Wastewater Collection & Treatment · Study · PE Civil: Water Resources and Environmental · FE → PE Prep
Wastewater Collection & Treatment
10% of exam
Collection systems and lift stations, preliminary/primary/secondary treatment, activated-sludge F/M and SRT, nutrient removal, solids handling, and disinfection.
9 concepts
A. Wastewater collection systems
Collection Systems, Flow Rates and Inflow/Infiltration
Sizing sanitary sewers with Manning for self-cleansing velocity, converting population to peak design flow, and the inflow/infiltration and wet-well cycling that drive lift-station design.
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B. Wastewater treatment systems
BOD Loading, Removal Efficiency and Treatment Trains
Converting concentrations to mass loads with the 8.34 factor, tracking BOD and TSS removal unit-by-unit through the treatment train, and meeting characteristic secondary effluent limits.
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Bar-screen velocity and head loss, horizontal and aerated grit-chamber design, and flow equalization that smooths the load before primary and secondary treatment.
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D. Primary treatment
Primary Treatment and Clarifier Design
Sizing primary clarifiers by surface overflow rate and detention time, predicting BOD/TSS removal, checking weir loading, and the scraper and sludge withdrawal that close the mass balance.
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E. Secondary treatment
Activated Sludge: F/M, SRT and Secondary Clarifiers
The F/M ratio, mean cell residence time (SRT), MLSS mass balance, hydraulic retention time, RAS/WAS flows, and the coupled secondary clarifier that together control a biological treatment plant.
Activated sludge is the heart of modern wastewater treatment and the densest source of secondary-treatment points on the PE-CIV-WRE exam. The idea is elegant: grow a concentrated population of microorganisms (the 'activated' floc) in an aerated reactor, let them eat the dissolved and colloidal BOD that primary treatment could not settle, then separate the biomass in a clarifier and recycle most of it to keep the population high. The whole process is controlled by just two ratios — the F/M ratio and the solids retention time — and a mass balance that ties the reactor to its clarifier. Master those and you can answer almost anything in this area. The biological relations are in the NCEES PE Civil Reference Handbook §6.8.5.3 Activated Sludge Treatment; the secondary-clarifier overflow and solids-loading design split follows standard Metcalf & Eddy practice.
The system: reactor, clarifier, recycle, and waste
Picture four flows. Influent (Q0, BOD S0) enters the aeration basin (volume V, holding a microbial concentration X = MLSS). The mixed liquor flows to the secondary clarifier, where biomass settles. Clarified effluent (Qe, BOD Se) leaves the top. Settled sludge is split: most returns as RAS (Qr, concentration Xr) to reseed the reactor, and a small waste stream (WAS, Qw, Xw) is removed to hold the population steady. Every design quantity below is a statement about one of these flows or a balance among them.
The food-to-microorganism ratio is the rate of BOD fed divided by the mass of organisms available to eat it — pounds of BOD per day per pound of MLSS. It is the operator's throttle. A high F/M (overfed, >0.5) grows dispersed, poorly-settling biomass; a low F/M (underfed, <0.2) drives the population into endogenous respiration, producing dense, well-settling floc but more aeration demand. Conventional plants target 0.2–0.4day−1. Because the numerator and denominator are both masses-per-time and mass, the 8.34
Solids retention time: the master variable
Mean cell residence time — SRT, sludge age, or θc — is the average number of days a microbe stays in the system. It is the total mass of solids in the reactor divided by the mass of solids leaving per day (in the waste plus the effluent). SRT is ·the· design and control variable: it sets which organisms can survive (slow-growing nitrifiers need θc≳8–10days), the effluent quality, the sludge settleability, and the waste rate. You adjust SRT primarily by changing the waste flow
MLSS, HRT, and the kinetic link
Two more quantities round out the reactor. The hydraulic retention time θ=V/Q0 is how long the ·liquid· stays in the basin (hours), in sharp contrast to the SRT (days) of the solids — decoupling those two is the whole point of recycling sludge. The MLSS concentration X that the basin actually carries is fixed by the kinetics: it rises with SRT and with the BOD removed, and falls with longer HRT and higher decay. This equation ties the biological growth constants (Y, kd
RAS and WAS: holding the population
Return activated sludge (Qr) reseeds the reactor; its rate is set by a solids balance at the reactor inlet. Treating the underflow concentration Xr as the maximum the clarifier can thicken to, the recycle ratio follows directly, and the limiting Xr
The coupled secondary clarifier
The clarifier must do two jobs at once, and each has its own loading limit. As a clarifier it must not let solids escape upward, so its surface overflow rate is checked on the influent wastewater flow Q0 ALONE — the recycle does not leave over the weir, so it does not count toward overflow. As a thickener it must move the solids down to the underflow, so its solids loading rate is checked on Q0+Qr
Exam strategy
Label the four flows on the figure before computing anything — most errors are flow-bookkeeping errors. For F/M, plug concentrations straight in; the 8.34s cancel. For SRT, mass-in-reactor over mass-wasted; if effluent solids are given, include QeXe, otherwise the RAS-line waste QwXw
Key equations
Food-to-microorganism ratioMF=VXQ0S0
Worked examples
F/M ratio and loadings
Problem. An aeration basin of V=1.0MG treats Q0=5.0MGD with influent BOD5=200mg/L
Common pitfalls
•Including RAS in the clarifier OVERFLOW rate. Surface overflow uses Q0 only; the recycle leaves through the underflow, not over the weir. Solids loading is what uses Q0+Qr
Recommended Standards for Wastewater Facilities (Ten States Standards), 2014 — secondary clarifier overflow and solids loading criteria
Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery — activated-sludge kinetics, design tables, and the secondary-clarifier overflow vs. solids-loading split
F. Nutrient removal
Nutrient Removal: Nitrification, Denitrification and Phosphorus
Nitrification oxygen and alkalinity demand, anoxic denitrification, biological (A2O) and chemical phosphorus removal, and the SRT and stoichiometry that hit effluent N and P limits.
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G. Solids treatment, handling, and disposal
Sludge Production, Digestion and Solids Handling
Compute dry-solids mass from TSS and BOD removal, convert to wet-sludge volume at a percent solids, and size thickening, digestion, gas production, and biosolids disposal.
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H. Disinfection
Wastewater Disinfection (Chlorine and UV)
Set chlorine dose from demand and residual, size contact using CT and the baffling factor, dechlorinate, and apply UV dose and transmittance to meet coliform limits.
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I. Advanced treatment
Advanced Treatment: Filtration, AOP and Reuse
Tertiary effluent filtration, membrane bioreactors and reverse osmosis for reuse, advanced oxidation, and the multi-barrier quality criteria that govern water reuse.
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factors cancel and you can compute F/M directly from concentrations.
MF=VXQ0S0
Qw
. Conventional plants run
θc≈5–15days
; extended aeration
20–30
.
θc=QwXw+QeXeVX≈QwXwVX(when effluent solids are small)
) to the solids the plant must manage.
X=θ(1+kdθc)θcY(S0−Se),θ=Q0V
comes from the sludge volume index:
Xr,max=106/SVI
. Waste activated sludge (
Qw
) is the deliberate removal of biomass that holds SRT constant — wasting from the concentrated RAS line needs a much smaller flow than wasting dilute mixed liquor. Get the recycle ratio from
X
and
Xr
; get
Qw
from the SRT you want.
R=Q0Qr=Xr−XX,Xr,max=SVI106
at the MLSS concentration. This split is the single most-tested subtlety of secondary clarification: overflow on
Q0
, solids loading on
Q0+Qr
. A clarifier that passes overflow but fails solids loading will thicken poorly and 'lose its blanket,' sending solids over the weir.
v0=AQ0(influent only),SLR=A8.34(Q0+Qr)X
dominates. Keep HRT (hours, on
Q0
, the liquid) rigidly separate from SRT (days, the solids). For the clarifier, write the two checks side by side: overflow rate on
Q0
only, solids loading on
Q0+Qr
. Verify SRT lands in the right band for the process (and above ~
8–10days
if nitrification is required).
Q0 influent flow, S0 influent BOD (mg/L), V aeration volume, X MLSS (mg/L). Units day−1. Conventional target 0.2–0.4.
Solids retention time (SRT, $\theta_c$)θc=QwXw+QeXeVX
Sludge age in days = solids inventory ÷ solids wasted per day. Master control variable; nitrification needs θc≳8–10d.
MLSS from kineticsX=θ(1+kdθc)θcY(S0−Se)
Y = yield (mg VSS/mg BOD, typical range 0.4–1.2), kd = endogenous decay (day−1, ~0.06), θ=V/Q0
Hydraulic retention timeθ=Q0V
Liquid residence time (hours). Distinct from SRT: HRT is on influent flow; SRT is on the solids. Conventional aeration θ≈4–8hr.
Process efficiencyE=S0S0−Se×100%
BOD removal across secondary; conventional activated sludge 85–95%.
Recycle (RAS) ratioR=Q0Qr=Xr−XX
From a solids balance at the reactor inlet. X = MLSS, Xr = RAS concentration. Typical R≈0.25–1.0.
SVI and limiting RAS concentrationSVI=MLSSSV×1000,Xr,max=SVI106
SVI (mL/g) from the 30-min settled volume SV (mL/L). Good sludge: 80–120. Sets the maximum thickening of the underflow.
Secondary clarifier overflow ratev0=AQ0
Surface overflow rate uses INFLUENT flow only (Q0), NOT Q0+Qr. RAS does not leave over the weir. Limit ~400–800 gpd/ft2 avg.
Thickening check uses Q0+Qr at MLSS X. Units lb/day-ft2
Clarifier solids mass balance(Q0+Qr)X=QeXe+QrXr+QwXw
Steady-state solids in = solids out. Closes the system; solve for an unknown flow or concentration.
Solids (system) loading massSL=8.34QX
Mass of suspended solids per day (lb/day); Q in MGD, X in mg/L. General form of the 8.34 conversion for solids.
at an MLSS of
X=2,500mg/L
. Find the F/M ratio and the hydraulic retention time.
Solution. F/M: MF=VXQ0S0=1.0×2,5005.0×200=25001000=0.400day−1.
HRT: θ=V/Q0=1.0/5.0=0.200day×24=4.80hr.
Final: F/M=0.400day−1, θ=4.80hr. Sanity check via masses: BOD load =8.34(5.0)(200)=8,340lb/d; MLSS mass =8.34(1.0)(2,500)=20,850lb; ratio =8,340/20,850=0.400 — identical, confirming the 8.34s cancel. Both values are squarely conventional.
MF=1.0(2500)5.0(200)=0.400day−1
SRT and the waste sludge rate
Problem. For the same basin (V=1.0MG, X=2,500mg/L, Q0=5.0MGD), the plant wastes from the RAS line at Xw=10,000mg/L, and the effluent carries Xe=15mg/L at Qe≈Q0. What waste flow Qw holds the SRT at 10days?
Solution. Rearrange θc=QwXw+QeXeVX
Qw=XwVX/θc−QeXe=10,000250−75=0.0175MGD
RAS ratio from SVI
Problem. The mixed liquor in the same plant has an SVI of 120mL/g at X=2,500mg/L. Estimate the maximum RAS concentration, the required recycle ratio, and the RAS flow.
Solution. Limiting RAS concentration: Xr=106/SVI=106/120=8,330mg/L.
Recycle ratio: R=Xr−XX=8,330−2,5002,500=5,8302,500=0.429.
RAS flow: Qr=RQ0=0.429×5.0=2.14MGD.
Final: Xr≈8,330mg/L, R≈0.43, Qr≈2.14MGD. Sanity check: R=0.43 sits in the typical 0.25–1.0 recycle band, and an SVI of 120 is a fair-to-good settling sludge, consistent with a moderate recycle.
R=Xr−XX=8,330−2,5002,500=0.429
Secondary clarifier: overflow vs. solids loading
Problem. Size-check a 90-ft diameter secondary clarifier for the plant: Q0=5.0MGD, MLSS X=2,500mg/L, recycle Qr=1.67MGD (R=0.33). Compute the surface overflow rate and the solids loading rate, and compare to typical limits.
•Confusing SRT (sludge age, days, on the solids) with HRT (liquid time, hours, on Q0). They are deliberately decoupled by recycling — never substitute one for the other.
•Computing F/M with the 8.34 factor left in. The factors cancel; F/M =Q0S0/(VX) uses concentrations directly (or masses on both top and bottom).
•Wasting from mixed liquor vs. RAS without changing Xw: Qw from the dilute reactor (X≈2,500) is several times larger than from the thick RAS line (Xr≈8,000–10,000) for the same SRT.
•Forgetting nitrification needs a long SRT. If the permit requires ammonia removal, θc must be roughly ≥8–10days — a short-SRT high-rate design will not nitrify.
•SVI sign error: Xr,max=106/SVI, so a HIGH SVI (bulking sludge) gives a LOW maximum RAS concentration and forces a high recycle ratio.
•Mixing F/M units: it is per day (mass BOD per day per unit mass MLSS). Volumetric loading (mass BOD per unit reactor volume) is a different parameter — do not interchange them.