Selecting the right storage container and filtration train is the central technical decision in any residential rainwater harvesting installation. The choice is shaped by available space, intended water uses, budget, and — in Italy — the physical characteristics of the site: soil type, seismic zone classification, water table depth, and the distance between the tank and the points of use.
Two Main Tank Categories
Residential installations in Italy fall into two broad categories: underground concrete or polyethylene cisterns, and above-ground storage units. Each has distinct installation requirements and maintenance profiles.
Underground Cisterns
Underground cisterns are the traditional choice in regions with dry summers — notably Puglia, Basilicata, and parts of Sicilia — where the cistern serves as the primary household water reserve. Modern versions use monolithic polyethylene tanks ranging from 1,500 to 10,000 litres for single-family homes, or reinforced precast concrete for larger volumes.
The advantages of underground installation are significant for Mediterranean climates: the soil insulates the stored water from summer heat, reducing algae growth and slowing bacterial activity. Temperatures in buried cisterns typically remain below 18°C year-round in most Italian soil conditions at depths of 1.2 metres or more.
Installation requires excavation, which means the process touches the building permit system (see the regulations article for permit categories). The tank must sit on a stable sand or gravel bed to distribute load evenly, and the surrounding fill must be compacted in layers to prevent settlement that would stress the inlet and outlet pipe connections.
Above-Ground Polyethylene Tanks
Above-ground tanks — also called rain barrels at smaller scales — are the simpler, lower-cost option. Capacities range from 200 litres (domestic barrel) to 5,000 litres for slimline vertical tanks designed for narrow side passages. Installation requires no excavation permit below certain size thresholds and can be completed without specialist contractors.
The main limitation is temperature sensitivity. In direct sun, water temperature in an above-ground tank can exceed 30°C in summer, accelerating bacterial proliferation. Dark-coloured UV-stabilised polyethylene reduces algae growth but does not solve the thermal problem. For this reason, above-ground tanks are more appropriate for garden irrigation use — where water quality requirements are lower — than for toilet flushing circuits.
Sizing the Tank
A simplified sizing approach used by Italian hydraulic engineers starts with the roof catchment area and the local rainfall distribution. The formula accounts for the collection efficiency of the roof surface (typically 0.8–0.9 for tiles and 0.9–0.95 for metal sheeting after the first-flush loss), the demand for non-potable water per day, and a storage buffer for dry periods.
For a house with a 100 m² roof in central Italy, a common starting point is a cistern in the 3,000–5,000 litre range for combined toilet flushing and garden use. A house in the drier south with a seasonal gap between October rains and the next meaningful precipitation event in autumn may require a larger buffer.
The Filtration Train
Water collected from a roof surface carries particulate matter, bird droppings, dissolved organic material from roofing tiles and gutters, and — particularly after a dry period — an initial flush of concentrated contaminants. A properly designed filtration train addresses each of these in sequence.
First-Flush Diverter
The first-flush diverter is a downpipe attachment that captures and discards the initial rainfall runoff — typically the first 1–2 mm of rain over the roof surface — before directing cleaner water into the storage tank. It is the single most cost-effective quality improvement measure. Self-cleaning versions reset automatically after each rain event. Italian manufacturers including Wisi and Graf Italia produce units compatible with standard 100 mm downpipes.
Calmed Inlet
The tank inlet should be designed to slow incoming water velocity and prevent the resuspension of settled sediment at the tank base. A submerged inlet pipe with a diffuser head achieves this. Some prefabricated cisterns include this feature; others require it to be installed separately.
Sediment Pre-Filter
An in-line sediment filter — typically a 100–200 micron mesh unit housed in the downpipe or at the tank inlet — removes particles that pass the first-flush diverter. These filters require periodic cleaning, usually every three to six months depending on the local tree cover and air quality.
Floating Suction with Fine Filter
For indoor uses (toilet flushing, washing machines), water is drawn from the middle of the tank column via a floating suction device, which avoids the sediment layer at the bottom and the surface biofilm. A fine mesh filter (typically 50–100 microns) is fitted at the pump intake.
UV Disinfection
For applications with direct human contact — washing machines handling laundry for sensitive users, or any use where droplet inhalation is possible — UV disinfection is added downstream of the fine filter. UV systems in the 6–25 watt range are standard for residential flow rates of 1–3 m³/hour. Effectiveness depends on maintaining low turbidity upstream; UV does not function correctly through turbid water.
| Component | Removes | Required for |
|---|---|---|
| First-flush diverter | Initial concentrated runoff | All installations |
| Sediment pre-filter | Particles 100–200 µm+ | All tank-fed circuits |
| Floating suction | Bottom sediment, surface film | Indoor use circuits |
| Fine filter (50–100 µm) | Fine suspended solids | WC, washing machine |
| UV disinfection | Bacteria, viruses, algae | Body contact or sensitive use |
Pump Selection and Pressure
Residential systems typically use a submersible pump inside the cistern or an external pressure pump connected to a small pressure vessel. The pressure vessel — usually 8–24 litres for residential use — reduces pump start-stop cycles and maintains stable supply pressure in the non-potable distribution circuit. Italian brands Pedrollo and DAB produce widely used units in this category.
The non-potable circuit must be physically separated from the drinking water supply and labelled throughout — Italian technical standard UNI 9182 requires identification of non-potable pipework with labels or colour coding to prevent inadvertent cross-connection.
Maintenance Schedule
The most common cause of system failure in residential installations is neglected maintenance. A practical schedule for a system used for toilet flushing and garden irrigation:
- Every 3 months: clean downpipe pre-filter mesh
- Every 6 months: inspect first-flush diverter, clean fine filter cartridge
- Annually: inspect tank interior via access hatch, check for sediment accumulation
- Every 2 years: service pump and pressure vessel
- Annually (for UV systems): replace UV lamp and quartz sleeve if turbidity has increased