In 1980, Miami-Dade County undertook a major regionalization effort that required expansion of the three major wastewater treatment facilities (South District, Central District, and North District), upgrades to new wastewater disposal solutions, and consolidation of many small-sized treatment plants that were replaced by master lift stations, while expanding service to a rapidly increasing population of 1.6 million people. At the time, the Central District Wastewater Treatment Facility was generating approximately 100 dry tons of raw sludge per day, which was reduced to about 65 dry tons per day after digestion. In the 1980s, the County created an alternative solids plan that included use of an electron beam prototype unit procured with 95% funding provided by USEPA. Thus, a pilot-scale electron beam system was installed and operated at the Miami-Dade County Central District Wastewater Treatment Plant in Virginia Key, FL. The installation was designed to achieve pathogen reduction and removal of organic priority pollutants in sludge and wastewater. This system was a 1.5 MeV, 75 kW (50 mA) unit capable of delivering 8.3 kGy at 120 gpm, which was only 0.1% of the plant’s capacity. The beam was scanned out to a window with dimensions of approximately 150 cm × 5 cm (60″ × 2″) in a horizontal configuration that passed through a constant flow over a weir. The wastewater fell by gravity through the electron beam with treatment achieved in less than one second exposure. The efficiency of this system was measured to be 66% (Kurucz et al. 1995).
During pilot testing operations, both pathogens and viruses were evaluated. The anaerobic digestion process generated a product with ~10,000 mg/L of ammonia, which is a concentration that is known to inactivate pathogens and viruses. Spiking sludge with viruses to monitor direct treatment performance was not acceptable with the regulatory agency (Florida Department of Heath). The facility achieved a 5-7 log reduction in bacteria, but it could not demonstrate effectiveness on Ascaris ova because of limited analytical capabilities. Likewise, because there were no viruses in the feed sludge, effectiveness on viruses could not be demonstrated. Similarly, with respect to organic priority pollutants, they were detected consistently but the removal efficiency could not be determined because concentrations were near zero initially and spike tests could not be performed (Cooper et al. 1998; Kurucz et al. 1995).
Laboratory and pilot tests were conducted in 1998-2001 using electron beam treatment of sludge collected from multiple sampling points at this facility (Meeroff et al. 2004). Operational improvements were investigated with respect to bulking control, thickening enhancement, anaerobic stabilization, and dewaterability. Electron beam processing caused permanent effects in measured sludge parameters including solids content, chemical oxygen demand, ammonia-nitrogen, zeta potential, specific surface area, resistance to filtration, sludge volume index, pH, organic acid production during anaerobic digestion, and digester gas evolution and methane content. Findings from sludge parameter analyses indicated that treatment should enhance certain flocculation and settling mechanisms by permanently altering electrokinetic sludge properties, rupturing cells, and increasing biodegradability of recalcitrant material.
For doses higher than 10 kGy, pilot testing generally showed a reversal of dose-response trends indicating undesirable effects. At moderate doses (3-4 kGy), pilot testing demonstrated several benefits:
- Ammonia nitrogen stayed below toxic levels (<1000 mg/L as N)
- COD solubilization increased slightly (3%)
- Surface charge became more neutral by 40%
- Specific surface area decreased by 30%
- Resistance to filtration was reduced by 50%
Taken together, these results indicated that treatment induced more efficient compaction and improved filterability; however, the rate of water release did not necessarily show a corresponding improvement, and bench scale settling tests were not sensitive enough to detect any differences, such that surface area requirements and loading rates in settling column studies were not affected, indicating no adverse impacts to sludge thickening. With respect to bulking control, feasibility was deduced from relative inactivation kinetics. Since the indicator filament Sphaerotilus natans (D10 = 0.66 kGy) was inactivated at a lower dose than the bulk flora (D10 = 0.94 kGy), selective elimination of bacterial filaments is possible. In summary, pilot testing results suggest that electron beams will enhance operational efficiency of certain processes within an activated sludge wastewater treatment plant with residuals processing. Analysis of beneficial effects from preliminary studies and pilot tests demonstrate that a dose of 2-3 kGy would be potentially successful for bulking control and to a lesser degree, enhanced thickening and improved anaerobic digestion. A cost analysis based on preliminary tests determined that a centralized electron beam accelerator in an integrated approach could potentially pay for itself at an estimated annual savings of $0.2-2.7 million depending upon the application (Meeroff et al. 2004).