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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 consumedsubstrateproducedmasscellYIELD = Y also nrateconsumptiosubstraterategrowthYIELD = 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): ecosglugcellsg.,g.eompoundsubstratecg)weightdry(cellsg Y(COD): ecosglug*07.1cellsg*42.1consumedCODgCODcellg Y(e-): eCODcellg Electron equivalent of COD from Oxygen reduction half-reaction: O2 + 4H+ + 4e-  2H2O Ratio: eCODsubstrateg8eOg8e4O)g32(*122 Y(e-) = 8*Y(COD) For glucose: Y(e-) = 8*(1.42/1.07)*Y(mass) = 10.6 Y(mass) Where Y(e-) > 1Factors 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 organismACTIVITIES 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-CODCell-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

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