12 October 2026

Underground rainwater tanks: a technical guide to water harvesting and corporate water management

The water crisis that hit Italy this summer, between climate change and extreme temperatures, has shown once again how efficient management of water resources in the industrial, agricultural and construction sectors has become increasingly strategic in recent years. 

The need is increasingly to store substantial volumes of water safely. Companies, however, do not always have the space required for large tanks and reservoirs. Installing underground rainwater tanks is therefore today the engineering standard for optimising surface space and ensuring that resources are preserved over time. 

The advantages of underground water storage 

Placing storage tanks below ground level solves two fundamental problems: one logistical (optimising surface space) and one biological (preserving the fluid). 

The surrounding ground acts as a natural thermal insulator, keeping the water at a constant temperature between 8 °C and 12 °C. This thermal stability, combined with the complete absence of direct solar radiation, inhibits photosynthesis and prevents the proliferation of algae and photosynthetic bacteria. 

Compared with above–ground solutions, underground storage offers further structural benefits: 

• no footprint on yards and goods-handling areas; 
• protection from the weather, winter frosts and accidental mechanical damage; 
• stability of the non-potable water intended for secondary uses, which retains its organoleptic and physico–chemical properties. 

Construction materials: the properties of high–density polyethylene (HDPE) 

Tanks intended for burial for rainwater harvesting are mainly produced by rotational moulding. The resulting monolithic structure, free of joints or welds, ensures complete water–tightness and eliminates the critical points subject to leaks or structural failure under load. 

The high–density polyethylene (HDPE) they are made of offers several advantages that make it suitable for contact with the ground: 

• electrochemical stability: unlike metals, polyethylene does not suffer from galvanic corrosion or rust caused by stray currents or soil moisture; 
• chemical inertness: the material does not alter the chemical composition of the stored water and withstands any aggressive substances present in the backfill; 
• structural flexibility: the external reinforcing ribs distribute the lateral earth pressures and the internal hydrostatic pressures evenly. 

Sizing criteria and filtration system 

The efficiency of a water harvesting system depends on correctly calculating the tank volume. Sizing must balance the yield of the catchment with the water demand of the user. 

The key parameters for determining the useful volume are: 

• mean annual rainfall (mm), taken from the historical rainfall data of the local weather station; catchment area (m2): the horizontal extent of the roofs or yards draining to the tank; 
• runoff coefficient (C): a dimensionless value that accounts for the surface material (e.g. C = 0.9 for tiled or membrane roofs; C = 0.8 for asphalt paving); 
• annual water demand (m3): calculated from the users to be served or from the planned washing/irrigation cycles. 

A widely used empirical formula for calculating the tank volume (V) suggests sizing the storage for between 21 and 30 days of demand or of rainfall yield: 

V = min (annual supply; annual demand) × 0.06 

Di Camillo Srl support for corporate water management 

Choosing the most suitable storage system requires careful analysis of rainfall volumes, soil type and local regulatory constraints. Di Camillo Srl offers farms, industrial plants and construction companies a dedicated technical assistance and consultancy service. 

Our technicians assist you in selecting the underground high-density polyethylene tanks, the filtration systems and the certified first–flush tanks best suited to the specific needs of your site or plant. 

Di Camillo Srl technicians and engineers are available for technical advice and a tailored quotation.