Section 620. Sludge drying beds

A. Actual performance data from similar facilities should be provided for bed sizing. If such data is unavailable, the following general guidelines shall been used as the minimum:

Stabilization Process	Loading Rate*
i. Anaerobic Digestion	20.0
ii. Aerobic Digestion and	15.0
iii. Other Stabilization Processes	15.0
*lbs dry solids/sq ft/year

B. Design. Area requirements for covered beds or greenhouse beds may be reduced if polymer is used to condition the sludge prior to application to the beds, or performance data from similar designs is provided. Covers should extend beyond the bed area sufficiently to keep out rain and snow.

1. Not less than two beds shall be provided and they shall be arranged to facilitate sludge removal. Concrete pads serving as vehicle support tracks should be provided for all percolation type sludge beds. Pairs of support tracks for percolation type beds should be on 20-foot centers.

2. Sludge drying beds should be rectangular and separated from adjacent beds by permanent or removable dividers. Bed width should be determined by a rational basis considering the sludge handling and treatment and sludge management options. If polymers or other chemicals are used to enhance sludge dewatering, the effects of the polymer dosage on uniform distribution of sludge on the bed shall be considered.

3. The sludge pipe to the beds shall terminate at least 12 inches above the surface and be arranged so that it will drain. Concrete splash plates shall be provided at sludge discharge points.

4. Interior walls shall be watertight and extend 15 to 18 inches above and at least 6 inches below the bed surface.

5. Exterior walls shall be watertight and extend 15 to 18 inches above the bed surface or ground elevation, whichever is higher. They shall extend 12 to 15 inches below the drain pipes.

6. The bottom of the drying bed shall be relatively impervious, consisting of a minimum of one-foot layer of clayey subsoil having a permeability of less than one-millionth (10^-6) cm/sec. In locations where the ground water table is within one foot of the bottom, a watertight concrete pad should be considered.

C. Media. The bed media top course shall consist of at least 12 inches of sand with a uniformity coefficient of less than 4.0 and an effective grain size between 0.3 and 0.75 millimeters. The bed media lower course shall consist of gravel around the underdrains that conforms to the Virginia Department of Transportation's Road and Bridge Specifications, 1974. The gravel layer should be 12 inches in depth, extending at least six inches above the top of the underdrains. It is desirable to place this gravel in two or more layers. The top layer of at least three inches shall consist of number 8 sized gravel 1/8 inch to ¼ inch in size and the bottom layer should consist of number 3 sized gravel.

1. Underdrains shall be clay pipe, concrete drain tile or other underdrain material acceptable to the department and shall be at least four inches in diameter and sloped not less than 1.0% to drain. Underdrains shall be spaced not more than 20 feet apart.

2. Vacuum assisted, wedgewire, or other variations to the gravity drying bed concept will be considered on a case-by-case basis. Actual performance data or pilot studies with appropriate scale-up factors shall be provided.

Historical Notes

Former 12VAC5-581-680 derived from Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-620, Virginia Register Volume 20, Issue 09, eff. February 12, 2004.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.