Troubleshooting Activated Sludge Processes

PNCWA - Southeast Idaho Operators Section Pocatello, ID | February 11, 2016| Jim Goodley, P.E.

Outline

• Process Types & Kinetics

• Influent Monitoring

• Process Monitoring & Control

• Nitrification

Intensive Sampling?

Process Flow Regimes

Complete Mix (CSTR) and Plug Flow Processes

Complete Mix (aka CSTR)

• ‘Ideal’ CSTR

• Dilution Effects

• Influence on Kinetics

• Pros:

• Stable Process

• Can Handle ‘Shock’ Loads

• Cons:

• Low Rate

• Larger Tanks Time

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Influent

Effluent

Growth Kinetics

• Kinetic

Considerations

• Ks – Half

Saturation

Coefficient

• Low DO

• Low substrate

Ks

𝜇𝑚𝑎𝑥2

µmax

Limiting Nutrient Concentration

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Growth Kinetics

• How does this affect operation/performance?

• Process Kinetics= ‘Low Rate Process’

• Monod Kinetics:

(Substrate Limited Growth)

• Where:

• µ = Specific growth rate (day-1)

• µmax = Maximum Specific Growth Rate (day-1)

• Ks = Half Saturation Constant (mg/L)

• S = Limiting Substrate Concentration (mg/L)

𝝁 = 𝝁𝒎𝒂𝒙𝑺

𝑲𝒔 + 𝑺

Example – Low DO on Autotroph

Growth Rate

µmax = 1.0 /day

Ks = 0.5 mg/L

S = 0.25 mg/L

µ = 0.33 /day

1/3 of maximum growth rate

Plug Flow Processes

Plug Flow Processes

• Ideal/Theoretical PFR

• No Longitudinal

Mixing

• CSTR’s in Series

Plug Flow Processes

• Concentration Gradient through tank length, High to Low

• Favorable Kinetics:

• Substrate Concentration High

• S >> Ks

• Maximum Growth Rate

• High Rate Process

•

• Pros:

• High Efficiency

• Low Effluent Concentrations

• Smaller Reactors

• Cons:

• Susceptible to Shock Loads

• Requires Tighter ControlTime

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Influent

Effluent

Influent Monitoring

Influent changes are often the cause of operational issues.

Influent Monitoring

• Changes in loading

– Seasonal

– Industries

– Nutrient Balance

• Sidestreams

• Temperature

• Representative Sampling

Influent Monitoring

• Changes in loading

– Nutrient Balance

– Macronutrients

• Phosphorus

• Nitrogen

C:N:P

100:5:1

Ratio of macronutrients required

for microbial growth

Influent Monitoring

• Nutrient Balance

BOD5= 500 mg/L

TKN = 20 mg/L

TP = 5 mg/L

Is this OK for normal growth?

Influent Monitoring

• Chemical Oxygen Demand- COD

• Develop Relationship with BOD5 for your facility

• COD Typically 2 to 2.5 x BOD5

• COD analyses is fast compared to BOD5

• 3hrs compared to 5 days min

Influent Monitoring

• Process Sidestreams

– Biosolids Processing- Typically a high strength waste stream

0

2

4

6

8

10

12

0:00 2:24 4:48 7:12 9:36 12:00 14:24 16:48 19:12 21:36 0:00

Time

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Try to avoid return

during peak loading

periods

Influent Monitoring

• Temperature Changes

Microrganisms are highly sensitive to temperature

Daily Influent Temperature Readings Recommended

Spring & Fall Foaming Events

High Runoff Events- Snowmelt

Industrial Dischargers

Process Monitoring & Control

• Dissolved Oxygen

• F:M

• SVI

Process Monitoring & Control

• Dissolved Oxygen

– Aerobic heterotrophs (BOD5 removal) and autotrophs (nitrification) require oxygen to respire

– Rule of thumb 2.0 mg/L

– Lower concentrations can cause low DO Bulking

• Competition for limited oxygen-Filaments vs Rotifers

• Filaments will outcompete

Process Monitoring & Control

• Dissolved Oxygen

– In Plug Flow Processes monitor DO residual along length of aeration tank- create DO profile.

– Focus at head of tank where demand is highest

– Not true for CSTR process- DO should be nearly the same at all points in tank

– Monitor during peak loading periods

Process Monitoring & Control

• F/M (Food to Microorganism Ratio)

– Key parameter for maintaining normal operation

Plug Flow Process 0.2-0.4/day

CSTR Process 0.05-0.3/day

Process Monitoring & Control

• F/M (Food to Microorganism Ratio)

– Low F/M is common- Why?

– Engineers are conservative

• Facility designed for future conditions

• Designed to handle peak conditions

• Tends to be oversized for normal conditions

Process Monitoring & Control

• F/M (Food to Microorganism Ratio)

– Problems Associated with Low F/M

Sludge Bulking

Foaming - Nocardia

Process Monitoring & Control

• F/M CalculationHigh F/M Problems not as common (my experience)

Typically found at industrial WWTF’s

or

Industrial discharges to municipal WWTF’s

High F/M problems similar to low F/M

• Sludge Bulking (Filamentous & NonFilamentous)

– Viscous Bulking

• Foaming

Process Monitoring & Control

• F/M Calculation

Food= lbs of BOD5 per day into process

Microorganism= lbs of microorganisms in the process

F/M= lbs BOD5/day

lbs MLVSS

Process Control

• F/M CalculationA WWTF has an influent flow of 5.5 MGD with a BOD5 concentration of 200 mg/L. The aeration tank has a volume of 3.2 MG and an MLSS concentration of 2,800 mg/L. Assume the MLSS volatility is 85%. What's the F/M for the plant?

Food= 5.5MGD * 200mg/L * 8.34lb/gal = 9,174 lb BOD5/day

Microorganism = 3.2MG * 2,800 mg/L * .85 * 8.34lb/gal = 63,517 lb MLVSS

F/M = 9,174/63,517 = 0.144/day

Process Monitoring & Control

• MCRT – Mean Cell Residence Time

– Measure of how long microorganisms stay in the system

– Key operating parameter

– Longer MCRT’s tend to be more susceptible to bulking & foaming

Plug Flow Process 5-15 days

CSTR Process 10-30 days

Process Monitoring & Control

• MCRT – Mean Cell Residence Time

• MCRT (days) = Total Solids in System (lbs)

Solids Removed From System (lbs/day)

Total Solids- Can include solids in clarifier blankets

Solids Removed- WAS + TSS in effluent

Process Monitoring

• SVI- Sludge Volume Index

– Measure of how well sludge settles and compacts

– Accounts for MLSS (compared to just SV30 test)

SVI <100 Good

SVI 100-150 OK

SVI >150 Bulking

Process Monitoring

• SVI- Sludge Volume Index

SVI= SV 30 (ml/L)

MLSS (g/L)

An SV 30 test settled to 450ml in a 1L settleometer in 30 minutes. The MLSS was 3,200 mg/L. What’s the SVI?

SVI = 450 ml/L = 140 ml/g

3.2g/L

Process Control

• Nitrification

– Autotrophs- Slow Growers,

• Primary Requirements– Carbon INORGANIC- NOT BOD!

– Oxygen (4.6 parts/ part NH4-N oxidized)

– Alkalinity (7.2 parts/part NH4-N Oxidized)

– pH > 7 (7.5-7.8 best)

Questions?