274
R. Majumdar
In a research article published in 1983, Breer mentioned that the application of
gamma rays or accelerated electrons could be a better alternative as compared to the
thermal processes, such as the aerobic-thermophilic fermentation of liquid sludge
and the drying of sewage sludge [64]. The advantage with the gamma rays is that
they exhibit adequate penetration into the water and the concentrated liquid sludges.
The half-value thicknesses of 1.33 meV γ-rays from Co-60 source, for water and
the normal liquid sludge are about 28 cm and 25 cm, respectively [65], and such
penetration depths ensure effective delivery of intended irradiation dose across the
thick layers of slurry, into the interior parts of liquid sludge.
Although during 1990s the future prospects of irradiation-based sludge hygienisation facilities looked quite promising, with several facilities being in various stages
of planning and some under construction; many of those plants were shut down
later on. However, most of them did not cite any particular operational difficulties
[66]. The most common γ-ray sources used in the industrial-scale radiation facilities
are Co-60 (Cobalt-60) and Cs-137 (Caesium-137). Co-60 is produced by exposing
non-radioactive Co-59 to a neutron flux in a nuclear reactor, whereas, Cs-137 can
be obtained as a by-product of the spent-fuel reprocessing cycle in the form of
CsCl (Caesium Chloride). However, as Co-60 has a lesser half-life (~5.3 years) as
compared to Cs-137 (half-life ~30.17 years), and the γ-photons emitted by the Co60 (1.17 and 1.33 meV) are more energetic as compared to that emitted by Cs-137
(0.662 meV); therefore, Co-60 has emerged as a preferred γ-radiation source in the
high activity irradiation facilities, such as the sludge hygienisation plant [65]. Disintegration of
60 Co atoms produces two photons with isotopic yield of 100% and the
cumulative energy released by the two gamma photons is 2.5 meV. Another advantage of using Co-60 is that it cannot make artificial radiation, which means it does
not produce any radiation on being bombarded with high speed (i.e. high energy)
particles [67]. On the other hand, CsCl is soluble in water and therefore, it poses great
risk of widespread radioactive contamination in case of a leakage from the sludge
hygienisation facility [65].
In order to promote the use of γ-radiation for hygienising municipal sewage
sludge, an indigenous Sludge Hygienization Research Irradiator (SHRI) was set
up in 1992 at Baroda (now Vadodara) in the State of Gujarat, by the Department
of Atomic Energy (DAE) in India. It was a collaborative program between Bhabha
Atomic Research Center (BARC) at Mumbai, Vadodara City Municipal Corporation,
and the Government of Gujarat, India. The installed irradiator had a provision for
a Co-60 source with radiation strength of 18.5 PBq (1 PBq = 10
15 Bq) to disinfect
the digested sludge. As reported in a more recent study, the strength of Co-60 source
in the SHRI facility was about 220 kCi in 2005, which was provided by 13 pairs of
Co-60 pencils housed in horizontally placed pencil slots to impart irradiation into
the stainless-steel vessel [68]. The main objective of SHRI project was to treat about
110 m
3 sewage-sludge output per day, so that the hygienised sludge can be used
as safe fertilizer on agricultural farmland [69]. Over an operational period of about
six months, it was found from the results obtained from SHRI facility that 3 kGy
dose of gamma radiation (Note: The thermal equivalent of 1 Gy radiation dose is
1 J/kg) is adequate for making the normal sludge free from pathogen and odour
R. Majumdar
In a research article published in 1983, Breer mentioned that the application of
gamma rays or accelerated electrons could be a better alternative as compared to the
thermal processes, such as the aerobic-thermophilic fermentation of liquid sludge
and the drying of sewage sludge [64]. The advantage with the gamma rays is that
they exhibit adequate penetration into the water and the concentrated liquid sludges.
The half-value thicknesses of 1.33 meV γ-rays from Co-60 source, for water and
the normal liquid sludge are about 28 cm and 25 cm, respectively [65], and such
penetration depths ensure effective delivery of intended irradiation dose across the
thick layers of slurry, into the interior parts of liquid sludge.
Although during 1990s the future prospects of irradiation-based sludge hygienisation facilities looked quite promising, with several facilities being in various stages
of planning and some under construction; many of those plants were shut down
later on. However, most of them did not cite any particular operational difficulties
[66]. The most common γ-ray sources used in the industrial-scale radiation facilities
are Co-60 (Cobalt-60) and Cs-137 (Caesium-137). Co-60 is produced by exposing
non-radioactive Co-59 to a neutron flux in a nuclear reactor, whereas, Cs-137 can
be obtained as a by-product of the spent-fuel reprocessing cycle in the form of
CsCl (Caesium Chloride). However, as Co-60 has a lesser half-life (~5.3 years) as
compared to Cs-137 (half-life ~30.17 years), and the γ-photons emitted by the Co60 (1.17 and 1.33 meV) are more energetic as compared to that emitted by Cs-137
(0.662 meV); therefore, Co-60 has emerged as a preferred γ-radiation source in the
high activity irradiation facilities, such as the sludge hygienisation plant [65]. Disintegration of
60 Co atoms produces two photons with isotopic yield of 100% and the
cumulative energy released by the two gamma photons is 2.5 meV. Another advantage of using Co-60 is that it cannot make artificial radiation, which means it does
not produce any radiation on being bombarded with high speed (i.e. high energy)
particles [67]. On the other hand, CsCl is soluble in water and therefore, it poses great
risk of widespread radioactive contamination in case of a leakage from the sludge
hygienisation facility [65].
In order to promote the use of γ-radiation for hygienising municipal sewage
sludge, an indigenous Sludge Hygienization Research Irradiator (SHRI) was set
up in 1992 at Baroda (now Vadodara) in the State of Gujarat, by the Department
of Atomic Energy (DAE) in India. It was a collaborative program between Bhabha
Atomic Research Center (BARC) at Mumbai, Vadodara City Municipal Corporation,
and the Government of Gujarat, India. The installed irradiator had a provision for
a Co-60 source with radiation strength of 18.5 PBq (1 PBq = 10
15 Bq) to disinfect
the digested sludge. As reported in a more recent study, the strength of Co-60 source
in the SHRI facility was about 220 kCi in 2005, which was provided by 13 pairs of
Co-60 pencils housed in horizontally placed pencil slots to impart irradiation into
the stainless-steel vessel [68]. The main objective of SHRI project was to treat about
110 m
3 sewage-sludge output per day, so that the hygienised sludge can be used
as safe fertilizer on agricultural farmland [69]. Over an operational period of about
six months, it was found from the results obtained from SHRI facility that 3 kGy
dose of gamma radiation (Note: The thermal equivalent of 1 Gy radiation dose is
1 J/kg) is adequate for making the normal sludge free from pathogen and odour