Virginia Law

A. Gravity drying beds, centrifuges, and various filtration equipment can be used to remove liquid from treated sewage sludge in order to reduce the amount of sludge that is to be managed. Drainage from beds and centrate 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 treatment works design, and alternatives for handling these loads may be considered similar to those for thickening and treatment supernatant. The design of dewatering equipment used for municipal sludges containing significant industrial waste shall consider the release of constituents such as free metals, organic toxicants, or strong reducing/oxidizing compounds, especially when thermal or chemical stabilization processes are employed.

B. Capacity. Where mechanical dewatering equipment is employed, at least two units shall be provided unless adequate storage (separate or in-line) or an alternative means of sludge handling is provided. Whenever performance reliability and sludge management options are dependent on production of dewatered sludge, each of the mechanical dewatering equipment provided should be designed to operate for less than 60 hours during any six day period. The facility shall be able to dewater in excess of 50% of the average design sludge flow with the largest unit out of service. The requirements for excess capacity will depend upon the type of equipment provided, peak sludge factor, and storage capability not otherwise considered. All units shall have bypass capability for maintenance.

1. Where mechanical dewatering equipment will not be operated on a continuous basis and the treatment works is without digesters with built-in short-term storage, separate storage shall be provided.

2. In-line storage of stabilized or unstabilized sludge shall not interfere with the design function of any of the treatment unit operations. Separate sludge storage from primary digestors shall be aerated and mixed as necessary to prevent nuisance conditions. The effect of storage on the sludge dewatering characteristics shall be considered.

3. All dewatering facilities should be properly ventilated to protect operator personnel in accordance with VOSH requirements and this chapter and standards contained in this chapter. The potential for odors or obnoxious gases being released within or without the building and grounds and the control of such should be addressed in accordance with applicable state and federal requirements.

Sampling stations before and after each dewatering unit or any appropriate segment of the unit shall be designed to allow the periodic evaluation of the dewatering process.

C. Conditioning. Adequate mixing time for the dispension of reaction between the chemical or other additives shall be provided. Subsequent handling should avoid floc shearing. The injection or addition point should be carefully considered in relation to downstream equipment and to the combined effect of other additives. Chemical handling shall be in accordance with this chapter.

1. Solution storage or day tanks should provide for the design dosages, if the equipment design does not require continuous operation. A minimum of eight hours storage shall be provided unless the specific chemical or additive selected is adversely affected by storage. Storage for batch operations shall be adequate for one batch at maximum chemical or additive demand. Storage volume reductions shall be justified, and other methods to ensure a continuous supply of chemicals or additives through the operating day or batch shall be provided. If conditioning tanks are employed, mixers may be necessary and the design should consider the capability for variable detention times.

2. Pilot plant testing or full size performance data shall be utilized to determine the characteristics and design dosage of the additives. In-stream flocculation/coagulation systems design shall be supported by comparable performance data or pilot plant testing.