Biome Trust: Puttenahalli-Yelahanka Lake Report 2020

Summary

Puttenahalli-Yelahanka Lake is spread over 34 acres in North Bangalore. Known for its bird population and bird diversity it was declared as a “Bird Conservation Reserve’ in 2015. For a long time, with heavy inflow of sewage into the lake water quality had deteriorated. The Forest department, Citizens, Institutions and Ornithologist came together to design and rejuvenate the lake to help restore the lake to its original glory. When we visited the lake in August 2020 the rejuvenation process was complete.

The lake's main source of water was a combination of rain and treated sewage. It receives runoff from its upstream source, Attur Lake. However, due to urban growth this source had become a trickle and very little run-off reached the lake from 2012. Instead, wastewater, predominantly sewage began flowing in and filled this Lake. From 2013 plans were initiated first, to remove the sewage sludge from the lake and second, provide for an alternative source of water to keep the lake filled throughout the year. To achieve this, a diversion drain for sewage was designed and executed to divert incoming sewage into a 3m wide diversion drain that runs along the South and East side of the lake. In this way primary sewage is prevented from directly entering the Lake. A runoff diverting sluice was designed and built upstream at the mouth of the lake so that only storm water or heavy runoff can enter the lake upstream. Whereas the daily flow of sewage continues to flow in the diversion drain. The stormwater or runoff entering the lake through this sluice is allowed to flow on a boulder and reed bed so as to trap the plastics and other floating wastes and thus allowing only clean water to enter the lake during the rainy season. The work on restoration began in 2016 and was completed in July 2018. As a part of the above restoration, a ‘Passive Conditioning Plant’ was constructed at the South East corner of the lake and commissioned with a capacity of providing one million liters of good quality water into the lake to compensate for evaporation losses and fall in the water levels in the Lake. Wastewater from the diversion drain is channelled into this plant which then gets treated before it enters the lake. The quantity of wastewater entering the plant can be controlled by the balancing weir and a sluice gate. Water is treated and let into the lake to replenish the water lost due to evaporation, mainly in summer while also allowing a defined quantity of nutrients for supporting primary algal biomass to grow and support a high fish density for the birds’ food.

Location with series of the lake

Plan of the lake

A A’

Conceptual Section AA’

Overview and Observations

Puttenahalli lake is located in North Bangalore in Yelahanka, on the way to Doddaballapura, near KPTCL office.

Puttenahalli Lake

Custodian	Forest Department
Lake Area	34 acres
STP	Passive Conditioning Plant
Wetlands	Yes
Upstream lake	Attur Lake
Downstream lake	Yelahanka Lake

The Forest department which is the custodian of the lake, in partnership with YPLBCT (Yelahanka Puttenahalli Lake and Bird Conservation Trust) lake group overlooks the maintenance of the lake in consultation with water experts from IISc and Bird experts from GKVK. The entire lake is imagined as an urban forest, designed to keep human interference to the minimum. The walking path covers only the northern part while the Southern and Eastern and Western boundaries do not have public access. There are benches placed and gazebos planned for photographers and birdwatchers to pursue their interests. The lake is designed to be generally shallow in nature with a maximum depth of 8 feet with three depth zones keeping in mind different types of birds will inhabit and benefit from the shallow (waders), mid-depth (swimmers) and two metre deep (diving birds) zones. The depth of the lake increases gradually from 2ft to 8ft West to East.

YPLBCT is an active citizen lake group. They have various activities like Idol making for Ganesha festival, Kere Habba, Bird census at the lake to encourage citizens to get involved with the lake.

Inlet

There are two sources of water for this lake. As sewage flows along the run-off channel, there is a need to separate the daily flow of sewage from the rain-water entering as run-off from upstream. Upstream of the Lake, the daily flow of sewage is let into a diversion drain that flows along the southern boundary for nearly a kilometre and then flows past the eastern bund towards the Yelahanka Lake. At the mouth of the Lake, there is a diversion system such that when the flow is low and the water level in the run-off channel is low, it flows into the diversion channel. However, when there is heavy runoff from heavy rain and the water level in the drain is high, a large part of the runoff flows past the barrier into the shallow end of the lake over a boulder trap that arrests silt and debris. On the other hand when there is no rain and the water levels in the lake is low due to drying up, the sewage is let into the PSTP (passive sewage treatment plant), gets treated to bath water quality and then flows into the lake at the deep end. A balancing weir is built slightly downstream of the PSTP inlet such that water is taken into the lake only if the water level in the lake is low. When the Lake is full, no sewage enters the PSTP and all the sewage flows down the diversion drain. Thus fresh water from run-off enters the Lake upstream at this mouth at the shallow end while sewage is taken in only when the Lake water level is low through the PSTP at the deep end of the Lake. The diversion drain prevents the sewage directly entering the lake. There are sluice gates at both the inlets to control the inflow of water that may be needed during maintenance of Lake.

Sluice gate at the Southwestern side of the lake

Passive Sewage Treatment Plant (PSTP)

A passive conditioning plant with a capacity of one million liters per day (1MLD) is constructed in the lake at the deep end on the Southeast corner of the lake abutting the diversion drain. A balancing weir helps control the intake of sewage into the plant. The balancing weir lets sewage to flow into the PSTP only if the water level in the lake is low and the Lake is drying up. The passive conditioning plant has 3 zones or tanks. In the first zone incoming sewage is subject to anaerobic digestion and most of the organic matter in the sewage is converted to biogas while a small part of the digested material, predominantly anaerobic bacteria settle down as sludge. The treated supernatant stripped off the digestible organic material, now flows to the next zone where it is treated with algae in two stages for further purification. In the second tank, specialized algae called Euglena are allowed to grow and these feed on the suspended bacteria still remaining in the water coming out of the anaerobic digester. After a day of treatment by Euglena in this tank, the clarified water enters the second algal tank. This tank permits a high growth of algae and in bright light, due the rapid photosynthesis, the pH of the water rises to about 9.0 and supersaturating the water with photosynthetically derived oxygen. These two phenomena of high pH and supersaturated dissolved oxygen in water firstly kill most of the potential pathogens (if found) and second remove a large part of the nitrogen and phosphorus from the water. The treated water at this stage reaches recreation quality or wild-life quality and after this the water enters the lake. The treatment and filling up the lake with treated sewage is necessary to replenish water lost due to evaporation and during summer and prevent the lake going dry. This will ensure water at the lake all through the year and helps to recharge the groundwater level in the vicinity. The water quality is so managed so as to provide for food for the higher levels of fish numbers maintained to attract more water birds.

Passive Sewage Treatment Plant

Outlet

There is one main outlet or spillway at the Northeast of the lake. Lake water overflows from an overflow weir. The diversion drain diverts sewage flowing into Puttenahalli lake and flows along its boundary without allowing it to mix up with the freshwater in the Lake. The excess water from the lake falling over the spillway as well as the diversion drain join after crossing the lake and flows into a reed bed before entering the Yelahanka lake.

Outlet

Islands and Bird Nesting Sites

There are 4 mini islands created at the lake. Two islands existed prior to restoration and two more were created during the restoration process. The islands are planted with trees for creating bird habitats keeping in mind the roosting and nesting activities. A gazebo is being built opposite these islands for photographers and bird watchers for their activities. There is no direct access to the island and therefore birds roosting and nesting remains undisturbed.

Island

There are seatings provided along the pathway.

Benches

Livelihood Activities

Various livelihood activities are being added slowly to sustain people dependent upon the Lake. Grass cutters are allowed into the lake to cut grass and Alternanthera and take it for feeding their cattle. This helps keep the weed level low in and on the banks of the Lake and also reduce human-snake encounters.

A Grass cutter at the lake

Fishing

The lake has been seeded with four kinds of fishes while catfish has inadvertently entered this lake from upstream lakes. Controlled fishing has been taken up to reduce the population of large fish (more than half kilo), so as to maintain fish at sizes appropriate for birds to feed on. Only large fish are allowed to be removed from the Lake through use of large nets and any small fish caught are returned to the Lake. Large fish usually prey on smaller fish as their main food and thus need to be selectively removed. Some extent of emergent weeds need to be maintained as fish shelters to maintain the fish population especially of the fingerlings and hatchlings. However, there is a need for a more scientific management of the fish population and types in the lake to favour bird feeding. Studies are on to show that fish are breeding and growing the lake.

Biodiversity at the lake

Puttenahalli lake was declared as a ‘Bird Conservation Reserve’ in 2015. The rejuvenation and design for the lake was planned and executed with this in mind. After the rejuvenation, the lake sees many birds roosting and nesting. Bird Surveys are carried out twice a month and species sighted are recorded and discussed with the management group as well as interest groups. There are a lot of insects such as butterflies and dragonflies, beetles and wasps that supplement various other bird feed such as Myna, bee-eaters, etc. One could also see different species of plants around the lake. For a complete list please visit YPLBCT website.

Below are few pictures of the biodiversity that we noticed

Birds

Coots

Purple Swamphen

There are over 120 species of birds spotted at the lake; Coot, Purple moorhen, Spot billed ducks, Pond heron to name a few. We observed Coots nesting near the island.

Flora

Few plants that we observed are Kadamba tree, Canon ball, Ficus, Acasia, Rain trees, Jamoon tree, Honge, Bamboo. Some of the wetland species were Alternanthera (alligator weed), Typha, Ipomea (morning glory), Vetiver, Colocasia, Cyprus, duckweed, Knot weed, Wolffia, Bamboo to name a few. Wolffia size depends on the amount of ammonia in the waterbody. Less ammonia encourages Wolffia-major to grow and more ammonia encourages wolffia-minor to grow. The same plant behaves differently on soil and on water. Taking knotweed as an example, the stem of knotweed is hollow when in water and solid/filled when on soil.

There are date palm trees on the island which have mile-a-minute creepers covered over it with beautiful pink flowers.

Duckweed at the outlet

As indicated earlier, the Lake was designed to have three depths to meet the needs of three types of birds. At the shallow end, it was observed that the alligator weed was growing in abundance and has created a niche for various types of swamp hens (purple moorhen, etc). On any given day there are over a hundred swamp hens to be sighted. They also created nests among these weeds and are breeding extensively. The occurrence of these alligator weeds is due to the legacy sludge and the difficulty during lock down for Lake maintenance and weed removal. We were told that this excess growth will slowly subside once the sediment stabilises and submerge vegetation sets up, which then takes care of the weeds. It can take up to 2 years to see the effect of the restoration.