GROWTH-RELATED PROCESSES
Concepts:
Balanced growth: substrate (energy source, carbon source, nutrients, electron
acceptors available). Cells use some substrate electrons for energy production
(consuming electron acceptor) and some for synthesis of new cells.
In general
Energy for cell activities = substrate energy available – energy potential of cells
Yield
consumed substrate
produced mass cell
YIELD
= Y
also
nrate consumptio substrate
rate growth
YIELD
= Y
Electron donor
(COD, NH4, etc)
Nutrients (N, P and
trace elements)
Cells
Electron
Acceptor
O2, NO3,
SO4, etc.
H2O, N2,
H2S, etc.
generally
0 < Y < 1
Units of Y:
Y(mass):e cos glu g
cells g
., g . e
ompound substratec g
) weight dry ( cells g
Y(COD):e cos glu g * 07 . 1
cells g * 42 . 1
consumed COD g
COD cell g
Y(e-):e
COD cell g
Electron equivalent of COD from Oxygen reduction half-reaction:
O2 + 4H+ + 4e- 2H2O
Ratio:e
COD substrate g 8
e
O g 8
e 4
O ) g 32 ( * 1 2 2
Y(e-) = 8*Y(COD)
For glucose:
Y(e-) = 8*(1.42/1.07)*Y(mass) = 10.6 Y(mass)
Where Y(e-) > 1
Factors affecting yield:
Substrate type: Connelly et al found for heterotrophs
Y(amino acids) > Y(sugars, alcohols) > Y(NOM)
Y(autotrophs) < Y(heterotrophs)
0.48 < Y(COD, heterotrophs) < 0.72 (Grady et al)
0.0.06 < Y(COD/N, autotrophs) < 0.35 (Grady et al)
Electron acceptor:
Y(COD, aerobic) > Y(COD, denitrifiers) > Y(COD, fermenters)
Growth Conditions:
Temperature
pH
species of organism
ACTIVITIES THAT DO NOT PRODUCE OR EVEN REDUCE GROWTH:
MAINTENANCE ENERGY, DECAY AND CELL LYSIS, PREDATION
Maintenance energy used for: regeneration of enzymes, membrane integrity,
motility, etc.
Use of Y in COD stoichiometry for cell growth:
COD - (1-Y) O2 Y cell-COD
Check: 1 – (1-Y) = Y
Cells
Debris
Biodegradable
particulates
Soluble
COD
NH4-N,
PO4, etc.
Maintenance energy, decay, predation all reduce yield:
Define “observed yield” = Yobs < Y
Example:
Let Y(COD) = 0.5 g cell COD/g-substrate COD consumed
Suppose 50% of biomass grown is lysed and 70% of lysed COD can be recovered
as substrate.
Yobs = 0.5*Y(COD) + 0.5*Y(COD)*0.7 = 0.25 + 0.35*0.5 = 0.495
(Orig. Growth) (lysed and recovered)
Note that debris are non-viable particulate products of decay and accumulate in
biomass, introducing concept of viable fraction.
KEY PROCESS IN DECAY CYCLE IN TREATMENT SYSTEMS:
HYDROLYSIS
Extracellular enzymes hydrolyze biodegradable particulates to soluble COD.
Factors that affect hydrolysis and recovery of COD
Cell mass (to produce enzymes)
Diffusion of products
Two models for oxygen utilization for full growth-decay cycle:
1. Decay produces either debris or new cells directly (no hydrolysis).
Secondary oxygen utilization in decay step for direct production of cells.
Compartments: external COD source, cells, and debris
COD + O2 cell-COD
Cell-COD + O2 cell-COD + debris-COD
2. Decay produces either debris or biodegradable particulates, followed by
hydrolysis. Oxygen utilization is only in the step soluble COD cells.
Compartments: substrate COD, cells, debris, and biodegradable particulates.
Soluble COD + O2 cell-COD
Cell-COD debris-COD + biodegradable particulate-COD
Biodegradable particulate-COD soluble COD
Advantages of second model.
Accounts for influent particulate and soluble COD
Allows for rapidly and slowly degradable COD
Simplifies oxygen consumption calculation
Probably more realistic.