Presentation

for

MWEA Process Committee Seminar

November 6, 2013

SCREENING AND CONSEQUENCES

Tom Grant, Hubbell Roth and Clark

Sandra Diorka, Delhi Charter Township

Need for Project

Class A Sludge Anaerobic Digestion System Heat

Exchangers Plugging

Plugging Heat Exchangers

All raw sludge is macerated such that fibers, plastics,

etc. are relatively small, about ¼” wide x 3” to 6”

long

During sludge recirculation and pumping through 1

1/4” and 3” diameter heat exchanger tubes, small

pieces become entwined and form ropes that plug the

heat exchangers

Heat exchangers required cleaning every other day

Plugging Heat Exchangers

Existing Not Effective

Existing headworks grinding with capture was not

effectively removing screenable materials from

wastewater

Requires seasonal cleaning of primary clarifiers and

disposal of wet settlings to drying beds

Causing early wear on POTW equipment

Excessive labor spent removing rags from all process

equipment

Wasting pumps

Telescopic valves

Decant valves

Digester heat exchangers

Existing Not Effective

Existing Not Effective

Removal of about 40 pounds wet material per day

New Screen Selection:

Site Visits, Round 1

Traditional bar screens

Visited Michigan WWTPs to view their headworks

screens:

South Lyon, ¼” bar

Grand Rapids, ¼” bar

Bar Screen Observations

During site visits noticed:

Bar screens had excessive pass through

Equipment had grease balls and strings hanging

Resulting screenings were very dirty

Bar screen room and environment was gross

Primaries, aeration tanks, and secondaries had floaters

Conclusion: Bar screens are not so good. What else

can we get?

Pilot Testing at POTW

How do we pick between round holes and slots (bar

screen)? How big should the holes be?

Devised an in house experiment

Fabricated a slide gate with four different “screens”

built in

1/8 inch round hole (perforated plate)

1/4 inch round hole

1/4 inch “bar” hole

Window screen (around 1mm hole size)

Pilot Testing at POTW

Placed each gate in the influent channel for a

specified time

Weighed the material collected

Window screen was considered “100%” capture

Roughly 8000 pounds per day

Round holes collected roughly 4 times the amount of

material as the bar or slot shaped openings

Conclusion: We want round holes!!

New Screen Selection:

Site Visits, Round 2

North Kent, 1 to 3 mm drum screen

New Screen Selection:

Site Visits, Round 2

Kalamazoo, 6 mm

bandscreen

Flow Out

Screen entrance is

sealed to channel

Flow In

Discovered and Purchased

U.K. Study

Purchased “National Screen Evaluation Facility, Inlet

Screen Comparative Report” performed between

1999 and 2011 by the UK Water Industry Research

Installed and compared the performance of 27 different

headworks treatment products

Finescreen

Stepscreen

Combined screen (bar or round with processing)

Bandscreen

Reported % capture of material

Maintenance frequency

Finescreen

•Screen rotates

around to carry

stuff to the top

•Can have round

or bar openings

Stepscreen

•Like a fine screen

•Steps move and

transport stuff to the

top – body of the

screen holds still

Discovered and Purchased

U.K. Study

Bandscreens showed the best performance

Highest screen capture rate

Lowest risk of “pass through” or “carry over” because of

seal between opening and the channel

Study showed two brands had the highest screen capture

rate

Screens incorporating slots or bar spacing showed the

lowest performance

Conclusion: We want one of the bandscreens with

the highest capture rate!

Site Visits

Round 3

Made a three day trip to visit four additional screens

Ishpeming, MI – Jones and Atwood band, 6 mm holes

Sheboygan, WI – Brackett Green band, 6 mm holes

Madison, WI – Brackett Green band, 6 mm holes

Fond du Lac, WI – JWC Fine Screen, 3 mm holes

Site Visits

Ishpeming

Site Visits

Sheboygan

The Final Decision

How did we decide?

The cleanest end product

3mm because

Madison had 6mm hole size with the same type of heat

exchanger

Heat exchangers still plugged

Fond du Lac had 3mm hole size with the same type of

heat exchanger

Heat exchangers did NOT plug!!

What Happened After ?

Heat exchangers no longer routinely cleaned

Primaries have no rags or floating “salad”

Grease hauled less often (contains only grease)

Aeration tanks do not have to be “skimmed”

No more condoms

Wasting pumps no longer need to be routinely

cleaned

Sludge storage decant valves have remained clean

Telescopic valves no longer have to be cleaned

Screenings Production

0

200

400

600

800

1000

1200

70.0

72.0

74.0

76.0

78.0

80.0

82.0

84.0

86.0

88.0

Before After

Raw Sludge % Volatile

0

50

100

150

200

250

300

350

400

450

500

Grit Production

Before – NOT Gritty

After – Actually Grit

Engineering and Construction

Design Engineering

2 screens, each handle 6 mgd, bypass/overflow

manually cleaned channel 5 mgd

Tight site – fit between existing influent screw

pumps, grit tank and primary clarifiers

Existing hydraulic grade line is fixed

Poor soils for building & channels foundation

Tight Site

Poor Soils

Helical Screw Piles Foundation

Construction

Extended schedule due to piles, originally June 2012

completion, first screen started in October 2012

Alternate bypass

Construction

Gravity Bypass

Construction

Construction

Construction

Construction

Results

Results

Questions??