| (a) General requirements.
(1) Disposal requirements, agreement with. Sludge processing
and treatment shall be in agreement with the requirements of the ultimate
form of disposal.
(2) Control of sludge and supernatant volumes. Provisions
shall be made to insure that waste sludge will be discharged to the
sludge digester in such a manner so as to minimize the volume of digester
supernatant liquor. Provisions shall be made for the return of supernatant
from sludge thickeners and digesters to the head of the treatment
works or to the aeration system accounting for the impact on the treatment
units.
(3) Piping. All piping from clarifiers to thickeners,
digesters, or other sludge processing facilities shall be arranged
for ease of maintenance and with sufficient hydraulic gradient to
insure the flow of sludge. Piping under stationary structures shall
be arranged so that stoppages can be readily eliminated by rodding
or with sewer cleaning devices. The sludge piping within the digester,
including the sludge drain line, shall be a minimum of four inches
in diameter. Appropriate facilities for transfer of supernatant liquor
shall be provided. Piping shall include means to observe the quality
of the supernatant from each of the withdrawal outlets provided. All
units shall be capable of being drained independently of one another.
(4) Sludge pumps. Selection of sludge transfer pumps
shall be based on both the quantity and character of the anticipated
solids load to be handled by them. Where mechanical pumps are used,
a sufficient number of pumps shall be provided so that the design
pumping capacity is available with the largest sludge pump out of
service. Air lift pumps are an acceptable mechanism for sludge transfer.
Duplicate design pumping capacity is not required when air lift pumps
are used. Pumps used for pumping sludge shall be specifically designed
for that purpose. Centrifugal sludge pumps shall have a positive suction
head unless they are self-priming or equipped with some other priming
device acceptable to the commission.
(5) Sludge stabilization. Sludge stabilization is required
for all biological treatment processes with the exception of extended
aeration processes (with a solids retention time (SRT) of 20 days
or more) in which case the sludge may be drawn directly to a sludge
dewatering facility.
(6) Sizing. Sizing requirements must be determined
using the five-day biochemical oxygen demand (BOD5 )
and design flow of the raw sewage influent to the plant.
(b) Aerobic digesters.
(1) Sludge thickening. Aerobic digesters should be
provided with sludge thickening capability.
(2) Aeration. Air supplied from air compressors or
blowers through diffusers shall be not less than 30 standard cubic
feet per meter (scfm) per 1,000 cubic feet of aerobic digester volume.
If a separate system of air compressors or blowers will supply air
to the digester, then the compressor or blower system shall be designed
so that the air requirements can be met with the largest single unit
out of service. If mechanical aerators are used, a minimum of 1.5
horsepower per 1,000 cubic feet must be provided.
(3) Mixing. Adequate mixing of the sludge shall be
provided to keep the solids in suspension and to bring the deoxygenated
liquid continuously to the aeration device. The amount of mixing shall
be based upon the sludge characteristics, the tank geometry, and type
of aeration mixing devices.
(4) Volume. A digester shall provide a minimum sludge
retention time of 15 days. The design volume of the aerobic digesters
may be calculated using 20 cubic feet per pound BOD
5 per day. This volume should be provided in two cells capable
of operating as a single or two-step unit.
(5) Sludge withdrawal. Provisions shall be made to
include an effective means of removing solids from the digester.
(c) Anaerobic digesters.
(1) Volume. The following minimum design criteria shall
be used in computing the capacity of digesters with and without facilities
for heating the sludge undergoing digestion and without sludge thickening
ahead of the digester. Variances to the table referenced as follows
for minimum digester volume may be granted provided that it can be
demonstrated to the satisfaction of the commission that a minimum
SRT of 30 days will be provided for unheated digesters and a minimum
SRT of 15 days will be provided for heated digesters. Heating of the
digester means that adequate facilities shall be provided for heating
and mixing the sludge and maintaining a year-round temperature of
at least 95 degrees Fahrenheit. Heating coils inside the digester
are not acceptable. All heated digesters shall include a thermometer
with not less than a four-inch dial to indicate the temperature of
digester contents. The use of flat-bottomed digestion chambers is
not acceptable. In sewage treatment plants employing sludge thickeners,
the volume of the digester may be reduced, with sufficient justification,
as a result of the thickeners reducing the volume of sludge going
to the digester. The calculations for the required sludge digestion
volume shall be based on the minimum percent solids in the sludge
expected to be encountered.
Attached Graphic
(2) Mixing. Adequate mixing of digester contents is
required for all first-stage and all single-stage digesters. Mixing
may be performed by mechanical equipment, including external pumps,
or by gas recirculation. The rate of mixing shall be such that the
flow created in the digester is sufficient to completely mix the incoming
sludge with the digester contents and prevent the formation of a scum
layer.
(3) Digester covers. Uncovered anaerobic digesters
are not acceptable. The sludge and supernatant withdrawal piping for
all single-stage and first-stage digesters with fixed covers shall
be arranged in such a manner so as to minimize the possibility of
air being drawn into the gas chamber above the liquid in the digester.
All digester covers shall include a gas chamber adequate for the gas
production anticipated. Digester covers shall be gas tight and the
specifications shall require a test of every digester cover for gas
leakage.
(4) Gas piping and safety equipment. The gas piping
shall be adequate for the volume of gas to be handled and shall be
pressure tested for leakage (at 1.5 times the design pressure) before
the digester is placed into operation. All gas piping shall slope
at least 1/8 inch per foot to provide drainage of condensation in
the gas piping. The main gas line from the digester shall have a sediment
trap equipped with a drip trap. Drip traps shall be provided at all
other low points in gas piping. The gas piping to every gas outlet
including the pilot line to the waste gas burner shall be equipped
with flame checks or flame traps. A natural or bottled gas source
shall be utilized for the burner pilot. Flame traps with fusible shutoffs
shall be included in all main gas lines. The gas line to the waste
gas burner shall include a suitable pressure, vacuum and relief valve.
Digester covers shall be equipped with an air vent which includes
a flame trap, a vacuum breaker, and pressure relief valve. The main
gas line shall be provided with a manometer or other acceptable device
which measures the gas pressure in inches of water. Manometers may
be used to measure the gas pressure in other gas lines. All manometers
shall be vented to the atmosphere outside digester buildings. A gas
meter to measure the rate of gas production is desirable and is mandatory
on all anaerobic digester systems designed for 1.0 mgd facilities
or larger. All rooms in digester buildings with floor level below
grade shall be ventilated. Ventilation may be either continuous or
intermittent. Ventilation, if continuous, shall provide at least six
complete air changes per hour; if intermittent, at least 30 complete
air changes per hour.
(5) Other requirements. The discharge end of sludge
inlet piping shall be separated from the overflow of the supernatant
liquor withdrawal point by a minimum distance equal to the radius
of the digester tank. Every digester shall be provided with an overflow.
A means shall be provided by which the level can be varied from which
supernatant liquor is withdrawn either automatically or by the operator.
If this means is by withdrawal pipes at different levels in the digester,
at least three different levels of supernatant liquor withdrawal shall
be provided. All supernatant liquor withdrawal systems shall be provided
with sampling cocks or other means of inspecting and testing the supernatant
liquor from each level. Piping for hot water heating systems may be
of any size adequate for the flow. The fresh water supply to hot water
heating systems shall be from a tank with an air gap between the top
of the tank and the fresh water supply pipe to prevent a cross connection
between the digester hot water system and the fresh water supply system.
(6) Treatment of digester supernatant liquor. Supernatant
liquor from anaerobic digesters may be treated by chemical means or
other acceptable methods before being returned to the plant. If the
commonly used method of dosing with lime is employed, the following
criteria shall apply: lime shall be applied to obtain a pH of 11.5.
The lime feeder shall be capable of feeding 2,000 mg/liter of hydrated
lime or its equivalent. The lime shall be mixed with the supernatant
liquor by a rapid mixer or by agitation with air in a mixing chamber.
After adequate mixing, the solids shall be allowed to settle. The
supernatant liquor treatment system may be a batch or continuous process.
If a batch process is used, the mixing and settling may be in the
same tank. The sedimentation tank shall have a capacity to hold 36
hours of supernatant liquor but not less than 1.5 gallons per capita.
If a continuous process is used, the sedimentation tank shall have
a detention time of not less than eight hours. Solids settled from
the supernatant liquor treatment are to be returned to the digester
or conveyed to sludge handling facilities. The clarified supernatant
liquor shall be returned to the head of the treatment works or to
the aeration system.
(d) Other stabilization processes.
(1) Incineration and heat treatment. The equipment
shall be housed in a fireproof building. Adequate facilities shall
be provided for storage of sludge during the longest period that drying
and or incineration units might normally be out of service for repairs
or maintenance. Plans for control of odors, insects, fly ash, and
for adequate facilities for the disposal of dried sludge or ash shall
be provided to the commission. Prior to construction of an incineration
or heat treatment facility, consultation should be made to the Texas
Air Control Board for applicable emission standards and the possible
requirement for a separate Texas Air Control Board permit.
(2) Composting, wet oxidation, and other processes.
Design information given to the commission shall include the demonstrated
level of stabilization achieved by the process to be employed. Test
results to verify the degree of stabilization may be required. In
addition, design information shall address design and/or operational
methods to minimize odor, insects, and other nuisance conditions.
Sludge storage requirements for each process shall be provided to
the commission. Also, the ultimate disposal method for the processed
sludge shall be reflected in the waste disposal application.
(e) Sludge dewatering facilities. Sludge shall be dewatered
sufficiently to meet the requirements of the ultimate form of disposal.
(1) Sludge drying beds.
(A) Required area. The area of sludge drying beds to
be provided will vary in accordance with the average rainfall, average
humidity, and type of treatment process used. The required area for
aerobic sludge dewatering shall be determined from §317.12 of
this title (relating to Appendix D) (for anaerobic sludge dewatering,
the value obtained from §317.12 of this title) may be reduced
35% to determine the required area) using a waste load based on sewage
strength and the daily average flow of the raw sewage. The bed area
sizing requirements shown in §317.12 of this title are for sludge
drying beds utilizing a continuous underdrain media as specified in
this subsection. Concrete (or similar impervious material) sludge
drying beds which do not use an underdrain media may require additional
area and will be evaluated on a case-by-case basis; however, in those
counties of the state which experience both high rainfall and high
relative humidity (Brazoria, Chambers, Fort Bend, Galveston, Hardin,
Harris, Jasper, Jefferson, Liberty, Newton, and Orange), other methods
of sludge dewatering should be utilized in lieu of sludge drying beds.
Where sludge drying beds are used in those counties of high rainfall
and humidity, provisions shall be made in the design of these beds
for covering the beds, for means of accelerated dewatering, or for
extra storage capacity and alternate dewatering methods to effectively
dewater the sludge during inclement weather.
(B) General design features. At least two sludge drying
beds shall be provided and they shall be constructed at elevations
above groundwater level. Construction shall be such as to exclude
surface water runoff from the beds and seepage from the beds into
the ground. Channels shall be of sufficient grade and size to facilitate
the flow of the sludge to the various beds. Runners should be provided
to facilitate sludge handling.
(C) Filtrate. The filtrate (or drainage) from the sludge
drying beds shall be returned to the head of the treatment works or
to the aeration system.
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