Several of the lakes in Bangalore face a common problem of inflow of sewage from neighboring communities into the lake. Though violating a law shying away from the ground reality wouldn’t solve the problem in any way. Hence, a group of people which included citizens, developers, organizations from the Sarjapur/bellandur region of Bangalore, embarked on a visit to Herohalli Kere to understand the mechanisms of Soil Biotechnology which has been used to treat wastewater entering the lake. The below write up summarizes the observations made during the visit by the participants:
Lake description
The lake is spread across 25 acres. The jurisdiction of Herohalli Lake is with BBMP. In other words, the maintenance of the lake is the responsibility of BBMP. Couple of years back, the lake had completely dried up. Being a landlocked lake without any natural inflows or outflows, BBMP thought of piloting a solution. The plan was to restore the lake by diverting sewage from a major sewage channel into the lake, treat it in a local STP (Soil Biotechnology in this case) and then release the treated sewage into the lake. In this case, the technology for treating the sewage is called Soil Biotechnology which works on similar principles of sewage treatment yet in a different way.
The lake is surrounded by independent houses which means that their sewage is not treated by privately run STPs. Moreover, majority of sewage inflow is domestic. However, there are some polluting industries in the neighborhood, which release chemicals (dye, etc.) periodically (illegally!) into the sewage. At that time, the inlet sluice gate is closed to prevent the polluted sewage from entering the plant. The staff has to keep a constant watch on the incoming sewage.
Process description
The process is called soil biotechnology (SBT). The technology is developed in-house by IIT-Bombay researchers and been implemented at Herohalli through Vision earthcare.
According to the design engineer, the plant is designed for 1.5 MLD and works at approximately 70% capacity.
Components of the process/system:
1. Jackwell:
1.5 MLD sewage is pumped from Jackwell. A screen chamber separates materials like plastic, etc.
2. After initial pumping from Jackwell, the sewage passes through following chambers for further screening:
- Manual screen chamber
- Mechanized 10 mm screen chamber
- Grit chamber
- Oil layer skimming chamber
3. Primary sedimentation tank (PST)– Settling/Anaerobic/Hydrolysis reaction
Tank capacity 200 cu.m. 3-4 hours retention time.
4. Soil bioreactor:
Area: 50m*40m
Volume of bioreactor: 3000 cu.m
Water holding capacity: 300 cu.m
The plant has two SBT tanks of equal size, one of which is considered as secondary stage and the second tank is considered as the tertiary stage. The roles are reversed every 3 months.
The Soil bed consists of:
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A thin layer of gravel at top (prevents soil erosion)
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1.25m deep active media in the middle, primarily made of crushed laterite rock, soil, and clay brick. The active media also contains bacterial culture. A large number of earthworms are released in this layer, which keep the layer aerated and feed on the biodegradable material to reduce it quickly.
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0.2 mm Jelly at the bottom (acts as filter media)
The water is pumped as follows:
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Water coming from the primary filtering stage is pumped by 2x 5BHP pumps into the first SBT tank.
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A grid of 2” CPVC pipes is laid on the top of the SBR tank, in which pipes are laid parallel to each other and 1m apart.
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Each pipe has 8mm dia holes on its sides, 50 cm apart.Sewage pours out of these holes onto the gravel (top layer of the SBT tank).
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The sewage passes through the three layers, and reaches the bottom.
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The tank bottom has a gentle slope towards the rear end of the plant. The water is collected at the back of the SBT tank, and again pumped to a second identical tank that is used as tertiary treatment stage.
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According to the design engineer, the first tank treats the BOD/COD, and only then the second tank can treat the nitrates and phosphates.
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Treated water from secondary treatment chamber collected in the recycling tank and diverted to tertiary treatment plant and finally let into the lake through 4” diameter pipes. Slope has been provided to do so.
Total process time: approximately 12 hours
Approximately 3 ton of sludge is settled in primary sedimentation tank. In course of time the sludge which settles will be hydrolyzed and will dissolve in the water itself.
Discussion points:
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Quality of treated sewage
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Inlet BOD 250, OUtlet BOD 4. Outlet COD 37.3
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Power requirement
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3 Pumps, total 7 kw power
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Land required
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General rule: 1 sq.m per KLD. Possible to customize
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Capital cost
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2-3 cr
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Operation and Maintenance costs
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60-70 k per month operating fee, 6-7k electricity cost per month
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Maintenance requirement
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Cleaning of screens, removal of sludge, de-clogging of pipe holes
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Operator attention
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Throughout attention by operator needed. Checking the blocked holes of the pipe in the soil bioreactor, color of the incoming sewage as garment industries nearby
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Reliability
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Resource recovery
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Sludge is recovered. Emptied on the soil bed itself. Plastic, paper recovered burnt here but can be recycled in other cases.
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Ability to handle load fluctuations/seasonal changes
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Chemicals/Industrial effluents
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Inflow of garment industry, plant is switched off.
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Questions:
Following are some of the questions (some have been responded) that would help evaluate the technology in a better way:
1. BOD/COD and other water quality parameters after Primary sedimentation tank?
Typically about 70%-75% of incoming BOD/COD. However this is to be tested
2. Final output water quality all parameters? Especially nitrates and phosphates
Nitrate and Phosphate data not available
3. Approved by KSPCB or not?
4. Existing sewage in the lake. What can be done?
Over a period of time, plan is to visit and understand few more technologies like conventional sewage treatment, phytorid, etc.
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