Untreated sludge from sewage treatment plants can pose a risk to environmental health and, hence, human health. However, if safely used, this sewage sludge can be a rich source of nutrients for agriculture.
There has been significant growth in the urbanisation pattern in India in recent years, and the urban population is projected to cross 50 crores in 2030 and 75 crores in 2050. However, the unplanned nature of urbanisation has come with substantial ecological and health risks. The major cities in India are estimated to generate about 72,368 million litres per day (MLD) of wastewater, of which only 28% is getting treated. Most wastewater is discharged into water bodies - polluting rivers, lakes, and other water sources.
The government of India is aware of the challenge. It has launched several programs, including AMRUT, Smart City, and SBMU, to focus on wastewater treatment in the country's urban settlements. However, wastewater treatment invariably generates massive amounts of sewage sludge, which, if untreated, is a high ecological risk. Sewage sludge is also referred to as ‘biosolids’, but here, biosolids specifically refers to treated sewage sludge that meets the Environmental Protection Agency's (EPA) pollutant and pathogen requirements for land application and surface disposal.
<p>It is assumed that even at the most conservative estimate, nearly 104,210 tons of sewage sludge are generated daily from these Sewage Treatment Plants (STPs). This figure is expected to increase to 186,347 tons per day if India treats 50% of the wastewater generated in the coming years. Currently, most of this untreated sludge is disposed of in landfills, which is one of the biggest threats to the environment.</p>
However, sewage sludge is a rich source of organic matter and inorganic plant nutrients and contains micronutrients in small portions. The nutrient content of sewage sludge is comparatively higher than that of farmyard manure. Excreta's organic matter content serves as a soil conditioner and humus replenisher. This helps upgrade the soil fertility, especially the organic carbon content, as Indian soil is poor in organic carbon content.
The use of sludge for land application has been predominately practiced in countries like the US (since the early 1970s), the UK, and EU-15 countries. Sewage sludge is extensively used in agriculture in Australia due to the low nutrient content of the soil. The findings from the sludge-applied areas have shown improved water-holding capacity and increased nutrient and organic matter content (Abdul R et al.; Katerina S et al.). Japan strives to achieve a 'zero sludge' disposal scenario by thermal treatment of sludge for fuel generation, and residues are used in phosphorus recovery and as soil amendments. The phosphate recovered from these STPs is further used to produce organic fertilisers. Nevertheless, land application of sewage sludge for agriculture shares a significant share of overall sludge reuse across the globe.
<p>However, there are some concerns about using sludge in agriculture due to pollutants like pathogens, heavy metals, and emerging contaminants. Countries are exploring advanced technologies to treat these pollutants and ensure their safety in application. Also, governments are adopting region-specific standards to allow sludge application with acceptable levels of pollutant concentration.</p>
<p>A considerable quantity of sludge is generated from Sewage Treatment Plants (STPs) across the country, but the data on the utilisation of this sludge is yet to be available. At a few STPs, this sludge is being used for gardening inside the premises of the treatment plant. Some STPs claim to use the sludge for horticultural and forestry crops, but most STPs have no plans for utilisation, so the sludge is disposed of in an unsafe manner. However, safe treatment and use of sewage sludge will combat public health and environmental health risks and address food security issues as a substitute for chemical fertilisers, thereby mitigating climate change effects by reducing greenhouse gas emissions.</p>
Farmers, especially small farmers in India, face water scarcity, increased production costs, and decreased crop yield. Around 50% of India is under water stress, and the soil's organic carbon content gradually decreases. As per the National Rainfed Area Authority, it is now at 0.3%, which is way too low and a matter of concern. The reduced soil productivity is mainly attributed to the continuous application of chemical fertilisers.
“Chemical fertiliser is an addiction. We need to keep increasing the dose of chemical fertiliser applied year by year to get the exact yield, as soil health is deteriorating due to decreased microbial activity," says Ramesh, Secretary of Farmers Association, Devanahalli taluk, Bengaluru rural.
In the last 50 years, there has been a significant increase in fertilisers; there has been a rapid growth in the production of fertilisers in India, showing an increase of more than 11% (2019–20) compared to the previous year. Also, overall fertiliser consumption has grown at a rate of 2% in 2020, while urea sales alone grew by 5.9% in 2020, as per the Annual Report, 2019-20, Government of India, Ministry of Chemicals and Fertilisers. Another alarming issue is the availability of phosphorous fertilisers, given that phosphorous is a limited resource and will be exhausted by 2050, as per estimates. There is a need to look for alternative sources of phosphorus.
The Russian invasion of Ukraine has affected the fertiliser supply in India. According to an estimation, both nations contribute to 10–12% of India's fertiliser needs. The current developments have led to a shortage of fertilisers and a spike in price.
However, increased fertiliser usage has its own environmental cost. Due to increased demand for meat and dairy products, nitrogen fertilisers (urea) are applied to animal feed and fodder crops. Urea-ammonia is the second most common chemical produced and used in considerable quantities in the world, and hence, there is a big market out there. However, urea-ammonia production is energy-intensive as it is produced under high temperatures and pressure. Hence, there are increased greenhouse gas emissions, as most of this energy comes from burning fossil fuels.
<p>The agriculture sector is India's second-largest greenhouse gas emitter. Our world data reports around 220 MT per capita per year of nitrogen oxide (N2O) emissions, a potent greenhouse gas produced mainly from synthetic fertilisers for crop cultivation. Nitrous oxide is a long-lived greenhouse gas that is 300 times more potent than CO<sub>2</sub>. India is one of the top nitrous oxide emitters at the global level, apart from China and Brazil.</p>
It's time to revisit our approach and look for alternatives to synthetic fertilisers. These fertilisers are slow nutrient releasers, low or no greenhouse gas emitters, enhance soil quality by enriching the microbial biomass, and upgrade soil fertility.
The non-judicious application of chemical fertilisers pollutes water bodies as these get into the waterbodies by run-off from fields and through chemical processes in the soil, releasing greenhouse gases into the atmosphere. Apart from adding to climate change effects through greenhouse gas emissions, synthetic fertilisers have significantly affected the soil microbial ecosystem, which is called the 'life of soil'. Long-term continuous application has degraded soil quality, reduced productivity, and organic matter content.
<p>In this context, safely treated sludge can be an excellent source of nutrition, reduce our dependence on chemical fertilisers, save foreign reserves, and help reduce greenhouse emissions. At the current rate of sludge production, it is estimated that sewage sludge can substitute ~4,531 tons of urea/day (synthetic fertiliser); this number increases with increased wastewater treatment, that is, the sludge use potential. However, the application is limited due to the legislative restrictions imposed due to the risks of pollutants.</p>
The US EPA (Environmental Protection Agency) has given guidelines for sewage sludge application in agriculture, including the pollutant limit concentrations for heavy metals and pathogens and the management measures for application. The UK, Australia, and New Zealand have formulated regulations for land applications based on the USEPA's Part 503 guidelines. However, the EPA report is a very old guideline (1994) that provides limit concentrations for a few parameters for a specific application. Hence, guideline documents and amendments should be revisited based on evidence from today's scenario, particularly regarding the context and application.
Owing to the scarcity of land, Japan introduced the New Sewerage Law in the 1970s to minimise sludge production and restrict unsafe disposal of sewage sludge. Hence, sophisticated technologies are being used for power and compost generation for sludge utilisation. Different laws and acts govern the quality standards for various applications, which is a good move by the country to encourage the use of sludge. European Union legislation has provided directives for sewage sludge application in agriculture. EU countries have adopted these directives with some revisions, which has led to stringent and stricter rules for land application. National legislation on sewage sludge in Greece abandons landfilling and promotes its use in agriculture as compost.
<p>There are no regulations for using sewage sludge for land applications in India. The Fertilizer Control Order of 2009 provides quality guidelines for organic compost where the feedstock is vegetable and yard waste, not sewage waste. Hence, there is a need to review the existing policies and procedures and develop a clear regulatory framework for sewage sludge application.</p>
In many regions of the world, local ordinances have banned the land application of sewage sludge, mainly due to the fear of high organic carbon loads, heavy metals, pathogens, emerging contaminants, and a longer processing period. However, safe treatment of sludge reuse can help in nutrient recovery for agriculture and closing the loop for circularity and sustainability. Many examples exist where the same has been safely achieved.
The risks of the above contaminants could be removed at the treatment level through pre-treatment technologies, operating regimes, and post-treatment technologies. Still, these are not sufficiently addressed to the best of our knowledge. Some technologies treat sludge further to reduce pollutants. Thermal-temperature inactivation of pathogens through heat treatment will help reduce pathogens. Different composting technologies and phyto-bioremediation technologies have shown a reduction of heavy metals and emerging contaminants in the final treated sludge.
Countries like Australia, South Africa, the United Kingdom (UK), and the US use sludge as fertiliser or soil conditioner for land application. Some of the best management measures developed countries follow to ensure safety are stabilisation, dewatering, composting, pellet production, and heat drying of sludge for broader applications. The two most commonly used stabilisation methods are anaerobic digestion and lime stabilisation.
<p>However, lime stabilisation is not widespread as it restricts application. Mechanical technologies such as the centrifuge and filter press are generally used for dewatering. Apart from technologies, other best operational practices would help put barriers in place and mitigate risks. For example, in the UK, the Code of Practice for Agriculture Use of Sewage Sludge provides operational practices to be followed by farmers, growers, and land managers, which includes discussion on good sludge application practices to prevent pollution and safeguard human health.</p>
Swachh Bharat Mission-Urban 2.0 has undertaken complete liquid waste management in cities to ensure the safe treatment of all the wastewater generated. With the AMRUT (Atal Mission for Rejuvenation and Urban Transformation) coming up with plans to increase the capacity of the sewage treatment plant, there will be an increase in wastewater treatment in the coming days. If the wastewater treatment is increased to 50% from the current 30%, then the sludge generated would be ~ 1,86,348 tons/day, which, if safely recovered, can substitute around 8,102 tons/day of urea.
<p>Sewage sludge generation is growing persistently and will continue to grow with the increased population, adding to a huge mass of solids. Stringent quality standards and guidelines make resource recovery even more challenging. All these calls for urgent, cost-effective, environmentally friendly, sustainable options to achieve our SDG goals of public and environmental health on the one hand and addressing food security issues and mitigating climate change effects on the other hand.</p>
Furthermore, not many studies have been done to develop treatment technologies or modules based on resource recovery as an objective. Hence, there is a need to work towards end products' application-based, cost-effective, and efficient treatment technologies, especially the processes and technologies for resource recovery and reuse.
The recent developments by the National Mission for Clean Ganga (NMCG, India) to handle sludge from the Ganga River to avoid the problem of sludge dumping and the plan to process and provide it to farmers are an appreciable move. To make the circular economy approach of safe sludge resource recovery a sustainable business model, we need the following actions: a regulatory framework, quality guidelines, market identity, and the development of innovative and cost-efficient technologies. Also, the promotion of the product through government subsidies, mandating fertiliser companies to sell these sludge compost and synthetic fertilisers, and incentives for farmers to safely use the sludge are some ways to go.
All these would help farmers with the availability and accessibility of nutrient-rich organic sources, which are cost-effective and eco-friendly. Safe use of sludge will reduce the use of chemical fertilisers, thereby reducing soil degradation and greenhouse gas emissions—sludge is not a waste but wasted; otherwise, it is an asset.
Authors
Girija Ramakrishna is Technical Expert (Reuse and Circularity) at CDD India, Bengaluru. Her academic background is in agriculture and agribusiness management. She has experience in sustainable sanitation and agriculture practices, safe resource recovery of nutrients from wastewater, sludge-based inputs, and municipal solid waste.
Harshvardhan is the Chief Executive Officer of CDD India, Bengaluru. Harsh is a development professional with extensive experience working across producer-owned collectives, governments, multilaterals, and grassroots NGOs. Harsh has designed and executed large-scale development projects both within and outside government systems and has been involved in policy-making by the national and state governments.