Filter Media Treatment

Abstract

The post sedimentation addition of polyaluminum chloride (PACl) was investigated as a means to enhance particle removal efficiency in rapid sand filtration. The process modification was evaluated in laboratory studies and at the Cornell Water Filtration Plant (CWFP). PACl was continuously metered into CWFP filter influent to increase concentrations by 0.06 to 4.2 mg/L (as aluminum) during the filter-to-waste stage of the filter operation cycle to accelerate filter ripening. Lower influent PACl concentrations ranging from 0.056 to 0.43 mgAl/L were also continuously applied during filtration. In comparison to a control filter that received no PACl addition, the ripening time required decreased with PACl dose, and the incremental improvement in particle removal during filtration increased with PACl dose. The addition of 0.056 mg Al/L of PACl (the lowest concentration tested) significantly reduced initial filter ripening time at the CWFP from 10 hours to 2.5 hours, and effluent turbidity in the test filter over the 77 hour filter run was lower than the control filter by an average of 17%. Incremental head loss increase caused by the PACl feed was dose dependent and was negligible for the lowest dosage tested.

Advantages of Filter Media Treatment

The filter media treatment process can significantly accelerate filter ripening, reduce the turbidity spikes when restarting the plant, effectively improve the filter performance, and make the filter less sensitive to fluctuations in influent turbidity. After 60 hours of operation addition of PACl to the test filter was terminated.

Method of application in Drinking Water Treatment Plants

The filter media treatment process was conducted in one of the rapid filters at the CWFP. An electrical metering pump (PULSAtron Series MP) proportionally controlled by the CWFP flow-paced computer system was employed to inject PACl. The PACl injection port was in the settled water pipe before the water reached the test filter. The filter media treatment process included two stages: (1) A high concentration of liquid PACl was applied to settled water during the filter-to-waste stage of operation for 13 min. (2) After filter-to-waste when the filter was placed in operation, a lower concentration of PACl was continuously injected during filter run. PACl addition was terminated when the test filter head loss reached 115 cm (45 inch) according to the standard operating procedure for the facility, but the filter was not backwashed until the filtered water volume reached 1.5 million gallons, filter online time reached 90 hours, or filter water quality deteriorated. Different concentrations of PACl were applied at both stages to evaluate the effect of PACl on filter ripening and long term performance. Table 2 provides details of PACl concentrations used in each trial. The PACl concentrations reported at each stage were dependent on the plant flow rate during filter-to-waste (0.9 mm/s or 240 gpm) and the average flow rate during filtration.
A control filter in the CWFP was operated simultaneously with the test filter to compare the filter performance, head loss accumulation, and aluminum concentration in the filtered water.
Table 2. Summary of full-scale experiments
Trial PACl used during FTW *† (mg Al/L) PACl used in Filtration (mg Al/L) PACl metering time (hr) Total Al mass used (g/m2) Incremental head loss ** (cm) Average filtration rate (mm/s)
1 0 0 0 0 -10 1.28
2 4 0.43 32.5 59.6 80 1.11
3 4.2 0.19 65.9 52.2 65 1.13
4 0.23 0.19 60 44.9 71 1.09
5 0.14 0.12 70 34.9 63 1.15
6 0.06 0.056 77 19.2 30 1.23

• FTW: Filter to waste
  † The test PACl was 5.5% Al by mass and the PACl density was 1260 kg/m3
  **Incremental head loss = final head loss in test filter – final head loss in control filter

Facilities Currently Using the Technology

The CWFP has implemented the PACl filter media treatment process as its normal operating procedure.

For more information

For more information please contact Po-Hsun Lin

Documents

Enhanced Filter Performance by Fluidized-Bed Pretreatment with Al(OH)3(am): Observations and Model Simulation
Po-Hsun Lin, Leonard W. Lion, and Monroe L. Weber-Shirk, Journal of Environmental Engineering 1, 389 (2011).

Enhanced Particle Capture through Aluminum Hydroxide Addition to Pores in Sand Media
Po-Hsun Lin, Leonard W. Lion, and Monroe L. Weber-Shirk, Journal of Environmental Engineering 1, 284 (2011).

Comparison of the Ability of Three Coagulants to Enhance Filter Performance
Po-Hsun Lin, Leonard W. Lion, and Monroe L. Weber-Shirk. Journal of Environmental Engineering 137, 371 (2011).