Section 580. Heat treatment

A. The design of heat treatment systems shall be based on the anticipated sludge flow rate (gpm) with the required heat input dependent on sludge characteristics and concentration. The system should be designed for continuous 24-hour operation to minimize additional heat input to start up the system. Measures for the adequate control of odors shall be stipulated for review.

B. Design. 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 grinders, fuel pumps, air compressor (if applicable), and dual sludge pumps shall be required.

1. 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. Control parameters shall be adequately monitored. Continuous recorders to monitor temperatures shall be provided.

2. Due to the large variation in incineration processes, equipment types, and configurations characteristic of currently available incineration systems, it is not feasible to stipulate specific design criteria. Therefore, these systems shall be considered on a case-by-case basis. Design of these systems should be based on pilot plant studies or data from comparable facilities.

C. Features. The process should provide heat stabilization in a reaction vessel within a range of 175°C or 350°F for 40 minutes to 205°C or 400°F for 20 minutes at pressure ranges of 250 to 400 psig, or provide for pasteurization at temperatures of 30°C or 85°F or more and gage pressures of more than one standard atmosphere (14.7 psia) for periods exceeding 25 days. The conventional heat drying system involves either direct or indirect contact between a dewatered sludge cake and hot gases in order to reduce the moisture content of the cake to 10% or less. The sludge cake temperature is typically 800°F or more during this process.

1. Sludge grinders shall be provided to protect heat exchangers from rag fouling. An acid wash or high pressure water wash system shall be available to remove scale from heat exchangers and reactors. Materials of construction of heat exchangers shall be selected to minimize corrosion.

2. The decant tank shall be equipped with a sludge scraper mechanism and shall be covered to prevent odor release.

3. Separate, additional grit removal (in addition to grit removal at the treatment works influent) should be considered to prevent abrasion of piping.

4. Adequate treatment works or sidestream treatment shall be provided for the recycle streams from heat treatment.

5. Odor control shall be addressed for exhaust and off-gas from decant tanks in accordance with state and federal air pollution control requirements.

D. Incineration. Sludge incinerator ash may be used as either a material additive or an ingredient for the manufacture of construction materials and other products. Due to the large variation in incineration processes, equipment types, and configurations characteristic of currently available incineration systems, it is not feasible to describe a conventional design. Design of these systems should be based on pilot plant studies or data from comparable facilities.

Historical Notes

Former 12VAC5-581-640 derived from Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-580, 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.