| (a) Oxygen Requirements (O2R)
of Wastewater.
(1) An aeration system must be designed to provide
a minimum dissolved oxygen concentration in the aeration basin of
2.0 milligrams per liter (mg/l).
(2) Mechanical and diffused aeration systems must supply
the O2R calculated by Equation F.2. in
Figure: 30 TAC §217.155(a)(3), or use the recommended values
presented in Table F.3. in Figure: 30 TAC §217.155(a)(3), whichever
is greater.
(3) The O2R values in
Table F.3. in Figure: 30 TAC §217.155(a)(3) use concentrations
of 200 mg/l five-day biochemical oxygen demand (BOD
5 ) and 45 mg/l ammonia-nitrogen (NH3-N) in Equation F.2. in
Figure: 30 TAC §217.155(a)(3):
Attached Graphic
(b) Diffused Aeration System. An airflow design must
be based on either paragraph (1) or (2) of this subsection.
(1) Design Airflow Requirements - Default Values. A
diffused aeration system may use Table F.4. in Figure: 30 TAC §217.155(b)(1)
to determine the airflow for sizing aeration system components:
Attached Graphic
(2) Design Airflow Requirements - Equipment and Site
Specific Values. A diffused aeration system may be based on calculations
of the airflow requirements for the diffused aeration equipment in
accordance with subparagraphs (A) - (D) of this paragraph.
(A) Determine Clean Water Oxygen Transfer Efficiency.
(i) A diffused aeration system design may be based
on a clean water oxygen transfer efficiency greater than 4%, only
if the clean water oxygen transfer efficiency is supported by full
scale diffuser performance data. Full scale performance data must
be developed by an accredited testing laboratory or a licensed professional
engineer. Data developed by a professional engineer must be sealed
by the engineer.
(ii) A testing laboratory or licensed engineer shall
use the oxygen transfer testing methodology described in the most
current version of the American Society of Civil Engineers (ASCE)
publication, A Standard for the Measurement
of Oxygen Transfer in Clean Water.
(iii) A diffused aeration system with a clean water
transfer efficiency greater than 18% for a coarse bubble system or
greater than 26% for a fine bubble system is considered an innovative
technology and is subject to §217.7(b)(2) of this title (relating
to Types of Plans and Specifications Approvals).
(iv) A design for clean water transfer efficiencies
obtained at temperatures other than 20 degrees Celsius must be adjusted
for a diffused aeration system to reflect the approximate transfer
efficiencies and air requirements under field conditions by using
the following equation:
Attached Graphic
(B) Determining Wastewater Oxygen Transfer Efficiency
(WOTE).
(i) The WOTE must be determined from clean water test
data by multiplying the clean water transfer efficiency by 0.65 for
a coarse bubble diffuser or by multiplying the clean water transfer
efficiency by 0.45 for a fine bubble diffuser.
(ii) The executive director may require additional
testing and data to justify actual WOTE for a wastewater treatment
facility treating wastewater containing greater than 10% industrial
wastes.
(C) Determining Required Airflow (RAF). The RAF must
be calculated using the following equation to determine the size needed
for a diffuser submergence of 12.0 feet. If the diffuser submergence
is other than 12.0 feet, a diffused aeration system must correct the
RAF, as detailed in subparagraph (D) of this paragraph.
Attached Graphic
(D) Corrections to RAF based on varying diffuser submergence
depths. The engineer shall provide the manufacturer's laboratory testing
data if the diffuser submergence depth in the design is the same as
the diffuser submergence depth in the manufacturer's testing. The
engineer shall apply a correction factor from Table F.5. in Figure:
30 TAC §217.155(b)(2)(D) to the required airflow rate calculated
using Equation F.4. in Figure: 30 TAC §217.155(b)(2)(C) if the
manufacturer's laboratory testing data is not available for the design
diffuser submergence depth. Linear interpolation is allowed for diffuser
submergence depths not shown in Table F.5. in Figure: 30 TAC §217.155(b)(2)(D).
Attached Graphic
(3) Mixing Requirements for Diffused Air. The air requirements
for mixing must be calculated using an airflow rate:
(A) from Design of Municipal
Wastewater Treatment Plants, Fifth Edition, Chapter 11, a joint
publication of the ASCE and the Water Environment Federation, for
mixing requirements; or
(B) greater than or equal to 20 standard cubic feet
per minute (scfm) per 1,000 cubic feet for a coarse bubble diffuser
and greater than or equal to 0.12 scfm per square foot for a fine
bubble diffuser.
(4) Blowers and Air Compressors.
(A) A blower and an air compressor system must provide
the required design airflow rate for biological treatment and mixing,
based on paragraphs (1) - (3) of this subsection, and the air requirements
of all other supplemental units where air must be supplied.
(B) The engineering report must include blower and
air compressor calculations that show the maximum air requirements
for the temperature range where the wastewater treatment facility
is located, including both summer and winter conditions, and the impact
of elevation on the air supply.
(C) A diffused aeration system must have multiple compressors
arranged to provide an adjustable air supply to meet the variable
organic load on the wastewater treatment facility.
(D) The air compressors must be capable of handling
the maximum design air requirements with the largest single air compressor
out of service.
(E) A blower unit and a compressor unit must restart
automatically after a power outage, or have a telemetry system or
an auto-dialer with battery backup to notify an operator of any outage.
(F) The design of a blower and air compressor system
must specify blowers and air compressors with sufficient capacity
to handle air intake temperatures that may exceed 100 degrees Fahrenheit
(38 degrees Celsius), and pressures that may be less than standard
(14.7 pounds per square inch absolute).
(G) The design of a blower and air compressor system
must specify the capacity of the motor drive necessary to handle air
intake temperatures that may be 20 degrees Fahrenheit (-7 degrees
Celsius) or less.
(H) A blower must include a governor or other means
to regulate airflow.
(5) Diffuser Systems - Additional Requirements.
(A) Diffuser Submergence.
(i) For a new wastewater treatment facility, the submergence
depth for any diffuser must meet the minimum depths in the following
table:
Attached Graphic
(ii) For an alteration or expansion of an existing
wastewater treatment facility, the diffuser submergence depth may
vary from the values in Table F.6. in Figure: 30 TAC §217.155(b)(5)(A)(i)
to match existing air pressure, delivery rate, and hydraulic requirements.
(iii) The submerged depth for a diffuser must be at
least 7.0 feet. A wastewater treatment facility with a design flow
of less than 5,000 gallons per day may have a diffuser submergence
depth of less than 7.0 feet, but only if justified by the engineer
and approved in writing by the executive director.
(B) Grit Removal. A wastewater treatment facility that
uses diffusers and has wastewater with concentrations of grit that
would interfere with the operation of a diffuser must either include
a grit removal unit upstream of an aeration process, or include multiple
aeration basins so that one basin may be taken out of service to allow
for grit removal.
(C) Aeration System Pipes.
(i) Each diffuser header must include an open/close
or throttling type control valve that can withstand the heat of compressed
air.
(ii) A diffuser header must be able to withstand temperatures
up to 250 degrees Fahrenheit.
(iii) The capacity of an air diffuser system, including
pipes and diffusers, must equal 150% of design air requirements.
(iv) The design of an aeration system must minimize
head loss. The engineering report must include a hydraulic analysis
of the entire air pipe system that quantifies head loss through the
pipe system and details the distribution of air from the blowers to
the diffusers.
Cont'd... |
