Section 590. Chemical treatment

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.

Historical Notes

Former 12VAC5-581-650 derived from Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-590, Virginia Register Volume 20, Issue 09, eff. February 12, 2004; amended, Virginia Register Volume 24, Issue 06, eff. January 1, 2008.

Statutory Authority

§ 62.1-44.15 of the Code of Virginia.