Rediscovering rainwater as a precious resource

Stormwater falling on roofs and roads in urban areas can be safely reused for flushing toilets, laundry, washing floors, industrial use and irrigation and even in recharging the underground water table – without posing any significant health hazards.

That is what new guidelines on Green Stormwater Infrastructure (GSI) from the ministry for public works and planning propose in a comprehensive document that includes contributions of hydrologist Marco Cremona and architect Philip Grech.

The document aims at putting to better use the 175 million cubic metres of rainwater which falls on Malta, a substantial part of which is lost by falling on impermeable surfaces in urban areas.

This is over five times the total volume of water produced by the Water Services Corporation (WSC) from desalination and groundwater sources.

And even one single storm, like that which occurred on 25 November 2021 during which 75mm of rain fell in few hours in the morning, can provide an enormous amount of water: an estimated 23.6 million cubic metres of freshwater in a single morning.

“Although it is unrealistic to design systems that can manage all this rainwater within few hours for good use, the percolated rainwater can make a positive difference,” the document states, proposing this rediscovery of a “precious resource” through sustainable stormwater infrastructure for new or retrofitted buildings, and infrastructural projects.

Urban runoff safer than rural one

Sampling in October 2021 and January 2022 shows that contrary to public perception, despite high traffic volumes and the occasional overloading of sanitary sewers during extreme storms, urban storm water is not exceptionally contaminated and generally contains low concentrations of heavy metals (including lead), fluoride and PAHs (Polycyclic Aromatic Hydrocarbons).

Samples of runoff water from roofs of schools in densely populated, heavy-traffic urban areas also showed very low concentrations of heavy metals. Urban runoff does not contain high levels of chloride and nitrate which are the major contaminants of groundwater in Malta.

The results, though not exhaustive, indicate that urban stormwater is not exceptionally contaminated, facilitating the construction of infiltration systems such as soakaways, pervious pavements, filter-strips and other means of indirect groundwater recharge in urban areas.

On the other hand, studies on the quality of runoff along valleys present a rather negative picture, due to severe contamination from animal waste nutrients, particularly nitrates. The study does not recommend groundwater recharge systems using rural runoff water.

Crucially, despite a €51 million investment in the National Flood Relief Project (NFRP) – a system of tunnels diverting rainwater to the sea – the system cannot cope with massive storms, whose frequency is bound to increase as climate patterns change radically

How to recover rainwater

Various technologies can be designed to divert, capture, and store rainwater, the most common being traditional harvesting for rain falling on roofs, which is then channelled to underground, rock-cut cisterns or tanks, for storage as second-class water.

By meeting the demand for second-class water within residences, office blocks, gardens, farms, or industrial concerns, the use of groundwater could be avoided, reducing stress on the underground aquifers. “The potential for indirect aquifer recharge systems (using urban runoff) to make a significant positive impact on the recovery and rehabilitation of Malta’s groundwater systems is huge”.”

It could also help in reducing flooding, because modern buildings have changed the way rainwater finds its way into the ground and watercourses. When rain falls on impervious surfaces, much more of it turns into runoff, which can cause flooding, pollution, and land erosion.

And GSI technologies could be utilised in urban greening projects. “It would be thoughtless to begin to plan for green spaces without provision of appropriate, properly engineered and managed water supplies, and it would be tragic if such a supply were to be dependent on abstracted groundwater or mains water, which are currently the main water sources for much of Malta’s current landscaping,” the report states.

Green roofs are also recommended as a way of preventing rainwater from being diverted into the streets or the sewer system.

Trees in urban areas, apart from beautifying surroundings, also have a fundamental role in filtering out pollutants from runoff water. In fact robust resilient trees can metabolize contaminants (heavy metals, inorganic and organic compounds) into their carbon-rich heartwoods, removing them from the runoff. Many trees are also able to remove a wide variety of pollutants from soil including metals, pesticides and organic compounds

One suggestion is the development of ‘pervious’ – porous – pavements which allow rainwater to infiltrate beneath its layers to be stored underground or diverted downstream.

A typical rainwater harvesting system schematic: a disregarded legal obligation is for all residences to have a cistern. Up until 2012, this was regulated by the Code of Police Laws, where all houses were obliged to have cisterns that could hold at least 3 cubic metres for every 5sq.m of floor area, of each room. In 2015, new rules demanded suitable wells for new or renovated buildings

Havoc of diverting rain to sewers

The report laments that over the last decades, urban runoff has been allowed to be collected and channelled into roads or sanitary sewers. “Conveying water away as quickly as possible from a development may adequately protect it from flooding but increases the risk of flooding occurring downstream.”

Although disposal of rainwater to sewers has always been illegal, for the past decades rainwater connections have often been made to the sewers, creating periodic overflows out of manhole covers and backflows in pipes during heavy storms. These overflows result in health hazards, damage to a properties and financial losses due to insurance claims and settlements.

Another disregarded legal obligation is for all residences to have a cistern. Up until 2012, this was regulated by the Code of Police Laws, where all houses were obliged to have cisterns that could hold at least 3 cubic metres for every 5sq.m of floor area, of each room. In 2015, new rules demanded suitable wells for new or renovated buildings.

A 2014 study for the Valletta 2018 Foundation had found only 15.2% of residences in Valletta had a cistern. Of these, just one in three houses collected the rainwater, which means 95% of the rain falling on residential rooftops in Valletta ends up in the sewers or as runoff.

If nothing is done to address the situation flooding can only get worse, according to the experts drafting the report.

Apart from the risks posed by climate change and a greater frequency of storm events, the four-storey blocks replacing townhouses are obliterating gardens and cisterns. “The dramatic increase in urban development in the last decades has drastically altered the physical characteristics of the landscape, increasing the sealing of land and, thereby, reducing infiltration processes.” The consequence of this has been a decrease in natural groundwater recharge.

There is now an obvious need to effectively manage the consequential increase in runoff generation, to reduce flood risks and safeguard – and where possible augment –freshwater sources.

Crucially, despite a €51 million investment in the National Flood Relief Project (NFRP) – a system of tunnels diverting rainwater to the sea – the system cannot cope with massive storms, whose frequency is bound to increase as climate patterns change radically. A case in point is the 2021 storm in November: “As a result, there was flooding across the country, with dozens of vehicles submerged, swept away, an elderly man needing airlifting, and various walls collapsing.”

The flooding was also extreme in the areas protected by the NFRP, with the report concluding that “it is evident that the NFRP does not cope with events with return period higher than five years.”