The Plant

Catchment Area and Dimensioning Principles

Catchment Area

Compilation if the keeping threshold values
Parameter EU-guidelines Dimensioning of the plant
CSB 125 75
BSB5 25 15
Nges 10 10
Pges 1 1
GUS 35 30

Imm_01.jpgARA Tobl serves 14 communities in the Puster Valley and has a catchment area of 900 km2. 
These communities make up 10% of the total area of South Tyrol. During the initial operation in 1996, the Central Puster Valley associated wastewater treatment plant clarified the wastewater of the following communities: St. Lorenzen, Bruneck, Percha, parts of Olang and Rasen-Antholz. 
In 1997, Sand in Taufers, the community Ahrntal to Steinhaus and the remaining part of Rasen-Antholz followed. In 1998, Prettau und Mühlwald joined the association. In 2000 where connected the communities of St. Vigil, Wengen and St. Martin.

Dimensioning Principles
 

Population Data
Hydraulic load: 95.000 inhabitants and population equivalent respectively
Specific sewage flow rate per head: 300 l/Day
Dry weather flow QDWF: 570 l/s
Wet weather flow QWWF: 1.140 l/s
Daily quantity of sewage 28.500 m3/day
Biological Load: 130.000 inhabitants and population equivalent respectively
BOD5 - loads:
Specific load value:

60 g/E day 7.800 kg/day
COD - load:
Specific load value:

110 g/E day 14.300 kg/day
NKj -loads:
Specific load value:

12 g/E day 1.560 kg/day
NH4-N -loads
Daily loads:

910 kg/day
NO3-N -loads
Daily loads:

60 kg/day
Ntotal -loads
Daily loads:

1.620 kg/day
Phosphor P - loads
Daily loads:

3 g/day 390 kg/day

Lay-out Plan / Sketch of Plant

Operation Procedure

In the central gallery, coarse screens with spacings of 15 mm, an aerated grit chamber and fat trap, fine screens with spacings of 6 mm, screw conveyors, a screenings bale press and grit classifiers are found in twin operation. 
Next, there is a division in four lines: each of them has two preliminary clarifiers, one activated sludge tank with primary denitrification and nitrification as circulation basin and two final clarifiers.

Verfa123.jpg
 
The phosphate precipitaition is done in the third cycle of the nitrification. 
The dosing device for lime milk and ferrous sulphate, the ventilation system centre with exhaust air treatment by means of a biological crosscurrent washer, the transformer box, as well as the power and control systems are built into the central gallery. 
The sludge line consists of four pre-thickeners, two mechanical excess sludge drainage systems, two strainpresses, two mesophilic digestion towers, which can be operated either sequentially or parallel to each other, three post thickeners and two belt filter presses.

Verfa456.jpg
 
The drying plant has been in operation since 1999. Digester gas is taken from the digestion tower and used to produce electrical power in three heating power stations (150 kW per tower), the warmth is used for heating up the sludge and the galleries. 
In 2004 installation of the thermal recycling plant (pyrolysis rotary kiln) followed by Start up in 2005. 

Verfa07.jpg Verfa08.jpg

Ventilation System

Ventilation System

Safety 
Because construction took place underground, higher standards were placed on safety. The aeration and deaeration ventilators were designed for an air exchange of one to six times per hour. The ventilators are divided into three control loops (side gallery 1, central gallery and side gallery 3). Gas sensors for explosive agents, butane-propane, benzene, methane and a petroleum-derived hydrocarbon measuring instrument are installed at the entrance of the mountain. The same gases are measured where the 960 m long inlet gallery meets the plant and also before the coarse screen. The explosive materials, methane, H2S, CO and O2, the temperature and the moisture content are measured in the air outlet pipe.

Ventilation System of the Galleries
Air exchange:
Preliminary treatment area: 1 - 6 times1
Side galleries area: 1 - 3 times1
Technical chambers area: 1 - 15 times1
Maximum amount of air: approx. 150.000 m3/h



Alarm 
When the level of the above mentioned gases reaches 20 % of the lower explosion limit, the warning alarm is activated through the process control system. Then the fresh air and the exhaust air ventilators automatically operate with the maximum quantity of air. When 40 % of the lower explosion limit are reached, the main alarm is activated. The inlet valve is closed manually and everything in the central gallery is operated without electricity in order to prevent an explosion in the mountain. If neither the warning alarm or the main alarm go off, the ventilators are regulated according to an adjustable temperature and/or moisture level. The ventilators can be regulated between 12,000 m3/h to 48,000 m3/h, as required. 

Exhaust Air 
The exhaust air is treated biologically and chemically with a three-stage crosscurrent washer. CO, O2, the temperature and the moisture level are measured in the side galleries' exhaust air pipes. The ventilators are regulated according to an adjustable temperature and/or moisture level. Oxygen and carbon monoxide have no regulation functions, but they represent important indicators for the work safety. During two years of operation, there have been three minor petroleum accidents which ran their course without great difficulties.

Operational Costs 
The operational costs of the plant are approx. 10 % higher than those of a sewage purification plant with an open building method. This is due to the regular calibration of the gas sensors in three months, the comparatively short life-time of the measuring heads (about one to two years) and the high current demand of the ventilators, which have to be run constantly because of safety reasons.

On-line Regulation

On-line Regulation

Reliable and Safe

Autoanalyzerstation.jpgAltogether, there are seven on-line devices located in a central room in the central gallery. PO4-P is continually measured in the influx and PO4-P and NO3-N in the outflow, NO3-N at the denitrification outflow, PO4-P and NH4-N at the nitrification outflow. 
At all of the above mentioned sites in the lines, testing pumps are installed. The testing pumps transport the respective sample in PE-pipelines to the appropriate analysing instrument. 
One measuring cycle lasts half an hour. If there are two lines in operation, then a current measured value is given every hour per line. 
The method of operation, the maintenance and the expenses are regularly checked.

Regulation strategies

a) Prioritised Regulation of Compressors with NH4-N Measuring Instrument 


AG.jpgUpon output of nitrification, the NH4-N is measured. With the help of this measured value of NH4-N, the desired value of oxygen is passed on from the process control system where all lines can be controlled independently of each other. 
If the measured value of NH4-N is smaller than an adjustable minimum (1 mg/l), an adjustable minimal desired value of oxygen (0.8 mg/l) is passed on from the process control system; if the measured value of NH4-N is larger than an adjustable maximum value (2 mg/l), an adjustable desired value of oxygen (2.5 mg/l)is passed on from the process control system; if the measured value of NH4-N is in an interim (1.0 < NH4-N < 2.0), an adjustable mean desired value of oxygen (1.5 mg/l) is passed on from the process control system. 
The desired value of oxygen controls the glare-regulating valve in the air conductors, with reliance on the principal deviation. These end in a collector where a manometer controls a frequency regulated rotating piston blower. 

The operation has confirmed that through the different desired value of oxygen guidelines, which are determined by the NH4-N measurements, a complete nitrification (98.5 % decomposition) can be guaranteed. For the most part, a minimal desired value of oxygen of 0.80 - 1.0 mg/l (70% of the time) is taken, and only during NH4-N daily maximums and seasonal maximum, a higher desired value of oxygen is controlled. Through this an optimal nitrification, with minimal transfer of oxygen, occurs and thus a minimal demand of electricity from the compressors is guaranteed. 


b) Regulation of the Denitrification with NO3-N Measuring Instruments 

B.jpgNO3-N is measured upon outlet of the denitrification. With the help of this measured value of NO3-N, the re-circulation pumps and the bypassing of the preliminary clarification are piloted by the process control system where all lines can be controlled independently of each other. If the measured value of NO3-N is smaller than an adjustable value (3 mg/l), the re-circulation pumps are regulated by the process control system at a maximal value (400% of QDW in influx). 
If the measured value of NO3-N is larger than an adjustable value (3 mg/l), the re-circulation pumps are controlled by the process control system at a minimal value (100% of QDW in influx). 
If the measured value of NO3-N is still larger than an adjustable value (3 mg/l) at the next measurement (one hour later), the primary clarifiers are additionally dealt with by the process control system so that more carbon enters the denitrification zone. T
he operation has shown that with this regulation strategy, the concentration of nitrate in the outlet can be kept low (yearly mean = 3.65 mg/l). 


c)Regulation of Phosphate Precipitation with PO4-P Measuring Instruments 

C.jpgPO4-P is measured upon outlet of nitrification in influx and outlet of the wastewater treatment plant. 
The precipitant dosing takes place in the third cycle of the nitrification. The frequency controlled metering pumps can be regulated according to the following possibilities: 

- according to the amount of influx 
- according to the PO4-P concentration in the nitrification outlet 
- according to the PO4-P loading in influx 
- according to pre-set maximal PO4-P loading in outlet 
- according to work and break periods 

In operation, the regulation of the precipitant metering pumps, according to the PO4-P concentration, has proved to be the most optimal plant management. 


d)Regulation of Excess Sludge Extraction 

D.jpgThe excess sludge is extracted from the return-sludge stream. A solid matter meter, which is recorded in the process control system, is placed in the nitrification basins. 
The regulation of the frequency controlled excess sludge pumps occurs automatically over a given dried matter content. This dried matter content is dependent on the deviation, although it shouldn't fall under a certain adjustable daily minimum. 

In addition to these possibilities, a daily quantity can be given, or work and break period can be selected through the process control system. As a further possibility, the regulation of the excess sludge extraction was programmed according to a pre-determined sludge age. 
The computer adopts the dried matter excess from the laboratory protocol, the sludge age is pre-determined, dried matterExcess is measured and so the amount of excess sludge is automatically subtracted. This method of regulation covered a period of about four months and functioned flawlessly. At this time, the excess sludge extraction is controlled according to dried matterAeration tank. 


e) Regulation of pH Value Activated Sludge Tanks 

E.jpgA pH measuring devise is installed in each activated sludge tank. A set point value is given in the process control system and depending on the principal deviation, the lime milk metering pumps are controlled. These pumps are equipped with frequency rotary converters. 
A lime milk dissolving station with a lime silo, in which lime Ca(OH)2 exists in powder, is added. The metering of lime milk is necessary in order to keep the acid carryover capacity upright and to guarantee nitrification.

a) Prioritised Regulation of Compressors with NH4-N Measuring Instrument 

AG.jpgUpon output of nitrification, the NH4-N is measured. With the help of this measured value of NH4-N, the desired value of oxygen is passed on from the process control system where all lines can be controlled independently of each other. 
If the measured value of NH4-N is smaller than an adjustable minimum (1 mg/l), an adjustable minimal desired value of oxygen (0.8 mg/l) is passed on from the process control system; if the measured value of NH4-N is larger than an adjustable maximum value (2 mg/l), an adjustable desired value of oxygen (2.5 mg/l)is passed on from the process control system; if the measured value of NH4-N is in an interim (1.0 < NH4-N < 2.0), an adjustable mean desired value of oxygen (1.5 mg/l) is passed on from the process control system. 
The desired value of oxygen controls the glare-regulating valve in the air conductors, with reliance on the principal deviation. These end in a collector where a manometer controls a frequency regulated rotating piston blower. 

The operation has confirmed that through the different desired value of oxygen guidelines, which are determined by the NH4-N measurements, a complete nitrification (98.5 % decomposition) can be guaranteed. For the most part, a minimal desired value of oxygen of 0.80 - 1.0 mg/l (70% of the time) is taken, and only during NH4-N daily maximums and seasonal maximum, a higher desired value of oxygen is controlled. Through this an optimal nitrification, with minimal transfer of oxygen, occurs and thus a minimal demand of electricity from the compressors is guaranteed. 


b) Regulation of the Denitrification with NO3-N Measuring Instruments 

B.jpgNO3-N is measured upon outlet of the denitrification. With the help of this measured value of NO3-N, the re-circulation pumps and the bypassing of the preliminary clarification are piloted by the process control system where all lines can be controlled independently of each other. If the measured value of NO3-N is smaller than an adjustable value (3 mg/l), the re-circulation pumps are regulated by the process control system at a maximal value (400% of QDW in influx). 
If the measured value of NO3-N is larger than an adjustable value (3 mg/l), the re-circulation pumps are controlled by the process control system at a minimal value (100% of QDW in influx). 
If the measured value of NO3-N is still larger than an adjustable value (3 mg/l) at the next measurement (one hour later), the primary clarifiers are additionally dealt with by the process control system so that more carbon enters the denitrification zone. T
he operation has shown that with this regulation strategy, the concentration of nitrate in the outlet can be kept low (yearly mean = 3.65 mg/l). 


c)Regulation of Phosphate Precipitation with PO4-P Measuring Instruments 

C.jpgPO4-P is measured upon outlet of nitrification in influx and outlet of the wastewater treatment plant. 
The precipitant dosing takes place in the third cycle of the nitrification. The frequency controlled metering pumps can be regulated according to the following possibilities: 

- according to the amount of influx 
- according to the PO4-P concentration in the nitrification outlet 
- according to the PO4-P loading in influx 
- according to pre-set maximal PO4-P loading in outlet 
- according to work and break periods 

In operation, the regulation of the precipitant metering pumps, according to the PO4-P concentration, has proved to be the most optimal plant management. 


d)Regulation of Excess Sludge Extraction 

D.jpgThe excess sludge is extracted from the return-sludge stream. A solid matter meter, which is recorded in the process control system, is placed in the nitrification basins. 
The regulation of the frequency controlled excess sludge pumps occurs automatically over a given dried matter content. This dried matter content is dependent on the deviation, although it shouldn't fall under a certain adjustable daily minimum. 

In addition to these possibilities, a daily quantity can be given, or work and break period can be selected through the process control system. As a further possibility, the regulation of the excess sludge extraction was programmed according to a pre-determined sludge age. 
The computer adopts the dried matter excess from the laboratory protocol, the sludge age is pre-determined, dried matterExcess is measured and so the amount of excess sludge is automatically subtracted. This method of regulation covered a period of about four months and functioned flawlessly. At this time, the excess sludge extraction is controlled according to dried matterAeration tank. 


e) Regulation of pH Value Activated Sludge Tanks 

E.jpgA pH measuring devise is installed in each activated sludge tank. A set point value is given in the process control system and depending on the principal deviation, the lime milk metering pumps are controlled. These pumps are equipped with frequency rotary converters. 
A lime milk dissolving station with a lime silo, in which lime Ca(OH)2 exists in powder, is added. The metering of lime milk is necessary in order to keep the acid carryover capacity upright and to guarantee nitrification.

Operation experience

Operation Experience with On-Line Analysis Equipment

Previous operation experience has shown us that on-line measuring instruments are an essential support to secure adherence to the demanded limit in outflow. They are of enormous importance for the early recognition of anomalies in the biocenosis and, with the help of the recorded course, give an extensive overview of the function in the stimulation. With conscientious and professionally competent care by motivated company staff, the reliability of the operation and the measurement accuracy of the on-line measuring instruments are guaranteed. The expenditure for the purchase and the care of the measuring instruments is calculated into plant business. 


a) Time Spent 

Time needed for the servicing of the instruments (inspection, duplication tests in the laboratory, calibration, prevention of pollution in the pipelines and preparation of the reagents) was all together 170 hours in the business year 2003, 172 hours in the year 2004, 144 hours in the year 2005, 138 hours in the business year 2006 and 140 hours in the business year 2007. 
The time involved per instrument and per week amounted to an average of 33 minutes in the year 2003, 33 minutes in 2004, 28 minutes in 2005, 27 minutes in the year 2006 and 27 minutes in 2007. The time required drops with the years because one becomes always more familiar with the instruments. 


b) Reliability of the Operation 
The reliability of the operation was established when the downtime of the on-line measuring instruments was recorded. The downtime of all instruments together amounted to 657 hours in 2003, 919 hours in 2004, 760 hours in 2005, 424 hours in 2006 and 775 hours in 2007. 
The average downtime per on-line measuring instrument was 109 hours in 2003 (4.45 days per year and instrument) 153 hours in 2004 (6.38 days per year and instrument), 127 hours in 2005 (5.28 days per year and instrument), 71 hours in 2006 (2.94 days per year and instrument) and 129 hours in 2007 (5.38 days per year and instrument). All on-line measuring instruments are very reliable. 


c) Accuracy of Measurement 

From 1998 to 2007, a total of 1,097 countermeasures for all on-line measuring instruments were carried out int the laboratory. 
From these 1,097 countermeasures 934 measures (85%) were within the permissible deviation guaranteed by the manufacturer. With 163 of the 1,097 measurements (15%), the permissible deviations were slightly too large. 


d) Operational Costs

Impianto_reg_Esp.jpg

For Viewing

Virtual Tour

We invite you to a virtual tour of the facility: cross section of the 3 caverns

 

 

Technical Data

1. TUNNEL
Rock mass class: IV
Excavated cross sectional area: approx. 250 m2
Rock mass loading: 250 kN/m²
Supporting resistance: Rock mass carrying ring 280 kN/m2
Shotcrete and reinforcement
300 kN/m2
Anchors 120 kN/m2
Total sum of supporting resistance
700 kN/m2
Anchors around the galleries:
10-15 pieces
Distance between the rows of anchors:
2-2,5 m
Length of the anchors: 4.5 / 6 / 7,5 m
Bearing capacity of the mortar anchors:
30 Tonnes
Stabilisation and Support System
1° layer shotcrete 10-15 cm, welded wire meshes diameter 5/20/20, grouted anchors, N = 4,5-6,0-7,50m GEWI bar diameter 28 mm
2. layer shotcrete 10 cm HS Cement, welded wire meshes diameter 5/20/2
Central gallery
Cross sectional area: 180 m2
Length: 325 m
Side Galleries
Cross sectional area: 250 m2
Length 403 m
2. CONSTRUCTIONAL PART
Aerated grit chamber and grease trap 2 x 200 m3
Preliminary clarifier 8 x 260 m3 = 2.080 m3 (retention period during dry weather flow approx. 1 h - lateral flow)
Denitrification basin 4 x 3.250 m3 = 13.000 m3
Aeration basin 4 x 6.350 m3 = 25.000 m3 (retention period during dry weather flow approx. 3.7 h - fine bubbly in depth aeration)
Final clarifier 8 x 1.400 m3 = 11.200 m3 (surface load during dry weather flow approx. 0.75 m/h - retention period during dry weather flow approx. 2.8 h)
TOTAL EFFECTIVE VOLUME: 50.000 m3
Service Building
Built over area: approx. 2.700 m2
Excavated material: Rock excavation ca. 13.000 m3
Loose material ca. 12.000 m3
Rock and slope stabilisation ca. 3.000 m2
Need of: Concrete ca. 5.000 m3
Lining ca. 20.000 m2
Ceilings 4.000 m2
Reinforcing steel ca. 400.000 kg
Gasometer
Effective volume 2.000 m3
Digestion Chambers
Effective volume: 2.000 m3
Excavated material: Rock excavation approx. 13.000 m3
Loose material approx. 7.000 m3
Need of: Shotcrete approx. 500 m3
Anchors approx. 2.000 m
Welded wire mesh approx. 3.000 m2
Concrete approx. 2.000 m3
Reinforcing steel approx. 150.000 kg
Lining approx. 5.000 m2
Concrete Works
Concrete 50.000 m3
Reinforcing steel ca. 1.500 tonnes = approx. 4.160 mixer conveyor trucks = approx. 60 truck loads
3. THE PLANT
Catchment area 900 km2 = 10% of the total area of South Tyrol 
Dimensioning principles:
Population Data
Hydraulic load 95.000 inhabitants and population equivalent respectively
Specific sewage flow rate per head 300 I/Day
Dry weather flow QDWF 570 I/s
Wet weather flow QWWF 1.140 I/s
Daily quantity of sewage 28.500 m3/Day
Biological load 130.000 inhabitants and population equivalent respectively
Biological load
BOD5 - loads Specific load value 60 g/Day
Daily load 7.800 kg/Day
COD - loads Specific load value 110 g/Day
Daily load 14.300 kg/Day
NKJ - loads Daily load 12 g/Day
Daily load 1.560 kg/Day
NH4-N - loads Daily load 910 kg/Day
NO3-N - loads Daily load 60 kg/Day
Ntotale - loads Daily load 1.620 kg/Day
P - loads Specific load value 3 g/Day
Daily load 390 kg/Day
Sewerage
Main sewer Total length approx. 90 km - Diameter 300 - 1200 mm
Branch sewer (transportation sewer and collection sewer) Total length approx. 90 km - Diameter 250 - 600 mm
Ventilation System Caverns
Air exchange Preliminary treatment area 1 - 6 times
Side galleries area 1 - 3 times
Technical chambers area 1 - 15 times
Maximum amount of air approx. 150.000 m3/h
Sludge dryer
Capacity Capacity evaporation 2,0 t/h
Thermal recycling plant
Capacity Trought put 550 kg/h