WHAT IS YOUR IDEAL RAINWATER CISTERN

webmaster59208
Dec 7, 2023
5 min read

Have you read the recent new regional urban development (GSVH 2023) regulation and/or have questions regarding the choice of your rainwater cistern or tank? This info sheet will provide you your answers, split into the following topics:

· Should I choose a plastic or concrete rainwater cistern?

· What size or shape should my cistern be and how should I place it?

· Are load classes important and what other options should I add to my cistern?

One of the most important components in your rainwater system is the cistern. This is where the rainwater is stored, after collection from your roof and any pre-filtration. From the well, a pump and an optional post-filter feed the recovered rainwater into the house for use in toilets your washing machine and garden. The aim of a rainwater system is to minimize the use of 'expensive' tap water and maximize reusing free rainwater. In this sheet, we discuss the different types of cisterns and tanks that exist, their placement and options that may be useful. If you want more information on the other important components of your rainwater system, please refer to our information sheets on filtration and pumps.

The new regional urban development regulation on rainwater [1] has been in force in Flanders since the second of October 2023. Specifically, with every new construction, re-build, extension, and renovation with drainage works, a rainwater tank/cistern must be placed. The minimum size of this tank depends on your available roof area, as you can see in the table below [2]. You can calculate this legally required minimum volume yourself via the tool on the groenblauwpeil website.

The minimum volume of your well is certainly an important parameter to consider, and Livios [3] recommends reviewing this volume choice carefully. A tank that is too small empties quickly, requiring you to top up with expensive tap water. A tank that is too large is not economically viable and does not get enough overflow during heavy rainfall to keep the water clean. Apart from the minimum volume, there are other aspects of your cistern to think about. In the following pages we will discuss the difference between wells made of concrete and plastic, what load classes are, different extra options for your cistern and how best to maintain it. The optimal choices in all these matters are not always straightforward. At SmartWaterLoop, we are happy help you review exactly which cistern your project needs, and how we can maximize the returns on your sustainable investment.

CONCRETE RAINWATER CITERNS

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Concrete cisterns are a commonly used method of storing rainwater underground. Concrete is the most economical option in standard situations on easily accessible project sites. Concrete walls also soften rainwater by neutralizing its natural acidity and promoting the mineralization process [4]. The smallest concrete cisterns are usually around 5000L, which is also the minimum volume in Flanders. Other sizes vary from 7500L, 10000L or sometimes even larger. For the average house, one of these 3 versions may suffice, but for plots with larger roof surfaces, a combination of several wells is necessary.

With concrete cisterns, there are two important factors to consider, namely the weight and volume of the cistern. Correctly sizing a (concrete) cistern is important if you want to optimally match it to your home, family situation and requires sufficient expertise, which we can certainly provide via our SmartWaterCalculator. Usually, these wells are cylindrical or oval and depending on the volume, they can also be quite deep. As a result, they often require a solid amount of excavation, sometimes down to the groundwater level, and the well must be placed in the excavated hole with a heavy crane [5]. Ideally, there should be a layer of stabilizing gravel under the pit and, depending on the desired permissible load, sufficient soil above it. The pictures below show a typical concrete cistern, and an accompanying technical drawing for installation.

PLASTIC TANKS

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Rainwater cisterns are also offered in plastic. These wells, often rather called tanks, are available in a whole range of shapes, colors, and volumes. In principle, you can buy a rainwater tank from as little as 100L, but for underground versions, the volume varies rather between 1000-10000L. Due to the plastic fabrication materials, they are a lot lighter than their concrete alternatives, but this also reflects in a higher price tag.

Despite this higher price tag, these tanks are much easier to install. They usually do not need to be installed with a crane, and the installation depths are also lower and thus require less excavation. It is important, however, that a layer of stabilized sand/gravel is also laid under them, and the groundwater level is considered [10]. This is particularly the case for tanks manufactured as 'ultra-flat' (see figure above). There is also a wide variety of different shapes: cylindrical, horizontal, canotto shapes and there are even bag-like shapes that can easily fit in your basement.

EXTRA'S & MAINTENANCE

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Concrete rainwater cisterns are available in different load classes, the most common for private homes being A15 and B125, according to EU standards [12]. A15 pits are suitable to handle pedestrian and bicycle traffic, while B125 can also easily handle the carrying load of a car. These load classes also apply to the necessary manhole covers that provide easy access to the inside of the tank. With A15-class lids, several installation options are available; lids with grass cladding, aluminum/cast iron lids, or even lids with stone inlays that fit nicely into your driveway [13]. In your cistern itself, there are also several useful options that can be built in. These include a ladder for easy access, a quiet inlet, odor control siphons, check valves and pre-filters that fit inside the cistern (see our filter sheet). You can find some images of such options in the figure table below.

Maintaining your cistern/tank is something that is best done periodically. Buildwise's maintenance guide for sustainable buildings [14] recommends an annual check to see if the smell of your rainwater is still neutral. If not, it is best to clean the bottom of the tank. In addition, you can always measure the acidity of your water with PH strips; if it’s too acidic, you should first clean your roof and gutters. A final extra exercise is to clean your pre-filter at least once every six months, so less green waste ends up in your tank.

Do you still have questions about cisterns or tanks after reading this sheet? Do you want a reliable partner who will give you clear advice on which cistern is best suited for your project? Then don't wait any longer and contact SmartWaterLoop via our website!