NivaarExam PrepOfficial exam papers ↗

18-Env-A4 Water and Wastewater Engineering · May 2016

Question 4 of 5: Alkalinity Titration Classification; Anaerobic Digestion

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

Notes on this paper

National Exams / EGBC — May 2016 — 04-ENV-A4 Water and Wastewater Engineering. Three-hour exam; Question 1 is compulsory (25 marks) and any three of the remaining four questions are required (25 marks each); all five are solved below for completeness. Closed book, one double-sided aid sheet permitted, approved calculator permitted.

Reference texts: Metcalf & Eddy, Wastewater Engineering: Treatment and Resource Recovery (5th ed.) — nitrification, BOD test theory, alkalinity/anaerobic digestion, phosphorus removal, disinfection chemistry; Davis & Cornwell, Introduction to Environmental Engineering (6th ed.) — the Streeter–Phelps oxygen sag, pH and coagulation–flocculation chemistry, water hardness; MWH's Water Treatment: Principles and Design (3rd ed.) — granular filtration (headloss, backwash) and ion exchange.

Question 4: Alkalinity Titration Classification; Anaerobic Digestion (25 marks: a 10, b 15)

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.

a. Phenolphthalein / total alkalinity classification (10 marks)

Given. A 50 mL water sample is titrated with 0.02N $H_2SO_4$: 5 mL is required to reach the phenolphthalein endpoint (pH 8.3), and a further titration to the Bromocresol Green endpoint (pH 4.5) requires 8 mL total from the start.

Find. The alkalinity indicated by each endpoint (phenolphthalein $P$ and total $T$), and any further alkalinity species (hydroxide/carbonate/bicarbonate) calculable from $P$ and $T$ together, with their values.

Approach. Convert each titrant volume to an alkalinity (as $CaCO_3$) using the standard normality relation, then classify the hydroxide/carbonate/bicarbonate split from the $P$-vs-$\tfrac{1}{2}T$ comparison in the Standard Methods table.

  1. Phenolphthalein alkalinity $P$ (titration to pH 8.3, the endpoint of $OH^-$ neutralization and the first equivalence point of $CO_3^{2-}\rightarrow HCO_3^-$). $$P=\frac{V_P\,N\,(50{,}000)}{V_\text{sample}}=\frac{5\times0.02\times50{,}000}{50}=\boxed{100\ \text{mg/L as }CaCO_3}.$$
  2. Total alkalinity $T$ (titration to pH 4.5, the second equivalence point of $HCO_3^-\rightarrow H_2CO_3$, i.e. all $OH^-+CO_3^{2-}+HCO_3^-$). $$T=\frac{V_T\,N\,(50{,}000)}{V_\text{sample}}=\frac{8\times0.02\times50{,}000}{50}=\boxed{160\ \text{mg/L as }CaCO_3}.$$
  3. Classify. Compare $P$ to $\tfrac{1}{2}T=80\ \text{mg/L}$: since $P=100>\tfrac{1}{2}T$, the sample falls in the hydroxide-plus-carbonate regime (no bicarbonate present), for which the standard table gives $\text{OH}=2P-T$ and $\text{CO}_3=2(T-P)$.
  4. Hydroxide and carbonate alkalinity (the “other” values calculable from $P$ and $T$). $$\text{OH-alkalinity}=2P-T=2(100)-160=\boxed{40\ \text{mg/L as }CaCO_3},$$ $$\text{CO}_3\text{-alkalinity}=2(T-P)=2(160-100)=\boxed{120\ \text{mg/L as }CaCO_3},\qquad \text{HCO}_3\text{-alkalinity}=0.$$ Check: $40+120+0=160=T$. ✓
Check
The two endpoint titrations are read as the standard sequential titration of one 50 mL sample (5 mL to pH 8.3, a further total of 8 mL from the start to pH 4.5), the conventional way this classification test is run and reported in Standard Methods 2320B.
QuantityResult
Phenolphthalein alkalinity $P$100 mg/L as CaCO3
Total alkalinity $T$160 mg/L as CaCO3
Hydroxide alkalinity40 mg/L as CaCO3
Carbonate alkalinity120 mg/L as CaCO3
Bicarbonate alkalinity0 mg/L as CaCO3

b. Anaerobic digestion of municipal wastewater sludge (15 marks)

Anaerobic digestion stabilizes primary and secondary (waste-activated) sludge in an oxygen-free reactor through a sequential microbial food chain of three stages. In hydrolysis, extracellular enzymes break complex particulate organics (proteins, lipids, carbohydrates/cellulose) down into soluble monomers (amino acids, fatty acids, sugars) — this is usually the rate-limiting step for raw sludge. In acidogenesis/acetogenesis, fermentative and acetogenic bacteria convert those monomers into volatile fatty acids (VFAs), hydrogen and $CO_2$, and ultimately to acetate. In methanogenesis, strictly anaerobic archaea convert acetate ($CH_3COOH\rightarrow CH_4+CO_2$, roughly two-thirds of the methane produced) and $H_2/CO_2$ ($CO_2+4H_2\rightarrow CH_4+2H_2O$, the remaining third) into biogas, typically 60–65% $CH_4$ and 35–40% $CO_2$. Methanogens are the most sensitive, slowest-growing organisms in the chain, so digester stability is governed by keeping conditions within their narrow tolerance.

The key operating parameters follow directly from that sensitivity. Temperature is held either in the mesophilic range (30–38 °C, most common) or thermophilic range (50–57 °C, faster kinetics and better pathogen kill but less stable and more energy-intensive); temperature must be kept steady since methanogens tolerate only small, slow fluctuations. Solids retention time / hydraulic retention time (typically 15–30 days mesophilic for a conventional completely-mixed digester, without recycle SRT=HRT) must be long enough for the slow-growing methanogens to be retained faster than they are washed out. pH is maintained near neutral (6.8–7.2) — methanogenesis is severely inhibited below about pH 6.2 — buffered by the digester's own bicarbonate alkalinity, which is why alkalinity (and the volatile-acids-to-alkalinity ratio, ideally $\lesssim0.3$–0.4) is tracked as the standard early-warning indicator of a souring/overloaded digester (VFA accumulation outrunning the buffering capacity). Mixing keeps the feed, biomass and heat uniformly distributed and prevents scum/grit accumulation, and the digester must be kept strictly anaerobic (any oxygen intrusion inhibits or kills methanogens) and free of toxic loads such as high ammonia, sulfide or heavy metals, which can also inhibit methanogenesis at elevated concentrations.