The objective of this study was to evaluate how much phosphorus and metals in sludge can be released into solution by microwave irradiation when applied to sludge prior to anaerobic digestion, and to determine the effectiveness of subsequent lime precipitation. For thickened sludge, the fraction of phosphorus in the soluble form increased to 23%~28% while for unthickened sludge, to 31%~38%, after raising temperatures by microwave heating to 50~70°C. Microwave irradiation also caused the release of arsenic, molybdenum, nickel, and selenium into solution to 33%, 15%, 13%, and 28% for thickened sludge and 63%, 61%, 37%, and 27% for unthickened sludge, respectively. Microwave irradiation has been found to destruct pathogens in sludge to meet Class A biosolids requirements. Therefore, the reduction of phosphorus and metals in biosolids using microwave heating is economically attractive when considered as a secondary benefit to the use of microwave heating to generate Class A biosolids.
An increasing number of municipal wastewater treatment plants have recently switched to enhanced biological phosphorus removal (EBPR). However, as effluent phosphorus concentrations decrease, phosphorus concentrations in waste activated sludge (WAS) from the plant increase. Increased phosphorus concentrations in WAS lead to two significant problems commonly faced by plant operators: struvite formation and undesirably high levels of phosphorus in sludge intended for land application.
United States (U.S.) municipal solid waste facilities generated approximately 6.9 million tons of biosolids in 1998. Future biosolids production was projected to increase by 0.1 million tons per year (U.S. Environmental Protection Agency, 1999). In the U.S., approximately 60% of all biosolids are land applied (National Research Council, 2002). Land applied biosolids are categorized as Class B, Class A, or Exceptional Quality (EQ) based on pathogen levels and metal concentrations. EQ biosolids meet the highest standards of these criteria and Class A and EQ biosolids have the least restrictions placed on their enduse (U.S. Code of Federal Regulations, 2003).
Although there are currently no restrictions placed on phosphorus concentrations on biosolids used for land application, the regulatory climate suggests that this may change in the not-so-distant future. This lead to pressure on municipal wastewater treatment plants to generate biosolids with lower phosphorus concentrations. Industrial applications of microwaves have existed since the mid-20th century, and have since been applied to areas such as the sterilization of medical instruments and the drying of materials. Microwave irradiation uses electromagnetic energy in the frequency range of 300 MHz ~ 300 GHz. Domestic and industrial microwave ovens generally operate at a frequency of 2.45 GHz corresponding to a wavelength of 12.2 cm and energy of 1.02ⅹ10-5 eV (Jacob et al., 1995). In many applications, microwave heating is preferred over conventional heating due to its ability to heat rapidly, instant on/off control, energy efficiency due to its ability to selectively heat substances, and environmentally friendly nature of the process from the standpoint of renewable energy use. Microwave heating is particularly better for solids and semi-solids such as sludge that depend on the slow thermal diffusion process in conventional heating (U.S. Food and Drug Administration, 2000; Jones et al., 2002).
Typically, sludge entering an anaerobic digester is composed primarily of a mixture of thickened primary clarifier sludge (PS) and WAS. Hong (2001) and Hong et al. (2004 & 2006) proposed the use of microwaves to destruct pathogens in biosolids and enhance the performance of anaerobic digesters. By simply irradiating microwaves to PS and WAS until bulk temperature reaches 65°C (30 seconds to 2 minutes of irradiation time depending on the volume of sludge), Hong et al. and Pino Jelcic et al. (2006) found that microwave irradiation of sludge lead to 2.3 to > 4 log reduction in fecal coliform (below the detection limit of 10~360 CFU/g TS) and increase in methane production during anaerobic digestion by approximately 30~50% compared with untreated raw sludge, meeting Class A biosolids requirement with mesophilic anaerobic digestion only. Kuroda et al. (2002) claimed that when WAS was heated, phosphorus stored in the sludge was released into solution. These studies suggest that a microwave system designed to create Class A biosolids can also potentially be used to reduce phosphorus concentrations in biosolids. The objectives of this research were: (1) to evaluate the ability of microwave heat, when applied to sludge prior to anaerobic digestion, to cause phosphorus and metal release into solution and (2) to investigate the effectiveness of lime precipitation to remove phosphorus from a phosphorusrich supernatant sidestream.