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The VERTAD Technology
PROCESS DESCRIPTION
KEY PROCESS FEATURES AND ADVANTAGES
The VERTAD™ sludge digestion system is a state-of-the-art aerobic thermophilic process that converts municipal primary and secondary sludges to Class A Biosolids, as defined by EPA, CFR-503. It uses an in-ground hyperbaric aeration reactor; a device that has been proven effective through more than 25 years of commercial operation in biological processes. The VERTAD™ reactor’s patented design features give it the following advantages over conventional ATAD and anaerobic systems:
· A treatment plant with a VERTAD™ system and subsequent dewatering will typically produce 50% less sludge volume than a plant with dewatering alone.
· VS reductions of 40% to 45% are readily obtained with a 4-day detention time, and a 7-log reduction in pathogens is achieved, guaranteeing that EPA requirements for Class A biosolids are met.
· The decanted product thickens to approximately 10% solids and dewaters readily to 30% to 35% solids with minimal polymer addition. Low polymer consumption and reduced trucking and disposal costs that result from the high solids content, give significant operating cost savings over conventional ATAD processes.
· Heat is recovered from the process using inexpensive heat exchangers and can be used to heat buildings without installing the expensive gas cleaning systems and boilers required to recover heat from anaerobic processes.
· Overall capital cost is lower than conventional processes in most applications.
· Due to the pressure in the shaft, the process has enhanced oxygen transfer, lowering the operating costs due to a lower power requirement than conventional aerobic digestion processes.
· The system is very compact and has a low space requirement, usually less than 20% of the space used by conventional processes. The subsurface nature of the aeration shaft results in very compact, low visual impact systems.
· The system can be economically enclosed in a building in locations where climatic conditions are unfavorable or if it is desirable for the plant to blend architecturally with the surrounding environment.
· The low aeration requirements and enclosed nature of the process help eliminate nuisance odors, and significantly reduces the potential for foaming. Odor, VOC, and ammonia emissions are minimal compared to conventional processes.
· The system is uncomplicated, easy to operate and maintain, and well suited to fully automated unattended operation.
· The leak tight aeration shaft casing, comprised of steel and surrounded by cement grout, eliminates any chance of ground water contamination.
· The submerged aeration shaft is much safer in an earthquake, and less likely to sustain damage than above-ground aeration basins and lagoons.
IDEAL APPLICATIONS
The VERTAD™ process is ideal for treating sludge from VERTREAT™ systems or from conventional biological treatment plants treating municipal sewage or industrial wastes. It is suitable for greenfield sites or can be used to expand existing facilities by incorporating it as an aerobic pre-treatment before conventional digestion systems.
It has particular advantages in applications with the following conditions:
· Applications in which Class A Biosolids are required
· Sites with high sludge disposal and/or trucking costs
· Sites with space constraints
· Retrofits and plant expansions
· Sites with high precipitation or extreme temperatures
· Sites close to residential areas or where large unsightly plants are undesirable
· Sites in areas with high seismic activity
VERTAD™ Sludge Digestion System, Renton, Washington (5,000 population equivalent capacity)
REACTOR FEATURES
The principal difference between VERTAD™ and conventional ATAD systems is its in-ground hyperbaric aeration reactor. Installed by conventional drilling techniques, the VERTAD™ reactor is typically 110 m (350 ft) deep, occupying only a fraction of the area used by conventional surface digestion systems. The diameter of the reactor, which can range from 0.75 m to 3 m, (2.5 ft to 10 ft) is determined by the quantity of sludge treated.
The VERTAD™ reactor has three separate treatment zones:
· The oxidation zone is the upper portion of the reactor and includes a central concentric draft tube for circulation.
· The mixing zone is immediately below the oxidation zone. Air required for bio-oxidation within the upper zone is injected into the mixing zone. The injected air also provides airlift circulation.
· The lower plug-flow zone provides the high-temperature residence time required to kill pathogens such as salmonella and fecal coliform, ensuring that the product meets the Class A Biosolids requirements set forth by the EPA in CFR-503.
VERTAD™ Class A Sludge Digestion Process Schematic
PROCESS DESCRIPTION
1. Screened sludge feed is delivered into the mixing zone where it is mixed with partially digested recirculating sludge.
2. Compressed air is continuously added below the mixing zone to provide the oxygen required by the microorganisms to digest the sludge. The high hydrostatic pressure ensures a high oxygen transfer rate.
3. Air bubbles rising up the outer annulus create circulation up the annulus, into the head tank, and down a central draft tube.
4. Off-gas containing excess air and carbon dioxide formed by microbial respiration disengages in
the head tank and vents through a feed tank or off-gas biofilter which effectively breaks any foam
and removes odors.
5. A small fraction of the recirculating sludge moves from the mixing zone into the lower plug flow zone
of the reactor where residual organic materials are digested and high temperature ensures that
pathogens are destroyed.
6. Digested sludge is withdrawn from the bottom of the reactor through a central discharge pipe and
transferred rapidly to a product tank at the surface.
7. The rapid depressurization of the digested sludge as it travels to the surface causes the solids to
separate in the product tank by flotation, and yields Class A Biosolids pre-thickened to around 10%
solids. The subnatant liquid is recycled back to the sewage treatment plant for processing prior to
discharge.
PERFORMANCE
PRODUCT
The VERTAD™ system produces a biosolids product that can be land applied safely as fertilizer without odors or pest attraction. The VERTAD™ reactor will reliably exceed the pathogen reduction requirements of EPA CFR-503 and produce Class A Biosolids with a minimum 40% reduction in volatile solids. Average VS reductions of approximately 45% are typically achieved. Over 90% of the FOG (fats, oils, and grease) contained in the raw feed is removed.
PRODUCT DEWATERING CHARACTERISTICS
The VERTAD™ product readily decants to approximately 10% solids. It can be dewatered to 30% to 35% solids using a conventional centrifuge or filter press, with less than 20 lb/ton polymer addition.
The high solids content of the dewatered product reduces trucking and disposal costs and the exceptionally low polymer consumption reduces operating costs considerably.
GASEOUS EMISSIONS
Gaseous emissions from the VERTAD™ system are considerably less than those from conventional aeration processes. Ammonia is converted to ammonium bicarbonate in the reactor, eliminating ammonia emissions.
Odor panel testing has indicated that the process is generally odor-free, with the untreated feed sludge being the only stream that emits significant odors.
POWER CONSUMPTION
The oxygen transfer efficiency of a VERTAD™ reactor operating at 4% solids is approximately 50%. The resulting aeration power requirement is less than 1.4 kW·h/kg (1,250 kW·h/ton) of volatile solids destroyed or 0.44 kW·h/kg (400 kW·h/ton) of total solids treated. No additional mixing energy is needed; therefore, the power requirement is much lower than the combined aeration and mixing power consumed by conventional aerobic digestion processes.
OPERATING AND MAINTENANCE REQUIREMENTS
Applying current instrumentation technology significantly reduces both operator labor and lab technician labor. The VERTAD™ system operates well with a few hours of operator attention on day shift. Operation can be left unattended evenings and weekends.
As an option, the PLC can be connected through a data link to NORAM’s offices in Vancouver to facilitate assistance in troubleshooting process upsets.
The maintenance requirement is minimal and is limited to routine maintenance of mechanical equipment and instrument calibration.
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