In Austria, 1,840 wastewater treatment plants (WWTPs) with design size >50 population equivalent (PE) serve about 95% of the population. The remaining 5% of the population live in single houses and small settlements that require on site and decentralized wastewater treatment technologies. There is no common database on small WWTPs with design size <50 PE; thus data had to be collected from the Austrian federal states and compiled in a database. The total number of small WWTPs in Austria is about 28,700 comprising 1,300 WWTPs with design size 51–500 PE and 27,400 with design size <50 PE, respectively. The total number of treatment wetlands implemented in Austria is 5,450. Due to legal requirements (nitrification), only vertical flow wetlands are implemented in Austria. From the 5,450 treatment wetlands, about 100 are of design size larger than 50 PE and about 2,800 treatment wetlands have a design size of 5–10 PE. The peak of wetland implementation was in the years 2007–2011 with 2,200 implemented systems in 5 years. Since about 2000, about 30–40% of the new implemented small WWTPs are treatment wetlands.

INTRODUCTION

The population in Austria is about 8.4 million and the area is about 84,000 km². About one-third of the population live in cities, one-third in villages and one-third in rural, mountainous areas, respectively. Data on wastewater treatment plants (WWTPs) with design size >50 population equivalent (PE) is available for all Austria as they are reported biennially to the EU (Table 1). There are in total 1,842 WWTPs with capacity >50 PE that serve about 95% of the population. According to the sector report on wastewater management in Austria (ÖWAV 2015), 923 WWTPs or more than 50% of total number have size of 51 to 500 PE.

Table 1

Existing WWTPs with capacity >50 PE in Austria (BMLFUW 2014)

Design size (PE60No. of WWTPs % WWTPs PE connected % PE connected 
51–1,999 1,204 65.4 462,087 2.1 
2,000–10,000 373 20.2 1,762,099 8.2 
10,001–15,000 45 2.4 572,675 2.6 
15,001–150,000 202 11.0 8,887,740 41.1 
>150,000 18 1.0 9,929,267 45.9 
Total 1,842 100 21,613,868 100 
Design size (PE60No. of WWTPs % WWTPs PE connected % PE connected 
51–1,999 1,204 65.4 462,087 2.1 
2,000–10,000 373 20.2 1,762,099 8.2 
10,001–15,000 45 2.4 572,675 2.6 
15,001–150,000 202 11.0 8,887,740 41.1 
>150,000 18 1.0 9,929,267 45.9 
Total 1,842 100 21,613,868 100 

The remaining 5% of Austria's population lives in single houses that require on site and decentralized wastewater treatment technologies. Due to the federal structure of Austria, there is no common database on small WWTPs. ÖWAV (2015) reports 13,836 existing WWTPs with capacity less than 50 PE. However, this number includes only those small WWTPs that received subsidies for construction since 1999. Thus the exact number of small WWTPs in general and treatment wetlands in particular is not known.

The aim of this work is to gather information from all nine federal states to develop a database aiming to get an overview on the number of small WWTPs (<500 PE) in Austria. Additionally, the database should give information on technologies applied, the size distribution of the plants, age of the plant, etc. This paper focusses on the evaluation of the dataset regarding treatment wetlands.

MATERIAL AND METHODS

Data have been collected from the water information systems of the seven federal states (Carinthia, Lower Austria, Salzburg, Styria, Tyrol, Upper Austria, and Vorarlberg), which is available online for the public. Data for the remaining two federal states with a low number of small WWTPs (Burgenland and Vienna) have been received from the responsible persons at the federal governments.

Data collected for each treatment plant included design size, treatment technology, date of commissioning, etc. As data are recorded slightly differently in all federal states, especially the categorising of the applied technology had to be checked and made uniform.

RESULTS AND DISCUSSION

Total number of small WWTPs, capacity and technologies used

The estimated number of small WWTPs in Austria is 28,730 (Table 2), whereby 1,280 WWTPs have size of 51–500 PE. 24,880 WWTPs have been identified with design size less than 50 PE and for 2,580 WWTPs the design size is unknown. As data for WWTPs larger than 50 PE are reported to the EU and thus well known, it can be assumed that the 2,580 WWTPs with unknown design size are below 50 PE. Thus the total number of WWTPs with design size less than 50 PE is estimated to be 27,460.

Table 2

Total number and size distribution of small WWTPs (design size <500 PE) in Austria

Federal state No. of WWTPs Design size 51–500 PE Unknown 
< 500 PE < 50 PE 
Burgenland 23 20 
Carinthia 7,279 6,343 318 618 
Lower Austria 4,982 4,513 465 
Salzburg 1,732 1,612 75 45 
Styria 10,718 9,178 53 1,487 
Tyrol 1,294 673 197 424 
Upper Austria 2,524 2,398 126 
Vienna 25 13 12 
Vorarlberg 156 129 27 
Total 28,733 24,879 1,276 2,578 
100% 86.6% 4.4% 9.0% 
Federal state No. of WWTPs Design size 51–500 PE Unknown 
< 500 PE < 50 PE 
Burgenland 23 20 
Carinthia 7,279 6,343 318 618 
Lower Austria 4,982 4,513 465 
Salzburg 1,732 1,612 75 45 
Styria 10,718 9,178 53 1,487 
Tyrol 1,294 673 197 424 
Upper Austria 2,524 2,398 126 
Vienna 25 13 12 
Vorarlberg 156 129 27 
Total 28,733 24,879 1,276 2,578 
100% 86.6% 4.4% 9.0% 

Table 3 shows the treatment technologies applied for WWTPs with design size <500 PE. For almost 3,000 plants (about 10%) the technology is not recorded in the databases. The highest number of unknown technologies is in Lower Austria (about 2,000 plants, i.e. 40% of the 5,000 plants) and Carinthia (about 720 plants, i.e. 10% of the 7,200 plants).

Table 3

Treatment technologies applied for WWTPs with design size <500 PE in Austria

Federal state Primary Activated sludge SBR Trickling filter Fixed bed Soil filter VF wetland Unknown + othera 
Burgenland 14 
Carinthia 1,911 3,137 573 55 306 563 716 
Lower Austria 161 250 1,712 27 11 98 718 1,999 
Salzburg 313 255 285 84 55 373 290 26 
Styria 2,408 2,541 1,046 381 334 378 3,287 297 
Tyrol 709 132 128 53 87 75 93 
Upper Austria 383 686 764 104 27 490 
Vienna 14 
Vorarlberg 19 12 36 68 10 
Total 5,905 7,033 4,550 662 469 1,337 5,450 3,135 
20.6% 24.5% 15.8% 2.3% 1.6% 4.7% 19.0% 10.9% 
Federal state Primary Activated sludge SBR Trickling filter Fixed bed Soil filter VF wetland Unknown + othera 
Burgenland 14 
Carinthia 1,911 3,137 573 55 306 563 716 
Lower Austria 161 250 1,712 27 11 98 718 1,999 
Salzburg 313 255 285 84 55 373 290 26 
Styria 2,408 2,541 1,046 381 334 378 3,287 297 
Tyrol 709 132 128 53 87 75 93 
Upper Austria 383 686 764 104 27 490 
Vienna 14 
Vorarlberg 19 12 36 68 10 
Total 5,905 7,033 4,550 662 469 1,337 5,450 3,135 
20.6% 24.5% 15.8% 2.3% 1.6% 4.7% 19.0% 10.9% 

aIncluding 48 membrane bioreactors and 144 rotating biological contactor plants.

Still, more than 20% of the plants are classified as primary treatment and are mainly old septic tanks from which mechanically treated wastewater is discharged. This technology is no longer state-of-the-art. However, most of these WWTPs were implemented before 1991 and still have a valid operation permission. Since 1991, nitrification is required for all WWTPs. For plants with design size <500 PE the maximum effluent concentrations are 25 mg BOD5/L (BOD5: biochemical oxygen demand); 90 mg COD/L (COD: chemical oxygen demand) and 10 mg NH4-N/L, respectively (1.AEVkA 1996).

The main types of technologies for secondary treatment are activated sludge (24.5%), treatment wetlands (19.0%) and sequencing batch reactors (SBRs, 15.8%). The number of treatment wetlands implemented as the main treatment stage is 5,450 and therefore much higher than former estimates that assumed ‘more than 3,000’ treatment wetlands in Austria (Langergraber & Haberl 2012).

Treatment wetlands in Austria

Due to the legal requirements (1.AEVkA 1996), only vertical flow (VF) wetlands can be implemented in Austria since the 1990s. Thus, only a small number of treatment wetlands implemented in the late 1980s and early 1990s are with horizontal flow. However, information on the type of treatment wetland is not available in the database. The main design size is 5–10 PE (Figure 1); less than 100 treatment wetlands have design size larger than 50 PE. Figure 2 shows the number of treatment wetlands have implemented over time; a peak of implementation can be observed between 2007 and 2011.
Figure 1

Design size distribution of treatment wetlands in Austria.

Figure 1

Design size distribution of treatment wetlands in Austria.

Figure 2

Treatment wetlands implementation in Austria over time.

Figure 2

Treatment wetlands implementation in Austria over time.

Since the release of the first Austrian design guideline in 1997 (ÖNORM B 2505 1997), treatment wetlands have gained popularity. In the 1997 design guideline the required specific surface area for VF wetlands was 5 m² per person connected. Based on results presented by Langergraber et al. (2007, 2009), the design guideline was revised and the required specific surface area for VF wetlands was reduced to 4 m² per person connected, i.e. the maximum applicable organic load was increased to 20 g COD/(m2·d) (ÖNORM B 2505 2009). For the VF wetlands, the grain size of the sandy substrate used for the 50 cm main layer should be 0.06–4 mm (with d10 = 0.2 mm and d60 = 0.8 mm, respectively).

Although the number of implementations of treatment wetlands is less in the period 2012 and 2016, still the same percentage (about 40%) of the systems implemented are treatment wetlands. The decrease in implementations may be linked to the economic crises since 2008 resulting in less available public funds for subsidies and less available private funds for investments in wastewater treatment (Dopplinger 2016).

Comparison of implemented technologies

Figure 3 shows the different technologies implemented in Austria over time. Since 1992, the relative share of treatment wetlands has been increased over time and currently about 30% of the implemented small WWTPs are treatment wetlands. The real share is estimated to be higher as there is a high number of new treatment systems with unknown technology (caused by the lack of data on the technology in several districts of Lower Austria).
Figure 3

Percentage of implemented small WWTPs in Austria.

Figure 3

Percentage of implemented small WWTPs in Austria.

Figures 47 compare the timely evolution of the implemented technologies in the four federal states with most treatment wetlands implemented, i.e. Upper Austria (718), Styria (3,287), Lower Austria (563) and Carinthia (490), respectively.

In Upper Austria (Figure 4), treatment wetlands have become more popular during the last 10 years and the number of implementations is still increasing. During the last 5 years treatment wetlands have been the second most popular after SBR technology and almost 40% of the new systems implemented have been treatment wetlands.
Figure 4

Implemented small WWTPs in Upper Austria.

Figure 4

Implemented small WWTPs in Upper Austria.

In contrast, treatment wetlands have been always the most popular treatment technology in Styria, the federal state with most of the small WWTPs (Figure 5). Since 1992, treatment wetlands have always had a share of about 40% of the new small WWTPs implemented. The peak of implementation in the period 2007–2011 was caused be a change in the subsidy policy of the federal state. To receive a special subsidy, projects had to be submitted before the end of 2008, resulting in a high number of projects submitted and high number of implemented small WWTPs in 2009 and 2010 (Dopplinger 2016).
Figure 5

Implemented small WWTPs in Styria.

Figure 5

Implemented small WWTPs in Styria.

In Lower Austria (Figure 6), SBR technology is most popular. In comparison to other states, very few activated sludge systems have been implemented. Similar to Upper Austria, the number of treatment wetlands implemented is still increasing. Currently about 30% of the implemented small WWTPs are treatment wetlands.
Figure 6

Implemented small WWTPs in Lower Austria.

Figure 6

Implemented small WWTPs in Lower Austria.

In Carinthia (Figure 7) the highest number of small WWTPs (mostly activated sludge technology) has been implemented already in the 1990s. Since the end of the 1990s, SBRs and treatment wetlands have become more popular. Treatment wetlands account for about 30–40% of the implemented systems since 2000.
Figure 7

Implemented small WWTPs in Carinthia.

Figure 7

Implemented small WWTPs in Carinthia.

CONCLUSIONS

The following can be concluded:

  • The number of implemented treatment wetlands in Austria is 5,450 and much higher than estimated previously.

  • 20% of all small WWTPs (less that 500 PE) in Austria are treatment wetlands.

  • As nitrification is required for all WWTPs in Austria, only VF wetlands can be implemented. Only very few horizontal flow wetlands were implemented in the 1980s and the early 1990s.

  • The release of the first Austrian design guideline in 1997 increased implementation numbers of treatment wetlands significantly.

  • Since 2000, about 30% of new small WWTPs implemented are treatment wetlands.

ACKNOWLEDGEMENTS

The authors thank the Master's students Iris Dopplinger, Sophia Gerstorfer and Domink Feigl, for the baseline work in collecting the data, as well as the responsible persons at the federal governments for supporting the students.

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