7.2. Activated Sludge
A. General. The activated sludge process and
its several modifications may be used to accomplish varied degrees of removal
of suspended solids, and reduction of carbonaceous and nitrogenous oxygen
demand. The degree and consistency of treatment required, type of waste to be
treated, proposed plant size, anticipated degree of operation and maintenance,
and operating and capital costs determine the choice of the process to be used.
The design shall provide for flexibility in operation. Plants over 1 million
gallons per day (3,785 cubic meters per day) shall be designed to facilitate
easy conversion to various operational modes. In severe climates, protection
against freezing shall be provided to ensure continuity of operation and
performance.
B. Aeration
1. Capacities and Permissible Loadings
a. The design of the aeration tank for any
particular adaptation of the process shall be based on full scale experience at
the plants receiving wastewater of similar characteristics under similar
climatic conditions, pilot plant studies, or calculations based on process
kinetics parameters reported in technical literature. The size of treatment
plant, diurnal load variations, degree of treatment required, temperature, pH,
and reactor dissolved oxygen when designing for nitrification, influence the
design. Calculations using values differing substantially from those in the
table shown below must reference actual operational data.
b. The applicant must substantiate capability
of the aeration and clarification systems in the processes using mixed liquor
suspended solids levels greater than 5,000 milligrams per liter.
c. The applicant shall use the values shown
in Table R317-3-7.2(B)(1)(c) to determine the aeration tank capacities and
permissible loadings for the several adaptations of the processes, when process
design calculations are not submitted. These values are based on the average
design rate of flow, and apply to plants receiving peak to average diurnal load
ratios ranging from about 2:1 to 4:1.
2. Arrangement of Aeration Tanks
a. Dimensions. Effective mixing and
utilization of air must be the basis of dimensions of each independent mixed
liquor aeration tank or return sludge reaeration tank. Liquid depths should not
be less than 10 feet (3 meters) or more than 30 feet (9 meters) unless the
applicant justifies the need for shallower or deeper tanks.
b. Short-circuiting. The shape of the tank
and the installation of aeration equipment should provide for positive control
of short-circuiting through the aeration tank.
c. Number of Units. Total aeration tank
volume shall be divided among two or more units, capable of independent
operation, to meet applicable effluent limitations and reliability
guidelines.
d. Inlets and Outlets.
Inlets and outlets for each aeration tank unit shall be suitably equipped with
valves, gates, stop plates, weirs, or other devices to permit controlling the
flow to any unit and to maintain reasonable constant liquid level. The
hydraulic properties of the system shall permit the maximum instantaneous
hydraulic load to be carried with any single aeration tank unit out of
service.
e. Conduits. Channels and
pipes carrying liquids with solids in suspension shall be designed to maintain
self-cleaning velocities or shall be agitated to keep such solids in suspension
at all rates of flow within the design limits. Drains shall be installed in the
aeration tank to drain segments or channels which are not being used due to
alternate flow patterns.
f.
Freeboard. All aeration tanks should have a freeboard of not less than 18
inches (46 centimeters). Additional freeboard or windbreak may be necessary to
protect against freezing or windblown spray.
3. Aeration Requirements
a. Oxygen requirements must be calculated
based on factors such as, maximum organic loading, degree of treatment, level
of suspended solids concentration (mixed liquor) to be maintained, and
uniformly maintaining a minimum dissolved oxygen concentration in the aeration
tank, at all times, of two milligrams per liter.
b. When pilot plant or experimental data on
oxygenation requirements are not available, the design oxygen requirements
shall be calculated on the basis of:
(1) 1.2
pounds 02 per pound of maximum
BOD5 applied to the aeration tanks (1.2 kilograms
02 per kilogram of maximum BOD5),
for carbonaceous BOD5 removal in all activated sludge
processes with the exception of the extended aeration process,
(2) 2 pounds 02 per
pound of maximum BOD5 applied to the aeration tanks (two
kilograms 02 per kilogram of maximum
BOD5) for carbonaceous BOD5
removal in the extended aeration process,
(3) 4.6 pounds 02 per
pound of maximum total kjeldahl nitrogen (TKN) applied to the aeration tanks
(1.2 kilograms 02 per kilogram of maximum TKN), for
oxidizing ammonia in the case of nitrification, and
(4) oxygen demand due to the high
concentrations of BOD5 and TKN associated with recycle
flows such as, digester supernatant, heat treatment supernatant, belt filter
pressate, vacuum filtrate, elutriates, etc.
c. Oxygen utilization should be maximized per
unit power input. The aeration system should be designed to match the diurnal
organic load variation while economizing on power input.
4. Diffused Air Systems
a. The design of the diffused air system to
provide the oxygen requirements shall be done using data derived from pilot
testing or an empirical approach.
b. Air requirements for a diffused air system
may be determined by use of any of the recognized equations incorporating such
factors as:
(1) tank depth;
(2) alpha factor of waste;
(3) beta factor of waste;
(4) certified aeration device transfer
efficiency;
(5) minimum aeration
tank dissolved oxygen concentrations;
(6) critical wastewater temperature;
and
(7) altitude of
plant.
c. In the absence
of experimentally determined alpha and beta factors by an independent
laboratory for the manufacturer or at the site, wastewater transfer efficiency
shall be assumed to be 50 percent of clean water efficiency for plants treating
primarily (90 percent or greater) domestic sewage. Treatment plants where the
waste contains higher percentages of industrial wastes shall use a
correspondingly lower percentage of clean water efficiency and shall submit
calculations to justify such a percentage.
d. The design air requirements shall be
calculated on the basis of:
(1) 1,500 cubic
feet per pound of maximum BOD5 applied to the aeration
tanks (94 cubic meters per kilogram of maximum BOD5),
for carbonaceous BOD5 removal in all activated sludge
processes with the exception of the extended aeration process,
(2) 2,000 cubic feet per pound of maximum
BOD5 applied to the aeration tanks (125 cubic meters per
kilogram of maximum BOD5) for carbonaceous
BOD5 removal in the extended aeration process,
(3) 5800 cubic feet per pound of maximum
total kjeldahl nitrogen (TKN) applied to the aeration tanks (360 cubic meters
per kilogram of maximum TKN), for oxidizing ammonia in the case of
nitrification,
(4) corresponding
air quantities for satisfaction of oxygen demand due to the high concentrations
of BOD5 and TKN associated with recycle flows such as,
digester supernatant, heat treatment supernatant, belt filter pressate, vacuum
filtrate, elutriates, etc., and
(5)
air required for channels, pumps, aerobic digesters, or other uses.
e. The capacity of blowers or air
compressors, particularly centrifugal blowers, must be calculated on the basis
of air intake temperature of 40 degrees Centigrade (104 degrees Fahrenheit) or
higher and the less than normal operating pressure. The capacity of drive motor
must be calculated on the basis of air intake temperature of -30 degrees
Centigrade (-22 degrees Fahrenheit) or less. The design must include means of
controlling the rate of air delivery to prevent overheating or damage to the
motor.
f. The blowers shall be
provided in multiple units, so arranged and in such capacities as to meet the
maximum air demand with the single largest unit out of service. The design
shall also provide for varying the volume of air delivered in proportion to the
load demand of the plant. Aeration equipment shall be easily adjustable in
increments and shall maintain solids suspension within these limits.
g. Diffuser systems shall be capable of
providing for the maximum design oxygen demand or 200 percent of the average
design oxygen demand, whichever is larger. The air diffusion piping and
diffuser system shall be capable of delivering normal air requirements with
minimal friction losses.
h. Air
piping systems should be designed such that total head loss from blower outlet
(or silencer outlet where used) to the diffuser inlet does not exceed 0.5
pounds per square inch (0.04 kilogram per square centimeter) at average
operating conditions.
i. The
spacing of diffusers should be in accordance with the oxygen requirements
through the length of the channel or tank, and should be designed to facilitate
adjustment of their spacing without major revision to air header piping.
Removable diffuser assemblies are recommended to minimize downtime of aeration
tanks.
j. Individual assembly units
of diffusers shall be equipped with control valves, preferably with indicator
markings for throttling, or for complete shutoff. Diffusers in any single
assembly shall have substantially uniform pressure loss.
k. Air filters shall be provided in numbers,
arrangements, and capacities to furnish, at all times, an air supply
sufficiently free from dust to prevent damage to blowers and clogging of the
diffuser system used.
5.
Mechanical Aeration Systems
a. Oxygen
Transfer Performance. The mechanism and drive unit shall be designed for the
expected conditions in the aeration tank in terms of the power performance. The
mechanical aerator performance shall be verified by certified
testing.
b. Design Requirements.
The design requirements of a mechanical aeration system shall accomplish the
following:
(1) Maintain a minimum of 2.0
milligrams per liter of dissolved oxygen in the mixed liquor at all times
throughout the tank or basin;
(2)
Maintain all biological solids in suspension;
(3) Meet maximum oxygen demand and maintain
process performance with the largest unit out of service; and
(4) Provide for varying the amount of oxygen
transferred in proportion to the load demand on the plant.
c. Winter Protection. Due to high heat loss
and the nature of spray-induced agitation, the mechanism, as well as subsequent
treatment units, shall be protected from freezing where extended cold weather
conditions occur.
6.
Return Sludge Equipment
a. Return Sludge Rate
(1) The minimum permissible return sludge
rate of withdrawal from the final settling tank is a function of the
concentration of suspended solids in the mixed liquor entering it, the sludge
volume index of these solids, and the length of time these solids are retained
in the settling tank. Since undue retention of solids in the final settling
tanks may be deleterious to both the aeration and sedimentation phases of the
activated sludge process, the rate of sludge return expressed as a percentage
of the average design flow of sewage should be between the limits set forth in
Table R317-3-7.2(B)(6)(a)(1).
(2)
The rate of sludge return shall be varied by means of variable speed motors,
drives, or timers (in plants designed for less than one million gallons per day
- 3,785 cubic meters per day) to pump sludge at the above rates.
b. Return Sludge Pumps
(1) If motor driven return sludge pumps are
used, the maximum return sludge capacity shall be with the largest pump out of
service. A positive head should be provided on pump suctions. Pumps should have
at least 3 inch (7.6 centimeters) suction and discharge openings.
(2) If air lifts are used for returning
sludge from each settling tank hopper, no standby unit is required provided the
design of the air lifts are such to facilitate their rapid and easy cleaning
and provided standby air lifts are provided. Air lifts should be at least 3
inches (7.6 centimeters) in diameter.
c. Return Sludge Piping. Discharge piping
shall not be less than 4 inches (10 centimeters) in diameter, and should be
designed to maintain a velocity of not less than two (2) feet per second (0.61
meters per second) when return sludge facilities are operating at normal return
sludge rates. Sight glasses, sampling ports and rate of flow controllers for
return activated sludge flow from each settling tank hopper shall be
provided.
7. Waste
Sludge Facilities
a. The design of waste
sludge control facilities should be based on a logically developed solids mass
balance at the maximum design flow. Otherwise, a maximum capacity of not less
than 25 percent of the average design flow shall be provided, and function
satisfactorily at rates of 0.5 percent of average sewage flow or a minimum of
10 gallons per minute (0.63 liters per second), whichever is larger.
b. Sight glasses, sampling ports and rate of
flow controllers for waste activated sludge flow shall be provided.
c. Waste sludge may be discharged to the
concentration or thickening tank, primary settling tank, sludge digestion tank,
vacuum filters, other thickening equipment, or any practical combination of
these units.