Wastewater Treatment Plant Process Schematic: 18-Env-A4 Water and Wastewater Engineering, December 2019, Q3

Question 3: Wastewater Treatment Plant Process Schematic (25 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.

Given.

Raw sewage characteristics and effluent limits
Parameter	Raw sewage	Effluent limit
TSS	250 mg/L	10 mg/L
$BOD_5$ ($\approx cBOD_5$, raw)	220 mg/L	$cBOD_5\le$ 10 mg/L
Alkalinity (as $CaCO_3$)	100 mg/L	—
TKN	40 mg/L	Ammonia-N $\le$ 3.0 mg/L
Total phosphorus	not given (assumed 8 mg/L, typical medium-strength domestic sewage)	$\le$ 0.3 mg/L
Coliform	not given (assumed $1\times10^7$ CFU/100 mL, typical raw sewage)	$\le$ 150 CFU/100 mL

Find. A fully labelled process schematic (unit processes, streams, chemical injection points) for a conventional wastewater treatment train, sized to confirm it can meet all five stated effluent limits from this raw sewage.

Approach. Select a conventional liquid train — screening/grit removal, primary clarification, a nitrifying activated-sludge process with return activated sludge (RAS) for ammonia removal, secondary clarification, tertiary filtration for a final TSS/$cBOD_5$ polish, chemical phosphorus removal ahead of the tertiary filter, and UV disinfection — plus a parallel solids train (thickening, anaerobic digestion, dewatering), then check each effluent limit in turn using literature-typical removal fractions and size the two chemical injection points (alkalinity supplementation, ferric chloride) that the raw water quality specifically demands.

Conventional wastewater treatment schematic: screening/grit → primary clarifier (sludge draw-off) → nitrifying activated-sludge aeration tank (alkalinity supplement injected; RAS returned from the secondary clarifier) → secondary clarifier (WAS to solids train) → tertiary filter (FeCl3 injected upstream for chemical P removal; backwash to equalization) → UV disinfection → effluent. Solids train: primary sludge + WAS → thickening → anaerobic digestion → mechanical dewatering → biosolids.

TSS: check the overall removal the train must achieve. $$\text{Overall TSS removal required}=\frac{250-10}{250}=96\%$$ A primary clarifier alone typically removes about 60% of raw TSS: $250\times(1-0.60)=100\text{ mg/L}$ carried into the secondary process. Conventional secondary clarification alone typically achieves only 85–90% overall removal from the raw value — not reliably enough to guarantee 10 mg/L from 250 mg/L raw — which is exactly why the train adds a tertiary (granular or cloth media) filter after the secondary clarifier: filtration is the step that closes the gap and makes the 96% overall removal target routinely achievable. $\boxed{\text{TSS: met, by design of the tertiary filtration step.}}$
$cBOD_5$: check the overall removal the train must achieve. Assuming raw $BOD_5\approx$ raw $cBOD_5$ (nitrifiers have not yet developed in the raw sample), $$\text{Overall }cBOD_5\text{ removal required}=\frac{220-10}{220}=95.5\%$$ Primary clarification removes about 30% of raw $BOD_5$: $220\times(1-0.30)=154\text{ mg/L}$ enters the aeration tank, so the secondary process plus tertiary filter together must remove $$\frac{154-10}{154}=93.5\%$$ of the load reaching them — comfortably inside the 85–98% overall removal range documented for a nitrifying activated-sludge plant with tertiary filtration. $\boxed{cBOD_5:\text{ met.}}$
Ammonia-N: check whether the raw alkalinity can support full nitrification. With a fraction $f_s\approx0.20$ of influent TKN assimilated into new cell mass (typical for conventional activated sludge) and an ammonia-N effluent target of 3.0 mg/L, the nitrogen actually oxidized by the nitrifiers is $$N_{ox}=TKN(1-f_s)-NH_3\text{-}N_{eff}=40(1-0.20)-3.0=29.0\text{ mg/L as N}$$ Nitrification consumes 7.14 mg alkalinity (as $CaCO_3$) per mg of ammonia-N oxidized, so $$\text{Alkalinity consumed}=7.14\times29.0=207.1\text{ mg/L as }CaCO_3$$ against a raw alkalinity of only 100 mg/L, of which roughly 50 mg/L must remain as a residual floor to hold pH in the range nitrifiers tolerate — leaving only $100-50=50\text{ mg/L}$ available to be consumed. $$\text{Alkalinity deficit}=207.1-50=157.1\text{ mg/L as }CaCO_3$$ $\boxed{\text{Ammonia-N target is achievable only with alkalinity supplementation}\approx157\text{ mg/L as }CaCO_3\text{ (lime or soda ash) dosed ahead of/into the aeration tank.}}$ Without it, nitrification would stall as alkalinity and pH collapse well before 3.0 mg/L ammonia-N is reached.
Total phosphorus: size the FeCl3 dose for chemical polishing. Assumed raw TP of 8 mg/L (typical medium-strength domestic sewage, not given by the exam) is reduced by roughly 10% in the primary clarifier and a further 15% by biomass synthesis uptake in the secondary process (conventional, non-enhanced biological treatment): $$TP\text{ at tertiary dosing point}=8\times(1-0.10)\times(1-0.15)=6.12\text{ mg/L}$$ so chemical precipitation must remove $6.12-0.3=5.82\text{ mg/L}$ of phosphorus. Using a standard 1.5:1 Fe:P molar dosing ratio for a low (0.3 mg/L) target and the molar masses $M_{Fe}=55.85$, $M_P=30.97$, $M_{FeCl_3}=162.2\text{ g/mol}$: $$\text{Fe dose}=5.82\times\frac{55.85}{30.97}\times1.5=15.7\text{ mg/L as Fe}\;\Rightarrow\;\text{FeCl}_3\text{ dose}=15.7\times\frac{162.2}{55.85}$$ $$\boxed{\text{FeCl}_3\text{ dose}\approx45.7\text{ mg/L, injected ahead of the tertiary filter}}$$ so the precipitated $FePO_4$ floc is captured in the same filtration step already provided for TSS polishing.
Coliform: check the UV disinfection credit needed. From an assumed raw coliform density of $1\times10^7$ CFU/100 mL down to the 150 CFU/100 mL limit, the required log reduction is $$\log_{10}\!\left(\frac{1\times10^7}{150}\right)=4.8\text{-log}$$ Biological treatment plus tertiary filtration typically credits about 2 to 2.5-log of this on their own (die-off, predation, and physical capture with the solids removed), leaving roughly 2.3–2.8-log for UV to provide — well inside the 3–4-log inactivation of fecal coliform routinely achieved by a standard UV dose in the 30–40 mJ/cm² range. $\boxed{\text{Coliform: met, with margin, at a standard UV design dose.}}$

Question 3 — effluent quality vs. limits (final results)
Parameter	Raw	Train result	Limit	Met?
TSS	250 mg/L	≤ 10 mg/L (via tertiary filtration)	10 mg/L	Yes
$cBOD_5$	220 mg/L	≤ 10 mg/L (93.5% removal across secondary + filter)	10 mg/L	Yes
Ammonia-N	—	3.0 mg/L, requires $\approx$157 mg/L alkalinity supplement	3.0 mg/L	Yes, with dosing
Total phosphorus	≈ 8 mg/L (assumed)	0.3 mg/L, via $\approx$45.7 mg/L FeCl3 dose	0.3 mg/L	Yes, with dosing
Coliform	≈ $10^7$ CFU/100 mL (assumed)	≤ 150 CFU/100 mL (UV, standard dose)	150 CFU/100 mL	Yes

Check: the exam gives only TSS, $BOD_5$, alkalinity and TKN for the raw sewage, and only numeric limits for the treated side. Raw total phosphorus (assumed 8 mg/L), raw coliform density (assumed $1\times10^7$ CFU/100 mL), and every removal-efficiency/synthesis fraction used above (primary 60% TSS/30% $BOD_5$/10% TP; 20% TKN assimilated to biomass; 15% biological P uptake; 50 mg/L residual alkalinity floor) are literature-typical design values from Metcalf & Eddy, not exam-supplied data, and are stated explicitly here rather than presented as given.

Topic: Conventional wastewater treatment train selection and mass-balance verification against multiple simultaneous effluent limits.

Key relations: Alkalinity consumed by nitrification $=7.14\times\Delta NH_3\text{-N}$ (mg $CaCO_3$/mg N); chemical P precipitation $FeCl_3+PO_4^{3-}\rightarrow FePO_4\downarrow+3Cl^-$, sized on a molar Fe:P ratio; required disinfection log-reduction $=\log_{10}(N_0/N)$.

Why this works. Each of the five effluent limits maps to one specific, checkable design decision rather than being met "by default" from a generic textbook train: the TSS/$cBOD_5$ targets are what justify adding tertiary filtration beyond secondary treatment alone; the ammonia-N target is what forces a nitrification check, which in turn is what exposes the alkalinity deficit hidden in the raw-water data (100 mg/L alkalinity looks adequate until it is compared against the 207 mg/L nitrification actually consumes); and the tight 0.3 mg/L phosphorus target is well below what biological uptake alone reaches, which is why a chemical dosing point is unavoidable. Reading the raw-water table for what it implies, not just what it states, is the actual skill this question tests.

Common pitfall. Sketching a "textbook" activated-sludge train without checking whether the given alkalinity can actually sustain the required nitrification (a very common design oversight in low-alkalinity wastewaters); dosing FeCl3 upstream of the primary clarifier rather than just ahead of the tertiary filter, which wastes coagulant on primary solids capture instead of targeting the soluble phosphorus that remains after biological treatment.

Source: Metcalf & Eddy, Wastewater Engineering §7–8 (activated sludge, nitrification alkalinity demand), §6 (phosphorus removal), §12 (UV disinfection design); Davis & Cornwell Ch. 3 (typical raw wastewater characteristics).

Question 3 of 5: Wastewater Treatment Plant Process Schematic

Question 3: Wastewater Treatment Plant Process Schematic (25 marks)