Question 3 of 5

Question 3 (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.

(a) Rotating biological contactor (RBC): principle and working (15 marks)

A rotating biological contactor is an attached-growth (fixed-film) secondary treatment process. It consists of a series of closely-spaced, large-diameter circular discs (traditionally corrugated plastic media) mounted on a common horizontal shaft that rotates slowly, typically 1–2 rpm, in a form-fitted contact tank shaped to match the disc geometry. Roughly 40% of each disc's surface is submerged in the flowing wastewater at any instant, with the rest exposed to the atmosphere. A biofilm of attached microorganisms grows on the disc surfaces, much like the slime layer on a trickling filter's media, and it is this biofilm — not a suspended culture — that carries out the biological treatment.

As the shaft rotates, each point on a disc is carried alternately through the submerged wastewater, where the biofilm absorbs dissolved organic substrate (and, at longer SRTs and in later stages, ammonia for nitrification) and shears off a thin film of wastewater as it emerges, and then through the air, where that thin film is rapidly re-oxygenated by direct atmospheric contact and by the turbulence of rotation. This continuous, rapid cycling between substrate uptake (submerged) and oxygen transfer (exposed) is the defining working principle of the RBC: aeration is achieved passively through disc rotation rather than by diffused or mechanical aeration, so the process consumes very little energy compared with an activated sludge system treating the same load. As biofilm thickness grows, the innermost layer loses contact with both substrate and oxygen and biofilm periodically sloughs off the disc surface into the flow; this sloughed solids, together with the flow, then passes to a downstream secondary clarifier for solids-liquid separation, just as with a trickling filter. Multiple RBC stages are commonly arranged in series (each stage's discs on their own shaft, in successive tank compartments) so that organic loading — and hence biofilm character — progressively decreases from stage to stage, allowing the later stages to develop the slower-growing nitrifying organisms once the bulk of the carbonaceous BOD has already been removed upstream.

(b) Treatment recommendation for high-COD dairy wastewater (10 marks)

Given. A dairy processing wastewater with flow $Q = 1000\ \text{m}^3/\text{d}$ and $\text{COD}_{in} = 10{,}000\ \text{mg/L}$ must be treated to meet a municipal sewer-use limit of $\text{COD} \le 500\ \text{mg/L}$, with the constraint of minimum footprint and minimum energy use.

Find. The required overall COD removal, and a recommended treatment train that meets it within the stated footprint/energy constraint.

Approach. Compute the minimum overall removal required, then select a process family suited to this concentration, flow, and the stated footprint/energy priority.

1. Required overall COD removal. $$\eta_{required} = \frac{\text{COD}_{in}-\text{COD}_{limit}}{\text{COD}_{in}} \times 100 = \frac{10{,}000-500}{10{,}000}\times100 = \boxed{95\%}$$

Dairy processing wastewater at $\text{COD} = 10{,}000\ \text{mg/L}$ is a high-strength organic wastewater — well above the roughly 4000–5000 mg/L COD threshold above which anaerobic treatment becomes the energy- and footprint-favoured choice over aerobic (activated-sludge-type) treatment. An aerobic system sized to remove this load would need to supply oxygen for the full COD, at the roughly 1–1.5 kg O₂/kg COD removed typical of aerobic biomass yield, which at $1000\ \text{m}^3/\text{d} \times (10{,}000-500)\ \text{g/m}^3 \approx 9.5\ \text{t COD/d}$ would demand roughly $10$–$14\ \text{t O}_2$/d of blower capacity — large aeration basins, a large blower/energy draw, and a correspondingly large clarifier and footprint.

The recommended system is a two-stage train: (1) an Upflow Anaerobic Sludge Blanket (UASB) reactor as the primary high-rate step, followed by (2) a small aerobic polishing step (e.g., a sequencing batch reactor or an aerated lagoon sized only for the polishing duty) before discharge. In a UASB, wastewater flows upward through a bed of dense, granular anaerobic biomass; the granules' settling velocity lets a very high biomass concentration be retained without a separate clarifier or support media, so a UASB reactor achieves this same COD reduction in a fraction of the footprint of an aerobic basin, needs no oxygen-transfer energy at all (anaerobic conversion produces methane-rich biogas instead of consuming air), and the recovered biogas can offset the dairy's own energy use (boiler fuel or CHP), directly answering the "minimum footprint and energy" brief. A UASB alone typically removes 80–90% of influent COD from a readily-biodegradable, carbohydrate/fat-rich stream like dairy wastewater, which is not by itself reliable enough to guarantee the 95% needed to reach 500 mg/L at all times (dairy wastewater strength and character vary with production schedule); the small aerobic polishing stage downstream removes the UASB's residual soluble COD and any remaining volatile fatty acids, giving the train the margin needed to consistently meet the by-law limit while keeping the aerobic (energy-intensive) portion of the plant small, since it only has to treat the UASB's already-reduced effluent rather than the full raw load.