Berry’s ETSW Project Concludes

The Alabama Department of Environmental Management’s (ADEM) Drinking Water Branch officially closed the Extended Terminal Subfluidization Wash (ETSW) Project at the Town of Berry’s water treatment plant (WTP) on June 22, 2012. ADEM would like to thank Walt Taylor and his staff for their time and dedicated effort to see this project through to the end. The project, which started in May 2011, looked at how to implement ETSW at a WTP and if it was even possible to return a filter to service with a turbidity less than 0.10 NTU in less than 15 minutes and not have a rewash (filter-to-waste) spike.

The study conducted at the WTP looked a several aspects of the ETSW theory to determine what parameters were critical for success. The important parameters where narrowed down to two. The amount of water exchanged and the rate at which the water is backwashed through the filter. To minimize the variables evaluated changes were only made to the second half of the backwash sequence (e.g., the second low backwash duration and rate were changed). The air scour and high backwash rate were not changed during this project except during a period of time when the WTP was feeding more than fifty parts per million of powdered activated carbon to deal with a massive algae bloom. Then, the only change was to lengthen the high backwash rate by two minutes to ensure that any carbon fines were being removed from the filter.

After the initial attempt to implement ETSW resulted in a filter backwash sequence that allowed a filter to rewash in less than fifteen minutes and not have a turbidity spike, the study looked at how much water needed to be exchanged for ETSW to work. The second low backwash duration was altered to look at filter volume exchanges between one-half of a filter volume to little more than one filter volume of water. The study determined that at least one filter volume of water had to be exchanged for the filter to be rewashed to below 0.10 NTU in less than fifteen minutes without a turbidity spike.

After determining how much water needed to be exchanged, the rate at which the water was exchanged (second backwash rate) was examined. The low backwash rate was evaluated for two reasons, first to determine the maximum low rate range as not all backwash pumps or control valves are designed to run at these low rates. Secondly it was desired to determine the rate that would give the best performance from the filter following the backwash. The rate of backwash was varied between 5 gallons per minute per square foot of filter area (gpm/sq ft) to 10 gpm/sq ft. The results showed that flow rates of 9 gpm/sq ft or less did not impinge on the outcome of the ETSW procedure. The test at a flow rate of 10 gpm/sq ft did not result in a turbidity spike, but the rewash time extended out from an average of 11 minutes to 23 minutes before the turbidity dropped to below 0.10 NTU.

The results of varying the low backwash flow rate determined that the low backwash rate should be 9 gpm/sq ft or less and exchange at a minimum one complete filter volume of water. Further evaluation indicated that the preferable flow rate should be in the 5 – 7 gpm/sq ft range as these tests had the shortest rewash time to achieve filter water below 0.10 NTU. It is noted that some rewash turbidities never went above 0.10 NTU during these evaluations.

During the study the filter media was examined using the floc retention analysis test. The first set conducted in July 2011 showed that the backwash sequence was adequately cleaning the media. The second test conducted in June 2012 after one year of implementing ETSW also showed that the media was being adequately cleaned. Based upon these tests, the modified backwash procedure resulted in no adverse affect on media cleanliness.

Additionally, ETSW trials have been started at other water treatment plants in the state. Based upon these trials it appears that filters with air scour or surface sweeps can both benefit from ETSW as long as the surface wash mechanism works (no plugged nozzles or holes). The age of the media also does not appear to be a factor. One particular WTP which has surface sweeps and old media (about 15 years old) was able to implement ETSW and have incredible results. The filters rewash to approximately 0.03 to 0.04 NTU in less than ten minutes.
In conclusion, based upon the Berry ETSW Project and other ETSW trials, the procedure can be expanded to any surface water treatment plant utilizing conventional filters if they are able to control the backwash flow rate in the desired range (5-7 gpm/sq ft).

The benefits of pursuing ETSW are minimized backwash spikes and backwash and rewash water savings. Alabama has rewash capability on all of its conventional surface water plants so spike control is not as significant an objective as plants without rewash capability. However, the water savings during a backwash/rewash cycle is significant and offers a benefit to implementing ETSW.