Open access peer-reviewed chapter - ONLINE FIRST

By Roberto M. Uribe and Jean Engohang-Ndong

Submitted: May 21st 2021Reviewed: October 7th 2021Published: November 18th 2021

DOI: 10.5772/intechopen.101119

Abstract

A review of the current status of sewage sludge decontamination using electron beam irradiation at industrial scale is presented. The chapter includes a historical development of the technology using both gamma and electron beam sources, a description of a facility using an electron accelerator, a discussion of the quality control techniques used to certify that satisfactory decontamination levels for safe use of treated sludge have been achieved, the effect of electron beam irradiation on the bacteria and virus present in a typical sample of municipal sewage sludge, and an analysis of the costs of decontaminating sewage sludge using electron beam irradiation compared to traditional and more routine technologies. Finally, the chapter concludes by emphasizing on the fact that electron accelerators described in this chapter are capable to decontaminate a typical municipal sewage sludge at competitive costs which are shown to be comparable and/or lower than routinely used technologies to achieve class A biosolids by the Environmental Protection Agency standards.

Keywords

• sludge decontamination
• e-beam applications
• microbial pathogens

1. Introduction

The use of radiation to reduce the microbial contamination in foodstuffs and medical supplies is a well-established technology. Both gamma rays and high energy electrons are used with this purpose and their main effect is to damage the DNA of harmful microorganisms. Although the technology has also been used in environmental applications like the removal of contaminants from fume stacks and the decontamination of wastewater and sewage sludge, their application at a commercial stage has not been as successful as in the case of medical supplies or food irradiation.

This chapter presents first an historical overview of the area of sewage sludge irradiation and the technologies developed over time. Then a description of the technologies used for the electron beam treatment of sewage sludge will be presented. A typical facility that has been developed to irradiate sewage sludge with electrons will be described as well as a current state of the use of the technology in several places around the world.

The techniques used to assess the quality control of the process will be described including dosimetric techniques and the analysis of the effect of the electron beam on the reduction of microorganisms that contaminate sewage sludge.

2. Historical development of the technology

Sludges from municipal sewage systems are good soil fertilizer because of their high content of organic matter, nitrogen, phosphorus, and many trace metals essential to plants pointing to its potential in agricultural applications, however they also contain a large density of pathogenic microorganisms, parasites and parasitic eggs that cause diseases to human beings, pets, and farm animals. To make the use of sewage sludge as a safe valuable soil fertilizer in agricultural applications it needs to be disinfected. To that end, several approaches have been used over the years including old techniques such as incineration and more modern techniques such as irradiation.

The use of irradiation to disinfect sludge started in 1973 when an industrial gamma ray facility from Geiselbullach near Munich (Germany) used Co-60 and Cs-137 sources [1]. The facility used 90,000 Ci of Co-60 and 570,000 Ci of Cs-137 and treated up to 180 m3/day of sludge. Similar activities were undertaken in the United States by the US Department of Energy Sandia National Laboratories using Cs-137 and capable of operating up to a maximum capacity of 7250 kg/day [2].

The use of electron accelerators to disinfect sludge was also started in the 1970’s with the work by Trump and collaborators [3] in Cambridge Massachussets, USA. The system which was installed at the Deer Island Wastewater Treatment Plant in Boston Massachussets originally in 1976 consisted of a 50 kW High Voltage Engineering (HVE) electron accelerator and originally delivered up to 375,000 l/day (100,000 gpd) of sludge irradiated at a dose of 4 kGy. In 1980 the system was completely restructured to increase the capacity of the plant to 637,500 l/day (170,000 gpd). In 1982 a similar system was installed in Miami, Florida with a 1.5 MeV, 50 mA accelerator used to treat sludge at a throughput of 645 m3/day [4]. Aqueous streams were presented to the beam in a falling stream about 114 cm wide and 0.4 cm thick. The system was capable to irradiate water streams to doses up to 8 kGy by changing the beam current from 0 to 50 mA [5]. A similar system was installed in Brazil where a 1.5 MeV, 37.5 kW accelerator, with a maximum throughput of 45 l/min were described [4]. Chmielewski and collaborators [6] have described the activities developed in Poland in this area including feasibility studies on the technique and then the design of a 70 tons/day treatment plant for doses of 5–6 kGy using a 300 kW, 10 MeV electron accelerator.

Similar studies have been conducted in Japan since the 1970’s by the Japan Atomic Energy Research Institute (JAERI). In this respect Washino [7] has compared electron and gamma ray treatment of wastewater in order to determine their bactericidal effect and found a larger reduction in the concentration of microorganisms for the latter one, and mention that this is due to an oxygen destruction effect produced by the higher dose rate generally used in the case of the electron irradiation. To overcome this problem, he proposed a reaction chamber for the irradiation with electrons consisting of a concentric dual-tube bubbling column with a 50 micron thick stainless steel window at the top to allow for the entrance of electrons. Oxygen is bubbling from the bottom of the reaction chamber to compensate for this reduction effect. Similarly, Hashimoto and co-workers [8] have described the use of process-control techniques to make the electron irradiation of waste waters more effective.

In 2014 through a collaboration between Arlington County in Virginia, USA and Kent State University, in Kent Ohio, USA a sample of sludge was irradiated to demonstrate the feasibility and the economic value of the process. The sample consisting of 33,750 l (9,000 gal) of sludge was irradiated at 6.7 and 25.7 kGy under typical production conditions using 3 MeV electrons provided by a Dynamitron accelerator and a flow rate of 184 l/min (50 gallons/min) and showed that the process is effective in reducing the concentration of some microorganisms and more economical than conventional disinfection techniques [9].

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

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].

5. Bactericidal effect

Until a recent past, the radiation effects on the bacterial load and removal of noxious chemical compounds had only been performed in small samples of sludge irradiated under laboratory conditions and mainly address either only the microbiological or the chemical effect of radiation in a sample of sludge [16, 17, 18, 19, 20]. Processing and disposal of wastewater sludge is a critical problem worldwide [18] and especially in large metropolitan such as Washington DC in the USA and Tokyo in Japan. Therefore, new technologies to solve the problem of safely disposing of sewage sludge are constantly being sought. An attractive solution for the disposal of wastewater sludge is its utilization in crop fields and landscaping as a fertilizer due to its high content in natural nitrogen compounds. However, in order to be used as such, it must be converted into a class A biosolid [21] which is a form of treated sludge that is deemed safe for humans and animals. In recent years, electron beam technology have shown to be an economically alternative that could be used to meet these regulations by considerably reducing the number of potentially harmful bacteria such as the fecal coliform bacteria and helminth ova such as Ascaris ovawhich are both ubiquitous contaminants present in the sewage sludge. The fecal coliform bacteria are a group of bacteria that are released in the environment through the fecal excrement of humans, wildlife, and livestock. They typically occur in the digestive tract of humans and warm-blooded animals. When present in sewage sludge, they are indicative of the presence of human and animal pathogens including some strains of Escherichia coli, Shigella flexneri, and Salmonella typhimurium. Among the fecal coliforms, the E. coli O157:H7strain which was responsible for many outbreaks has retain attention of media in the past several years. Indeed, E. coli O157:H7was found contaminating drinking water and vegetables, causing cases of stomach cramping, vomiting, and bloody diarrhea in patients. However, in most cases infections with the E. coli O157:H7strain were mild or with no symptoms. Most people infected with S. typhimuriumdevelop diarrhea, stomach cramping, and fever which could cause the patient to experience fatigues and dehydration. An infection with S. flexneriis much more serious with severe stomach pain, dehydration, severe diarrhea, and fever. Patients suffering for shigellosis feel very sick and stay in bed at the pick of the infection. In farming areas where livestock is grown, beside occurrence of fecal coliforms, ascaris ova that when ingested by a person or an animal would hatch and develop into adult worms, Ascaris lumbricoidescause stomach pain, nausea, vomiting, and fatigue. In patients who are heavily infested with ascaris worms, the parasites could be expelled through feces and/or vomit.

Additionally, the electron beam technology could be used to reduce the concentration of volatile organic sulfides and other volatile organic compounds (VOCs) responsible for the unpleasant odor in sewage sludge. Moreover, electron beam treatment of sludge is not an energy intensive process which means that it has a small footprint and the processing times are usually short. The main effect of radiation on sewage sludge is in the radiolysis of water producing OH* and H* radicals and hydrated electrons, highly reactive chemical species which rapidly react with organic compounds in the sludge. The main intermediates in this process are:

H2O2.7OH+2.6eaq+0.6H+2.6H3O++0.7H2O2+0.45H2E1

6. Economic aspects

An important aspect in the implementation of a new technology such as an electron accelerator in a wastewater treatment plant is to anticipate its impact on the operation costs of the facility and on the environment. With respect to the irradiation of sewage sludge several authors have addressed this issue. A team in Florida reported a cost of $2.50 per 1000 gallons of sludge for a 1.5 MeV electron irradiation facility running at 160 gallons per minute [24], while another group compared gamma and electron beam irradiations for a sample of activated sludge and obtained treatment costs of$4.20/m3 for gamma irradiation and $2.10/m3 for electron beam irradiation, which are lower compared to$4.85–$5.19 when using conventional technology at the Central District Wastewater Treatment Facility in Miami Dade County [25]. Furthermore, a comparison was made between irradiation at a dose 6 kGy and incineration of sludge samples and showed a cost of$60.87/m3 for this latter compared to $3.12/m3 when using gamma radiation. Similar results have been obtained by Engohang-Ndong et al. using a 3 MeV electron accelerator. In this case these authors reported that the cost to irradiate one cubic meter of sludge to a dose of 25.7 kGy will be$1.26 [9]. That comparative cost analysis tends to show that electron beam irradiation of sludge consumes less energy than other technologies. Furthermore, electron beam irradiation requires processing times, and very importantly more environmentally friendly technology compared to other technologies such as gamma radiation and incineration.

7. Conclusions

While the use of irradiation to control microbial populations in foodstuffs, on medical devices, and on the environment is relatively old, the use of electron beam irradiation to eliminate potential human and animal pathogen from sewage sludge is relatively new. At the present times electron beam irradiation is a mature technology and is capable to provide the required power to irradiate sludge at the flow rates used in a sewage sludge decontamination municipal plant. The quality control techniques are readily available to provide measurements of the dose needed to decontaminate the sludge and the technology has proved its capability to eliminate bacterial populations and ascaris ova to levels that are considered safe for humans and animals in landscape, agricultural, and landfill applications. Analysis of the costs involved in using electron beam technology to decontaminate sewage sludge showed that they are competitive with usual costs involving mechanical and chemical means to treat the sewage sludge. Application of this technology opens new possibilities for major agglomerations worldwide to safely repurpose municipal sewage sludges.

chapter PDF

More

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution 3.0 License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

How to cite and reference

Cite this chapter Copy to clipboard

Roberto M. Uribe and Jean Engohang-Ndong (November 18th 2021). Irradiation of Sewage Sludge [Online First], IntechOpen, DOI: 10.5772/intechopen.101119. Available from: