Rainwater Harvesting

– what stands in its way?

While permeable paving is most frequently used as a water attenuation facility, it can also be used for rainwater harvesting

When constructing new commercial, residential and public buildings, a rainwater harvesting system is one of the last elements to install. At this stage, many projects are either over budget or have run out of money completely, and so the idea is often abandoned, or a cheaper, shorter-lifespan option is adopted.

“Another reason is that rainwater harvesting systems are still underquoted in South Africa at the tender phase. If the requirement is for 50 000 litres of harvesting, we often note at a later stage that 10 plastic, above-ground 5 000 litre storage tanks have been used as the pricing guideline. Consideration might not have been given to the site plans and hence an understanding as to where the engineers envisaged these tanks to be placed. On sites where the tanks are designed to be placed below ground, due to durability and overall size and/ or space requirements, the plastic guideline pricing can be out by a factor of 10 or even 20 once you include below-ground installation work.”

Kretzmar maintains that rainwater harvesting is sometimes viewed as a nice-to-have as opposed to a necessity and hence gets dropped at the final stages due to updated overall cost implications. “There is little financial incentive, and no legislation that forbids the use of potable water when non-potable water could be used; however, with parts of our country either suffering from drought, or experiencing water shortages and restrictions, there has been an increased interest in residential rainwater harvesting. Unfortunately, many people only consider rainwater harvesting in times of drought, which, of course, is too late. There has to be rain to fill the tanks. We need to embrace a longer-term philosophy.”

Product offering Technicrete and Rocla, part of the Infrastructure Specialist Group, both offer different water-harvesting-friendly options that can be used independently or combined for a holistic solution. These include permeable paving solutions to capture surface water and below-ground and concrete collection tank systems for roof water capture. Rocla offers two concrete rainwater harvesting solutions that are both installed underground: • A (smaller) modular system, made up of 6 000 litre units, that can accommodate storage requirements from about 24 kℓ up to hundreds of thousands of litres. • A (larger) modular system made up of 60 kℓ rectangular tanks. These are more cost-effective than the smaller modular option but installations must be in increments of 60 000 ℓ. Below-ground concrete tanks are considerably more hygienic when compared to above-ground plastic alternatives due to the naturally cool and dark environment where most microorganisms cannot survive. With

With water shortages and the longer-term security of water supply being a serious concern for South Africans, one would think that every business, school and home would have a rainwater harvesting system. Justin Kretzmar, sales engineer at Rocla, explains why this is not always the case.

the tanks underground and out of site, vandalism is reduced and there are no problems associated with UV reduction in the overall lifespan because they are not exposed to sunlight.

Technicrete’s Aqua range of permeable paving products subscribes to a methodology whereby vertical slots between adjacent pavers provide drainage channels, allowing for stormwater to flow from the surface, through the paving and bedding layers, into a suitably designed stone layer works below. This stone layer becomes the storage ‘tank’, with the captured water replacing air cavities between adjacent stones. It is interesting to note that these air cavities can be up to 40% of the overall volume of this layer, or 40 litres of water stored per square metre in every 100 mm thick stone layer.

While permeable paving is most frequently used as a water attenuation facility, it can also be used for rainwater harvesting, by placing an impervious membrane at the base of the layerworks, capturing the water instead of infiltrating it. This water would typically be used for reuse in greywater and irrigation systems.

Potential problems “Even though permeable paving technology has been around for well over 40 years (locally for over 20 years), some engineers are still hesitant to make use of this option. Permeable paving can and does fail when it has been designed or installed incorrectly or has been placed in unsuitable locations,” says Kretzmar.

For instance, one should not use permeable paving in places where there is a lot of fine dust and pollution that can be expected to blow over the area on a regular basis. As such, there is a justifiable concern about the long-term clogging of permeable paving. With the finest material at the top and grading to larger stones at the bottom, clogging occurs when tiny particles are transported by stormwater and get caught by the smaller gravel at the top of the pavement layers. This reduces the filtration capacity through the paving layer and the functionality of permeable paving.

Other causes of failure include design and installation shortcomings where the permeable layer must contend with more surface water than it can physically process. One should ensure there is a limit of a 2:1 ratio of impermeable to permeable layering. Furthermore, contractors often do not follow correct installation instructions as regards compaction and contamination. “Maintenance and cleaning of permeable paving is very important. Maintenance should include annual sweeping of the paved area prior to the rainy season, as this will remove most of the silt that has settled on to the area over the dry season – therefore avoiding contamination of the top grit layer.

“Typical cleaning regimens should be carried out every five to ten years, or as and when the system is no longer functioning. A high-pressure spray will loosen the upper 25 mm of clogged, polluted gravel/grit, followed by a high-strength vacuum to remove this material. The fine grit layer between pavers must then be reinstated with new, clean and washed material.” adds Kretzmar.

He concludes that rainwater harvesting tanks and permeable paving have been used very successfully internationally and moderately so in South Africa, “Both systems have been very successful in other countries, especially where the requirement has been driven by local legislation and municipalities.”

Justin Kretzmar, sales engineer at Rocla

Below-ground concrete tanks are considerably more hygienic when compared Below-ground concrete tanks are to above-ground plastic alternatives considerably more hygienic when compared to above-ground plastic alternatives