Rainwater harvesting, method and its types

RAINWATER HARVESTING 

Rainwater harvesting is the process of collecting, storing and then using rainwater as an alternative or complementary source to mains water. Rainwater is usually collected from rooftops and other hard surfaces which allow the volume of water to run off into channels, guttering or downpipes which divert the water into a collection tank and sometimes these are underground tanks. 

From there the water can be gravity fed or pumped into a system for use around your home or business premises. Rainwater is commonly used for watering plants and flowers, washing cars, flushing toilets and washing clothes. Filters can be added to a system to remove certain contaminants from the water such as leaves and debris. 

Methods of Rainwater Harvesting 

Broadly there are two ways of harvesting rainwater 

1. Surface runoff harvesting 

2. Roof top rainwater harvesting 

Rainwater harvesting is the collection and storage of rainwater for reuse on-site, rather than allowing it to run off. These stored waters are used for various purposes such as gardening, irrigation etc. Various methods of rainwater harvesting are described in this section. 

1. Surface runoff harvesting 

In urban area rainwater flows away as surface runoff. This runoff could be caught and used for recharging aquifers by adopting appropriate methods. 

2. Rooftop rainwater harvesting 

It is a system of catching rainwater where it falls. In rooftop harvesting, the roof becomes the catchments, and the rainwater is collected from the roof of the house/building. It can either be stored in a tank or diverted to artificial recharge system. This method is less expensive and very effective and if implemented properly helps in augmenting the groundwater level of the area. 

Need for Rooftop Rain Water Harvesting 

1. To meet the ever-increasing demand for water  

2. To reduce the runoff which chokes storm drains  

3. To avoid flooding of roads 

 4. To augment the ground water storage and control decline of water levels  

5. To reduce ground water pollution 

 6. To improve the quality of ground water  

7. To reduce the soil erosion  

8. To supplement domestic water requirement during summer, drought etc. 

Components of the Rooftop Rainwater Harvesting 

The illustrative design of the basic components of roof top rainwater harvesting system is given in the typical schematic diagram shown in Fig 1. 

The system mainly constitutes of following sub components: 

• Catchments 
• Transportation 
• First flush 
• Filter 

Catchments 

The surface that receives rainfall directly is the catchment of rainwater harvesting system. It may be terrace, courtyard, or paved or unpaved open ground. The terrace may be flat RCC/stone roof or sloping roof. Therefore, the catchment is the area, which actually contributes rainwater to the harvesting system. 

Transportation 

Rainwater from rooftop should be carried through down take water pipes or drains to storage/harvesting system. Water pipes should be UV resistant (ISI HDPE/PVC pipes) of required capacity. Water from sloping roofs could be caught through gutters and down take pipe. At terraces, mouth of each drain should have wire mesh to restrict floating material. 

First Flush 

First flush is a device used to flush off the water received in first shower. The first shower of rains needs to be flushed-off to avoid contaminating storable/rechargeable water by the probable contaminants of the atmosphere and the catchment roof. It will also help in cleaning of silt and other material deposited on roof during dry seasons Provisions of first rain separator should be made at outlet of each drainpipe. 

Filter 

There is always some skepticism regarding Roof Top Rainwater Harvesting since doubts are raised that rainwater may contaminate groundwater. There is remote possibility of this fear coming true if proper filter mechanism is not adopted. 

Secondly all care must be taken to see that underground sewer drains are not punctured and no leakage is taking place in close vicinity. 

Filters are used for treatment of water to effectively remove turbidity, color and microorganisms. After first flushing of rainfall, water should pass through filters. A gravel, sand mesh filter is designed and placed on top of the storage tank. This filter is very important in keeping the rainwater in the storage tank clean. It removes silt, dust, leaves and other organic matter from entering the storage tank. 

Advantages of rain water harvesting 

 (a) Promotes adequacy of underground water 

 (b) Mitigates the effect of drought  

(c) Reduces soil erosion as surface run-off is reduced  

(d) Decreases load on storm water disposal system  

(e) Reduces flood hazards  

(f) Improves ground water quality / decreases salinity (by dilution) 

 (g) Prevents ingress of sea water in subsurface aquifers in coastal areas 6  

(h) Improves ground water table, thus saving energy (to lift water)  

(i) The cost of recharging subsurface aquifer is lower than surface reservoirs 

 (j) The subsurface aquifer also serves as storage and distribution system 

 (k) No land is wasted for storage purpose and no population displacement is involved 

 (l) Storing water underground is environment friendly. 

Components of rain water harvesting 

 The rain water harvesting system consists of following basic components – 

 (a) Catchment area  

(b) Coarse mesh / leaf screen  

(c) Gutter 

 (d) Down spout or conduit 

 (e) First flushing device 

 (f) Filter 

 (g) Storage tank 

 (h) Recharge structure 

1 Catchment area 

 The catchment area is the surface on which the rain water falls. This may be a roof top or open area around the building. The quality of water collected from roof top is comparatively much better than collection from the ground. Rain water harvested from catchment surfaces along the ground should be used for lawn watering, flushing etc., because of increased risk of contamination. This water can also be used for recharging ground aquifers after proper filtration. 

Coarse mesh / leaf screen 

 To prevent the entry of leaves and other debris in the system, the coarse mesh should be provided at the mouth of inflow pipe for flat roofs. For slope in roofs where gutters are provided to collect and divert the rain water to downspout or conduits, the gutters should have a continuous leaf screen, made of ¼ inch wire mesh in a metal frame, installed along their entire length, and a screen or wire basket at the head of the downspout. 

Gutter 

 Gutter is required to be used for collecting water from sloping roof and to divert it to downspout. These are the channels all around the edge of a sloping roof to collect and transport rain water to the storage tank. Gutters can be of semi-circular, rectangular or trapezoidal shape. Gutters must be properly sized, sloped and installed in order to maximize the quantity of harvested rain. Gutter can be made using any of the following materials: 

 (a) Galvanized iron sheet  

(b) Aluminum sheet 

 (c) Semi-circular gutters of PVC material which can be readily prepared by cutting these pipes into two equal semi-circular channels  

(d) Bamboo or betel trunks cut vertically in half (for low-cost housing projects) 

Down Spout / Conduit 

 The rain water collected on the roof top is transported down to storage facility through down spouts / conduits. Conduits can be of any material like PVC, GI or cast iron. The conduits should be free of lead and any other treatment which could contaminate the water. The idea about the diameter of pipe required for draining out rain water based on rainfall intensity and roof area. 

 First flushing device 

 Roof washing or the collection and disposal of the first flush of water from a roof, is very important if the collected rain water is to be used directly for human consumption. All the debris, dirt and other contaminants especially bird dropping etc. accumulated on the roof during dry season are washed by the first rain and if this water will enter into storage tank or recharge system it will contaminate the water. Therefore, to avoid this contamination a first flush system is incorporated in the roof top rain water harvesting system. The first flushing device, dispose of the first spell of rain water so that it does not enter the system 

 Filter 

 If the collected water from roof top is to be used for human consumption directly, a filter unit is required to be Diversion valve to recharge/ storage to drain 17 installed in RWH system before storage tank. The filter is used to remove suspended pollutants from rain water collected over roof. The filter unit is basically a chamber filled with filtering media such as fiber, coarse sand and gravel layers to remove debris and dirt from water before it enters the storage tank. The filter unit should be placed after first flush device but before storage tank. There are various type of filters which have been developed all over the country. The type and selection of filters is governed by the final use of harvested rain water and economy. Depending upon the filtering media used and its arrangements 

Storage tank  

Whenever the rain water collected from roof top is used directly for various purposes, storage tank is required. The storage tank can be cylindrical, rectangular or square in shape. The material of construction can be RCC, ferrocement, masonry, PVC or metal sheets. Depending upon the availability of space, the storage tank can be above ground, partially underground or fully underground. The design of storage tank is dependent on many factors which are listed below:  

(a) Number of persons in the household – The greater the number of persons, more will be requirement of water. 

 (b) Per capita requirement – varies from household to household, based on standard of living. The requirement also varies with season. In summer the requirement is more in comparison to winter. Similarly, the per capita requirement is more in urban areas in comparison to rural areas. 

 (c) Average annual rainfall  

(d) Rainfall pattern – It has a significant impact on capacity of storage tank. If the rainfall is uniformly spread throughout the year, the requirement of storage capacity will be less. But if the rainfall is concentrated to a limited period in a year, the storage tanks of higher capacity will be required. 

 (e) Type and size of catchment – Depending upon the type of roofing material, the runoff coefficient varies which affect the effective yield from a catchment area. The size of the catchment also has a bearing on tank size. The more the catchment area, larger the size of storage tank. 

NECESSITY FOR RAINWATER HARVESTING 

Rain water harvesting reduces the requirement to find clean water. Most commonly used for flushing toilets and laundry, rain water can also be used for irrigation and much more. 

As for the sewer systems found in many developed areas – changing climate often makes the pressure on the sewers much higher. Flooding of lower areas occur more and more often in certain regions. Upgrading the capacity of sewers are expensive – so if every household had a rain water tank in the ground, a lot of water can be captured, rather than flooding the sewers. 

SOME MORE METHODS OF RAINWATER HARVESTING 

By collecting rainwater, we have a direct positive impact on the environment that all can see. Rainwater collection obviously reduces municipal water use, the average household is using 18% of their water for flushing the toilet and 23% for the washing machine! 

1. Rainwater Harvesting and Utilization is an alternative, and in many cases a better, water supply to the normal surface water-based supplies from reservoirs or groundwater-based supplies from wells tapped into underground aquifers. 

2. Rainwater harvesting helps manage stormwater run-off to prevent erosion, flooding and poor water quality in our lakes an stream. 

3 Rainwater can have a major impact on water supply and has been shown to be able to reduce municipal water demand enough that large new public water sourcing projects become unnecessary. 

4 In urban settings, up to 90% of water runs off because of sealed surfaces like roads, parking lots and roofs. In nature, up to 90% soaks into the ground. Rainwater harvesting systems counteract stormwater runoff and thereby reduce flooding, erosion and ground water contamination. When combined with an infiltration system like rain gardens or an in-ground system. 

5 Taking water out of our lakes, reservoirs and rives effects groundwater, irrigating with rainwater helps replenish groundwater supplies. 

6 Infrastructure projects to increase water supply use massive amounts of energy and natural resources in addition to the obvious cost to ratepayers. Rainwater harvesting and utilization helps minimize the need for these projects. 

CONS OF RAINWATER HARVESTING 

Rainwater Harvesting (RWH) is the process of collecting rainwater and storing it for future use. It is the next best thing when it comes to trying to save water, especially groundwater. But just like every good thing, RWH has its own sets of drawbacks. Let us look at some of them – 

• Initial Cost – To set up an RWH system requires an initial cost. This may vary depending on the kind of system that is being created. 
• Maintenance – An RWH system requires maintenance to ensure that no contaminants enter the storage tanks. If the tanks are not cleaned properly, it could act as a breeding ground for many pathogens and animals. 
• Unpredictable Rains – The biggest disadvantage of RWH is that the rains are not predictable. One may run out of water before the next spell of rainfall. 
• Chemicals Seepage – At times, the chemicals in the roofing material might seep into the water, thus contaminating it. Even other factors like bird droppings might pollute the water. 

• Storage Limits – The success of an RWH system depends on the size of the storage tank that is constructed/purchased. 
• Water Treatment – Most of the time, the water collected might require a few rounds of purification processes to make it completely safe to drink. 

RWH is an ancient process of collecting and storing water. It has been around since the Indus Valley civilization. It is only because of the success of this process that it has been around for so long. Yes, there are a few drawbacks, but the overall advantage of the system can benefit us in the long run.