With the rapid advancement of water treatment technologies, the control systems for large-scale ultrafiltration sewage treatment have become increasingly complex. This complexity introduces more challenges in automated operations, especially in large ultrafiltration systems. Ensuring efficient, stable, reliable, and safe operation has become a critical research focus for future developments in such systems. This paper examines the 600,000 tons/day ultrafiltration system at Beijing Xiaohongmen Wastewater Treatment Plant. It addresses key issues like corridor competition during backwashing and air scrubbing, which not only protect the ultrafiltration membranes but also ensure smooth system operation.
To handle fluctuations in influent flow, an adaptive step water intake control method is employed, implementing a "demand-based" approach. This means adjusting the system’s processing capacity according to the actual amount of wastewater being treated, ensuring continuous and stable performance. Additionally, water hammer—a persistent challenge in large-scale ultrafiltration systems—has been effectively mitigated through the use of variable frequency S-curve pump shutdowns and PWM valve control techniques. To further enhance system stability, a queue scheduling algorithm was introduced to manage corridor competition efficiently. The implementation of dual ring network technology (a large ring combined with small rings) also contributes to the overall system reliability.
The ultrafiltration sewage treatment process relies on membrane pore size as a filtering mechanism, using pressure differences across the membrane to separate contaminants from the water. The system uses large-scale ultrafiltration membranes to filter significant volumes of wastewater, achieving purification and screening. The control system utilizes Rockwell Automation PLC as the main controller, with remote I/O valves managing pneumatic valves across the system. The network is structured as a “main ring + sub-ring†configuration, enabling seamless communication between 3 main stations and 48 sub-stations.
Each sub-station connects via Ethernet, forming smaller local networks that are linked through switches and fiber optics. The monitoring system is integrated into the main ring network, providing real-time oversight of the entire ultrafiltration process. In terms of control, the system is divided into three key areas: inlet control, water production and cleaning, and water hammer management. For inlet control, maintaining a stable sump level is crucial. The system adjusts its processing rate based on the inflow volume, avoiding unnecessary parameter changes that could destabilize the system.
A step-wise adaptive control strategy is used to balance the inflow and outflow, ensuring the sump remains within safe levels without frequent adjustments. Experimental data and real-world testing were used to determine optimal parameters for different flow conditions. During filtration and cleaning cycles, the system goes through four stages: water production, backwashing, air scrubbing, and emptying. However, conflicts arise when multiple corridors require backwashing simultaneously, leading to what is known as "corridor competition."
To address this, a queuing system was implemented, following a first-in-first-out (FIFO) principle. This ensures fair and timely handling of backwashing requests, minimizing disruptions and maximizing system efficiency. Regarding water hammer, the system employs slow pump shutdowns using S-curves and PWM-controlled valve closures. These methods significantly reduce pressure surges, enhancing pipeline safety and extending equipment lifespan.
In conclusion, the application of adaptive water intake, queue scheduling, and advanced control strategies has ensured the long-term efficient and stable operation of the ultrafiltration system at Xiaohongmen Wastewater Treatment Plant. These innovations contribute to safer, more reliable, and sustainable wastewater treatment processes.
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