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Handling Alum Residuals In the Main - A technical assistance newsletter for Public Water Suppliers
Winter 2008
By Lewis Zediana, Tewksbury Water Department
History
In 1995 the Tewksbury Water Department was informed that the alum residuals pumped into the sewer system were grossly out of compliance with local pretreatment regulations. Excessive levels of aluminum, low pH values, and high total settleable solids (TSS) were causing problems with the receiving sewer plant. The alum residuals were causing problems with wastewater treatment processes and affecting the discharge of the plant with excessive aluminum levels. As a result the Tewksbury Water Treatment Plant was told that the discharge could not continue.
The water system quickly researched various ways to handle the residuals discharge. Due to site restrictions, lagoons and drying beds would not be usable. Simply trucking the waste to another facility was very expensive, even if another facility would consider taking the waste. The final option was to mechanically dry the alum residuals and then dispose of them in an offsite landfill. Several mechanical options were investigated which included filter presses, centrifuges, belt presses and thermal drying. Most of the processes required addition of polymers, bulking agents (such as lime or perlite) and one process even needed additional alum to produce a reasonable cake. However, the economics and not being able to recycle the filtrate made these processes unattractive.
The Solution
The Venturi Aeration Corporation suggested a process called Precoat Rotational Vacuum Filtration as a possible solution to our problem. They offered to process a sample and return with results. About 1 week later, they returned with a plastic bag of dried residuals and 1 liter of clear filtrate. The dried residuals had about 32 % solids content and the filtrate had a turbidity of less than 1 NTU. In addition, the process required no pretreatment. Based on these results, the project began in earnest.
What is Vacuum Filtration?
Precoat rotational vacuum filtration consists of a drum, which is equipped with a screen onto which a fine polypropylene cloth is applied. This acts as the base to which diatomaceous earth (DE) is applied. A vacuum is applied at both ends of the drum, which supplies the driving force for the filtration process.
The drum is rotated in a tub into which slurry of DE is pumped. The coating, which varies from 2-5 inches according to the amount of DE used, provides an excellent barrier that is rated at 0.5 microns or 6-log reduction according to EPA. Once the drum is coated, alum residuals are then added to the tub and the filtration process begins. As the drum rotates, fresh new DE surface plunges below into the residuals and the vacuum draws through water, leaving solids on the surface of the DE. The drum emerges from the submergence zone and filtration continues as the force of the vacuum and air percolation dewaters the residuals, now in gel form.
The dried residuals are then sliced off, along with a few thousands of an inch of DE, by a stellite blade. The dried residuals, containing between 25-32% solids, fall off the blade and into a hopper which guides the dried residuals into a 30-yard dumpster. The fresh, newly exposed DE is now ready to repeat the process over and over until the DE coat is exhausted.
The filtrate is mixed with cooling water from the vacuum pump and recycled back into the headways of the plant for retreatment. The filtrate has been characterized as meeting all of the water regulations for finished drinking water but is still retreated. The dried residuals have constantly met all of the requirements for disposal at landfills and for use in beneficial use designations (BUD) to produce topsoil.
The New Facility
The three-year-old facility built by CDM and R.H White Company now employs two drying units purchased from the Alar Corporation. The units are designated as 690 Autovac's indicating a drum size of 6 feet in diameter and a length of 9 feet. Each drum has about 250 square feet of filtration surface. Individual Allen Bradley PLCs control each Autovac and two equipment sleds contain two liquid seal vacuum pumps, which supply the vacuum for the autovacs.
The operation of the system includes loading 1000 pounds of DE into a 2000-gallon tank containing water. This is accomplished using a Flexicon bag lifting system. The super-sack bags of DE contain 1000 pounds of DE. These bags or supersacks have nipples on the bottom to allow gravity draining of the DE into the Flexicon ribbon conveyor, which lifts the DE into the 2000-gallon tank.
The slurry is made using a simple mixer. Once the slurry is made the operator sets up the various processing parameters and then the PLC program is started and the process is carried out automatically. The equipment can be made to run semi-automatically as long as a supply of DE slurry is available.
The building is totally self-sufficient and contains various rooms needed for processing the residuals. A covered receiving dock contains a hydraulic dock plate, which allows easy unloading of the 2000-pound pallets of DE. Storage of one truckload of DE can be contained in the storage bay, which also doubles as the slurry-making room. On the second floor are the actual processing units and a small control room which houses the motor control console, electrical distribution panels, and the control boards for each autovac. A slot cut into the floor allows the residuals to fall into a 30-yard dumpster located directly below the autovacs in a heated garage. Finally, next to the garage section is the equipment room where all of the vacuum pumps are located with associated equipment.
A Typical Run
A typical run using 1000 pounds of DE will take between 8-10 hours. During that time about 6,000 to 14,000 gallons of alum residuals will be processed (0.05-0.1 gallons per minute per square foot). Approximately 90 % of the water volume is reduced and a rough estimation of 1 pound of DE is used for every 1-1.1 pound of solids processed.
What Affects the Processing Rate?
Processing rates are determined by the concentration of the residuals, the temperature, and to a lesser degree the condition of the cloth and knife blades. Once the optimum drum speed (RPM) is determined, the operator will vary the knife blade speed to fine-tune the run. The knife speed is the forward motion of the blade as it is pushed slowly into the DE, cutting off several thousands of an inch per revolution of the drum.
A "good run" will produce an almost continuous sheet of dried residuals with only a very small amount of white DE showing on the underside of the sheet as it falls off the drum (our technical term is "wallpaper"). A balance is desired between the processing rate (in gallons/minute/sq. foot) vs. the dryness of the "cake". Slower drum speeds can produce drier cakes but will result in lower production. Conversely, faster knife speeds will produce a drier cake but shorter runs and therefore lower production.
The Final Product
The Tewksbury Water Treatment Plant processes over 1.5 million gallons of liquid residuals each year, producing about 700 tons of dried alum residuals. This semi-dried solid material is transported on a weekly schedule using a 30-yard roll-on/roll off container. The material is odorless and resembles moist loose dirt. The process has been very reliable in over three years of operation.
The resulting dried sludge has never been rejected by the hauler and is very consistent in physical properties. The cake has always passed the paint filter test, contains no free draining liquids and the solids are typically always over 20% wt./wt.
The whole facility occupies a small footprint and is very quiet with very little noise or odor emanations. Cost to run the facility to make about 1 billion gallons of water is about $120,000 per year for DE and disposal of dried cake ($120.00/MG of water produced).
This is not the fastest process nor is it the cheapest. However, it does fill a gap for land restrictive areas or sensitive locations.
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