Wastewater Treatment Principles, ESP and Baghouse Design: 23-Chem-B2 Environmental Engineering, December 2016, Q1

Problem 1: Wastewater Treatment Principles, ESP and Baghouse Design (20 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(i) Aeration, secondary clarifier and disinfection design principles

Unit process	Design/engineering principle 1	Design/engineering principle 2
(a) Aeration system	Oxygen transfer rate must meet the biological oxygen demand: the standard oxygen transfer rate (SOTR) is sized from the actual field oxygen requirement (AOTR) corrected for temperature, altitude, fouling (α-factor) and dissolved-oxygen setpoint, $SOTR = AOTR/(\alpha F \cdot \beta C^_{s,T,alt}/C^_{s,20} \cdot 1.024^{T-20})$.	Complete mixing/no dead zones so the biomass stays in suspension and in contact with the incoming substrate; basin geometry and diffuser or mechanical-aerator placement are sized to keep the mixing energy above the solids-suspension threshold at every point in the tank.
(b) Secondary clarifier	Surface overflow rate (SOR = Q/A) is sized below the zone-settling velocity of the mixed liquor so flocculated biomass settles out before the clarified effluent leaves over the weir — too high an SOR causes solids carryover (washout).	Solids (mass) loading rate on the clarifier floor, $SLR = (Q+Q_r)X/A$, must stay below the sludge's thickening capacity or the blanket rises and clarification fails even though the SOR itself is acceptable — the clarifier is really sized by whichever of SOR or SLR governs.
(c) Disinfection	CT concept: the product of disinfectant concentration and contact time (C×t) at the design flow sets the achieved log-inactivation; contact-tank geometry (baffling factor, length:width ratio) is designed to approach plug flow so the effective t₁₀ (time for the first 10% of flow to pass) is close to the theoretical hydraulic retention time.	Residual disinfectant/dose must be sized against the demand exerted by the wastewater matrix itself (chlorine demand from organics/ammonia, or UV transmittance for a UV system) so the required CT is delivered even after the matrix's own demand is satisfied.

(ii) ESP and baghouse design/operational considerations

The exam's schematics show the ESP as a duct with grounded collecting plates alternating with high-voltage discharge electrode wires, and the baghouse as a housing of hanging filter bags with a pulse-jet cleaning-air header above the tube sheet:

Fig. 1a — ESP cross-section: alternating discharge-electrode wires (corona) and grounded collecting plates.

Fig. 1b — Baghouse cross-section: hanging filter bags below the tube sheet, pulse-jet cleaning-air header above.

Equipment	Design/operational consideration 1	Consideration 2	Consideration 3
Electrostatic precipitator	Corona voltage and electrode spacing are set to maximize particle charging without sparkover; collection area A and migration velocity w are sized to the target efficiency via the Deutsch–Anderson relation $\eta = 1-e^{-wA/Q}$.	Flue-gas resistivity must stay in the 10⁴–10¹⁰ Ω·cm window (condition with SO3/moisture injection if it drifts too high) or back-corona sparking collapses collection efficiency.	Rapping cycle (mechanical or electric) timed to dislodge the collected cake into the hopper without re-entraining fines back into the gas stream.
Baghouse (fabric filter)	Air-to-cloth (face-velocity) ratio sized to the dust/media pair (typically 2–4 ft/min for a pulse-jet unit) so the dust cake — not the bag fabric — becomes the true fine-particle filter.	Cleaning (pulse-jet) triggered from a ΔP setpoint rather than a fixed timer, balancing cake retention (needed for fine capture) against blinding.	Bag media selected for gas temperature/moisture/chemical compatibility, with continuous opacity/bag-leak monitoring per compartment to catch tears before a stack exceedance.

Topic: Core wastewater unit-process design parameters, and ESP/baghouse particulate control design.

Key relations: Aeration SOTR/AOTR oxygen-transfer correction; clarifier SOR = Q/A and SLR = (Q+Qr)X/A; disinfection CT (concentration × contact time); ESP Deutsch–Anderson η=1−e^−wA/Q; baghouse air-to-cloth ratio.

Why this works: Each unit process is governed by a rate-limiting physical mechanism (oxygen transfer, gravity settling, disinfectant contact, electrostatic migration, or cake filtration), and the corresponding design parameter is simply the dimensionless or rate form of that mechanism — sizing to it is what prevents the process from being the bottleneck in the treatment train.

Common pitfall: Sizing a clarifier on SOR alone and missing that a high recycle ratio can make SLR the true limiting parameter, or running an ESP with too-high flue-gas resistivity without conditioning, which silently destroys collection efficiency even though voltage/area look adequate on paper.

Source: Metcalf & Eddy, Wastewater Engineering, 4th ed., Ch. 8 (aeration, clarification, disinfection); Davis & Cornwell, Introduction to Environmental Engineering, 5th ed., Ch. 5 (ESP/baghouse design).

Question 1 of 7: Wastewater Treatment Principles, ESP and Baghouse Design

Problem 1: Wastewater Treatment Principles, ESP and Baghouse Design (20 marks)

(i) Aeration, secondary clarifier and disinfection design principles

(ii) ESP and baghouse design/operational considerations