Waste Water Treatment

2004 Biological Wastewater Treatment Operators School Advanced Treatment Systems May 13, 2004 Dean Pond, Black & Veatch

Advanced Treatment Systems

What are the forms of nitrogen found in wastewater?

What are the forms of nitrogen found in wastewater?  TKN = 40% Organic + 60% Free Ammonia

 

Typical concentrations: Ammonia-N = 10-50 mg/L Organic N = 10 – 35 mg/L No nitrites or nitrates Forms of nitrogen: Organic N TKN Ammonia Nitrite Nitrate

Total N

Advanced Treatment Systems

Why is it necessary to treat the forms of nitrogen?

Why is it necessary to treat the forms of nitrogen?

 Improve receiving stream quality  Increase chlorination efficiency  Minimize pH changes in plant  Increase suitability for reuse  Prevent NH4 toxicity  Protect groundwater from nitrate contamination

Advanced Treatment Systems

What are the effects of N and P in receiving waters?

What are the effects of N and P in receiving waters?

 Increases aquatic growth (algae)  Increases DO depletion  Causes NH4 toxicity  Causes pH changes

Advanced Treatment Systems

Why is it sometimes necessary to remove P from municipal wastewater treatment plants?

Why is it sometimes necessary to remove P from municipal WWTPs?

 Reduce phosphorus, which is a key limiting nutrient in the environment  Improve receiving water quality by: • Reducing aquatic plant growth and DO depletion • Preventing aquatic organism kill

 Reduce taste and odor problems in downstream drinking water supplies

Advanced Treatment Systems

How is P removed by conventional secondary (biological) wastewater treatment plants?

How is P removed by conventional secondary (biological) WWTPs?

 Biological assimilation BUG = C60H86O23N12P  0.03 lb P/lb of bug mass  GROW BUGS, WASTE BUGS = REMOVE P

Advanced Treatment Systems

Where in the treatment plant process flow could chemical precipitants be added?

Where in the treatment plant flow could chemical precipitants be added?

 At pretreatment  Before primary clarifiers  After aeration basins  At final clarifiers  Ahead of effluent filters  Considerations: • Effective mixing • Flexibility • Sludge production

Advanced Treatment Systems

How is N removed or altered by conventional secondary (biological) treatment?

How is N removed or altered by secondary (biological) treatment?

 Biological assimilation BUG = C60H86O23N12P  0.13 lb N/lb of bug mass  Biological conversion by nitrification and denitrification

Nitrification

 NH4+  Nitrosomonas  NO2 NO2-  Nitrobacter  NO3 Notes: • • • •

Aerobic process Control by SRT (4 + days) Uses oxygen  1 mg of NH4+ uses 4.6 mg O2 Depletes alkalinity  1 mg NH4+ consumes 7.14 mg alkalinity • Low oxygen and temperature = difficult to operate

Denitrification  NO3-  denitrifiers (facultative bacteria)  

N2 gas + CO2 gas Notes:

• Anoxic process • Control by volume and oxic MLSS recycle to anoxic zone • N used as O2 source = 1 mg NO3- yields 2.85 mg O2 equivalent • Adds alkalinity  1 mg NO3- restores 3.57 mg alkalinity • High BOD and NO3- load and low temperature = difficult to operate

Advanced Treatment Systems

What are typical flow application rates in tertiary filters?

What are typical flow application rates in tertiary filters?

 Automatic backwash filters (1-2 ft media depth) = 2 to 4 gpm/sf  Deep bed filters (4-6 ft media depth) = 4 to 8 gpm/sf

Advanced Treatment Systems

What are typical backwash rates for a tertiary filter (in gpm/sf)?

What are typical backwash rates for a tertiary filter (in gpm/sf)?

 Automatic backwash filters • 20 to 25 gpm/sf • 5 to 10% of throughput

 Deep bed filters • 15 to 20 gpm/sf • 3 to 5% of throughput

Advanced Treatment Systems

Define advanced treatment…

Define advanced treatment …

 Treatment that improves or enhances secondary treatment processes  Further removal of organics, nutrients and dissolved solids

Advanced Treatment Systems

Explain circumstances under which advanced treatment may be necessary…

Explain circumstances under which advanced treatment may be necessary…

 Limited assimilative capacity of stream  Toxicity reduction / elimination  Nutrient control  Closed systems  Water reuse

Advanced Treatment Systems Identify and explain the objectives of the following advanced treatment systems:

• • • •

Further removal of organics Further removal of suspended solids Nutrient removal (N and P) Removal of dissolved solids

Identify and explain the objectives of the following advanced treatment systems:

 Further removal of organics • Reduce effluent BOD to reduce receiving stream DO depletion • Improve disinfection • Reduce effluent N to improve water quality

 Further removal of suspended solids • Removing TSS removes BOD • Removing TSS removes N and P (BUG = C60H86O23N12P) • Protects stream  sediment oxygen demand • Improves efficiency of disinfection

Identify and explain the objectives of the following advanced treatment systems:

 Removal of nutrients (N and P) • Reduce oxygen demand of receiving stream • Control nutrients and algae • Control taste and odor in downstream drinking water • Suitability for reuse (examples: boiler water recycle, irrigation – N&P control of runoff, groundwater recharge)

Identify and explain the objectives of the following advanced treatment systems:

 Removal of dissolved solids • Removal of specific pollutant – zinc, chromium, lead • Pretreatment of industrial waste • Control effluent toxicity • Make suitable for reuse

Advanced wastewater treatment… Describe the purpose or procedure and mechanism by which it is done for each of the following:

 Activated carbon      

adsorption Chemical coagulation Flocculation Phosphorus removal Nitrogen removal Effluent Filtration Polishing lagoons

      

Nitrification Denitrification Ammonia striping Alum or ion precipitation Lime precipitation Reverse osmosis (RO) Electrodialysis

Activated Carbon Adsorption

 Purpose • Tertiary treatment • Removal of low concentration organic compounds

 Application: Influent Primary Trt Biological Trt  Filtration Carbon Disinfection

• Many variations

Continued …

Activated Carbon Adsorption  Carbon Regeneration • • • •

5 to 10% loss Less capacity than new carbon Hot air @ 350oF Chemicals (sodium hydroxide) • Fire / Explosion

• Carbon usually replaced after 5 regenerations

 Mechanism: • Active sites “Activated Carbon” • Molecular bonding • Particles adhere to surface

Chemical Coagulation

 Purpose • Enhanced removal of organics and fine particles • Addition of lime, alum, iron, polymer to change ionic charge

 Application • Chemical feed with rapid mix • Ahead of final clarifiers • Ahead of filtration

Continued …

Chemical Coagulation Lime+ Heavy metals SS removal P removal Polymer + - SS control Mechanism:

Alum+ SS removal Aluminum sulfate P removal Iron+ Ferric chloride Ferric sulfate Ferrous sulfate

SS removal P removal

• Destabilization by ionic charge neutralization • Reduce charge that keeps small particles apart + + +

_ + + _ _ _ + + + + + + + +++ + _ + _ _ + + _ + + + + + + + +

_ _

_ _ _ _

+ + _ + + +_ _ + _ + _ _ ++ _

_

_ _

_ _

_

_

_

Flocculation

 Purpose • Produce larger, more dense floc particles that will settle or filter easily

 Application • Gentle mixing after rapid mix (coagulation) • Mixing – Mechanical or Aeration Infl Q Rapid Mix / Coagulation

Gentle Mix / Flocculation

Q

Sludge

Continued …

Flocculation

 Mechanism • Coagulated particles strung together into larger floc particles (snow flake floc)

+ + + + + +

+ + +++ + + + +

+ + +

+ + +

+ ++ + + + + + ++ ++ + + +++ + + + + + + + + + +

Phosphorus Removal

 Purpose • • • • •

Reduce effluent P Biological or chemical method Reduce nutrient load on stream Reduce algae growth Reduce oxygen depletion

 Application / Mechanism • Biological • Chemical

Continued …

Phosphorus Removal

 Biological Q

Anaerobic Zone

Aerobic Zone

P Release

P Luxury Uptake

Final Clarifier

RAS

Effl

WAS P Removal

Continued …

Phosphorus Removal

 Chemical Q

Primary Clarifier

Chemical Coagulant

Aerobic Zone

Final Clarifier

Effl

Chemical Coagulant

RAS

WAS P Removal

Nitrogen Removal

 Purpose • • • • •

Reduce effluent N (ammonia and nitrates) Biological or chemical Reduce nutrient load on stream Reduce algae growth Reduce oxygen depletion

 Application / Mechanism 1. Advanced Activated Sludge Processes 

Nitrification (remove ammonia) NH4  NO2  NO3

Continued …

Nitrogen Removal 

Denitrification (remove nitrate) NO3  NO2  NO, N2O or N2 gas

2. Deep Bed Filtration 

Anaerobic fixed film bacteria (denitrify)

Q Methanol (carbon)

Media

6-8’

Q

3. Air Stripping  

Removes ammonia Elevated pH 10.8 to 11.5

NH4 as gas

Effluent Filtration  Purpose 

• Remove SS (usually after FC) • Reduce BOD and insoluble P Application 1. Deep Bed     

4-6’ sand and gravel Large cells 10’ x 30’ Similar to WTP (batch backwash) hL = 4 - 6 ft $$$

2. Traveling Bridge    

1-2’ sand and anthracite Small cells 1’ x 14’ Contiuous backwash hL = 2 - 3 ft

Continued …

Effluent Filtration

 Loading Rate • Backwash • • • • • •

2 – 4 gpm/sf Frequency depends on loading 20 – 25 gpm/sf 5 – 15% of throughput Must clean beds Air scour

 Mechanism • Filtration by granular media

Polishing Lagoons

 Purpose • To further treat or polish the effluent • After final clarifier • Facultative pond (aerobic and anaerobic)

 Application • Typical volume = 1 day average flow i.e., 1 mgd plant = 1 mgd lagoon 24 hour detention time • Surface aerators

Continued …

Polishing Lagoons Sunlight Surface Aerator M

Algae Settling

Aerobic Anaerobic

 Sunlight  Photosynthesis  Algae + Organics & Nutrients  Organic Matter  Anaerobic Decomposition

 Mechanism

methane gas

Algae and bacteria grow in pond consuming organics and nutrients in FC effluent. Algae settles and degrades by anaerobic process.

Nitrification

 Purpose • Reduce ammonia on plant effluent • High ammonia concentrations are toxic to streams • Quickest impact on DO versus nitrates

 Application

• SRT > 3 days in activated sludge process • Grow Nitrosomonas and Nitrobacter • NH4  NO2 NO3

 Mechanism

Biological conversion of ammonia to nitrate

Denitrification  Purpose • •

Reduce nitrate on plant effluent Usually in combination with nitrification to reduce Total N to the stream

 Application

1. Activated Sludge Process Q

Anx

FC

Oxic Oxic Recycle

2. Deep Bed Filters

RAS

 Mechanism Biological conversion of nitrate to N2 gas

WAS

Ammonia Stripping

 Purpose • •

Reduce ammonia either before or after biological treatment Not commonly used in the US

 Application / Mechanism • •

Raise pH  10.8 to 11.5, usually by adding lime Move equilibrium point to ammonia gas @ 250C and pH 11 •

NH4 gas = 98%

Continued …

Ammonia Stripping • •

Break wastewater into droplets and strip off ammonia gas with air Freefall through tower that circulates a lot of air to remove ammonia to atmosphere NH4 Air

Lime Q

Floc Precip.

NH4 Stripper

Lime Sludge Air

Q

Alum or Iron Precipitation  Purpose •

To remove orthophosphate

• • •

As a backup to Bio-P process As chemical P removal As chemical process

•

Al+ or Fe+ + PO4  Aluminum or Iron Phosphate

 Application  Mechanism Al+ or Fe+ Q

Q

Filtration Optional Precipitate

Rapid Mix RAS

WAS + Precipitate

Lime Precipitation  Purpose •

P removal before primary clarifier or following biological treatment

 Application • As a backup to Bio-P process • As chemical P removal • As chemical process High pH can be a problem in effluent or in biological treatment

 Mechanism •

Chemical conversion of phosphorus to calcium phosphate is in pH range of 9.5 to 11.0

Reverse Osmosis (RO)  Purpose •

High quality removal of various salts – calcium, sodium, magnesium

 Application • •

Water reuse AWT

 Mechanism • • •

Chemical separation / filtration across a semipermeable membrane High pressure Tertiary process

Used in Gulf War to treat sea water sodium removal

Electrodialysis  Purpose •

Removal of ionic inorganic compounds

• • • •

AWT Medical WTP Clinical

• •

Apply electrical current between two electrodes Water passes through semi-permeable membranes (ion-selective) Alternate spacing of cation and anion permeable membranes Cells of concentrated and diluted salts are formed

 Application

 Mechanism

• •

Electrodialysis  Purpose

 Mechanism

• Removal of ionic inorganic compounds

 Application • • • •

AWT Medical WTP Clinical _

+

H20

+

Cl-

H+

OH-

Na+

Bipolar Membranes

_

•

Apply electrical current between two electrodes • Water passes through semipermeable membranes (ionselective) • Alternate spacing of cation and anion permeable membranes • Cells of concentrated and diluted salts are formed Sludge – concentrated salt waste stream as process reject water Problems – plugging, fowling of membranes, MUST pretreat activated carbon, multi-media filtration

Advanced wastewater treatment… What would be the effect on sludge production for each of the following advanced treatment processes?

 Activated carbon      

adsorption Chemical coagulation Flocculation Phosphorus removal Nitrogen removal Effluent Filtration Polishing lagoons

      

Nitrification Denitrification Ammonia striping Alum or ion precipitation Lime precipitation Reverse osmosis (RO) Electrodialysis

What would be the effect on sludge production for each of the advanced treatment processes?

 TANSTAAFL (tanstaffull) •

“There ain’t no such thing as a free lunch.”



REMOVE MORE STUFF = GET MORE SLUDGE

   

More BOD & TSS Removal  MORE SLUDGE Add chemicals  MORE SLUDGE N & P Removal  MORE SLUDGE Some processes produce more sludge than others: • • •

Electro/mechanical – some sludge Biological – more sludge Chemical – MOST sludge