Trickling Filter, MBBR or MBR: When Suspended Growth Is Not the Answer
Suggested readership: designers benchmarking biological options for a tropical plant where land, energy or effluent-solids limits make a conventional activated-sludge box unattractive. Reading time: ~7 min. Companion tools: MBBR · Trickling filter · MBR
Three ways to hold biomass without an aeration-tank MLSS
Suspended-growth processes earn their keep by being flexible. But every one of them eventually hits a wall — the clarifier. Fixed-film and membrane processes answer differently: hold the biomass on a plastic surface (trickling filter, MBBR), or hold it behind a barrier that also does the solid–liquid separation (MBR).
Sized for a 10,000 m³/d, 250 mg/L BOD₅, ammonia-nitrogen ~40 mg/L municipal feed, the three options look like this:
| Trickling filter | MBBR | MBR | |
|---|---|---|---|
| Biology volume | 1,667 m³ media | 1,017 m³ reactor | 2,083 m³ |
| Nitrification area | media surface | 254,000 m² carrier | membrane is not the biology |
| Hydraulic residence | HLR 36 m³/m²·d | 2.4 h | ~5 h aeration + membrane |
| Clarifier after | humus tank 200 m² | yes (or combined) | none — membrane replaces it |
| Aeration energy | near zero (natural draft) | blowers for fine bubbles | blowers + membrane air scour |
| Operator skill | lowest | medium | highest |
Trickling filter: the zero-energy biology people keep rediscovering
A high-rate plastic trickling filter treats the whole 10,000 m³/d load (plus 65 % recirculation for the NRC kinetics) on 1,667 m³ of media in an 18.8 m diameter tower, at a hydraulic loading of 36 m³/m²·d. The distributor does one pass of ~75 mm every 0.17 rev/min, and the oxygen comes from natural draught — the largest energy consumer on the whole unit is the influent pump.
The honest limits: removal is first-order and incomplete. The default calculation lands at 79 % BOD removal, 42 mg/L effluent — fine for discharge to a polishing step, not fine as a final effluent alone. Media fouling is the tropical disease: at 30 °C the film grows thick, snails and filter flies colonise the wet zones, and the distributor bearings need real maintenance. Sloughing sends daily batches of solids to the humus tank, which is why there is always one — 200 m², 16 m diameter, SOR 25 m³/m²·d — quietly doing the "clarifier" job the filter was supposed to avoid.
Use a trickling filter when energy is dear, operator skills are scarce, and a downstream polishing stage exists. Its specific energy can be a tenth of an activated-sludge plant's.
MBBR: nitrification in a shoebox, if you count the m²
The MBBR's trick is arithmetic: nitrification needs 1.57 g N per m² of carrier per day at 30 °C (r_A(T)), and 400 kg N/d of ammonia therefore needs 254,000 m² of protected biofilm surface. At 500 m²/m³ of carrier that is 508 m³ of plastic — suspended at 50 % fill in a 1,017 m³ reactor with a 2.4 h HRT.
That is the pitch: nitrifying volume is an order of magnitude smaller than a suspended-growth plant would need. Two caveats in tropical service:
- Carrier fill is a hard constraint. You cannot "add biomass" past the fill fraction — the carriers need to circulate. If your 50 % fill assumption is wrong, the whole reactor is wrong; there is no RAS flow to lean on.
- The biofilm is the SRT. A carrier does not get wasted, so the film holds whatever grows — including slow nitrifiers, which is why it works — but you surrender the operator's SRT lever. Solids control moves to the downstream clarifier and to periodic media cleaning.
MBR: delete the clarifier, adopt the membrane
An MBR holds 10,000 mg/L MLSS in a 2,083 m³ tank and pulls 9,500 m³/d of permeate through 19,792 m² of membrane (660 modules) at 480 L/m²·d — flux on the low side because tropical sludge is less filterable at high temperature. In return for the membrane area you remove the secondary clarifier entirely and get effluent solids near zero.
The operating ledger is unforgiving: membrane permeability degrades, so design TMP is 0.4 bar and cleaning frequency is a real cost line; aeration splits between biology and air scour for the membranes; and any surge of non-biodegradable solids (sand, coagulant flocs) fouls irreversibly. The MBR is a process, not a tank — it lives or dies by the operators' membrane hygiene.
Which one, in a tropical municipal context
- Choose the trickling filter for small-to-medium plants with plentiful land, expensive electricity and modest effluent standards — then give it a real humus tank and a sloughing-tolerant downstream step.
- Choose the MBBR to retrofit nitrification into an existing tank that has no room for more volume, or when you need the smallest new aeration footprint.
- Choose the MBR for reuse-grade effluent or space-critical sites, and only if the operating utility will run a serious cleaning regime. An MBR in the hands of a stretched crew fails expensively.
Each of the three needs surprisingly few instruments to run well — flow (SE10A class) for the load and recirculation in every case; DO and pH for the MBBR reactor; and for the MBR, pressure/transmembrane monitoring plus MLSS, with turbidity (NT6000G/NT6000L class) guarding what the membrane sends out. The biology is different; the measurement basics are not.