Design Manual › Biological › UCT / Bio-P
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UCT / MUCT — Enhanced Biological P Removal
UCT exists to fix one specific flaw in A²/O: the return sludge carries nitrate straight into
the anaerobic zone, where it steals the volatile fatty acids the phosphorus-accumulating organisms need.
UCT routes the return sludge through an anoxic zone first. This tool sizes the zones off the nitrification
SRT, then shows you exactly how much nitrate load that rerouting saves.
1Duty & water quality
Bio-P performance is driven by the ratio of readily biodegradable COD to phosphorus —
as much as by the phosphorus number itself. Keep an eye on the BOD/TP ratio in the results.
2Kinetics — the SRT that nitrification demands
The aerobic SRT is set by how fast nitrifiers grow at your water temperature. Everything
else in the reactor follows from it.
3Zone HRTs & recycles
UCT = Anaerobic → Anoxic 1 → Anoxic 2 → Aerobic. Return sludge enters Anoxic 1;
the nitrate recycle from the aerobic end enters Anoxic 2. Nothing nitrified reaches the anaerobic
zone until it has been denitrified.
Results — reactor & effluent
Total volume
—m³
Total HRT
—h
Effluent TP
—mg/L
| Item | Value | Basis |
|---|
4Why UCT — nitrate load on the anaerobic zone
Every kilogram of nitrate entering the anaerobic zone consumes volatile fatty acids that
the P-accumulating organisms would otherwise use. This is the entire reason UCT exists.
| Item | A²/O | UCT |
|---|
5Nitrogen & phosphorus balance
| Item | Value | Basis |
|---|
Oxygen & alkalinity
Two things quietly limit BNR plants: blower capacity, and alkalinity. Nitrification
destroys alkalinity; denitrification gives some of it back.
| Item | Value | Basis |
|---|
6Instrument schedule
A BNR plant is controlled at its zone boundaries — that is where the instruments belong.
| Point | Instrument | Why here |
|---|
Quote the UCT instruments at factory price
Zone DO, ORP at the anaerobic/anoxic boundaries, MLSS, nitrate and the two recycle flowmeters — all mappable to our catalogue and supplied at the manufacturer's ex-works price (our margin is the export rebate, not a markup on the device).
Request a quote for this duty ← Primary Clarifier Chemical Phosphorus Removal & Carbon Dosing →Response within 24 hours · Inputs stay in your browser — nothing is uploaded.
Sources & parameter basis
| [1] | METCALF & EDDY / EPA — BNR configuration: anaerobic 0.5–2 h, anoxic 1–3 h, aerobic 4–12 h; MLSS 2.5–4.5 g/L; RAS 0.5–1.5×; effluent TP achievable biologically 0.5–2 mg/L. |
| [2] | Nitrifier growth μN = 0.47·1.103(T−20)·DO/(1.3+DO) d⁻¹; design SRT = SF·(1/μN) with SF typically 2–6. Aerobic SRT governs nitrification. |
| [3] | Observed yield Yobs = Y/(1 + kd·SRT), kd,T = kd20·1.04(T−20); sludge N content ≈0.10 kg N/kg VSS. |
| [4] | Bio-P sludge carries 3–6 % P (vs 1.5–2 % in a conventional plant); bio-P needs roughly 10 kg BOD per kg P taken up. |
| [5] | Alkalinity: nitrification consumes 7.14 kg CaCO₃/kg NH₃-N; denitrification returns 3.57 kg CaCO₃/kg NO₃-N. Keep ≥70–80 mg/L residual for stable nitrification. |
| [6] | UCT (University of Cape Town) and MUCT: RAS routed to anoxic 1, nitrate recycle to anoxic 2; achievable denitrification ≈ Rir/(1+Rir) limited by available carbon; anoxic-1 nitrate removal ≈85 %. |
| [7] | Oxygen: carbon 0.9, nitrification 4.57, denitrification credit 2.86 kg O₂/kg NO₃-N, biomass 1.42 kg O₂/kg VSS; blower SAE 1.8–2.5 kg O₂/kWh. |
Full derivation and worked example: UCT-Bio-P-Calculation-Sheet.md. Compare the
A²/O and UCT columns at a high internal recycle — the nitrate load gap is what you are paying the
extra anoxic zone for.
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