| (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.
(D) Sludge removal. A splash block or slab shall be
provided at the point where digested sludge is discharged onto each
of the beds. Appropriate means shall be provided to facilitate the
removal of the dried sludge from the beds for disposal without bed
damage resulting. Every sludge drying bed should include a removal
gate or stop planks in one end to provide access for machinery and
trucks to remove and haul away the dried sludge.
(E) Media. A minimum depth of 12 inches of filtering
material, of which four to six inches is coarse sand, is required.
To exclude surface water and eroded earth, the bed shall be protected
by a permanent wall which shall extend at least 12 inches but not
more than 24 inches above the finished surface of the beds.
(2) Vacuum filters, belt filters, belt filter presses,
and other mechanical dewatering filters.
(A) Multiple units. Where dewatering of sludge is proposed,
the design engineer shall provide data to document sufficient capacity,
alternate disposal means, or storage facilities capable of maintaining
normal daily operations during breakdowns, upsets, etc.
(B) Filtrate. The filtrate from the filters shall be
returned to the head of the treatment works or to the aeration system.
Consideration shall be given to the impact of the returned filtrate
on the treatment units and to providing odor and insect control facilities.
(3) Portable dewatering units. If sludge is to be treated
using portable mechanical dewatering units, provisions shall be made
in the facility plan or preliminary engineering report for the location
and connection of the portable dewatering unit(s) during facility
operation.
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