Rainwater recovery and reuse system. Design, sizing and standards

This is the English translation of the article taken from the InfoBuild website

A truly sustainable design cannot be separated from providing, in addition to energy efficiency measures, also a or rainwater recovery and treatment plant. We see the technical and legislative others currently

Since 2009, the issuance of the building permit is subject, in addition to the energy certification of the building, also to the structural characteristics of the building aimed at saving water and reusing rainwater. (Article 1, paragraph 288, Law 244/2007). 

The law, however, without the implementing measures, does not allow a clear and uniform application of the building rules. Fortunately, many regions have legislated on the issue of water saving, in some cases providing for obligations for new construction. In other circumstances, on the other hand, the focus has been on a voluntary scheme by encouraging, with economic contributions and facilitations, the application of the principles of green building. 

Rainwater, as well as gray water derived from washing, such as those draining showers or bathtubs, sinks, bidets, washing machines, dishwashers, can be recovered and reused to cover more than half of the drinking water needs that are normally used in a home. What? Through a purification and recycling system that re-circulates reclaimed water (with characteristics similar to distilled water) which, without limestone, would not spoil the sanitary equipment, consuming less energy. This would reduce the need for drinking water, with immediate benefits for the environment and for the pockets. 

Water: a scarce precious resource 

In the world more than 1 in 4 people (about 2.2 billion people, especially in poor countries) do not have access to safe water sources and half of the world's population (4.2 billion people) live without sanitation due to poverty and wars. These are alarming figures, which indicate the urgency of action. The implementation of the UN 2030 Agenda in Goal 6 (Ensuring the availability and sustainable management of water and sanitation for all) presides over the achievement of many other Sustainable Development Goals.

Access to drinking water, so obvious to us Westerners, is not so obvious for many peoples of the world. The ancient peoples knew well the value of water and have always worked to collect and preserve the rains.

From the ancient Romans (they exploited the impluvium of the roofs to convey the rains in a large basin located in the center of the domus, named precisely impluvium) to the Apulian Trulli (they had a similar system: the rainwater was however conveyed by the cone roofs and through the channels hidden in the masonry, they arrived in a cistern located below the floor which, in addition to being used directly for water purposes, it also had the function of thermo-hygrometric regulation), to self-built water tanks still scattered in many rural areas of the world, up to the most recent architectural inventions, such as the Warka Water by Arturo Vittori, which manages to capture the moisture contained in the air and, condensing it, collects it in the form of rainwater.

If increasingly uncertain and irregular water supplies are not enough, climate change will aggravate the situation in regions already subject to water stress, generating water stress even in those regions where resources are currently abundant. Global water use, which has already grown 6-fold in the last hundred years, is estimated to continue to grow steadily at a rate of about 1% per annum. The scale of the challenge is unprecedented. An efficient and sustainable water supply chain could provide immediate answers to the problem. The recovery of rainwater or the reuse of gray ones would guarantee a huge water saving (at least 50%) for uses that do not require drinking water, such as flushing the toilet, bidet, washing laundry, or even for irrigation of the garden.

Italy first in Europe for drinking water withdrawal 

With 9.2 billion cubic meters, Italy holds the record among the 27 EU countries, for the total volume of fresh water taken for drinking use (Istat data referring to the year 2018). In per capita terms, it is second in Europe, with 153 cubic meters per inhabitant per year (twice the European average), just behind Greece (157) and at a great distance from the following countries in the ranking: Ireland (128), Bulgaria (119) and Croatia (111). Most Member States (20 out of 27 countries) took between 45 and 90 cubic metres of fresh water per person.

In 2018, to ensure the level of consumption, 8.2 billion cubic meters were fed into the network, compared to 4.7 billion for authorized uses. The percentage of total water losses of the national drinking water distribution network is 42%: for every 100 liters introduced into the system, as many as 42 do not reach end users. Due to the poor condition of the water infrastructure, 3.4 billion cubic meters are dispersed: 156 liters per day per inhabitant. Estimating a per capita daily consumption of 215 liters (national value), the losses could guarantee water needs for about 44 million people a year. 

In the comparison between the Italian regions the gap is wide and, apart from some exceptions (Friuli, Tuscany) there is the usual and clear geographical distinction north-south. It is in fact in the south that the greatest water losses are concentrated, from Sicily going up to Umbria, with Abruzzo in the lead which totals 55.6%. The most virtuous regions are all located in the north of the country, with the Aosta Valley that stands out for "just" 22.1% of losses in the network.

This is a huge problem, which highlights the need and urgency for radical renewal interventions on the entire national water supply network, much of which is now old and dilapidated. 

The recovery of rainwater or rainwater 

At national level, the first legislation that deals with the issue is Legislative Decree 152/1999 (or framework law on water), concepts then taken up by Legislative Decree 152/2006 or environmental code. Art. 98 (water saving): "Those who manage or use the water resource adopt the necessary measures to eliminate waste and reduce consumption and to increase recycling and reuse, also through the use of the best available techniques". 

Rainwater, not drinkable, in order to be reusable for uses other than human consumption must come from the roofs of buildings, through the adoption of collection, filtering and accumulation systems. 

In Italy, per capita water consumption is estimated to be, excluding water network losses, on average equal to 215 liters per day (Istat data for the year 2018), mostly attributable to toilet drainage (29%), shower and personal hygiene (30%).

Of these, at least 50% can be replaced with rainwater recovery and wastewater reuse - for uses related to: 

toilet drain 

domestic cleaning 

washing machine 

garden irrigation

Design and sizing of the plant 

The project of a plant for the recovery and reuse of rainwater, is regulated by the UNI / TS 11445: 2012 standard ("Plants for the collection and use of rainwater for uses other than human consumption – design, installation and maintenance") which follows the European legislation, in particular the German ONE DIN 1989 of 2002.

The system must be proportionate to the supply of rainwater and the need for water, to ensure that it is balanced from an economic and efficiency point of view. The criteria for the sizing and calculation of the plant are therefore essentially two: 

need for service water 

rainwater supply

The need for service water must be calculated taking into account the complexity of the system, and is directly proportional to the number of people present (users) and the amount of equipment to be served (irrigation, toilet, washing machine, etc ...).

The contribution of rainwater is closely linked to the geographical context, to the location where the building that will host the rainwater collection system is located. 

average annual rainfall of the locality 

size (area) and type of coverage

The UNI/TS 11445:2012 standard introduces specific sizing criteria for Italian locations based on the rainfall characteristics.

Once the value of annual rainfall has been estimated, it is necessary to evaluate the percentage of precipitation that can be captured by the roof of the building. The legislation, based on the type of roof (flat, garden, sloping), gives us a corresponding coefficient of outflow, varying from a minimum of 0.4 for the green roof to a maximum of 0.9 for the sloping roof with polished tiles.

Finally, the collection area of the roof must be calculated. This will be equivalent to the projection on the horizontal plane of its perimeter, regardless of the shape and complexity of the building.

Now we have all the data available. By multiplying the value of precipitation, the collection area and the runoff coefficient [(average annual precipitation) x (collection area) x (runoff coefficient)] we can obtain the value of the rainwater supply that - together with the service needs - will allow us to correctly size the system. For the calculation of the volume of the tank it will then be useful to take into account the average dry period (number of days in the absence of rain), generally considered equal to 21 days. The resulting volume will be [(average usable volume)x(average dry period)/(days of the year)].

Components of the rainwater recovery system A rainwater recovery and reuse plant generally consists of the following basic components: 

reservoir 

filter pump 

integration with drinking water and second pipeline network 

overflow drain

The filters have the function of purifying the water from the impurities contained therein due in large part to the pollution and debris it encounters from the moment it touches the roof until it reaches the collection tank. Depending on the use of the collected water, the system can be more or less complex and consist of several elements. It may be absent if the purpose of reuse is exclusively watering the garden. 

But if the recovery of rainwater is also for domestic use (toilet and washing machine), the system can be equipped, in addition to a storage tank and by-pass for overflow, with a removable basket filter for PVC grating, submerged centrifugal electric pump for water distribution, control inverter, multi-stage filter, debacterizer with UV lamp for disinfection and mains water replenishment system.

The filters can be installed directly on the downpipe, in the tank, or in a filtering unit. They exist of the type: 

  Anti leaves 

  Self-cleaning 

  Not Self-cleaning

For pipes, the most common material is galvanized iron pipe even if synthetic materials - gerberit, polyethylene, PVC, polypropylene -, which are not afraid of rust and are easy to work, have now invaded the market of the sector.

Collection tanks: internal, underground or external

The tanks for the accumulation of rainwater generally have a capacity ranging between 1,000 and 10 thousand liters. The most common form is the cylindrical one because it allows, with the same surface exposed to air, to contain more water. The tanks are usually made of high-density polyethylene (HDPE or PE-HD), a recyclable material that complies with the regulations regarding the storage of water intended for human consumption but can also be made of steel or cement.

Depending on the location relative to the building, tanks for collecting rainwater are distinguished into: 

Underground

locations 

interiors 

A tank inside the building must have a special room in the house, but it will enjoy advantages such as ease of installation and entry into circulation. An underground tank, on the other hand, provides for higher costs and consumption due to the use of pumps, typically immersion, to lift water.

Regulatory aspects, bonuses and tax breaks 

With regard to the regulatory aspects related to the construction of rainwater recovery plants, art. 96 paragraph 4 of Legislative Decree no. 152/06, provides that "The collection of rainwater in reservoirs and cisterns at the service of agricultural funds or individual buildings for civil or industrial use is free and does not require a license or concession of water derivation, while remaining the realization of the related artifacts regulated by the laws on construction, construction of seismic areas, dams and dams and other special laws ". 

Rainwater collection systems are included among the domestic plumbing systems (DM 37/08).

The design is regulated by the following regulations: 

UNI/TS 11445:2012 – Systems for the collection and use of rainwater for uses other than human consumption – Design, installation and maintenance 

UNI 10724:2004. Rainwater collection and disposal systems. Instructions for designing and running with discontinuous elements 

UNI EN 120563:2001 Gravity exhaust systems inside buildings. Systems for the evacuation of rainwater, design and calculation.

The Superbonus 110%, at present, does not take into account the systems of recovery and reuse of domestic water (rainwater or gray). The maximum that the Government has been able to do with respect to the issue is the newborn water bonus, a contribution of 1,000 euros for the replacement of sanitary vessels with reduced drain appliances, sanitary taps, shower heads and shower columns with flow limitation appliances. 

Too bad because a real building redevelopment, a sustainable design attentive to the environment can not ignore the consideration, in addition to energy efficiency, also the aspects related to water saving. 

Water, energy and waste (including asbestos) are essential components of the circular economy, of NZEB buildings built according to the most rigorous principles of green building, the keystone for achieving a zero impact in the construction sector, with a view to achieving the European Union's 2050 decarbonization objectives. We hope that this and other regulatory chasms (such as asbestos remediation) will soon be transposed into an ad hoc and structural measure on a national scale.