L4- Anaerobic Treatment

Question 1

Anaerobic Digestion (AD) and traits

Answer 1

Convert organic substances into gas via anaerobic m/o

Traits: high treatment times, low biomass production, high biogas production, high volumetric loads, low content in pathogen m/o

Question 2

Aerobic Digestions traits

Answer 2

Aerobic m/o, oxygen supply, high growth rates, low treatment times, high biomass production, low gas production, high content in pathogen m/o

Question 3

Advantages and disadvantages of AD

Answer 3

Adv: Energy producer, low sludge formation, less space/smaller reactors

Dis: long treatment times, sensitivity to toxic chemicals, potential for odour production, corrosiveness of digester gas, high cost to construct, effluent cannot be directly discharged readily

Question 4

Differences in aerobic to anaerobic processes

Answer 4

Sankey diagram- AD: more effluent, less sludge, more gases, CH4 instead of H2O

Question 5

AD Stages Overview (3 stages)

Answer 5

1st stage: hydrolysis - low reaction rate;
Biomass -> glucose + H2
2nd stage: acetogenesis - high growth rate;
Glucose -> Acetic acid + other acids
3rd stage: Methanogenesis - low growth rate;
Acetic acid-> methane/biogas

Question 6

Factors affecting AD process

Answer 6

-Temperature: affects reaction rate and bacteria composition
-pH: bacterial reactions stop below pH=6.5
-Type/quantity of organic load
-Stirring/mixing: contact m/o with organic load
-Alkalinity, presence of nutrients & toxic substances

Question 7

Typical AD Digester traits

Answer 7

Cylindrical tank
Diameter of 6-35m
Height 14m

Question 8

Types of AD reactor

Answer 8

Anaerobic lagoon;
UASB- upflow anaerobic sludge blanket
EGSB- expanded granular sludge blanket
ANF- anaerobic filter
ANMBR- anaerobic membrane process

Question 9

Residence time (tow c) equation, when its equal to hydraulic residence time and effect of increase in tow

Answer 9

t = V/Q

tow c = tow (hydraulic residence time) when no recycling

Increase tow- decrease volatile acids concentration & increase biogas production

Question 10

Define variables of the Recycling ratio

Answer 10

tow- hydraulic residence time
Tow c- residence time of m/o
MR - sludge concentration in recycling stream
M - sludge concentration in digester

Question 11

Define variables of sludge concentration in digester, M

Answer 11

E- degradation of volatile solids
XVSS- concentration of volatile solids (VSS)
XTSS- concentration of total solids (TSS)

Question 12

Define variables of total production of methane, VCH4

Answer 12

VCH4- volume of produced methane (m3/d)
E- conversion factor of organic load during AD (0.6-0.9)
Q- volumetric flow (m3/d)
S0- COD influent (mg/L)
Px- produce biomass (kg/d)

Question 13

Define variables of produced biomass, Px

Answer 13

Y- efficiency factor
E- conversion factor of organic load during anaerobic digestion (0.6-0.9)
Q- volumetric flow (m3/d)
S0- initial organic load (mg/L)
kd- endogenous respiration factor (1/d)
tow c- residence time of m/o (d)

Question 14

Methane production

Answer 14

Volume produce CH4: 0.35m3 CH4/kg BOD
(1 mol CH4 -> 22.4 dm3)

Question 15

Define variables of Energy requirements (q1) for waste water heating

Answer 15

q1- required heat for wastewater heating (J/d)
Q- volumetric flow (kg/d)
Cp- specific heat of wastewater (4200 J/kg C)
Delta T- temperature difference between reactant liquid and influent (deg C)

Question 16

Define variables of q2

Answer 16

q2 - heat loss through reactor wall (W = J/s)
U- overall heat transfer coefficient (W/degC m2)
A- Surface area through heat transfer (m2)
Delta T- temp difference of two sides of surface (deg C)

Question 17

Produced biogas reactor heating criteria

Answer 17

Qbiogas > q1+q2