Flocculation and Clarification:
·
Clariflocculator
unit is combination of a flocculation tank and clarification tank. This unit is
a circular tank with concentric compartment for flocculation. The effluent in
distributed uniformly over the surface of flocculation chamber for effective
utilization of the available volume for flocculation. The specially designed
flocculation paddles enhance flocculation of the feed solids. The flocculated
effluent enters the settling zone from bottom.
·
As water
moves upwards, heavy particles settle at bottom, the liquid flows radially
outwards and upwards and the clarified liquid is discharged over a peripheral
weir into the launder. The deposited sludge is raked to the bottom near the
central pocket from which will be discharged into the sludge sump.
·
The
clarified water will flow into the storage tank by gravity.
Coagulants and Flocculation
Objective:
Coagulants and flocculants enhance dissolved metal removal
and reduce sludge volume during conventional acidic drainage and high-density
sludge treatment.
Description:
Coagulants and flocculants
are chemicals that can be added during acidic drainage treatment. Although some chemicals can be considered
both coagulants and flocculants (iron and aluminum salts), coagulation and
flocculation are two distinct processes.
Coagulation describes the
consolidation of smaller metal precipitate particles into larger metal
precipitate particles (flocs).
Coagulants reduce the net electrical repulsive force at the surface of
the metal precipitate particles. The
purpose of adding coagulants to acidic drainage waters is to increase the
number of flocs present in the treatment water.
As floc density increases, interparticle contact increases due to
Brownian motion, promoting agglomeration of colloidal particles into larger
flocs for enhanced settling (Qasim et al., 2000).
Coagulants are widely used in water treatment systems and
but are not commonly used at conventional acidic drainage treatment
operations. The most common coagulants
are aluminum and iron salts. Aluminum
and iron coagulants react with bicarbonate alkalinity (HCO3-)
in acid drainage creating aluminum, ferric or ferrous hydroxide flocs which
attract metals in solution through co-precipitation. Formation of hydroxide flocs with alum and
ferrous sulfate can be represented by the following equations (Faust and Aly,
1999):
Flocculation involves the
combination of small particles by bridging the space between particles with
chemicals (Skousen et al., 1996).
Essentially, coagulants aid in the formation of metal precipitate flocs,
and flocculants enhance the floc by making it heavier and more stable. For this reason, flocculants are sometimes
referred to as coagulant aids at water treatment operations (Tillman, 1996;
Faust and Aly, 1999).
Two
main groups of flocculants exist: mineral which includes activated silica,
clays, and metal hydroxides and synthetic which include anionic, cationic, and
nonionic compounds. Activated silica has
been used as a flocculant since the 1930’s to strengthen flocs and reduce the
potential of deteroriation (Skousen et al., 1996). It is usually produced on-site by reacting
sodium silicate with an acid to form a gel (Tillman, 1996). When using activated silica, the resultant
floc is larger, denser, more chemically stable, and settles faster than iron
and aluminum flocs.
Metal
hydroxides, produced from neutralization and precipitation reactions during
acidic drainage treatment, can be recycled to serve as flocculants. This is commonly implemented at high
density sludge treatment operations to increase solids content and
stability.
Clay
particles have large surface area to mass ratios. Clays have numerous negative exchange sites
on their surfaces for metal adsorption.
The addition of clays into the acid drainage treatment reduces metals
and results in more dense flocs which increase settling velocities of the
flocs.
Synthetic
flocculants consist of polymers which produce negative (anionic), positive
(cationic), or both (polyampholytes or nonionic polymers). Polyampholytes are neutral but release both
negative and positive ions when dissolved in water. The ions released from synthetic polymers
(flocculants) adsorb to destabilized particles to form larger flocs. According to Tillman (1996) cationic polymers
are most often used for charge neutralization and are usually used in
conjunction with a metallic coagulant to reduce the dose required and amount of
sludge produced. Anionic polymers
dissolve in water to provide more reaction sites for positively charged
coagulants. A drawback to using
synthetic flocculants is that over-dosage may hinder their efficiency.
Both coagulants and
flocculants enhance particle setting rates as particle settling velocity is
directly related to particle size and weight.
Naturally, large relatively heavy particles settle more rapidly than
smaller, light particles. In addition to
decreasing settling times, heavier flocs can reduce the amount of sludge volume
produced in acid drainage treatment facilities.
This is the reason flocculants are used during high-density sludge
treatment operations.
Climate:
Conventional lime and
limestone and high density sludge treatment of acidic drainage waters can be
implemented in all climates.
Treatment Process:
Coagulants and flocculants are typically released into
acidic drainage waters after the addition of the neutralizing reagent, prior to
the aeration basin (if present) or settling basin if an aeration basin is not
used. Refer to conventional lime and
limestone treatment and high-density sludge.
Requirements and Limitations of Use:
Coagulants and flocculants are used in existing acid
drainage treatment facilities when: 1) unique water chemistry or high metal
concentrations exist in the acid drainage water or 2) when residence time in
the settling basin is not adequate to facilitate efficient metal precipitation
and sludge accumulation (Skousen et al., 1996).
Many trace metals such as As (a metalloid), Cd, Cu, Mo, Ni,
Pb, and Zn do not precipitate as metal hydroxides, although they can be removed
from acidic drainage from co-precipitation on the surfaces of metal
hydroxides. An acidic drainage effluent
could contain high amounts of trace metals with inadequate dissolved iron and
aluminum concentrations for effective co-precipitation. Aluminum or iron coagulants can be added
during conventional treatment of the above effluent to enhance
co-precipitation. MEND (1994) recommends
a 2:1 iron to arsenic ratio for effective arsenic removal. Additionally, barium sulfate is used to
precipitate radionuclides such as radium.
Coagulants and flocculants may also be helpful at acidic
drainage treatment operations which either do not use mechanical aerators or
have insufficient residence time in the settling basin. Coagulants and flocculants form larger,
heavier, metal precipitate particles which can settle rapidly and scavenge
dissolved metals which have not been oxidized.
Alum (aluminum sulfate or Al2(SO4)3
is the most commonly used coagulant in water treatment. Alum is an effective coagulant for pH values
ranging from 5.5 to 7.5 (Table 1). Alum
can be applied as a solid or liquid.
Iron coagulants are ferric sulfate (Fe2(SO4)3),
ferrous sulfate (FeSO4) and ferric chloride (FeCl2). Iron compounds are generally cheaper, produce
a heavier floc, and perform over a wider pH range than aluminum coagulants
(Tillman, 1996). However, iron
coagulants are not used as much as aluminum due to staining equipment,
corrosiveness, and they require more alkalinity than alum. Ferric sulfate is active over a larger pH
range (4.0-6.0, 8.8-9.2) than ferrous sulfate (8.8-9.2) (Table 1) and produces
heavier flocs which settle more quickly.
Ferric chloride reacts in a manner similar to ferrous sulfate but is
commonly used as an oxidant (Skousen et al., 1996). Ferric chloride is effective over a much
greater pH range than aluminum sulfate, ferric sulfate, and ferrous sulfate
(Table 1).
Table 1. Coagulants and their effective pH ranges. From Tillman (1996).
|
Effective pH
Range
|
Coagulant
|
|
5.5 to 8.0
|
Aluminum Sulfate
|
|
4.0 to 6.0, 8.8-9.2
|
Ferric Sulfate
|
|
8.8 to 9.2
|
Ferrous Sulfate
|
|
4.0 to 11.0.
|
Ferric Chloride
|
The above coagulants are also
considered flocculants because they form metal hydroxides. Reactions involving iron and aluminum
coagulants require alkalinity and can decrease pH. Therefore, additional alkalinity in the form
of a neutralizing reagent must be added to account for the extra alkalinity use
(Refer to calculations).
Alkalinity requirements for coagulants (Faust and
Aly, 1999):
Assumptions
1) all
alkalinity in system is bicarbonate (HCO3-)
2) calculated
alkalinity expressed as CaCO3
3) 100%
coagulant content-additional calculations may be required based on state of
coagulant (solid or liquid) and coagulant content.
Alkalinity required for
alum
Reaction between alum and
natural alkalinity:
From the equation, it is apparent that 1 mole alum reacts
with 3 moles Ca(HCO3)2
On a weight basis, 1 mg/L alum reacts with:
Converting Ca(HCO3)2 to CaCO3:
The above calculation indicates that a 1 mg/L
addition of hydrated alum at an acidic drainage treatment plant requires an
additional 0.45 mg/L CaCO3 alkalinity.
Alkalinity required for ferrous sulfate
Reaction between ferrous sulfate and natural
alkalinity:
From the equation, it is
apparent that 1 mol Fe2(SO4)3 reacts with 3
moles Ca(HCO3)2
On a weight basis, 1 mg/L
Fe2(SO4)3 reacts with:
Converting Ca(HCO3)2 to CaCO3:
The above calculation indicates that a 1 mg/L
addition of hydrated alum at an acidic drainage treatment plant requires an
additional 1.10 mg/L CaCO3 alkalinity.
Predicted Performance:
Coagulants are not commonly used
in acidic drainage treatment operations.
However, iron and aluminum salts can be used for trace metal (and
metalloid) removal under special circumstances.
Water treatment operations have experienced good results using
coagulants to remove dissolved metals (Tillman, 1996; Faust and Aly, 1999).
Metal hydroxide flocculants,
formed during acidic drainage treatment, is recycled to create high density
sludge. High density sludge treatment
processes can increase solids content from 1-5% to as high as 40% (MEND,
1994).
Synergistic Technologies:
Conventional Lime and Limestone Treatment
High-Density Sludge (HDS)
Oxidants
Costs:
List costs of aluminum and iron salts
Flocculants used in high density sludge operations are
recycled metal hydroxides. Refer to high
density sludge for additional costs associated with this technology.
Case Studies:
No available case studies on the use of coagulants or
flocculants were found in the acidic drainage treatment literature. Refer to high density sludge for metal
hydroxide flocculants.
References:
Faust, S.D., and Aly, O.M., 1999, Chemistry of Water
Treatment: New York
Publishers, 581 p.
MEND, 1994. MEND Report
3.32.1: Acid mine drainage-Status of chemical treatment
and sludge management practices,
Mine Environmental Neutral Drainage (MEND), Canada
Qasim, S.R., Motley, E.M., and Zhu, G., 2000, Water Works
Engineering: Planning,
Design, and Operation: Upper Saddle River , New
Jersey
Skousen, J., Lilly, R., and
Hilton, T., 1996, Special chemicals for treating acid mine
drainage, in Skousen, J.,
and Ziemkiewicz, P.F., eds., Acid Mine Drainage: Control and Treatment: Morgantown , West Virginia ,
West Virginia University
and National Mine Land Reclamation Center
Tillman,
G.M., 1996, Water Treatment: Troubleshooting and Problem Solving: Chelsea
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