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Article 1

Infiltration proof NRW: what the water authority checks in 2026

What must a DWA-A 138-1 infiltration proof contain? Which mistakes lead to rejection? A step-by-step guide from the engineer who writes them daily.

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Infiltration & Heavy-Rainfall Preparedness

What is the infiltration verification under DWA-A 138-1?

The infiltration verification is the calculated design of an infiltration system in accordance with the DWA-A 138-1 code of practice (October 2024). It demonstrates that the stormwater arising on a property can be infiltrated close to source without endangering neighbouring properties or the groundwater. The verification is part of the building application documents as soon as an infiltration system is planned — and a prerequisite for the water-law permit under Section 8 WHG.

From what degree of sealing is infiltration verification mandatory?

There is no nationwide threshold — it is governed by state law. In NRW, Section 44 LWG NRW applies: stormwater must be infiltrated close to source insofar as this is technically feasible and economically reasonable. In practice, the water authority requires the infiltration verification for every newly sealed surface arising within a coherent construction project — typically from single-family homes upwards. For properties with more than 800 m² of sealed surface, or where discharge is made into a surface water body, an additional emission verification under DWA-A 102-2 is required.

What soil data does the infiltration verification require?

At minimum a geotechnical soil report with a determined permeability coefficient (kf-Wert) in accordance with DIN 18130, plus a figure for the mean highest groundwater level (MHGW). For the design under DWA-A 138-1, the kf-Wert must lie between 1·10⁻⁶ m/s and 1·10⁻³ m/s — below that, infiltration is not permissible (soil passage too short), above that it is too permeable (groundwater protection not ensured). In addition, we need the site plan with elevation data and the KOSTRA-DWD data for your location.

What is the kf-Wert and how is it determined?

The kf-Wert is the soil's saturated permeability coefficient in m/s — it describes how quickly water seeps through the soil. It is determined either in the laboratory (permeameter test in accordance with DIN 18130-1) or in the field (open-end test, infiltration test in accordance with the DWA-A 138-1 annex). Important: a single value is rarely sufficient — with a heterogeneous subsoil at least two exploratory borings are needed, and the governing value is applied at the least favourable soil horizon.

What is KOSTRA-DWD and why does every project need this data?

KOSTRA-DWD 2020 is the official statistical reference of the German Meteorological Service (Deutscher Wetterdienst), providing location-specific design rainfall intensities for all of Germany — gridded into 8×8 km cells. Every infiltration verification must base its design on the KOSTRA data for the specific location, not on generic values. In its current version (White Print October 2024), DWA-A 138-1 requires by default that the design be carried out against a 30-year design event (T = 30 years), with a safety surcharge for climate change.

To the service: Infiltration verification

Sponge City & EU Water Law

What is a sponge city — and what is it not?

The sponge city concept refers to the urban-development principle of retaining, infiltrating, evaporating or reusing stormwater at the point where it arises — rather than draining it away through sewers into the receiving watercourse. Concrete building blocks include swale-trench systems, green roofs, permeable surfaces, infiltration swales, retention cisterns, tree trenches and rainwater harvesting. What a sponge city is not: a marketing label for any green construction project. A sponge city is a quantifiable hydrological strategy with a design basis — and, in NRW, increasingly a prerequisite for funding and approval.

What does EU Directive 2024/3019 mean for German construction projects?

EU Directive 2024/3019 (the recast of the Urban Wastewater Treatment Directive, in force since 1 January 2025) must be transposed into national law by Germany by 31 July 2027. Three key points: (1) The treatment obligation no longer begins at 2,000 but at 1,000 population equivalents — many municipalities and industrial sites fall under a treatment obligation for the first time. (2) A quaternary treatment stage (removal of micropollutants) becomes mandatory for large plants. (3) Energy neutrality of all municipal wastewater treatment by 2045. For SME and industrial construction projects this means: anyone planning in 2025 should already account for the stricter requirements, because approvals must remain valid beyond the transposition deadline.

What obligations arise from EU 2020/741 (water reuse)?

EU Regulation 2020/741 has been directly applicable since 26 June 2023 — it sets minimum requirements for reclaimed wastewater used in agricultural irrigation. For private and commercial construction projects in NRW, this currently means above all: anyone wishing to use greywater or treated rainwater for irrigation (including in the garden) must comply with quality classes A to D. The regulation also has an indirect effect on constructed wetlands with a downstream irrigation function.

What does Section 55(2) WHG say about stormwater disposal?

Section 55(2) WHG requires that stormwater be infiltrated, trickled away or discharged directly into a surface water body close to source, unless water-law, technical, geological or topographical reasons preclude this. This is the federal-law obligation for decentralised stormwater disposal — connection to the combined sewer system is expressly subordinate. In NRW this is given concrete form by Section 44 LWG.

What is DWA-A 102-2 and when does it apply?

The DWA-A 102-2 code of practice (December 2020) governs the emission verification for stormwater discharges into surface water bodies — specifically for fine suspended solids (AFS63) and dissolved substances. It applies as a mandatory requirement to every discharge of stormwater requiring treatment into a water body. Specifically: for sealed surfaces with an elevated pollutant load (traffic areas, industrial areas) or for direct discharge into sensitive water bodies. The complementary standard DWA-M 179-1 (2025) governs the design of decentralised treatment systems.

On the topic: Sponge City NRW

Constructed Wetlands & Nature-Based Systems

When is a constructed wetland the right solution?

Constructed wetlands (PKA) are an economical and ecologically sound solution for properties without a sewer connection — typically in rural areas, in the outlying (undeveloped) zone under Section 35 BauGB, or in the servicing gap. They are approved up to approx. 50 population equivalents (PE) under individual verification; larger systems are possible with an extended water-law permit. Advantages: no electricity costs, no pump failure, 25+ years of service life, visually unobtrusive. Disadvantage: they require space (approx. 4–6 m² per PE).

Which standards apply to constructed wetlands?

The governing standard is DWA-A 262 (November 2017, revision in progress) for planted soil filters treating domestic wastewater. The standard covers vertical-flow and horizontal-flow systems as well as hybrid systems. Additionally: DIN EN 12566-3 for the upstream small wastewater treatment plants (usually a three-chamber tank ahead of the PKA), and DWA-A 226 for sludge treatment. Treatment performance under DWA-A 262: BOD₅ > 96 %, COD > 90 %, NH₄-N > 80 % (seasonally dependent).

How much space does a constructed wetland need for a single-family home?

Rule of thumb: 4–6 m² of filter area per population equivalent. A typical single-family home with 4 people therefore needs 16–24 m² of pure filter area, plus primary treatment (three-chamber tank, approx. 6–8 m³) and a collector/distributor. In total, a property should allow 35–50 m² for the entire system, ideally with a slight gradient and no deep-rooted plants within a 3 m radius. The exact design depends on soil conditions, climate and primary-treatment performance.

Does a constructed wetland work reliably in winter?

Yes — the biological treatment in vertical-flow constructed wetlands works year-round. The microorganisms in the filter body are temperature-tolerant; the plants (reed, cattail, iris) die back above ground, while the root system and microbial activity remain active. Frost damage practically never occurs because the water circulates beneath the mulch layer. In the winter months the nitrogen turnover rate declines, but the organic load is reliably broken down. With correct design, there is no seasonal restriction on operation.

To the service: Constructed wetlands & natural pools

Rainwater Harvesting & Greywater

When is rainwater harvesting economically worthwhile?

A cistern typically pays for itself after 12–18 years when used for toilet flushing, the washing machine and garden irrigation — for garden use alone, rarely. The economics depend on four factors: roof area (water availability), annual rainfall (approx. 750–900 mm in NRW), the household demand profile and the municipality's water/wastewater charges. In municipalities with a split wastewater charge and a high freshwater price, the payback period drops significantly. For every project we calculate the specific payback as part of the hydraulic analysis.

What distinguishes DIN 1989 from DIN EN 16941-2?

DIN 1989 (national) has governed rainwater harvesting systems in Germany since 2002 — components, design and hygiene. DIN EN 16941-2 (April 2021) is the European standard for greywater reuse systems (Part 2: greywater; Part 1: rainwater). Both standards are valid alongside each other and are applied together in the planning when rainwater and greywater are used in combination. In addition, DWA-M 277 applies to the hygienic assessment of the treatment.

May rainwater be used for toilet flushing?

Yes, expressly so. Section 17 TrinkwV permits the use of rainwater for toilet flushing, the washing machine, garden irrigation and cleaning purposes — provided complete separation from the drinking-water network is ensured (network separation in accordance with DIN EN 1717). Labelling all draw-off points as “not drinking water” is mandatory. The system must be notified to the public health office before commissioning (an informal letter suffices).

What is greywater recycling and for whom is it worthwhile?

Greywater refers to lightly contaminated domestic wastewater from showers, bathtubs and washbasins — excluding toilet water (blackwater) and kitchen wastewater. Once treated, it can be reused for toilet flushing and garden irrigation. Greywater recycling is economically worthwhile above all for multi-family buildings, hotels, sports facilities and commercial properties with a high demand for toilet-flushing water. For single-family homes the investment usually cannot be amortised — here, rainwater harvesting is more economical.

To the service: Rainwater harvesting & greywater

Planning Process & Fixed Price

How does the 24-hour fixed-price offer come about?

We provide a binding fixed-price offer within 24 hours of the initial consultation, because we can cost standardised planning services very precisely. The prerequisite: the initial consultation clarifies the construction project, location, available documents and the required planning depth. On this basis you receive an offer with clearly defined services, delivery time and fixed price. Once you accept, we begin planning the same day.

What does an infiltration verification cost in NRW?

Infiltration verifications start from €700 net (€833 incl. VAT, infiltration concept) and from €1,500 net (€1,785 incl. VAT, full design); the plausibility check of an already existing verification costs €600 net (€714 incl. VAT) — each as a fixed price. You receive the binding fixed price for your specific property within 24 hours of the initial consultation, once the construction project, location and planning depth have been clarified.

What does “ready for examination” or “ready for submission” mean?

Ready for examination — also called ready for submission — means: the documents are complete and formally prepared such that the responsible water authority can examine them without further requests for additional information. It is expressly not an assurance that the authority will approve — the decision on the water-law permit rests with it alone. Specifically, we deliver: a signed and stamped calculation, a complete site plan with dimensions, a statement of conformity with the applied standard with the relevant DWA standard, a KOSTRA data extract, a soil-report reference and an effectiveness forecast with a source citation. You submit this package directly with the building application.

What documents do I need for the enquiry?

For a precise offer, three documents suffice: (1) a site plan with the planned building areas marked, (2) a geotechnical soil report or at least an indication of the planned soil investigation, (3) building-application documents or the development plan. If no geotechnical soil report is yet available, we recommend and arrange suitable experts from the NRW region. Without a report, sound planning is not possible.

What distinguishes engineering design from landscaping practical experience?

The landscaping contractor builds the system — and generally does not provide a standard-compliant infiltration verification, because that requires a professional engineering qualification (registration with the Chamber of Engineers for Construction) and the calculated design in accordance with DWA standards. A client who submits only a landscaper's sketch risks rejection in the water-law procedure or a subsequent order to make corrections. The division is clear: the engineer plans and signs, the landscaper builds. Both work hand in hand — on request we supply ready-made tender documents for the landscaping contractor of your choice.

Who is responsible for the planning?

Responsibility rests with Alvaro Burgos Cifuentes, M.Sc., a registered member of the Ingenieurkammer-Bau NRW (Nr. 754661). Every calculation is personally reviewed, signed and stamped by him — with professional indemnity insurance. You therefore always know who is liable for the planning: a named, chamber-registered individual, not an anonymous office.

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