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Mechanical Biological Treatment - MBT

MBT Ljubljana: In Slovenia arises one of the largest and most modern plants in Europe

The new MBT in Slovenia’s capital gets ready to become one of the largest and most modern waste treatment plants in Europe. It will produce biogas, recover heat and power as well as SRF and other recyclable products while avoiding landfilling. The new facility is implemented into the existing waste management centre of Ljubljana


1 Introduction and scope of work
2 Technical concepts and planned process steps
3 Economic characteristic data
4 Current status of implementation

1 Introduction and scope of work

Already in 2012 the Municipality of Ljubljana has awarded the contract for the new Regional Centre for Waste Management RCERO Ljubljana (Regijskega centra za ravnanje z odpadki Ljubljana) to produce biogas from organic waste with the patented STRABAG LARAN® plug-flow technology, for production of SRF and recovery of valuable materials. Since then this waste treatment and biogas plant is constructed for € 112,2 million and thereby one of the largest and most modern MBT plants arises in Europe.

Figure 1 Model of RCERO Ljubljana at Barje landfill site (2012)
On the one hand the project comprises the construction of replacement buildings for objects that had to be removed at the construction site near the landfill Barje. On the other hand all building permits had to be obtained so that the new turnkey mechanical-biological waste treatment plant (MBT) can be handed over in 2016. The integrated plant concept will enable an efficient processing of approx. 171.000 Mg/a (Phase 1).
Due to intensive activities in accordance with Slovenian legislation resulting from the current European Union Directives, significant changes in quantities and waste structure are expected. Major changes are expected until 2020 when certain collection/recycling goals need to be reached. Considering these facts, the plant will be constructed in a way that the highest flexibility will be provided and all collected waste types are going to be treated also in Phase

2 Technical concepts and planned process steps

2.1 Technical concepts

Prime target of the project is to avoid landfilling at the existing landfill site Barje and to recover as much as reasonable waste products which are reusable and to obtain biogas resp. to produce heat and power from organic waste fractions. The residuals from waste treatment are utilised as SRF, on the one hand as high calorific fuel applicable in industrial thermal processes (SRF A) and on the other hand as low calorific fuel (SRF B).
Table 1 Waste input data (2012)
Depending on specific waste types and waste input characteristics, obtained from field surveys carried out by the Municipality of Ljubljana, the technical concepts have been generated.
Figure 2 Block diagram of MBT Ljubljana (2012)
The MBT is designed for two phases of expansion, whereas the initial stage includes already a complete and fully functional treatment facility for approx. 171.000 Mg/a. Possible future extensions are for extending the capacity, e.g. up to 60.000 Mg/a biogenic waste, including space for an additional digester for AD.
2.2 Planned process steps
2.2.1 Mechanical pre-treatment, sorting and SRF production

The delivered municipal waste (MHW), commercial waste (PTSS), bulky waste (BW) and packaging material are first split in the mechanical pre-treatment in several lines by means of shredding or just bag splitting, screening and ballistic separation in 2D, 3D and fine fraction. Also classical methods are applied for the recovery of scrap iron and non-ferrous metals.
Then, the particular mass flows are further processed in order to recover valuable materials and for the production of solid recovered fuels (SRF). For this purpose innovative sorting methods are implemented by using optical sorters (NIR). This sensor-based sorting technology with an additional manual follow-up check allows to produce high-quality, almost pure product streams of HDPE and PET bottles for recycling. The recyclables are pressed into bales finally. By means of compressing the volume and the storage space are decreasing and the logistics significantly simplify.
However MHW, PTSS, BW or packaging material are not processed together, meaning that the different waste types are not fed simultaneously, but in different shifts with the particular material, to optimise the product quality of recyclables.
Special applications, e.g. for sorting PET bottles by colour, may be carried out by feeding a mixed PET fraction once again and appropriate adjustment of the NIR sorter.
In order to improve the quality of refuse derived fuels optical sorters (NIR) are also used to separate PVC. Depending on the fuel specification different shredding aggregates are applied, too. The alternative fuels (SRF) can be transported for recycling either loose or baled. The baling enables a substantial reduction in volume and is in compliance with the requirements for storability of the SRF.
2.2.2 Biological treatment

In the mechanical pre-treatment is recovered also an organic fraction from the delivered municipal waste (MHW), which is utilised in the biological treatment to produce biogas in two STRABAG LARAN® plug flow digesters and finally in the biological stabilisation with drying of the digestate.
The delivered biogenic waste (BHW) and yard waste are prepared for biological treatment depending on the type of waste in the mechanical processing, on the one hand to produce biogas in a STRABAG LARAN® plug flow digester and secondly for composting of the digestate (intensive composting and curing phase) and compost refining.
Figure 3 Process scheme of a STRABAG LARAN® Dry Digester
The two horizontal plug flow reactors for organic fraction of MHW (mesophilic operation) and the one for BHW (thermophilic operation) are of identical design and type TF 2200. Therefore a second plug flow reactor can be made available for BHW easily substituting one plug flow reactor from MHW digestion (Phase 2).
Dry anaerobic digestion has proven its capability for drier waste types with higher structure content, like organic fractions from municipal solid waste and residual household waste as well as for typically wet food waste and biowaste. According to the experience dry anaerobic digestion provides a less complicated operation as it avoids the higher water volumes and additional reject streams typically for wet anaerobic processes. The main characteristics of the STRABAG LARAN® plug-flow technology are:

- horizontal, rectangular plug flow digester providing large surface for biogas release
- plug-flow – providing a controlled residence time within digester and
- this reduces mixing and movements inside the digester
- possibility of product and centrate recirculation giving a stable biological process
- reduced space requirements, modular system making the units very service-friendly
- operation either in meso- (»37ºC) or thermophilic (»57ºC) temperature ranges whilst
- using the same heating circuit for both temperature ranges
- flexibility in treatment capacity due to
- digester filling level and residence time can be varied according to necessary capacity
- flexibility in operation according to waste quality – even dry waste can be treated
The digesters are fed one after another with organic fraction from MHW or BHW from the intermediate storage buffer (continuous operation mode on 7 days per week and 24 hours per day in contrast to the mechanical pre-treatment which is feeding the intermediate storage buffer on 260 working days per year and 16 operational hours per day) which ensures continuous and even anaerobic digestion and biogas production.
Additionally, the waste is inoculated with centrate from the downstream dewatering units. The centrate is "anaerobically active” water re-circulated to activate the fresh material and to adjust the dry substance moisture content at the digester inlet. At the required design throughput, the retention time in the anaerobic digesters would be approx. 25 days. After the AD the material is taken out with vacuum system to dewatering.
The three-stage dewatering of digestate from BHW and MHW will be done consecutively to avoid a mixture of the different press cakes for ongoing stabilisation/maturation and the different centrates. To avoid contamination of each waste stream, the entire dewatering sequence cascade is flushed with fugate from biowaste fugate tank which will then (after the flushing) be returned into the MHW processing. This assures "biowaste fugate” quality level throughout the entire dewatering sequence cascade at the beginning of both dewatering cycles. Three screw presses, two vibrating screens and two decanter machines are available and used parallel for dewatering either BHW digestate or MHW digestate. No surplus water will remain from MHW and from BHW line besides emergency overflow.
The press cake-/screen-overflow (screw press, vibrating screen, decanter) from MHW is transported to a mixing device where it is unified with structure material from MHW and then the stabilisation boxes are filled by means of an automatic feeding system to the stabilisation boxes. The solids from dewatering of BHW are conveyed straight to the composting boxes whereas structure material (e.g. shredded green waste or coarse fraction from compost refining and shredded untreated wood, if adequate) is dosed from an intermediate storage conveyor directly onto the conveyor system transporting the digestion residues.
The boxes for stabilisation and composting are of same type and provide redundancy and also the biological process for MHW and BHW is similar. The plant is equipped with process ventilators and heat exchangers for the intermittent aeration of the boxes.
2.2.3 Biogas and energy recovery

The energy recovery of the biogas takes place in a common plant technology. The biogas produced from the three STRABAG LARAN® plug flow digesters is buffered after desulphurisation and siloxane-cleaning in a biogas storage and then passed for energy recovery in three cogeneration units (CHP). The supply of electrical energy (about 2 MW) takes place into the public network. The waste heat - both from the existing CHP (landfill gas) and the new gas engines - is fed into a common central heating system and utilised in the plant (e.g. for heating the digesters or drying the digestates).
2.3 Accessories and infrastructure

2.3.1 General

Also included in the scope of work are the soil preparation, construction of an administration building and the setting up of the entire infrastructure, including roads and supporting structures, and the renewal of the bridge on the access road.
To the infrastructure belong amongst others the demolition of the existing car wash plant and the erection of the new car wash plant, washing area for heavy mobile equipment and installations for the workshop facilities and laboratory. A visitor gallery throughout the entire mechanical pre-treatment and sorting plant is also included.
High performance requirements are to be fulfilled for ventilation respectively exhaust air treatment as well as fire protection and firefighting, too.
2.3.2 Fire protection and firefighting

Various installations are provided to achieve utmost fire protection and highest efficiency for firefighting to prevent spreading of the fire inside the building and on neighboring objects, e.g.: - subterranean water tank for fire-fighting and hydrant system with diesel pumps

- several fire zones with internal firewalls and de-smoking openings
- statics of roof constructions and emergency ladders considering thermal loads
- manual alarm buttons at the doors of halls
- fire access paths and safety lighting - ventilation piping and
- conveyors passing fire zones equipped with fire retarding devices
- CO-detection for early detection of hidden fires in waste bunker
- fire detection, alarm system and sprinkler system in treatment halls
- independent fire extinguishing with foam for shredders
- detection of sparks and separate fire extinguishing units and
- standard ex-protection measures on specific equipment
2.3.3 Ventilation and exhaust air treatment

The ventilation management and exhaust air treatment concerns to the whole MBT plant consisting of the mechanical pre-treatment and the biological treatment. Amounts of waste air from the buildings and single equipment are partly providing the process air flow into the aeration system of stabilisation and composting. The principle of ventilation of the MBT is to provide for negative pressure in the processing halls to avoid uncontrolled emissions.
Ventilation is carried out for the processing halls as well as storage halls and locally at specific equipment. For reduction of the dust load in the ventilation system a dust filter is installed prior the air is reused again. The final treatment of exhaust air from stabilisation and composting is an enclosed biofilter system. Before the exhaust air enters this final treatment stage it is pre-cleaned and conditioned by means of a chemical acid scrubber stage. The latest Slovenian legislation for exhaust air from biological waste treatment is limiting emissions on the stack, e.g. 50 mg non-methane TOC/m³.

3 Economic characteristic data

In Phase 1 the treatment capacity of the MBT is approx. 171.000 Mg/a consisting of household waste (MHW), commercial waste (PTSS), bulky waste (BW) and packaging material. The efficiency of the system is greatly influenced on the one hand by the recovered energy and the other by products. Thus, approximately 30.000 Mg/a recyclable material and 60.000 Mg/a SRF, approximately 41.500 Mg/a stabilised digestate and landfill material and around 7.000 Mg/a compost will be produced. Due to the biological treatment of waste a mass loss of about 32.500 Mg/a contributes a positive impact, too. The supply of electrical energy will be about 2 MW which are fed to the public network. Working places for operation in 2 shifts will be approx. 60 (without management).
In Phase 2 the treatment capacity will be increased to a maximum with extension of operational time for approx. 256.000 Mg/a, with the focus on growth of capacity of biowaste and additionally the maturation area for compost production will be extended accordingly.

4 Current status of implementation

Actually in early March 2015 the technological civil construction works of the three digesters are finished, of the mechanical pre-treatment halls are under finalisation and of stabilisation/composting as well as maturation are ongoing.
Figure 4 STRABAG LARAN® plug-flow digester under construction (September 2014)
Figure 5 Waste treatment hall under construction (December 2014)
In December 2014 the mounting of the two drum screens in the mechanical pre-treatment hall and also of the bridge cranes in the waste bunker hall has been carried out.
Starting with January 2015 the assembly of ventilation piping works as well as equipment in the mechanical pre-treatment is ongoing.
Figure 6 Drum screen for mechanical pre-treatment (January 2015)

RCERO Ljubljana Strabag Webcam

Live webcam of the construction

Biological Treatment


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Created by Dipl.-Ing. Gerhard Pilz (STRABAG AG), (), last modified by ()


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