LOW D.O. OPERATION: EFFECTS ON BIOLOGICAL PHOSPHORUS REMOVAL, OXYGEN TRANSFER EFFICIENCY, DENITRIFICATION, AND ENERGY SAVINGS

IN 1980 THE PLANT WAS DESIGNED FOR 50 MGD NITRIFICATION IN 1996 THE PLANT WAS MODIFIED FOR BIO-P ABOUT 20% OF THE TANKAGE WAS REMOVED FROM AERATION IT WAS STILL ESTIMATED THE PLANT CAPACITY WAS 50 MGD TOO GOOD TO BE TRUE???

2002 UW SPONSORED STUDY STUDY POSSIBILITY FOR INCREASED SIMULTANEOUS NITRIFICATION AND DENITRIFICATION INCREASED TANK LOADINGS TO ACCOMPLISH GOAL PROVIDED THE OPPORTUNITY TO EVALUATE PLANT CAPACITY

NITRIFICATION/DENIT UW STUDY IN ‘83 & ‘84 LOW D.O. WAS MAINTAINED IN AERATION TANKS (<0.5 MG/L) DENITRIFICATION WAS THE GOAL FILAMENTOUS GROWTH (TYPE 021N) AND NOCARDIA INTERRUPTED STUDY FILAMENTOUS CONTROL WAS A CONSTANT BATTLE UNTIL BIO-P

-The plant is a Modifed UCT process without mixed liquor recycle -Prior to study the D.O. setpoint for pass 2 was set at 1.5 mg/l and air flow controlled in passes 1 & 2 to meet it. The study setpoint was set at 0.3 mg/l -The D.O. setpoint in pass 3 controlled air flow to pass 3 and was normally set at 2.5 mg/l. The study setpoint was 1.0 mg/l.

In August, ‘01 one half of plant 4 was removed from service in order to increase the organic loading and enable operating at lower D.O.’s Plant 3 was the control plant with the same volume of tankage in service during both time periods.

The goal was to maintain the same sludge age in both plants for both time periods. The higher organic loadings in plant 4 allowed operating at lower D.O.’s. The effluent TN was about 3 mg/l lower in plant 4 than plant 3 and the TP levels about the same. The air usage/lb of BOD removed was 40% less in plant 4 than plant 3.

Operating D.O. Levels & Oxygen Transfer Efficiency

The District owns an off-gas apparatus for testing oxygen transfer efficiency. This tool has been used for several research projects with the UW on fine bubble diffusion. The purpose for these investigations was to check if alpha was suppressed in the tankage as a result of maintaining lower D.O.’s.

The points 1-1 thru 3-6 are points where the off-gas testing was carried out, in both plants 3 & 4, before and during the test period.

-D.O. measurements are continuously recorded and compressed to hourly averages. -Prior to the test period the D.O. setpoint at the first point could be maintained to + or mg/l about 70% of the time, but because of the low organic loadings the D.O.’s were always much higher at the tail end of the tank than the setpoint. -This was because minimum air flow setpoints controlled.

During the test period the D.O. at the first probe in the tank could be maintained very close to the setpoint of 0.3 mg/l and at the tail end of the tank was within 0.2 mg/l of the 1.0 mg/l setpoint 75% of the time. The other 25% of the time were low loading periods and minimum air flow setpoints still controlled.

The profile of alpha in the plant 4 tanks was approximately the same with normal, low loadings and high D.O.’s as during the test with higher loadings and low D.O.’s

The D.O. profiles were much different with the two conditions

-The field transfer efficiencies were much different because of the D.O. profiles. -If the demand can be satisfied in passes 2 & 3 and take advantage of low D.O.’s and high alpha values, significant air can be saved.

BOD Loadings have increased 62% from Jan, ‘88 to Jan, ‘06

TKN Loadings have increased 56% from Jan, ‘88 to Jan, ‘06

Air use has increased 45% from Jan, ‘88 to Jan, ‘06, much less than 60% ’02 The drop in air use was due to changing control strategy following this study--a savings of about $32,000 in ‘03 ‘85-’88 Reduced air use was due to replacement of coarse bubble diffusers with fine bubble and implementation of D.O. control

-Air flow is maintained constant at locations 1-1 & 1-2 regardles of D.O.’s. at about 1.3 cfm/diffuser -The D.O. setpoint for the second probe is 0.5 and controls air to pass #2 -The D.O. setpoint for the final probe is 3.0 mg/l, controls air in the last pass, and is set as such to assure an effluent D.O. of 5.0 mg/l

A real time process calculator/simulator in the plant process control system was used to project plant capacity with the knowledge gained as a result of the study.

The aeration capacity of the plant likely won’t be reached until the average Annual daily flow exceeds 60 MGD.

Plant clarifier capacity may be reached sooner, when the average annual daily flow exceeds 50 MGD.

Operational Observations Diffusers have shown no more of a tendency to plug since Bio-P Occassionally there has been minor foaming organism growth on the tanks A related process, UV disinfection, has benefited from Bio-P; quartz tube are acid cleaned once/year versus every 3-4 weeks prior to Bio-P

SUMMARY Plant was capable of N & P removal at higher loadings and much lower D.O. concentrations Settling remained good Oxygen Transfer efficiency remained high at lower operating D.O. level It appears optimistic projections for future plant treatment capacity look good

A paper prepared and presented at Central States on this project can be found at click on Published Papers, “Effect of Biological Phosphorus Removal On Plant Operations and Capacity at the Nine Springs WWTP