Virginia Law

A. Conventional sludge composting facilities aerobically process digested, or otherwise treated, sewage sludge that is uniformly mixed with other organic materials and bulking agents to facilitate biological decomposition of organics. The treated sewage sludge will be exposed to temperatures at or above 55°C for three consecutive days or more. The method of mixing and aeration, and the carbon to nitrogen characteristics, of the compost mix are critical to the process design.

B. General design. Unless the facility is totally enclosed, an appropriate buffer shall be established on a case-by-case basis by considering both the locations of any residential area, hospitals, nursing homes for the elderly and serum production centers and the prevailing wind at such locations. Local jurisdictions impacted by this restriction shall be so notified.

1. All compost facilities shall be provided with adequate means to prevent and control odors as necessary.

2. All compost facilities shall be provided with all-weather roads to and from the facility, as well as between the various process operations.

3. The receiving, mixing, composting, curing, drying, screening, and storage areas shall be paved with asphaltic concrete, reinforced concrete, or other impervious, structurally stable material that provides similar site characteristics.

4. The facility shall be graded to prevent uncontrolled runoff and a suitable drainage system shall be provided to collect all process wastewater and direct it to storage and treatment facilities. Process wastewater includes water collected from paved process areas. The capacity of the drainage system, including associated storage or treatment works system shall be based on the 24-hour rainfall of a 10-year return frequency.

5. All facility process wastewater and sanitary wastewater shall be collected and treated prior to discharge.

C. Facilities. A weigh scale, volumetric method, or other means shall be provided for determining the amount of sludge or residuals delivered to the facility and the amount of compost material removed from the facility. Adequate space and equipment must be provided for mixing operations and other material handling operations.

1. Where liquid, or dewatered, sludge or residuals are processed by the compost facility, all receiving of such inputs shall occur in either:

a. An area that drains directly to a storage, treatment, or disposal facility.

b. A handling area which shall be hard-surfaced and diked to prevent entry of runoff or escape of the liquids.

c. A sump with an adequately sized pump located at the low point of the hard-surfaced area shall be provided to convey spills to a disposal or holding facility.

2. Provisions for cleaning all sludge transport or residual hauling trucks that return to public roads, shall be provided at all compost facilities. The facility shall be capable of effective operation regardless of weather conditions. Wash water shall be collected for necessary treatment.

3. At all compost facilities handling liquid or dewatered residual materials that must be mixed prior to composting, a mixing operation shall be provided. The operation shall have sufficient capacity to properly process the peak daily waste input with the largest mixer out of operation. Volumetric throughput values used to establish necessary mixing capacity may be based on the material volume resulting from the sludge to bulking agent ratio, or may be estimated from previous experience or pilot scale tests.

4. Effective mixing equipment should be provided for use at all compost facilities. The ability of all selected equipment to produce a compostable mix from sludge of an established moisture content, residual material, and the selected bulking agent shall be documented from previous experience or pilot tests.

5. Except for windrow composting wherein mobile mixers are used, an area with sufficient space to mix the bulking agent and sludge or residuals and store half of the daily peak input shall be provided. The mixing area shall be covered to prevent ambient precipitation from directly contacting the mix materials.

6. Where conveyors are used to move the compost mix to the composting area and or help provide mixing, either sufficient capacity shall be provided to permit handling of the mix with one conveyor out of operation, or a backup method of handling or storing shall be provided. Runoff shall be directed to a storage or treatment facility. Capacity of the drainage system shall be based on the 24-hour rainfall producing a peak rate expected once in 10 years.

D. System design. The system design shall be sufficient to provide the level of treatment required for protection of public health and welfare in relation to the anticipated management method. Consideration should be given to covering the compost mixing pad and curing area in order to allow for handling of bulking agents and treated sludge and the finished compost, during extended periods of precipitation. If a roof type cover is not provided, operation of the facility during critical weather periods shall be addressed. Sufficient equipment shall be provided for routinely measuring the temperature and oxygen at multiple points and depths within the compost piles.

1. Windrow method. The area requirements shall be based on the average daily compost mix inputs, a minimum detention time of 30 days on the compost pad, and the area required for operation of the mixing equipment. Sufficient compost mix handling equipment shall be provided to turn the windrows daily. In addition, proper drainage and space shall be provided to allow equipment movement between compost pile sections and access around the working areas.

2. Aerated-static pile method. The aerated-static pile area requirement shall be based on the average daily compost mix inputs, along with storing base and cover material, with a composting time of 21 days, unless the applicant can demonstrate through previous experience or pilot scale studies that less time is necessary to achieve the requirements.

a. The compost mix pile shall be provided with a means of uniformly distributing air flow. One foot or more thick base of friable material may be utilized under the deepest sections of compost mix. A 1-1/2 foot or more thick covering blanket of unscreened compost or a one foot thick or more blanket of screened compost may be utilized over the compost mix pile.

b. Compost mix piles should be configured to provide adequate aeration of the mix using either positive or negative pressure for air flow through the piles.

3. Confined composting methods. Due to the large variation in composting processes, equipment types, and process configuration characteristic of currently available confined systems, such as enclosed operations or in-vessel systems, it is not feasible to stipulate specific design criteria. However, a confined composting system will not be approved unless the applicant can demonstrate, through previous operating experience or pilot scale studies, that the material removed from the enclosed container or compost process, after the manufacturer's suggested residence time, has an equivalent or higher degree of stabilization than would be achieved after 21 consecutive days of aerated static pile composting.

E. Aeration. Sufficient blower capacity shall be provided to deliver the necessary air flow through the compost mix, but the delivered air flow shall not be less than a minimum aeration rate of 500 cubic feet per hour per dry ton (CFH/DT). Where centralized aeration is utilized, multiple blower units shall be provided and shall be arranged so that the design air requirement can be met with the largest single unit out of service. Where individual or separated blowers are used, sufficient numbers of extra blowers shall be provided so that the design air requirement can be met with 10% of the blowers out of service. For facilities that are not continuously manned, the blower units should be equipped with automatic reset and restart mechanisms or alarmed to a continuously manned station, so that they will be placed back into operation after periods of power outage.

1. Each pile aeration distribution header shall be provided with a throttling control valve. The aeration system shall be designed to permit both suction and forced aeration. The piping system shall be capable of delivering 150% of the design aeration rate. The aeration piping may be located in troughs cast into the compost pad.

2. The aeration system shall be designed to permit the length of the aeration cycle to be individually adjusted at each pile header pipe.

F. Compost handling. The design of the curing area shall be based on a minimum retention time of 30 days unless the applicant can demonstrate through previous experience or pilot studies that less time is required. Daily input shall be based on the average daily input of mix to the composting area.

1. A drying stage is optional, but is usually required if compost is to be recycled as a bulking agent or if screening is required. Consideration should be given to covering the drying area. If a cover is provided, it can be designed so that sunlight is transmitted to the composting materials while preventing direct contact with ambient precipitation. Efficient drying may be accomplished by drawing or blowing air through the compost mixture or by mechanical mixing of shallow layers with stationary bucket systems, mobile earth moving equipment, or rotating discs.

2. Screening shall be provided for all compost facilities where the compost disposition necessitates the use of a screened product or where the bulking agent must be recycled and reused. When dry compost is used as a bulking agent screening is not typically provided.

a. A daily screening capacity of 200% of the average daily amount of compost mix shall be provided when screening is required.

b. Based on previous composting facility performance, or on pilot tests, the ability of the specified equipment to screen compost at the projected moisture range shall be demonstrated.

c. The area used for screening should be covered unless operations are not hindered when screening is temporarily discontinued.

3. Storage areas shall be provided for six months storage of compost unless the applicant can demonstrate (through previous experience, pilot studies or letters of intent to accept compost offsite) that less storage area is required. Storage for curing or drying biosolids compost is usually provided if compost is to be recycled for public use.

For all compost facilities where a separate bulking agent is required, storage area for a six-month supply of the bulking agent shall be provided, unless the applicant can demonstrate that bulking agent supplies can be replenished more frequently.