Rainwater Harvesting Cisterns with EPDM — 50 Years of Water Autonomy
Maximise rainwater capture and storage with EPDM-waterproofed cisterns — the membrane that resists acid water, fill/empty cycles and soil pressure for 50+ years. WRAS/ACS certification for irrigation, washing and flush systems. 20-year warranty.
The Water That Falls and the Water That Leaks — The Problem with Conventional Cisterns
Portugal has one of the most unequal rainfall profiles in Western Europe: Minho receives up to 3,000 mm/year, the Alentejo interior less than 500 mm concentrated in five months. The cistern logic — capturing winter rain to survive summer — is ancient in Portugal. The problem is that most existing cisterns are built with materials that do not last: uncovered concrete that leaches lime, mortars that crack, plastic cisterns that degrade. The harvested water escapes, becomes contaminated, or cannot be safely used.
- Unlined concrete cisterns leach calcium hydroxide that raises water pH and forms limescale deposits; the porous surface is an ideal substrate for biofilm and algae that degrade the quality of harvested water
- Pre-fabricated plastic cisterns (PE, PP) are limited to 10-20 m³ and are not scalable — for serious harvesting on an Alentejo estate, required capacity is 50 to 500 m³
- Cementitious waterproofing mortars crack with less than 0.5 mm of movement — and cisterns always move: concrete shrinks during curing, the ground settles, seasonal thermal cycles expand and contract the structure annually
- PVC liners for buried cisterns degrade under surrounding soil pressure, acid rainwater (pH 5.5-7.0) and fill/empty cycles — Membriko documents PVC liners in buried cisterns with less than 15 years of real service life
- Open or partially exposed cisterns suffer accelerated degradation of linings without intrinsic UV stability: paints, PVC liners and polyethylene become brittle in 5-10 years of direct UV exposure
- Water loss through infiltration in poorly waterproofed cisterns can represent 10-30% of the annually captured volume — precisely the resource that justified the harvesting investment
EPDM Membrane — The 21st Century Cistern for Any Volume
EPDM is the reference material for rainwater harvesting cisterns because it simultaneously solves all the problems with alternatives: 400% elongation for substrate movements, saturated polymer chain stable in acid water contact (pH 4 to 12), intrinsic UV stability through incorporated carbon black, and no plasticisers migrating into stored water. Membriko installs cisterns of any volume — from 5 m³ domestic to 500 m³ agricultural — with a 20-year warranty and documented service life of 50+ years.
- 400% elongation accommodates any concrete crack or movement without compromising watertightness — the same 3 mm movement that cracks a waterproofing mortar goes unnoticed by an EPDM membrane
- Chemical resistance to acid rainwater (pH 5.5-7.0) documented by ASTM D471 tests at pH 4.5 and 70°C — equivalent to decades of real exposure — with no property changes
- WRAS and ACS certification for water that can be used for irrigation, vehicle washing, toilet flush systems and other non-potable uses — with documentation available for LEED, BREEAM and LiderA sustainability certifications
- Dimensioning for any cistern volume — from 5 m³ for individual housing to 10,000 m³ for large-scale municipal or agricultural reservoir
- Intrinsic UV stability (carbon black 25-35 phr) — no UV stabilisers that become exhausted; the membrane maintains the same UV resistance in 2075 as in 2025
- Resistance to soil pressure and hydrostatic pressure in buried cisterns — EPDM withstands hydrostatic pressure of 60 kPa (EN 1928 Method B), equivalent to 6 m of water head
EPDM Benefits
Preserved Water Quality — No Contamination from Lining
Rainwater harvested in an EPDM cistern maintains its physico-chemical quality throughout the entire storage period. EPDM does not leach lime (like concrete), does not release plasticisers (like PVC), and does not develop micro-cracks that are substrate for biofilm. WRAS certification guarantees no compound migration into the water. For cisterns whose water is destined for orchard, vegetable garden or garden irrigation, this absence of lining contamination is directly relevant to the quality of agricultural products.
Return on Investment in 3-8 Years
In Portugal, the price of mains water has been rising above inflation in all major municipal systems — with an outlook of further increases given growing water scarcity. A 50 m³ cistern in a Lisbon home can save €800-1,500/year in water for irrigation and washing. A 200 m³ cistern on an Alentejo estate with intensive garden irrigation saves €3,000-8,000/year. The total ROI of the EPDM cistern investment (structure + membrane) is achieved in 3 to 8 years, depending on volume, location and use.
Contribution to Sustainability Certifications
Rainwater harvesting systems with EPDM membrane contribute to the most demanding sustainability certifications: LEED (WE credit — Water Efficiency), BREEAM (Wat 01 — Water Consumption), and LiderA (Water category). Membriko provides the technical documentation required for LEED/BREEAM/LiderA certification on each project. Additionally, rainwater harvesting is mandatory in many Portuguese municipalities for new constructions above certain roof area thresholds.
Permanent Watertightness — No Infiltration for 50+ Years
The EPDM membrane installed by Membriko creates a completely watertight cistern for the service life of the concrete structure. While waterproofing mortars crack with 0.5 mm of movement and PVC liners degrade in 15-20 years, EPDM with 400% elongation accommodates any crack or settlement without loss of watertightness. The investment in rainwater harvesting — collectors, gutters, filters, concrete structure — only generates a return if the cistern actually retains the captured water.
Scalable from 5 m³ to 10,000 m³
Membriko installs EPDM rainwater harvesting cisterns of any size: domestic cisterns of 5 to 50 m³ for individual homes (buried under garden or in basement), agricultural cisterns of 50 to 500 m³ for vineyard, orchard and olive grove irrigation, condominium or commercial cisterns of 100 to 1,000 m³ for combined reuse systems, and large-scale reservoirs of 1,000 to 10,000 m³ for municipal or industrial use. The same material, the same warranty, for any scale.
Fast Installation — 1 to 5 Days Depending on Volume
Installing an EPDM membrane in an already-built cistern is significantly faster than applying waterproofing mortars (which require 3 to 7 coats with curing time between each) or PVC liner (which requires heating for conforming to irregular cistern geometries). A 20-30 m³ domestic cistern is installed in 1 to 2 days. A 200-300 m³ agricultural cistern in 3 to 5 days. The cistern can be filled immediately after work completion and seam verification.
Technical Specifications
Thickness (buried cistern)
1.5 mm (EN 1849-2)
Thickness (surface cistern / UV exposure)
1.5-2.0 mm (EN 1849-2)
Tensile strength
≥ 9 N/mm² (EN 12311-2)
Elongation at break
≥ 400% (EN 12311-2)
Tear resistance
≥ 20 N/mm (EN 12310-2)
Hydrostatic watertightness
Pass at 60 kPa (EN 1928 Method B)
Supported pH range
4 to 12 — covers all rainwater range
Service temperature
-45°C to +70°C (air) / 0°C to 50°C (immersion)
UV resistance
No cracking at 2,000 hours (EN ISO 4892-3)
Water contact certification
WRAS (UK, BS 6920) / ACS (France)
Soil pressure resistance
Pass with protection geocomposite (EN 14150)
Maximum available width
15.25 m — seam minimisation
Dimensional stability
< 1% at 70°C/6h (EN 1107-2)
Projected service life
50+ years (ERA/SKZ/Arrhenius)
CE marking
EN 13956 / EN 13967 (as applicable)
Installation Process
- 1
Technical Visit and Cistern Sizing
Membriko assesses the cistern location (buried, semi-buried or surface), dimensions and geometry (rectangular, circular, irregular), condition of existing concrete or new-build project, and projected harvesting volume based on local rainfall and catchment roof area. Membrane thickness (1.5 or 2.0 mm), certification level (WRAS/ACS for sensitive uses) and required accessories (inlet/outlet flanges, level probe, access cover) are defined.
- 2
Concrete Structure Preparation
High-pressure water jet cleaning to remove biofilm, limescale deposits and concrete laitance. Active crack injection with hydro-expansive polyurethane resin. Inactive crack filling with epoxy mortar. Sharp edge rectification (any sharp edge can progressively puncture the membrane under hydrostatic load). Surface flatness verification — irregularities greater than 5 mm are corrected. New concrete must have minimum 28 days curing before installation.
- 3
Cut Layout Design and Membrane Preparation
Cut layout is planned in the office to minimise the number of seams. For rectangular cisterns up to 15 m wide, it is frequently possible to cover the floor with a single panel without underwater seams. Wall panels are cut with 100 mm overlap onto the floor panel. Pre-formed EPDM pieces are ordered for interior and exterior corners. All penetration accessories (flanges, washers) are verified for WRAS/ACS certification where applicable.
- 4
EPDM Installation — Floor and Walls
Installation starts at the floor with the largest panel, bonded to 100% of the surface (fully adhered method) with contact adhesive specific for immersion in rainwater and WRAS certified where applicable. After the floor, walls are lined from bottom to top, overlapping the floor panel. In buried cisterns with possible external hydrostatic pressure, wall panels are equally bonded to the full surface to eliminate the risk of lifting from differential pressure.
- 5
Corners and Seams with QuickSeam
Interior and exterior corners receive pre-formed EPDM pieces (90° corners, 150×150 mm) ensuring continuous transition without stress concentrations. All seams are executed with QuickPrime Plus primer and 150 mm QuickSeam tape, applied under pressure with a silicone roller. After curing (minimum 30 minutes at 20°C), each seam is tested with a metal probe. Seams are documented with photographs. For WRAS cisterns, the seam photographic record is included in the certification documentation delivered to the client.
- 6
Penetrations — Water Inlet, Outlet, Overflow and Level Probe
Each membrane penetration (captured water inlet pipe, outlet pipe to pump, overflow, level probe, inspection access) receives an EPDM flange with compression ring creating a hermetic seal. For WRAS cisterns, flanges are equally WRAS certified. Membriko does not use mastic or silicone as primary sealant in any penetration — only EPDM certified compression flanges and rings.
- 7
Access Cover and Cistern Covering
The EPDM membrane on the walls is terminated at the cistern coping level with a mechanical termination bar in anodised aluminium or stainless steel. The access cover — in HDPE, aluminium or concrete with stainless ring — is coordinated with the membrane installation to ensure watertightness at the membrane/cover transition. For buried cisterns with concrete covering, Membriko coordinates with the main contractor the installation of the protection geocomposite before the covering slab is cast.
- 8
Fill Test, Verification and Final Documentation
The cistern is filled with water (from the mains or from harvesting after rain) and the level is monitored for 48-72 hours. Any level drop above normal evaporation triggers a leak inspection. Membriko delivers the as-built photographic record, fill test report, WRAS/ACS certificates for membrane and accessories (where applicable), and 20-year warranty certificate. Documentation is formatted for inclusion in LEED/BREEAM/LiderA sustainability dossiers.
Installation Techniques
Buried Cistern — Bonded EPDM with Protection Geocomposite
For cisterns buried under garden, pavement or structure, Membriko installs EPDM bonded to the full interior surface with a drainage and protection geocomposite on the outer face (between EPDM and surrounding concrete, on the walls). The geocomposite protects the membrane from point soil loads and roots, and drains any groundwater that may accumulate between the cistern and the ground, preventing external hydrostatic pressure that might try to lift the membrane. Standard method for all buried cisterns.
Vantagens
- Membrane protection against point soil loads and roots — especially important in cisterns buried near trees
- Drainage of external groundwater preventing differential pressure on the membrane
- Completely invisible installation after backfilling — the cistern integrates into garden or structure
Desvantagens
- Requires additional protection geocomposite (extra cost and installation time)
- Not suitable for cisterns with groundwater level above the cistern floor without an active drainage system
- Inspection or repair access requires opening the cover — inherent limitation of all buried cisterns
Surface or Semi-Buried Cistern — EPDM with UV Exposure
For cisterns above ground or semi-buried with part of the membrane exposed to solar radiation, Membriko uses 1.5-2.0 mm EPDM with intrinsic UV stability through carbon black (25-35 phr). The membrane is installed with mechanical perimeter fixing at the cistern coping and bonded across the full underwater surface. The UV exposure zone (above maximum water level) is checked at each annual inspection.
Vantagens
- No protection geocomposite requirement on exposed faces
- Easier access for membrane inspection and maintenance in the exposure zone
- Intrinsic UV stability — no UV stabilising additives that become exhausted over time
Desvantagens
- The direct UV exposure zone must be inspected annually to detect superficial mechanical damage
- Greater heating of stored water in sun-exposed cisterns (may promote algae growth without opaque cover)
- Requires higher-resistance mechanical perimeter fixing for the non-underwater zone
Large-Volume Agricultural Cistern — EPDM with Field Seams
For agricultural cisterns of 100 to 10,000 m³ with geometries exceeding the available membrane width (15.25 m), Membriko executes field seams on site. Field seams in EPDM are executed with the same QuickSeam system as factory seams — the difference is execution on site rather than in a controlled environment. Membriko trains installers specifically for field seams in large cisterns, with systematic verification of each seam.
Vantagens
- Dimensioning for any volume — there is no practical upper limit for EPDM cisterns
- Field seams with strength equivalent to factory seams when correctly executed
- Systematic verification of all seams with probe before installation completion
Desvantagens
- Field seams require controlled temperature and humidity conditions — must not be executed below 5°C or in rain
- Higher risk of installation error in field seams versus factory seams — mandatory training and supervision
- Installation time proportional to number of seams — large cisterns require more working days
Comparison with Other Membranes
| Característica | EPDM | Cementitious waterproofing mortar | PVC liner | Pre-fabricated PE cistern |
|---|---|---|---|---|
| Resistance to acid water (rainwater pH 5.5-7.0) | Excellent — stable from pH 4 to 12, no documented degradation (ASTM D471) | Moderate — progressive carbonation in contact with acid water over 10-20 years | Good — but plasticisers migrate and contaminate stored rainwater | Good — but limited to small volumes and degrades with UV in surface cisterns |
| Crack and concrete movement accommodation | Excellent — 400% elongation, accommodates cracks up to 20 mm without watertightness loss | Poor — cracking with 0.3-0.5 mm movements; no elastic accommodation capacity | Moderate — 150-250% elongation; fails at wide cracks | N/A — independent cistern without concrete substrate |
| UV stability (surface cistern) | Excellent — carbon black 25-35 phr; 50+ year service life in direct UV exposure | Good — UV resistant but cracks with thermal movements | Poor — UV degrades plasticisers, making liner rigid in 5-10 years of direct exposure | Moderate — PE can be UV stabilised but with additives that become exhausted over time |
| Maximum achievable volume | Unlimited — field-seamed membranes for any size; examples of 10,000 m³ | Unlimited — but execution quality difficult to guarantee over large surfaces | Large — but underwater PVC seams are common failure points | Limited to ~20 m³ — transport and installation of larger cisterns is impractical |
| WRAS/ACS certification for non-potable use | Yes — WRAS and ACS certification available; LEED/BREEAM/LiderA documentation | Generally not — product-specific, verify with manufacturer | Some products — verify individually; plasticisers in water are a concern for food irrigation | Yes for food-grade PE — limited to small volumes |
| Accumulated cost over 30 years (relative index) | 100 — single installation, no specific membrane maintenance | 150 — reapplications for cracking and surface retreatment | 160 — full replacement after 15-20 years | 200 — total replacement after 20-25 years; demolition/excavation cost |
Performance in the Portuguese Climate
Alentejo and Algarve — Water Scarcity and Agricultural Cisterns
With less than 500 mm of annual rainfall concentrated between October and March, the Alentejo and Algarve are the regions of Portugal where investment in rainwater harvesting cisterns has the greatest economic return. An Alentejo estate with 5 hectares of olive grove or vineyard can capture 150 to 400 m³ per year from roof and shed precipitation — enough to irrigate gardens, supply animals and reduce dependence on boreholes in drought years. Membriko has extensive experience in agricultural cisterns of 50 to 500 m³ in this region.
North and Centre — Abundant Rainfall and Domestic Cisterns
In northern Portugal (Minho, Douro Litoral), rainfall can reach 2,000-3,000 mm/year. This context inverts the problem: it is not scarcity but management of peak rainfall flows that motivates the cistern. A 20-50 m³ cistern buried under the garden of a house in Minho captures water from a 100 m² roof, reduces runoff flows that overload municipal networks and supplies water for garden irrigation during the dry summer months. EPDM for buried cisterns in the North must resist the high water table in this region — external hydrostatic pressure that the fully bonded membrane accommodates without lifting.
Lisbon and Metropolitan Area — Cisterns for Condominiums and New Buildings
The Legal Framework for Urbanisation and Building (RJUE) and several municipal plans require rainwater harvesting in new buildings in the Lisbon metropolitan area. Membriko installs 50 to 500 m³ cisterns in collective housing, commercial and mixed-use buildings, generally buried in the basement or under the courtyard. WRAS certification of the membrane satisfies water quality requirements for permitted uses (irrigation, washing, toilet flushing) and the technical documentation is formatted for municipal licensing processes.
Portuguese Interior — Rural Tourism with Certified Sustainability
Rural tourism and eco-tourism developments in Portugal have increasingly adopted rainwater harvesting as a component of their ecological value proposition. Boutique hotels in the Alentejo, biodynamic estates in the Douro and eco-lodges in the Algarve use rainwater harvesting systems as a demonstration of sustainable practices — and LEED/BREEAM certification requires technical documentation that Membriko's WRAS EPDM provides in full. The EPDM cistern is simultaneously an efficient water management solution and a certifiable differentiation argument in the responsible tourism market.
Portugal — Industrial and Commercial Buildings with Large Roof Areas
Warehouses, distribution centres, factories and commercial properties with roofs of 1,000 to 50,000 m² have a rainwater harvesting potential of 600 to 30,000 m³/year that is rarely exploited. Membriko sizes and installs large-volume cisterns (100 to 2,000 m³) for these buildings, with simple treatment systems (pre-filtration filters, first-flush diverters) for use of captured water in industrial process cooling, facility washing and green area irrigation. Reduction in industrial water cost can represent 15 to 40% of the cistern investment per year.
Frequently Asked Questions
The annual harvestable volume is calculated by the formula: V = P × A × Cf × E, where P is mean annual rainfall (mm/year), A is the captured roof area (m²), Cf is the roof runoff coefficient (0.75-0.90 for pitched roofs with tile or sheet, 0.80-0.95 for flat roofs with membrane) and E is the filtration system efficiency (typically 0.85-0.95). Example: house in Évora with 120 m² of roof: 550 mm/year × 120 m² × 0.85 × 0.90 ≈ 50 m³/year. Membriko performs this sizing calculation free of charge during the technical visit.
Yes. Rainwater is chemically purer than mains water in many parameters: no residual chlorine, no agricultural nitrates (which mains water can contain in rural areas), no distribution heavy metals. WRAS/ACS EPDM membrane introduces no contaminants. For irrigation of food products (vegetables, fruit), it is recommended that the first flush (first-flush diverter) be discarded to eliminate roof deposits accumulated during the dry period, and that stored water be periodically verified by bacteriological analysis if storage is prolonged (over 30 days).
The EPDM membrane installed by Membriko has a documented service life of over 50 years — the same or greater than the service life of the cistern concrete structure. Membriko issues a 20-year installation warranty. EPDM longevity data (ERA studies, SKZ, Arrhenius modelling) document negligible degradation in permanent water immersion over 50+ years — no replacement, no reapplication, no specific membrane maintenance.
Yes — this is one of the most common applications for Membriko. Installing EPDM in an existing concrete cistern (even 30-50 years old) permanently solves the problems of cracking, lime leaching and biofilm. Membriko assesses the concrete condition during the technical visit: active cracks are injected with polyurethane resin before installation; the surface is cleaned and prepared; EPDM is bonded to the full surface. The cistern recovers 100% watertightness and water quality improves immediately.
In a growing number of Portuguese municipalities, urban planning regulations require rainwater harvesting in new buildings above certain construction area thresholds. Lisbon, Porto, Cascais, Sintra and other AML municipalities have provisions to this effect. Additionally, RJUE (Legal Framework for Urbanisation and Building) and SCE (Energy Certification System) are increasingly incorporating water management requirements. Membriko verifies the specific regulatory requirements of each municipality during the project phase.
Yes. Membriko always recommends a pre-filtration system before water enters the cistern: an automatic first-flush diverter to eliminate roof deposits accumulated during the dry period, a coarse particle filter (100-200 µm) to remove leaves and large debris, and a sand or cartridge filter (50-100 µm) for fine particles. This system is not part of Membriko's supply (which focuses on the cistern and membrane) but Membriko can recommend specialist suppliers and coordinate installation.
Yes. EPDM with 400% elongation adapts to any cistern geometry — rectangular, circular, trapezoidal, irregular. For circular cisterns, the membrane is installed in gores with vertical seams or in a spiral, depending on size. For free-form or irregular geometry cisterns, EPDM is cut and seamed on site to conform to the exact cistern shape. Membriko designs the cut layout for each cistern individually, minimising the number of seams.
WRAS EPDM (certified for water contact) is formulated without plasticisers, halogenated solvents or heavy metals that could migrate into stored water. Standard roofing EPDM may contain these compounds in quantities irrelevant for roofing applications (no water contact) but potentially problematic in prolonged contact with water for irrigation or consumption. Membriko specifies WRAS/ACS EPDM in all cisterns whose water is destined for food crop irrigation, animal contact or any sensitive use. The additional cost of WRAS EPDM versus standard EPDM is marginal — typically 5-10%.
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