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Block diagram of sludge route

Sludge compression

Compression is the first step to reduce sludge volume in order to ensure correct processes. The two types of sludge (raw and excess) generated in the plant are compressed by two different technical solutions.

Gravitational compression – raw sludge compression method

There are three gravitational compressors in the plant, one of which is a backup. Gravitational compression is one of the most economical methods of compression and it requires very little energy. In this compression process, the sludge settles at the bottom of the gravity compressor due to the gravity. Compression of the sludge is helped by rodded excavation equipment which is mounted within the concrete structure. The leachate is removed through the weir mounted on the wall of the structure.

The bottom excavator shovel delivers the sludge in the sludge sump located in the centre of the gravitational compressor, where the screw pumps turn it over in the mixed sludge tank.

In order to avoid stink emission, compressors are covered, and the contaminated air sucked from the airspace is treated in odour treatment units. The gravitational compressor operates constantly, 24 hours a day, 7 days a week.

Compressor tables – for the compression of biological excess sludge

The sludge is delivered by screw pumps from the biological compressor basins to the dewatering table. So-called polyelectrolytes are needed for the compression on the dewatering tables. Polyelectrolytes are soaked with drinking water in two fully automatic processing/dissolution installations. Deflectors on the filter table ensure better distribution of the sludge on the filter screen and thus better water delivery capacity is achieved.

The water between the flakes passes through the filter screen and when the sludge reaches the end of the equipment due to gravity, the water leaves the compressed sludge. Filtrate water is collected at the bottom of the equipment and then returned to the beginning of the plant.

There are six (five plus a backup) pre-dewatering/compressor belt filters in the plant, which operate continuously, just like the gravity compressors.

Compressed sludge mixing tank

The two types of compressed sludge are mixed here. In order to achieve good mixing, the basin is equipped with 7 submersible mixers which hold the sludge in a suspended state and prevent sludge deposition. The volume of the compressed sludge mixing basin is 385 m3. After that, 3 screw pumps deliver the mixed compressed sludge to the pasteurizers.

Warming the compressed sludge

Warming is carried out in three stages using a complex sludge-water “pipe in the pipe” heat exchanger, which reduces the energy needs of the system. In the first step, the heat of the warm sludge from the digester is used to warm up the compressed sludge (using a heat recovery loop). In the second step, the hot sludge (70°C) pumped out from the pasteurizer tank raises the temperature of the already pre-heated compressed sludge with the help of a heat recovery loop. The control system ensures that the pasteurized sludge is cooled down to a temperature that corresponds to the technological heat needs of the thermophilic digesters. Cooling water from the gas engine is also used in the last heating cycle; this helps to increase the maximum temperature of the compressed sludge to 70°C.

Heat exchangers warm up and cool down the treated sludge and provide the appropriate feed temperature of the digesters in order to maintain the desired technological temperature in them. The heat exchanger system uses “pipe in the pipe” sludge-water-sludge heat exchangers instead of the traditional error-prone spiral sludge-sludge heat exchanger.


During this process, the mixed sludge is heated up to 70°C, then it is kept at this temperature for 30 minutes. Reduction of the number of pathogens is achieved through their destruction by heat. As there are 3 pasteurization tanks operating at the same time and they operate in cascade configuration (loading-pasteurisation-emptying), this construction allows the continuous and uniform feed of the digesters. Two operational and one backup pasteurization lines can be found in the plant, each of which includes three pasteurizer tanks and heat exchangers.

Thermophilic digestion

The next technological step in the sludge treatment is digestion, which occurs at a temperature of 55°C. The anaerobic bacteria turn the organic matter into biogas (methane, carbon dioxide and other ingredients of different ppm volumes) in the digesters. Thermophilic digestion (as opposed to mesophilic digestion) ensures good utilisation of the solid biological materials, because it can split proteins and polymers outside of the cells further, and thus converts more organic material into biogas. In this way, the volume of the sludge is reduced and better sludge stability can be achieved with the digestion of the organic material. However, this operation requires more caution as the microorganisms in it are much more sensitive than their mesophilic counterparts and they react quickly to any changes.

Three digesters are in operation in the plant, each with a volume of 6,056 m3. The towers are cylinder-conical reinforced concrete structures, equipped with insulation and their floor plan is shaped like a triangle. The top of the digesters can be reached from a central stair tower.

Sludge in the digester is mixed by a vertical axis mechanical mixer. The sludge is fed through to the bottom of the digester and drifted upwards toward the top of the digesters. Feeding the sludge to the bottom of the digester helps the mixing through the temperature gradient in the digester.

It is possible to add iron (III) chloride in each digester. This is only necessary when the hydrogen sulphide level is higher than the value allowed for the operation of the gas engines (150 ppm).

The biogas obtained is led into a washer, where the adhering particles and foam residues are removed. In addition to this, biogas is treated in active carbon filters directly before it is used by the gas engines. In this way, the risks associated with siloxane can be minimized.

Digested sludge reservoir

This is an approximately 1,700 m3 buffer tank before dewatering. It is a covered, cylindrical concrete structure. There are immersion blenders in the tank to provide appropriate mixing and keep the sludge in a suspended state in order to prevent it from settling at the bottom of the structure.


An average of 24-26,000 Nm3 biogas is produced at the plant daily, which covers about 55-60% of our own energy use.

Thanks to the combined energy production, biogas produced in the thermophilic digesters can be used for three purposes:

  • electricity production,
  • heat generation for the pasteurizer system and for the heating of the digesters (technological heat demand),
  • heat production for the heating of the buildings in the wastewater treatment plant (plant heat demand).

The plant includes two operational gas engines and a backup gas engine; these are suitable for the utilization of biogas and natural gas. These are able to produce up to 1.4 MW/400 V electrical energy and 1.4 MW heat energy.

In addition to the gas engines, three boilers capable of producing 2.5 MW heat energy are installed in the plant. These are able to provide additional heat energy to the quantity of heat produced by the gas engines. Their operation depends on the heat demand. The boilers can be operated with both natural and biogas. In the event of operational failure, there is a flare available to burn the biogas safely.

In order to settle the periodic variation between production and consumption, two double membrane gas tanks were built, each with a volume of 3,500 m3.

Sludge dewatering

In accordance with the standards, the solid matter content of the dewatered sludge must be above 26% on a monthly average. To ensure this, we use high-performance centrifuges for sludge dewatering.

Digested sludge is delivered up to the centrifuges by screw pumps. Coagulant (iron (III) chloride) and polyelectrolyte solution are added to the digested sludge before the centrifuges for helping to dewater the sludge.

Dewatered sludge reservoir

After the centrifuges, the dewatered sludge is stored in 4 silos – 210 m3 each – until its final disposal. From the silos, the sludge is placed on the vehicles with the help of mechanical equipment in an enclosed shed, and the cleaning of the vehicles is carried out in another shed.

Sidestream Nitrogen Removal (3rd treatment stage)

This development of wastewater treatment technology aiming to reduce nitrogen emission is about using a sidestream biological nitrogen removal technology – independent of the main purification units – called DEMON for treating the sludge water (centrifugal leachate) high in ammonia generated during the dewatering of the mixed sludge resulting from the basic technology and digested at thermophilic temperature. Its capacity is 1,500 m3/day. The leachate treated here is eventually returned to the mainstream treatment line.

The technology is an SBR-type ammonia removal treatment carried out with a special active sludge. The bacteria typically form high density red granules. Cyclicality is the quintessence of SBR technologies, which in this case refers to the alternation of aeration, agitation, dispatching (feeding), settling and decantation processes in accordance with the strictly defined cycle times.