The Prime Minister of India, Shri Narendra Modi launched the “Swachh Bharat Abhiyan” on 2nd October, 2014to fulfil Mahatma Gandhi's vision of Clean India. He said that Swachh Bharat would make a significant impact on public health, and in safeguarding income of the poor, ultimately contributing to the national economy. As a part of Swachh Bharat Abhiyaan, various ministries and departments are taking various steps to contribute to a “Clean India”. In this light, recently in April, 2015, the Bahbha Atomic research Centre (BARC) has signed an MoU withAhmedabad Municipal Corporation to set up a 100 tonnes per day Gamma Irradiation sludge hygienisation facility under its scientific and technical support. Such radiation hygienisation facilities would greatly contribute to achieve the objectives of the mission of “Swachh and Swastha Bharat”.
Sewage is the wastewater discharged from domestic premises consisting mainly of human waste. Sewage typically contains more than 99.9% water and about 0.05% solid. The solid part results in the formation of sludge. Largely, sludge is disposed in unorganized manner resulting in environmental pollution and spread of diseases. The sludge produced carries a heavy microbiological load and therefore its disposal has been a challenge to the urban development authorities. Indian cities and towns together are generating an estimated sewage load of 38,254 million liters per day (MLD). Considering 0.05 % solid content, the total potential of sludge generation from the sewage is 19127 tonnes per day(1). Considering the objectives of Clean India Mission and Smart Cities, many of upcoming Sewage Treatment Plants would further add to the sludge volume.
Disposal of municipal sewage sludge, especially in large metropolitan cities is a serious problem due to presence of potentially infectious microorganisms that can be a serious threat to public health. The present sludge disposal methods have their own limitations. For example, disposal into sea is site specific, incineration is an extremely energy intensive process and land filling involves transporting the sludge to faraway places due to scarce land availability in urban areas. On the other hand sludge is an important source of macro and micro nutrients such as N, P, K and Zn, Fe, Cu etc. respectively. Interest in the use of sludge for application in agriculture has increased among the farming community as well as among the sewage treatment plant (STP) operators. The farming community has realized that the excessive use of chemical fertilizers is not sustainable for long term agriculture. Dry sewage sludge (Dry Sludge) can be beneficially utilized for supplying nutrients to the crop, improving soil physical properties and above all increasing the soil organic matter. This can result in increased crop productivity as well as restoration of soil fertility. For STP operators, it may offer a way of generating a value added by product from waste whose disposal otherwise is a matter of environmental concerns and economic loss to the nation. Therefore, recycling of the sewage sludge for agriculture applications can emerge as an important outlet provided it is carried out in a manner that protects human and animal health as well as environment at large.
The sludge after conventional treatment processes at Sewage Treatment Plants (STP) still contains a heavy pathogenic microbial load and needs to be hygienised before it is applied to agricultural land or distributed in bagged form. This necessitates development of technologies that can hygienise the sludge in a reliable, efficient and cost effective manner. World over, mix of methodologies are applied for disposal of sludge which includes 40-50% of its use in agriculture(2). Lime stabilization, heat pasteurization and composting are some of the known methods of treatment for use of sludge in agriculture. In general, EPA or country specific norms are followed for sludge disposal(3).
The high energy radiation has the unique ability of inactivating microorganisms present in the sewage sludge in a simple, efficient and reliable manner. Ionizing radiation emitted by radiation source such as 60Co (Cobalt-60) interact with the critical molecules like DNA and proteins present in the cell resulting in the inactivation of pathogens(Table 1.). Indirectly, radiolytic products of water also makes the treatment more lethal to microorganisms. Due to this property, radiation technology is used worldwide for sterilization of medical products. There are currently 18 Cobalt-60 based gamma radiation facilities in India and more than 400 world over.
The end product of a standard STP is dry sludge which contains about 75-80% solid( Fig. 1.) and 20-25% water. It should not exceed the specified limits for:
1. The presence of pollutants (Arsenic, cadmium, chromium, copper, lead, mercury, nickel, selenium and zinc). Domestic sewage is not expected to have high concentration of these metals. Ministry of Urban Development (MoUD) and United States Environmental Protection Agency(US EPA) have described similar limits. Radiation processing does not alter the concentration of the heavy metals.
2. The presence of pathogens (e.g. bacteria, viruses, parasites)
3. The sewage sludge attractiveness to vectors e.g. rodents, flies, mosquitoes, birds etc. which could transfer pathogens to other places and human. STP process reduces this factor(2).
On meeting the above criteria, dry sludge can be safely used in agriculture. US EPA and MoUD have described gamma and electron beam irradiation as one of the effective methods to further reduce bacteria, viruses, protozoan cysts ,helminthes, ova to below detectable level.
Experience gained at Sludge Hygienisation Research Irradiator (SHRI) Facility at Vadodara has established that sludge can be applied on land for farmer's benefit. SHRI employs liquid sludge irradiation process (96% water and 4 % solids). Dry sludge irradiation is more economical, reliable and scalable to large scale sludge hygienisation. Other solid waste can also be hygienised using the process of dry sludge irradiation. Inoculation of the hygienised sludge with Rhizobium, Azotobacter and Phosphate solubilizing bacteria showed 100-1000 times higher growth in comparison to growth in unhygienised sludge and making it a value added bio-fertilizer.
Fig 1 Schematic of Municipal Sewage Treatment Plant With Irradiation Facility
Table 1 Microbiological Analysis of Irradiated Dry Sludge
Results of field trials carried out by Krishi Vigyan Kendra (ICAR) and local farmers in Gujarat using radiation hygienised sludge established:
• Increased crop yield - direct benefit to the farmers. Improved soil conditions - soil conservation & restoration.
• Reduced health risks associated with sludge which reduces potential pressure on the country's health care system.
• Reduced demand of water due to higher water holding capacity of the sludge.
• The nutrient rich sludge which otherwise is wastefully discarded can be gainfully recycled for economic gain.
• Improved overall quality of life.
Thus, such radiation hygienisation facilities can be utilised in other parts of the country also can really contribute in making India Cleaner, healthier and providing better quality of life to the people of India.
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