Lakes contain a variety of nutrients. Aquatic plants take these nutrients from the sediments through their roots or leaves. However when these nutrients are in excess, algal blooms overcrowd the lake and the lake flora and fauna suffers as a result of the high levels of nutrients and an increase in the biological oxygen demand (BOD). Thus the removal of the excess nutrients via techniques like flushing with nutrient-poor waters, deep water abstraction, sludge removal, etc are required. This article will explain to you about the Removal of the Nutrients from a Lake which will be helpful in preparing the Environment syllabus for the UPSC Civil service exam.

How do Nutrients Enter the Lake?

• Aside from internal loading, there are numerous external loading options. This occurs when nutrients from the surrounding land enter a body of water.
• When water washes over land, nutrients are pushed into a nearby body of water, which is usually a stream.
• The stream collects nutrients as it flows, but it doesn't keep them for long. It eventually dumps them into the first lake it comes across.
• This can occur over the course of several streams, each of which empties into the next. A stream transports nutrients in a variety of ways as it flows.
• Nutrients can sometimes bind to other materials, such as sediment or eroded soil from a streambank. They can take the form of branches, leaves, or grass at times.
• These organic materials decompose and release nutrients over time. When they reach a larger body of water, such as a lake, they decompose.
• Nutrient-rich streams tend to load lakes with a large amount of nutrients. This can happen quickly, for example, during a strong thunderstorm. Sometimes it happens gradually, such as when lawns are watered or a streambank erodes.

Sources of Nutrients

Natural Sources

• Although nutrients occur naturally in lakes and streams, human activity has significantly increased the amount of nutrients entering surface waters.
• Nitrogen and phosphorus levels in the background are typically measured in milligrammes per litre.
• The primary natural sources of phosphorus, usually in the form of phosphates, are soil and rocks.
• Natural nitrogen sources include riparian vegetation's leaves and other organic debris.

Sewage Treatment Plants

• Wastewater (or sewage) treatment plants are a source of nutrients because their effluent is discharged directly into rivers and streams.
• It can be a significant contributor unless the effluent has undergone tertiary treatment, or treatment to remove nutrients.

Household Detergents

• Previously, household detergents brought large amounts of phosphorus to treatment plants, which were then discharged with the effluent.

Septic Systems

• Septic systems can contribute a significant amount of nutrients, especially if they are near water.
• Nitrates are not removed by standard septic systems; however, special systems such as sand filters that remove nutrients are becoming more common.

Sediment

• Excessive erosion produces sediment, which transports phosphorus in the form of particles attached to soil.
• Construction sites without effective erosion control systems can significantly increase the amount of sediment reaching lakes and streams, bringing in large amounts of phosphorus.

Animal Manure

• Manure contributes significantly to nutrient pollution in lakes and streams.
• If livestock manure is not properly managed, it can enter streams via runoff or direct deposits by animals in the water.
• Pet waste is also a nutrient source in urban and suburban areas, and swarms of ducks, geese, and other waterfowl have caused issues.

Commercial Fertilizers

• Commercial fertilisers are a significant source of phosphorus and nitrogen.
• Nutrients not needed by crops either run off the land into lakes and streams, build up in the soil, or seep down into groundwater depending on the composition of the soil in an area, irrigation amounts and application methods, and the amount of rainfall.
• Groundwater can seep into a stream and serve as a source of nutrients.

Atmospheric Nitrogen

• Around 78 percent of the air we breathe is made up of atmospheric nitrogen.
• When fossil fuels are burned, oxidised nitrogen compounds are produced, which then fall to Earth as rain or snow.

Removal of Nutrients From a Lake

The following methods are employed in a lake to remove the nutrients.

1. Using nutrient-depleted water as a flush

• Dilution/flushing has been shown to be a successful restoration technique for restoring eutrophic Moses and removing nutrients.
• The nitrogen and phosphorus content of the dilution water added to lakes is low in comparison to the lake or normal input water. As a result, the nutrient content of lakes decreases predictably.

2. Deep water abstraction

• It refers to extraction of water from any natural source, such as a lake, aquifer, river, stream, or spring.
• Water abstraction is most commonly used for irrigation, with more than two-thirds (71%) of all freshwater on the planet diverted for agricultural purposes.
• Water is abstracted for a variety of reasons, including treatment for use as drinking water and numerous industrial applications.

3. P-elimination on-site using flocculation/flotation with water backflow or floating Plant NESSIE with adsorbents

• Flocculation is a water treatment process in which solids combine to form larger clusters, or flocs, which are then removed from the water. This process can occur naturally or with the assistance of chemical agents.
• It is a common method of stormwater treatment, wastewater treatment, and drinking water purification.
• The removal of suspended solids is one of the requirements for treated water leaving wastewater treatment plants. Small solid particles change the colour of water and transport impurities into natural water sources such as rivers and the ocean.
• Phosphorus content in wastewater must also be limited because phosphorus release into rivers promotes algae growth. Uncontrolled phosphorus releases have been linked to mass die-offs of fish and other aquatic life.
• Some industrial applications produce high levels of phosphorus in their wastewater, which may necessitate pre-treatment before being discharged to wastewater treatment plants.
• Flocculation is a chemical procedure. It entails sequentially adding chemicals to wastewater and allowing tiny solid particles to aggregate into a larger mass known as a floc.

4. Filters and P-adsorbers for on-site algae removal

• Bioadsorption is regarded as one of the most promising low-cost wastewater treatment processes.
• Metal oxides, activated carbon, zeolite, chitin, metal sulphide, resin, and other materials were used as adsorbents to remove heavy metal ions from water.
• Because algae have a diverse biochemical composition, their biomass is a promising material for use as a bioadsorbent to decontaminate water and wastewater by removing pollutants like heavy metals.
• Marine algae, also known as seaweeds, have a high potential for use as a promising bioadsorbent material in pollutant removal processes.
  • This is because they are renewable, have distinct properties, and have a high biosorption capacity.
• Seaweeds are classified into three broad categories: (i) Green Algae (Chlorophyta), (ii) Red Algae (Rhodophyta), and (iii) Brown Algae (Phaeophyta).
• Due to the structure and components of their cell walls, brown algae provides the best adsorption capacities.
  • Brown algae cell walls contain numerous active chemical functional groups such as hydroxyl, carboxylic acid, amine, imidazole, phosphate, phenolic, thioether, and sulfhydryl, which provide selective binding and interaction with metals and pollutants during the bioadsorption process.

5. Destratification (permanent or intermittent) / Artificial mixing

• Destratification is the process of obstructing thermal and/or chemical layers in bodies of water, most notably lakes and reservoirs.
• Destratification is frequently associated with the mixing process because both have the same goal of homogenising the water column in order to improve water quality.
• Destratification is classified into two types based on the occurrence of the mixing process. There are two types of destratification: natural and artificial.
• Destratification can occur naturally when surface water uses wind energy to circulate water in a lake or reservoir.
• Water circulates completely and there is no stratification during the winter season because the wind is strong enough to naturally destratify the water column.
• In the summer, however, with warm weather and little wind, circulation occurs only partially. This results in stratification. In this case, artificial destratifiers are required to add energy to completely circulate water.

6. Fish and Macrophyte Harvesting

• Lakes and ponds surrounded by swamps and marginal vegetation are widely used to treat municipal, industrial, and agricultural wastewater. Aquatic macrophytes can also be used to filter aquaculture effluent.
• This helps to keep the water quality in the affected body of water at an acceptable level for fish in natural bodies of water and reservoirs.
• Aquatic macrophytes have also been used successfully to remove nutrients from fish production.
• Water hyacinth was the fastest growing plant in the wastewater treatment tank and removed the most nitrogen per square metre of growing area in the experimental use of macrophytes for removal of nutrients originating from fish production tanks.
• Submerged Egeria densa is also an efficient nitrogen-removal plant, whereas Vallisneria sp. grows slowly and removes the least amount of nitrogen.
• Macrophyte harvesting has frequently been proposed as a method of improving fish growth and size structure in lakes with dense populations of submerged macrophytes and stunted fish.

7. Sludge removal

• Sludge removal is an important process for disposing of waste in an environmentally responsible and safe manner.
• Sludge is a type of industrial waste that is most commonly found in sewage systems.
• Sludge can be solid or semisolid, and it is classified as primary or secondary.
• Natural processes such as sedimentation or chemical precipitation produce primary sludge. Secondary sludge is something that occurs from an industrial treatment or process.
• The following are the most common kinds of sludge:
  • Drinking water sludge - Wastewater treatment plants produce drinking water sludge. Because drinking water sludge is made from treated water, it is usually simple to remove.
  • Septic sludge - It is produced by sanitation systems or septic tanks. It contains contaminants such as human waste, which must be treated and removed carefully.
  • Industrial sludge - It is generated in warehouses or manufacturing plants. Because it contains high levels of metals and chemicals, it must be carefully removed.
• Organic sludge is produced by facilities that treat wastewater contamination, including sewage.
• Inorganic sludge is produced by wastewater treatment plants that treat water with sand or metal components.
• Sludge removal can be accomplished in two ways: consolidation and destruction.
  • Consolidation is the process of removing water along with sludge. This is accomplished by thickening or dewatering the sludge. Allowing the solids to settle and then draining or evaporating the water is the process.
  • Destruction is the process of oxidising sludge in order to convert it to methane. Lime dosing, which is the process of conducting oxidation or hydrothermal processes in water, can help with this.

Conclusion

In addition to the above methods, controlling external sources of nutrients is also equally important. Bioengineered shoreline technologies and the addition of a beneficial vegetation buffer can assist in absorbing runoff from the surrounding area. By keeping grass clippings, leaves, and garbage out of the water, the nutrient input from this organic material is reduced. Limiting what enters the water by switching to low-phosphate, slow-release fertilisers near resources or halting fertiliser use entirely within the watershed is advisable. It's also a good idea to keep water birds to a minimum and avoid overfeeding fish.