Current through Register Vol. 41, No. 3, September 23, 2024
A. The
fundamental design areas to be considered include chemical feeding, mixing, and
storage capacity. Chemical treatment operation controls may include pH, contact
time and mixture temperature.
B.
Alkaline treatment. The three design parameters typically considered
fundamental for design of an alkaline stabilization system include pH, contact
time, and mixture temperature. The alkaline additive dosage required to
stabilize sludge is determined by the type of sludge, its chemical composition
and the solids concentration. Performance data taken from pilot plant test
programs or from comparable facilities should be used in determining the proper
dosage.
1. For conventional treatment, the
design objective shall be to furnish uniform mixing in order to maintain a pH
of 12 or above for two hours or more in the alkaline additive-sludge mixture.
The design criteria for accomplishing adequate treatment may include:
a. Adding a controlled dosage of alkaline
agents to sludge and providing uniform mixing of the sludge and
agents.
b. Bringing the alkaline
additive-sludge mixture pH to the design objective, such as a mixture pH of
12.5 or more and maintaining the mixture pH above 12.5 for 30 minutes or
more.
c. Providing capacity to
achieve a temperature of the alkaline-sludge mixture of more than 52°C, if
desired, and maintaining a sufficient temperature over a measured contact
period to ensure pasteurization.
d.
Maintaining conditions so that the sludge is not altered or further distributed
for two hours or more after alkaline treatment.
2. For Class I or Class II treatment to
achieve pathogen reduction and control for beneficial uses of sewage sludge,
including biosolids, the design objective shall be to meet the operational
standards for pH during pasteurization, contact time, temperature, pH following
pasteurization, and any other applicable requirements specified in the Virginia
Pollution Abatement Permit Regulation (9VAC25-32) for the alkaline
treatment.
3. Multiple units shall
be provided unless nuisance-free storage or alternate stabilization methods are
available to avoid disruption to treatment works operation when units are not
in service. If a single system is provided, standby conveyance and mixers,
backup heat sources, dual blowers, etc., shall be provided as necessary. A
reasonable downtime for maintenance and repair based on data from comparable
facilities shall be included in the design. Adequate storage for process, feed,
and downtime shall be included.
4.
Storage facilities and chemical handling shall be designed in accordance with
this chapter. Either mechanical or aeration agitation should be provided to
ensure uniform discharge from storage bins. Alkaline additive feeding equipment
shall meet the requirements of this chapter. Hydrated lime should be fed as a
6% to 18% Ca(OH2) slurry by weight. Other suitable means should be developed
for controlling the feed rate for dry additives.
5. The additive/sludge blending or mixing
vessel shall be large enough to hold the mixture for 30 minutes at maximum feed
rate. In a batch process, a pH greater than 12 shall be maintained in the
mixing tank during this period. In a continuous flow process, the nominal
detention time (defined as tank volume divided by volumetric input flow rate)
shall be used in design, and a pH greater than 12 shall be maintained in the
exit line. Slurry mixtures can be mixed with either diffused air or mechanical
mixers. Mixing equipment shall be designed to keep the alkaline slurry mixture
in complete suspension.
6. Coarse
bubble diffusers should be used for mixing with compressed air. A minimum air
supply of 20 scfm per 1,000 cubic feet of tank volume should be provided for
adequate mixing. The mixing tank shall be adequately ventilated and odor
control equipment shall be provided.
7. Mechanical mixers should be sized to
provide 5 to 10 HP per 1,000 cubic feet of tank volume. Impellers should be
designed to minimize fouling with debris in the sludge.
8. Pasteurization vessels shall be designed
to provide for a minimum retention period of 30 minutes. The means for
provision of external heat shall be specified.
C. Chlorine treatment. The stabilization of
sludge by high doses of chlorine should be considered on a case-by-case basis.
Process equipment that comes into contact with sludges that have not been
neutralized after chlorine oxidation shall be constructed of acid resistant
materials or coated with protective films. Caution should be exercised with
recycle streams from dewatering devices or sludge drying beds which have
received chlorine stabilized sludge due to the creation of potential toxic
byproducts which may be detrimental to the treatment process or receiving
stream.
D. Other treatment. Other
processes for chemical treatment can be considered in accordance with this
chapter.
