Application of Horizontal Decanter Centrifuge in Chemical Wastewater Treatment (2)

3. Application of Horizontal Decanter Centrifuge

3.1 Equipment Selection

3.1.1 Wastewater Treatment Process

The wastewater treatment process adopts the route of pretreatment + anaerobic treatment + aerobic treatment + advanced treatment.

Pretreatment mainly includes: coagulation sedimentation + dissolved air flotation + EBR reaction, which is used to remove part of suspended solids and large molecules that are not easily degraded.

Biochemical treatment consists of anaerobic and aerobic systems, capable of removing most organic matters that are not easily degraded.

Advanced treatment applies terminal chemical phosphorus removal combined with activated carbon filters to eliminate most phosphorus and fine suspended solids in wastewater.

The feed material of the horizontal decanter centrifuge is the mixture of phosphorus-containing sludge and wastewater generated after degradation by anaerobic-aerobic systems and subsequent terminal chemical phosphorus removal.

Before equipment selection, samples were collected and sent to external institutions for analysis. The results confirmed that the solid content of incoming wastewater is 0.86%, and the ash content of sludge reaches 80.33%.

3.1.2 Selection Method

The selection of horizontal decanter centrifuge shall comprehensively take into account processing capacity, separation effect, rotating speed, materials, structural design, automation level, energy consumption, maintenance, safety, cost and after-sales service, so as to ensure the equipment meets production requirements with good economic efficiency.

At present, there are two commonly used selection methods:

(1) Theoretical calculation: It requires sampling analysis and laboratory tube separation tests, with accurate process parameters provided for model selection.

(2) On-site test verification: The selection data obtained from on-site tests and actual production trials are more reliable, yet this method features high cost and heavy workload.

In view of cost control, the theoretical calculation method is adopted in this project.

The Σ theory is widely applied for calculating feeding capacity and determining model specifications of horizontal decanter centrifuge, focusing on two key indicators:

(1) Hydraulic load: the feed volume processed by the centrifuge per unit time.

(2) Solid load: the sludge discharge volume processed by the centrifuge per unit time.

Solid load shall be prioritized when the solid content of incoming wastewater exceeds 1.5%, while hydraulic load shall be taken as the main consideration when the solid content is lower than 1.5%.

The normal wastewater treatment capacity is 15~16 tons per hour. In consideration of subsequent capacity expansion of the facility, the centrifuge is designed to handle 24 tons per hour of wastewater after expansion, with an operational flexibility of 20% to 50%, reaching a maximum treatment capacity of 28.8 tons per hour. Given the inlet wastewater solid content of 0.86%, hydraulic load is the core factor for equipment selection.

Finally, the LW430EB horizontal decanter centrifuge is selected, which consists of a chemical dosing system and a material conveying system to form a complete technical process.

3.2 Equipment Installation

The installation of horizontal screw decanter centrifuge is relatively simple, which is divided into three steps: site selection, foundation fabrication and equipment assembly.

3.2.1 Site Selection

The installation site shall have sufficient vertical height and proper surrounding space, and be equipped with corresponding hoisting equipment. A steel structure awning shall be built for outdoor installation. As this is a renovation project, the available space in the wastewater treatment area is limited. Besides, the unit is not equipped with a conveyor for sludge discharge, so the selected site is relatively narrow and arranged close to the edge to facilitate forklift sludge handling.

3.2.2 Foundation Fabrication

It is only required to pour the foundation with embedded parts in accordance with the foundation drawings. After the equipment arrives, weld the shock absorber base under the unit firmly to the embedded parts. Steel structure or other types of foundations are also acceptable.

According to the layout drawing provided by the manufacturer, the original designed foundation height was only 0.5 meters. Considering the height of sludge ton barrels and sludge splashing during discharging, the actual foundation height was raised to 1.5 meters. This height is suitable for sludge discharge, allowing smooth access of ton barrels and effectively reducing sludge splashing, yet it is unfavorable for water discharge.

In this process, separated water flows into the clean water tank by gravity without pressure from the centrifuge. Excessively low installation height leads to large pipeline resistance, which hinders timely water drainage. The separated water even overflowed to the sludge discharge port through the connecting pipe of the knife gate valve, restricting the treatment capacity in the early commissioning stage. Normal operation was realized only after adjusting the water discharge process to reduce pipeline resistance.

It is recommended to determine the installation height by comprehensively considering both sludge discharge and water discharge conditions, with the minimum suggested height above 2 meters. If the installation height is restricted by space, install a conveyor at the sludge outlet and add a booster pump for the water discharge pipeline.

3.2.3 Installation Precautions

Read the operation manual carefully before installation and strictly follow the below requirements:

(1) During hoisting, sling ropes shall be firmly connected to the dedicated lifting lugs on the unit, and the included angle between slings shall not exceed 90 degrees.

(2) Place the equipment on the foundation at the correct position, adjust its orientation and spacing, and conduct full inspection in accordance with relevant specifications before firmly welding the vibration dampers to embedded parts.

(3) Check the levelness of the machine based on the base plane. Place a level gauge on the guide rail for preliminary measurement, and the levelness deviation in both length and width directions shall be no more than 2 mm.

(4) Flexible connections shall be adopted for process pipelines directly connected to the equipment to reduce vibration transmission and avoid damage to equipment and pipelines.

3.3 Equipment Commissioning

There are multiple factors affecting the separation performance of horizontal screw decanter centrifuges. The purpose of commissioning is to achieve the optimal and most economical operating condition. The main commissioning items include determining the optimal liquid layer depth, rotating speed, flocculant dosage and differential speed.

3.3.1 Liquid Layer Depth

The liquid layer depth is adjusted by replacing liquid layer regulating plates. Larger plates lead to deeper liquid layers, lower solid content in separated liquid and higher moisture content of discharged sludge. Smaller plates result in shallower liquid layers, higher solid content in separated liquid and lower moisture content of discharged sludge. Selecting proper liquid layer plates is critical to guarantee separation effect. Multiple tests shall be carried out during commissioning to confirm the most suitable plate matching the actual working conditions.

3.3.2 Rotating Speed Analysis

Theoretical analysis shows that higher rotating speed brings greater separation factor and better separation effect. Nevertheless, excessively high speed is not advisable in actual operation, as it will greatly raise requirements on equipment materials, accelerate wear and increase power consumption. Therefore, the rotating speed can be properly reduced on the premise of ensuring separation effect to realize economical operation. Both main and auxiliary motors of LW430EB horizontal screw decanter centrifuge are frequency-conversion designed. After commissioning, the operating frequency of the main motor is set at 32 Hz with a rotating speed of 2300 r/min.

3.3.3 Flocculant Dosage

Increasing flocculant dosage can raise sludge output and improve the clarity of separated liquid. However, excessive dosage will increase operating cost and elevate the COD value of discharged water. Hence, the flocculant dosage shall be adjusted according to actual separation status and feed flow rate.

3.3.4 Differential Speed

Differential speed refers to the speed difference between the drum and the screw conveyor. A larger differential speed enables faster sludge conveying with lower torque, resulting in higher sludge moisture content and clearer effluent. A smaller differential speed slows down sludge conveying with higher torque, leading to lower sludge moisture content and turbid effluent. Accordingly, the differential speed shall be adjusted according to the solid content of feed materials; higher sludge content corresponds to increased torque, and vice versa.

3.4 Handling of Abnormal Working Conditions

3.4.1 Rotor Locking Failure

When material blockage occurs in the horizontal screw decanter centrifuge, audible and visual alarms will be triggered on the electric control cabinet, and alarm codes will appear on the frequency converter.

Solutions:

(1) Lock the output shaft of the auxiliary motor, and manually turn the pulley in the operating direction of the centrifuge until it rotates smoothly.

(2) Fasten the differential gear and run the auxiliary motor in reverse. Keep observing the sludge discharge condition at the knife gate valve and stop the auxiliary motor in due course. After troubleshooting, perform manual flushing of the centrifuge via the touch screen.

3.4.2 Rise of Sludge Moisture Content

If the moisture content of discharged sludge increases, first check changes in feed materials, as well as the normal status of differential speed and flocculant dosage. Prioritize adjusting the differential speed and chemical dosage.

If no obvious improvement is achieved, check whether there is accumulated water in the connecting pipe of the knife gate valve. Water accumulation indicates blocked drain pipelines which need dredging. If no water exists, sludge deposition on liquid layer plates may deepen the liquid layer and further raise sludge moisture content.

Since the commissioning seven months after renovation, sludge buildup on liquid layer plates has repeatedly caused higher sludge moisture content. Therefore, cleaning of liquid layer plates is arranged synchronously during regular shutdown and grease lubrication maintenance.

3.5 Operation Performance

Put into service in August 2024, the horizontal screw decanter centrifuge has maintained stable operation so far. The separated water is clear, and the actual measured moisture content of dewatered sludge is 66%, far below 80%.

The renovation successfully transformed the original batch-type sludge discharge process into continuous discharge for the sludge drying system of the wastewater plant, which greatly reduces labor intensity of operators and optimizes the on-site working environment.

3.6 Economic Benefit Analysis

Before renovation, the daily power consumption of sludge drying was 613.5 kW·h, with an annual low-pressure steam consumption of 456 tons. After renovation, the daily power consumption reaches 1046.25 kW·h.

Based on the average electricity price of 0.68 yuan per kW·h and steam price of 180 yuan per ton:

The annual increased operating cost is calculated as follows:

1046.25×0.68×365−613.5×0.68×365−456×180=25328.6 yuan

The solid content of sludge was about 20% before renovation and rises to around 30% after modification. According to the annual sludge output of 657 tons from phosphorus removal facilities in 2023 and sludge disposal cost of 930 yuan per ton:

Annual saved sludge disposal cost:

(657−657×0.30.2)×930=203760 yuan

The net direct economic benefit brought by the centrifuge is:

203760−25328.6=178431.4 yuan

4. Conclusion

After the application of horizontal screw decanter centrifuges in the sludge dewatering section, favorable sludge dewatering performance has been achieved. This technology solves multiple existing problems including batch-type sludge drying, difficult sludge extraction from underground tanks and strong odor emission of dried sludge in the wastewater treatment unit. It also generates tangible economic benefits, effectively cuts down labor intensity of staff and improves the working environment.

This paper elaborates on the working principle, equipment selection and installation specifications of horizontal screw decanter centrifuges, and summarizes practical experience in commissioning operation and common fault troubleshooting. It can serve as a reliable reference for solving bottleneck problems encountered in sludge dewatering projects under similar working conditions.