**3. Description of a typical electron beam treatment facility**

Usually, sludge consists of wastewater with a low concentration of solid particles (around 15%). This is a fluid that can readily move from the storage area to the irradiation zone and back to the storage area and in a typical sludge irradiation facility this is done using a hydraulic system consisting of pipes and pneumatic pumps. Inside the electron beam room, the sludge is irradiated using a bulk irradiation system that will present a laminar like fluid to the electron beam with a thickness of a few millimeters depending on the energy of the electrons. An example of this could be a rectangular stainless-steel tank divided in the middle by a wall. At the top of this wall, a weir is built to produce a cascade of sludge. This weir is aligned with the length of the scanner of the accelerator in such manner that the sludge is uniformly irradiated by the system. The influent (pre-treated) sludge flows from the bottom of the first section of the tank, rising up to the height of the middle wall over the weir and then overflowing into the second section (**Figure 1** shows an example of this system, as installed for the Arlington County experiment described above). The effluent (post-treated) sludge is taken out from the bottom of the second section flowing through a second set of rubber pipes to a second storage tank.

## **4. Quality control of the process**

The effect of radiation on the reduction of bacterial load and decontamination of sludge depends on the amount of energy from the radiation that is absorbed by the sludge. This energy is used to produce chemically active species that eventually disrupt structural integrity of DNA in bacteria, parasites, and viruses causing their partial or permanent inactivation and eventually death of microorganisms. The amount of energy from the radiation that is absorbed by the sludge is measured by the physical term dose which is defined as the amount of energy absorbed in a volume of sludge divided by the mass of that volume and is measured in kilogray (kGy). A dose of several kGy could be enough to cause a disruption of DNA molecules and inactivate a virus or kill a bacterium. In order to apply the technique in a municipal installation one needs to be sure that all the sludge that gets irradiated will receive the minimum amount of dose needed to inactivate the microorganisms. Several techniques have been proposed to determine the real dose received by the sample during the irradiation of

sludge. One of them makes use of the fact that the interaction of the radiation with the sludge will cause a certain amount of temperature increase in the sludge, therefore, the dose can be determined by measuring the difference in temperature in it before and after irradiation using thermocouples located in the influent and effluent parts of the water piping system near the falling stream; this set of thermocouples can also be used to monitor the irradiation treatment. An advantage of this technique is that once the thermocouples are installed the temperature difference between the input and output portions of the system is easy to be determined; the only two parameters needed to know are the specific capacity of the sample which needs to be determined experimentally for each type of sludge treated and the temperature difference. With these two values the energy absorbed in a sample of sludge can readily be obtained. One problem with this technique however is that it only provides an average value of the dose and does not measure the dose that individual samples of sludge will get when going through the system neither does it measure how uniform the dose delivered to the sludge is really. To overcome the first problem, a small dosimeter can typically be placed inside a small water tight plastic capsule and be allowed to run through the system from the supply tank to the collection tank. Several candidates of dosimeters can be used in this respect; one of them consists of alanine pellets which have dimensions of several mm. These dosimeters rely on the fact that under irradiation a concentration of relatively stable free radicals are produced in them; the free concentration of free radicals can be determined by the technique of electron paramagnetic resonance [9, 10]. Another dosimeter that can be used is Lithium fluoride in crystals or in powder; the crystals have dimensions of a few milimeters per side and two or three could be accommodated in a single capsule. When using it in powder a few mg can be placed in a capsule for irradiation. Upon irradiation electrons from the valence band of the crystal jump to the conduction band and when trying to be de-excited, some of these electrons get trapped in the energy gap of the material. The electrons can be liberated from these traps and allowed to return to the valence band by applying heat to the material. When the electrons return to the valence band, they emit light to lose the excess of energy they have in them. The technique to heat the crystals in a controlled way and measure the output light in the de-excitation process is called thermoluminescence and when applied to dosimetry it is called thermoluminescence dosimetry (TLD) [11]. Uribe and co-workers described the use of this technology in a similar application in the irradiation of corn kernels in a pilot plant using 1.0 MeV electrons [12].

To address the second issue and as part of the operation qualification (OQ ) activities of the system [13] a verification of the dose uniformity along the weird where the electrons irradiate the sludge needs to be performed. This is useful to remove "dark" spots where the sludge does not get irradiated or to correct for instabilities in the electron beam scanning system that makes the electrons to stay longer times or to correct for misalignment of the sludge delivery system with the scanned electrons coming out of the electron accelerator. Film systems are the best candidates to perform dose uniformity measurements along the scanner of the electron accelerator. Several examples of these are available in the market; good examples of them are the cellulose triacetate film (CTA) [14] and the radio chromic films [15]. Both of these work on the principle that radiation induces a change in the optical absorption of the film that can be quantified using a spectrophotometer. Through a suitable calibration with a primary dosimeter these systems can be used to measure the dose along the length of the weird of the sludge delivery system. For instance, when performing the OQ activities using CTA film it was found that the length of the weird where the sludge was irradiated was longer than the extent of the scanned electrons. So, there were "dark" spots at both ends of the weird where the sludge was not irradiated. The situation was corrected by placing a couple of plates at both ends of the weird that reduced its length so all the sludge going through the weird was irradiated [9].
