In the field of water treatment, multi-media filters are indispensable devices widely used in the purification processes of various water qualities. However, during actual operation, multi-media filters may encounter various problems that affect their filtration efficiency and service life. This paper will analyze the issues encountered in the operation of multi-media filters through a case study from a practical project and explore corresponding solutions to provide references for relevant engineering and technical personnel.
In a water treatment project, multi-media filters were employed for daily water quality filtration tasks. The equipment operated during daytime hours, and initially, the effluent turbidity met the predetermined requirements. However, after approximately 20 days of cumulative operation, a situation arose where the influent water was clear, but the effluent water was turbid. It required a sufficiently long backwash duration to meet the water quality requirements, sometimes even up to 30 minutes. After the operation and commissioning were completed, the incoming water quality was often poor, with the produced water turbidity being substandard and even exceeding the measurement range of the turbidimeter. These issues severely impacted the normal operation of the entire water treatment system, making it crucial to conduct an in-depth analysis and resolution of these problems.
Before delving into solutions, it is essential to conduct a comprehensive and in-depth analysis of the issues encountered in the operation of multi-media filters. Only by accurately identifying the root causes can effective strategies be formulated. The following is a detailed analysis of these issues.
When designing a multi-media filter system, there are clear requirements for the influent turbidity, which is typically not higher than 20 NTU. However, during actual operation, the incoming water turbidity sometimes exceeds the measurement range of the turbidimeter (0 - 200 NTU), which can significantly impact the stable operation of the multi-media filter. When the influent water quality deteriorates, a large amount of suspended solids and impurities enter the filter, causing rapid clogging of the filter layer, reduced filtration efficiency, and deteriorated effluent water quality. Therefore, it is necessary to strictly control the influent water quality. When the influent turbidity exceeds 20 NTU, the equipment should be immediately shut down, and the causes of the deteriorated influent water quality should be detected and analyzed. The operation status of the relevant systems should be checked to ensure that the issue of poor influent water quality is resolved.
Turbid backwash effluent is one of the common issues in the operation of multi-media filters, and the main reasons are as follows:
Multi-media filters typically use a dual-layer filter media packing of anthracite + quartz sand. If the interstitial spaces between the filter media are too large, fine suspended particles may penetrate the filter layer during the filtration process, thereby affecting the effluent water quality. Moreover, if the particle size distribution of the filter media is not reasonable, it will also lead to poor filtration performance.
The intensity of the backwash is crucial for the backwash effect. A low backwash intensity cannot sufficiently loosen the filter layer, thereby affecting the backwash effect. At the same time, a low backwash intensity will also leave sediment in the filter layer. In severe cases, the sediment and filter media will form hard lumps, causing uneven water flow in the filter layer during operation, rapid deterioration of effluent water quality, increased water flow resistance through the filter layer, and rapid head loss during filtration. On the other hand, a high backwash intensity will carry out fine filter media with the backwash water, causing filter media loss.
The designed filter media layer height is 1200 mm, but the actual installation is 1300 mm. Although a higher filter media layer is somewhat beneficial to the effluent water quality, it will increase the pressure difference and head loss, reduce the output, and thereby affect the normal operation of the filter.
After thoroughly analyzing the issues encountered in the operation of multi-media filters, we will now explore targeted solutions. These solutions aim to address the aforementioned problems, optimize the filter's operation, and ensure its long-term stable operation. The following are specific measures.
In response to the deterioration of raw water quality, the original system should be retrofitted and upgraded. Particularly when the desulfurization wastewater is turbid, the operation status of the desulfurization wastewater system should be checked, and the desulfurization wastewater treatment system should be adjusted and retrofitted accordingly to ensure that the influent water quality meets the design requirements of the multi-media filter.
Adjust Backwash Flow: Within the design parameters, increase the backwash flow of the multi-media filter to enhance the backwash intensity. This prevents sediment from remaining in the filter layer and avoids the formation of hard lumps between sediment and filter media.
Add Air Washing: Consider adding an air washing step, first supplying air for 1 - 3 minutes (air velocity 10 - 15 L/(m²・s)), followed by water washing to enhance the cleaning effect. This is applicable to highly polluted water qualities and can effectively improve the backwash performance.
Re-Sieve Filter Media: Re-sieve the filter media to achieve the optimal particle size distribution, ensuring the filtration performance of the filter media.
Control Operating Parameters: Strictly control the flow and pressure during operation to ensure that the filter operates within the design parameters. When not in operation, do not drain all the water from the filter; retain a certain height of water cushion to protect the filter media and equipment.
After identifying the issues in the operation of multi-media filters and proposing corresponding solutions, it is also necessary to focus on the daily operation and maintenance of the equipment. Good maintenance practices are key to ensuring the long-term stable operation of the equipment. The following will detail the key points for the daily operation and maintenance of multi-media filters.
Inspect Filter Media: Confirm that the particle size and grading of the filter media (such as quartz sand, anthracite, manganese sand, etc.) meet the design requirements, and that there is no damage, lumping, or impurities mixed in. The quality of the filter media directly affects the filtration performance, so it is essential to ensure the quality of the filter media.
Observe Filter Media Height: Regularly observe the height of the filter media. If the filter layer thickness is insufficient due to backwash loss (e.g., below 80% of the designed height), it should be replenished in a timely manner. Insufficient filter layer thickness will affect the filtration performance and lead to a decline in effluent water quality.
Check Equipment Sealing: Check for leaks in the filter body, pipe connections, valves, especially at flange connections and pressure gauge interfaces. Leakage issues not only affect the filtration performance but can also lead to equipment damage.
Check Valve Flexibility: Ensure that the exhaust valve and sludge discharge valve operate smoothly without blockages or sticking. The normal operation of the valves is crucial for the proper functioning of the filter.
Calibrate Instruments: Calibrate the inlet and outlet pressure gauges and flow meters to ensure accurate data (with an error not exceeding ±5%). Accurate instrument data is the foundation for monitoring the operation status of the filter.
Test Automatic Control Programs: Test whether the automatic control programs (such as backwash cycle and time settings) are functioning normally and that the manual/auto mode switching is correct. The normal operation of the automatic control system can improve the operational efficiency and stability of the filter.
Strictly Control Influent Water Quality: Strictly control the influent turbidity (usually ≤10 NTU), suspended solids concentration (≤50 mg/L), and water temperature (generally 5 - 40℃). Avoid exceeding the design range, which can lead to filter layer clogging or degradation of filter media performance. If the influent contains oil, colloids, or other substances, they should be removed through pre-treatment to prevent the filter media from "clumping" and becoming ineffective.
Control Filtration Velocity: Control the filtration velocity within the design range (typically 8 - 15 m/h, depending on the type of filter media). Excessive velocity can lead to a sharp increase in head loss or penetration of the filter layer. At the same time, monitor the inlet and outlet pressure difference. When the pressure difference reaches the set value (e.g., 0.05 - 0.1 MPa), backwashing should be performed promptly to avoid compaction or damage to the filter layer.
Set a Reasonable Operating Cycle: Set a reasonable operating cycle (usually 8 - 24 hours), adjusting according to changes in water quality. When the water quality is poor, shorten the cycle to prevent the filter layer from excessively retaining impurities, which can affect the effluent water quality.
Initiate Backwash Conditions: Initiate backwashing when any of the following conditions are met: the inlet and outlet pressure difference exceeds the set value (e.g., 0.1 MPa); the operating time reaches the set cycle; the effluent turbidity exceeds the standard (e.g., >3 NTU).
Control Backwash Velocity: The backwash velocity is typically 2 - 3 times the filtration velocity (15 - 30 m/h) to ensure sufficient loosening of the filter media. However, excessive flushing should be avoided to prevent filter media loss. Backwash duration: 10 - 15 minutes, with the effluent turbidity ≤5 NTU as the criterion. If turbidity remains high for an extended period, check for filter media damage or improper proportioning.
Air-Water Combined Backwash: If the air-water combined backwash function is available, first supply air for 1 - 3 minutes (air velocity 10 - 15 L/(m²・s)), followed by water washing to enhance the cleaning effect. This is suitable for highly polluted water qualities. After backwashing, slowly open the inlet valve to perform a forward wash (velocity 5 - 10 m/h) until the effluent turbidity is qualified (e.g., ≤2 NTU), then switch to normal operation.
Record Operating Data: Record the inlet and outlet pressure, flow, water temperature, and effluent turbidity every hour. Immediately investigate the cause of any abnormal fluctuations (such as pipe blockages or valve failures). Timely detection and handling of abnormal conditions can prevent further deterioration of problems.
Observe Equipment Operating Conditions: Observe whether the tank body vibrates or makes unusual noises, and check for water seepage at flange connections. Timely handling of hidden dangers ensures the safe operation of the equipment.
The service life of filter media is typically 3 - 5 years. Replacement is required if the following conditions occur: severe wear of the filter media (particle size reduction of more than 30%); organic or metal ion contamination leading to filter media compaction and failure (which can be detected through a hydrochloric acid soaking test). When replenishing filter media, add it in layers according to the original grading to avoid mixing layers that can affect filtration performance.
Through an in-depth analysis of the operation issues of multi-media filters in a specific project, we identified that the deterioration of raw water quality, filter media problems, insufficient backwash intensity, and incorrect filter media height are the main causes of abnormal equipment operation. In response to these issues, we have implemented a series of solutions, including system retrofitting and upgrading, optimization of backwash operations, re-sieving of filter media, and control of operating parameters. We have also summarized the key points for the daily operation and maintenance of multi-media filters, such as pre-operation preparation and inspection, calibration of instruments and control systems, key control parameters during operation, backwash condition judgment and operation, operation record keeping and hidden trouble investigation, as well as replacement and replenishment of filter media. Through these measures, we have successfully found a relatively ideal operating mode, achieving the standard of reclaimed water quality and ensuring the stable operation of the entire water treatment system.
