Sewage treatment wetland in making
To demonstrate the use of wastewater in agriculture, the Indian Agricultural Research Institute (IARI) is constructing a wetland in its Delhi campus. Spread over 1.42 hectares, the wetland is capable of holding and treating two million litres of sewage a day from the Krishi Kunj Colony adjoining IARI’s campus. The institute plans to use the treated sewage in its farms to grow crops.
Developed by IARI at a cost of Rs 1.4 crore, the wetland consists of three treatment tanks/ cells and one treated-water holding tank. IARI’s Water Technology Centre project director, Ravinder Kaur under whose initiative this facility was developed, says that the institute had wasteland in a corner with the remnants of an old pond, sumps and pumps. The wastewater treatment cells/ tanks were developed from this detritus.
Sewage from the Krishi Kunj Colony is first let into one sump where any lumps or larger pieces settle down to the bottom. The wastewater is then pumped into the second sump for further sedimentation. These sumps are cleaned out periodically. From here it goes to a grit chamber filled with broken pieces of bricks and then to the tanks of the wetland. The effluent entering the tanks are now clear of any large floating pieces that could clog inlet pipes. The wastewater is stored in these treatment tanks for 2.2 days. Each treatment tank is layered with 60 cm thick layer of stones, on which different species of plants such as phragmites, typha and acorus are planted.
These aquatic plants grow by absorbing micronutrients (nitrates, phosphates, heavy metals, potassium and sulphate) from the sewage, thereby disinfecting the wastewater. Water flow into these tanks is regulated so that its level does not rise beyond the gravel layer. This prevents mosquitoes from breeding. Kaur says that so far only one tank has been planted with typha plants, which have covered half the area. The cleaned water is then collected in the holding tank, from where it is pumped through a riser pipe to IARI’s fields.
The cleaning action is quite significant, says Kaur. While biological oxygen demand (BOD) of the effluent at the inlet is about 460 milligrammes per litre (mg/l), BOD of effluent is 100 at the outlet. Similarly, the system helps reduce the total suspended solids (TSS) from 220 mg/l to about 2.2 mg/l. And this is when the system is not fully functional. Kaur expects these values to improve once the system is fully operational.
Kaur’s confidence stems from an older model of the wetland that she had built in 2009 on an experimental basis. With a capacity of 1,500 litres a day, the system is essentially a series of 18 plastic water tanks of 500 litre capacity, connected parallel to a pipe that carries sewage from a sump. The sewage is retained in tanks planted with phragmites, typha or acorus for 14 to 17 hours and in unplanted tanks for 54 hours, before releasing it to another sump. The treated water is then pumped as required to a test bed where its use in agriculture is evaluated.
The unplanted tanks essentially work as a control system, explains Kaur. “We have found filamentous micro-organisms grow in these tanks and decompose the sewage. Their performance over time is the most effective in reducing TSS and BOD compared to the plants. The disadvantage is the long retention time. These organisms improve dissolved oxygen through photosynthesis.”
The plants not only remove pollutants but also aid in oxygenation through their roots, she adds. However, their efficacy decreases with time. From the first year to the second year, the system’s nitrate removal efficiency fell from 86 per cent to 50 per cent, phosphate removal efficiency from 69 per cent to 26 per cent and that for metals by varying amounts. However, periodic harvesting restores the system’s efficiency, says Kaur. Harvesting takes place before the monsoons.
According to Kaur, the environmental stress of such a sewage treatment system is 33 times less than that of a conventional sewage treatment plant (STP). Constructed wetlands are therefore a viable option for treating domestic sewage in peri-urban areas where land may be available but other resources such as electricity, human resources and other inputs are not.
IARI is now testing the harvested grass for the presence of heavy metals. If devoid of heavy metals, the wetland can simultaneously be a perennial source of mulch and fodder.