Question 3 of 5: Ammonia in Treated Effluents; Stages of Anaerobic Sludge Digestion

Question 3: Ammonia in Treated Effluents; Stages of Anaerobic Sludge Digestion (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.

The paper labels both sub-parts "a."; the second is treated as part (b) below, as its mark allocation and content require.

Part (a) — Significance of ammonia, the reported forms, and the receiving-water cases (15 marks).

Given. A secondary effluent discharged to a surface water body; illustrative effluent total ammonia nitrogen 20 mg N/L with cBOD₅ 20 mg/L; receiving water at 20 °C. Find. Why ammonia matters, which analytical forms are reported, which form governs in each of the three receiver cases, and how pH controls the toxicity.

Ammonia matters for three distinct reasons, and keeping them separate is the whole of this question. First, it exerts an oxygen demand. Nitrifying bacteria oxidise ammonium to nitrate,

so the stoichiometric nitrogenous oxygen demand is \(2 \times 32.00/14.01 = 4.57\) g O₂ per g N (about 4.33 g/g once cell synthesis is allowed for). For the illustrative effluent,

which is 4.6 times the carbonaceous demand of the same effluent. A discharge that meets a 20 mg/L cBOD₅ permit can therefore still deoxygenate the reach, because the permit measures only the smaller of the two demands. Second, it is directly toxic through the un-ionised species, as developed below. Third, it is a nutrient, contributing to eutrophication and to nuisance algal growth in nitrogen-limited receivers such as estuaries and coastal waters. A fourth, plant-side significance is that ammonia consumes chlorine, so a chlorinated effluent leaves only a combined residual.

The forms usually determined and reported in an effluent analysis are:

• Total ammonia nitrogen (TAN), the analytical sum NH₄⁺-N + NH₃-N, reported as mg N/L. This is the routine measurement and the usual permit parameter.
• Un-ionised (free) ammonia, NH₃-N, not measured directly but calculated from TAN with the pH and temperature of the sample. This is the toxicologically active form.
• Ionised ammonium, NH₄⁺-N, obtained by difference; essentially non-toxic but fully oxygen-demanding.
• Supporting parameters — TKN (organic N + ammonia), from which organic nitrogen is obtained by difference, since organic nitrogen ammonifies in the receiver and becomes a delayed ammonia load.

The three receiver cases are then answered by asking which of the two mechanisms binds:

1. Case (a) — high DO, an endangered species sensitive to toxicity. Oxygen is not limiting, so the NOD is not the controlling mechanism; what matters is the un-ionised NH₃-N concentration in the receiver after mixing, compared against the CCME chronic guideline of 0.019 mg/L NH₃-N. The permit should be written on un-ionised ammonia at the receiving water's pH and temperature, and the critical condition is the warm, high-pH summer afternoon when photosynthesis has driven the pH up — not the annual average.
2. Case (b) — low DO, no toxicity concerns. Here the controlling quantity is the total ammonia nitrogen, because every mole of nitrogen — ionised or not — is eventually nitrified and draws 4.57 g O₂ per g N from an already stressed reach. The permit should be a mass load of TAN, and the analysis is a Streeter–Phelps-type oxygen balance with a nitrogenous term. Speciation is irrelevant to oxygen demand.
3. Case (c) — both low DO and toxicity concerns. Both forms bind, and they must be checked separately, because they respond to pH in opposite operational directions. The governing limit is whichever of the two produces the lower allowable effluent concentration — and note the coupling: as the reach deoxygenates, nitrification stalls, TAN persists further downstream and the toxic exposure is extended. This is the case that justifies full nitrification (or nitrification–denitrification) at the plant rather than a limit written on a single parameter.

Impact of pH, and its control. The toxic fraction is set by the ammonium dissociation with pK_a = 9.40 at 20 °C (Emerson). For the illustrative 20 mg N/L:

Temperature acts the same way, though more weakly: at pH 8.0 the free ammonia rises from 0.761 to 1.07 mg/L between 20 and 25 °C, because pK_a falls with temperature. Control therefore has two routes. The direct route is to remove the ammonia: biological nitrification in the plant (the standard answer, and the one that also removes the NOD), or nitrification–denitrification where total nitrogen is capped, or breakpoint chlorination and air stripping for special cases. The indirect route is to manage the speciation: lower the effluent pH before discharge (chemical addition, or CO₂ recarbonation), avoid discharging into a shallow, weed-choked reach where afternoon photosynthesis pushes the pH above 9, and design the outfall diffuser so that the plume mixes rapidly while its pH is still that of the effluent. Air stripping of ammonia is the reverse operation and makes the point plainly: it works only after the pH is raised to 10.5–11, precisely because that converts the ammonium to the volatile free form.

Part (b) — The four biochemical stages of anaerobic sludge digestion, and the rate-limiting step (10 marks).

Anaerobic digestion is a consortium process in which four groups of organisms act in series, each living on the products of the one before. The sequence is:

1. Hydrolysis. Extracellular enzymes secreted by fermentative bacteria break the particulate, polymeric substrate — carbohydrates, proteins, lipids and the cell walls of the waste activated sludge — into soluble monomers: sugars, amino acids, long-chain fatty acids and glycerol. Nothing crosses a cell membrane until this has happened, so hydrolysis is the gateway to the whole process.
2. Acidogenesis (fermentation). The soluble monomers are fermented by acidogenic bacteria to volatile fatty acids (propionate, butyrate, valerate), alcohols, lactate, ammonia, hydrogen and carbon dioxide. This step is fast — growth rates an order of magnitude above the methanogens — which is why an overloaded digester accumulates VFAs.
3. Acetogenesis. Obligate hydrogen-producing acetogens convert the higher VFAs and alcohols to acetate, H₂ and CO₂, e.g. \(\mathrm{CH_3CH_2COO^-} + 3\mathrm{H_2O} \rightarrow \mathrm{CH_3COO^-} + \mathrm{HCO_3^-} + \mathrm{H^+} + 3\mathrm{H_2}\). This reaction is thermodynamically unfavourable unless the hydrogen partial pressure is kept below about 10⁻⁴ atm, so the acetogens depend on interspecies hydrogen transfer to the methanogens — a syntrophic coupling that is the process's chief vulnerability.
4. Methanogenesis. Strictly anaerobic archaea produce methane by two routes: the acetoclastic path \(\mathrm{CH_3COO^-} + \mathrm{H_2O} \rightarrow \mathrm{CH_4} + \mathrm{HCO_3^-}\), which yields about 70 % of the gas in a municipal digester, and the hydrogenotrophic path \(4\mathrm{H_2} + \mathrm{CO_2} \rightarrow \mathrm{CH_4} + 2\mathrm{H_2O}\), which also performs the hydrogen scavenging that the acetogens require.

The methane yield that ties the four stages together is 0.35 m³ CH₄ per kg COD stabilised at STP, or 0.395 m³/kg at a mesophilic 35 °C.

Which stage is rate-limiting, and why. The answer depends on the substrate, and saying so is what distinguishes a complete answer. For municipal sludge — the case this question is about — hydrolysis is rate-limiting, because the feed is predominantly particulate: primary sludge is fibrous and lipid-rich, and waste activated sludge is bacterial cell mass whose walls must be lysed before anything inside is available. Enzymatic attack on a solid surface proceeds at a rate set by the available surface area, not by a soluble-substrate concentration, so it follows first-order kinetics with a low rate constant. This is precisely why sludge pre-treatment technologies — thermal hydrolysis, ultrasonic and mechanical disintegration, alkaline lysis — target this step and can cut the required SRT roughly in half. For a soluble, readily fermentable feed (a food-industry wastewater in a UASB, say), hydrolysis is trivial and methanogenesis becomes rate-limiting instead.

Methanogenesis is, in either case, the most vulnerable stage, and the distinction between "slowest" and "most vulnerable" is worth marks. Acetoclastic methanogens have a maximum specific growth rate of roughly 0.3 d⁻¹ at 35 °C, so their doubling time is

which is the origin of the familiar 15–20 d mesophilic digester design. They are also inhibited by free ammonia above roughly 100 mg/L NH₃-N, by sulphide, and by pH outside 6.8–7.4. If they are lost, the fast acidogens keep running, VFAs accumulate, the pH falls, and the digester "sours" — a failure whose signature (rising VFA/alkalinity ratio, falling methane fraction) is monitored precisely because the recovery takes weeks at a 2.3 d doubling time.