Rainwater Harvesting | Geology Optional Notes for UPSC PDF Download

Introduction

• Water is essential for all life on Earth, yet only a tiny fraction is available for human use due to growing demands and population growth.
• In India, per capita water availability has significantly decreased over the years, leading to concerns about water scarcity.
• Rainwater harvesting, a traditional practice in India, involves collecting rainwater for various uses, especially in regions with limited access to clean water sources.
• There are two main types of rainwater harvesting systems: rooftop and surface systems, each serving the purpose of collecting and storing rainwater for direct use and groundwater recharge.
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  • Roof-top rainwater harvesting involves collecting runoff from roofs and storing it in tanks.
  • Surface rainwater harvesting includes collecting runoff from the ground and storing it in reservoirs or catchment systems.
• These systems are crucial for water conservation and can help mitigate water scarcity issues in various regions.

Traditional Techniques for Rainwater Harvesting

• Rainwater Harvesting in Ancient India
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  • Water was revered in ancient Indian civilization.
• Structures for Rainwater Collection
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  • Tankas in Bikaner, Rajasthan: Underground tanks lined with polished lime to store rainwater from rooftops.
  • Kundis/Kunds: Underground covered tanks in western Rajasthan, Gujarat, and parts of Uttar Pradesh for drinking water.
  • Khadins in Rajasthan: Earthen embankments to harvest rainwater for crop production and groundwater recharge.
  • Nadis: Village ponds to store water during the rainy season.
  • Kuis/Beris: Deep pits near tanks to collect seepage and harvest rainwater in areas with low rainfall.
• Water Conservation Techniques
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  • Water Soak Pits: Used to conserve and recharge groundwater, known by different names in various regions.
  • Johads in Rajasthan: Simple barriers built to store rainwater and recharge aquifers.
  • Zabo of Nagaland: Pond-like structures on terraced hills to collect rainwater for agricultural and fish rearing purposes.
  • Bamboo Drip Irrigation System: Developed by tribal farmers in Northeast India for irrigating crops with minimal water requirements.

Modern Techniques for Rainwater Harvesting

• A modern rainwater harvesting system consists of various components and processes, including a catchment surface, conveyance system, pre-storage filtration, storage container, pump, post-storage filtration/treatment, and post-storage distribution system.
• The components of a modern rainwater harvesting system are adaptable based on the intended use of the harvested rainwater.

Classification of Modern Rainwater Harvesting Techniques

• 1. Collection and Storage for Direct Use
• Rainwater is collected and stored for immediate utilization.
• Example: Capturing rainwater from rooftops and storing it in tanks for household purposes like watering plants or washing vehicles.
• 2. Groundwater Recharging
• Excess rainwater is directed to recharge groundwater reservoirs.
• Example: Allowing rainwater to percolate into the ground, replenishing underground aquifers and wells.
• The two purposes of rainwater harvesting can also be combined, where rainwater is stored for direct use, and surplus water is channeled into groundwater recharge systems.

By implementing modern rainwater harvesting techniques, communities can efficiently manage water resources by collecting rainwater for immediate needs and replenishing groundwater sources for long-term sustainability.

Surface Water Harvesting System

• Catchment Area: This area directly receives rainfall and supplies water to the system. For instance, the roof of a building is commonly utilized due to its cleanliness and safety in comparison to paved surfaces. The efficiency of water collection and quality is influenced by factors like the roof's material and effective area.
• Coarse Mesh: Placed on the roof to prevent debris from entering the system.
• Gutters: Channels located along the edges of a sloped roof to collect and direct water to storage via conduits. These can be made from materials like galvanized iron (GI) sheets, PVC pipes, bamboo, or betel trunks.
• Conduits: Pipes that transport rainwater from the roof to storage containers. It's crucial to use chemically inert materials like wood, plastic, aluminum, or fiberglass to maintain water quality.
• First Flushing: A valve designed to prevent the initial runoff, laden with pollutants from the atmosphere and catchment, from entering the system.
• Filters: Chambers filled with materials like fiber, coarse sand, gravel, and charcoal to eliminate debris and dirt particles from harvested water, ensuring water quality by removing color, silt, clay, and microorganisms.
• Storage Tank: The final destination for collected water, constructed from inert materials such as reinforced concrete, fiberglass, wood, aluminum, or stainless steel. Tanks can be underground or above ground, with construction depending on factors like daily demand, dry spell duration, catchment area, local material availability, and space constraints.

The tank is equipped with the following features for its setup and operation:- Manhole Details:

• A manhole of dimensions 0.50 m × 0.50 m is provided with a cover for access.

- Ventilation and Overflow System:

• A vent pipe or overflow pipe, featuring a screen, with a diameter of 100 mm is included.

- Drainage Arrangement:

• There is a drain pipe at the bottom of the tank, with a diameter of 100 mm, for effective drainage.

For tanks constructed underground, it is essential that a minimum of 30 cm of the tank structure remains above the ground level. This design allows for accessibility and maintenance. Additionally, to facilitate water withdrawal, a hand pump can be installed on the tank.These arrangements ensure proper functionality and accessibility of the tank, making it suitable for various applications.

Rainwater Harvesting Techniques

• Overflow Pipe:
  • Positioned at the top of a reservoir to drain excess water during heavy rainfall.
  • Should match the inlet pipe's size and fitted with wire mesh to prevent entry of insects or animals.
• Groundwater Recharging:
  • An indirect method involving replenishing aquifer supplies.
  • Requires understanding hydrological and geological aspects for selecting suitable recharge sites.
  • It entails constructing various structures.
• Recharge Pits:
  • Pits designed for refilling shallow aquifers.
  • Dimensions typically 1-2 meters wide and 3 meters deep, filled with boulders, gravel, and coarse sand.
  • An aquifer refers to water-saturated underground layers.
• Recharge Trenches:
  • Constructed where permeable strata exist at shallow depths.
  • Dimensions vary based on water availability.
  • Trenches are backfilled with filter materials.
• Dug Wells:
  • Unused dug wells repurposed for recharging.
  • Water should pass through filter media before entering the well.
• Hand Pumps:
  • Abandoned or active hand pumps utilized for shallow or deep aquifer recharging.
  • Water diverted from rooftops to hand pumps through pipes.
  • Filter media must be used to prevent clogging of recharge wells.

Estimating Rainwater Harvested Quantity

• When implementing a rainwater harvesting system, the first step involves collecting the rainfall accumulated on a surface catchment. This water is then directed into a storage tank for later non-potable use. Any excess water can either flow through a surface drainage system or a wastewater network.
• The size of the catchment area and the storage tank should be carefully considered to ensure an adequate supply of water for users during dry periods.
• The capacity of the storage system can be determined based on factors such as the available roof area and the amount of rainfall received.
• The total amount of rainwater received over an area is known as the rainwater endowment, while the portion of rainwater that can be effectively harvested is referred to as the rainwater harvesting potential.
• The rainwater harvesting potential can be calculated using the formula: Rainwater Harvesting Potential = Rainfall (mm) x Area of the catchment x Runoff Coefficient.
• The rainwater endowment of an area can be calculated as: Rainwater endowment of an area = Area of the plot (sqm) x Rainfall height (m).
• Calculations related to determining the rainwater harvesting potential of a catchment involve the utilization of the runoff coefficient. This coefficient is crucial for accounting for runoff losses due to various factors like spillage, leakage, infiltration, surface wetting, and evaporation.
• Runoff refers to the water that flows away from the catchment area following rainfall and is influenced by factors such as surface characteristics, area size, and catchment type.
• The runoff coefficient for any catchment represents the ratio of runoff to rainfall and is influenced by parameters like the materials used in roof or catchment construction, slope, soil type, land use, degree of imperviousness, surface roughness, and the duration and intensity of rainfall.

Rainwater Harvesting: Key Concepts

• Effective Harvested Quantity Calculation:
  • Rainwater harvesting potential is determined by factors such as rainfall, catchment area, and runoff coefficient.
  • Formula: Harvested Quantity = Height of rainfall x Area x Runoff Coefficient x Constant Factors
  • Example Calculation: Given data yields an effective harvested quantity of 40800 litres.
• Volume of Storage Tank Calculation:
  • The volume of the tank is calculated based on the dry season length, number of users, and daily consumption per capita.
  • Formula: V = (t x n x q)
  • Factors such as evaporation loss are considered in determining the required tank volume.
  • Example Calculation: For a family of 5 with specific consumption rates, a storage volume of 16,000 litres or 16 m3 is needed.

Quality of Harvested Water

• Rainwater, often considered the purest form of water, is suitable for direct use in various household activities.
• However, upon contact with atmospheric components and rainwater harvesting systems, it becomes contaminated with heavy metals, nutrients, dirt, sediments, and various microbiological contaminants like bacteria, viruses, and protozoa.
• Understanding the potential contaminants associated with rainwater from harvesting systems is crucial.

Contaminants of a Rainwater Harvesting System

• Quality of harvested rainwater is primarily influenced by environmental factors like topography and weather conditions where the system is located.
• The materials used in constructing the system also play a significant role in determining the quality of the harvested rainwater.

Categories of Contaminants:

• Physical Contaminants: These include dirt, dust, ash, debris, and plant materials. For instance, leaves and twigs that fall onto the roof can be washed into the harvesting system during rainfall.
• Chemical Contaminants: This category comprises reactive chemical substances released from sources like automobiles and industrial activities. An example is the deposition of pollutants from traffic emissions onto the roof surface.
• Microbiological Contaminants: Bacteria, protozoa, and viruses are part of this group. These contaminants can originate from sources like bird droppings on the roof or from the materials used in constructing the system.

Sources of Contaminants:

• Atmospheric Pollutants: Wet deposition from the atmosphere can introduce contaminants into the rainwater harvesting system.
• Roof Surface Accumulation: Dry deposition on the roof surface, such as dust and pollutants settling over time, can contribute to contamination.
• Construction Materials: Contaminants may also arise from the materials used in building the roof, potentially leaching harmful substances into the harvested rainwater.

Wet Deposition

• When rain falls to the earth's surface, it collects various substances from the atmosphere such as gases, aerosols, dust, and ash.
• Factors Influencing Rain Composition:
  • The proximity and intensity of emission sources play a crucial role. Areas near industrial zones, agricultural fields with pesticide spraying, and busy roads are more likely to have rainwater containing sulfates, nitrites, nitrates, carbon dioxide, and pesticides.
  • The interaction of rainwater with chemicals in the atmosphere and air movements is significant. Acid rain formation is a common occurrence, initiated by the absorption of sulfur oxides (SOx) and nitrogen oxides (NOx) by rainwater.
• Causes of Acid Rain:
  • Anthropogenic activities like burning fossil fuels (coal, oil), vehicle emissions, as well as natural events such as forest fires and volcanic eruptions release sulfur oxides and nitrogen oxides into the air.
  • Normally, rainwater is slightly acidic, but when it mixes with SOx and NOx, its pH drops further, resulting in acidic rain.

Dry Deposition

• Definition: Dry deposition, also known as atmospheric deposition, occurs when pollutants from both human activities and natural sources settle onto surfaces.
• Components: Dry deposition includes various elements such as dust, nitrates, nitrites, heavy metals like lead, copper, zinc, aluminum, iron, and calcium.
• Impact on Water: Accumulated dust and particulates on surfaces, such as rooftops, can contaminate rainwater with sediments, nutrients, and heavy metals.
• First Flush Phenomenon: The concentration of dry deposits is notably high in the initial moments of precipitation, known as the "first flush."

Roofing Materials

• Roofing material selection significantly impacts the quality of roof runoff as it introduces various contaminants into the collected water.
• Weathering processes and chemical reactions between rainwater and roofing components contribute to the pollution of roof runoff.
• Metals like iron-zinc, aluminum, galvanized iron, and zinc in roofing materials can lower the pH of rainwater, as evidenced by various studies.
• The increased acidity in rainwater due to roofing materials can lead to the leaching of chemicals and metals.
• Materials such as concrete, gravel, asphalt shingles, clay, or pantile tend to be alkaline, raising the pH of rainwater, which can aid in the precipitation of heavy metals.

Conveyance and Storage in Rainwater Harvesting

Conveyance (Distribution Pipes)

• Materials used in distributing pipes and plumbing fixtures play a crucial role in the quality of harvested rainwater.
• Elemental loads found in rainwater may include nickel, iron, copper, zinc, lead, arsenic, strontium, and molybdenum.
• Nickel is commonly utilized in plating taps and plumbing fittings.

Storage Tank

• The material of the storage tank significantly impacts the quality of harvested water.
• Concrete or plaster tanks aid in the precipitation and elimination of heavy metals, a process challenging in plastic or metal tanks.
• Concrete tanks can raise the pH of stored water, while plastic tanks lower it.
• Metal tanks have the potential to introduce metal loads into the water, whereas plastic tanks might contain organic compounds if they don't meet specific manufacturing standards.

MICROBIOLOGICAL CONTAMINANTS IN A RAINWATER HARVESTING SYSTEM

Microbiological contaminants in a rainwater harvesting system originate from various sources:

• Soil: Microbes can be carried into the system from the surrounding soil.
• Plant Material: Decomposed plant material on the roof and in the drainage system can introduce contaminants.
• Organisms: Dead insects and other organisms on the catchment area and storage tank can contaminate the water.
• Fecal Deposition: Droppings from birds, mice, lizards, rodents, etc., can introduce harmful bacteria.
• Airborne Microbes: Microbes can be transported by winds and deposited into the system.

The microbial contaminants found in rainwater harvesting systems include:

• Enterococci
• Fecal Coliform
• Fecal Streptococci
• E. coli
• Salmonella
• Giardia
• Cryptosporidium
• Campylobacter
• Viruses

These contaminants can pose health risks if not properly managed and treated.

Maintenance of Water Quality in Rainwater Harvesting System

• Fabricating or coating the catchment area with non-toxic materials is essential to maintain water quality. This prevents the contamination of rainwater by pollutants accumulated on the rooftop catchment.
• Ensuring the roofing surface is smooth helps in reducing the trapping of particles and pollutants, thus improving the overall quality of harvested rainwater.
• Avoiding materials like zinc and copper in roofing is crucial as they can leach and contaminate the water.
• Covering all inlets with nylon or wire mesh prevents the entry of insects, debris, and bird droppings into the storage tank, which can compromise water quality.
• Installing mesh filters at the mouth of drainpipes prevents the entry of leaves and debris into the system, maintaining the purity of collected rainwater.
• Keeping the storage tank covered to avoid sunlight exposure is important as sunlight promotes algal growth in water, affecting its quality.
• Facilitating easy cleaning of the tank by fitting an outlet pipe at the bottom level helps in maintaining water quality and system efficiency.
• Improving collected rainwater quality significantly can be achieved by diverting the first flush. Installing a first flush device before the storage container can efficiently enhance water quality.
• Proper treatment methods like chlorination, boiling, filtration, and sunlight disinfection are essential to maintain hygienic conditions and prevent the development of pathogens in stored water.

Stored Water Treatments

• Chlorination is a preferred method for treating rainwater containing color or odor. It involves using calcium hypochlorite, also known as stabilized bleaching powder, to disinfect the water by killing bacteria. The recommended dosage of calcium hypochlorite is 1 gram per 200 liters of water.
• Boiling water for 10 to 20 minutes is an effective way to eliminate pathogens present in the water.
• Exposing stored water to direct sunlight before consumption can help remove many pathogens. Solar disinfection (SODIS) is a method that utilizes the UV radiation of the sun to kill microbial contamination in water.
• Optimal disinfection through SODIS can be achieved by considering factors such as temperature, weather conditions, bottle material, water quality, and bottle shape.
• Using transparent or PET bottles is preferred over PVC bottles for SODIS, as PET bottles contain fewer UV stabilizers.
• Maximizing the efficiency of SODIS can be done by using bottles with a larger surface area.
• SODIS is particularly effective for cleaning water with high oxygen content due to the production of oxygen free radicals and hydrogen peroxides from sunlight exposure.

Rainwater Harvesting

• The Government of India emphasizes water conservation to secure water resources. Both the National Water Policy and State Water Policy recognize the importance of rainwater harvesting for national water security.
• State governments and urban development authorities across India have mandated the implementation of rainwater harvesting systems in new and existing buildings to prevent groundwater depletion.
• For instance, Tamil Nadu and Gujarat have made rainwater harvesting mandatory for certain building sizes, and the Ministry of Urban Affairs has imposed regulations based on roof area and plot size.
• Various governmental and non-governmental organizations are actively involved in developing manuals and promoting rainwater harvesting systems.

Rainwater Harvesting Overview

• Importance of Rainwater Harvesting: Rainwater harvesting is crucial for sustainable water management, especially in regions facing water scarcity. It involves collecting and storing rainwater for various uses such as irrigation, household chores, and groundwater recharge.
• Types of Rainwater Harvesting Systems: There are different methods of rainwater harvesting, including rooftop rainwater harvesting, surface runoff harvesting, and groundwater recharge systems. Each system is designed to capture rainwater efficiently based on specific needs and environmental conditions.
• Maintenance of Water Quality: Ensuring the quality of harvested rainwater is essential to prevent contamination and promote safe usage. Proper maintenance practices involve regular inspection, cleaning of storage tanks, and treatment if necessary to maintain water quality standards.

Rainwater Harvesting Systems

• Rooftop Rainwater Harvesting: This system involves collecting rainwater from rooftops and directing it to storage tanks through gutters and pipes. It is a cost-effective method commonly used in residential buildings and schools.
• Surface Runoff Harvesting: Surface runoff harvesting captures rainwater from open surfaces like roads, pavements, and landscapes. The collected water can be stored in underground tanks or used to recharge groundwater aquifers.

Maintaining Water Quality

• Regular Inspection: Periodic checks of storage tanks and filtration systems to ensure no sediment buildup or contamination.
• Treatment Processes: Employing filtration, disinfection, or other treatment methods to eliminate impurities and pathogens from harvested rainwater.