Elevated particle counts or turbidity immediately following the restart of a backwashed filter (shown above) is commonly known as the filter ripening period, which can be responsible for more than 90% of particles passing through a filter during a filter run. Extended Terminal Subfluidization Wash (ETSW) has been around for nearly 20 years, but it is still not well-understood or widely practiced.
ETSW is an acronym that defines itself:
Extended (typically 5-10 min.)
Terminal (last step of a backwash)
Subfluidization (low flow rates with no media expansion or fluidization)
Wash (washes out backwash remnant particles generally left in the filter after backwashing).
State regulations still commonly prevent or hinder its use by requiring filter-to-waste, and consulting engineers and operators alike often struggle to perform it correctly. The USEPA Partnership for Safe Water Program has led to multiple states successfully promoting and using ETSW. The purpose of this document below is to educate the drinking water industry on both understanding and efficiently dealing with filter ripening pathogen passage and implementing ETSW properly. The goal is to use as few words as possible to communicate the facts. There will not be a wordy introduction, no literature review, no equations, no methods section, or even any references (although those things have all been published previously in six peer-reviewed journal articles on ETSW that are not freely available to the general public due to copyrights). A collection of short factual statements of just a few lines with figures to back them up to follow (with a companion spreadsheet to do ALL of the math for you).
This paper will introduce a totally new concept of optimizing coagulation (and all drinking water treatment processes) by using optimized ETSW backwashes and monitoring the filtered water quality carefully during the first 60 minutes of filter operation.