Activated sludge settling problem resolved

Al ParrellaWestern Lake Superior Sanitary

District

Sludge storage

Anaerobic digesters

Flare

Duluth influentKnowlton sta. Influent (industrial flow)

Oxygen production

Liq. O2 storage

Unox reactors

Biofilter

Secondary settling tanks

Flocculation tanks

Multimedia filters

Chlorine contact tanks

Dechlorination

Effluent

Sludge thickeners

Boilers (LP steam)

Dewatering centrifuges Grit removal

Screening

Typical SVI values

• 65-70 range (annual average)

SVI values December-February (2012-2013)

SVI values December-February (2012-2013)

Trouble above this line

Problems in plant

• High sludge blanket levels• Thin sludge• Appearance of settleometer• Other indicators to follow:

Return sludge solids concentration

Thickened sludge solids concentration (sludge sent to digestion)

Operating parameters, centrifugesMonth/parameter % capture Polymer dosage,

lb./ton active% cake solids

December 2012 85 56 24

January 2013 98 46 25

February 2013 98 49 26

Operating parameters, centrifugesMonth/parameter % capture Polymer dosage,

lb./ton active% cake solids

December 2012 85 56 24

January 2013 98 46 25

February 2013 98 49 26

Problem

What started it all?

• Problem with dewatering centrifuge polymer system

• Common to both centrifuges– Corrected early January

SVI values December-February (2012-2013)

Mechanical problem repaired here

SVI values December-February (2012-2013)

Mechanical problem repaired here

Activated sludge process adjustment made here

Initial response to problem

• As plant became upset with recycled digested sludge, response was to increase the aeration units in service

• This may have been proper to address the amount recycled septic sludge entering the aeration system but caused a classic “low nutrient” problem when the recycle flow stopped

My point:

• Even though mechanical problem was resolved in early January we still had deteriorating activated sludge problem because we had too many aeration units in service!

Adjustment made to number of Unox reactors in service

Result of taking reactor off line

• BOD was driven deeper into the basins• F/M increased (by roughly 25%)• Sludge regained its darker-brown color• SVI came within range and settling improved

in the nick of time (before the spring floods)

Through it all

• Only effluent limit that we struggled with was phosphorus– But we met all limits