Question 3 of 5: Ammonia in Effluents; Activated Sludge with Anaerobic Digestion

Question 3: Ammonia in Effluents; Activated Sludge with Anaerobic 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.

Part (a) — Significance of ammonia, the reported forms, and the pH effect (12 marks)

Significance. Ammonia discharged in a treated effluent damages a receiving water in three distinct ways, and the three do not act on the same organisms or on the same timescale. First, it exerts a nitrogenous oxygen demand: nitrifying bacteria in the receiving water oxidise it by NH₄⁺ + 2O₂ → NO₃⁻ + 2H⁺ + H₂O, consuming 4.57 g of oxygen per gram of nitrogen. A 25 mg N/L effluent therefore carries 114 mg/L of eventual oxygen demand — ten times the carbonaceous demand of a well-treated 10 mg/L cBOD₅ effluent, and it is exerted slowly and far downstream, producing a second dissolved-oxygen sag that a BOD₅ test never sees. Second, the un-ionised fraction is acutely toxic to fish, damaging gill epithelium and disrupting ion regulation at concentrations below 0.1 mg/L as N. Third, ammonia is a nutrient that contributes to eutrophication, and it exerts a large chlorine demand on any downstream water intake, converting free chlorine to chloramine.

Forms determined and reported. Effluent analyses report total ammonia nitrogen (TAN) — the sum NH₃ + NH₄⁺ expressed as mg N/L, which is what the standard distillation or ion-selective-electrode method actually measures; the un-ionised ammonia, NH₃-N, which is not measured directly but computed from TAN with the measured pH and temperature; and total Kjeldahl nitrogen (TKN), the sum of ammonia and organic nitrogen, which bounds the ammonia that may still be released by hydrolysis. Nitrite and nitrate nitrogen are reported alongside to close the nitrogen balance and to reveal partial nitrification.

Which form governs, by receiver. The three cases in the question separate cleanly once the mechanism behind each is identified.

• (a) High DO, but an endangered species sensitive to toxicity. Oxygen demand is not the constraint, because the receiver has the assimilative capacity to absorb the nitrogenous demand. The governing form is the un-ionised NH₃-N, because that is the species that crosses the gill membrane; TAN alone is meaningless here without the pH and temperature that convert it. The permit should be written on NH₃-N, or on TAN with an explicit pH-and-temperature dependence.
• (b) Low DO, no toxicity concern. The governing form is total ammonia nitrogen, because every milligram of nitrogen — ionised or not — will eventually be nitrified and will consume 4.57 mg of oxygen. The relevant permit parameter is a TAN mass loading in kg/d, and the relevant analysis is a dissolved-oxygen sag computation combining carbonaceous and nitrogenous demand.
• (c) Both low DO and toxicity concerns. Both forms must be reported and both limits met, with the more stringent governing. These two limits pull in opposite directions in an important way: raising pH lowers the ionised fraction and therefore worsens toxicity, while doing nothing to the oxygen demand. In practice the plant is driven to full nitrification, which removes both problems at source by converting ammonia to nitrate in the plant rather than in the river — at the cost of a longer solids retention time, more aeration and, if nitrate is itself a concern, a denitrification stage.

Effect of pH. Total ammonia distributes between the two species according to NH₄⁺ ⇌ NH₃ + H⁺, and the fraction that is un-ionised follows the Emerson relation:

Given. Receiving water at 20 °C, so pK_a = 9.40; CCME guideline for the protection of freshwater aquatic life, 0.019 mg/L of un-ionised NH₃ as N. Find. The un-ionised fraction and the equivalent total-ammonia limit across the normal pH range.

Across a pH range that any river experiences between night and a sunny afternoon, the toxic fraction changes by a factor of

and the permissible total ammonia therefore falls twenty-eight-fold, from 4.8 to 0.17 mg N/L. Temperature acts the same way but more weakly: pK_a rises from 9.40 at 20 °C to 9.73 at 10 °C, so a cold winter river is roughly half as sensitive at the same pH. This is why an ammonia limit that is not tied to pH and temperature is scientifically meaningless, and why the federal Wastewater Systems Effluent Regulations define acutely lethal effluent by an un-ionised ammonia concentration below 1.25 mg N/L at 15 °C.

Control. The toxicity can be managed on four fronts. In-plant nitrogen removal is the durable answer — operating at a solids retention time above the nitrifier washout value (a safety factor of 2 to 2.5 on the winter growth rate), with anoxic denitrification where nitrate is also limited. Effluent pH adjustment works but is a last resort: lowering pH by one unit cuts un-ionised ammonia roughly threefold, and is defensible only where the receiver is well buffered. Discharge management — a diffuser that achieves rapid dilution, or seasonal limits keyed to summer low-flow and high-pH conditions — addresses exposure rather than load. And side-stream treatment of the digester supernatant, which can carry 20 to 30 per cent of the plant's nitrogen load in one or two per cent of its flow, is usually the cheapest first move.

Part (b) — Process flow diagram and description of a conventional activated-sludge plant with anaerobic digestion (13 marks)

Preliminary treatment. Bar screens (25–50 mm coarse, 6 mm fine) remove rags, plastics and debris that would foul pumps and blind diffusers, and a grit chamber — aerated, vortex or horizontal-flow — removes sand, gravel, cinders and bone fragments at a design settling velocity chosen to capture 0.2 mm particles while returning organic solids to the flow. The purpose is entirely protective: nothing is treated here, but everything downstream depends on it.

Primary treatment. The primary clarifier is a Type 1 and Type 2 gravity settling basin operating at a surface overflow rate of 30–50 m/d and a detention time of 1.5–2.5 h. It removes 50–70 per cent of suspended solids and 25–40 per cent of BOD as primary sludge, and skims floating grease. Its economic function is to cut the load on the aeration basin, which is the most energy-intensive unit in the plant, and to deliver a concentrated, highly digestible sludge to the digesters.

Secondary (biological) treatment — the activated-sludge reactor. Settled sewage enters an aerated basin holding a mixed culture of bacteria and protozoa at a mixed-liquor suspended solids concentration of 2000–4000 mg/L. Diffused or mechanical aeration supplies oxygen and keeps the floc in suspension while the organisms oxidise soluble and colloidal organic matter to carbon dioxide and water and synthesise new cell mass. The two variables that define the process are the hydraulic retention time (4–8 h in a conventional plant) and the solids retention time, $\theta_c = VX/(Q_wX_w)$, which is the true control parameter: 4–6 d for carbon removal only, 10–20 d if nitrification is required in a cold climate.

Secondary clarification and return sludge. The mixed liquor flows to a secondary clarifier, whose two duties are clarification (producing a low-solids effluent, sized on surface overflow rate, typically 16–28 m/d at average flow) and thickening (sized on solids loading rate, 4–6 kg/m²·h at peak). Most of the settled biomass is returned as return activated sludge at 50–100 per cent of the plant flow to maintain the reactor inventory; the small remainder, the waste activated sludge, is the deliberate withdrawal that fixes the solids retention time. This recycle is what distinguishes activated sludge from every once-through process: without it the biomass would wash out at the hydraulic retention time.

Disinfection and discharge. The clarified effluent is disinfected by ultraviolet irradiation or by chlorination followed by dechlorination — the latter mandatory in Canada, since the federal effluent regulations cap total residual chlorine at 0.02 mg/L — before discharge.

Sludge handling and anaerobic digestion. Primary sludge and waste activated sludge are thickened, by gravity for the primary and by dissolved-air flotation or a gravity belt for the biological sludge, from roughly 1 to 4–6 per cent solids; this step matters because digester volume is inversely proportional to feed concentration. The thickened sludge enters a heated, mixed anaerobic digester held at 35 °C with a solids retention time of 15–20 d, where hydrolysis, acidogenesis, acetogenesis and methanogenesis proceed in series. The process destroys 45–55 per cent of the volatile solids, produces biogas at 0.75–1.1 m³ per kg of volatile solids destroyed containing 60–65 per cent methane — usually enough to heat the digesters and generate a useful share of the plant's electricity — and delivers a stabilised, far less odorous product that meets pathogen-reduction requirements for land application. Digested sludge is dewatered by centrifuge or belt press to 20–30 per cent solids as biosolids. The two liquid sidestreams, digester supernatant and dewatering filtrate, are returned to the head of works and carry a disproportionate ammonia and phosphorus load, which is why they appear explicitly on the diagram.