Discussion
Started 25 October 2024

Phytoremediation for reducing TDS value in wastewater

I am conducting an experiment on the effectiveness of water lettuce (Pistia stratiotes) in reducing TDS in wastewater. Over the first 7 days, TDS increased significantly (from 1200 ppm to 1500 ppm), while other parameters like COD, nitrate, phosphate, TSS, and turbidity decreased. Between days 8 to 10, TDS began to slowly decrease (by 30-40 ppm per day). Based on biochemical processes or plant absorption mechanisms, what could explain these results in my phytoremediation experiment?

All replies (2)

Raghad Mouhamad
Ministry of Higher Education and Scientific Research
The results you're observing can be explained by several biochemical processes and physiological responses of *Pistia stratiotes* (water lettuce) during phytoremediation:
1. **Initial TDS Increase (Days 1-7)**:
- **Leaching of Ions**: When water lettuce is first introduced to wastewater, it can initially release ions or dissolved organic compounds from its tissues, which can temporarily increase TDS. This is common when plants adapt to a new environment, as they may release exudates, ions, and metabolites as part of a stress response.
- **Microbial Activity**: Water lettuce roots support a variety of microbes that help with nutrient breakdown and decomposition. This microbial activity can release certain ions or breakdown products, contributing to TDS.
- **Decomposition of Existing Organic Matter**: As the plants begin to uptake nutrients and remove organic contaminants (evident in reduced COD and turbidity), microbial processes may convert complex organic materials into simpler, soluble ions, leading to a rise in TDS.
2. **TDS Decrease (Days 8-10)**:
- **Plant Uptake of Dissolved Ions**: Once the initial adjustment period has passed, water lettuce can begin effectively absorbing dissolved ions, including some components contributing to TDS. Commonly absorbed ions include nitrates, phosphates, and certain metals. The slight decline in TDS aligns with the phase where the plant has adapted to the wastewater environment and begins actively reducing dissolved contaminants.
- **Enhanced Root Zone Microbial Activity**: As the water lettuce adapts, it may enhance root zone microbial populations, which in turn can immobilize certain ions or convert them into forms that precipitate out of solution, reducing TDS.
- **Reduction of Other Contaminants**: As TDS is often indirectly influenced by the levels of organic and inorganic compounds, the continued reduction in COD, nitrate, phosphate, TSS, and turbidity may indicate that the overall pollutant load is declining, leading to a gradual decrease in TDS as well.
3. **Steady-State or Equilibrium Phase**:
- If the experiment continues, you might observe a stable or reduced TDS over time as the plant and microbial communities reach a steady-state, balancing the removal of dissolved substances with any residual release of ions.
The observed pattern suggests that water lettuce initially undergoes an adaptation phase, after which it begins to actively reduce both dissolved and particulate contaminants in the wastewater. This aligns well with phytoremediation mechanisms, where a lag period is typical before substantial pollutant removal begins.
Ketul Kumawat
Jai Narain Vyas University
Phytoremediation uses plants to reduce Total Dissolved Solids (TDS) in wastewater by absorbing, accumulating, or precipitating salts and dissolved ions. Specific plants like cattails, water hyacinths, and vetiver grass are effective in this process. These plants improve water quality by reducing salinity and mineral content. It's an eco-friendly, cost-effective method for wastewater treatment.

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CO2 Leakage: A Matter of Concern?
Discussion
1 reply
  • Suresh Kumar GovindarajanSuresh Kumar Govindarajan
CO2 Leakage: A Matter of Concern?
1. If CO2 emission remains to be dangerous, then, CO2 leakage is not a concern?
2. Should CO2 migrate from the storage reservoir, multiple processes will attenuate the CO2, such that the likelihood of CO2 reaching the surface always remains to be low?
3. Did ‘CO2 injected at ZERT’ NOT arrive at the surface within ~5 h?
4. Were CO2 bubble streams NOT observed at QICS within 3 h of injection commencing?
5. Given the spatial and temporal distribution of CO2 degassing, is it NOT difficult to take continuous or detailed measurements of the seep site?
6. How about CO2 getting migrated beyond the initial monitoring boundaries and which makes it difficult to estimate the relative proportions of CO2 that leaked to surface, remained in soil gas or dissolved?
7. Whether, the loss of injected CO2 volume greater than 0.1% per year really negate any benefits from carbon geo-sequestration, highlighting the need for quick and accurate leak detection?
8. Whether, improperly sealed wellbores present the most likely pathways of CO2 leakage?
9. With moderate to high leakage risk from existing bore-wells in CCUS; and with variable seal risk from high formation pressures in CCS (CCS/CCUS being a low-risk method for long-term CO2 storage but not a zero-risk method); don’t we have a possibility of ending up with CO2 poisoning upon breathing air with 5% or more CO2 by volume?
10. Are we completely avoiding (a) areas with large numbers of old wells? (b) storage site areas having complex geo-mechanical attributes (c) areas with deteriorated seal rocks? And (d) areas with elevated formation pressure?
11. As of now, predominantly, whether, the studies from oil and gas industries remain to be more focused on loss of integrity rather than leaks (containing leak rates, probabilities and their duration)?
12. No possibility on the leakage of CO2 from transport pipelines, presenting a hazard to humans and animals in the area of the released gas cloud?
13. No possibility on the leakage of CO2 from the installation and from intermediate storage points?
14. No possibility on the leakage of CO2 from the CO2 storage tank in ports during sea transport?
15. In the absence of having details on True Abandoned Well Density; and also on the details of number of active wells that got converted into plugged and intact wells, how could we forecast, whether, we are going to have a continuous leakage or discrete leakage (either minor or major with blowouts) of CO2, following CO2 injection (as a function of number of injection wells and the area of the injected CO2 volume)?
16. How about a scenario, where, there is a sudden, fast and short-lived release of CO2 as would occur in the case of well failure during injection or spontaneous blowouts?
17. How about the local leakage scenario, where, the leak remains to be more gradual, and occurring along undetected faults, fractures or well linings? If such diffusive leakage remain undetected for prolonged periods of time, then, would it have the greatest potential to cause broad-scale environmental impacts?
18. If monitoring by remote sensing provides large coverage and rapid detection without the need for invasive or disruptive installations, then, whether, the existing spectroscopic methods suffer from absorption interference when attempting to detect CO2 directly from storage sites? Or, a ground surface deformation detection would be able to provide a precise leak location?
19. Feasible for a sudden depressurisation caused by a leak could result in a boiling liquid expanding vapour explosion (BLEVE), upon CO2 causing pipeline corrosion?
20. How about the CO2 leakage occurred in 1986 in Cameroon (not from a pipeline or industrial operation but from a lake)?
21. How about the magma chamber underneath Lake Nyos releasing 1.6m t of CO2 which killed more than 1,700 people, thousands of livestock, and destroyed trees and vegetation around the lake?
22. How about 15 t of CO2 release from a fire-extinguishing installation in Mönchengladbach in Germany in 1008 that spread outside into the still air intoxicating 107 people, 19 of whom were hospitalised?
23. How about the leak of CO2 from a pipeline operated by Denbury in the US in 2020 that hospitalised 45 people and led to the evacuation of residents from the nearby town of Satartia? [the likely cause being the pipeline rupturing by a landslide following heavy rains and the cloud of CO2 dissipated slowly because of local weather conditions and the contours of the surrounding land]
24. As on date, is there a standard unit for reporting CO2 release rates into the subsurface (mass or volume per unit time); and CO2 fluxes (CO2 leaked per unit area) from land surface or seabed to atmosphere or water column? Can we differentiate between CO2 release rates and CO2 fluxes in a typical CO2 storage site?
25. Upon CO2 injection into an aquifer, where brine containing not only extremely high salt levels but often heavy metals, hydrocarbons, and radioactive elements; whether, brine leaks, therefore, can be even more worrying than the escape of CO2?
Suresh Kumar Govindarajan, Professor [HAG]
IIT Madras 18-Dec-2024

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