Biological Filter or BAF? Matching Loading Rates to Carbon Removal, Nitrification and Denitrification Duty

The biological filter descends from the intermittent sand filter and the contact filter, built on the soil self-purification principle and long practice with sewage irrigation. What it asks of its media is specific: large specific surface area, high porosity, high material strength, stability and low cost. What it offers is a large buffering capacity that absorbs concentration peaks, fully automatic and stable operation with few vulnerable parts, and modular construction — capacity grows by adding units, which suits separate treatment under dispersed conditions.

The structure carries more of the performance than the name suggests. The tank structure provides the support; the tank bottom houses the ventilation system, the sludge discharge system and the supporting and percolating structure; and the water distribution system — typically a rotating water distributor — spreads the flow evenly across the media surface. Because the media stay fixed and the flow moves through them, the biomass attached to the grains sees a steadier diet than suspended-growth systems provide, which is where the buffering capacity and shock-load resistance come from in practice.

Choose the configuration before choosing the numbers

• Carbonaceous organic matter only: a single-stage carbon-oxidation biological aerated filter is enough.
• Carbon removal plus nitrification: the single-stage filter at appropriately reduced load, or a two-stage series of a carbon-oxidation filter and a nitrifying biological aerated filter.
• Enough influent carbon source with strict effluent total nitrogen: the pre-denitrification filter plus nitrification filter combined process.
• High influent total nitrogen, short carbon, strict limit: post-denitrification with carbon source supplementation, or a pre-denitrification filter with external carbon dosed by calculation; the nitrified-liquor recirculation ratio of the pre-denitrification filter follows the design NO3-N removal rate and the influent carbon-to-nitrogen ratio.
• Denitrification needed but carbon short: place the denitrification tank ahead of the nitrification tank and recirculate part of the nitrified effluent. The wastewater's own organics become the carbon source, organic removal in the denitrification tank protects the nitrification capacity behind it, aeration demand drops and sludge yield falls — for domestic sewage with sufficient BOD5, pre-denitrification wins clearly on operating cost.

The working load ranges

Volumetric loading and filtration rate travel together. For carbon oxidation duty, 3.0~6.0 kgBOD5/(m·d) with 2.0~10.0 m/(m·h); for nitrification, 3.0~6.0 kgNH3-N/(m·d) with 3.0~12.0 m/(m·h); for denitrification, 3.0~6.0 kgNO3-N/(m·d) with 6.0~12.0 m/(m·h). Deeper denitrification stages work at 1.5~3.0 kgNO3-N/(m·d) and 8.0~12.0 m/(m·h). Dissolved oxygen in the effluent of carbon-oxidation and nitrification filters should be controlled at 3.0~4.0 mg/L.

Filtration rate cuts both ways

For pure carbon oxidation, raising the filtration rate is unfavourable — part of the insoluble organic matter discharges before it degrades, and 6 m/h is the usual recommendation. Within a certain volumetric loading range, though, a higher rate helps nitrogen removal three ways: mass transfer improves, giving air, wastewater and biomass more contact; the biofilm renews faster and stays more active; and the media clogs less, so the backwash cycle stretches — which nitrifying bacteria, slow growers all, will thank you for.

Backwash practice deserves its own attention: frequent backwashing is very unfavourable for nitrifying bacteria, which reproduce slowly and take weeks to rebuild a film that a careless wash strips in minutes. Setting backwash triggers on pressure or turbidity rather than a fixed clock, and keeping the air-scour phase gentle, protects the slow-growing populations that do the nitrogen work while still clearing the headloss that carbon-oxidation media accumulate.

Setting the load by effluent target

For carbon-oxidation duty, aim the volumetric load at 3.5~5.0 kgBOD5/(m·d) for effluent BOD5 of 10~20 mg/L, and at 2.5~3.2 kgBOD5/(m·d) for 5~10 mg/L. Combined carbon-oxidation and nitrification wants the BOD5 volumetric load kept to 3.0 kgBOD5/(m·d) or less: research shows ammonia nitrogen removal is inhibited beyond that value, and at 4.0 kgBOD5/(m·d) ammonia removal is clearly suppressed. On CODcr, an effluent of 60 mg/L calls for an influent load of 4.0~5.0 kgCODcr/(m·d), while 50 mg/L or better pulls the load under 3.0 kgCODcr/(m·d). Where both nitrification and denitrification must be checked, suggested volumetric loads are below 2.0 kgNH3-N/(m·d) and 5.0 kgNO3-N/(m·d), with 0.3~0.8 kgNH3-N/(m·d) and 0.8~4.0 kgNO3-N/(m·d) recommended in practice.

Post-denitrification, C/N tanks and carbon dosing

Post-denitrification fits two cases: wastewater with markedly low BOD5 content — a high proportion of industrial effluent — and plant upgrades where nitrification targets were never considered and effluent BOD5 is low but ammonia is relatively high. To keep carbon removal from stifling nitrification, post-denitrification should remove part of the BOD5 at the pretreatment stage. The C/N tank designs best at a filtration rate of 6~10 m/h, with the nitrification load set by influent strength: about 0.3 kgNH3-N/(m·d) for influent BOD5 at 60 mg/L or above, about 0.6 kgNH3-N/(m·d) at 20~50 mg/L, and about 1.0 kgNH3-N/(m·d) below 20 mg/L. When methanol serves as the external carbon source, the DN stage doses at 3.3 kgCH4O per kgNO3-N — a figure worth pinning on the wall, since overdosing methanol costs money and underdosing costs compliance.