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Recent events have demonstrated that excess phosphorus in receiving waters can create many serious problems including impairment of drinking water supplies. For this reason and others, incorporation of phosphorus limits into NPDES discharge permits is occurring in many states. Many water resource recovery facilities (WRRFs) are being required to remove phosphorus for the first time and will need to add a process to the flow sheet. A discharge limit of 1.0 mg/L may be achieved most cost-effectively with chemical addition. Enhanced biological treatment may be needed to meet lower limits down to 0.5 mg/L and below. Additionally, biological treatment has other potential benefits. Regardless of the treatment method, continuous monitoring is essential. Critical parameters include orthophosphate, dissolved oxygen, oxidation-reduction potential (ORP), total suspended solids, and nitrate.
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The Phosphorus Problem: Treatment Options and Process Monitoring Solutions YSI WATER RESOURCE RECOVERY WEBINAR
1
What is Phosphorus?
• Essential to life – all living organisms require it • No substitutes • Major component of fertilizer • Creates nuisance conditions in excess • Limiting nutrient in fresh water • High quality reserves are depleting
It’s more than just the letter ‘P’
2
‘P’ Promotes Growth of Algae
• Human health • Environmental • Economic
Excessive algal growth has many undesirable effects
3
How Does ‘P’ Get Into Surface Water?
4
Diffuse sources Point sources
Regulating ‘P’ in Point Source Discharges
• Technology-based limits (TBL) • Typically 1.0 mg/L TP monthly average
• Total Maximum Daily Load (TMDL) • Mass-based limit – as treated water flow ↑ concentration must ↓
• Water quality based emission limits (WQBEL) • Numeric concentration limit to not cause adverse effects
3 types
5
Status Of Numeric Nutrient Criteria (WQBEL)
Current
6
Status Of Numeric Nutrient Criteria (WQBEL)
2016
7
The Wisconsin Example
Adverse effects threshold depends on surface water type
8
What Are You Going To Do?
Options for complying with ‘P’ limits
• Variance based on economic feasibility • Water quality trading
• Trade with your neighbor • Adaptive management
• Watershed based • Operational changes / add chemical
• Improve treatment process • Significant upgrades likely if WQBEL
is < 0.6 mg/L • Compliance schedule will extend 5+
years (not more than 9 years)
9
Phosphorus Removal Treatment Options
10
Terminology
• ‘P’ = phosphorus • TP = total phosphorus = particulate + dissolved phosphorus • Orthophosphate = dissolved phosphorus = PO4
3-
• (E)BPR = (Enhanced) Biological Phosphorus Removal • Oxic = aerobic = DO • Anoxic = DO; NO3 • Anaerobic = DO; NO3
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How is ‘P’ Removed?
2 ways
1. Biological 2. Chemical
Basic concept: ‘P’ dissolved ‘P’ Particulate
‘P’
How is ‘P’ Removed?
2 ways
1. Biological 2. Chemical
Basic concept: ‘P’ dissolved ‘P’ Particulate
‘P’
Effluent TP
Most WRRFs Are Not Designed to Remove ‘P’ Some ‘P’ removal occurs normally
14
Soluble - P (Ortho-P)
Particulate P
Influent
Soluble - P
Particulate P
Secondary Effluent
TP
Biological Transformation
WAS
Bio or Chem P Removal
Most WRRFs Are Not Designed to Remove ‘P’ Some ‘P’ removal occurs normally
15
Particulate P
Treated Effluent
Effluent TP
WAS
Soluble - P
Chemical Removal – How It Works
Addition of ferric or alum to water triggers a complex chain reaction
16
Takacs, et al (2011), “Chemical P removal – from lab tests through model understanding to full-scale Demonstration“, Influents, Water Environment Association of Ontario.
‘baby’ ferric hydroxide floc
Fe:P ratio (moles)
Dis
solv
ed P
targ
et, m
g/L Model Prediction
Plant Data
Surface complexation + Co-Precipitation + Other competing reactions Increased sludge production & alkalinity consumption
Relationship Between Dosage and Ortho P
Surface complexation
Lower sludge production & alkalinity consumption
Chemical P Removal
Control strategies • Pre-precipitation
Fe/ Al
‘P’
Chemical P Removal
Control strategies • Simultaneous
precipitation Fe / Al Fe / Al
‘P’
Chemical P Removal
Control strategies
• Post precipitation
‘P’
Fe / Al
Chemical P Removal
Control strategies
• Multiple dosing points
Why EBPR works? Energy Released by PHB oxidation is 24-36 times energy required for PHB storage
EBPR (Enhanced Biological Phosphorus Removal) Mechanism
Aerobic Anaerobic
Waste Sludge Loaded with P
BOD (VFA) uptake & C (PHB) Storage P release
Feed condition Battery charging
Ortho- P
• PHB Oxidized
• Excess P Uptake
Starved condition Battery discharging
Anaerobic/Oxic (A/O) Process Configuration
RAS
Anaerobic Aerobic
Net P Removal
Ortho-P ≥3 x Infl. Ortho-P
BOD PHB Storage
BOD Oxidized
Concentrations in Bioreactor
Location in Bioreactor
Biological Phosphorus Removal
1. Excess phosphorus 2. Readily degradable carbon 3. Cyclic anaerobic/oxic conditions
3 requirements
24
Phosphorus Removal Monitoring Solutions
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Orthophosphate Cabinet Analyzers
Chemical or biological removal
• Wet chemistry • 4 main components:
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• Electronics • Photometer & tubing • Sample transport • Reagent & solutions
Features of an Online Analyzer
• Low reagent consumption • Suitable for outdoors • Automatic calibration • Integrated permeate pump • Filter module
27
Monitoring for Chemical P Removal
• Effluent monitoring
P 700 IQ
Monitoring for Chemical P Removal
• Feedback control • Pre-precipitation
P 700 IQ
Monitoring for Chemical P Removal
• Feedback control
• Simultaneous precipitation
P 700 IQ
P 700 IQ
Monitoring for Chemical P Removal
• Feedback control
• Post precipitation
P 700 IQ
Monitoring for Chemical P Removal
• Feed forward control
P 700 IQ
Watertown, WI Simultaneous precipitation
Chemical Cost Reduction
Analyzer installed in 2012
$0
$20,000
$40,000
$60,000
$80,000
$100,000
2011 2012 2013 2014 todate
Ferric chloride costs
Annual expense
• 3.0 mgd • Limit = 1.0 mg TP /L
(for now) • Paid for itself in 1 year
+ other benefits • Decreased sludge
production
34
EBPR Monitoring
• COD/BOD • DO • Nitrate • TSS • ORP • Blanket depth
Everything is important!
35
Biological ‘P’ Removal
Orthophosphate release and uptake • ‘P’ - release / anaerobic
• Adjust mixing • Activate swing
zone
36
P 700 IQ
Biological ‘P’ Removal
Orthophosphate release and uptake
• ‘P’ – uptake / oxic
37
P 700 IQ
0.0
2.0
4.0
6.0
8.0
10.0
0.0% 20.0% 40.0% 60.0% 80.0% 100.0%
Aeration Volume (% of Total Aeration Vol.)
Ort
ho-P
, mg/
L
10-Sep-07
11-Sep-07
12-Sep-07
Courtesy City of Xenia
Dissolved Oxygen
‘P’ uptake is rapid when conditions are right
‘P’ Uptake in Oxic Zone
Too low DO concentration limits performance
39
Anaerobic Aerobic
‘P’ Release
Infl. TP
Ortho-P
Location Along Bioreactor
‘P’ Uptake in Oxic Zone
Too low DO concentration limits performance
40
Anaerobic Aerobic
‘P’ Release
Infl. TP
Ortho-P
Location Along Bioreactor
~55%
~30%
EBPR Monitoring
ORP
Reproduced G Olsson, M Nielsen, Z Yuan, A Lynggaard-Jensen, J-P Steyer (2005) Science & Technical Report No. 15, Instrumentation, Control, and Automation in Wastewater Systems, with permission from the copyright holders, IWA Publishing
ORP Control of Intermittent Aeration
42
Biological ‘P’ Removal
• Too low – not enough time for PAO’s • Too high
• Secondary release • Competition • Settleability
The role of SRT
43
‘P’ Removal and Sludge Settleability
Don’t let the ‘P’ get away!
10% P
6% P
8% P
4% P
2% P
Effluent TP = Dissolved P + Particulate P
Process Control Strategy for Achieving the Lowest Effluent TSS SRT control and sludge blanket control
45 Wahlberg, E. “What makes secondary clarifiers work”, WEFTEC 2013
Further reading
• Neethling, et al, Factors influencing the reliability of enhanced biological phosphorus removal, WERF report 01-CTS-3ASP, 2005.
• Jeyanayagam, S. and Husband, J., Chain Reaction: How chemical phosphorus removal really works, Water Environment & Technology, 2009.
• USEPA, Phosphorus Removal Design Manual (purple book), EPA/625/1-87/001, 1987.
• Narayanan, B. et al, Critical role of aerobic uptake in biological phosphorus removal, WEFTEC proceedings, 2006.
46
