The clarifier is used, as the name suggests, for the clarification of a dilute suspension to obtain water containing minimal suspended solids, while producing a concentrated sludge. A thickener is used to thicken a suspension to produce an underflow with a high solids concentration, while also producing a clarified overflow. Primary clarification is the most economical unit process for pollutant removal from a cost per unit weight of biochemical oxygen demand (BOD) or solids removed. For this reason, it is the most widely used process for wastewater treatment.

Forms of Solids in Wastewater

Total suspended solids (TSS) in water are defined by the U.S. Environmental Protection Agency (EPA) as those dry solids retained on a 0.45 µm filter from a total water sample and reported as dry mg/L or percent dry solids by weight (The suspended solids include floatable material.). The soluble or dissolved solids in the water are those that pass through the 0.45 µm filter and reported as dry mg/L or percent dry solids by weight. The sum of these two is the total solids in the water. By burning the solids of each, the suspended solids and dissolved solids are further classified into volatile and nonvolatile. The volatile fraction is assumed to represent the organic fraction in the suspended and dissolved solids.

The total suspended solids in the water are additionally divided into settleable solids and nonsettleable solids. The nonsettleable solids are the fine, colloidal particle fraction held in suspension by surface charges that do not settle by gravity. Clarification processes remove only the settleable fraction of the suspended solids. If nonsettleable sol ids are to be removed by clarification, chemical conditioning (coagulation and flocculation) of the colloidal solids is necessary.

Standard Methods for the Examination of Water and Wastewater (American Public Health Association, Washington , DC ) includes two methods for the measurement of settleable solids. The first is reported by volume as mL/L and the second is by weight in mg/L.

A one-liter Imhoff cone is used to settle a mixed sample of the wastewater for one hour. The volume of solids that settle in the cone are read by the milliliter graduations on the cone apex. The settleable suspended solids are reported by volume as mL/L.

The nonsettleable solids by weight are measured by settling a minimum one-liter sample in a minimum 90 mm diameter glass vessel.

A larger diameter vessel and sample may be used, but the sample depth in the container must be deeper than 200 mm. The sample is allowed to settle for one hour. A 250 mL sample is siphoned from the center of the container at a point halfway between the surface of the settled solids and the water surface without disturbing the settled or floating material. The suspended solids by weight (mg/L) are determined on the 250 mL sample, and this represents the nonsettleable solids. The settle able solids by weight are then the difference between the mg/L total suspended solids and the mg/L nonsettleable suspended solids.

Ill. 1 The four types of settling.

Sedimentation Phenomena

Four basic classes or types of sedimentation processes take place, depending on the particle concentration and the degree of particle interaction. These settling classes are discrete particle, flocculant, hindered (also called zone), and compression (Ill. 1). More than one class of settling can take place at any one time, and it is common to have three occurring in the clarification of dilute solutions. Thickening of solutions having suspended solids concentrations greater than about 500 to 1000 mg/L typically utilize hindered and compression settling. Tbl. 1 describes these types of settling phenomena.

Discrete Particle Settling

The analysis of discrete, non-flocculating particle settling is by means of the classic laws of sedimentation formed by Newton and Stokes.

Newton 's Law relates the gravitational force on the particle, with the frictional resistance or drag, and the difference between the densities of the particle and liquid. The value of the drag coefficient varies depending on turbulent or laminar flow conditions and is a function of the Reynolds number. The drag coefficient becomes a constant for the low laminar flow conditions found in clarification. Stokes incorporated this constant drag coefficient into Newton 's gravitational force equation to arrive at the classic Stokes' Law for sedimentation:

Vo = G (ρ_1 - ρ_2)D² / 18µ

where Vo = settling velocity, cm/s

G = gravitational force, cm/s²

ρ_1= particle density, g/cm³

ρ_2= liquid density, g/cm3

D = particle diameter, cm

µ= liquid viscosity, dyne s/cm²

TBL. 1 Types of Sedimentation in Wastewater Treatment

Type of settling | Description | Application

[Discrete particle (Class 1) Flocculant (Class 2) Hindered or Zone (Class 3) Compression (Class 4)]

[Description:

Particles in a low solids concentration settle as individual entities not interacting with adjacent particles.

A dilute suspension of particles that coalesce or flocculate during sedimentation. The particles increase in size and mass by agglomeration, thereby increasing the settling rate.

Suspensions of intermediate concentration in which the forces between particles are sufficient to hinder the settling of adjacent particles. The particles adhere together, and the mass settles as a blanket, forming a distinct inter face between the floc and the supernatant.

The settling rate starts to decrease.

The particle suspension has reached a concentration such that a structure is formed and further settling can occur only by compression of the structure. The weight of the particles being constantly added to the top of the structure by sedimentation provides this compression. The settling rate of this zone becomes very slow.]

[Application:

Removal of grit, sand, and inorganic particles such as slag in steel mills.

Most solids in wastewater are of a flocculant nature including pulp and paper, food processing, municipal, and biological treatment. Chemically treated solids exhibit Class 2 sedimentation.

Hindered settling is characterized by biological solids and flocculated chemical suspensions, when the concentration exceeds 500-1000 mg/L, depending on the type of particle.

Occurs in the lower layers of deep sludge masses, such as in the bottom of deep clarifiers and in sludge-thickening facilities.]

Ill. 2 Discrete particle settling.

A terminal velocity V_o is selected for the sizing of a sedimentation basin so that all particles having a velocity equal to or greater than V_o are removed. Class 1 settling in an ideal basin is shown in Ill. 2.

The particle having a settling velocity Vo that settles through a distance equal to depth D in length L (theoretical detention period) is removed.

This is called the basin overflow rate:

V_o= Q/A

where Q = inlet flow rate, gpm or gpd (m³/h)

A = surface area of tank, ft² (m² )

V_o = overflow rate, gpm/ft² or gpd/ft² (m3/[h m² ] or m/h) In the ideal basin of Ill. 2, it is assumed that particles entering the tank are evenly distributed over the inlet cross-section, and a particle is considered removed when it enters the sludge zone. Thus, all particles with a settling velocity greater than V_o are removed, and particles with lower settling velocity (Vp) are removed in the ratio Vp/V_o as illustrated in Ill. 2.

The flow capacity of basins for discrete particle settling is independent of depth, as shown in Eq (2). The length of the basin and the time that a unit of water is in the basin for continuous flow sedimentation needs to be such that all particles with the design velocity of Vo settle to the bottom. The design velocity (V_o) for a full scale continuous flow clarification device, has to be adjusted for the effects of influent and effluent turbulence, hydraulic short-circuiting, sludge storage, velocity gradients due to temperature changes, and operation of sludge removal equipment.

Ill. 3 Flocculant settling.

Flocculant Settling

Flocculant settling is created by the coalescence of particles in a dilute solution. The settling velocity of the particle increases as it settles through the tank depth, because of this agglomeration that increases its size and density. Ill. 3 illustrates the curvilinear settling path of the particle as it increases in settling velocity and size. Most of the suspended solids in industrial wastewaters are flocculant in nature.

The efficiency of removal for discrete particles is related only to the overflow rate, while flocculant particle removal is dependent on both overflow rate and depth of basin (detention time).

The degree of flocculation that takes place is dependent on the opportunity for particle contact, the overflow rate, the depth of basin, initial particle concentration, range of particle sizes, and flocculating nature of the particles. A mathematical analysis concerning these parameters for basin design and operation is generally considered impossible. Long column settling tests are required to deter mine the effects of these variables, when historical data concerning the type of solids in a specific type of wastewater is unknown. The columns used are typically 6 in (152 mm) in diameter by 8 to 10 ft (2.4 to 3 m) high.

Hindered Settling

Hindered settling is typified by activated (biological) sludges and chemically flocculated suspensions, when the concentration of solids exceeds 500 to 1000 mg/L. Hindered settling takes place following the concentration of the suspension during flocculant settling or in gravity thickening, when the initial solids concentration is greater than about 500 mg/L. Since flocculated particles are in close proximity to each other, they settle as a mass or blanket, and form a distinct water/solids interface at the top of the blanket. The friction produced as the water moves up through the interstices of the blanket slows the settling rate of the mass (Ill. 4). The reduction in settling rate continues as the solids blanket concentrates. The rate of settling in the hindered region is a function of the concentration of solids and their characteristics.

Ill. 4 Hindered settling.

Ill. 5 Rake action in compression zone.

Compression Settling

A compressed layer of solids starts to form beneath the hindered settling zone as sedimentation continues. A structure is formed in this region by the close physical contact between the particles. Additional water is forced out and upward from the hindered settling zone, by the increasing weight of solids on the top of the zone as sedimentation continues. The maximum solids concentration of the solids at the bottom is limited by the characteristics of the solids and the weight of solids above the lowest layer.

Due to the many variables between the various types of solids encountered in wastewater treatment, settling tests are usually required to determine the settling rate and detention time when hindered and compression settling are a consideration. The test cylinder is equipped with a slow-speed stirrer rotating at 4 to 5 revolutions per hour. The stirrer simulates the rake action and corresponding hydraulic motion in a clarifier. This breaks the arching action of the solids in these zones, to increase the release of water from the sludge bed, and increase solids concentration ( Ill. 5).

Mass Loading

Both the clarification of the water overflow and the thickening of the sludge underflow are involved in the separation of flocculant suspensions. The overflow rate for clarification requires that the rise velocity of the liquid overflowing the tank be less then the settling rate of the suspension. The tank surface area requirements for thickening the underflow to a desired concentration, are related to the solids loading or solids flux, usually expressed in terms of mass loading (pounds per square foot per day or kilograms per square meter per day) or a unit area (square feet per pound of solids per day or square meters per kilogram of solids per day). The mass loading with corresponding underflow concentration for a particular application, can be calculated from a stirred laboratory test.

In most clarification applications, the limiting flux and underflow concentration from the laboratory, testing must equal or be greater than the solids loading rate to the clarifier:

GL = CQ/A = M/A

where Qo = influent flow, ft³/d (m³/d)

Co = influent solids, lb/ft³(kg/m³ )

M = solids loading, lb/d (kg/d)

GL= limiting solids flux, lb/[d · ft²] (kg/[d m²])

A = area, ft² (m² )

Both gravity, and the velocity resulting from sludge removal at the tank bottom in a continuous clarifier or thickener, removes solids:

U = G_L/C_u

… where U is downward sludge velocity due to sludge removal, ft/d (m/d) and Cu is underflow concentration, lb/ft³ (kg/m^3) Increasing the underflow pumping rate U in Eq. (22.4) decreases the underflow concentration Cu. The solids flux G_L is not the limiting factor for most clarification applications, since the influent suspended solids are generally less than 500 mg/L.

Clarifier Hydraulic Capacity

The parameters that affect the settling rate are of prime consideration for the sizing and operation of a sedimentation device. The larger the particle diameter, and the heavier the particle, the faster it settles. The colder the water (higher viscosity), the slower the settling rate. Wastewaters contain a mixture of particle sizes and densities. Therefore, the sedimentation device has to be sized for the smallest, lightest settleable particle, in order to maximize the settleable solids capture. Conversely, a change in an industrial process to one that produces a larger quantity of smaller lighter particles, results in a decrease in solids capture.

A sedimentation device that includes inlet baffling for the dissipation of the influent energy, a quiescent zone for particulate settling, mechanical means for the removal of settled solids, and low flow velocity to the outlet, is commonly called a clarifier. The hydraulic capacity of a clarifier is based on the discreet and flocculant type settling velocity expressed as feet per minute (meter/hour). In U.S. units, multiplying the settling rate by 7.48 gal/ft³ yields the hydraulic loading expressed as gallons per minute per square foot. The hydraulic loading can also be expressed as gallons per day per square foot.

The metric units are cubic meters per hour or day per square meter.

This hydraulic loading is also called the overflow rate, and it is used to size a new clarifier or ascertain the capacity of an existing clarifier.

A settling curve is developed for a sample of the wastewater to arrive at an estimate of the overflow rate. Preferably, a minimum 2L graduate is used for the settling test, and the fall of the suspended solids in milliliters is recorded for each 30 s of elapsed time. Readings are taken for at least 30 min or until the change in solids height per minute is relatively small or constant. The results in milliliters are converted to inches (millimeters) by measuring the graduate (in/2000 mL or mm/2000 mL) and graphed as illustrated in Ill. 6. The slope of the linear portion of the curve is the settling rate for that sample in inches per minute (mm/min).

Ill. 6 Settling curve.

In the Ill. 6 example, the slope is (16-2)/(9-0) = 1.56 in/min (39.51 mm/min). It is necessary to apply a scale-up factor to account for the dynamics of an operating clarifier versus the quiescent state of the settling test. The scale-up factor is generally in the range of 0.65 to 0.85, depending on the actual application conditions. The hydraulic loading can then be calculated as follows:

U.S. Units

(1.56 in/min)(1 ft/12 in)(7.48 gal/ft³)(0.7)= 0.68 gpm/ft² or 980 gpd/ft²

Metric Units

(39.51 mm/min)(1 m/1000 mm)(60 min/h)(0.7) = 1.66 m³/[h · m²] or 39.8 m³ /[d · m²]

The sedimentation device can be referred to as a clarifier-thickener, when thickening of the settled sludge is desired. Determining the settling rate of the sludge bed to achieve the target sludge concentration is important to learn if this rate controls the capacity of the clarifier thickener. The dry solids loading or solids flux, expressed as pounds dry solids per square foot per hour or day (kilograms dry solids per square meter per hour or day), is the parameter used to describe the thickening capacity (A typical example is a secondary clarifier following a biological aeration basin, where the solids loading and not the hydraulic loading is the controlling factor.). The concentration of the settling sludge bed takes place during and after the knee in the settling curve (Ill. 6), where hindered and compression zone settling take place. Generally, the solids flux is not a controlling factor in clarification of wastewaters having dilute solids concentrations less than about 500 to 1000 mg/L.

Another major parameter for sizing a clarifier is the minimum tank depth or detention time. The tank volume has to provide the required flocculant settling depth plus 3 to 4 ft (0.91 to 1.2 m) for deceleration of the water as it approaches the effluent weirs, plus depth for sludge storage. The water depth at the tank wall is commonly 8 to 15 ft (2.4 to 4.6 m), with a minimum basin floor slope of 1 on 12.

The typical detention time is in the range of 1.5 to 3 h. Slow-settling solids usually require a deeper water depth with correspondingly longer detention times.

TBL. 2 Temperature Correction for Rise Rate

Ill. 7 Clarifier operating zones

Temperature

Warm water has a lower viscosity (less dense) than colder water.

Therefore, the particle settling rate is faster at higher water temperatures than colder ones. Raising the temperature from 32°F to 80°F (0°C to 27°C) doubles the settling rate for a given particle, since both the density and viscosity of the water are reduced. Multiply the rise rate by the correction factor in Tbl. 2 to adjust for the change of water temperature from 60°F (15.6°C).

Another temperature concern is the possibility of developing thermal gradients within the clarifier. If developed, they can pre vent the sedimentation of the suspended solids or lift settled solids, from the sludge blanket up to the outlet launders. This is of concern when treating warm waters in above grade steel tanks, in cold climates. The water at the tank wall and floor cools, so that the bottom of the tank has a colder water temperature than the incoming water.

The warmer influent water can short-circuit to the effluent launders, resulting in high solids carryover. In warmer climates, above ground clarifiers can absorb radiant heat from the sun, and clarifier water temperatures increase. When cooler influent water enters the clarifier, the temperature difference can create thermal currents, which can result in resuspension and carryover of settled floc.

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