Important Process Design Variables for Five Treatment/Disposal Unit Processes: 18-Env-B5 Industrial & Hazardous Waste Management, December 2015, Q16

Question 16: Important Process Design Variables for Five Treatment/Disposal Unit Processes (10 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.

Unit process	Key design variable(s)	Why important
16.1 Incinerator	Combustion temperature & residence time	Must be high enough, and held long enough, to fully destroy hazardous organics and minimize products of incomplete combustion.
	Excess air ratio	Ensures complete combustion (avoids fuel-rich, sooty/PIC-forming conditions) without wasting auxiliary fuel on excess heating of air.
	Waste heating value / moisture content	Governs whether the waste is self-sustaining or requires auxiliary fuel to maintain design temperature.
16.2 Membrane separator	Pore size / molecular weight cut-off	Sets which contaminants are physically rejected versus pass through with the permeate.
	Transmembrane pressure & flux	Governs throughput per unit membrane area and drives the rate of fouling.
	Feed pretreatment / cross-flow velocity	Controls fouling rate and cake-layer formation, which otherwise degrade flux and shorten cleaning intervals.
16.3 Conventional activated sludge	F/M ratio & MLSS/MLVSS	Balances organic loading against available biomass; governs removal efficiency and sludge settleability (bulking risk).
	Solids retention time (SRT)	Controls whether nitrifiers are retained (SRT must exceed their minimum growth rate) and sets net sludge production.
	Hydraulic retention time & dissolved oxygen	Sets available reaction time and ensures adequate oxygen supply for aerobic metabolism.
16.4 Anaerobic digester	Organic (VS) loading rate & HRT/SRT	Controls reactor sizing and prevents overloading, which can sour the digester (VFA accumulation).
	Temperature (mesophilic ~35°C or thermophilic ~55°C)	Governs methanogen kinetics; must be held stable since methanogens are highly temperature-sensitive.
	pH / alkalinity	Methanogens tolerate only a narrow pH band (~6.8–7.2); alkalinity buffers against VFA-driven pH drop.
16.5 Aerobic digester	Solids retention time	Sets the degree of volatile-solids destruction (stabilization) achieved.
	Dissolved oxygen & mixing/oxygen-transfer capacity	Must sustain aerobic conditions throughout the digester volume for continued endogenous respiration.
	Temperature	Governs the rate of biological VS destruction, as with any biological process.

Topic: Governing design variables across thermal, membrane and biological treatment unit processes

Key relations: thermal process (incinerator) → temperature/time/excess-air; physical separation (membrane) → pore size/pressure-flux/fouling control; biological processes (activated sludge, anaerobic and aerobic digestion) → loading, retention time, and the specific environmental condition (DO, temperature, pH) the responsible organisms need

Why this works: every design variable listed controls either the rate of the governing mechanism (combustion kinetics, membrane flux, microbial kinetics) or the environmental condition the mechanism depends on — get either wrong and the unit process fails to perform its intended function regardless of how well-sized the tankage or equipment otherwise is.

Common pitfall: listing only "size" or "flow rate" as the design variable for a biological process, when the governing variables are actually the loading rate, retention time and the specific environmental condition (pH, DO, temperature) the organisms require — size follows from these, not the other way around.

Source: Metcalf & Eddy, Wastewater Engineering, 5th ed.; Nemerow & Dasgupta, Industrial and Hazardous Waste Treatment, 2nd ed.; LaGrega, Buckingham & Evans, Hazardous Waste Management, 2nd ed. (incineration).

Question 16 of 18: Important Process Design Variables for Five Treatment/Disposal Unit Processes

Question 16: Important Process Design Variables for Five Treatment/Disposal Unit Processes (10 marks)