Virginia Law

Article 5. Sludge Processing and Management

9VAC25-790-540. Sludge stabilization.

Article 5
Sludge Processing and Management

The selection and operation of the sludge treatment or stabilization process shall be based on the ultimate utilization of the final sludge product. Land based management of treated sewage sludge may require the production of biosolids as described in the Virginia Pollution Abatement Permit Regulation (9VAC25-32) or Virginia Pollutant Discharge Elimination System Permit Regulation (9VAC25-31). The design requirements for the treatment and stabilization processes described in this chapter are based on the assumption that they must accomplish the necessary processing of sewage sludge at the treatment works. Consideration will be given to reducing design requirements, on a case-by-case basis, for treatment works employing series operation of two or more stabilization processes or methods in accordance with the means of sludge management. The standard buffer distance of 200 feet shall be provided between the walls of open and exposed sludge treatment operations and the boundaries of the site area in which either controlled use or access restrictions apply.

Statutory Authority

§ 62.1-44.15 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-600 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-540, Virginia Register Volume 20, Issue 9, eff. February 12, 2004; amended, Virginia Register Volume 24, Issue 6, eff. January 1, 2008.

9VAC25-790-550. Anaerobic digestion.

A. The design of anaerobic digesters should provide an optimum environment for effective microbial degradation of the organic matter in sewage sludge. The digester system design shall address separation and removal of liquid or supernatant. The production of methane gas (CH₄) should be optimized. Digester gas should be utilized as a fuel whenever practical.

B. Design. A minimum of two anaerobic digesters, or enclosed reactors, shall be provided, so that each digester may be used as a first stage or primary reactor for treating primary and secondary sludge flows generated at a treatment works with a design flow exceeding 0.5 mgd. Additional digesters are provided to treat the total flow of primary and secondary sludge generated at treatment works with sewage design flows exceeding one MGD.

1. Where multiple digesters are not provided, a storage facility or adequate available sludge processing system shall be provided for emergency use so that the digester may be taken out of service without unduly interrupting treatment works operation.

2. Each digester should have the means for transferring a portion of its contents to other digesters. Multiple digester facilities should have means of returning supernatant from the settling digester unit to appropriate points for treatment.

3. Provisions for side-stream treatment of supernatant shall be included when the supernatant load is not included in the treatment works design.

4. Multiple sludge inlets and draw-offs and multiple recirculation section and discharge points (minimum of three) to facilitate flexible operation and effective mixing of the digester contents to optimize treatment for pathogen control and vector attraction reduction shall be provided. One inlet shall discharge above the liquid level and be located at approximately the center of the digester to assist in scum breakup. Raw sludge inlets should be so located as to minimize short circuiting between the inlets and either the supernatant draw-off, or sludge withdrawal points.

5. The proportion of depth to diameter should provide for a minimum of six feet storage depth for supernatant liquor, or the proportion of total volume allocated for supernatant should be 10% or more.

6. The digester bottom shall slope to drain toward the withdrawal pipe. At least one access manholes shall be provided in the top of the digester in addition to the gas dome. One opening shall be large enough to permit the use of mechanical equipment to remove grit and sand. A separate side wall manhole shall be provided at the basin floor level.

a. To facilitate emptying, cleaning, and maintenance, the digester design shall provide for access and safety features.

b. In accordance with VOSH requirements and these regulations, the operation and maintenance manual should specify: nonsparking tools, rubber soled shoes, safety harness, gas detectors for inflammable and toxic gases, and at least one self contained breathing apparatus.

C. Loadings. Where the composition of the sewage has been established, digester capacity shall be computed from the volume and character of sludge to be digested. The total digestion volume shall be determined by rational calculations based upon such factors as volume of sludge added, its percent solids and character, the temperature to be maintained in the digesters, the degree or extent of mixing to be obtained, expected formation of inactive deposits, and the size of the installation with appropriate allowance for sludge and supernatant storage. These detailed calculations shall be submitted to justify the basis of design.

1. The design average detention time for sludge undergoing digestion for stabilization shall be a minimum of 15 days within the primary digester, but longer periods may be required to achieve the levels of pathogen control and vector attraction reduction necessary for the method used for sludge management.

2. The digester shall be capable of maintaining a minimum average sludge digestion temperature of 35°C (95°F) with the capability of maintaining temperature control within a 4°(+/-)C range.

3. If unheated digesters are utilized, they shall have the capacity to provide a minimum detention time of 60 days within the digestion volume in which sludge is maintained at a temperature of at least 20°C (68°F).

4. For digestion systems where mixing is accomplished only by circulating sludge through an external heat exchanger, the system shall be loaded at less than 40 pounds of volatile solids per 1,000 cubic feet of volume per day or at a volumetric rate that provides not less than a 30 day detention time in the active digestion volume. The design volatile solids loading should be established in accordance with the degree of mixing provided.

5. Where mixing is accomplished by other methods, loading rates shall be determined on the basis of information furnished by the design consultant.

D. Completely mixed systems. For digesters providing for intimate and effective mixing of the digestion volume contents, the systems shall be designed for an average feed loading rate of less than 200 pounds of volatile solids per 1,000 cubic feet of volume per day or at a volumetric loading that provides 15 days or more detention time in the active digestion volume.

1. Confined mixing systems include (i) arrangements where gas or sludge flows are directed through vertical channels; and (ii) mechanical stirring, or pumping systems. Both confined mixing and unconfined continuously discharging gas mixing systems shall be designed to ensure complete turnover of digestion volume every 30 minutes. For tanks over 60 feet in diameter, multiple mixing devices shall be used.

2. Unconfined, sequentially discharging gas mixing systems shall be designed using the number of discharge points and gas flow rates shown for the various tank diameters as listed in this section, unless sufficient operating data is submitted and approved to verify the performance reliability of a alternative designs.

3. Gas discharge lines (lances) mounted on a floating cover or top designed to accumulate gas emissions shall extend to the base of the vertical side wall while the cover is resting on its landing brackets. For floor mounted diffuser boxes or lances mounted to a fixed cover, gas discharge shall extend to the base of the vertical side wall.

4. The minimum gas flow supplied for complete mixing shall be 15 cubic feet/min./1000 cubic feet of digestion volume. Flow measuring devices and throttling valves shall be used to provide the minimum gas flow.

5. The design power supplied for mechanical stirring or pumping type complete mixing systems shall be capable of achieving a minimum of 0.5 horsepower per 1,000 cubic feet of digestion volume.

6. Where low speed mechanical mixing devices are specified, more than one device shall be provided unless other mixing devices are also provided.

E. Gas collection. All portions of the gas system, including the space above the liquid surface in the digester, storage facilities and piping shall be so designed that under all normal operating conditions, including sludge withdrawal, the gas will be maintained under positive pressure.

1. All enclosed areas where any gas leakage might occur shall be adequately ventilated.

2. All necessary safety facilities should be included where gas is produced in accordance with VOSH requirements.

3. Pressure and vacuum relief valves and flame traps, together with automatic safety shut-off valves, may be provided.

4. Water seal equipment shall not be installed on gas piping.

5. Gas piping shall be of adequate diameter to provide a velocity less than 12 feet per second at a flow of two times the average rate and shall slope to condensation or drip traps at low points.

6. The use of float controlled condensate traps is not permitted. Condensation traps shall be placed in accessible locations for daily servicing and draining.

7. Electrical fixtures and equipment located in enclosed places where gas may accumulate will be evaluated in accordance with the National Board of Fire Underwriters' specifications for hazardous conditions and other applicable codes and regulations.

8. The electrical equipment provided in sludge-digester pipe galleries containing gas piping should be designed and installed to eliminate potential explosive conditions. The design of electrical equipment located in any location where gas or digested sludge leakage is possible will be evaluated in accordance with applicable codes and regulations.

9. Waste gas burners shall be readily accessible and should be located at least 50 feet away from any structure, if placed at ground level. Gas burners may be located on the roof of the control building if sufficiently removed from the digester and gas storage tank and will comply with all applicable state and federal air pollution control requirements. Waste gas burners shall not be located on top of the digester or gas storage tank.

10. In remote locations it may be permissible to discharge small quantities of digester gas (less than 100 CFH) to the atmosphere through a return bend screened vent terminating at least 10 feet above the walking surface, provided the assembly incorporates a flame trap and is in compliance with all applicable state and federal regulations.

11. A gas meter with bypass shall be provided to meter total gas production. Gas piping lines for anaerobic digesters shall be equipped with closed type indicating gauges. These gauges shall read directly in inches of water. Normally, three gauges will be provided: (i) one to measure the main line pressure; (ii) a second to measure the pressure to gas-utilization equipment; and (iii) the third to measure pressure to waste burners. Gas tight shut-off and vent cocks shall be provided. The vent piping shall be extended outside the building, and the opening shall be screened and arranged to prevent the entrance of rainwater. All piping of the manometer system shall be protected with safety equipment.

12. Any underground enclosures connecting with anaerobic digester tanks or containing sludge or gas piping or equipment shall be provided with forced ventilation in accordance with VOSH requirements and this chapter and standards contained in this chapter. Tightly fitting, self closing doors shall be provided at connecting passageways and tunnels to minimize the spread of gas.

F. Energy control. If practical, digesters should be constructed above the water table and should be suitably insulated to minimize heat loss. The use of digester gas as a heating fuel source is encouraged.

1. Sludge shall be heated by circulating the sludge through external heaters unless effective mixing is provided. Piping shall be designed to provide for the preheating of feed sludge before introduction to the digesters. Provisions shall be made in the layout of the piping and valving to facilitate cleaning of these lines. Heat exchanger sludge piping shall be sized for design heat transfer requirements.

2. Sufficient heating capacity shall be provided to maintain consistently the design temperature required for sludge stabilization. For emergency usage, an alternate source of fuel shall be available and the boiler or other heat source shall be capable of using the alternate fuel.

3. The heating system design shall provide for all controls necessary to ensure effective and safe operation. Facilities for optimizing mixing of the digester contents for effective heating shall be provided.

4. Sludge heating devices with open flames should be located above grade in areas separate from locations of gas production or storage.

G. Supernatant handling. Supernatant withdrawal piping shall not be less than six inches in diameter, and piping shall be arranged so that withdrawal can be made from three or more levels in the tank. A positive, unvalved, vented overflow shall be provided.

1. On fixed cover digesters the supernatant withdrawal level should preferably be selected by means of interchangeable extensions at the discharge end of the piping.

2. If a supernatant selector is provided, provision shall be made for at least two other draw-off levels located in the supernatant zone of the digester in addition to the unvalved emergency supernatant draw-off pipe. High pressure backwash facilities shall be provided.

3. Provisions shall be made for sampling at each supernatant draw-off level. Sampling pipes shall be at least 1-1/2 inches in diameter.

4. Management of digester supernatant must be addressed in the treatment works design. Also, sidestream treatment alternatives for digester supernatant shall be considered in the preliminary engineering design.

Statutory Authority

§ 62.1-44.15 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-610 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-550, Virginia Register Volume 20, Issue 9, eff. February 12, 2004; amended, Virginia Register Volume 24, Issue 6, eff. January 1, 2008.

9VAC25-790-560. Aerobic sludge digestion.

A. Aerobic sludge digestion reactors shall be designed for effective mixing and aeration. If diffusers are used, they shall be of the type that minimizes clogging and they should be designed to permit removal for inspection, maintenance and replacement without dewatering the tanks.

B. Design. Multiple aerobic digesters are required at all treatment works having a design flow capacity of more than 0.5 mgd. The size and number of aerobic sludge digesters shall be determined by rational calculations based upon such factors as (i) volume of sludge added; (ii) type and percent solids; (iii) the required volatile solids reduction for stabilization; (iv) allowance for sludge and supernatant storage; and (v) the minimum design temperature of the digester contents.

1. Calculations shall be submitted to justify the design and shall include design digester temperature based on the type of mixing equipment and other factors.

2. Digester volume shall be a minimum of 20% of the average design flow of the treatment works. The design digester volume should be increased up to 25% of the average design flow if the wastewater temperature will remain below 10°C (50°F) for an extensive period of time (60 days/year).

3. A reduction in requirements for hydraulic detention time may be given for treatment works designed to be operated in the extended aeration mode, or coupled with additional stabilization processes, or operated at elevated temperatures.

4. Facilities shall be provided for effective separation and withdrawal, or decanting of supernatant.

C. Loadings. The volatile solids loading should be in the range of one- to two-tenths (0.1 to 0.2) pounds of volatile solids per cubic foot per day.

1. Dissolved oxygen concentration maintained in the liquid shall be in the range of one to two milligrams per liter.

2. Energy input requirements for mixing should be in the range of 0.5 to 1.5 horsepower per 1,000 cubic feet, where mechanical aerators are utilized, and 20 to 30 standard cubic feet per minute per 1,000 cubic feet of aeration tank, where air mixing is utilized.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-620 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-560, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

9VAC25-790-570. Composting.

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.

Statutory Authority

§ 62.1-44.15 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-630 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-570, Virginia Register Volume 20, Issue 9, eff. February 12, 2004; amended, Virginia Register Volume 24, Issue 6, eff. January 1, 2008.

9VAC25-790-580. 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.

Statutory Authority

§ 62.1-44.15 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-640 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-580, Virginia Register Volume 20, Issue 9, eff. February 12, 2004; amended, Virginia Register Volume 24, Issue 6, eff. January 1, 2008.

9VAC25-790-590. Chemical treatment.

A. The fundamental design areas to be considered include chemical feeding, mixing, and storage capacity. Chemical treatment operation controls may include pH, contact time and mixture temperature.

B. Alkaline treatment. The three design parameters typically considered fundamental for design of an alkaline stabilization system include pH, contact time, and mixture temperature. The alkaline additive dosage required to stabilize sludge is determined by the type of sludge, its chemical composition and the solids concentration. Performance data taken from pilot plant test programs or from comparable facilities should be used in determining the proper dosage.

1. For conventional treatment, the design objective shall be to furnish uniform mixing in order to maintain a pH of 12 or above for two hours or more in the alkaline additive-sludge mixture. The design criteria for accomplishing adequate treatment may include:

a. Adding a controlled dosage of alkaline agents to sludge and providing uniform mixing of the sludge and agents.

b. Bringing the alkaline additive-sludge mixture pH to the design objective, such as a mixture pH of 12.5 or more and maintaining the mixture pH above 12.5 for 30 minutes or more.

c. Providing capacity to achieve a temperature of the alkaline-sludge mixture of more than 52°C, if desired, and maintaining a sufficient temperature over a measured contact period to ensure pasteurization.

d. Maintaining conditions so that the sludge is not altered or further distributed for two hours or more after alkaline treatment.

2. For Class I or Class II treatment to achieve pathogen reduction and control for beneficial uses of sewage sludge, including biosolids, the design objective shall be to meet the operational standards for pH during pasteurization, contact time, temperature, pH following pasteurization, and any other applicable requirements specified in the Virginia Pollution Abatement Permit Regulation (9VAC25-32) for the alkaline treatment.

3. 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 conveyance and mixers, backup heat sources, dual blowers, etc., shall be provided as necessary. 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.

4. Storage facilities and chemical handling shall be designed in accordance with this chapter. Either mechanical or aeration agitation should be provided to ensure uniform discharge from storage bins. Alkaline additive feeding equipment shall meet the requirements of this chapter. Hydrated lime should be fed as a 6% to 18% Ca(OH2) slurry by weight. Other suitable means should be developed for controlling the feed rate for dry additives.

5. The additive/sludge blending or mixing vessel shall be large enough to hold the mixture for 30 minutes at maximum feed rate. In a batch process, a pH greater than 12 shall be maintained in the mixing tank during this period. In a continuous flow process, the nominal detention time (defined as tank volume divided by volumetric input flow rate) shall be used in design, and a pH greater than 12 shall be maintained in the exit line. Slurry mixtures can be mixed with either diffused air or mechanical mixers. Mixing equipment shall be designed to keep the alkaline slurry mixture in complete suspension.

6. Coarse bubble diffusers should be used for mixing with compressed air. A minimum air supply of 20 scfm per 1,000 cubic feet of tank volume should be provided for adequate mixing. The mixing tank shall be adequately ventilated and odor control equipment shall be provided.

7. Mechanical mixers should be sized to provide 5 to 10 HP per 1,000 cubic feet of tank volume. Impellers should be designed to minimize fouling with debris in the sludge.

8. Pasteurization vessels shall be designed to provide for a minimum retention period of 30 minutes. The means for provision of external heat shall be specified.

C. Chlorine treatment. The stabilization of sludge by high doses of chlorine should be considered on a case-by-case basis. Process equipment that comes into contact with sludges that have not been neutralized after chlorine oxidation shall be constructed of acid resistant materials or coated with protective films. Caution should be exercised with recycle streams from dewatering devices or sludge drying beds which have received chlorine stabilized sludge due to the creation of potential toxic byproducts which may be detrimental to the treatment process or receiving stream.

D. Other treatment. Other processes for chemical treatment can be considered in accordance with this chapter.

Statutory Authority

§ 62.1-44.15 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-650 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-590, Virginia Register Volume 20, Issue 9, eff. February 12, 2004; amended, Virginia Register Volume 24, Issue 6, eff. January 1, 2008.

9VAC25-790-600. Sludge thickening.

A. Sludge thickening to decrease the liquid fraction should be considered for volume reduction and conditioning of sludges prior to treatment and management. Biological sludges returned to reactors should be thickened to provide for effective control of biomass. Prior to conventional treatment of biosolids, thickening should be provided to reduce volume and to condition the raw sludge flow.

B. General design. Thickener design shall provide adequate capacity to meet peak demands. Thickeners should be designed to prevent septicity during the thickening process.

1. A sludge handling bypass around the thickening process is required. Dual units or alternate storage is required for all treatment works of greater than 1 mgd capacity.

2. Thickeners shall be provided with a means of continuous return of supernatant for treatment. Provisions for side-stream treatment of supernatant should be considered.

3. Consideration should be given to any potential treatment advantages obtained from the blending of sludges from various treatment processes.

4. Odor control shall be addressed with consideration being given to flexibility of operations and changes of influent sludge characteristics.

C. Gravity systems. Clarifiers or gravity thickeners sufficiently sized for clarification will provide for thickening. However, the use of mechanical stirring devices will significantly improve the performance of gravity thickeners. Mechanical thickeners employ low speed stirring mechanisms for continuous mixing and flocculation within the zone of sludge concentration. In this manner, liquid separation is enhanced.

1. Conventional overflow rates for gravity thickeners should be in the 400-800 gpd per square foot range. The engineer shall provide the basis and calculations for the nonconventional surface loading rates. The side water depth of conventional gravity thickeners shall be a minimum of 10 feet. Circular thickeners shall have a minimum bottom slope of 1-1/2 inches per radial foot.

2. A gravity sludge thickener shall be so designed as to provide for sludge storage, if sufficient storage is unavailable within other external tankage. Sludge withdrawal from gravity thickeners should be controlled and adjusted, and variable speed pumps should be provided.

3. Gravity thickeners should be provided with bottom scraping equipment to enhance sludge removal. The scraper mechanism peripheral velocity should be in the 15 to 20 feet per minute range.

a. The scraper mechanical train shall be capable of withstanding extra heavy torque loads. The normal working torque load shall not exceed 10% of the rated torque load.

b. A method to correct blockage of the scraper mechanism and restore operation from a stalled position should be provided in accordance with the Operation and Maintenance Manual.

4. Alternative designs should be based on data obtained from a pilot plant (relatively small scale test equipment) program. Chemical addition and dilution water feed systems should be evaluated for use to optimize performance.

D. Dissolved air flotation. Dissolved air flotation (DAF) basins shall be equipped with bottom scrapers to remove settled solids and surface skimmers to remove the float established through release of pressurized air into the sludge inflow. The bottom scraper should function independently of the surface skimmer mechanism. Dissolved air flotation units should be enclosed in a building. A positive air ventilation system and odor control shall be provided.

1. Conventional design parameters include:

a. Maximum hydraulic loading rates of 2.0 gallons per minute per square foot of surface area (gal/min/sq. ft.).

b. A solids loading rate in the range of 0.4 to 1.0 pounds per hour per square foot of surface area (lb/hr/sq. ft.) without chemical addition. A solids loading rate of up to 2.5 lbs./hr./sq. ft. may be used if appropriate chemical addition is provided (9VAC25-790-860).

c. An air supply to sludge solids weight ratio in the range of 0.02 to 0.04.

2. The recycle ratio should be in the 30% to 150% range. The recycle pressurization system should utilize DAF effluent or secondary effluent if use of potable water is not available. The retention tank system shall provide a minimum pressure of 40 psig.

3. A polymer feed system shall be provided. The feed system shall meet the requirements of this chapter.

4. Alternative design should be based on data obtained from a pilot plant test program if sufficient operational performance data is not available.

5. Skimmer design shall be multiple or variable speed such as to allow normal operation in the less than one fpm range, with the capability of a speed increase to 25 fpm.

E. Mechanical separation. Filters or centrifuge can be used to thicken sludges. The process shall be preceded by pretreatment to remove material that can plug the media, nozzles or cause excessive wear.

1. Provisions for the addition of appropriate coagulants to the sludge inflow to the filter or centrifuge shall be considered.

2. The design basis and calculations for nonconventional loading rates shall be submitted for evaluation.

3. Filtrate or centrate shall be returned to the head of the primary units, aeration basins, or a separate side-stream treatment system.

Statutory Authority

§ 62.1-44.15 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-660 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-600, Virginia Register Volume 20, Issue 9, eff. February 12, 2004; amended, Virginia Register Volume 24, Issue 6, eff. January 1, 2008.

9VAC25-790-610. Sludge dewatering.

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.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-670 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-610, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

9VAC25-790-620. 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:

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.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

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

9VAC25-790-630. Filtration.

A. Rotary drums. The following rates of vacuum filtration, in pounds of dry solids per square foot of drum filter area per hour, for various types of sludge, may be considered conventional loading with proper prior sludge conditioning. A variable speed drive shall be provided.

1. Unless dual trains are provided, the following appurtenant equipment shall be provided in duplicate with necessary connecting piping, and electrical controls to allow equipment alternation. Spare filter fabric shall be provided except when metal coils are utilized.

a. Feed pump.

b. Vacuum pump.

c. Filtrate pump.

2. Wetted parts should be constructed of corrosion-resistant material. Drum and agitator assemblies shall be equipped with variable speed drives and provisions for altering the liquid level shall be made.

3. Vacuum pumps having a capacity of at least 1.5 cfm per square foot for metal-covered drums should be provided. Vacuum receivers are required with dry type vacuum pumps.

4. Each filter shall be fed by a separate feed pump to ensure a proper feed rate. Filtrate pumps must be of adequate capacity to pump the maximum amount of liquid to be removed from the sludge.

5. Careful consideration to filter washing and variable sludge pickup depth should be made.

B. Plate and frame presses. Actual performance data developed from similar operational characteristics should be utilized for design. The impact that anticipated sludge variability will have on the design variables for the press as well as chemical conditioning shall be addressed. Appropriate scale-up factors shall be utilized for full size designs if pilot scale testing is done in lieu of full-scale testing.

1. The following appurtenant equipment shall be considered for duplicate operation unless multiple units are provided:

a. Feed pump.

b. Air compressor.

c. Washwater booster pump.

2. The following spare appurtenances shall be provided where multiple units are not installed:

a. At least one extra plate for every ten required for startup, but a total of not less than two extra plates required.

b. One complete filter fabric set.

c. Closure drive system.

d. Feed pump (when duplicates are not provided).

e. Air compressor (when duplicates are not provided).

f. Washwater booster pump (when duplicates are not provided).

3. Filter feed pumps shall be capable of a combination of initial high flow, low pressure filling followed by sustained periods of operating at 100 to 225 psi. An integral pressure vessel to produce this initial high volume flow should be considered. Operating pressures less than 225 psi will be considered if actual performance data using similar sludges is provided.

4. Provisions for cake breaking to protect or enhance downline process shall be incorporated where necessary.

5. Crane or monorail services capable of removing the plates should be considered. In some installations, the capability to remove other press parts should also be considered.

6. Provision for a high pressure water or acid wash system to clean the filter shall be considered. Booster pumping should be addressed.

C. Belt presses. Actual performance data developed from similar operational characteristics should be utilized for design. The impact that anticipated sludge variability will have on the design variables for the press as well as chemical conditioning shall be addressed. A second belt filter press or an approved backup method of dewatering shall be required whenever a single belt press is operated 60 hours or more within any consecutive five day period or the average daily flow received at the treatment works equals or exceeds four mgd. Appropriate scale-up factors shall be utilized for full-size designs if pilot plant testing is performed in lieu of full-scale testing.

1. The following appurtenant equipment shall be considered for duplicate operation unless multiple units are provided:

a. Feed pump.

b. Washwater booster pump.

2. Requirements for spare appurtenances should include the following:

a. Complete set of belts.

b. One set of bearings for each type of press bearing.

c. Tensioning and tracking sensors.

d. One set of wash nozzles.

e. Doctor blade.

f. Conditioning or flocculation drive equipment if duplicate units are not provided.

3. A polymer selection methodology, accounting for sludge variability and anticipated sludge loading to the press shall be provided.

4. Sludge feed shall be as constant as possible to eliminate difficulties in polymer addition and press operation. The range in feed variability shall be identified and equalization shall be provided as necessary. A method for uniform sludge dispersion on the belt shall be provided. Grinders for the sludge feed to the flocculation system shall be considered. Thickening of the feed sludge should be an integral part of the design of the filter press. Separate thickening or dual purpose thickening will be considered on a case-by-case basis.

5. The filter press design shall consider the following:

a. Variable belt speed mechanism.

b. Belt tracking and belt tensioning equipment.

c. Belt replacement availability based on evaluation of the belt equipment selection especially if the weave, material, width, or thickness cannot be reasonably duplicated.

6. Rollers specified for the press design should provide:

a. Rubber coating or other protective finish.

b. Maximum frame and roller deflection and operating tension.

c. Roller bearings that are watertight and rated for a B-10 life of 100,000 hours.

7. The washwater system should provide for:

a. High pressure washwater for each belt with a specified operating pressure.

b. Booster pumps if necessary.

c. Spray wash systems designed to be cleaned without interference with the system operation.

d. Particular care in nozzle selections and optional nozzle cleaning systems when recycled wastewater is used for belt washing.

e. Replaceable spray nozzles.

f. Spray curtains.

D. Additional design features to be considered include:

a. Drip trays under the press and under the thickener to readily remove filtrate if gravity belt thickening is employed.

b. Adequate clearance to the side and floor for maintenance and removal of the dewatered sludge.

c. Location of all electrical panels or other materials that are subject to corrosion out of the area of the press.

d. Adjustable doctor blade clearance.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-690 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-630, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

9VAC25-790-640. Centrifuges.

Successful application of centrifugation similar to sludge thickening applications for dewatering of municipal type sludges requires consideration of certain design factors. Proper scale-up data pertaining to the particular sludge to be dewatered and the necessary polymer and coagulant dosage to achieve the design solids content shall be provided. The abrasiveness of each sludge supply shall be considered in scroll selection. Adequate sludge storage shall be provided for proper operation.

1. Unless dual trains are provided, the following spare appurtenant equipment shall be provided, with necessary connecting piping and electrical controls to allow easy installation:

a. Drive motor.

b. Gear assembly.

c. Feed pump.

2. Each feed pump shall be variable speed. A pump for each centrifuge shall be provided within the feed system.

3. Each centrifuge shall be equipped with provisions for variation of scroll speed and pool depth.

4. A crane or monorail shall be provided for equipment removal or maintenance.

5. Provision for adequate and efficient wash down of the interior of the machine shall be an integral part of the design.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-700 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-640, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

9VAC25-790-650. Sludge pumping.

Pump capacities shall be adequate but not excessive. Provisions for varying pump capacity are desirable.

1. Duplicate units shall be provided where failure of one unit would seriously hamper treatment works operation.

2. Positive displacement pumps or other types of pumps with demonstrated solids handling capability shall be provided for handling raw sludge.

3. The minimum positive head necessary for proper operation shall be provided at the suction side of centrifugal type pumps. A positive head of 24 inches or more may be desirable for all types of sludge pumps. Maximum suction lifts shall not exceed 10 feet for positive displacement pumps.

4. Adequate sludge sampling facilities shall be provided. Provision of quick closing sampling valves installed at the sludge pumps would be an adequate means of sampling. The size of valve and piping shall be at least 1-1/2 inches.

5. Sludge withdrawal piping for anaerobic digesters and gravity thickeners shall have a minimum diameter of six inches for gravity withdrawal and four inches for pump suction and discharge lines. Where withdrawal is by gravity, the available head on the discharge pipe shall be at least four feet and preferably more. Also, where gravity withdrawal is to be used as the primary withdrawal method, the piping for the primary sludge clarifier pump should be so arranged as to permit use of the pump for removal of digested sludge. Downstream gravity piping for transport of sludge shall be laid on uniform grade and alignment. Slope on gravity discharge piping should not be less than 3.0%. Provisions shall be made for draining and flushing discharge lines, and special consideration shall be given to the corrosion resistance and continuing stability of supporting systems for piping located inside the digestion tank.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-710 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-650, Virginia Register Volume 20, Issue 9, eff. February 12, 2004.

9VAC25-790-660. Sludge management.

Sludge management activities not specifically provided for through approval of design plans and specifications shall be described in a sludge management plan submitted by the owner to the area engineer and the DEQ regional office for review and approval. The use or disposal of treated sewage sludge shall be addressed through either the sludge management plan required by the VPDES permit, or a permit issued through the Virginia Pollution Abatement Permit Regulation (9VAC25-32) or Virginia Pollutant Discharge Elimination System Permit Regulation (9VAC25-31).

Statutory Authority

§ 62.1-44.15 of the Code of Virginia.

Historical Notes

Former 12VAC5-581-720 derived from Virginia Register Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-660, Virginia Register Volume 20, Issue 9, eff. February 12, 2004; amended, Virginia Register Volume 24, Issue 6, eff. January 1, 2008.