TM 5-814-3/AFM 88-11, Volume III
c. Typical design. Example C-3 illustrates a typical clarifier design.
Chemical treatment of wastewater may be advantageous when the following conditions exist:
Wastewater flow and strength are intermittent and vary greatly;
Space available for additional facilities is limited;
Industrial waste that would interfere with biological treatment is present;
The plant is overloaded;
Plant odor is a problem;
Phosphorus removal is desired; and
Experience has shown that adding alum, iron or polyelectrolyte at either the primary or secondary clarifier
is effective in increasing pollutant removal efficiencies. Lime addition is also effective if the effluent pH is
adjusted (by recarbonation or acid addition) to acceptable limits for the subsequent treatment process or for
final disposal. Jar tests will be made to determine optimum coagulants and dosages. Pilot studies should be
made before selecting a coagulant.
a. Chemical used. The EPA Process Design Manual for Suspended Solids Removal provides criteria
for the application of the chemistry and the use of the chemical precipitants discussed. Sample problem
number C--4 illustrates the calculation of sludge volume resulting from chemical precipitation.
(1) Aluminum salts. Alum (hydrated aluminum sulfate) is the most widely used aluminum salt. It is
effective in many wastewater applications but the precipitate sludge is difficult to dewater. The primary use
of aluminum salts is for the removal of suspended solids and phosphorus. When alum is used, clarifier
overflow rates will not exceed 600 gallons per day per square foot.
(2) Iron salts. Experience has shown that ferric salts are better coagulants than ferrous salts. Both
ferrous and ferric salts are effective in the removal of suspended solids and phosphorous, but iron hydroxide
carryover in the effluent can affect the effluent quality.
(3) Lime. Lime addition will improve grit separation, suspended solids removal, phosphorus removal,
and oil and grease removal, as well as reduce odors from dried sludge. Dosage of lime equal to the suspended
solids in wastewater is a common practice.
(4) Polyelectrolytes. These are used frequently, by themselves and in conjunction with other coagulant
aids, to improve the solids-removal performance of sedimentation units. Their use should be based on jar test
results and be reconfirmed by results in situ. They are more expensive on a unit-weight basis than the other
chemicals in general use, but the required dosage is much lower. Polyelectrolytes--high molecular weight,
water-soluble polymers classified as cationic, anionic, and nonionic--are highly ionized proprietary
compounds. The cationic polymers are positively charged and will neutralize the negative surface charges
on suspended particles, thus permitting agglomeration. Anionic (negatively charged) and nonionic (no charge)
polyelectrolytes function as flocculants and must be used with a cationic material. The use of polyelectrolytes
has been justified on the basis of improved water quality rather than cost savings. They can also permit higher
flow rates through existing equipment.
b. Equipment for chemical precipitation
. The following brief
discussion on basic equipment required
for chemical precipitation is useful for the design of such systems.
(1) Mixing tanks. The method for mixing wastewater and the chemical will be a flash mixing device
in a mixing tank designed for 2 minutes detention time. The propeller will be specified so as to provide for
the anticipated maximum flow in the mixing tank.
(2) Flocculation. Design criteria for air and mechanical flocculators are given in paragraph 10-6.
Flocculation tanks will be designed for a detention time of 30 minutes.
(3) Settling tanks. The settling tanks involved in chemical treatment of wastewater will be designed
for a minimum 2 hours detention time or the applicable maximum overflow rate stipulated in table 11-3.