Virginia Law

A. Sludge thickening to decrease the liquid fraction should be considered for volume reduction and conditioning of sludges prior to treatment and management. Biological sludges returned to reactors should be thickened to provide for effective control of biomass. Prior to conventional treatment of biosolids, thickening should be provided to reduce volume and to condition the raw sludge flow.

B. General design. Thickener design shall provide adequate capacity to meet peak demands. Thickeners should be designed to prevent septicity during the thickening process.

1. A sludge handling bypass around the thickening process is required. Dual units or alternate storage is required for all treatment works of greater than 1 mgd capacity.

2. Thickeners shall be provided with a means of continuous return of supernatant for treatment. Provisions for side-stream treatment of supernatant should be considered.

3. Consideration should be given to any potential treatment advantages obtained from the blending of sludges from various treatment processes.

4. Odor control shall be addressed with consideration being given to flexibility of operations and changes of influent sludge characteristics.

C. Gravity systems. Clarifiers or gravity thickeners sufficiently sized for clarification will provide for thickening. However, the use of mechanical stirring devices will significantly improve the performance of gravity thickeners. Mechanical thickeners employ low speed stirring mechanisms for continuous mixing and flocculation within the zone of sludge concentration. In this manner, liquid separation is enhanced.

1. Conventional overflow rates for gravity thickeners should be in the 400-800 gpd per square foot range. The engineer shall provide the basis and calculations for the nonconventional surface loading rates. The side water depth of conventional gravity thickeners shall be a minimum of 10 feet. Circular thickeners shall have a minimum bottom slope of 1-1/2 inches per radial foot.

2. A gravity sludge thickener shall be so designed as to provide for sludge storage, if sufficient storage is unavailable within other external tankage. Sludge withdrawal from gravity thickeners should be controlled and adjusted, and variable speed pumps should be provided.

3. Gravity thickeners should be provided with bottom scraping equipment to enhance sludge removal. The scraper mechanism peripheral velocity should be in the 15 to 20 feet per minute range.

a. The scraper mechanical train shall be capable of withstanding extra heavy torque loads. The normal working torque load shall not exceed 10% of the rated torque load.

b. A method to correct blockage of the scraper mechanism and restore operation from a stalled position should be provided in accordance with the Operation and Maintenance Manual.

4. Alternative designs should be based on data obtained from a pilot plant (relatively small scale test equipment) program. Chemical addition and dilution water feed systems should be evaluated for use to optimize performance.

D. Dissolved air flotation. Dissolved air flotation (DAF) basins shall be equipped with bottom scrapers to remove settled solids and surface skimmers to remove the float established through release of pressurized air into the sludge inflow. The bottom scraper should function independently of the surface skimmer mechanism. Dissolved air flotation units should be enclosed in a building. A positive air ventilation system and odor control shall be provided.

1. Conventional design parameters include:

a. Maximum hydraulic loading rates of 2.0 gallons per minute per square foot of surface area (gal/min/sq. ft.).

b. A solids loading rate in the range of 0.4 to 1.0 pounds per hour per square foot of surface area (lb/hr/sq. ft.) without chemical addition. A solids loading rate of up to 2.5 lbs./hr./sq. ft. may be used if appropriate chemical addition is provided (9VAC25-790-860).

c. An air supply to sludge solids weight ratio in the range of 0.02 to 0.04.

2. The recycle ratio should be in the 30% to 150% range. The recycle pressurization system should utilize DAF effluent or secondary effluent if use of potable water is not available. The retention tank system shall provide a minimum pressure of 40 psig.

3. A polymer feed system shall be provided. The feed system shall meet the requirements of this chapter.

4. Alternative design should be based on data obtained from a pilot plant test program if sufficient operational performance data is not available.

5. Skimmer design shall be multiple or variable speed such as to allow normal operation in the less than one fpm range, with the capability of a speed increase to 25 fpm.

E. Mechanical separation. Filters or centrifuge can be used to thicken sludges. The process shall be preceded by pretreatment to remove material that can plug the media, nozzles or cause excessive wear.

1. Provisions for the addition of appropriate coagulants to the sludge inflow to the filter or centrifuge shall be considered.

2. The design basis and calculations for nonconventional loading rates shall be submitted for evaluation.

3. Filtrate or centrate shall be returned to the head of the primary units, aeration basins, or a separate side-stream treatment system.