by Hilary Haskell
In the developing world, municipal solid waste management has become increasingly problematic. Remediation of open dumps and waste incineration sites present an additional issue, especially in removing contaminants from the soil that pose a threat to ecosystems and surrounding water resources. Ash et al. (2013) worked with a university in South India to remediate an open dump and incineration site, focusing on remediation techniques involving vermicomposting and phytoremediation to remove heavy metal compounds from the soil. The ultimate goal of the project is to demonstrate the feasibility of remediating similar dump sites so that native ecosystems can be restored and continue to function in the future.
India is now the world’s sixth largest generator of municipal solid waste, producing 110,000 tons each day according to a 2012 World Watch report. This report indicated that only 25% of the world’s waste is recycled or composted, while the remainder is incinerated or landfilled. In another report, the World Bank found that 1.3 billion tons of waste were generated by 3 billion urban residents. Low and middle-income countries spend a great deal on municipal solid waste management. India is classified as one of these types of countries. Without effective waste management, municipalities struggle with other services such as public health and transportation.
Ash et al. worked with a university in South India that had been researching zero-waste initiatives in order to assist local governments with waste management. The university had begun to remediate a former landfill site, and the authors of this study worked with them to further implement and assess this effort. The landfill site studied is an open dump on a wetland island in the backwaters of the Arabian Sea, near the Ernakulam-Kochi urban areas of Kerala, India. Due to its proximity to the Sea, the ground is muddy and waterlogged. Cleanup efforts were initiated in late 2010.
Through a number of restoration methods, the authors worked to restore the landfill site in three phases. Phase 1 consisted of site cleanup and compost spreading over the landfill after dumping and burning concluded in early 2010. The goal of this Phase was to initially re-cultivate healthy soil. For Phase 2, compost and soil were mixed to cover the black mud. The authors also introduced vermicompost during this phase. Phase 3 involved planting of vetiver grass and continuation of cleanup efforts.
The authors built thermophilic compost windrows using food waste and other dry organic material. Organic matter for the compost was brought in from the Amrita Institute of Medical Sciences on a daily basis. To ensure optimal composting conditions, the windrows retained adequate moisture content, maintained a carbon:nitrogen ratio of approximately 30:1, and were provided with plenty of oxygen. Organic material was applied to prevent the waste from rotting anaerobically. These windrows were kept at about 70 °C, and were turned consistently. A roof was constructed to keep the windrows dry during the monsoon.
Mature compost was spread over the toxic black mud, and new windrows were built on top of the mud. This process resulted in a compost layer of 12–18 inches within 6 months. After this layer of compost had been completed, vegetation began to sprout and wildlife returned to the site. Next, the authors added clean soil and planted vegetation. Invasive weeds that clogged water passage were used as additional compost after being mixed with manure. Vermicompost was used to detoxify soil containing compounds such as lead and cadmium. Vetiver grass maintained soil composition after dredging and also removed additional heavy metal compounds.
The authors collected samples of island soil and river mud, and compared the results from before and after the restoration effort. Soil samples taken after the restoration were collected from the black mud layer below the compost and clean soil, and were then analyzed using Inductively Coupled Plasma Optical Emission Spectrometry. Vetiver grass and vegetation were also sampled to measure for uptake of toxic compounds and heavy metals.
Because the site is in close proximity to sea water, during tidal inflows, water would fill into the site and bring up buried waste to the soil surface. Before the restoration, when open dumping and burning still occurred at the site, few species were found in the surrounding area. After 18 months of restoration, much of the ecosystem appeared to have been restored due to clean-up, vermicomposting, and vegetation planting efforts. Although buried waste was not removed from below the soil surface, the main restoration efforts focused on remediating the soil were effective. Soil from the island where the dump was located and its adjoining backwaters demonstrated high levels of heavy metals prior to remediation. After remediation, dramatic reductions in heavy metal concentrations were observed for all contaminants, except for cadmium. Arsenic, mercury, and nickel were no longer detectable, while copper, cobalt, lead and chromium decreased substantially as well. Cadmium concentrations were reduced by 29% overall, which represented the average reduction between samples taken at both the north and south ends of the site.
Compost samples from the site revealed that the compost had nitrogen and potassium levels similar to typical values as indicated by NPK Values of Manures and Compost. Phosphorous levels were slightly higher than normal. Electrical conductivity in the compost was found to be 0.26 mS/cm at 25 °C, indicating that the compost was not saline.
Hyperaccumulating plants used to eliminate soil contamination, like vetiver grass, were studied in order to determine plants’ effectiveness in uptaking heavy metals. The authors did not intend for the vegetation to be used as a food crop. Overall, the authors found somewhat low levels of heavy metal accumulation by vetiver grass and vegetables compared to actual contaminant levels in the soil. Therefore, hyperaccumulating plants may not be that effective in phytoaccumulation or phytoextraction in some cases. The plants did provide an improved environment for phytostimulation of microorganism growth in the rhizosphere, which thus resulted in reduced levels of soil contamination.
In the future, the authors intend to increase vermicomposting, and encourage soil microorganism and fungi remediation processes. By adding additional fungi to compost, fungal types and populations increase, thus resulting in biological processes like chelation or deactivation of soil contaminants. More native trees and nonfood crops will also be added to the site to increase microorganism and fungi populations. The site will continue to be used for education and awareness purposes for students and the community, given the growing need for soil pollution remediation, appropriate waste disposal, and sustainable agriculture.
Ash, P., Sullivan, D., Kothurkar, N.K., Bist, A., Chandran, S., 2013. Rehabilitating former landfill sites: A case study in habitat restoration, Global Humanitarian Technology Conference (GHTC), 2013 IEEE. IEEE, pp. 452-456. Full paper: http://bit.ly/1x83OCh