Restoration of urban lakes through aeration (A case study of Bhopal lakes).
Water is the "Elixir of Life". It makes up over one half of the human body. All living things, from the tiniest insect to the tallest tree, need water to survive. Water is dynamic resource, which keeps moving in the nature's "Hydrological cycles" natural process of the continuous movement of water between ocean, atmosphere and the land that means the same water molecules have been transferred time and time again from the oceans and the land surface into the atmosphere by evaporation, dropped on the land as precipitation and transferred back to the sea by rivers and groundwater. This endless circulation is known as the "hydrologic cycle" (Fig 1).
[FIGURE 1 OMITTED]
We know that freshwater lakes and rivers, ice and snow, and underground aquifers hold about 2.5% of the world's water only. By comparison, saltwater oceans and seas contain 97.5% of the world's water supply. Nearly 70% of the earth's fresh water exists in the form of glaciers and permanent snow cover. Only 0.3% of total global fresh water is stored in lakes and rivers.
Lakes and reservoirs are vital to the economic development process. They are important sources for food, fresh water and building materials and provide valuable services such as water treatment and erosion control. They form vital ecosystems for aquatic biodiversity; and provide livelihood and social, economic and aesthetic benefits that are essential for improving the quality of life of the basin communities. Human activities are profoundly impacting their ecological integrity. Lakes are closed systems with relatively long retention times, which can trap pollutants for extended periods. They have complex dynamics and characteristics, and are particularly vulnerable to a range of anthropogenic stresses. The distribution of lakes is governed primarily by variations in geology and climate. Although there is no definitive count, there are at least several million lakes on the planet. A complete lake system, however, consists both of the depression in the land surface that contains the water (the lake itself) as well as the land surface (drainage basin) which surrounds the lake.
Water Quality and Clarity
Water quality is defined in terms of the chemical, physical, and biological content of water. The water quality of rivers and lakes changes with seasons and geographical areas, even when there is no pollution present. There is no single measure that constitutes good water quality. For instance, water suitable for drinking can be used for irrigation but water used for irrigation may not meet drinking water guidelines.
Rain reaches the earth's surface and, as runoff, flows over and through the soil and rocks, dissolving and picking up other substances. Consequently, the rivers and lakes in these areas have very low concentrations of dissolved substances. Urban runoff worsens the water quality in rivers and lakes by increasing the concentrations of substances such as nutrients (phosphorus and nitrogen), sediments, animal wastes (fecal coliform and pathogens), petroleum products, and road salts. It should be noted that there is a difference between "pure water" and "safe drinking water". Pure water, often defined as water containing no minerals or chemicals, does not exist naturally in the environment. Under ideal conditions, water may be distilled to produce "pure" water. Safe drinking water, on the other hand, may retain naturally occurring minerals and chemicals, such as calcium, potassium, sodium, or fluoride, which are actually beneficial to human health and may also improve the taste of the water. Water serves as the universal medium for all metabolic reactions and life cannot sustain without water. The quality of surface water is currently arousing considerable interest due to the importance of this resource for human life and activities (U.S. EPA, 2004).
Water clarity is closely related to water quality, and provides a basis upon which the actual state of safety of water is judged. (Smith et. al., 1995a; 1995b). The depth of light penetration in water influences visual aesthetics and in addition to nutrients and temperature regulates the growth of phytoplankton (Effler et. al., 1998). Water clarity also affects the depth over which solar radiation heats the water (Mazumder and Taylor, 1994).
The clarity of water is a function of how efficiently light is transmitted (Davies-Colley et. al., 1993). The behavior of light is determined by the optical properties of the medium, and the optical properties, in turn, depend upon the constituent composition of the medium (Davies-Colley et. al., 1993). Prediction of water clarity is somewhat unique, insofar as it represents the integrated and coupled effects of a broad range of individual water quality components. These include the biological components such as phytoplankton, together with the associated cycles of nutrients that are needed to sustain their populations and abiotic components such as suspended particles that may be introduced by streams, atmospheric deposition or sediment resuspension. Changes in clarity induced by either component will feed back on the phytoplankton dynamics, as incident light also affects biological growth.
Water clarity is also unique in that it may be one of the earliest and most easily detected warnings of the acceleration of the process of eutrophication in a water body. Long before changes in nutrient levels are at readily detectable levels, clarity may start to be impacted. The water quality components reflect biochemical processes such as nutrient uptake and cycling, algae growth and zooplankton dynamics, and dissolved oxygen cycling. Effler (1988) reviewed the optical principles that govern the Secchi disc transparency and turbidity as a common measures of clarity, and demonstrated that these measurements differ fundamentally in their sensitivity to light attenuating processes and that they cannot be uniquely specified by each other. Natural waters contain a heterogeneous mixture of dissolved and particulate matter, which are both optically significant and highly variable in type and concentration (Mobley, 1994). These can be divided up into water molecules, dissolved organic substances (gelbstoff or yellow substances), biological organic matter, and inorganic particles (tripton) (Kirk, 1994).
The City of Lakes: Bhopal
Bhopal is popularly known as the city of lakes because of innumerable water resources. The Upper and Lower Lakes (twin lakes) are the urban lakes together known as Bhoj Wetland. The 284.90 square kilometer of undulating landscape of Bhopal, well punctuated with water bodies lie sandwiched between the Malwa plateau on the North and the Vindhya to its South. A cartographer's definition of Bhopal would read 23[degrees]16'N and 77[degrees]36'E, with a maximum elevation of 550 meters above MSL spreading over seven hills. Bhopal as said is bestowed with large number of water bodies of which the Upper Lake is the most magnificent lake amongst all these lakes and reservoirs and is a major source of drinking water supply to about 6 lac population of the capital.
The Upper Lake is an elongated water body, constructed by Raja Bhoj in 11th century whereas Lower Lake was constructed by Nawab Chhote Khan in 1794 A.D. It is situated towards the east end of the Upper Lake and is an integral part of the latter. The Lower Lake receives a large amount of raw sewage from it's densely populated habitation. Lower Lake water is used for secondary purposes. The water quality of Lower Lake has far more deteriorated than in the Upper Lake (Pani et. al., (2000). Phytoplanktons are ecologically significant, as they trap the radiant energy of sunlight to convert chemical energy i.e. organic materials. The role of Phytoplankton in managing bioenergetics of lake, their role as bio-indicators and their secretions of enzymes in purification of polluted aquatic habitats has known for a long time (Shastree, 1993). Different groups of phytoplankton population are found in Upper and Lower Lakes i.e. chlorophyceae, bacillariophyceae, cyanophyceae, euglenophyceae, dinophyceae. In Upper Lake bacillariophyceae species was found dominant while in Lower Lake chlorophyceae species was found dominant in Lower Lake in almost all season (Bajpai et. al., 2001). In the catchments area number of activities such as religious, agricultural, colonization, etc. takes place as a result of these variety of materials, including residual, fertilizer, pesticides, wastewater, silt find their way into the lake. It is an established fact that influence of such factors in the catchment area of any water body largely responsible for the eutrophication or deterioration of water quality.
Aeration is the process of adding oxygen to water. Maintaining healthy levels of dissolved oxygen (DO), one of the most important water quality parameter of the water bodies aids in the breakdown of decaying vegetation and other sources of nutrients. This breakdown of bottom silt is carried out by microorganisms at the water/soil interface and continues to proceed a few centimeters deep in the soil. This decomposition can be carried out in two ways, aerobically and/or anaerobically. Aerobic decomposition requires a continuous supply of oxygen and proceeds more rapidly as dissolved oxygen concentrations near saturation levels. The rate of degradation of organic matter in anaerobic conditions is not as rapid as under aerobic conditions, and the end products are organic compounds, such as alcohols and foul-smelling organic acids (the sulfury pond muck smell). In other words, the decomposition is slower and less complete in anaerobic environments than in aerobic habitats where the primary end product of decomposition is carbon dioxide. So aeration helps to increase water clarity, prevent algal growth and decrease in fish kills. Apart from these it promotes healthy aquatic life maintaining the ecological balance. Dissolved oxygen is found in microscopic bubbles of oxygen that are mixed in the water and occur between water molecules (Murphy, 2002). Because dissolved oxygen concentration is affected by many water quality parameters, it is sensitive indicator of the health of the aquatic ecosystem. Oxygen is vital to the life cycle common to water. It is essential for keeping organisms alive, for sustaining species reproduction and for the development of population. Oxygen diffuses into the water column from the atmosphere and is produced by aquatic plants and algae as a by-product of photosynthesis by day in the presence of sunlight, and releases it into the water, while at night and on very cloudy days they consume oxygen through respiration (Addy and Green, 1997).
Aeration is an essential part of almost all wastewater treatment systems and is usually the major energy consuming unit process. The aeration process is also used to remove volatile substances and gases present in water and wastewater and to improve the dissolved oxygen (DO) content in the water and wastewater (Rao and Kumar, 2007). Aeration is the most important and indispensable operation unit for the treatment of wastewater (Chen et. al., 2003; Boyle, 2002; Reardon, 1995) and the main purpose of aeration is to dissolve the oxygen into the water to provide oxygen as the microorganisms decompose organic compounds as food. It improves water quality by increasing dissolved oxygen levels in the water especially where oxygen is needed i.e. at the bottom. Aeration method adds no chemicals to the water and is most effective in warm climate (Jain, 2006). Verma et. al., (2006) reported that the artificial aeration cum ozonizer unit contribute to the improvement of water quality in an ecofreindly way. Aeration is effected using low pressures and is continuously adjusted to meet oxygen demand so that energy consumption is low and constantly optimized in case of the landfill body (Heyer, 2005). Artificial aeration represents a promising approach to improve pollutant removal efficiency in horizontal subsurface flow constructed wetland especially for fresh water fish farms in cold climate (Ouellet-Plamondon et. al., 2006). According to Parker and Suttle (1987) water circulation as well as the aeration of ponds have been responsible for an increase in primary productivity, reducing stratification and causing greater soluble nutrient availability, decreases organic accumulation at the bottom and consequently, increasing fish production. Aeration speeds up this process of oxidizing organic and mineral pollution. In fact, if there is sufficient aeration, the fish and other aquatic organism will survive. Aeration systems transfer oxygen into a liquid media by either diffusing gas through a gas --liquid interface, or dissolving gas into the liquid solution using a semi-permeable membrane (Rosso and Stenstrom, 2006). According to Avinimelech et. al., (1992) ponds without water circulation and without a supplementary aeration mechanism, show higher concentration of organic nitrogen, ammonia and nitrate. Using aeration is often a safe and sound form of pollution removal. Aeration also helps to eliminate stratification, increase water clarity, prevent algal growth and decrease in fish kills.
Affects on Dissolved Oxygen
Barometric pressure, altitude, salinity, water purity, and biological oxygen demand all affect the amount of naturally occurring dissolved oxygen levels in water. The amount of additional oxygen water can hold through the aeration process is a function of temperature, altitude, and salinity. Colder water holds more oxygen than warm, water in higher elevations or with higher salinity levels has a decreased saturation level of oxygen. Once the saturation level reached, oxygen cannot be added without the help of photosynthetic activity or the introduction of pure oxygen.
In lakes/ponds, the introduction of oxygen via some type of aeration device can:
* Allow for greater densities of fish.
* Eliminate the potential for spring and fall turnover.
* Prevent winter kills caused by low oxygen levels.
* Improve overall water quality.
* Speed up the rate of organic decomposition.
* Reduce the amount of phosphorus, which would otherwise be available for plant growth.
* Thermally and chemically destratify the water.
* Cause circulation currents that might create favorable conditions for more desirable algae to out compete blue green algae.
* Decrease the severity of algae blooms and algae die-offs.
* Shift the level of carbon dioxide by venting it into the air, which could limit the amount available for plants
There are many types of methods to introduce oxygen to water through the process of aeration. Most manufacturers of equipment have tested their aeration devices for efficiency under standard conditions. One test gives the result as the Standard Oxygen Transfer Rate (SOTR). The unit of measurement is kilograms or pounds of oxygen per hour. Another measure is the Standard Aeration Efficiency (SAE), which is the SOTR divided by power. The resulting measurement is pounds of oxygen per horsepower per hour.
Various methods have been developed to achieve hypolimnetic aeration. The methods can be grouped into three different categories: mechanical agitation, oxygen injection, and air injection (Lorenzen and Fast, 1977; Cooke and Carlson, 1989; Mercier and Perret, 1949).
Mechanical agitation: During mechanical agitation, water is pumped from the hypolimnion into a splash basin that is located on the surface of the lake or reservoir. The water is then mechanically agitated, which creates a high level of turbulence at the air-water interface. This action increases oxygen transfer from the gas phase to the liquid phase. In addition, air is entrained into the water by the agitating motion. The entrained air forms bubbles in the water, which continue to transfer gaseous oxygen into the liquid. The aerated water is then returned to the hypolimnion. This type of aeration system is relatively inefficient, but it has proven to be successful in a limited number of cases (Cooke and Carlson, 1989). The movement in the water due to mechanical agitation quickly breaks stratification the temperature gradient (Tavares et. al., 1994). Mechanical aerators, used in many industrial plants for wastewater treatment, are the largest energy consumers in biological reactors (Tarshish, et. al., 2000).
Oxygen injection: It can be accomplished through several different methods. Hypolimnetic water can be withdrawn, exposed to pure oxygen under high pressure and then delivered back to the hypolimnion. Another method involves introducing pure oxygen into the hypolimnion through the use of a diffuser to form a rising unconfined oxygen bubble plume. However, measures must be taken to ensure that the oxygen bubbles formed are small enough to dissolve completely before reaching the epilimnion or destratification can occur.
Air injection: A third method involves pumping hypolimnetic water downward with sufficient velocity that injected oxygen is forced downward as well. The oxygen bubbles that do not dissolve entirely must then be separated from the water in the hypolimnion. A Speece Cone oxygenator, originally called a downflow bubble contactor, is an example of a hypolimnetic aerator that utilizes this principle (Speece et. al., 1973). Air injection systems include several different methods as well. Bubble plume diffusers can be used to aerate a water body by dispersing injected air into the hypolimnion. Using air as opposed to pure oxygen is less efficient, but the gas supply is less costly.
A relatively new technique, layer aeration, uses air injection to oxygenate and to redistribute available dissolved oxygen obtained from photosynthesis and contact with the atmosphere (Kortmann et al., 1994a, 1994b). Downflow systems are similar to the one described above for oxygen injection, with the exception of air being utilized instead of pure oxygen. Another method of air injection is an upflow system. Upflow systems can be classified into two principle types: partial airlift and full airlift. Partial airlift systems operate by injecting compressed air near the bottom of the hypolimnion. The air-water mixture travels up a vertical pipe to a given depth in the lake where gasses are vented to the atmosphere through a pipe to the water surface. The aerated water is then returned downward to the hypolimnion. In a full airlift injection system, the same process occurs except that the air-water mixture rises to the surface of the lake before gasses are vented to the atmosphere. This allows the air bubbles to be in contact with the water longer than in a partial-lift system and the water to be in contact with the atmosphere for a short time. These occurrences increase oxygen transfer from the gas phase to the liquid phase. Many of the hypolimnetic aerators studied have been of the full airlift type.
The two techniques most widely used in aquaculture to elevate production rates, as well as to conserve environmental conditions of cultivation, are continuous water flow and an increase in dissolved oxygen levels in the aquatic environment, by way of artificial aeration of the system (Hopkins et. al., 1993).All basic types of mechanical aerators have been used in aquaculture, but vertical pumps, pump sprayers, propeller-aspirator-pumps, paddle wheels, and diffused-air systems are most common in pond aquaculture (Jensen et. al., 1989; Boyd, 1998).
Each description includes advantages (+) and drawbacks (-) of each method.
Wind Aerators/Circulators: Wind powered units will either drive a small compressor that pushes air to a diffuser membrane or will be connected to some type of paddle that enters the water and moves as the wind blows (Fig.2).
+ No electricity is required so they can be used in remote areas.
+ Visually pleasing piece of equipment.
- Will not work in no-wind conditions.
- Not portable and installation time makes it necessary that the right installation point is decided the first try.
- Will not work when they are needed most in the lazy, hazy days of lat summer with little to no wind and overcast skies.
[FIGURE 2 OMITTED]
Vertical Pumps: A vertical pump aerator consists of a submersible, electric motor with an impeller attached to its shaft (Fig.3). The motor is suspended by floats, and the impeller jets water into the air to affect aeration. These aerators are manufactured in sizes ranging from less than<1 to >50 kW, but units for aquaculture are seldom larger than 2 kW. Units for aquaculture have high speed impellers, which rotate at 1730 or 3450 rpm (Boyd, 1998).
+ Ideal for smaller ponds specially aquaculture ponds.
+ Fairly decent oxygen transfer rate.
+ Portable and lightweight.
+ Inexpensive for the water movement.
- Not as efficient at moving water at depths greater than 10'.
[FIGURE 3 OMITTED]
Paddlewheels: The rotating paddle wheel of a paddle wheel aerator splashes water into the air to affect aeration. The device consists of floats, a frame, motor, speed reduction mechanism, coupling, paddle wheel, and bearings (Fig.4). Motors for paddle wheel aerators usually turn at 1750 rpm, but this speed is reduced so that the paddle wheel rotates at 70-120 rpm. There is considerable variation in the design of the paddle wheel and in the mechanism for reducing the speed of the motor output shaft (Boyd, 1998). Fast et. al., (1999), studied oxygen transfer efficiencies with paddlewheel aerators. Moulick et. al., (2002) studied the prediction of aeration performance of paddlewheel aerators.
+ Most efficient surface aerator.
+ Can cause directional flow while aerating.
- Will not be efficient in deeper ponds.
- Units are typically bulky and not very portable.
[FIGURE 4 OMITTED]
Horizontal Aspirators: These units employ an above water level motor, extended shaft, propeller, and draft tube to suck in air. They can be adjusted to point the propeller in several angles (Fig.5).
+ Cause directional flow to address dead spots.
+ The ability to angle the prop into the water makes these units more effective at moving water in deeper ponds.
- Oxygen transfer is not quite as good as other devices.
- Some units have premature failure in the area of the extended shaft.
[FIGURE 5 OMITTED]
Horizontal Prop Units: These units operate similar to aspirators without the extended shaft nor do they suck air. It can be position to point in any direction and angle them similar to an aspirator. These are known as Water Circulators (Fig.6).
+ Excellent for causing water movement.
+ Great for keeping ice off ponds and preventing winter kills.
+ Can be mounted at variable water depths and can mix deep water.
- Not as efficient as other devices at Oxygen Transfer.
[FIGURE 6 OMITTED]
Pumping or Cascading Water: This method is designed after Mother Nature. If water has to be pumped into a pond or body of water and as well splash it to take advantage of the oxygen transfer from the air/water contact (Fig.7).
+ Free aeration
+ Natural look to the pond or body of water
- Not as efficient as some mechanical devices
[FIGURE 7 OMITTED]
Diffused Air: This type of aeration will typically employ either a compressor or blower (Fig.8). A simple way of keep track of what is what is that a blower is high in volume of air produced but cannot pump air very deep. A compressor is low in air volume but can push air much deeper. In deep ponds, a compressor with a diffuser assembly can be very effective at moving the water and transferring oxygen at the air/water interface. This device has an extremely high efficiency for transferring oxygen from air bubbles to water (Boyd, 1995a). Of course, the individual units are small, so several units must be placed in a pond to cause uniform aeration and mixing.
+ Most efficient in deeper ponds.
+ No electricity in the water.
+ Not much surface movement.
- Not very portable.
- Not great for emergencies.
- Not as efficient in shallower ponds.
[FIGURE 8 OMITTED]
Pure Oxygen: Pure oxygen is typically added to high-density aquaculture systems. This can be accomplished via an oxygen generator as well as through purchasing oxygen in cylinders (Fig.8).
+ If the concentration of the oxygen is close to the saturation level, this is the best method to add more oxygen.
- Expensive and elaborate set up.
- Nitrogen supersaturation can result and cause fish stress or mortality analogous to humans getting the "bends".
Tractor-powered aerators: Large aerators such as the paddle wheel aerator have been widely used for emergency aeration in large ponds. Such aerators are driven by the power-take-off (PTO) of farm tractors (Boyd, 1998) (Fig.9).
+ They can quickly raise DO concentrations.
+ They are mobile and can be easily moved from pond to pond.
+ They do not require an electrical service.
- They require a large tractor to power each unit.
- They are less efficient than electric aerators. Therefore, the use of tractor-powered aerators is rapidly diminishing.
[FIGURE 9 OMITTED]
Solar pond aeration: Solar power pond aeration systems are a good alternative to a windmill pond aeration system (Fig.10). Solar has been providing solar pond aeration systems for years.
+ Solar aeration is extremely important to the health of isolated ponds.
+ Golf course pond and farm pond stagnation problems are eliminated with solar pond aeration systems.
- Will not work in no-sun/solar conditions.
[FIGURE 11 OMITTED]
Effect of Aeration
The majority of the investigators listed previously have examined the effects of hypolimnetic aeration on water quality and the lake ecosystem. In their extensive review of literature on hypolimnetic aeration, McQueen and Lean (1986) summarized the results of the studies: (1) well-designed aeration systems have maintained stratification and have not increased hypolimnetic water temperature significantly; (2) hypolimnetic oxygen levels increased, (3) iron, manganese, hydrogen sulfide, and methane levels decreased; (4) zooplankton populations were generally unaffected; (5) chlorophyll levels were usually not altered; and (6) depth distributions of cold-water fish populations increased. The effects of hypolimnetic aeration on phosphorus levels have been more variable. McQueen et. al., (1986) attribute this to pH levels and iron availability for phosphorus sedimentation. The published effects of aeration on nitrogen levels have not been consistent either; ammonium and total nitrogen decreased in some studies, but they increased in others. McQueen and Lean (1986) also concluded that this occurrence is related to pH levels. It has been reported that gaseous nitrogen concentrations were elevated to supersaturation levels during hypolimnetic aeration with compressed air, and some concern has been expressed over causing gas bubble disease in fish (Fast et. al., 1975b). However, no adverse effects of hypolimnetic aeration on fish populations have been reported (McQueen and Lean, 1986). A literature survey published since the review of McQueen and Lean (1986) supports the results presented above (Favre, 1991;Gibbons, 1994; Jaeger, 1994; Soltero et. al., 1994; Gemza, 1995; Nordin et al., 1995).
Rao and Kumar (2007) studied the aeration experiments were conducted in different sized baffled and unbaffled circular surface aeration tanks to study their relative performance on oxygen transfer process while aerating the same volume of water. Dixit et. al., (2005) and Bahl et. al., (2006) reported that dissolved oxygen level in increases while biological oxygen demand and chemical oxygen demand decreases by artificial aeration units in Upper Lake, Bhopal.
The potential uses of ozonization/aeration of lake water for treatment are being practiced very recently (Rao and Prabhakaran, 1998). Reddy and Praksh (1996) studied the use of ozone in aquaculture system to control pathogenic diseases and maintain optimum water quality. Pani and Misra, (2003) reported that artificial aeration/ozonisation is very effective in lake ecosystem for increasing oxygen concentration in hypolimnion and improvement of water quality of a eutrophic lake. Verma et. al., (2006) reported that the artificial aeration cum ozonizer unit is very effective in improving the water quality of Lower Lake, Bhopal. Varughese et. al., (2004) studied that aeration and ozonization have significant impact in increasing oxygen concentration and reducing the pathogenic microbial population.
Sengupta and Jana (1987) reported that aeration induced the net photosynthetic activity of the phytoplankton by substantially reducing their demand for oxygen for respiration.
Tavares et. al., (1999) studied that the effect of aeration caused a direct alteration in the concentration of nutrients, increasing nitrite and orthophosphate and decreasing total phosphorus, ammonia and nitrate. Ding X and Xia L (2006) found that the two-phase aeration process was more effective than the one-phase aeration process in xylitol production. Ouellet-Plamondon et. al., 2006 studied that artificial aeration represents a promising approach to improve pollutant removal efficiency in fresh water fish farms in cold climate. Chen et. al., (2003) had demonstrated that preozonation followed by polyaluminum chloride (PAC) coagulation could enhance the COD removal of phenolic wastewater treatment. Ozone can effectively decompose phenolic compounds (Hsu et. al., 2004). Tsujimura (2004) reported that the aeration system is helpful to reduce internal phosphorus loading and for the destratification of the lake.
Restoration of Bhopal Lakes
Bhoj Wetland of Bhopal comprises of two lakes i.e. Upper and Lower Lakes, India. These wetlands are listed amongst the 25 lakes recognized by Ramsar (2007). The twin lakes have a total water spread area of 32.29 sq. km and catchment area of 370.6 sq. km and both lakes support a rich and diverse range of flora and fauna. With the help of 7.055 billion Yen soft loan from Japanese Bank for International Cooperation (JBIC), a comprehensive project called the Bhoj Wetland Project had been implemented for conservation and management of these twin lakes and this is one of the most reputed projects of its kind that has been undertaken in India (Kodarkar and Mukerjee, 2006).
The conservation measures done under this project, like diversion and treatment of sewage, reduction of harmful inflows of chemicals from activities such as washing activity, shifting of idol immersions site, desilting and dredging operations, improvement in the water quality by reduction of pollution, deweeding operations, nutrient removal by dredging, increased aquaculture activities, restoration of Takia island, catchment area treatment, large scale afforestation, implementation of solid waste management, awareness programme and one the most important measure was installation of aeration units in these twin lakes.
The Upper Lake, Bhopal is having floating fountain type of aeration units while the Lower Lake, Bhopal consist of floating fountain, ozonizer and floating fountain cum ozonizer (dual system) types of aeration unit.
Floating fountain: These are mechanical/electrical devices to facilitate pumping of the lower level anoxic/low oxygenated water of the lake to expose them to the atmosphere. This type of equipment is suitable for improving oxygen level in deep water with less pollution load. Submersible pumps take up water from 15'-20' depth and throw it to a height of 80-90' through the central jets. For side fall, pumps take up water from 5'-10' depth and throw up to 5'-7' in height and horizontally up to 8'-10' wide. The height of water jets allows water to come in contact with air and fall back in the lake. The continuous recycling of water and mixing helps in increasing dissolved oxygen level in the lake water.
Ozonizer: In this system atmospheric oxygen is converted into ozone, which is injected in to the hypolimnion to control bacterial growth and to facilitate increase in dissolved oxygen level in water. In this system ambient air is passed through an ozone generating equipment and the air mixed with ozone is diffused in the water to a depth of 10-15 feet. In this process, initially ozone controls bacterial growth, subsequently ozone is converted into oxygen, which helps in increasing the dissolved oxygen level of the water. These devices are reportedly very effective in highly polluted and shallow water system.
Floating fountain cum ozonizer: This floating fountain and ozonizer combination equipment is reportedly very efficient in deep and less polluted water.In this system water is being ozonized through ozonizer and after ozonation, pumps the lake water from a depth of 10-15' to the atmosphere upto a height of about 80-90'. This facilitates much faster circulation of deep polluted water.
Aeration to make oxygen available for microbial degradation and recycling of organic matter is one of the effective techniques in water quality management. These devices apart from beautification are effective in improvement of the water quality and biodiversity. The installation of aeration units improves water quality by increasing dissolved oxygen concentration consequently reduction of biochemical oxygen demand and chemical oxygen demand. At the same time bacterial population also decreased.
On an average an increase of 40-60% in dissolved oxygen concentration was recorded during the operation of aeration units when compared with values of before operation of aeration unit. Similarly reduction of 30-50% in biochemical oxygen demand and chemical oxygen demand was recorded. The increased biological stabilization of the waste material becomes particularly evident considering the reduction in nutrient concentration particularly of Phosphate and Nitrate.
The floating fountain cum ozonizer (dual aeration system) installed in Bhopal lakes has the best performance for reduction of Biochemical oxygen demand and Chemical oxygen demand. This aeration unit had more influence in improving the water quality by way of higher increase in dissolved oxygen concentration and deactivating active nutrients particularly of phosphorus and nitrate. Ozonizer is effective for controlling the growth of MPN count. The performance of floating fountain cum ozonizer type of aeration unit is significant.
Author is thankful to Dr. D.D. Mishra, Principal Govt. College, Udaipura, Raisen and Dr. Avinash Bajpai, Makhanlal Chaturvedi University, Bhopal, India.
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Department of Chemistry, Govt. Geetanjali Girls P.G. College
Barkatullah University, Bhopal, India.
Table 1 : Salient features of the Bhopal lakes i.e Upper and Lower Lakes. Lakes Description Upper Lake Lower Lake Constructed in 11th Century A.D. 1794 A.D. Type of Dam Earthen Earthen Latitude 23[degrees] 12' 23[degrees] 14' -23[degrees] -23[degrees] 16' N 16' N Longitude 77[degrees] 18' 77[degrees]24'-77 -77[degrees] [degrees] 25' E 23' E Hydrology: Catchment Area 361 9.6 (Sq.km.) Submergence Area at FTL 36.54 0.90 (Sq.km.) Reservoir features: Full Tank 508.65 499.88 Level (MSL) (m) Dead Storage Level (MSL) 503.53 499.88 (m) Storage Capacity (Million 117.05 4.3 Cum.) Maximum Depth (m) 11.7 9.4 Source of Water Rain water Rain water, Seepage from Upper Lake and Domestic Sewage