Current through Register Vol. 41, No. 3, September 23, 2024
A.
General design. Design unit operation detention time shall be estimated as the
ratio of the design basin volume to the design flow rate (into that basin)
unless adequate test data is made available verifying that a different value of
detention time can be utilized. Multiple unit operations for mixing,
flocculation and clarification, including duplicate basins and equipment used
for chemical feeding, controlled mixing and for final clarification, shall be
provided as follows:
1. Advanced treatment
works having a rated capacity greater than 40,000 gallons per day.
2. Treatment works consisting of
physical-chemical unit operations.
3. Unit operations for controlled mixing
shall be in series or parallel.
4.
Provisions for unit operations to be taken out of service without disrupting
operation shall be included.
5.
Multiple stage unit operations shall be provided when a conventional operation
cannot be achieved otherwise.
B. Mixing. All treatment works shall provide
appropriate mixing unit operations upstream from required chemical
clarification and filtration unit operations.
Rapid or high intensity mixing may be accomplished either
within basins or in-line within closed channels. Basins should be equipped with
mechanical mixing devices; other arrangements, such as baffling, are acceptable
only under special conditions. Where mechanical mixing devices are utilized,
duplicate mechanical mixing units or spare mixing equipment shall be
provided.
The rapid or high intensity detention period (T) should
not be less than 10 seconds.
1. The
design of the rapid mixing unit operations should be based upon the mean
temporal velocity gradient (G) (expressed in inverse units of seconds). Typical
values for G and T are:
| T (Seconds) |
G (Seconds-1) |
| 10 |
1,100 |
| 20 |
1,000 |
| 30 |
900 |
| 40 |
790 |
| 41 |
700 |
For optimization, the design should establish the proper
values of (G) and (T) from appropriate test or performance data.
2. Multiple points of application
shall be provided to enable the provision of maximum mixing
intensity.
3. The physical
configurations of the mixing basin shall be designed to eliminate
vortexing.
4. The speed variation
of rapid mix equipment should be approximately 50% of the average speed
requirement range.
C.
Flocculation. Flocculation basins shall be designed to optimize the effects of
coagulation through increased opportunity for solids contact, and thus inlet
and outlet design shall prevent short-circuiting and destruction of the
developed suspended particles or floc.
Flocculation and sedimentation basins shall be as close
together as physically possible. The velocity gradient of the flocculated water
through pipes or conduits to settling basins shall not be greater than the
velocity gradient utilized in flocculating the water. Where velocity gradient
is not used as a design parameter, the linear velocity in pipes and conduits
from the flocculators to the settling basin shall not exceed 1/2 foot per
second. Allowances shall be made to minimize turbulence at bends and changes in
direction.
1. A drain and overflow
shall be provided for each basin.
2. Multiple unit operations shall be provided
for continuous operability for design flows greater than 40,000 gallons per
day.
3. Baffling may be used to
provide for flocculation in small scale unit operations (less than 2,000
gallons in volume).
4. Flocculation
basins shall be provided separately from other unit operations except where a
reactor clarifier or clarifiers are provided.
D. Low intensity mixing. The minimum
detention time for the low intensity mixed volume shall be 20 minutes, unless
acceptable operational or test data establishes that adequate flocculation can
be accomplished within a reduced detention time.
1. The design of the low intensity or contact
type flocculation units shall be based upon the value of the product of the
mean temporal velocity gradient times the detention time (GT), which is
ordinarily in the range of 20,000 to 200,000.
2. The design should also establish the
optimum value of GT for flocculation from appropriate test data. Variable speed
drive units shall be designed to allow speed variation throughout the design
range.
3. Successive mixed or
contact compartments should be provided. Special attention shall be given to
providing properly sized ports effectively located between compartments to
minimize short-circuiting.
Tapered flocculation should be provided. Wing walls or
stators shall be provided to prevent vortexing in basins utilizing vertical
shaft flocculators.
E. Conventional clarifiers. Circular
clarifiers of the center feed, peripheral feed and spiral flow type will be
considered on an individual basis for gravity settling of coagulated and
flocculated sewage effluent (chemical clarification).
1. Multiple basins shall be provided as
required for continuous operability of treatment works with design flow
capacity of more than 40,000 gallons per day or for treatment works utilizing
chemical-physical unit operations.
2. The design surface loading (overflow rate)
shall be established on a case-by-case basis as a function of the types of
coagulants or use of enhanced settling devices or configurations, such as
modular tube-type sections utilized within shallow depth clarifiers. Surface
loading rates shall not exceed 600 gpd/square foot for alum sludges, 800
gpd/square foot for iron sludges and 1,000 gpd/square foot for lime sludges, in
processes utilizing flocculation, unless adequate pilot plant data is presented
verifying that higher loading rates are acceptable.
3. Conventional chemical clarification shall
provide a minimum of four hours effective settling time unless adequate
operational data is submitted to verify that adequate treatment can be achieved
at a reduced value of detention time. Effective settling time will be
calculated using the settling zone volume of the basins extending from the
inlet entrance to the basins to the submerged effluent orifices or
weirs.
4. Rectangular sedimentation
basins shall be designed with a length to width ratio of at least four to
one.
5. Inlets shall be designed to
distribute the wastewater equally and at uniform velocities. Open ports,
submerged ports, stilling walls or similar entrance arrangements are required.
Where stilling walls are not provided, a baffle shall be constructed across the
basin in a manner to redirect flow from the inlet and shall project several
feet below the water surface to dissipate inlet velocities and provide uniform
flows across the basin settling zone.
6. Outlet devices shall be designed to
maintain velocities suitable for settling in the basin and to minimize
short-circuiting. The use of submerged orifices or submerged weirs shall be
provided where flocculation precedes filtration. The maximum velocity gradient
in pipes and conduits from the settling basins to the filters shall not exceed
that used in the flocculation. Where velocity gradient is not used as a
parameter in the design of outlet devices, the linear velocity in pipes and
conduits from settling basins shall not exceed one foot per second.
7. The velocity through settling basins shall
not exceed one foot per minute. The basins shall be designed to minimize short
circuiting.
8. An overflow weir (or
pipe) shall be installed to be compatible with the maximum water level desired
above the filter media where filters follow sedimentation. The overflow shall
discharge with a free fall at a location where the discharge may be
observed.
9. Settling basins used
for chemical clarification shall be provided with a means for dewatering. Basin
bottoms shall slope toward the drain not less than one foot of fall in 12 feet
of length.
10. Automatic continuous
sludge removal equipment shall be provided for chemical clarification.
Provision shall be made for the operator to observe or sample sludge being
withdrawn from the clarifier.
11.
Consideration shall be given to the provision of control of climatic factors,
such as wind and temperature through use of enclosures or
superstructures.
F.
Reactor clarifiers. Reactor type flocculation and chemical clarification basins
may be considered where wastewater characteristics are evaluated by the
department and deemed to be uniform.
Reactor clarifiers shall be designed for the maximum
uniform flow rate and shall be adjustable to changes in flow which are less
than the design rate.
1. Multiple
reactor clarifiers are required to maintain continuous operability.
2. For reactor clarifiers a minimum of 30
minutes shall be provided for flocculation and mixing. The clarification
detention time shall be established on the basis of the raw wastewater or
sewage characteristics and other local conditions that affect the operation of
the unit. Based on design flow rates, the minimum detention time shall be two
hours for reactor clarifiers.
3.
Reactor clarifiers shall be equipped with orifices if they precede filtration.
Orifices shall produce uniform rising or overflow rates over the entire area of
the tank and shall provide an exit velocity not to exceed one foot per second.
Upflow rates shall not exceed one gallon per minute per square foot of area of
the horizontal zone of sludge separation (blanket), for the design mode of
operation of the clarifier.
4. The
following operating equipment shall be provided:
a. A complete set of necessary tools and
accessories.
b. Adequate piping
with suitable sampling taps so located as to permit the collection of samples
of wastewater from critical portions of the units.
c. Conventional equipment to maintain
feeding, mixing, and flocculation operation.
5. Weirs should be designed so that surface
water does not travel over 10 feet horizontally to the overflow point or tops
of weirs (launders). Weir loading shall not exceed 20 gallons per minute per
foot of weir length. Where weirs are used they shall be:
a. Adjustable.
b. At least equivalent in length to the
perimeter of the tank.
6. Sludge removal design shall provide that:
a. Sludge pipes shall be not less than three
inches in diameter and so arranged as to facilitate cleaning;
b. Entrance to sludge withdrawal piping will
prevent clogging;
c. Valves are
located outside the tank for accessibility;
d. The operator may observe or sample sludge
being withdrawn from the unit;
e.
Automatic continuous sludge control shall be provided; gravity control should
be utilized.
7.
Superstructures. Consideration shall be given to providing a superstructure to
enclose the reactor clarifier and associated sampling valves and
piping.
Statutory Authority
§ 62.1-44.19 of the Code of Virginia.
