Upgrading a 12 MLD activated sludge plant to meet river discharge consent

Loads

• BOD: 12,000 × 0.28 = 3,360 kg/d; TSS 3,600 kg/d; NH4-N 384 kg/d.
• Biomass inventory: 3,000 m³ × 3.2 kg/m³ = 9,600 kg MLSS; wasting at SRT 4.5 d means about 2,100 kg MLSS/d, so the plant is being run as a high-rate unit with F/M ≈ 3,360/7,200 ≈ 0.47 kg BOD/kg VSS·d.

Nitrifier washout Nitrifier growth is slow. At 22–25 °C the maximum specific growth rate is about 0.8–1.0 per day, and the minimum sludge age for nitrification is roughly 1.5–2.5 days; design practice uses 2.5–3 times that, i.e. SRT 10–15 days, to survive low DO, cold spells and load shocks. At SRT 4.5 d the nitrifiers are being washed out faster than they grow. The measured ammonia removal is only (32 − 26)/32 = 19%, which is consistent with partial, unstable nitrification.

Alkalinity - the binding constraint after SRT Oxidising 1 mg of NH4-N consumes 7.14 mg of alkalinity as CaCO3. Complete nitrification of 32 mg/L needs 228 mg/L against 220 mg/L available: the pH would collapse to about 5.5–6 and stop nitrification. Denitrification recovers 3.57 mg alkalinity per mg NO3-N reduced, so an anoxic zone with 70% nitrogen removal recovers about 75 mg/L and provides the necessary buffer.

Oxygen Nitrification adds 4.57 mg O2 per mg N; oxidising 30 mg/L of ammonia adds about 137 mg/L of oxygen demand on top of the carbonaceous demand, and the existing blowers were sized for carbonaceous treatment alone.

Conclusion: the binding constraints are SRT (tank volume/solids inventory), alkalinity/pH (needs denitrification), and oxygen transfer - in that order. More diffusers alone will not fix it.

Target SRT: 12 days (about 3× the minimum at the coldest month) with MLSS 3.5 kg/m³.

Solids inventory (approximate, confirm with plant data)

• Biomass from BOD: Yobs ≈ 0.35 kg VSS/kg BOD × 3,300 kg/d ≈ 1,150 kg/d.
• Inert influent solids: 0.25 × 3,600 ≈ 900 kg/d.
• Total ≈ 2,050 kg MLSS/d; inventory at SRT 12 d ≈ 24,600 kg.
• Volume at 3.5 kg/m³ ≈ 7,000 m³. Existing 3,000 m³, so about 4,000 m³ of new volume; at 5.0 m depth this needs roughly 800 m² of land - well within 1.5 ha.

Configuration

• New anoxic tank of about 1,800 m³ (25% of the total), ahead of the aerobic tanks, with internal recycle of 2–3 Q and return sludge; this gives denitrification, recovers alkalinity and reduces the aeration demand by about 2.9 mg O2 per mg N.
• Aerobic volume 5,200 m³ (existing + new), DO control at 1.5–2.0 mg/L, new fine-bubble diffusers, and blowers 2 working + 1 standby; aeration is an essential load, so it must be on DG.
• Re-check the secondary clarifiers: at MLSS 3.5 kg/m³ the solids loading rate will rise; keep the peak surface settling rate below about 1.0 m/h and the solids loading below about 5 kg/m²·h, otherwise add clarifier capacity.

Tertiary treatment

• Dual-media filtration for TSS ≤ 10 mg/L; without filtration the clarifier alone is unlikely to guarantee 10 mg/L.
• Disinfection: chlorination becomes inefficient once ammonia is 5 mg/L (chlorine demand and chloramine formation), so use UV or a properly designed chlorination with a CT check, targeting under 100 MPN/100 mL.
• Sludge: the extra sludge must be thickened and dewatered; check the existing sludge handling capacity for the increased mass.

Power: keep the aeration blowers and the recycle pumps on DG; the plant can tolerate a short aeration break but not 8 hours a day of it.

The claim is half true, and the half that is untrue is the dangerous half.

Where the vendor is right: in a moving-bed biofilm reactor the nitrifiers live in a biofilm with a long solids retention time, decoupled from the sludge age of the suspended solids and from the clarifier. That protects nitrification against washout and permits a retrofit of the existing tank volume.

Where the claim fails

• Surface area is still needed. Design ammonia removal rates for nitrification in MBBR are about 0.5–1.0 g NH4-N per m² of media per day at 20 °C. For 384 kg N/d at 0.75 g/m²·d you need about 480,000 m² of media; at a specific surface area of 500 m²/m³ and 50% fill, that is about 1,900 m³ of reactor volume. It physically fits in 3,000 m³, but only just, and with no allowance for the carbonaceous duty or for winter.
• Oxygen transfer must be re-checked - the diffusers and blowers must supply the nitrification demand - as must mixing and the media retention screens.
• 'Sludge age does not apply' is false for the suspended sludge: the clarifier still sets the solids inventory, the return sludge and the effluent TSS. Filtration is still needed for TSS 10 mg/L.
• The alkalinity problem is untouched. If pH falls, the biofilm stops nitrifying too. An anoxic/denitrification stage or alkali dosing is still required.

Decision rule: accept only with (i) a process design showing media volume, fill fraction, oxygen demand and clarifier loading; (ii) a performance guarantee on BOD 10/TSS 10/NH4-N 5 at 22 °C minimum with a defined influent; (iii) proven media with a stated service life and a retention screen design; (iv) a paid trial or a retention sum. Compare it on life-cycle cost with the anoxic–aerobic–filtration option, and do not release the existing tank for media filling until the clarifier and aeration checks are complete.