Analysis and Design

The balances that treatment and conveyance design rest on: mass balances, hydraulic loading, solids loading, and measuring flow. The NCEES spec gives it 6–9 of 80 questions, and the same tools resurface inside the water, wastewater, and water-quality areas — time here pays twice.

What the problems look like

Short, arithmetic-heavy setups. Recurring shapes — a steady-state mass balance on a river reach, pond, or blend (dilution, evaporation, a treated-plus-bypass blend); reactor behavior for a first-order constituent in a completely mixed tank or tanks in series; hydraulic loading of clarifiers, filters, weirs, and land application; solids loading on secondary clarifiers and thickeners, plus sediment and sludge production; and flow measurement with weirs, Parshall flumes, orifices, venturis, and Pitot tubes.

By exam day you should be able to —

• write a mass balance around the right control volume, including streams that enter or leave inside it;
• predict effluent from a completely mixed reactor or N reactors in series, and know when plug flow applies;
• compute overflow rate, weir loading, filtration rate, and hydraulic loading on the correct plan area or length;
• compute solids loading and size a unit that must satisfy two limits at once, and convert a sediment or sludge mass rate to the volume it occupies;
• pick and apply the right rating for a weir, flume, orifice, or venturi.

Where people lose points

• Control-volume boundaries — a tributary entering below the station, a diversion upstream of the outfall, an evaporation stream that carries no dissolved solids. Draw the box first.
• Completely mixed ≠ plug flow. A CSTR gives C₀/(1 + kθ); tanks in series split the total detention time among the tanks before raising to the Nth power.
• Which flow goes where on a clarifier. Surface overflow uses the flow leaving over the weirs; solids loading uses the mixed-liquor flow, influent plus return sludge.
• Geometry slips — diameter used as radius, π dropped from the circle area, weir length taken as 2πD, or the filter in backwash counted as in service.
• Two limits, one area. When hydraulic and solids loading both govern, size to the larger required area — don't add them.
• Conversion constants — 8.34 for MGD × mg/L, not 7.48 or 1.547.
• Meter ratings — the throat width selects the Parshall equation; a free-flowing flume reads upstream head, not the drop; a V-notch goes as H^(5/2); a submerged orifice runs on the head difference.

Have these at your fingertips

The completely mixed and plug-flow reactor relations, the lb/day conversion, and the weir, flume, and orifice ratings — plus where they live in the Handbook so you're navigating, not hunting.

Going deeper

• Wastewater Collection and Treatment — clarifier and solids-handling design in context
• Drinking Water Distribution and Treatment — filtration and sedimentation loading

Part of the Water Resources & Environmental exam. Practice this area with exam-real problems — the tutor walks any one you miss. → Start practicing free — free with an account.