Technical guide · Updated June 2026
Greywater Recycling in Buildings – Compliance & Savings under Spain's RD 1085 and Barcelona's Mandate
Practical Guide
12 min read
Executive Summary
Barcelona mandates greywater recycling in buildings.
Barcelona 2026 Greywater Ordinance
From July 19, 2026, greywater recovery becomes mandatory for new buildings and major renovations:
- Residential buildings (16+ dwellings)
- Hotels, offices, gyms & other sites (595+ m³/year)
Without a plan for a greywater recycling system in your basic project submittal, your building license will not advance.
Spain's new water reuse regulation
Real decreto 1085/2024
RD 1085/2024 replaces the older 2007 framework with a risk-based approach and formal risk plans (PGRAR, Plan de Gestión del Riesgo del Agua Regenerada), clearly defined points of compliance, and referenced analytical methods.
It is closely aligned with EN 16941-2:2021, the EU's standard for building-scale greywater, which sets the framework for design, sizing, commissioning, and O&M for building-scale greywater (WCs, garden watering, cleaning, laundry).
Up to ~60% savings for offices
Leading studies show that recycling greywater can cut potable water use by approximately 30% in homes (toilet-only) and up to 60% in offices. While most OEMs claim savings of 25–45%, Caskade recommends planning for a prudent range of 30–50% in multi-family, hospitality, and student accommodation projects (showers, sinks → WCs, irrigation), validated by a technical review for each building.
Water-as-a-service Contract
Keep infrastructure off the balance sheet
Many real estate firms, building owners, and project developers do not want additional infrastructure to strain their balance sheet. Yet, the vast majority of OEMs offer only lump-sum CAPEX purchase + O&M (by the owner or a maintenance firm). This results in CAPEX burden on the balance sheet, fragmented responsibility and the QA risk may stay with the owner.
Modern service models:
- DBO (Design-Build-Operate)
- WaaS (Water-as-a-Service)
Design late = redesign
Why act early:
Late design equals plumbing re-work and licence delays in Barcelona. Nationally, RD 1085 expects a documented risk plan (PGRAR) and accredited lab methods. Treat water recycling as part of the design, not an afterthought.
Scope
Greywater • Buildings • Spain
Spain • Europe • US • MENA
Geography: Spain (national water reuse regulation RD 1085/2024), with a deep dive on Barcelona (first municipal mandate).
Buildings • Industry • Agriculture
Sector: Buildings, multifamily buildings, hotels, resorts, offices, student housing, gyms/sports centres, especially new builds and large refurbishements. Industrial/process reuse is out of scope.
Greywater • Process Water • Drinking Water
Greywater is lightly contaminated wastewater from showers/baths, washbasins and laundry, excluding kitchens and blackwater, which can be treated on-site for non-potable end uses:
Covered
- Streams (sources): Showers/baths (priority), washbasins, laundries.
- Uses (non-potable): Toilet flushing, irrigation (non-edible), cleaning.
Excluded
- Streams (sources): Kitchen water, blackwater, industrial effluents (covered in another guide)
- Potable uses
These use cases directly reflect EN 16941-2.
Why Now
Compliance • Restrictions • Costs
1 - Compliance
2025
Barcelona mandates greywater recycling in buildings.
2024
Spain revamps water reuse regulation
RD 1085/2024 both extends the legal foundation for on-site water recycling but also sets strict rules for doing so safely.
2 - Restrictions & Operations
Climate change makes extreme droughts more likely and more severe
The 2024 restrictions due to drought emergency in Catalonia in a nutshell:
- 120–200 L/person/day cap across all uses (homes, hotels, offices, municipal)
- Hotel/tourist water capped at 90–115 L per guest/day
- Swimming pools banned from filling/refilling with freshwater (very few exemptions)
- Green space irrigation banned, except minimal “survival” watering with non-potable sources
On-site reuse reduces dependence on utility water and keeps essential flows during restrictions while stabilising expenditure.
3 - Costs & Economics
30–50% potable water savings are consistently achievable
in multi-residential, hospitality, and student housing
Offices with WC-dominant loads can reach savings of up to ~60%.
Heat recovery adds extra savings: ~20–37% of building energy use.
ARCHITECTURES THAT PASS REVIEW
How It Works
In a Nutshell
Greywater (showers, sinks, laundries) is treated on-site and the purified water used for non-potable purposes: toilets, irrigation and cleaning.
in Detail
Capture
- Prioritise showers & bathtubs (large share of water footprint in buildings), add washbasins
- Incorporating laundries into the building-level recycle scheme leads to significant gains. For even better efficiency & ROI, choose recycling solutions closer to the washing machines & dryers (Caskade offers solutions ↗).
- Rainwater capturing is a great way to add available water volumes but may require civil works. Decide per project.
Treatment
- Filtration + disinfection (UV/chlorine), compact, proven for WC/cleaning.
- MBR (membrane bioreactor), tighter, steadier quality; preferred for variable-load mixed-use.
- Building: label greywater and blackwater networks
- Connectors: greywater (rainwater) buffer tank with gravity overflow, freshwater input with air gap to prevent backflow, output with overflow (and often bypass) to safely ensure supply.
- Operation: Make sure O&M handbook and logs are close to hand. Caskade's solutions come with remote monitoring based on live IoT data. Our O&M teams get notified in case of deviations allowing them to act before any impact.
Distribution
- Use the regenerated water for toilet flushing, irrigation of green spaces (non-edible), and cleaning.The distribution takes place via separate, non-potable piping (labelled) which is the standard for new buildings and major renovations anyways.
Economics
Building-level water recycling makes financial sense in most cases
Savings = current water bills* – (reduced water bills* + recycled water Tariff)
*Water bills = costs for potable water + sewer + canon
30-50% savings in utility water
across multi-residential, hospitality, and student housing when recycling shower water for toilet flushing.
Market figures and peer-reviewed studies shows ~30% (homes) and up to ~60% (offices) for toilet-only reuse. Higher rates are possible in office-heavy WC loads.
OEM figures and studies:
- Van de Walle (2023, Urban Water Research): reports up to ~30% potable reduction in households (toilet-only) and up to ~60% in offices where WC is dominant.
- OEMs claim ~25–45% reduction in potable water use. E.g. Hydraloop cites 25–45%; INTEWA reports ≈50% in some residential use-cases. (Indication; Do not use for design.)
- Smith et al. (2018, IWA/Case studies compendium): multi-residential WC reuse projects delivering ~35–45% potable reduction with automated disinfection and QA. (Grey literature but widely cited.)
- Friedler et al. (2011, Water Research): building-scale analyses show 30–50% potable substitution potential for WC reuse given typical occupancy and flush volumes; highlights operational controls as key to realising savings.
Compliance & QA
Building-level water reuse in Spain & Barcelona
BOPB 18/07/2025
Barcelona Greywater Ordinance
- Residential buldings with 16+ dwellings (new construction/significant renovation) – including multi-residential buildings, student housing, etc.
- Non-residential buldings with 595+ m³/y water demand (WCs, irrigation and cleaning) – this includes hotels, resorts, offices, spas, sports facilities, etc.
- Enforcement: 19 Jul 2026 (one year after BOPB publication)
- Design & operations must align with UNE-EN 16941-2 (EU standard for greywater reutlisation)
- Submission rule: the basic project must include the concept and pre-sizing calculations, drawings and technical specs; without it, the licence does not advance.
RD 1085/2024 (Spain's reuse regulation)
It replaces RD 1620/2007 as the national legal basis governing production, supply and use of regenerated water. Design, commissioning and operations must comply with this new, risk-based approach which requires amongst other things:
- A risk plan (Plan de Gestión del Riesgo del Agua Regenerada, PGRAR),
- defined points of measurement for compliance,
- referenced methods/quality classes and,
- in practice, ISO 17025-certified labs for quality assurance.
What to do:
- Define your point(s) of compliance in your piping & instrumentation plans.
- Build a risk plan (PGRAR) covering hazards, barriers, monitoring, corrective actions and roles.
- Align sampling to RD 1085 annex methods and keep chain-of-custody.
EN 16941-2:2021 (design & operation)
It's the base standard for building-level greywater in Europe. Use it to specify sizing, identification/labelling, commissioning (incl. cross-connection tests), O&M, and user information.
ISO 17025 (sampling & lab analysis)
Base standard for lab competence. Ensures sampling, testing, and reporting are valid, traceable, and defensible.
RD 487/2022 (aerosols)
Applies for uses that produce aerosols, for example spray/pressure cleaning. Toilets are typically low aerosol risk. RD 1085/2024 requires compliance with this regulation to protect against Legionella.
YOUR 7 NEXT DECISIONS & actions – AND HOW CASKADE HELPS
Playbook
In a Nutshell
Add greywater system to your building planning ASAP
Especially if you are within the scope of the Barcelona Greywater Ordinance, designing late can be costly.
Membrane Bioreactors (MBR) and Ultrafiltration (UF)
These are the two best choices for building-level greywater recycling. Caskade's free review helps determine which one suits your case best. Optionally add heat recovery for additional energy savings. Avoid MBBR and SBR purification.
Water-as-a-service eliminates CAPEX and reduces OPEX
WaaS (Water-as-a-service) provides a fully-managed service at zero upfront investment, reducing OPEX from day one. Alternatively, choose DBO (Design–Build–Operate) or buy the device upfront and mange O&M in-house if you want to be involved more.
Ensure compliance & safety
Both the national RD 1085/2024 regulation (referencing EN 16941-2:2021) as well as the Barcelona Greywater Ordinance require regular sampling & analysis from certified labs. Caskade's Water-as-a-service includes all required compliance and quality measurements and uses remote monitoring based on IoT data for predictive maintenance.
1) Are you in scope of the Barcelona Greywater Ordinance?
- Within Barcelona municipality, and
- New building or major renovation, and
- Residential buildings: 16+ dwellings (including multi-residential buildings, student housing), or
- Non-residential buildings: 595+ m³/y water demand* (including hotels, offices, gyms/sports facilities, student housing, malls/convention venues).
*Exempt are hospitals, nursery schools and sites already supplied with municipal regenerated water for those uses.
Measure annual WC/irrigation/cleaning demand or estimate via occupancy × days × (showers/day × litres/shower + flushes/day × litres/flush) + cleaning/irrigation. Use conservative values, e.g. litres/flush 4.5–6 L, and actual occupancy.
2) Integrate greywater recycling into your planning ASAP
Things you need to plan for:
- Dedicated greywater piping (showers, baths, washbasins)
- Plant room footprint and space for recycling device
- Non-potable, closed-loop distribution circuit & labelling (EN 16941-2)
- Points of sampling & compliance (EN 16941-2)
Caskade packages specifications aligned with EN 16941-2 so architects/MEPs can include them in the basic project definition.
3) Choose technology
Bioreactor + Ultrafiltration + Disinfection
A membrane bioreactor (MBR) provides advanced treatment. Afterwards, ultrafiltration (UF) and disinfection purify the water to highest standards.
The go-to choice for greywater recycling in buildings, especially when QA stringency is high.
Pros
- compact footprint, lower sludge volumes, consistent & low-turbidity effluent, resilient under load swings.
Cons
- higher CAPEX; membrane care.
Direct Ultrafiltration + Disinfection
Ultrafiltration membranes (0.01–0.1 µm) remove dissolved particles; UV or chlorine provides microbiological control.
Suitable when the greywater is only lightly contaminated, for example in shower-only or rainwater recycling schemes.
Pros
- compact, simple O&M, low CAPEX.
Cons
- more sensitive to variable loads; turbidity spikes can reduce effectiveness
Designs to avoid:
MBBR + UF (Moving Bed Biofilm Reactor + Ultrafiltration)
Some engineering firms scale down designs from larger municipal/industrial wastewater plants, but it's rare in buildings – too many tanks, more complex O&M.
SBR (Sequencing Batch Reactor) + Disinfection
Uncommon in buildings. Takes more operator attention, sequencing tanks don't fit well in basements, and introduces unreasonable O&M risk in real-world residential settings.
4) Choose contract model
| Buy upfront | DBO | WaaS | |
|---|---|---|---|
| Model | CAPEX + O&M | Design–Build–Operate | Water-as-a-Service |
| Cash Impact | High CAPEX; lower OPEX but requires O&M involvement (in-house/outsource) | Moderate CAPEX; bundled OPEX | Zero CAPEX; OPEX tied to performance (pay per m³, verified quality) |
| Accountability | Split (EPC vs O&M) | Provider across lifecycle | Provider; performance-based |
| Quality assurance | Owner defines PGRAR, books ISO 17025 lab, runs sampling/reporting; bears compliance risk & corrective-action costs | Operator executes QA plan, ISO 17025 sampling bundled; shared compliance with KPIs/LDs tied to pass rates | Caskade owns QA end-to-end: quarterly ISO 17025 sampling + telemetry included; |
| Time to operation | Medium (more contact points) | Faster (less contractual overhead) | Fastest (fully managed) |
| Best for | Owners with cheap capital & strong operations teams | Owners who want to reduce CAPEX strain and outsource O&M. | Owners who want a fully-managed service with zero CAPEX and lower OPEX from day one. |
5) Prepare documents for building license
Include:
- Water recycling concept, specification outline, & pre-sizing calculations,
- Layout for risers, plant room, recycling device footprint,
- QA plan, and user handbook.
Caskade procures a permit-ready bundle aligned with the Barcelona Greywater Ordinance.
6) Installation
Installation of MBR / UF devices can usually take place in 1-3 days with minimal disruption thanks to pre-developed modules. The procedure includes a cross-connection test, initial sampling and quality verification (RD 1085/2024), final certification, and the handover including maintenance instructions.
7) Operation & Verification
RD 1085/2024 requires a documented risk plan (PGRAR) and ongoing operational monitoring. Record keeping is expected and may be audited by the competent authority. The Barcelona ordinance mandates QA sampling and documentation (and availability for inspection).
- Quarterly sampling & lab analysis (E. coli at point-of-use; turbidity/SS at outlet; residual/UV logs)
- Annual legionella lab analysis if water aerosolising in cleaning
- Storage of records & ISO 17025 lab certificates
Volume metering is not explicitly mandated by regulation, but is strongly recommended (and often requested) to evidence performance, support billing/SLAs, and underpin certifications.
Caskade's Water-as-a-service offer includes all mandatory QA measurements and predictive maintenance on its platform for ease of mind.
FAQ
Common questions
Does this guide replace the technical project?
No. It prioritizes decisions and documentation teams usually demand before locking the basic project or selecting technology.
What role does UNE-EN 16941-2 play?
It structures assumptions and controls; it doesn’t replace site-specific data or comparisons across routes.
When is a comparative evaluation worthwhile?
When multiple routes matter or internal approval needs homogeneous evidence across options.
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