In an effort to manage municipal sewage sludge and meet the demands for construction materials, the Indian Institute of Technology – Gandhinagar (IIT-Gn) has developed an environmental-friendly technology of treating this waste with lime – calcium oxide – to build roads.
Sewage sludge is a bio-product of wastewater treatment plants, but can also occur in untreated sewage that is released into water bodies.
“India is building an extensive network of highways and roads. Therefore lime treatment can be used as an alternative to stabilise sewage sludge as a construction material for the sub-base layer of pavements,” said Manish Kumar, co-author and assistant professor, discipline of earth sciences, IIT-Gn. “The obtained quality or geotechnical strength of our final product meets the 2012 guidelines of the Indian Road Congress,” he said.
The study ‘Enhancement of geotechnical properties of municipal sewage sludge for sustainable utilization as engineering construction material’ is published in the Journal of Cleaner Production.
For their work, the four-member team added different proportions of lime (0, 2, 4 and 8%) based on the weight of dried sewage sludge samples (68% silt and 27% clay), and cured them for 7, 14 and 28 days. Results revealed that lime-modified sewage sludge after 28 days of curing with optimum lime content of 6% significantly increased unconfined compressive strength – it is a measure of a material’s strength – to 1102 kPa (kilopascal) owing to the formation of cementing compounds as compared to 207 kPa of untreated sewage sludge.
Further analysis showed that lime treatment controls leaching of metals from the sewage sludge into the environment – an increase in lime content resulted in a decrease in copper, chromium and iron leachates after 28 days of curing. For instance, copper leachate from 70mg/kg (milligrams per kilogram) sewage sludge decreased to 16 milligrams per litre (mg/L) with 6% lime from 19 mg/L with 2% lime. Similarly, iron leachate from 39502mg/kg sludge decreased from 19mg/L (2% lime) to 17mg/L (6% lime).
Four different analyses conducted after lime treatment confirmed the formation of new compounds responsible for improved engineering properties of the sewage sludge. Additionally, immediate reduction in swelling (54%) and plasticity (28%) indicates that lime-laden sewage sludge can be used as an engineering material.
“Enhancement of the geotechnical properties of SS after lime treatment and curing needs to be further investigated on a larger number of samples from various wastewater treatment plants to determine its potential wider utility,” the study stated.
Researchers said their work is important because unlike the developed world, developing countries such as India do not have lined drain systems leading to the high content of sediments in wastewater at sewage treatment plants. “The quality of sewage sludge produced in India is highly complex and unpredictable. Our poorly designed drainage systems also pose a problem for adequate and effective management and disposal of sewage sludge,” said Kumar.
Traditionally sewage sludge, which is rich in essential nutrients such as nitrates and phosphates, is used as a fertiliser for agriculture purposes, to manufacture bricks or filled in landfills.
“Using it as a fertiliser however has a flipside since the sludge can contain toxic metals that can affect yield in addition to contaminating soil and crops with implications on the food chain and human health,” said Kaling Taki, principal investigator, discipline of civil engineering, IIT-Gn.
Globally too, the scientific community over the last decade has been exploring sewage sludge for geotechnical applications. For instance, a 2019 study showed that drinking water sludge mixed with crushed concrete and incineration ash could be used as to build a road subgrade, which is material under a constructed road or a railway track.
Taki added, “High cost and lack of space in urban and semi-urban areas to build landfills is an unsustainable option. There is also a high possibility of heavy metals will leaching into the groundwater and reach sources of drinking water.”