1. Evaluation of alternative technologies for upgrading wastewater treatment plants in Minnesota for new phosphorus limits
H. David Stensel
University of Washington
Gary M. Grey
George J. Kehrberger
Hydroqual
11th Annual Education Seminar
Central States Water Environment Association
April 4, 2006

2. Upgrading WWTPs for phosphorus removal
Phosphorus effluent limits of <1.0 mg/L P expected in general for state of Minnesota
Minnesota Science and Economic Review Board
Identify the most appropriate cost effective phosphorus reduction strategies for retrofitting existing treatment plants for different types of biological treatment processes
Developed a protocol to systematically evaluate the effectiveness of phosphorus removal alternatives for various types of plants
Apply protocol to evaluate phosphorus removal alternatives for different types of WWTPs

4. Summary of Treatment Plant Characteristics
Different Sizes 0.5 – 19.1 MGD
8 Different Biological Treatment Processes
3 Activated sludge
2 Biological nutrient removal
2 Oxidation ditches
2 High purity oxygen
1 Trickling filter
4 Combined trickling filter & activated sludge
2 Lagoon systems
1 Rotating biological contactor

5. Continued Summary of Treatment Plant Characteristics
5 Plants with tertiary treatment (filters)
5 Plants dewater sludge
14 Plants land apply waste sludge
Nutrient Requirements
11 Plants monitor only for phosphorus
7 Plants monitor only for ammonia nitrogen
14 Plants receive industrial wastewater
4 Plants have 1 mg/L phosphorus discharge limit
8 Plants have discharge limit for ammonia

6. Protocol used for phosphorus Removal plternatives evaluation
Ehanced Biological Phosphorus Removal (EBPR) Only
Chemical precipitation only
EBPR +chemical addition
Key site-specific information was obtained for evaluation

7. Goals of retrofit process evaluations for P removal
Tank volumes required for process configuration selected – i.e. anaerobic, anoxic, aerobic
Feasible retrofit modifications within existing facility
Sludge production and P recycle
Effect of pre-activated sludge processes on design and performance
Primary treatment
Trickling filter
P removal possible with EBPR
Chemical dose required for P removal and alkalinity control

8. Impact of wastewater characteristics
Influent Parameter
On Chemical Treatment On
EBPR
>BOD
>Tank volume
>Sludge production
>Tank Volume >Sludge Production
>TSS
>Total P
>Chemical dose
>Effluent P conc.
>BOD/P
<Effluent P conc.
>rbCOD/P
<Effluent P conc
<Temperature
rbCOD=soluble readily biodegradable COD-VFA source

9. Impact of wastewater characteristics (continued)
Influent Parameter
On Chemical Treatment On
EBPR
>TKN, NH3
>Effluent P conc.
<BOD/TKN
>Loading variations

10. EBPR Protocol Select SRT Chemical addition option Size & Locate
Anaerobic
Tank
WWT
Character.
Select
SRT
If nitrification
locate and size
anoxic tank
Determine
amount of
P removed
Evaluate
Costs
Chemical addition
option

11. Chemical Addition Only Protocol
Determine P
used in
biotreatment
WWT
Character.
Identify
dose
points
Determine
chemical
Sludge
produced
Determine
chemical
dose
Evaluate
Costs

12. EBPR Protocol Size – 1.0 Hour detention time
Size & Locate
Anaerobic
Tank
Size – 1.0 Hour detention time
Ability to add to Existing System depends
On existing design, capacity, and layout
Easier to
Add to
Plug flow
Tanks with
Enough
capacity PC AT SC
Anaerobic PC AT SC

13. Oxidation Ditch and High Purity Oxygen (HPO) require an external tank
Size & Locate
Anaerobic
Tank
For some systems layout is not
Compatible for fitting into existing tanks AN SC AT
Oxidation Ditch and High Purity Oxygen (HPO)
require an external tank
HPO tanks PC AN SC AT

14. EBPR Protocol EBPR Select SRT Nitrification: NH3 to NO3 SRT@100C
Function of
temperature
EBPR
5.1 days
4.1 days
Nitrification
15.1 days
9.3 days

15. Tank volume needed is related to SRT and BOD removed
Primary treatment lowers Yn
Primary treatment with chemicals lowers Yn more
Use of anaerobic zone in EBPR produces lower
SVI and thus allows higher MLSS concentration
3500 mg/L possible

16. EBPR Protocol Nitrate reduced to N2 in anoxic tank
Less nitrate to anaerobic zone
1 mg/L NO3-N robs 0.70 mg/L P removal
Saves energy – use NO3 produced
Improves sludge settling
If nitrification
locate and size
anoxic tank
ANAEROBIC
INFL.
EFFL.
ANOXIC
AEROBIC
WAS

17. EBPR Protocol Typically 10-20% of aerobic volume
More influent TKN, more nitrate; larger tank
Less influent BOD/TKN; larger tank
Less soluble BOD; larger tank
If nitrification
locate and size
anoxic tank
ANAEROBIC
INFL.
EFFL.
ANOXIC
AEROBIC
WAS

18. EBPR Protocol P is removed by phosphorus accumulating organisms (PAOs)
Determine
amount of
P removed
P is removed by phosphorus
accumulating organisms (PAOs)
and exits system in waste sludge
Anaerobic
Anoxic and/or
Aerobic
Waste sludge
NO3
P release
P uptake
Influent
rbCOD
Influent
particulate
BOD
-Carbon storage-PHB
-poly P storage

19. How much P is removed by microbes?
P removal =f(PAO growth from rbCOD, % P in cells)
+ P removed for cell synthesis
Assumes 25% dry wgt of PAOs=P

20. Processes that deprive PAOs of rbCOD
Denitrification in anaerobic zone
Nitrate (NO3) may be present in return activated sludge
1 mg/L NO3-N uses ~ 7 mg/L equivalent to 0.70 mg/L P removal by EBPR
Trickling filter treatment prior to activated sludge in combined systems
Effluent rbCOD can be at very low concentration
Depends on influent rbCOD concentration and trickling filter loading

21. EBPR Protocol Evaluate Costs Preliminary costs only
external tankage needed
retrofit existing tanks for A2O process
recycle lines and pumps
mixers
chemical feed equipment and storage
O&M for mixing, pumping, labor
Some things not included?
site specific issues
aeration design
solids processing

22. General Preliminary Capital Costs Curves Used
Figure 4.9 – Preliminary Budgetary Retrofit Capital Costs – Enhanced Biological Phosphorus Removal

23. General Preliminary Operating Cost Curves Used
Figure 4.10 – Preliminary Budgetary O&M Costs – Enhanced Biological

24. What if EBPR does not provide enough P removal?
Provide chemical addition at secondary effluent or in primary treatment
Provide additional rbCOD (volatile fatty acid) by purchase of organics or produce VFA by on-site fermentation
primary AN
Aerobic SC
WPS
VFA
WAS
Fermenter
To digester or other

25. Fermenter Design Assumptions
Primary clarifier solids removal
Influent TSS = 200 mg/L
65% TSS removal
Primary sludge = 3% solids
SRT = 3 days in gravity thickener fermenter
VFA production = 0.15 g VFA/g TSS applied
Elutriation returns 70% of VFA produced
Additional P removal = 1 g P/ 12 g VFA added
Sugar cost = $0.18 per lb COD

26. Impact of obtaining rbCOD (VFA) from on-site primary sludge fermenter

27. Chemical Addition Only Protocol
Determine P
used in
biotreatment
WWT
Character.
Identify
dose
points
Al or Fe
Al or Fe PC
Biological
Process SC
waste
waste

28. Effect of dose point on chemical requirement

29. Chemical Addition Only Protocol
Determine
chemical
dose
At dose point select effluent P
From curve get Al/P ratio
(Infl P- Effl P)Al/P ratio = Al dose, mg/L
For primary step select effluent P
so that Al/P ratio ~ 1.0 M/M
Evaluate alkalinity consumed by alum/ferric addition
0.45 g alkalinity (as CaCO3) used per g alum

30. Chemical Addition Only Protocol
Determine
chemical
Sludge
produced
3 modes of sludge production
AlPO4
Precip.
Al(OH3)
Precip.
Increased Primary
sludge removal
3.93 g/g P
0.23 g/g P
%TSSr=65(0.0021*Al+1.0)

31. Impact of chemical addition to primary clarification step
Decreases overall chemical dose
Removes more suspended solids
% TSS removal from 65 to 90%
Removes more BOD
% BOD removal from 35 to 65%
Removes more on non degradable VSS
More primary sludge production
Decreases load to activated sludge process
Increases capacity of activated sludge process
More volume available for retrofit to biological phosphorus and nitrogen removal

32. Chemical Addition Only Protocol Cost Factors
$0.10 per lb of Alum as Al2(SO4)3.18H2O
$0.30 per lb of alkalinity as soda ash
$180 per dry ton of solids processing and disposal
$0.08 per kilowatt-hr
Labor at $20/hour
Present worth at 20 years and 5% interest rate

33. Capital cost for chemical feeding
Figure 4.11 – Preliminary Budgetary Retrofit Capital Costs – Chemical Precipitation

34. Alternative Analyses at Level of Facility Planning Costs not included in analyses
Specific site conditions
Land availability for expansion
Layout constraints for addition of tanks and piping
Needed improvements to existing system
Hydraulic profile limitations
Additional sludge handling and disposal equipment

35. EBPR Retrofit Analysis
Add anaerobic contact tank – 1.0 hour HRT
Is nitrification required or occurrring?
If yes, provide anoxic tank and recycle for A2O process
If no, use lower SRT and A/O process
Does activated sludge follow a trickling filter
Determine if sufficient rbCOD remains after trickling filter to allow EBPR
If not, bypass trickling filter
Or use chemical treatment only
Evaluate with single or two point chemical addition

36. Chemical Addition Retrofit Analysis
Determine possible chemical dose points
Evaluate chemical dose for different dose point options
Determine sludge production
Determine alkalinity depletion
If nitrification system, consider alkalinity addition to maintain system pH

37. Summary of P Removal Alternatives Selection for Attached Growth Systems
Selected
Alternative
Feed
BOD/P ratio
Trickling Filter
Detroit Lakes
RBC
Brainerd
Lagoons
Redwood Falls
Thief River Falls
Chemical (no action)
Chemical NA
Chemical NA

38. Summary of P Removal Alternative Selection for Combined Systems
Selected
Alternative
Feed
BOD/P ratio
Trickling/Activated Sludge
Faribault
Marshall
Glencoe (w/ Industry)
Glencoe (w/o Industry)
Little Falls
Chemical
EBPR + Chemical 12 28 20 40 36

39. Summary of P Removal Alternative Selection for Activated Sludge Systems
Selected
Alternative
Feed
BOD/P ratio
Conventional
Alexandria Lake
New Ulm
Grand Rapids
Chemical
EBPR + Chemical
Nutrient limited 27 23
>100
BNR
St. Cloud
Fergus Falls**
EBPR 26
** - demonstrated and proven for P < 1.0 mg/L

40. Summary of P Removal Alternative Selection for Activated Sludge Systems
Selected
Alternative
Feed
BOD/P ratio
Oxidation Ditch
Wadena
Whitewater River
Chemical
EBPR + Chemical 22 46
High Purity Oxygen
Moorhead
Rochester**
EBPR 32 30
** - demonstrated and proven for P < 1.0 mg/L

41. Major Factors Effecting EBPR Selection
EBPR higher capital – lower operating costs
Influent wastewater Characteristics
BOD/P ratio and soluble BOD fraction (rbCOD)
Aeration tank configuration that is easily retrofitted for anaerobic tank addition and anoxic tanks by baffles
Less sludge production with EBPR
Recycle flows from digesters or anaerobic unit processes less favorable for EBPR
Sludge processing and disposal methods
Sludge holding and land application with minimal recycle good for EBPR
Aerobic thickening processes
Chemical treatment easier to implement and quicker
Most EBPR applications also require chemical equipment and addition