18-Env-A4 Water and Wastewater Engineering · May 2018
Nivaar worked solution (AI-drafted; not reviewed by a licensed engineer)
National Exams — May 2018 — 04-Env-A4 / Water and Wastewater Engineering. 3 hours duration; closed book with one double-sided aid sheet; approved Casio/Sharp calculator permitted. Question 1 is compulsory; the paper instructs candidates to attempt any three of the remaining four questions — all five are solved below for completeness.
Reference texts. Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.) — population equivalent, oxygen sag/Streeter–Phelps, activated-sludge process control (RAS/WAS, HRT/SRT), secondary clarifier design; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.) — turbidity, alkalinity chemistry, digester fundamentals; MWH’s Water Treatment: Principles and Design (3rd ed.) — coagulation-flocculation mechanisms, ozonation, disinfection by-products, pH.
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.
An anaerobic digester stabilizes sludge in a sealed, oxygen-free tank where a consortium of anaerobic bacteria carries out sequential hydrolysis, acidogenesis, acetogenesis and methanogenesis, converting organic solids into biogas (60–70% methane, the rest largely CO2) that can be captured and used as an energy source; it operates at a much longer SRT (15–30 days mesophilic) and destroys 40–60% of volatile solids while producing comparatively little excess biomass. An aerobic digester instead continuously aerates the sludge (as in an extended-aeration activated-sludge tank), driving endogenous respiration of the biomass by aerobic organisms; it is simpler to build and operate and produces a more stable, more easily dewatered cake at similar VS destruction, but consumes substantial aeration energy and generates no usable biogas, making it more attractive at smaller plants where digester gas utilization is not economical.
Coagulation is the rapid, chemical destabilization step: a coagulant (a hydrolyzing metal salt or a polymer) is dosed and flash-mixed at high intensity for a short time (seconds to ~2 minutes) so it disperses uniformly and neutralizes or overwhelms the electrostatic repulsion holding colloidal particles apart, exactly by the bridging/sweep/compression mechanisms described in Question 2. Flocculation is the subsequent, physical aggregation step: the now-destabilized micro-particles are gently and slowly mixed (typically 20–45 minutes at low velocity gradient $G$) so that repeated, gentle collisions let them agglomerate into larger, settleable floc without the shear that would tear the floc apart. In short, coagulation destabilizes; flocculation aggregates — and the two require deliberately different mixing intensities and durations to work.
Total Kjeldahl Nitrogen (TKN) is a laboratory-defined measurement obtained by Kjeldahl digestion: it captures organic nitrogen (protein, urea, amino compounds) plus ammonia-nitrogen together, explicitly excluding the already-oxidized nitrite and nitrate forms, $TKN=N_{org}+NH_3\text{-}N$. Ammonia-nitrogen ($NH_3\text{-}N$) is narrower — the free-ammonia/ammonium fraction alone, one component of TKN measured directly (distillation/titration or ion-selective electrode) without digesting the organic fraction first. Tracking both across a biological nitrogen-removal train shows the process working: influent TKN is dominated by organic-N and ammonia-N, while a well-nitrified effluent shows TKN collapsing toward zero as ammonia is oxidized to nitrite/nitrate (which TKN does not capture).
Hydraulic Retention Time (HRT), $HRT=V/Q$, is the average time a parcel of liquid spends in a reactor of volume $V$ at flow $Q$ — a purely hydraulic sizing parameter. Solids (Sludge) Retention Time (SRT), also called mean cell residence time, $SRT=\dfrac{\text{mass of solids in the system}}{\text{mass of solids wasted per day}}$, is the average time the biomass itself remains in the treatment system. In a conventional activated-sludge process with RAS/WAS (Question 1(iii)), SRT is deliberately decoupled from and made far longer than HRT (typically 5–20 days SRT vs. 4–8 hours HRT), because return activated sludge continuously recycles the biomass while the liquid passes through once; SRT is the parameter that actually controls biomass age — and hence whether slow-growing nitrifiers survive in the system — while HRT governs tank sizing and hydraulic capacity.
