Treatment and Disposal of Process Wastes: 23-Chem-A5 Chemical Plant Design and Economics, December 2017, Q6

Question 6: Treatment and Disposal of Process Wastes (20 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.

Good practice follows the waste-management hierarchy — first reduce at source, then reuse/recycle, then treat, and only then dispose — so that the quantity and hazard of waste requiring final disposal are minimised.

(a) Eight general waste treatment / disposal techniques

Source reduction (waste minimisation) — process changes that generate less waste in the first place.
Recycling / reuse / recovery — recovering unreacted feed, solvent or by-product value for return to the process.
Physical separation — sedimentation, filtration, centrifugation, flotation to concentrate or remove contaminants.
Chemical treatment — neutralisation, precipitation, and oxidation/reduction to render species non-hazardous.
Biological treatment — aerobic (activated sludge) or anaerobic (digestion) breakdown of biodegradable matter.
Adsorption / absorption — activated-carbon adsorption or scrubbing to capture dissolved or gaseous contaminants.
Incineration / thermal oxidation — controlled combustion to destroy organics and reduce volume, with energy recovery.
Landfill / secure disposal — engineered (secure) landfill, stabilisation/encapsulation, or deep-well injection for the final residue.

(b) Gaseous wastes

Gaseous effluents are treated by absorption (wet scrubbing of acid gases or solvents into a liquid, as in the Q1 flowsheet), adsorption of trace organics onto activated carbon, condensation to recover volatile solvents, and thermal or catalytic incineration (thermal oxidisers and flares) to destroy combustible vapours. Particulate-laden gases are cleaned by cyclones, fabric (bag) filters, electrostatic precipitators or wet scrubbers; dilute biodegradable streams can be treated in biofilters.

(c) Liquid (organic / non-aqueous) wastes

Organic liquid wastes are best recovered by distillation where a solvent has value, or otherwise incinerated — high-calorific streams are often blended and burned as supplementary fuel with heat recovery. Phase separation (decanting) removes immiscible water or solids first, and any resulting aqueous phase is routed to aqueous treatment.

(d) Solid wastes

Solids are managed by recovery/recycling where practical, incineration to reduce volume and destroy organics, and, for the residue, secure landfill with prior stabilisation/encapsulation of hazardous solids; biodegradable organic solids may be composted.

(e) Aqueous wastes

Aqueous effluents are treated in the classic sequence: physical pre-treatment (equalisation, screening, sedimentation, flotation, filtration) to remove suspended solids and oils; chemical treatment (neutralisation of acids/alkalis, precipitation of heavy metals, chemical oxidation of refractory organics); biological treatment (activated sludge, trickling filters or anaerobic digestion) to remove dissolved biodegradable organics; and, for polishing or reuse, membrane processes (ultrafiltration, reverse osmosis) or activated-carbon adsorption, before discharge to sewer or watercourse within permit limits.

Concept & Theory

Topic. Waste management in process design — matching the treatment method to the phase and hazard of the waste.

Key ideas. The waste hierarchy (reduce > reuse/recycle > treat > dispose); phase-appropriate methods (gaseous → scrub/adsorb/incinerate; organic liquid → recover/incinerate; solid → recover/incinerate/landfill; aqueous → physical–chemical–biological train).

Why this matters. Treatment and disposal are a real design cost and a regulatory obligation; designing waste out at source is almost always cheaper than end-of-pipe treatment, and the phase of the waste dictates which unit operations even apply.

Common pitfall. Treating “liquid” and “aqueous” wastes as the same problem — concentrated organic liquids are usually recovered or incinerated, whereas dilute aqueous streams go through a physical–chemical–biological train.

Source. Towler & Sinnott, Chemical Engineering Design, Ch. 11 (waste management and pollution control); Turton et al., Ch. 27 (environmental performance).