By: Rich Papp, Crown Plant Manager

This article discusses the success of a series of filter-coring exercises performed at the Cleveland Division of Water’s (CWD’s) Crown Water Works. These filter corings, conducted while our overhead backwash tank was out-of-service, likely prevented the need to replace the 72-in. deep, anthracite media in our twelve filters. It is hoped that sharing this experience of the CWD will encourage other operators around the State to perform filter-coring exercises at their water treatment plants.

The twelve, deep-bed filters at Crown are backwashed using a combination air/water system. Water for the water portion of our backwash is provided jointly by an overhead tank and a dedicated pump. Results of filter corings, conducted while our overhead tank was out-of-service for repair, alerted our operators that the filter media was not being cleaned effectively. Modifying the filter-backwash procedure at Crown, allowing the filter media to be cleaned effectively, returned the media to nearly its original condition. Thus, potential replacement of the 1.0-mm anthracite filter media, which had been in place at Crown for only a few years, was avoided.

Introduction;

CWD’s Filter-Coring, Training/Evaluation Program;

Filter Backwashing at Crown; and

Results of Filter Corings at Crown.

CWD has implemented a comprehensive, filter-coring training/evaluation program to:

Instruct CWD operators at our Baldwin, Crown, Morgan and Nottingham Water Works how to conduct a filter-coring exercise (i.e., prepare operators to respond to any potential future upsets of an individual filter after the Interim Enhanced Surface Water Treatment Rule (IESWTR) goes into effect in January 2002);

Train CWD operators to analyze the results from a filter coring (i.e., prepare operators to develop the Exceptions Report material potentially required under the IESWTR); and

Assess the condition of existing filter media in CWD’s 108 filters at our four Water Works (i.e., prior to replacing the filter media at three of our Water Works as part of CWD’s extensive Plant Enhancement Program).

CWD’s filter-coring, training/evaluation program consisted of:

The filter-coring exercise, with assistance from an independent advisor:

Filter inspection and coring, conducted by operators, before backwashing a filter;

Inspection and measurements, performed by operators, during the filter backwash; and

Two filter corings, conducted by operators, after backwashing a filter (i.e., one to obtain incremental-depth media samples for developing a solids-retention profile, and one to obtain full-depth media samples for an independent lab to perform filter-media analyses).

Classroom instruction, performed by the independent advisor, two weeks after a filter-coring exercise had been conducted:

Development and analysis of the solids-retention profile based, on the filter-coring results from measurements taken before backwashing the filter - Figure 1;

Analysis of the physical-inspection results from before backwashing the filter;

Development and analysis of the backwash-turbidity profile from measurements taken during the filter backwash;

Analysis of bed-expansion measurements taken during the filter backwash;

Development and analysis of the solids-retention profile, based on the filter-coring results from measurements taken after backwashing the filter – see Figure 1; and

Analysis of results from the media analyses conducted by an independent laboratory (i.e., grain-size distribution curves, d10, d60, d90, uniformity coefficient, specific gravity, etc.).

Report preparation, by the independent advisor, for each individual filter that was cored at CWD’s four Water Works.

Reports were shared with operators by the independent advisor, four weeks after a filter-coring exercise had been conducted.

 

 

The backwashing of the twelve deep-bed, 72-in. anthracite filters at our Crown Water Works consists of:

Surface wash for two minutes;

Air backwash for two minutes;

Ramping up the water backwash for two minutes;

High-rate water backwash for six minutes (i.e., at the appropriate backwash rate to achieve a 25-percent, bed-expansion during all seasons of the year); and

Ramping down the water backwash for two minutes.

The deep-bed filters at our Crown facility are approved by the Ohio EPA to be operated at a filtration rate of 6.25 gpm/sf. Particles are stored (i.e., these particles are not truly removed from the water stream until a filter is backwashed) in the filters by one of two principal mechanisms: 1) the larger particles are strained out on the surface of the filter bed, and 2) the smaller particles stick to the surfaces of individual media grains in the upper reaches of the filter bed. The surface wash loosens the mat of larger particles that are strained out on the surface of the filter bed. The air backwash detaches most of the smaller particles that are stuck to the surfaces of individual media grains in the upper reaches of the filter bed. The high-rate water backwash, at a 25-percent bed expansion that results in an expanded-bed porosity of 0.7 for our anthracite filters, both: a) detaches additional smaller particles that are stuck to the surfaces of individual media grains in the upper reaches of the filter bed, and b) carries all of the detached particles up into the filter troughs for removal from the filter. This high-rate, water wash is accomplished at a backwash rate of roughly 15 gpm/sf in the winter, and at about 20 gpm/sf in the summer.

The water portion of the filter backwash at Crown is accomplished using a combination of water delivered from:

an overhead tank, and

a dedicated backwash pump.

Our overhead tank at Crown was removed from service for repairs. To account for the overhead tank being out-of-service, the distributive-control system (DCS) at Crown was adjusted. This adjustment was made to presumably fully open the rate-of-flow (ROF) control valve located on the line between the backwash pump and the filters. The filter-coring results (i.e., solids-retention profiles developed from data collected before, and after, backwashing a filter; Figure 2) showed the stored particles were being detached from the surfaces of individual media grains during a backwash. However, the water backwash was not removing these detached particles from the filter (i.e., the detached particles were being retained in the filter bed, as opposed to being carried up into the filter troughs for removal).

Evaluation of the percent bed-expansion results obtained while backwashing the filter showed the appropriate water backwash rate was not being achieved. A check of the DCS uncovered the fact that the ROF-control valve was not truly fully open (i.e., a fully-open indication on the DCS translated to a less-than-fully open ROF-control valve). The DCS was re-programmed to fully open the ROF-control valve.

Subsequent, filter-coring results (i.e., solids-retention profiles developed from data collected before, and after, backwashing a filter; see Figure 1) showed the stored particles were being detached from the surfaces of individual media grains during a backwash. Also, the water backwash was now removing these detached particles from the filter (i.e., the detached particles were being carried up into the filter troughs for removal).

Re-programming the DCS at Crown to fully open the ROF-control valve increased the water backwash rate; allowing particles that had been stored in the filter bed during a filter run to be detached and removed from the filter. The higher water backwash rate that was achieved returned the 1.0-mm anthracite media to nearly its original condition. It is not known how long we could have operated with an inadequate water backwash rate at Crown before the filter media would have deteriorated to a condition that would have necessitated replacing the 72-in. deep, anthracite media. It is, however, known by operators at the Crown Water Works that periodic filter corings are a practical and valuable tool for both: a) assessing the effectiveness of a backwash, and b) determining the current condition of filter media.

Our Crown Water Works has twelve, deep-bed filters with 1.0 mm anthracite media. Three filters are cored in January, April, July and October. Therefore, each of our twelve filters is cored at least once a year. We at CWD highly recommend that operators at other water treatment plants around the State consider using this valuable tool to maintain their filters in top operating condition.

This filter-coring, success story at Cleveland’s Crown Water Works would not have been possible without significant contributions from the following operators: Tim Mullen, John O’Donnell and Tim Stephens. Ms. Maggie Rodgers and Mr. Rolfe Porter organized and implemented this CWD filter-coring, training/evaluation program. We received assistance with the filter-coring training from our engineering independent advisors; including Stan Zachopoulos, Kristen Miller, Andrea Bell and Brian Kazyak.

For those operators interested in learning more about maintaining your filters in top operating condition; previous Ohio Section Newsletter articles concerning filtration, filter backwashing and filter coring by either Nick Pizzi or Tim Wolfe are recommended.

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