By Rohan Nath
Introduction
Calcutta has two important water bodies. One of them is the Hooghly river on the west which acts as a drinking water source and the other is a low-lying area towards the east called East Calcutta Wetland (Fig. 1), which plays the role of a sink. The East Calcutta Wetland has been declared as a Ramsar site in 2002.
Waste can be defined as an economically useless material or energy that cannot be recovered or recycled at a particular place and time. Consumption of resources by cities produces both solid and liquid waste. 20,000 farmers and fishermen of East Calcutta Wetland have devised a method to utilize these wastes as a resource rather than a hazard.
The East Calcutta Wetland acts as a resource recovery system where the city sewage is used for practices of agriculture and fisheries. Four principal resource recovery practices are involved in the recycling of waste:
i. Garbage vegetable farms.
ii. Fish ponds holding wastewater.
iii. Fishpond effluents used for paddy fields.
iv. Aquaculture with sewage-fed brackish water.
About 600 million litres of daily sewage and wastewater and more than 2500 tons of garbage is generated by the Municipal Corporation. Several underground sewers carry the wastewater to the eastern border of the city where the pumping stations are located. The pumping stations then pumps the wastewater into open channels. The wastewater is then distributed to sewagefed fisheries, solid waste and agricultural farms.
The bio-remedial activities are conducted in Bheri
Shallow flat-bottomed ponds with wastewater are locally called Bheris (Fig. 2). They have a depth and size of about 50-150 cm and 40-50 ha, respectively. The bio-remedial activities are primarily due to a high rate of photosynthesis in the basin. The shallow nature of the basin allows a high photosynthetic rate due to a better pond volume and pond surface ratio than ponds that have a deeper depth. The high amount of oxygenation in the pond leads to efficient BOD (Biochemical Oxygen Demand) and population reduction of pathogen or faecal coliform. The schematic of the sewage treatment is as follows:
The factors important for the purifying process are:
i. Shallow ponds – It plays a role of a stabilizing tank
ii. An abundance of water hyacinth – It helps in the accumulation of metals.
iii. The sunlight reaching the bottom of the pond.
iv. The wind.
v. Multiple types of algae, plankton and bacteria.
The East Calcutta Wetland is a rich source of 12 different bacterial phyla. This heavily indicates the high amount of bio remedial activity undergoing in the wetland. They play a varying role in the bio remedial activity (Table 1).
Table 1. Bacteria in the East Calcutta Wetland and their functions
The multiple types of bacterial phyla in the wetland indicate the probable presence of bacteria which can be useful commercially such as metal accumulating, antimicrobial compound producing, oil-degrading, and enzyme-producing bacteria.
Reason for water purification
The possible factors responsible for the water purification phenomenon of the East Calcutta Wetland are:
i. Cumulative efficiency of above 80% in reducing the BOD of the sewage water.
ii. Efficient photosynthesis due to solar radiation of 250 langlays a day.
iii. Algae can use solar radiation and accumulate nutrients from the dilute concentration of water.
iv. Commensal association between the algae and waste-oxidizing bacteria results in the release of oxygen and bacterial degradation products synthesis which is used as protein-rich plant material.
The algae-bacteria symbiosis is highly beneficial since it can reduce the BOD at a rate of 237 kg of BOD per day.
The fishes thriving in the wetland also plays a huge ecological role:
i. They maintain the population of planktons in a proper balance.
ii. They convert the available pond nutrients into an edible form (fish).
Removal of faecal coliform bacteria
Pond algae play a major role in the removal of faecal bacteria. The photosynthesizing algae have a high demand for CO2 which cannot be fulfilled by bacterial metabolism. This leads to the dissociation of carbonate and bicarbonate ions. The CO2 formed is fixed by the algae and the hydroxyl ions are accumulated which raises the pH of the water, killing the faecal bacteria. The algal photosynthesis also results in a high amount of O2 in the water, as well as the high light intensities from the sun is fatal to faecal bacteria.
However, there are certain limitations in this method:
i. The solar energy of the ambience must be greater than 200 cal/cm2/day for waste utilization process and algal growth, which is primarily possible in tropical countries.
ii. The photosynthetic oxygen production should be high enough which requires great algal growth potential of the wastewater.
Importance of aquatic weeds
The process by which green plants such as aquatic weeds remove, contain or convert environmental contaminants is known as phytoremediation. Water hyacinth is an important plant that accumulates and removes heavy metal ions from the waterbody (Fig. 3). It is a case of rhizofiltration where the roots of the plant act as a biological filter and absorb heavy metals present in the wastewater. It also prevents pond bank erosion and provides shade to the aquatic organisms during summer.
Conclusion
A developing country like India is in a dire need of proper environmental protection and management planning. The case study of the East Calcutta Wetland is of prime importance due to the sustainability of the ecosystem using inexpensive resource and the least possibility of side effects. Bioremediation is the main process that aids to sustain the environment. The city of Calcutta with 12 million citizens has no sewage treatment plant currently. The East Calcutta Wetland has been a boon for the people of Calcutta and has saved the city from building and maintaining wastewater treatment plant. The wastewater fed lagoons help the city to produce around 8000 tons of fishes annually. Moreover, 150 tons of vegetables daily and 16,000 tons of winter paddy annually is being produced by the garbage farms. The wetland is also a biodiversity-rich site forming a habitat to about 100 plant species, above 40 bird species (both indigenous and migratory), and 20 rare mammal species. However, the wetland is currently under threat with a reduction of 2/3rd of its area within the last 40 years. Hence, we must take immediate action considering the innumerable benefits the people of Calcutta receive from it.
Reference
- Raychaudhuri, S., Mishra, M., Nandy, P., & Thakur, A. R. (2008). Waste management: a case study of ongoing traditional practices at East Calcutta Wetland. American Journal of Agricultural and Biological Sciences, 3(1), 315-320.
Lakes of India 