Rainwater Harvesting

Overview

Rainwater Harvesting plant as water recycling reuse plant

Category / type: water recycling plant, non-potable use.
Application: domestic, commercial, industrial.
Capacity: from 1000 up to 10.000 litre.
Pump: flow 1-15 l/s – hydraulic head 10 – 45 m.
Normative references and guarantees: E DIN 1989-1:2000-12;
Material: concrete, polyethylene, polypropylene, glass-reinforced plastic (grp).
Drinking water resource protection and cost savings.


Rainwater harvesting is a very common system used all over the world since ancient time; in some areas, such as rural, it is the main water supply source. It is also often used in construction (public, residential or commercial), for irrigation, car washing and other non-potable use.
Building roofs are the main source for recycling rainwater. Rain water harvesting works diverting rainwater from drainpipe upstream to sewer or body of water.
It is possible to recycle water in basin or small lake near the building area.
An integrated approach combines the construction of houses with the installation, underground or above ground, of the storage tank.
Underground installation is a better solution because improves thermal insulation and reduce the plant footprint.
The amount of available water depends on local climatic conditions, rainfall and roof surface (and its physical characteristics like porosity).
Pumps or pressure systems provide water feeding into the house through an apposite network for non-potable uses. Storage tank can be integrated with water main or well pumps.
Screening and further treatment as filters and sterilizer are needed to a safe use of rainwater.

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How It Works

The main sizing parameter is the storage tank volume.
It is a balance between needs and available water. The annual water volume, availablefor use, depends on roof net surface, its construction material characteristics, average annual rainfall, water losses for filtration, cleaning operations and overflow.

∑i (Si Φi) I c

where:

Si [m2] horizontal surface;
Φi [-]: runoff coefficient (range 0-1)
I [mm per year]: annual rainfall
c [-]: loss coefficient

According to the surface type, the runoff coefficient PHI is evaluated, for example, as in following table:

  • 0.9 – sloping roof, clay smooth shingle;
  • 0.8 – sloping roof, slate concrete or rough shingle
  • 0.8 – flat roof, no gravel
  • 0.6 – flat roof, gravel
  • 0.5 – paved surface
  • 0.5 – extensive green roof
  • 0.4 – green roof
  • 0.3 – intensive green roof

Annual rainfall (l) changes according to local climatic conditions. A typical value medium value ranges from 250 up to 1200 mm per year for Europe countries.
Loss coefficient (c) depends on components efficiency (specified by the manufacturer). Values for c can teach even 98%.
Water requirement is calculated according to population equivalent served and/or to the garden surface (where collected water is used for irrigation). Watering needs approximately 0.5 cubic metres per square metre per year. Other uses, like toilet flush, car washing, washing machine, etc. are possible.
Project value (VM) corresponds to the lower of the two calculated values. If demand is greater than available water the deficit is bridged by water main or well water. Otherwise, surplus is discharged.
Mean duration of dry weather (T) is calculated as

T = (365-G)/12

where G is the number of rainy days in a year.

Minimum tank volume is:
W = T (VM/365)

where VM is the project value

Depending on use, filters and sterilizer (e.g UV rays) are added to ensure outlet water quality.
Pumps or pressure systems provide water for domestic use in a dual water networks.
Submersible or self-priming pumps are commonly installed, according to specific requirements.
Flow and hydraulic head are the main parameters affecting sizing: flow is the sum of water consumption values of each delivery point; the maximum theoretical requirement must be reduced according to the contemporaneity coefficient, because delivery point should not be used all together.
Hydraulic head depends on pipe type and length, diameter, flow and the height difference.
When tank is empty, it requests switches to the water main. The operation can be automated through a control system as PLC, valves, level and pressure sensors.

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Operation and maintenance

The periodical tank cleaning is the most important maintenance operation. It consists on sand and other sediments removal from the basin bottom. Drainpipes and gutters unclogging improves water quality helping filters and preserving the roof useful life. Regular cleaning is carried out yearly, before the greatest rainfall season. A general check should be performed after more intense events.
During designing phase a particular attention must be used to manhole covers accessibility for all maintenance operations, but also for unauthorized use. Padlocks and safety locks or manhole with warning sign can represent a good solution to prevent an improper stored water use.
Filters maintenance is periodically required (replacement or cleaning depending by models), as for disinfection system (UV lamp replacement), for pump or pressure system.
Measures to identify non-potable water network must be provided (e.g. warning sign on withdrawal points and different colour for pipes).

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Regulatory framework

Currently there are no rules governing use of rainwater. It is free and doesn’t require concession.
E DIN 1989-1:2000-12 German standard defines sizing methods and technical requirements: it specifies how to calculate storage tank depending on climatic conditions and on water demands.
Local regulations are increasingly demanding recycling water system for building environmental sustainability and efficiency. For some local regulations tax relief is expected.

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Features and benefits

Water scarcity is faced in many parts of the world, making livelihoods, human health and entire ecosystems under threat. Water recycling, if possible, can safeguard the environment and provides some cost saving. Lower drinking water demand reduces the operating costs of water supply and sewerage.
Rain water harvesting for recycling is an excellent solution for sustainable social housing, public, commercial and industrial constructions.
A significant portion of the water requirement (up to 50%) may be replaced by non-potable water. Rainwater, for example can be used for toilet flushing, washing machine, car washing (domestic and industrial), housecleaning, etc.
Use of recovered water for personal hygiene, washing dishes, and food preparation is absolutely prohibited.
Rainwater allows also to reduce the consumption of detergents and cleaners and limescale problems, thanks to its low hardness.

Advantages (benefits)

  • system simplicity;
  • drinking water resource saving;
  • free water source;
  • no limescale problem;
  • lower water hardness, saving detergents;
  • rainwater flow reduction.

Disadvantages

  • non-potable water network;
  • no drinking water (warning sign);
  • excavation and laying;
  • electric pump or pressure system needed;
  • periodic maintenance required.

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Products in category Rainwater Harvesting

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