Dyes are used for producing a variety of colours in the textile industry. But most of these are toxic and difficult to get rid of. Existing methods of water treatment have been unproductive in checking dye contamination. Now, a team of scientists from Shivaji University, Kolhapur have proposed using floating plant beds to remove dyes from industrial discharge. Their study was published in Environmental Research earlier this month.
Several studies (such as here and here) in the past have recommended using plants for bioremediation of wastewater. However, none have made their way out of the labs
One reason for this could be the obvious design flaw, explains Sanjay P. Govindwar the lead investigator of this study. “Plants need soil for growth. But a handful of rooted plants grown at the edge of wetlands cannot neutralise huge quantities of waste water. It is not practical”.
This has been a gaping hole in the research all along.
To plug this gap, Govindwar and his team tried a different approach. They reasoned that for a viable model, plants should have greater access to the industrial discharge. This sparked the idea of a floating plant bed that could be lodged into a treatment chamber. But the success of their design rested on finding plants that could satisfy 3 conditions- decompose the dye, were resilient to the dye and could grow on water beds.
In search of a candidate that would fulfil all the criteria, scientists ventured into a dye polluted site at Maharashtra Industrial Development Corporation, Kagal, India. From this area they selected two plants- Fimbristylis dichotoma and Ammannia baccifera- both of which are annual herbs well adjusted to dyes and grow in a marshy environment.
Next, they tested the plants for their decolourisation potential. Individual plants of F.dichotoma, A. baccifera, and their combination were placed in beakers filled with a commonly used dye- methylene orange. After four days, scientists found huge reductions in the amount of dye left behind in each beaker.
This colour disappearing act takes place in the roots. Once the dyes are absorbed, enzymes in roots degrade these chemicals. Bacteria living in the roots also add to this step by releasing a cocktail of enzymes. F.dichotoma and A. baccifera consortia harbour 5-7 folds greater bacteria than the plants alone. Together the combination group supports 170 unique types of bacteria and therefore shows greater dexterity in degrading dyes.
To prove that decolourised water is indeed safe, scientists analysed the toxicity of cleaved fragments on a common freshwater bivalve. The test confirmed that decolourised dye solution was less toxic.
After passing these benchmarks, the plants were sowed onto an elaborate rectangular floating structure created with dappled PVC pipes. “Each hole was fitted with plastic fillers holding the plant and the soil,” says Govindwar. Nine days after the plant beds were introduced in effluent chambers, the treated textile effluents were assessed for water purity.
F.dichotoma and A. baccifera consortia could remove about 79% of colour from textile wastewater while reducing 66% of the total dissolved solids. The oxygen level in the wastewater also increased and the pH was reverted to normal values.
“A strong point of this method is that it can be applied for in-situ treatment with lower cost,” says Kisan Mallesham Kodam, who was not involved with the study and is an Associate Professor of Biochemistry at the University of Pune. “Use of well-acclimatised plants and low cost material for construction makes these phyto-treatment units ideal for large-scale wastewater treatment,”
Govindwar and his colleagues are confident that their proposed design has practical relevance for bioremediation. Based on their findings from this study, they are developing plant reactors that can be combined with effluent treatment plants for neutralising industrial wastewater.