MANAGEMENT OF RAINWATER FOR THE SUPPLY OF ROOFTOP GREENHOUSES
By Bernard de Gouvello (CSTB)
In RTG operations, water requirements are often high, resulting in significant management costs. Therefore, it may be relevant to use rainwater from the greenhouses themselves, or even from the surrounding roofs located within the same plot*.
Two configurations are available to use the rainwater from the greenhouses and other surrounding roofs:
storage on the rooftop;
storage at the bottom of the building (on the ground, in the basement, or belowground).
*In the case of very large needs in terms of roof surfaces suitable for water collection, the recovery of water from surrounding roofs – i.e. outside the project plot – can be studied. A specific legal act may be required to implement this solution in order to be compatible with ongoing national regulations.
CONFIGURATION 1: ROOFTOP STORAGE
APPLICATION SCOPE AND PRINCIPLE
The storage tanks are placed directly on the flat roof near a downspout from which they collect water from the corresponding roof portion through a bypass system installed on the downspout that includes a 1-mm mesh filter (French decree of August 21st 2008).
The water inlet of the tank also serves as an overflow: excess water flows down the downspout and into the public rainwater network.
The tank is placed on an elevation stand so that the tap at its bottom can feed a watering can. This tap is also used to completely empty the tank during the winter period when the bypass system is bypassed.
A sign indicating "non-drinkable water" and an explicit pictogram (in accordance with the French NF X 08-003-1 standard) should be affixed near the tap.
In this configuration, the use of rainwater constitutes a rather marginal supplement from a quantitative point of view. Its main purpose is to educate and raise awareness of the use of alternative resources to drinking water for watering
Please note that this case may occur in particular when the downspouts are located inside the greenhouse. It may also be appropriate if the greenhouse floor is more robust than the flat roof floor and can therefore bear a higher load.
ease of installation: there is no need for a professional installer
very simple handling (no electricity required)
possibility to install several tanks (one per downspout)
Constraints and possible weak points
this system is only possible if the mechanical characteristics of the rooftop allow for it; it requires precautions when implementing it (see bellow possible weak points)
the outdoor tank(s) must be emptied in winter to avoid degradation by frost.
Possible weak points:
check the lid for tightness to prevent mosquitoes from developing. This is particularly important if the tank is located inside the greenhouse (increased risk of mosquitoes)
Prefer dark or black tanks to avoid algal growth
Ensure that the "non-drinkable water" signs are legible and replace them if necessary.
Make sure that the connection to the gutter – bypass and overflow – is made correctly (see figure bellow).
The tank must not rest on slabs because this entails several risks: tilting of the tank, breaking of the slabs, deterioration of the studs and even of the insulation material (see figure bellow). It is preferable to have it rest on a concrete slab which allows for the weight to be better distributed on the floor (see figure bellow).
Make sure that the connection to the gutter – bypass and overflow – is made correctly.
The tank must not rest on slabs because this entails several risks: tilting of the tank, breaking of the slabs, deterioration of the studs and even of the insulation material
It is preferable to have it rest on a concrete slab which allows for the weight to be better distributed on the floor.
CONFIGURATION 2: STORAGE AT THE BOTTOM OF THE BUILDING
Principle and application scope
Water from the various downspouts in the greenhouse (and from other inaccessible roofs if necessary) is channelled into a tank located at the bottom of the building. The buried network of pipes feeds the tank. A suction strainer attached to a mini buoy and connected to a booster pump by a flexible hose allows for the water to be drawn from the upper water table, which guarantees a better quality (less suspended solids). The booster pump allows for the water to be returned to the greenhouse for use via a pipe and a tap.
There are many possible variants as regards storage location and the organisation of the disconnection device. These different configurations are not specific to greenhouses. They are widely described in guides or leaflets, in particular the ASTEE (the French association of water and waste professionals) guide and the booklet published by the French Ministries of Ecology and Health (see references 2 and 4). These documents are essentially technical, so they are available in other countries.
allows to collect more water by collecting it from all the downspouts from the roofs of greenhouses and other inaccessible roofs
allows for the use of rainwater throughout the year, therefore greater substitution rates.
Constraints and possible weak points
significantly higher installation costs
a professional plumber experienced in this type of installation will be needed
a maintenance procedure – possibly performed by a third party – will have to be set up.
Possible weak points:
be careful not to connect water from the flat (accessible) roof itself to the tank, which can turn out to be complicated in practice in some cases
dedicate a clear space to the technical installation, in which the schematic diagram and the sanitary notebook of the installation can be found. These documents must be protected from deterioration by humidity
tanks should be cleaned yearly to prevent dead volumes from forming inside and the buildup of deposits
Please note metering to determine the volume of water saved is appropriate for large installations. However, if the installation includes a drinkable water supplement, two meters will be installed: one at the outlet of the booster pump, the other at the inlet of the supplemental tank (to subtract the volume of rainwater substituted for drinkable water).
 It is recommended to plasticize the schematic diagram.
References for further information on rainwater harvesting and use
Please note apart from the standard (which has a European outreach), the references given below relate to France. However, comparable elements are bound to exist in the other partner countries of the project.
AFNOR, 2018NF EN 16941-1, On-site non-drinkable water systems - Part 1 : systems for the use of rainwater.
This European standard has replaced the various standards previously existing in several European countries (Germany, Great Britain, France in particular).
ASTEE (Association Scientifique et Technique pour l’Eau et l’Environnement), 2015, Guide Technique Récupération et Utilisation de l’eau de pluie. Informations et recommandations relatives à la réalisation de dispositifs utilisant les eaux issues de toitures et stockées in situ (coordination: B. de Gouvello). Paris : ASTEE, 65 p. (https://www.astee.org/publications/guide-sur-la-recuperation-et-utilisation-de-leau-de-pluie/
This guide, intended for private individuals, construction and urban planning stakeholders (public or private project owners, project managers, design offices), as well as managers and executives of water and wastewater services, has a dual purpose: (i) to provide a synthesis of current knowledge on the subject; (ii) to present, in an organized and coherent manner, a set of information and recommendations for the implementation of a project.
de Gouvello B., Noeuveglise M., 2007, Récupération et Utilisation de l’eau de pluie dans les opérations de construction. Retour d’expériences et recommandations, Paris : ARENE IDF, 64 p.
Written before the 2008 French Order, this guide presents in detail 8 examples of projects and a methodology for carrying out projects from their design to their operation.
Ministères en charge de l’Ecologie et de la Santé, Systèmes d’utilisation de l’eau de pluie dans le bâtiment Règles et bonnes pratiques à l’attention des installateurs, 20 p.
This small booklet produced by a group of experts explains the framework of French regulations with numerous explanatory diagrams.
Arrêté du 21 août 2008 « relatif à la récupération des eaux de pluie et à leur usage à l’intérieur et à l’extérieur des bâtiments » Available on legifrance.fr, this text constitutes the core of current French regulations on rainwater harvesting and use.
de Gouvello B., 2010, La gestion durable de l’eau : Gérer durablement l’eau dans le bâtiment et sa parcelle (illustrations Jean-Marc Lauby), Paris : CSTB Editions, 129 p.
See in particular Chapter 2, specifically devoted to rainwater harvesting and use.
Alberto Campisano, David Butler, Sarah Ward, Matthew J. Burns, Eran Friedler, Kathy DeBusk, Lloyd N. Fisher-Jeffes, Enedir Ghisi, Ataur Rahman, Hiroaki Furumai, Mooyoung Han, Urban rainwater harvesting systems: Research, implementation and future perspectives, Water Research, Volume 115, 15 May 2017, Pages 195-209