Answers on water planning

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.

There is no nationwide threshold — it is governed by state law. In NRW, § 44 LWG NRW applies: stormwater from properties first built on, paved or connected to the public sewer after 1 January 1996 is to be disposed of in accordance with § 55 (2) WHG — infiltrated, allowed to seep away or discharged into a body of water close to where it falls, unless provisions or water-management concerns stand in the way. In practice, water authorities will as a rule require the infiltration verification as soon as new surfaces are sealed within a construction project — typically from single-family homes upwards. Where discharge is made into a surface water body, and for surfaces with elevated pollution loads (load categories II and III per DWA-A 102-2, Annex A — e.g. traffic and operational areas or metal roofs), additional treatment with an emission verification (AFS63) under DWA-A 102-2 is required.

At minimum a geotechnical soil report with a determined permeability coefficient (kf-value) in accordance with DIN 18130-1, plus a figure for the mean highest groundwater level (MHGW). For the design under DWA-A 138-1, the kf-value must lie between 1·10⁻⁶ m/s and 1·10⁻³ m/s — below that, the water infiltrates too slowly — pure infiltration is then usually ruled out, and hybrid or throttled-discharge solutions need to be examined — while above that, infiltration is possible, but additional measures for pollutant retention have to be examined case by case and agreed with the permitting authority. In addition, we need the site plan with elevation data and the KOSTRA-DWD data for your location.

The kf-value 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 — DWA-A 138-1 requires at least one test location per 150 m² of infiltration-system base area, and with heterogeneous subsoil and systems longer than 10 m at least one more. The governing layer is the one through which the water actually infiltrates; in layered soils, the lower permeability governs.

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 5 km cells, with durations from 5 minutes to 7 days. Every infiltration verification must base its design on the KOSTRA data for the specific location, not on generic values. DWA-A 138-1 (White Print October 2024) requires the return period appropriate to the specific verification — the design frequency ranges from n = 0.5/a to 0.02/a (T = 2 to 50 years) — depending on the protection category (DWA-A 138-1, Tables 8 and 12) — and higher for flooding verifications. The standard does not require a blanket climate surcharge: according to the standard itself, its design approaches already contain safety margins, and no validated surcharge factor can currently be proposed.

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.

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 — agglomerations of 1,000 PE and above must ensure secondary treatment by 31 December 2035. (2) A quaternary treatment stage (removal of micropollutants) becomes mandatory for large plants. (3) Energy neutrality for municipal treatment plants of 10,000 PE and above by 2045. For SME and industrial construction projects this means: anyone planning today should already account for the stricter requirements, because approvals must remain valid beyond the transposition deadline.

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 construction projects in NRW it generally does not apply directly: the regulation concerns reclaimed municipal wastewater used for agricultural irrigation — quality classes A to D apply within that scope. For rainwater use in the garden and domestic greywater recycling, the national rules apply instead (DIN EN 16941, DWA-M 277, TrinkwV); the EU classes serve here at most as a point of orientation. The regulation also has an indirect effect on constructed wetlands with a downstream irrigation function. In Germany, the technical implementation is described by the DWA-M 1200 series (water reuse for agricultural and urban purposes, 2025).

Stormwater is to be infiltrated, allowed to seep away or discharged into a body of water close to where it falls, directly or via a sewer without mixing with wastewater, unless water-law or other public-law provisions or water-management concerns stand in the way (§ 55 (2) WHG). This is the federal-law basis for decentralised stormwater disposal. In NRW this is given concrete form by § 44 LWG.

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 is the recognised code of practice for discharges into surface waters: stormwater of pollution categories II and III generally requires treatment — specifically from sealed surfaces with an elevated pollutant load (traffic areas, industrial areas). The complementary DWA-M 179-1 (Decentralised stormwater treatment facilities, Part 1) is currently available as a draft of September 2024.

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. DWA-A 262 covers small systems up to 50 population equivalents (PE) as well as larger plants; larger systems require an extended water-law permit. Advantages: very low energy demand (depending on the terrain, without a pump or with a small dosing pump), robust technology, long service life — with good maintenance, 25 years and more are achievable — and visually unobtrusive. Disadvantage: they require space (at least 4 m² of filter area per population equivalent for vertical-flow sand filters (DWA-A 262, Table 4); other designs from 1–3 m²/PE).

The governing standard is DWA-A 262 (November 2017) for planted soil filters treating domestic wastewater. The standard covers vertical-flow and horizontal-flow systems as well as two-stage systems. Additionally: a primary treatment to DWA-A 262 (multi-chamber septic tank) for the upstream primary settlement ahead of the treatment wetland, and DWA-A 222 for small wastewater treatment plants up to 1,000 PE. The achievable treatment performance depends on system type, dimensioning and season; what governs are the requirements of the water-law permit — a project-specific performance prognosis is part of the design.

Rule of thumb: at least 4 m² of filter area per population equivalent for vertical-flow sand filters (DWA-A 262, Table 4); other designs from 1–3 m²/PE. A typical single-family home with 4 people therefore needs at least 16 m² of pure filter area, plus primary treatment (three-chamber tank) and a collector/distributor. The total footprint follows from the design — 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.

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.

A cistern used for toilet flushing, the washing machine and garden irrigation, depending on the building, often pays for itself within one to two decades — reliable only with your figures; for garden use alone, rarely. The economics depend on four factors: roof area (water availability), annual rainfall (around 870 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.

DIN 1989 (national) has governed rainwater harvesting systems in Germany since 2002 — components, design and hygiene. DIN EN 16941-2 (November 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 provides guidance on the design of systems for treating and using greywater.

Yes, expressly so. The German Drinking Water Ordinance expressly allows non-potable water systems — for toilet flushing, the washing machine, garden irrigation and cleaning purposes; Section 13 TrinkwV sets the requirements: complete separation from the drinking-water network (network separation in accordance with DIN 1989-1) and permanent “not drinking water” labelling at all draw-off points. The system must be notified to the public health office at least four weeks before installation (Section 12 TrinkwV; an informal letter suffices).

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.

Promptly after the initial consultation you receive the binding fixed-price quote — or the specific question about what's still needed for it. That is possible because we can cost standardised planning services precisely — provided the initial consultation clarifies the project, the site and the planning depth. After you commission, the agreed turnaround runs from complete documents.

Infiltration verifications start from €700 net (from €833 incl. VAT, infiltration verification (concept)) and from €1,500 net (from €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. The price follows the scope of required verifications: the base price covers the standard case under the core code. If your project triggers further requirements — say, a discharge into a watercourse and with it additional verifications — the scope expands, and the fixed price in the quote reflects it. Which codes apply to your property is exactly what the free initial consultation clarifies. You receive the binding fixed price for your specific property promptly after the initial consultation, once the construction project, location and planning depth have been clarified.

Review-ready — also called ready to submit — means: the documents are complete and formally prepared such that the responsible water authority can examine them as far as possible, without avoidable follow-up requests. 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 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.

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.

The landscaping contractor builds the system — and generally does not provide a standard-compliant infiltration verification — for that you need the calculated design in accordance with DWA standards, and the authorities expect a verifiable, traceable design. 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.

Responsibility rests with Alvaro Burgos Cifuentes, M.Sc., a registered member of the Ingenieurkammer-Bau NRW (No. 754661). Every calculation is personally reviewed, signed and stamped by him — with professional liability insurance. You therefore always know who stands behind the planning: a named, chamber-registered individual, not an anonymous office.