Design Tips for Rainwater Harvesting Components

Design tips for rainwater harvesting components to collect and store the rainwater during rains in a overhead tank, or underground tank is discussed. Rainwater harvesting is natural in case of water collected in ponds in villages, but cities needs special methods to capture and store rainwater.

Design Tips for Collection of Rainwater

Water collected from roof can be a) stored for direct use b) allowed to infiltrate as groundwater recharge and c) combination of part storage and part recharge.

There are three structural components a) roof water collection system b) storage tank and c) recharge pit. The cost of the system will depends upon the cost of each of these components.

Water collected from the early showers are allowed to flow away since they contain some dirt. Once the roof top is washed clean by rainfall, the remaining water is collected. Nearly 80% of the rain fall can be easily harvested.

It is much easy to collect water from a sloping roof top simply by placing lateral gutters (semi-circular collecting pipe) to bring the collected water at one common point. Flat, concrete roof tops sometimes offer more serious challenges for collection of rain water.

Usually flat concrete roof tops are provided with at least 4 outlets (drain pipes) running along the four corners. This means the water collected from all the 4 corners pipes are to be brought at the common storage site through a network of additional pipe lines. This may spoil the aesthetics and increase cost. Also, there is a need to keep the roof top clean to prevent pollution of rain water by preventing people to use the roof top for various purposes including storing of junk items.

Design Tips for Storage Tank

Storage tanks could be constructed a) underground b) half underground and half over ground and c) over-ground depending upon its size and availability of land area.

Storage tanks could be made of a) PVC (ready made Syntex type), b) Ferro-cement tanks and c) cement concrete tanks. Cost not being a consideration, a) for small storage (<> 20,000 L), cement concrete tanks would be ideal.

Ferro-cement tanks are of least cost. These are tanks made from pasting rich cement mortar over a tank structure made from bending, shaping and binding 5 mm or so iron rods as reinforcement with an inside and out side layer of chicken mess (common wire net). Unlike cement concrete tanks, these are less prone to cracks/leakages and can be made in any shape (round, cylindrical etc.). Masons having experience in working with ferro cement are required to undertake this job. Many NGOs have such masons.

Storage tank to be used for drinking water must have a filter system at the entry point, a water extraction system for taking out water (tap, hand pump) and a draining system for cleaning the tank (manhole, drain pipe etc.) periodically.

A diversion arrangement (T-joint with valve) is required at the outlet of roof collected water pipe line to allow the water to be either disposed off (dirty water after initial rain) or stored in the storage tank through the filter system for use as drinking water.

Rain water is generally deficient in dissolved minerals necessary for human body. Some scientists have suggested that on prolonged use, low Calcium (Ca) in rain water may absorb some Ca from our body making our bones weak (also applies to some RO purified water). However, so far there has been no report of any adverse effect of using rain water as drinking water.

Design Tips for Groundwater Recharge

Artificial groundwater recharge is professed when a) there is excess source water available at site and b) quality of source water is compatible with the groundwater to be recharged.

It is always safe and desirable to construct a RWH structure, where water is partly stored for direct use and partly allowed to percolate to recharge groundwater. This helps in maintaining quality control of the water used for recharge.

The recharge structure is usually a 1- 2 cubic meter pit (could be larger when large quantity of water is to be recharged), filled with graded filter materials like cobbles, pebbles, gravel and sand in layers. The Objective is to let silt free good quality water to percolate.

Designing site specific filter beds are of critical importance. If the interstices of the filter are too small, water percolation will be slow. Conversely, filters with larger pores will allow faster percolation but water may not be free from suspended matters. For this reason, it is customary to have a pre-filter storage tank from where water is supplied to the pit as per its percolation capacity.

In hard rock areas (basalt and granite) where water occurs under water table condition in the weathered formation, there is no need to attach an extension pipe from the bottom of the recharge pit. Bur in areas where water occurs under water table condition at a deeper level (20 – 100 m bgl), it would be necessary to attach an extension pipe.

The purpose of the pipe is to allow the source water to join directly to the aquifer (water table) without being lost in the over-lying unsaturated zone. Ideally, the length of the pipe should be such that it ends near the water table which however is known to fluctuate seasonally. It is necessary, that the bottom most part of the pipe be a slotted pipe. Similarly, if the pipe extends upwards to the ground level, the top most pipe should also be a slotted pipe.

An abandoned (dry but clean) dug well in hard rock area can be used as an effective recharge pit provided, a) water table is not too far below from the bottom of the well and b) formation near the well bottom is porous and permeable and c) the water diverted in to the well for recharge is of good quality. Such recharge practice should be best done in consultation with a geohydrologist and better be avoided if there is a risk of contaminated water entering in to the well and finally to the groundwater.

Artificial groundwater recharge through a number of such recharge structures will not only increase water availability (raised water table) but will also improve water quality. If properly constructed and managed, it is possible to use a bore well both as a recharge well during the monsoon period and supply (abstraction) well during summer months.

All RWH structure will require regular monitoring and periodical (at least annual) cleaning and maintenance of its various components. It is advisable to rather not construct these RWH structures, if regular maintenance cannot be assured.

Components of Rainwater Harvesting System and their Uses

Rainwater harvesting system components are used for transporting rainwater through pipes or drains, filtration and tanks for storage of harvested water.

Components of Rainwater Harvesting System

The common components of a rainwater harvesting system are:

1. Cachments
2. Coarse mesh
3. Gutters
4. Conduits
5. First flush
6. Filters
7. Storage tanks and
8. Recharge structures

1. Catchments

The surface which directly receives the rainfall and provides water to the system is called catchment area. It can be a paved area like a terrace or courtyard of a building, or an unpaved area like a lawn or open ground. A roof made of reinforced cement concrete (RCC), galvanized iron or corrugated sheets can also be used for water harvesting.

2. Coarse Mesh

It prevents the passage of debris, provided in the roof.

3. Gutters

Channels which surrounds edge of a sloping roof to collect and transport rainwater to the storage tank. Gutters can be semi – circular or rectangular and mostly made locally from plain galvanized iron sheet. Gutters need to be supported so they do not sag or fall off when loaded with water. The way in which gutters are fixed mainly depends on the construction of the house, mostly iron or timber brackets are fixed into the walls.

4. Conduits

Conduits are pipelines or drains that carry rainwater from the catchment or roof top area to the harvesting system. Commonly available conduits are made up of material like polyvinyl chloride (PVC) or galvanized iron (GI).

5. First-flushing

A first flush device is a valve which ensures flushing out of first spell of rain away from the storage tank that carries a relatively larger amount of pollutants from the air and catchment surface.

6. Filters

The filter is used to remove suspended pollutants from rainwater collected from roof top water. The Various types of filters generally used for commercial purpose are Charcoal water filter, Sand filters, Horizontal roughing filter and slow sand filter.

7. Storage facility

There are various options available for the construction of these tanks with respect to the shape, size, material of construction and the position of tank and they are: -Shape : Cylindrical, square and rectangular.

Material of construction: Reinforced cement concrete (RCC), masonry, Ferrocement etc.

Position of tank: Depending on land space availability these tanks could be constructed above ground, partly underground or fully underground. Some maintenance measures like disinfection and cleaning are required to ensure the quality of water stored in the container.

If harvested water is decided to recharge the underground aquifer/reservoir, then some of the structures mentioned below are used.

8. Recharge structures

Rainwater Harvested can also be used for charging the groundwater aquifers through suitable structures like dugwells, borewells, recharge trenches and recharge pits.

Various recharge structures are possible – some which promote the percolation of water through soil strata at shallower depth (e.g., recharge trenches, permeable pavements) whereas others conduct water to greater depths from where it joins the groundwater (e.g. recharge wells).

At many locations, existing structures like wells, pits and tanks can be modified as recharge structures, eliminating the need to construct any fresh structures.

Some of the few commonly used recharging methods are recharging of dug wells and abandoned tube wells, Settlement tank, Recharging of service tube wells, Recharge pits, Soakaways /Percolation pit , Recharge troughs, Recharge trenches, Modified injection well.

Methods of Rainwater Harvesting -Components, Transport and Storage

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.

Rooftop Rainwater Harvesting System

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.

Fig 1: Components of Rainwater Harvesting

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 the 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, colour and microorganisms. After first flushing of rainfall, water should pass through filters. A gravel, sand and ‘netlon’ 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.

The filter media should be cleaned daily after every rainfall event. Clogged filters prevent rainwater from easily entering the storage tank and the filter may overflow. The sand or gravel media should be taken out and washed before it is replaced in the filter.

A typical photograph of filter is shown in Fig 2.

Fig 2: Photograph of Typical Filter in Rainwater Harvesting

There are different types of filters in practice, but basic function is to purify water. Different types of filters are described in this section.

Sand Gravel Filter

These are commonly used filters, constructed by brick masonry and filleted by pebbles, gravel, and sand as shown in the figure. Each layer should be separated by wire mesh. A typical figure of Sand Gravel Filter is shown in Fig 3.

Fig 3: Sand Gravel Filter

Charcoal Filter

Charcoal filter can be made in-situ or in a drum. Pebbles, gravel, sand and charcoal as shown in the figure should fill the drum or chamber. Each layer should be separated by wire mesh. Thin layer of charcoal is used to absorb odor if any. A schematic diagram of Charcoal filter is indicated in Fig 4.


Fig 4: Charcoal Filter

PVC –Pipe filter

This filter can be made by PVC pipe of 1 to 1.20 m length; Diameter of pipe depends on the area of roof. Six inches dia. pipe is enough for a 1500 Sq. Ft. roof and 8 inches dia. pipe should be used for roofs more than 1500 Sq. Ft. Pipe is divided into three compartments by wire mesh.

Each component should be filled with gravel and sand alternatively as shown in the figure. A layer of charcoal could also be inserted between two layers. Both ends of filter should have reduce of required size to connect inlet and outlet. This filter could be placed horizontally or vertically in the system. A schematic pipe filter is shown in Fig 5.

Fig 5: PVC-Pipe filter

Sponge Filter

It is a simple filter made from PVC drum having a layer of sponge in the middle of drum. It is the easiest and cheapest form filter, suitable for residential units. A typical figure of sponge filter is shown in Fig 6.

Fig 6: Sponge Filter

Methods of Rooftop Rainwater Harvesting

Various methods of using roof top rainwater harvesting are illustrated in this section.

a) Storage of Direct Use

In this method rainwater collected from the roof of the building is diverted to a storage tank. The storage tank has to be designed according to the water requirements, rainfall and catchment availability.

Each drainpipe should have mesh filter at mouth and first flush device followed by filtration system before connecting to the storage tank. It is advisable that each tank should have excess water over flow system.

Excess water could be diverted to recharge system. Water from storage tank can be used for secondary purposes such as washing and gardening etc. This is the most cost effective way of rainwater harvesting.

The main advantage of collecting and using the rainwater during rainy season is not only to save water from conventional sources, but also to save energy incurred on transportation and distribution of water at the doorstep. This also conserves groundwater, if it is being extracted to meet the demand when rains are on. A typical fig of storage tank is shown in Fig 7.

Fig 7: A storage tank on a platform painted white

b) Recharging groundwater aquifers

Groundwater aquifers can be recharged by various kinds of structures to ensure percolation of rainwater in the ground instead of draining away from the surface. Commonly used recharging methods are:-

a) Recharging of bore wells

b) Recharging of dug wells.

c) Recharge pits

d) Recharge Trenches

e) Soakaways or Recharge Shafts

f) Percolation Tanks

c) Recharging of bore wells

Rainwater collected from rooftop of the building is diverted through drainpipes to settlement or filtration tank. After settlement filtered water is diverted to bore wells to recharge deep aquifers. Abandoned bore wells can also be used for recharge.

Optimum capacity of settlement tank/filtration tank can be designed on the basis of area of catchment, intensity of rainfall and recharge rate. While recharging, entry of floating matter and silt should be restricted because it may clog the recharge structure.

First one or two shower should be flushed out through rain separator to avoid contamination. A schematic diagram of filtration tank recharging to bore well is indicated in Fig 8 .

Fig 8 :Filtration tank recharging to bore well

d) Recharge pits

Recharge pits are small pits of any shape rectangular, square or circular, contracted with brick or stone masonry wall with weep hole at regular intervals. Top of pit can be covered with perforated covers. Bottom of pit should be filled with filter media.

The capacity of the pit can be designed on the basis of catchment area, rainfall intensity and recharge rate of soil. Usually the dimensions of the pit may be of 1 to 2 m width and 2 to 3 m deep depending on the depth of pervious strata.

These pits are suitable for recharging of shallow aquifers, and small houses. A schematic diagram of recharge pit is shown in Fig 9.

Fig 9: Recharge pit

e) Soakway or Recharge shafts

Soak away or recharge shafts are provided where upper layer of soil is alluvial or less pervious. These are bored hole of 30 cm dia. up to 10 to 15 m deep, depending on depth of  pervious layer. Bore should be lined with slotted/perforated PVC/MS pipe to prevent collapse of the vertical sides.

At the top of soak away required size sump is constructed to retain runoff before the filters through soak away. Sump should be filled with filter media. A schematic diagram of recharge shaft is shown in Fig 10.

Fig 10 : Schematic Diagram of Recharge shaft

f) Recharging of dug wells

Dug well can be used as recharge structure. Rainwater from the rooftop is diverted to dug wells after passing it through filtration bed. Cleaning and desalting of dug well should be done regularly to enhance the recharge rate. The filtration method suggested for bore well recharging could be used. A schematic diagram of recharging into dug well is indicated in Fig 11 shown below.

Fig 11: Schematic diagram of recharging to dug well

g)Recharge trenches

Recharge trench in provided where upper impervious layer of  soil is shallow. It is a trench excavated on the ground and refilled with porous media like pebbles, boulder or brickbats. it is usually made for harvesting the surface runoff.
Bore wells can also be provided inside the trench as recharge shafts to enhance percolation. The length of the trench is decided as per the amount of runoff expected.

This method is suitable for small houses, playgrounds, parks and roadside drains. The recharge trench can be of size 0.50 to 1.0 m wide and 1.0 to 1.5 m deep. A schematic diagram of recharging to trenches is shown in Fig below 12.

Fig 12: Recharging to trenches

h) Percolation tank

Percolation tanks are artificially created surface water bodies, submerging a land area with adequate permeability to facilitate sufficient percolation to recharge the groundwater. These can be built in big campuses where land is available and topography is suitable.

Surface runoff and roof top water can be diverted to this tank. Water accumulating in the tank percolates in the solid to augment the groundwater. The stored water can be used directly for gardening and raw use. Percolation tanks should be built in gardens, open spaces and roadside greenbelts of urban area.