PATHOGEN INACTIVATION IN BIOSOLIDS WITH CAVITATION TO PRODUCE CLASS A BIOSOLIDS
Abstract:KADY® International manufactures the KADY® Mill, high-speed rotor-stator dispersion systems, for fluid/solid systems. KADY mills are capable of mixing, dispersing, blending, cooking, aeration, de-aeration, premilling, chopping, and emulsion. The mills are available in top or bottom entry, up to 3,000 gallons batch, or for continuous operation with thru-put rates of 3 to 500 gallons per minute. The rotor-stator produces high shear and cavitation forces that lyse particles and cells as they pass through the ports under high pressure. The proprietary KADY Bio-Lysis System has been used in wastewater applications to fractionate activated sludge causing reductions in the quantity of biosolids produced. This work examined the potential of the KADY system to produce Class A biosolids from digested sludge.
A 3 L bench-top mill system was analyzed for its ability to inactivate oocysts of Cryptosporidium parvum and eggs of Ascaris suum. A prototype flow-through system was examined for its ability to inactivate A. suum eggs. The effect of the process on oocysts in either water or activated sludge was examined only in the batch system. The ability of the mill to destroy A. suum eggs was monitored in water and sludges of different types with and without lime, quick lime, and sodium hydroxide.
The bench top experiments were performed in Maine using both a small batch mill and a continuous flow mill. In Illinois, further bench-top studies verified the killing of eggs added to sludges produced at the Woodridge-Greene Valley Wastewater Facility. After the preliminary batch tests, a prototype flow-through mill was tested.
Initially, all testing in the bench-top batch and continuous-flow mills was performed in water to which the eggs or oocysts were added. Heating was prevented in the batch system by the use of a water-cooled jacket. In later studies, batch runs were made in water, return activated sludge, primary sludge, and anaerobically digested sludge. Other variables included in these runs were the frictional generation of heat in non-cooled vessels, the addition of lime or quicklime while milling. Oocysts in water were either destroyed or rendered nonviable. The majority of the oocysts ruptured along the suture of the oocyst wall destroying the infectious sporozoites. Similar results were obtained when oocysts were added to return activated sludge. When the system was allowed to generate heat, all the oocysts were rendered nonviable within 180 seconds. A. suum eggs were more refractory to killing than oocysts. In water, the eggs seemed to deform and pass through the slots in the mill without rupturing. However, the process scrambled the morphology of the cell within the eggshell, and the eggs did not develop in culture. In water in the batch mill, 100% of the eggs were killed within 10 minutes. When cooling was not applied, all eggs were killed within 5 minutes. With primary and anaerobically digested sludge, heat builds more rapidly in the system and the eggs are inactivated even more rapidly. In the batch mill, egg survival in the presence of lime was only examined in cooled systems. The addition of lime to the system appeared to increase shear in the milling process, so unlike sludges without lime, the eggs were not only inactivated but also ruptured and destroyed.
In the samples processed in the batch mill at DuPage County, the ability of the mill to kill eggs was examined using thickened waste-activated sludge and sludge from the methane transfer digester. Runs were also made with quicklime being added to the sludge at 1 gram and 10 grams per gram of solids. In these processes, the milling process killed all eggs. The addition of 12 grams quicklime to the 3 L batch of methane transfer sludge resulted in a final pH of only 8.9. When 120 grams of the quicklime were added, the final pH was 12.8. The milling rapidly dispersed the lime and produced highly consistent mixture with a very rapid rise in pH throughout the system.
The prototype mill set up at the site was a flow-through mill run as a batch circulation system by attaching the mill to a 300 gallon tank wrapped in fiberglass insulation. Sludge from the mill was added back to the top of the tank and drawn into the mill from the bottom of the tank. The mill caused the sludge to be pumped at a rate of approximately 30 gallons per minute, so each 10 minutes represented a theoretical single pass through the rotor-stator. After treatment, waste was moved to a large holding tank where it could be inactivated by other processes. The 300-gallon tank was approximately 10 feet tall, and 10 temperature loggers were placed in the center of the tank at 1-foot intervals from the bottom to the top. The temperature of the sludge at the beginning of each run was about 34°C, and after milling for 2 hours (i.e., 10 theoretical passes), the temperature increased to around 50°C. Three runs were performed, and split samples were processed and analyzed in both IL and NY. The eggs added to the samples were 96% viable. The viability after each of the three runs was approximately 60%. Thus, the overall reduction in viability was only about 40%. At the end of the third run, 50 pounds of sodium hydroxide was added to the sludge entering the mill. The temperature spike was then almost instantaneous as the sludge passed through the mill. The sludge in the 300-gallon tank was not mixed other than by pumping, and after the hydroxide addition, the temperature at the top was about 80°C, about 55°C in the middle, and only about 50°C at the bottom of the tank. The pH of the hydroxide treated sludge was around 14, and out of 344 eggs examined after culture, only 1 egg was found to be viable. In additional runs with a temperature greater than 50 C, all eggs were killed within 1 hour. With the addition of lime, the process was fully mixed with the pH adjustment occurring in a very short period.
The KADY mill shows significant promise for producing Class A liquid biosolids. The reduced kill of the eggs in the first runs in the batch mill are unexplained, it is possible that the temperature did not increase as high because the assays were run in the winter. This could be easily be overcome using better insulation and larger batch units. The slots in the continuous flow rotor-stator system were larger than those occurring in the batch mill, and this may also have decreased the ability of the flowthrough mill to kill the eggs as readily and to cause their destruction as they passed through the mill.
Improvements may be made in the process by using a complete-mix batch system where the rotor-stator assembly is immersed in the holding tank or by the judicious addition of lime or caustic to the system to increase temperature and pH. It may also be necessary to try other rotor-stator heads. With the monitoring of these changes to the system, we believe that it is very capable of producing a Class A biosolid. The mill causes a complete dispersion and mixing, and thus, each particle is treated to the same set of conditions. The final product is a fully blended solution with small particle size that should make for very easy handling and application as a liquid product for soil emendation.
Document Type: Research Article
Publication date: January 1, 2003
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