The highly dewatered sludge is pumped into the granulation unit through an eccentric monopump, where it forms an average 7-9 mm diameter stick pasta shape and spreads homogeneously on the belt. The sludge sticks formed here are cut into specific lengths and spread onto the belt. This method ensures that the sludge is evenly spread on the belt, allowing efficient air circulation and drying.

The granulation unit has several nozzles. The unit can be washed with pressurized water when needed, and regular washing intervals are required. In case the unit becomes completely clogged due to foreign matter in the sludge, it may need to be disassembled and cleaned mechanically. This process is a simple maintenance task that takes approximately 1.5 hours.

The sludge spread on the belt is transported forward inside the tunnel via a conveyor belt. The conveyor belt is perforated, and hot air passes through these holes, making contact with the sludge and drying the wet sludge. In these types of convective drying plants, the slow initial heating and drying of the sludge in the first step ensure that the product dries without disturbing its structure. The heat required for evaporation is taken from the hot air supplied into the system. As a result, the hot air begins to cool down.

After the first belt, the product falls onto the second belt below. This belt moves in the opposite direction, covering the same distance to achieve approximately 90% dryness. During this time, the product maintains its homogeneous structure.

The belt width is exactly 2500 or 4000 mm. The belt speed can be adjusted in the range of 0.2-0.7 m/min. The belt material is perforated stainless steel. The part of the upper belt that comes into contact with the sludge is coated with PFA or PTFE. The perforation pattern is provided below.

Additionally, the system includes one or two product turnover systems near the middle of the belt to ensure more homogeneous drying of the product. This way, the product is both mixed and prevents sticking or clumping.

If necessary, a water-cooled screw conveyor can be added to the exit of the drying plant to lower the temperature of the outgoing product below the desired level. The product exiting the dryer is suitable for pelletizing, has a structure that does not produce dust, and is at the correct temperature. The final product can be transported without pelletizing if needed.

The thermal energy required for the drying process can be obtained from waste heat, biogas, natural gas, electricity, or any other energy source.

In cases where biogas or natural gas is used as the thermal energy source, hot air is produced in a hot air boiler.

Hot air is produced using a gas burner and a furnace. While part of the heated air becomes clean air, some of the air is obtained from the air circulating in the condensate line.

Circulation air is the air taken from the dryer in a closed loop, heated again in the gas burner, and returned to the system.

Part of the internal circulation air is drawn in and transferred to the condensate unit.

In the first step of the condensate system, the air is cooled in a water/air heat exchanger to bring it to the condensation point. The energy gained in the water/air heat exchanger is used to reheat the returning air.

The condensate unit is an air purification unit integrated with a moisture trap that has water spray nozzles. By cooling the waste air, the air temperature is reduced, and condensation is achieved.

The temperature of the returning air is approximately 40°C and is divided into two streams.

The waste air is the air released to the outside and can be treated again with an additional system. Clean air is required for the burner, equal to the amount of waste air.

The remaining air is referred to as circulation air. This air is reheated in the water/air heat exchanger (using the heat obtained from the cooling cycle of the condensate unit) and is sent directly to the combustion heat exchanger. Here, the air is heated to a higher temperature.

The heat recovery from the waste air recovery system increases thermal efficiency and reduces the amount of waste air.

Wet sludge is passed through nozzles in the granulation unit before drying to shape it into the appropriate pasta form. This pre-shaping helps prevent the formation of granules with different diameters and significantly reduces dust formation.

Since the product is transported on a conveyor belt, it is free from effects that could cause dust, such as friction, tension, or centrifugation.

Due to the very slow airflow rate, dust particles cannot be carried by air.

In the facility, dust emissions into the drying building air or mixing with flue gases are minimal. The small amount of dust that emerges accumulates at the bottom of the drying tunnel over time. This dust should be cleaned and removed from the environment once every 1 or 2 years.

Due to the very low amount of dust, the risk of combustion, ignition, or flame formation is also extremely low.

The drying facility will be equipped with one online dust measurement device. This device will trigger an alarm if the dust concentration in the drying tunnel reaches 30 mg/m³ during any operational period, and the system will shut down for safety. Combustion/explosion risks due to dust can only occur if the dust concentration reaches values like 50-100 g/m³. The dust sensor set to a maximum value of 30 mg/m³ eliminates the possibility of combustion or explosion-related incidents in the facility.

Since the drying temperature is considered low, between 130-140°C, the product is dried through convective heat transfer without altering its structure. This means that the heat required for evaporation is provided by hot air. This system, based on high-temperature heating with surface contact, offers significant advantages in terms of odor control. As a result, the amount of waste air is reduced, and consequently, the odor levels also decrease.

Additionally, the homogeneous distribution of the product prevents regional overheating, ensuring uniform drying across all areas.

The waste air is also cleaned with the Wet Cleaning System. Waste air is cooled using the water spray method in the gas washing unit (Wet Scrubber). This allows many of the gases to condense along with odor-causing substances. Additionally, fine dust particles are separated from the air during the cleaning process.

The water used in the Gas Washing Unit is sourced from the effluent of the Treatment Plant. All water discharged from the system is returned to the facility’s intake.

This waste air cleaning system guarantees the removal of odor and the separation of fine dust particles.

The incoming treatment sludge is dried to between 85% and 95%. As a result, weight and mass loss occur. The resulting sludge has a pellet structure suitable for various disposal methods (such as storage, incineration, fertilizer use, etc.). The temperature of the final product exiting the dryer is in the range of 85-90°C, and this temperature will be reduced to the range of 45°C with the help of a cooling jacketed screw conveyor at the drying exit before being transferred to the dry product silo.

The dewatered sludge, processed by belt filter presses in the treatment plant, will be transferred to the sludge silo by a monopump. From the bottom of the sludge silo, the dewatered sludge cake is pumped by another monopump, pressurized, and sent to the granulation unit at the beginning of the belt drying facility.

In the granulation unit, the sludge cake is formed into long, thin pasta-like strips and spread onto the first conveyor belt. The sludge, which begins to dry on the first belt, then falls onto the second belt.

At the end of the second belt, the sludge, now dried to the desired level, is transferred to the dry sludge conveyor. The dry product is then transferred to the dry product silo via this conveyor.

The product held in the dry product silo will be transported to the final disposal point by trucks as the silo fills up.

A portion of the hot air used in the system is continuously recirculated. As a result, a certain amount of air is drawn in as clean air from the atmosphere, and an equal amount of air is discharged as flue gas from the system.

The hot air will be produced using a direct-fired air heater. The main hot air flow, which reaches a temperature of 280°C at the exit of the air heater, is directed directly beneath the drying tunnel and is then evenly distributed along the entire length and width of the drying tunnel through an internal duct system.

The hot air supplied to the drying tunnel is recirculated approximately four times per hour inside the tunnel. This ensures that the air becomes saturated with moisture and that the water in the sludge evaporates and transitions into the air phase.

The moisture-laden air is drawn from the drying tunnel by the **flue gas fan**. This air flow, which contains moisture, will enter a condensation unit working on the principle of wet scrubbing to remove moisture and potentially wash out dust pollutants. In this unit, the air is washed with clean water, and then it is returned to the air heater to be reheated.

1 – Granulation Unit

2 – Drying Tunnel

3 – Belt Drive Group

4 – Recuperation Fan

5 – Hot Air Fan

6 – Hot Air Boiler

7 – Heat Exchanger

8 – Wet Washing Unit

9 – Discharge Conveyors