bio wastewater - WIP

wastewater treatment process

screening, grit removal, EQ basin, primary treatment, activated sludge/aeration, secondary treatment, disinfection, reairation

screening

removes large debris from raw water before entering the treatment process

Grit removal

After removal of large objects by the bar screen, incoming raw sewage passes through the grit removal basins where inorganic gravel/pebbles and dirt settle out by gravity in the shallow basins. Grit is removed, dewatered, and hauled off.

EQ basin

equalization basin, used to overcome the operational problems caused by flowrate variations and improve the downstream processes

primary treatment

when physically treated sewage water is passed into a settling tank, where suspended solids settle out as sludge; chemically treated polymers may be added to help the suspended solids separate and settle out. filters out main VSS and FSS

activated sludge/aeriation basin

removes organic carbon and turns it into waste, oxygen used as the electron acceptor

removal is by consumption and oxidation

secondary clarifier

the tank where organisms and other particles settle out after the aeration tank. bacteria is recycled back to primary treatment for reuse. removes TSS and VSS

disinfection

removes and inactivates pathogens in water using UV/chlorine

reairation basin

where air is introduced to replenish the dissolved oxygen (DO) level

major water quality parameters to characterize wastewater

TSS, VSS, BOD, COD, TN, NH4, P

TSS

Total Suspended Solids. A measure of the total undisolved solids. affect turbidity, burial, aesthetics. has an average of 210 mg/L in wastewater, typical standards are 10-20 mg/L

VSS

volatile suspended solids - organic

has an avg of 160 mg/L in wastewater

FSS

fixed suspended solids - inorganic and non reactive

BOD

measures the amount of oxygen consumed by microorganisms during the biological breakdown of organic matter

COD

measures the total amount of oxygen needed to chemically oxidize all organic and inorganic compounds

TN

total nitrogen. affects taste and odor, can cause algae blooms, blue baby syndrome. has an average of 40 mg/L in wastewater, with standards of less than 15 mg/L

P

phosphorus - limiting nutrient that also fosters algae growth. has an average of 7 mg/L in wastewater, with standards of 1-5 mg/L

important biological reactions

substrate being consumed (energy or biomass), nutrients to biomass, electron acceptor to energy, detoxification

aerobic carbon oxidation

(occurs in the activated sludge basin) carbon is converted into new cells + energy

organic carbon oxidation

crap (C10H19O3N) reacts with oxygen (1402) to create carbon dioxide, ammonia, and water (energy reaction) or reacts with ammonium + carbon dioxide for growth

nitrification

the two step biological process by which ammonia is converted first to nitrite and then to nitrate

denitrification

the biological process by which nitrate is reduced to nitrogen and other gaseous end products

reaction kinetics

the field of chemistry that deals with reaction mechanisms and reaction rates

0 order reaction

rate is independent of concentration

1st order reaction

rate=k[A], dependent of concentration

Monod Kinetics - biomass

specific rate of growth plataus

begins as a first degree reaction (increasing steadily) and then reaches a 0 degree reaction

qXa

(QcapS)/(Ks+S)Xa

mew (u)

specific rate of growth (y axis) (t^-1)

Xa

conc. of biomass (mg*vss/L)

S

substrate conc (mg/L)

Ks

half saturation coeff - where mew 1/2 is (x axis)

Kd

decay rate (t^-1)

q

max specific utilization rate (mgs/(mgvss*t)

yield

(mew cap / q cap)

completely mixed reactor

- no inflow/outflow

-completely mixed

concentration is the same everywhere

continuous flow stirred tank reactor

-completely mixed

- constant in/out flow

-concentration is the same at all locations

plug flow reactor

-constant in/outlflow

- fully mixed in the diameter perpendicular to flow

no horizontal mixing

mass balance: conservative continuous tracer

dc/dt * V = Q1C1-Q2C2

mass balance: conservative pulse tracer

dc/dt * V = -Q2C2

batch reactor 0 degree reaction

C = Co-kt

batch reactor 1 degree reaction

C=Co*e^-kt

CFSTR 0 degree reaction

C=Co-KΘ

CFSTR 1 degree reaction

C=(1/1+kΘ)Co

PFR 0 degree reaction

C=Co-kΘ

PFR 1 degree reaction

C=Co*e^-kΘ

mass balance, CFSTR, Xa

(dxa/dt)V=QXa0-QXa+μXaV

μ in Xa mass balance

((μ^*Se)/(Ks+Se))-Kd

steady state mass balance, CFSTR, Xa

1/θ+Kd=(μ^*Se)/(Ks+Se)

mass balance, CFSTR, S

ds/dtV=QS0-QSe-((q^Se)/(Ks+Se))Xa*V

steady state mass balance, CFSTR, S

Xa=(So-Se)/θ((Ks+Se)/(q^Se))

1/θ

washout rate

u^*Se/Ks+Se

rate of biomass produced, substrate utilization rate

So-Se/θ

rate of substrate loss per unit time

Ks+Se/q^*Se

rate of substrate loss per unit bacteria

cell is breathing it

acceptor (Ra)

cell is eating for

donor (Rd)

overall reaction

R = feRa + fsRc - Rd

anoxic basin rxn

OC + NO3 -> N2 + CO2

aerobic basin rxn

OC + O2 -> CO2

NH4+O2 -> NO3

as q^ increases

Se increases

Xa increases

θmin decreases

Smin decreases

Ks increases

Se increases

Xa decreases

Kd increases

Se increases

Xa decreases

Smin increases

Se >> Ks: defining a reactor

0th order

Se = Ks: defining a reactor

1st order

PxVSS

PXoca+PXocpp+Pxna+PxnppPxi

why do we use NO3 produced in the system to determine the production of nitrifiers rather than the amount of organic N lost

some of the organic N is incorperated into cells and not used for nitrification