An Engineered System Approach to Resource Recovery Straining
Many attempts have been made to filter white water for reuse on machine showers and in similar applications. Most of these attempts have resulted in various levels of success.
The design and implementation of a filtered white water system for use on non-critical showers starts with a detailed process review, including real-time measurements of the existing process. Additional measurements include flows, solids, temperatures, and pressures. This detailed evaluation is necessary to identify process variations to ensure proper application of technologies that will provide the lowest total cost of ownership.
The save-all is typically the first stage of white water filtration and is a critical area of a white water filtration system. If a failure occurs in this section, there is a high probability that rich white water will feed forward to the next stage and foul or plug the subsequent filtration step. Ultimately, the fines and contaminants can arrive at the machine shower nozzles or other services and plug these sensitive areas of the machine.
For example, if a save-all is underperforming or suffers from design deficiencies, these issues must be considered to ensure optimal performance of the mill’s water management system. This is particularly important when designing an engineered solution that is adapted to reflect actual mill operating conditions. Accommodations are made for process variations and to facilitate a continued production of quality filtrate with a minimum of process interruptions.
Once a thorough understanding of the process is gained, the application of equipment and automation can be considered. Many of the common inline process measurement devices are absent. The limited instrumentation increases the system’s vulnerability to upset conditions often resulting in establishing a narrow operational window before upsets occur.
A well-designed filtration system must include a measurement and control strategy. The measurement portion utilizes sensors to alert operators to excessive process variation and will initiate a control strategy to prevent any excessive process variation from reaching the resource recovery devices or machine showers.
A properly engineered filtration system would include two suspended solids sensors to measure and correct for unexpected system process variations. The sensors would measure suspended solids concentration (ppm) and initiate an alarm and corrective action whenever the variation exceeds system design parameters.
A suspended solids sensor and associated analyzer is shown below.
The most common system variations occur when suspended solids concentrations increase substantially during process upsets. One suspended solids sensor would be installed in the supply piping and the second would be installed in the filtrate piping exiting the resource recovery system.
If at any point the suspended solids concentration contained within the supply to the resource recovery devices exceeded system design parameters, flow to the resource recovery devices would be decreased from the source either to a manageable level or shut off entirely if the excessive suspended solids concentration continued to increase.
Using this approach, suspended solids can be actively monitored and the flow automatically restored to the resource recovery devices as the suspended solids concentration returns to manageable levels. The second suspended solids sensor is generally located in the filtrate piping from the resource recovery device to monitor unexpected damage to the resource recovery system screen.
This approach would be utilized to monitor the suspended solids contained in the resource recovery system filtrate; in the event the suspended solids exceeded system design parameters, alarms would be sent and flow to the resource recovery devices would shut off.
Following operator inspections of the resource recovery system screens for any potential damage or required cleaning, the system can then be brought back online after corrective action has been completed.
The basis for this strategy is to protect the machine showers from unexpected process upsets and prevent system failure. This automated approach is recommended to provide a reliable and stable filtered white water source for the machine showers.
The RotoFlex™ recovery strainer has a more robust filtration approach capable of operating effectively in an expanded range when compared to conventional strainers or filtration systems. A RotoFlex recovery strainer trial unit (100-150 gpm) is available for on-site simulations to validate the application and operating performance of the advanced filtration system using actual mill process conditions.