12VAC5-610-1170:5. APPENDIX J. Sludge Dewatering.
APPENDIX J.
Sludge Dewatering
The following section is reproduced from the Commonwealth of Virginia Sewerage Regulations State Department of Health, State Water Control Board, February 1977
25.05 Sludge Dewatering
25.05.01 General
Drainage from beds and concentrate or filtrate from dewatering units shall be returned to the sewage treatment process at appropriate points preceding disinfection. These organic loads shall be considered in plant design, and alternatives for handling these loads may be considered similar to those for supernatant (See Section 25.01.06d).
25.05.02 Sludge Drying Beds
a. Area
The sludge drying bed area required for dewatering aerobic and anaerobic digested sludge shall not be less than the following:
| Type of Treatment | Open Beds | Covered Beds |
| Primary | 1.25 | 0.75 |
| Trickling Filter | 1.50 | 1.25 |
| Activated Sludge | 1.75 | 1.35 |
| Chemical Precipitation | 2.00 | 1.50 |
| Aerated Plant with Aerobic Digesters | 1.50 | 1.35 |
For other types of sludge treatment, the drying bed area will be evaluated on a case by case basis.
b. Percolation Type
1. Gravel
The lower course of gravel around the underdrains shall be proper graded and should be 12 inches in depth, extending at the underdrains. It is desirable to place this in two or more layers. The top layers of at least three inches shall consist of gravel 1/8 inch to ¼ inch size.
2. Sand
The 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.
3. Underdrains
Underdrains shall be clay pipe, concrete drain tile or other underdrain material acceptable to the Department and the Board and shall be at least four inches in diameter and sloped not less than one percent to drain. Underdrains shall be spaced not more than 20 feet apart.
c. Impervious Types
Paved surface beds may be used if supporting data to justify such usage are provided.
d. Walls
Walls shall be watertight and extended 15 to 18 inches above and at least six inches below the surface.
e. Sludge Removal
Not less than two beds shall be provided and they shall be arranged to facilitate sludge removal. Concrete truck tracks should be provided for all percolation type sludge beds. Pairs of tracks for percolation type beds should be on 20-foot centers.
f. Sludge Influent
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.
25.05.03 Rotary Vacuum Filtration
a. Where units will not operate on a continuous basis and the plant does not have digesters, aerated storage tanks should be provided for the sludge.
b. A maximum amount of flexibility consistent with reasonable economy should be designed into the system. Design flexibility should include, but not be limited to, the following:
1. sludge and chemical dilution facilities;
2. separate chemical conditioning tanks;
3. variable speed filter pan agitator drives; and
4. effective filter media cleaning facilities.
25.05.04 Centrifugation
a. Where units will not operate on a continuous basis and plant does not have digesters, aerated storage tanks should be provided for the sludge.
b. Successful application of centrifugation of municipal type sludges requires consideration of numerous factors. Therefore, proper scale-up data pertaining to the particular sludge to be dewatered shall be obtained and submitted to the Department and Board for approval.
c. Provisions for addition of coagulants to the sludge before or during introduction of the centrifuge shall be considered.
25.05.05 Pressure Filtration
a. The addition of and mixing of coagulants before filtration shall be considered.
b. Design data shall be collected from laboratory tests and be properly scaled-up to plant size.
c. Adequate storage should be provided for single unit systems for down time and for multiunit systems on one or two shift cycles.
25.05.06 Lagooning
a. Lagooning for dewatering may be used where suitable land is available for this use.
b. The soil shall be reasonably porous, or underdrains shall be provided. The maximum water table level shall be 18 inches below the bottom of the lagoon. The surrounding areas shall be grades to prevent surface water from entering the lagoon. The maximum depth shall be 24 inches or less. There shall be two or more lagoons. Any underdrainage fluids shall be returned to the treatment plant and be treated.
c. Loading rates, buffer zones, odor control and groundwater protection will be addressed in the preliminary engineering conference.