The German Association for Water, Wastewater and Waste e.V. (DWA) has published a new standard for the dimensioning, construction, and operation of constructed wetlands for treatment of domestic and municipal wastewater. The changes to the standard are based on a wide range of experience gained in recent years in Germany and Europe. For the first time ever, the standard has been officially translated and published in English. This paper summarizes the new standard for secondary treatment of domestic wastewater with classical one-stage unsaturated vertical flow (VF) wetlands, VF wetlands with lava sand for treatment of wastewater from combined sewer systems, and actively aerated VF and horizontal flow (HF) flow wetlands. Two-stage unsaturated VF wetlands treating raw wastewater (French VF wetlands), are also included in the new standard. HF wetlands are no longer described in the standard for secondary treatment of domestic wastewater. This does not exclude their application. Existing HF wetland systems in Germany may continue to be operated so long as effluent parameters are met and proper operations and maintenance is ensured. This paper gives an overview of the new design standard, including key information on wastewater type and loading, as well as primary attributes of each wetland design.

In 2013, the DWA Working Committee KA-10 ‘Sanitation in rural areas’ decided the German DWA-A 262 design standard for constructed wetlands (DWA 2006) needed to be updated and commissioned the DWA Working Group KA-10.1 ‘Wastewater Treatment in Planted Filters’ to revise it. The 2006 standard included design advice for single-stage vertical flow (VF) constructed wetlands and horizontal flow (HF) constructed wetlands for use as a main biological treatment step for the following applications:

  • small wastewater treatment plants treating domestic wastewater for up to 50 Population Equivalents (PE);

  • wastewater treatment plants with separated sewer networks;

  • combined treatment plants for use as further biological treatment or polishing step.

The revision to the DWA-A 262 standard (DWA 2006) started with technical presentations and discussions at a public hearing in Potsdam, Germany in January 2014 and continued with regular meetings of the working group in 2014 and 2015. In April 2016, the draft standard was released for public review and comment. The review of the draft standard evoked responses from a wide range of professionals, and their feedback was presented at arbitration meetings held in 2016 and 2017. The standard was accordingly revised and recently published in German (DWA 2017a), and, for the first time, was officially translated and published in English (DWA 2017b). The title of the standard was modified to include both planted and unplanted filters.

For each technology in the new standard, specific design requirements are given, and an overview of the common combinations of different treatment steps is provided. Design advice is accompanied by schematics that show the most important dimensions for each wetland type. Specifications are also given for filter construction, selection and installation of liner, grain size characteristics of filter media commonly used in Germany, inlet and outlet construction, as well as operations and maintenance.

The new DWA-A 262 standard applies to small wastewater treatment systems up to 50 PE, as well as municipal treatment plants with combined or separated sewer networks, and treatment plants which use constructed wetlands as a polishing step. Guidance for wastewater treatment plants that are operated seasonally (during summer months only) is also included in the standard. The new standard considers other European standards and changes that have occurred in laws and regulations and takes into account recent findings on primary treatment using multi-compartment septic tanks.

In addition to the classical one-stage unsaturated VF wetland design, the standard has been expanded to include the following technologies:

  • two-stage unsaturated VF wetlands which receive raw wastewater in the first stage, i.e. French VF wetlands (based on experience in France);

  • VF wetlands with lava sand for treatment of combined sewer flows (based on experience in Germany);

  • two-stage unsaturated VF wetlands receiving primary-treated wastewater (based on experience in Austria);

  • two-layer filter trenches; and

  • actively aerated VF and HF wetlands (based on experience in the USA and in Germany).

HF wetlands are no longer described in the DWA-A 262 standard for secondary treatment of domestic wastewater since other wetland types have been shown to provide superior effluent water quality. This, however, does not exclude their application. Existing HF wetland systems in Germany may continue to be operated so long as effluent requirements are met and proper operations and maintenance of the system is ensured.

The standard provides design advice for achieving increased total nitrogen removal. Long-term phosphorus removal cannot be expected without a separate treatment step (not the subject of this standard). Pathogen removal is principally possible, but currently there is insufficient data to establish design specifications.

The DWA-A 262 design standard applies to central European climatic conditions only. Changes to design recommendations may be necessary for use in other climates. Regions with permafrost are considered fundamentally unsuitable for the technologies described in this standard. This paper summarizes and discusses the key design considerations for a variety of treatment wetland technologies. The reader is referred to the design standard (70 pages) in its entirety for all required design engineering information (DWA 2017a, 2017b), which can be purchased online in English at: http://www.dwa.de/dwa/shop.

Wastewater type and loading

The standard considers only constructed wetlands for treatment of domestic and municipal wastewater, including:

  • small wastewater treatment systems for domestic wastewater (up to 50 PE);

  • wastewater treatment systems serving fewer than 50 PE with significant sewer networks with ballast water;

  • municipal wastewater treatment plants with either separated or combined sewer networks;

  • combined wastewater treatment plants providing additional biological treatment or polishing; and

  • wastewater treatment plants that are only operated in the summer (seasonal operation).

Equations for calculating maximum wastewater flow for separate sewer networks, hourly peak flow for municipal wastewater treatment plants, determination of extraneous water and stormwater, and maximum wastewater flow, are given in the standard.

The presumptions for per capita pollutant loads are based on experience in Germany. Presumed wastewater generation for dimensioning is 150 L per person per day for wastewater and 75 L per person per day for greywater. The presumed specific pollutant loads for wastewater are presented in Table 1. Advice for wastewater flows from combined sewer networks are provided in the German Standard ATV-A 128 (DWA 1992).

Table 1

Specific mass loads per population equivalent in g/PE · d for wastewater and greywater

ParameterRaw wastewateraAfter pretreatment in a septic tank, settling pond, or Imhoff tank with a retention time of ≥2 h at QTr,h,maxdAfter primary treatment with a raw wastewater filterbAfter pretreatment with an aerated settling pondcGreywater, median valueseGreywater, average valuesf
BOD5 60 40 10 30 18 31 
COD 120 80 25 60 47 57 
TSS 70 25 16 13 
TKN 11 10 4.4 (>12 °C) 8.5 — 
TN 11 10 10 10 
TP 1.8 1.6 1.6 1.6 0.5 0.4 
ParameterRaw wastewateraAfter pretreatment in a septic tank, settling pond, or Imhoff tank with a retention time of ≥2 h at QTr,h,maxdAfter primary treatment with a raw wastewater filterbAfter pretreatment with an aerated settling pondcGreywater, median valueseGreywater, average valuesf
BOD5 60 40 10 30 18 31 
COD 120 80 25 60 47 57 
TSS 70 25 16 13 
TKN 11 10 4.4 (>12 °C) 8.5 — 
TN 11 10 10 10 
TP 1.8 1.6 1.6 1.6 0.5 0.4 

aFrom ATV-DVWK-A 198 (ATV-DVWK 2003).

cCalculated from Hasselbach (2013).

dQTr,h,max refers to the maximum hourly wastewater volumetric flow rate in a sewer network, for areas with a separated sewer network.

eFrom DWA-A 272 (DWA 2014).

All treatment plants described in the standard are intended to fulfill the wastewater treatment requirements according to Size Class 1, Appendix 1, Part C of the German Wastewater Ordinance (AbwV 2004). Size Class 1 refers to plants treating less than 60 kg BOD5 per day, which is approximately 1,000 PE when a raw wastewater BOD5 content of 60 g/PE · d is used. According to the German Wastewater Ordinance, required effluent concentrations for Size Class 1 treatment plants are 150 mg/L chemical oxygen demand (COD) and 40 mg/L BOD in four out of five randomly collected samples (i.e. 80% compliance). However, some VF filters and aerated HF filters are actually suitable for further nitrification according to Size Class 3 requirements (which is equal to 10,000 PE or a biochemical oxygen demand (BOD5) load of 600 kg/d) for producing NH4-N ≤10 mg/L at filter effluent water temperatures of at least 12 °C.

Pretreatment

For larger municipal wastewater treatment plants, an inlet sand trap and self-cleaning screen should be considered. For systems treating flows from combined sewer networks, a pebble trap should be installed upstream of the treatment plant. All pretreatment options must equalize peak flows and ensure safe and sustainable removal of suspended solids. Design specifications for multi-compartment septic tanks, rotting tanks, settling ponds, Imhoff tanks, and aerated settling ponds are given in detail in the standard (DWA 2017b). Raw wastewater filters are also defined as a pretreatment technology; these filters are used as part of the two-stage French VF wetland which receive raw wastewater in the first stage. Pretreatment technologies which generate sludge must receive routine maintenance and sludge removal as required. Specific details for sludge removal are also provided in the standard (DWA 2017b).

Liners

All planted and unplanted filters must be constructed with a watertight lining on the bottom of the filter basin and side walls. Polyethylene liners may be used; they should be water-resistant, ≥1.5 mm thickness, UV-resistant, and flexible, and must be protected on both sides of the liner with a mineral material (or, if suitable, filter material on the inside of the filter may be used), or a non-woven polypropylene or polyethylene fiber material according to DIN EN 13254. For installation of liners without welds in small wastewater treatment systems (4–50 PE), the thickness of a polyethylene-based liner can be ≥1.0 mm. Alternate liner materials include concrete or plastic basins, a mineral seal with clay material, or a subsoil with low permeability. Exact specifications for liners are given in the standard (DWA 2017b). A leak test must be conducted after the lining has been installed. Filter media can only be installed after a leak test has confirmed the basin is watertight.

Filter material

Filter media must be well-graded, have a smooth particle distribution curve, a uniformity coefficient less than five, and have a fines content (grain size <63 μm) less than 2%. The permeability of the filter media should be determined prior to installation. Layers of filter media must be chosen so as to avoid unwanted settling of one filter layer into the one beneath it. In the new standard, the following filter media are specified: sand (0–2 mm), coarse sand (0–4 mm), lava sand (0–4 mm), fine gravel (2–8 mm), and medium gravel (8–16 mm). Further details on the specific characteristics and grain size distributions for the filter media to be used is provided in the standard (DWA 2017b).

Inlet and outlet structures

Vertical filters must be loaded in evenly distributed, intermittent doses. The spacing of the distribution network depends on the effective permeability of the filter material; the higher the permeability of the filter material, the tighter the grid of the influent distribution must be. Dividing the influent distribution system into subsections can reduce the required capacities of associated pumps, siphons and pipes. To avoid blockage, the minimum diameter orifice in a distribution pipe is 8 mm. The design of the distribution system must drain completely after each dosing event and must otherwise be secure against freezing during cold periods.

Distance between drainage pipes in vertical filters must be less than 5 m, and appropriate measures should be taken to minimize root penetration into drainage pipes. Further design recommendations for the drainage system can be found in DWA-A 178 (DWA 2005).

For horizontal filters, uniform distribution of wastewater over the infiltration cross-section must be insured, and the inlet region must be designed to accept the maximum surge volume per inlet dose. The outlet structure for horizontal filters consists of a standpipe which must enable adjustment of the water level within the filter.

Vertical filter with sand

For small wastewater treatment systems (4–50 PE), unsaturated vertical filters with sand are sized according to the upper surface of the filter. The specific area is ≥4 m2/PE and the minimum filter area is 16 m2. A principal schematic of a vertical filter with sand is shown in Figure 1.

Figure 1

Vertical filter with sand (DWA 2017b).

Figure 1

Vertical filter with sand (DWA 2017b).

For municipal wastewater treatment plants, the upper surface of the vertical filter with sand is used for determination of the required filter area. The specific area of the filter is ≥4 m2/PE. The following criteria apply to the design of vertical filters with sand:

  • average daily specific COD loading rate on the total filter area, as measured on the upper surface of the filter: ≤20 g/m2 · d;

  • average daily specific COD loading rate on the filter area in operation, as measured on the upper surface of the filter: ≤27 g/m2 · d;

  • average specific hydraulic loading rate over the total filter area, as measured on the upper surface of the filter: ≤80 L/m2 · d;

  • average minimum time between dosing intervals: ≥6 h;

  • average specific hydraulic loading rate of the first-stage filter area in operation during a dosing event, as measured on the upper surface of the filter: ≥6 L/m2 · min; and

  • specific hydraulic loading of the first-stage filter area in operation, per dose, as measured on the upper surface of the filter (e.g. dosing height): 10 L/m2–20 L/m2.

The filter must be divided into at least two, but better four, equally sized subsections which can be individually and intermittently dosed. One subsection at a time is to be put into a resting phase; for larger systems it is recommended that one quarter of the total filter area is in resting phase. A resting phase of 1 week has been shown to be sufficient.

The minimum dose volume must be designed such that it is higher than the infiltration capacity of the dosed area; this will ensure sufficient distribution of water over the filter surface in operation. A dosing event should only begin after the water content of the filter body has reached the minimum. Hydraulic calculations or simulations must be used to prove that there is a uniform pressure distribution in the influent distribution system and that uniform distribution of wastewater is guaranteed.

Two-stage vertical filter with fine gravel and coarse sand

For small wastewater treatment systems (4–50 PE), two-stage vertical filters with fine gravel and coarse sand are sized according to the upper surface of the filter. The specific area of the first-stage filter is ≥1 m2/PE, the specific area of the second-stage filter is ≥1 m2/PE, and the minimum filter area is 8 m2 total (4 m2 per stage). A principal schematic of a two-stage vertical filter with fine gravel and coarse sand is shown in Figure 2(a) and 2(b).

Figure 2

Two-stage vertical filter: first-stage filter with fine gravel (a) and second-stage filter with coarse sand (b) (DWA 2017b).

Figure 2

Two-stage vertical filter: first-stage filter with fine gravel (a) and second-stage filter with coarse sand (b) (DWA 2017b).

For municipal wastewater treatment plants, the upper surface of the filter is used for determination of the required filter area. The following criteria apply to the design of two-stage vertical filters with fine gravel and coarse sand:

  • average specific hydraulic loading rate over the total filter area, as measured on the upper surface of the first filter: ≤80 L/m2 · d;

  • average minimum time between dosing intervals: ≥3 h;

  • average specific hydraulic loading rate of the first-stage filter area in operation during a dosing event, as measured on the upper surface of the filter: ≥10 L/m2 · min; and

  • specific hydraulic loading of the first-stage filter area in operation, per dose, as measured on the upper surface of the filter (e.g. dosing height): >20 L/m2.

Sizing is based on the specific area of the filter and the permissible hydraulic load for the first-stage filter. The larger of the two values determines the design. The filter area of the individual stages must be divided into at least two equally sized subsections which can be individually and intermittently dosed so that if needed, one of the subsections can be put into a resting phase without wastewater loading.

Two-stage vertical filter treating raw wastewater

Two-stage vertical filters treating raw wastewater are sized according to the upper surface of the filter. The specific area of the first-stage filter is ≥1.2 m2/PE for separated sewer networks, and the minimum filter area of the total first stage is 4.8 m2. The specific area of the first-stage filter is ≥1.5 m2/PE for combined sewer networks. The first-stage filter must be divided into a minimum of three (or integer multiple of three) identical cells that are separated at the filter surface and can be intermittently and individually dosed. Only one cell (a third of the total first-stage filter area) is loaded at any one time. The following criteria apply to the design of first-stage filters treating raw wastewater:

  • average daily specific COD loading rate on the total area of all first-stage filters, as measured on the upper surface of the filter: ≤100 g/m2 · d;

  • average daily specific hydraulic loading rate of the total area of all first-stage filters, as measured on the upper surface of the filter: ≤250 L/m2 · d;

  • average specific hydraulic loading rate of the first-stage filter area in operation during a dosing event, as measured on the upper surface of the filter: ≥10 L/m2 · min; and

  • specific hydraulic loading of the first-stage filter area in operation, per dose, as measured on the upper surface of the filter (e.g. dosing height): 20 L/m2–50 L/m2.

First-stage filters receiving raw wastewater are shown in Figure 3(a) for separated sewer networks and in Figure 3(b) for combined sewer networks. The resting period for each sub-area of first-stage filters treating raw wastewater must be at least seven days. A half-weekly rotation is recommended for the three first-stage filters (e.g., each filter is dosed for 3.5 days, and subsequently rested for 7 days).

Figure 3

Two-stage vertical filter treating raw wastewater: (a) first-stage filter for use in separated sewer networks and (b) first-stage filter for use in combined sewer networks (DWA 2017b).

Figure 3

Two-stage vertical filter treating raw wastewater: (a) first-stage filter for use in separated sewer networks and (b) first-stage filter for use in combined sewer networks (DWA 2017b).

If first-stage filters treating raw wastewater from combined sewer networks have no upstream stormwater tank with an overflow, but are connected to an upstream stormwater overflow, a filter overflow structure must be incorporated into the filter design in order to limit the retention volume in the filter (Figure 3(b)). The retention volume of the filter must be designed according to ATV-A 128 (DWA 1992).

A schematic of the second-stage filter is shown in Figure 4. The second-stage filter is to be used downstream of the first-stage filter treating raw wastewater. The upper surface of the filter is used for determining the required filter area. Sizing is based on the specific area per inhabitant and average permissible hydraulic load; the larger value determines the design. Dimensioning for systems connected to separated sewer networks and combined sewer networks differs. The following criteria apply to the design of second-stage filters (downstream of first-stage filters receiving raw wastewater):

  • for systems connected to a separated sewer network, the specific area of the second-stage filter, as measured on the upper surface of the filter, is ≥0.8 m2/PE;

  • for systems connected to a combined sewer network, the specific area of the second-stage filter, as measured on the upper surface of the filter, is ≥1.0 m2/PE;

  • average specific hydraulic loading rate of the second-stage filter area in operation during a dosing event, as measured on the upper surface of the filter: ≥6 L/m2 · min; and

  • specific hydraulic loading rate on the second-stage filter area in operation, per dose, as measured on the upper surface of the filter (e.g. dosing height): ≥20 L/m2.

Figure 4

Two-stage vertical filter treating raw wastewater: second-stage filter (DWA 2017b).

Figure 4

Two-stage vertical filter treating raw wastewater: second-stage filter (DWA 2017b).

The area of the second-stage filter must be divided into two, or an integer multiple of two, equally sized subsections that can be intermittently and individually dosed. Resting periods must be at least three days. A weekly rotation of the subsection(s) to be loaded is recommended.

Vertical filter with lava sand

Vertical filters with lava sand are used for secondary treatment of municipal wastewater for separated sewer networks and for combined sewer networks. The pretreatment step is an aerated settling pond (design specifications for the settling pond are provided in detail in the standard). For use with combined sewer networks, an overflow filter is required in addition to the two parallel main filters (Figure 5). The upper surface area of the filter is used for the determination of the required filter area, and sizing is based on the specific area per inhabitant and the permissible average hydraulic load (the larger value determines the design). The following criteria apply to the design of vertical filters with lava sand:

  • specific area per inhabitant for the total main filter area, as measured on the upper surface of the filter: ≥3 m2/PE;

  • average daily specific COD loading rate on the entire area of the filter, as measured on the upper surface of the filter: ≤20 g/m2 · d;

  • average daily specific hydraulic loading rate of the filter area in operation, as measured on the upper surface of the filter: ≤240 L/m2 · d;

  • average minimum time between dosing intervals: ≥4 h;

  • average specific hydraulic loading rate of the filter area in operation during a dosing event, as measured on the upper surface of the filter: ≥10 L/m2 · min;

  • specific hydraulic loading of the filter area in operation, per dose, as measured on the upper surface of the filter (e.g. dosing height): ≥20 L/m2;

  • specific area per inhabitant for the overflow filter, as measured on the upper surface of the filter: ≥1 m2/PE; and

  • average specific daily hydraulic loading rate on the overflow filter: ≤500 L/m2 · d.

Figure 5

Vertical filter with lava sand for treating combined sewer overflows: (a) overall schematic (b)and filter details (DWA 2017b).

Figure 5

Vertical filter with lava sand for treating combined sewer overflows: (a) overall schematic (b)and filter details (DWA 2017b).

For vertical filters with lava sand, the main filter area must be divided into two equally sized subsections that can be intermittently and individually dosed. The design must allow for one of the subsections to be put into a resting phase without loading. Larger treatment systems should divide the main filter area into subareas in a multiple of two and operated such that half of the subsections can be put into a resting phase without loading. The resting period of a subsection is to be at least three days, and a weekly rotation of the loaded filters is recommended.

Actively aerated vertical filter with gravel

Actively aerated vertical filters (Figure 6) may be used as small wastewater treatment plants (4–50 PE) and for treatment of municipal wastewater. These systems are permanently saturated and equipped with air compressors that deliver air to the bottom of the bed. The bottom of the filter basin is used for determination of the required filter area. Dimensioning is based on specific area per inhabitant and the average daily organic volumetric COD loading of the filter body. The following criteria apply to the design of actively aerated vertical filters:

  • specific area, as measured on the bottom of the filter: ≥1 m2/PE;

  • average specific daily COD volumetric loading: ≤100 g/m3 · d;

  • average minimum time between dosing intervals: 4 h; and

  • average specific hydraulic loading rate during a dosing event, as measured on the bottom surface of the filter: ≥6 L/m2 · min.

Figure 6

Actively aerated vertical filter with gravel (DWA 2017b).

Figure 6

Actively aerated vertical filter with gravel (DWA 2017b).

The aeration system must be designed such that homogeneous distribution of air in the filter body is ensured. Polyethylene drip irrigation tubing, 16 mm diameter, with integrated emitters and an emitter spacing of approximately 0.3 m has been shown to be suitable for use in the aeration system. Distance between lines of the air distribution system should not exceed 0.3 m. A specific air flow rate of ≥0.6 m3/h · m2 has proven to be effective. Vertical sidewalls may be used.

Actively aerated horizontal filter with gravel

Actively aerated horizontal filters (Figure 7) may only be used as small wastewater treatment plants (4–50 PE). These systems are permanently saturated and are equipped with air compressors that deliver air to the bottom of the bed. The bottom of the filter basin is used for determination of the required filter area. Dimensioning is based on specific area per inhabitant, the average daily organic volumetric COD loading of the filter body, and the areal COD loading on the cross-sectional area of the filter. The following criteria apply to the design of actively aerated horizontal filters:

  • specific area, as measured on the bottom of the filter: ≥1 m2/PE;

  • minimum filter area, as measured on the bottom of the filter: 4 m2;

  • average COD area loading on the cross-sectional area: ≤200 g/m2 · d;

  • average specific daily COD volumetric loading: ≤100 g/m3 · d; and

  • minimum filter length: 2 m.

Figure 7

Actively aerated horizontal filter with gravel (DWA 2017b).

Figure 7

Actively aerated horizontal filter with gravel (DWA 2017b).

Other applications

Vertical filters with sand may also be used as a secondary treatment step for seasonal wastewater flows (e.g. operation during summer months only). The DWA-A 262 standard provides guidance for reducing the required area for seasonally-operated VF wetlands treating domestic wastewater, treating wastewater from seasonal tourist facilities, and for small (4 PE–50 PE) wastewater treatment systems.

The DWA-A 262 standard also provides design guidance for treatment of domestic wastewater using two-layer filter trenches with fine gravel and coarse sand.

Traditional horizontal filters are no longer recommended in the standard for use as a main biological (secondary) treatment step. Design guidance for the use of horizontal filters as an additional (tertiary) treatment step is given. Horizontal filters may use either coarse sand (0 mm–4 mm) or fine gravel (2 mm–8 mm) as the main filter material. The design and construction of horizontal filters is well-known, so the detailed dimensioning is not mentioned here. Hydraulic loading and cross-sectional organic loading rates must be respected (the limiting factor determines the size of the filter).

Constructed wetlands can also be used for graywater treatment (see Table 1 for specific mass loads). The pretreatment step for a graywater treatment system must include removal of coarse particulates. Wetlands treating graywater require about 50% of the specific surface area compared to a conventional filter treating domestic wastewater, but the specific composition of the greywater must be taken into account. All hydraulic boundary conditions for a particular filter design must be respected. This applies to both small (4 PE–50 PE) and municipal treatment plants.

Common reed (Phragmites spp.) works best in the treatment wetlands described in this standard for central European climatic conditions. Planting of the filter can be conducted at any time of the year, but springtime is preferable. Reeds can be planted as clusters, rhizomes or seedlings; a higher initial planting density will minimize the growth of weeds and other invasive species. Rhizome planting is most successful when the rhizome has one or two shoots, 10 cm–60 cm tall, and are planted at the end of May or in June, in a density of four to six plants per square meter. Seedlings are preferred over seeds, should be planted at the end of May or in June, in a density of four to eight seedlings per square meter. During plant establishment, the water level in saturated filters must be kept high enough for the plant roots to reach the water, but not so high that the filter is flooded. Further planting advice can be found in DWA-A 178 (DWA 2005).

The system designer must provide the system operator with a simple-to-understand operation and maintenance manual for all operating conditions, including start-up and the phase of vegetation growth and establishment. All treatment plants require professional and regular inspection and maintenance. Maintenance activities must be carefully documented.

For small wastewater treatment systems in Germany (4–50 PE), self-conducted operational requirements can vary from state to state. Malfunctions of any kind (hydraulic, mechanical and electrical failure) must be indicated acoustically and/or visually with an alarm system that is independent from the power network. Maintenance checks must be conducted on a regular basis (generally monthly) to ensure proper functioning of the following components: pretreatment, pump shaft (as appropriate), influent structure, filter(s), air pumps (as appropriate) and effluent structure.

Operations and maintenance of municipal wastewater treatment plants, routine maintenance checks must include: measurement of sludge level in the pretreatment step, sufficient re-aeration of the filter body to prevent clogging (in unsaturated vertical filters), measurement or recording of flow (e.g. pumping events, pump run times, flow meters), and even distribution of water on/in the filter. Invasive plant species and weeds must be removed from the filter.

The new German standard DWA-A 262 is the first national treatment wetland design standard in the EU to include a wide selection of technologies for treatment of domestic and municipal wastewater, including single-stage VF wetlands, two-stage unsaturated VF wetlands which receive raw wastewater in the first stage (i.e. French VF wetlands), VF wetlands with lava sand for treatment of combined sewer flows, two-layer filter trenches, two-stage unsaturated VF wetlands, and actively aerated vertical and HF wetlands.

Design, construction and operation of wetlands according to this standard are safe as long as its limitations and details are respected. The voluntary or mandatory application of this standard in the tendering and implementation of a construction project will:

  • increase the decision-making reliability of local water authorities;

  • improve the construction quality, prevent malfunction and extend the life cycle of treatment wetland systems; and

  • increase the acceptance of wetland technology and overall customer satisfaction.

Design recommendations for total nitrogen removal are provided in the standard. Relevant long-term phosphorus removal cannot be achieved without a separate treatment step. Pathogen removal in treatment wetlands is possible, but currently there is insufficient data to establish design specifications. The DWA-A 262 standard was created based on information and data from central European climatic conditions (warm summers, cold winters) in areas without permafrost; therefore, must be taken not to extrapolate design advice outside the conditions set forth in the guideline. The adaptation of these technologies to other climatic conditions and the confirmation or adjustment of the design criteria could be a useful task for future research projects within the international wetland community.

All authors kindly acknowledge the contributions of the following members of the DWA Working Group KA-10.1: A. Albold, K. Bernhard, G. Fehr, C. Galander, B. Heise, V. Kühn, and M. Stockbauer. These members contributed to the guideline but were not involved in the preparation of the manuscript. J. Nivala acknowledges the German Ministry of Education and Research (BMBF) within the context of the SMART Project: Management of Highly Variable Water Resources in Semi-Arid Regions (FKZ 02WM1355B) and the Helmholtz Center for Environmental Research (Helmholtz Zentrum für Umweltforschung – UFZ) for funding and support.

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