With the continuous increase in environmental requirements and regulations for wastewater treatment a lot of wastewater treatment plants in Austria had to be extended. Extension of the plant requires in general an additional area, which is not always available. This study shows the extension of the WWTP Hard Hofsteig and doubling its capacity by using the Hybrid concept, which is a special two stage activated sludge process for extensive nutrient removal. The plant was extended from 170,000 PE to 270,000 PE without changing the footprint of the plant and the extension costs could be kept very low. Operational data shows that by operating the plant in the two stage Hybrid mode the energy balance of the plant can be improved.

plant extension, two stage Hybrid concept, nitrite

Because of a demand for more efficient wastewater treatment processes, a lot of wastewater treatment plants (WWTP) built in the 1970s and 1980s had to be upgraded and extended for complete nutrient removal. Apart from the fact that extension of the WWTPs requires an additional area, which is not always available, it is also associated with considerable investments.

The Hard-Hofsteig WWTP is situated in the Austrian province of Vorarlberg near Lake Constance. Catchment area of Hofsteig WWTP covers the municipalities of Bildstein, Fußach, Gaißau, Hard, Höchst, Lauterach, Lustenau and Wolfurt. Some of these communities have textile, metal and food industry, which contribute to a high organic pollution of the wastewater.

The original plant was built in the 1970s, as a conventional activated sludge plant for carbon and phosphorus removal only (WWTP1, Figure 1), with two parallel lines for 80,000 PE. Due to the increase of the catchment area, the load has increased to 138,000 PE and at the same time the effluent limits have become more stringent in respect of nitrogen and phosphorus removal.
Figure 1
Figure 1. WWTP1 Hard-Hofsteig, 80,000 PE, Start-Up 1974.
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WWTP1 Hard-Hofsteig, 80,000 PE, Start-Up 1974.

Figure 1
Figure 1. WWTP1 Hard-Hofsteig, 80,000 PE, Start-Up 1974.
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WWTP1 Hard-Hofsteig, 80,000 PE, Start-Up 1974.

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The existing biological stage was too small to fulfil these new effluent requirements, and a new biological stage with an aeration tank of the volume of 17,000 m3 and two new circular final tanks (Ø 38 m, total volume 9,000 m3) had to be built (WWTP2, Figure 2). The aeration tanks were built as rectangular tanks with circular flow, where nitrification/denitrification takes place simultaneously: nitrification in the oxygenated zones and denitrification in the anoxic zones when dissolved oxygen is depleted.
Figure 2
Figure 2. WWTP2 Hard-Hofsteig, 138,000 PE, Start-Up 1997.
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WWTP2 Hard-Hofsteig, 138,000 PE, Start-Up 1997.

Figure 2
Figure 2. WWTP2 Hard-Hofsteig, 138,000 PE, Start-Up 1997.
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WWTP2 Hard-Hofsteig, 138,000 PE, Start-Up 1997.

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At that time, biological stage of the WWTP1 was out of operation.

Due to further industrial development in the catchment area and increased demand for storm water treatment, the load to the plant has increased and WWTP2 was periodically overloaded. For a further extension, biological stage of WWTP1 was planned to be put in operation, which could increase the capacity of the plant to 170,000 PE. However, actual load to the plant was sometimes significantly higher and ranged up to 250,000 PE. Therefore it was decided to extend the plant to a capacity of 270,000 PE. For increasing the biological part of the single stage plant, the volume of the aeration tank would have been enlarged to approx. 40,000 m3, but the required area for such enlargement was not available. As a solution for this challenge, the two stage HYBRID® concept (Figure 3) for the extension of the plant was selected (WWTP3, Figure 4).
Figure 3
Figure 3. Two stage Hybrid concept, European Patent (EP-0-527-123-B1).
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Two stage Hybrid concept, European Patent (EP-0-527-123-B1).

Figure 3
Figure 3. Two stage Hybrid concept, European Patent (EP-0-527-123-B1).
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Two stage Hybrid concept, European Patent (EP-0-527-123-B1).

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Figure 4
Figure 4. WWTP3 Hard-Hofsteig, 270,000 PE, Start-Up 2014.
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WWTP3 Hard-Hofsteig, 270,000 PE, Start-Up 2014.

Figure 4
Figure 4. WWTP3 Hard-Hofsteig, 270,000 PE, Start-Up 2014.
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WWTP3 Hard-Hofsteig, 270,000 PE, Start-Up 2014.

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The HYBRID® concept consists of high loaded first stage and a low loaded second stage, with a combination of sludge cycles, to enable nitrogen removal (Matsché & Winkler 2014; Winkler et al. 2008). This two stage activated sludge process combines the advantages of single and two stage activated sludge plants achieving extensive nutrient removal. Main advantages of this concept are a small space demand, robustness against substantial load variations, high operational flexibility, and high potential of integrating the existing plant structures into an extension concept in case of upgrading the nutrient removal. The first stage is a high loaded activated sludge process, with a low sludge age of 1–3 days and a volumetric loading of 3–5 kg BOD5kg/m3,d. This results in a high excess sludge production, with low oxygen demand. Subsequently, this excess sludge provides a good substrate for anaerobic digestion and boosts biogas production, while the energy demand for aeration in the first stage can be kept low.

The second stage is a low loaded activated process with an SRT over ten days for secured nitrification and volumetric loading of 0.05–015 kg BOD5/m3,d. For denitrification, a side stream of high loaded sludge from the first stage (sludge-circle 1, Figure 3) is transferred into the second stage as carbon source for denitrification.

Excess sludge of the second stage is transferred into the first stage through the sludge-circle 2, transferring the nitrifying sludge into the first stage, which is operated at an SRT which is too short to maintain the autotrophic bacteria growth. Due to the high loading in the first stage, the nitrate produced is immediately denitrified by high loaded sludge. It can be estimated that the first stage can yield up to 40% of total nitrogen removal capacity of the plant.

Total nitrogen removal of the whole process can be further improved by recycling treated effluent into the first stage.

For the first high loaded stage of the Hybrid plant, the original biological stage with a volume of 5,600 m3 which was not operated was partially used: 4,200 m3 for the high loaded aeration tank and 1,400 m3 for engine room and intermediate pumping station. 50% of the final sedimentation tank (3,000 m3) of WWTP 1 was used as an intermediate sedimentation tank.

As the low loaded stage of the Hybrid plant, the existing biological stage of the WWTP2 was used. Final sedimentation of new Hybrid plant consists of 3,000 m3 volume of WWTP1 and 9,000 m3 volume of WWTP2.

Table 1 gives an overview of the two extensions to WWTP Hard-Hofsteig.

Table 1

The three phases of the extension of WWTP Hard-Hofsteig

 Original Plant WWTP1 Start-Up 19741st Extension WWTP2 Start-Up 19972nd Extension WWTP3 Start-Up 2015
Load 80,000 PE 138,000 PE 270,000 PE 
Primary Sedimentation 1,300 m3 630 m3 630 m3 
Biological stage original plant (Stage 1 Hybrid plant) 
Aeration Tank 5,600 m3 Not operated 4,200 m3 
Settling Tank 6,000 m3 Not operated 3,000 m3 
Biological Stage 1st extension (Stage 2 Hybrid plant) 
Aeration Tank  17,000 m3 17,000 m3 
Settling Tank (2 circular tanks)  9,000 m3 9,000 m3 
Settling Tank (2 rectangular tanks)   3,000 m3 
SBR tank for sludge liquor treatment  630 m3 630 m3 
Specific volume of Aeration Tank 70 L/PE 123 L/PE 78 L/PE 
Only C Removal CNP Removal CNP Removal 
 Original Plant WWTP1 Start-Up 19741st Extension WWTP2 Start-Up 19972nd Extension WWTP3 Start-Up 2015
Load 80,000 PE 138,000 PE 270,000 PE 
Primary Sedimentation 1,300 m3 630 m3 630 m3 
Biological stage original plant (Stage 1 Hybrid plant) 
Aeration Tank 5,600 m3 Not operated 4,200 m3 
Settling Tank 6,000 m3 Not operated 3,000 m3 
Biological Stage 1st extension (Stage 2 Hybrid plant) 
Aeration Tank  17,000 m3 17,000 m3 
Settling Tank (2 circular tanks)  9,000 m3 9,000 m3 
Settling Tank (2 rectangular tanks)   3,000 m3 
SBR tank for sludge liquor treatment  630 m3 630 m3 
Specific volume of Aeration Tank 70 L/PE 123 L/PE 78 L/PE 
Only C Removal CNP Removal CNP Removal 
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The planning of enlargement started during the year 2010 and construction of the plant was finished by the end of 2014. During the planning and construction period, some enterprises which were major contributors to the pollution load of the plant were closed down, thus certain load reductions occurred. During the years 2012 and 2013, peak loads were below 160,000 PE, but during 2014 they reached 200,000 PE.

The comparison of the design and operational data (Table 2) shows that the WWTP3 is significantly underloaded and has considerable reserves, which enables start-up of new industrial enterprises in the future.

Table 2

Comparison of design and operational data for WWTP2 and WWTP3

 Design DataOperational data
  WWTP2 WWTP3 WWTP2a WWTP2b 
BOD5 60 g/PE,d 138,000 270,000 139.311 123,428 
Flow m3/d 29,000 29,000 20,088 22,004 
BOD5 kg/d 8,288 16,298 7,146 7,406 
COD kg/d 14,400 32,596 16,717 15,055 
TN kg/d 1,271 1,800 1,054 1,014 
TP kg/d 289 318 265 120 
  First Stage Second Stage  First Stage Second Stage 
Volumetric Loading kgBOD5/m3,d 0.33 3.4 0.11 0.27 1.23 0.09 
MLSS g/l 3.4 2 2 3.5 1.6 2.9 
Sludge Age d 14.4 0.44 31.2 15 3 33 
 Design DataOperational data
  WWTP2 WWTP3 WWTP2a WWTP2b 
BOD5 60 g/PE,d 138,000 270,000 139.311 123,428 
Flow m3/d 29,000 29,000 20,088 22,004 
BOD5 kg/d 8,288 16,298 7,146 7,406 
COD kg/d 14,400 32,596 16,717 15,055 
TN kg/d 1,271 1,800 1,054 1,014 
TP kg/d 289 318 265 120 
  First Stage Second Stage  First Stage Second Stage 
Volumetric Loading kgBOD5/m3,d 0.33 3.4 0.11 0.27 1.23 0.09 
MLSS g/l 3.4 2 2 3.5 1.6 2.9 
Sludge Age d 14.4 0.44 31.2 15 3 33 

aOperational Data 01.01.2014–15.11.2014.

bOperational Data 15.11.2014–30.06.2015.

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Meanwhile, the operation of the plant has been changed by operating only one of two trains. This resulted in a significant increase of volumetric load of the first stage and approaching conditions according to the design of the process.

Plant performance

The enlarged plant (WWTP3) was put into operation in November 2014. Shortly after the start-up, nitrite concentration in the effluent started to increase, due to the fact that Nitrosomonas develop faster than Nitrobacter. Similar results were already obtained at the start-up of the Main Treatment Plant of Vienna, which was also operated by applying the Hybrid concept. The results are shown in Figure 5.
Figure 5
Figure 5. Nitrogen compounds during start-up of the WWTP3 Hard-Hofsteig.
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Nitrogen compounds during start-up of the WWTP3 Hard-Hofsteig.

Figure 5
Figure 5. Nitrogen compounds during start-up of the WWTP3 Hard-Hofsteig.
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Nitrogen compounds during start-up of the WWTP3 Hard-Hofsteig.

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For acclimated activated sludge and temperatures lower than 25°C, nitrification is complete and nitrite concentrations are lower than 0.1 mg/l. However, in transient conditions, e.g. start-up, NO2-N, concentrations of 5–20 mg/l are possible at low NH4-N concentrations (Metcalf & Eddy 2014).

Discharge of the effluent into Lake Constance is subject to very strict regulations. Effluent limits and the performance of WWTP2 and WWTP3 are shown in Table 3.

Table 3

Effluent limits and treatment efficiency of WWTP2 and WWTP3

 WWTP2WWTP3WWTP2/ WWTP3WWTP2aWWTP3b
 Influent mg/lEffluent Limits mg/l (Catchment Lake Constance)Removal Efficiency
BOD5 362 348 15 98% 99% 
COD 849 719 60 96% 97% 
TN 52 48 >70%c ∼80% ∼70% 
NH4-N 31 31 5 95% 96% 
TP 13.2 6 0.5 97% 93% 
 WWTP2WWTP3WWTP2/ WWTP3WWTP2aWWTP3b
 Influent mg/lEffluent Limits mg/l (Catchment Lake Constance)Removal Efficiency
BOD5 362 348 15 98% 99% 
COD 849 719 60 96% 97% 
TN 52 48 >70%c ∼80% ∼70% 
NH4-N 31 31 5 95% 96% 
TP 13.2 6 0.5 97% 93% 

aOperational Data 01.01.2014–31.10.2014.

bOperational Data 01.01.2015–30.06.2014.

cThe annual average for wastewater T > 12°C.

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It can be seen in Table 3 that the ratio of TKN/COD is relatively low, probably due to industrial contributions (0.063 in 2014 and 0.067 in 2015). However, there are several Hybrid plants in Austria which receive only domestic wastewater and obtain up to 85% nitrogen removal, as well.

Due to reduced organic load in the influent, the loading of the WWTP3 first stage was significantly lower than the design load, which resulted in a higher BOD5 removal and lower denitrification capacity in the second stage.

Nitrogen data before and after two stage operation are shown in Figure 6. It can be clearly seen from the NH4-N concentration that the nitrification is more stable in two stage period and that all the values are <3 mg/l. However, during the first three months of WWTP3 operation (until April 2015), denitrification is poor and reaches the values of WWTP2 only in May and June 2015. The reason for this is a comparatively low loading of the first stage (Table 2) and uncontrolled oxygen input due to leaking air valves in the denitrification zone of stage two.
Figure 6
Figure 6. Nitrogen species before and after two stage operation.
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Nitrogen species before and after two stage operation.

Figure 6
Figure 6. Nitrogen species before and after two stage operation.
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Nitrogen species before and after two stage operation.

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COD balance

One of the assumed advantages of the Hybrid process is lower energy demand and higher biogas production due to higher excess sludge production. COD balance of WWTP2 and WWTP 3 is shown in Figure 7.
Figure 7
Figure 7. COD Balance for the single stage and two stage – Hybrid operation.
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COD Balance for the single stage and two stage – Hybrid operation.

Figure 7
Figure 7. COD Balance for the single stage and two stage – Hybrid operation.
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COD Balance for the single stage and two stage – Hybrid operation.

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Out of the data presented in Figure 7, the assumed advantages of the two stage Hybrid process can be confirmed. Surplus sludge production is increased by 27%, energy consumption in the aeration tanks is reduced by 25% and biogas production is increased by 15%.

Specific energy demand for aeration and specific biogas production before and after the enlargement are summarized in Table 4, and they clearly indicate the advantages of the Hybrid process.

Table 4

Specific biogas production and oxygen demand of WWTP2 and WWPT3

 WWTP2WWTP3
Oxygen demand kgO2/kgCOD 1 0.75 
Specific biogas production l/PE,d 18 21 
 WWTP2WWTP3
Oxygen demand kgO2/kgCOD 1 0.75 
Specific biogas production l/PE,d 18 21 
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Sludge characteristics

The first stage of the Hybrid plant is highly loaded and usually characterized with a very low sludge volume index (SVI). Previous experiences have shown that properties of sludge from the high loaded stage essentially determine the properties of second stage sludge and that the SVI of the second stage is low, as well (Winkler et al., 2004).

In case of WWTP3, the SVI was actually higher than expected and ranged up to 200 ml/g. Due to the low incoming load to the plant, sludge load of the first stage is below the design values of the Hybrid process (1.54 vs. 3.4 kgCOD/kgSS,d). This is in good agreement with previous results from the pilot plant Main Treatment Plant of Vienna (Figure 8).
Figure 8
Figure 8. Sludge Characteristics vs. Sludge loading (Pilot Plant Vienna).
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Sludge Characteristics vs. Sludge loading (Pilot Plant Vienna).

Figure 8
Figure 8. Sludge Characteristics vs. Sludge loading (Pilot Plant Vienna).
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Sludge Characteristics vs. Sludge loading (Pilot Plant Vienna).

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Cost savings

In case of WWTP Hard-Hofsteig, specific construction costs of plant enlargement were extremely low. Bearing in mind that the capacity of the plant was extended from 138,000 PE to 270,000 PE and that construction costs were approx. 6.25 million €, specific costs per inhabitant amounted to as low as 47 €/PE, which is extremely favourable as compared to a one stage plant. According to the benchmarking project of ÖWAV (Österreichischer Wasser und Abfallwirtschaftsverband) published in report End Report Part A 2012, construction costs of a conventional single stage plant for nutrient removal were 160 €/PE, which is 70% higher than construction costs of the Hard-Hofsteig plant.

  • With application of the Hybrid concept to the WWTP Hard-Hofsteig, the treatment capacity could be significantly increased, without using an additional area to the original footprint. Since no additional space was available at the location of the treatment plant, the Hybrid concept was the only alternative for the enlargement. However, the need for the enlargement can only be implemented in the future, when the load increases further due to economic development.

  • As predicted, energy demand for aeration has decreased significantly and was only 75% as compared to the previous single stage operation.

  • Due to a higher surplus sludge production in the first stage, biogas production has increased by 15%. However, specific biogas production of 21 L/PE,d is comparatively low, since the organic load of the plant was only 50% of the design load.

  • The reduced load to the plant has also a negative influence on the SVI, which is normally bellow 120 ml/g in more than 10 existing plants in Austria, operated as Hybrid.

  • Maximum usage of the existing tank volumes and low specific volume demand of the Hybrid process resulted in extremely low construction costs, which were below 50 €/PE.

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