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23-Chem-B2 Environmental Engineering · May 2018

Question 1 of 7: Water Treatment Principles and VOC Adsorber Design

Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)

Notes on this paper

National Exam 16-Chem-B2, Environmental Engineering — May 2018. 3 hours, Closed-Book Exam with a candidate-prepared 8½×11" double-sided aid sheet. Any five (5) of the seven questions constitute a complete paper (100 marks); all seven are solved below for completeness.

Reference texts: Metcalf & Eddy (Tchobanoglous, Burton, Stensel), Wastewater Engineering: Treatment and Reuse, 4th ed.; Davis & Cornwell, Introduction to Environmental Engineering, 5th ed.; Turner, Workbook of Atmospheric Dispersion Estimates, 2nd ed.; Cooper & Alley, Air Pollution Control: A Design Approach, 4th ed.

Problem 1: Water Treatment Principles and VOC Adsorber 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) Coagulation-flocculation, sand filtration and disinfection design principles

Unit processDesign/engineering principle 1Design/engineering principle 2
(a) Coagulation-flocculation Rapid mix delivers a high velocity gradient (G ≈ 700–1000 s−1) for only a few seconds so the coagulant (e.g. alum, ferric chloride) hydrolyzes and disperses uniformly before it precipitates — charge neutralization/sweep-floc must happen while the coagulant is still molecularly dispersed. Flocculation then uses a much lower G (20–70 s−1) over 20–30 minutes so the destabilized particles collide and grow into settleable floc without shearing the floc apart — sized by the Camp number Gt (typically 104–105).
(b) Sand filter Hydraulic loading rate (filtration rate, typically 5–15 m/h, about 2–6 gpm/ft², for rapid sand) is sized below the rate that would push floc through the media bed, and the media grain size/uniformity coefficient is selected to balance headloss against depth-of-penetration capture. Backwash rate and duration are sized to fluidize and expand the bed (typically 20–50% bed expansion) enough to scour captured floc from the grains without carrying media out of the filter box.
(c) Disinfection CT concept: the product of disinfectant concentration and contact time (C×t) at the design flow sets the achieved log-inactivation of target pathogens (viruses, Giardia, bacteria) per the regulatory CT tables. Contact-tank geometry (baffling factor, length:width ratio) is designed toward plug flow so the effective t10 (time for the first 10% of flow to pass) approaches the theoretical hydraulic retention time, avoiding short-circuiting that would under-deliver CT.

(ii) Tertiary treatment example

A common tertiary component is granular-media or membrane filtration followed by phosphorus removal (chemical precipitation with alum/ferric salt, or biological P uptake) added downstream of a conventional secondary (activated-sludge) process. Tertiary filtration polishes the residual effluent TSS (typically from ~20–30 mg/L after secondary clarification down to <2 mg/L), which directly lowers particulate-bound BOD/phosphorus and improves disinfection efficiency (lower turbidity means less UV-shielding or chlorine-demand from particulates); the added chemical/biological P-removal step targets the nutrient (phosphorus) that secondary treatment alone does not reliably remove, protecting the receiving water from eutrophication.

(iii) VOC adsorber (activated-carbon) design and operational considerations

The two-bed cyclic adsorber shown below runs one bed on-line (adsorbing VOCs from the contaminated air stream through a filter and pressure regulator) while the other bed is regenerated off-line with steam; the steam-VOC vapour is condensed and gravity-separated for VOC recovery/disposal:

FilterPressureRegulatorAbsorber 1(On Line)Absorber 2(Off Line)CondenserSeparatorVOC contaminatedairClean airSteamSteam +desorbed VOCCondensateDecanting /Separation
Fig. 1 — Two-bed cyclic activated-carbon adsorber for VOC reduction: one bed on-line (adsorbing), one bed off-line (steam-regenerating), with condenser/separator recovering the desorbed VOC.
  1. Breakthrough-limited cycle switching. The on-line bed's switch-over time is set below the adsorption breakthrough time (the point the bed's mass-transfer zone reaches the outlet and VOC starts leaking through) — sized from the bed's equilibrium isotherm capacity, inlet VOC loading and superficial gas velocity, with a safety margin so the standby bed is always ready before breakthrough occurs.
  2. Regeneration steam ratio and bed drying/cooling. The steam-to-carbon ratio used for desorption must be enough to strip the adsorbed VOC (typically ~0.2–0.4 kg steam/kg carbon) without over-wetting the bed; residual moisture left in the carbon after regeneration reduces the working VOC capacity on the next adsorption cycle, so the bed is dried/cooled before being switched back on-line.
  3. Condenser/separator sizing for VOC recovery. The condenser must fully condense the steam-VOC overhead (sized on the regeneration steam flow and VOC/water latent heat loads) so the separator can gravity-decant a clean, recoverable VOC phase from the condensate rather than allowing residual VOC vapour or dissolved VOC to pass to sewer/vent, which would simply relocate the pollutant instead of abating it.
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