STP technology selection: choosing between MBBR, SBR, MBR and FAB on evidence, not preference.

MBBR is an attached-growth process: biomass grows on free-moving plastic carriers in a continuously flowing aeration tank, so it handles fluctuating load well and keeps a compact footprint. SBR is a suspended-growth process operated in timed batches (fill, react, settle, decant) in a single tank, which removes the need for a separate secondary clarifier and gives good control over nitrogen removal. MBBR suits variable, smaller flows in tight spaces; SBR suits larger, steadier flows where automation and nutrient control matter.

When effluent quality or footprint leaves no alternative. A membrane bio-reactor replaces settling with membrane filtration, so it produces a consistently clear, low-turbidity effluent well suited to reuse, in the smallest area of any option. The trade-off is higher power draw, membrane fouling management, periodic chemical cleaning and eventual membrane replacement, real recurring costs that should be priced in before the technology is selected.

FAB, fluidised aerobic bed: is an attached-growth process in the same family as MBBR, with biomass carried on media kept in suspension by aeration. In practice the two are close relatives and the choice between them often comes down to media type, supplier and hydraulic design rather than a fundamental process difference. Both are chosen for compactness and tolerance of intermittent load.

Where land is available and flow is large and steady, municipal plants in particular, ASP remains a proven, well-understood and relatively low-capital option with operators everywhere who know how to run it. Its disadvantages are footprint and sensitivity to shock loading. It is displaced on constrained sites, not on principle.

Substantially, and mostly through power. Aeration is the dominant electrical load in every aerobic process, and membrane systems add permeate pumping and cleaning chemicals on top. Sludge yield and disposal, chemical dosing for tertiary polishing, and the skill level of the operating team also move the number. A technology selected only on capital cost frequently loses the difference back over a few years of operation.

The board sets the discharge standard and the consent conditions; the technology is the proponent's choice provided it demonstrably meets those conditions. In practice, boards and appraisal committees do question a technology that looks unlikely to achieve the required quality for the site or the load, so a documented basis of selection is useful at consent and clearance stage.

It raises the bar. Treated sewage used for flushing and landscape irrigation is expected to be clear, low in suspended solids and disinfected, which pushes the design towards tertiary filtration and disinfection regardless of the biological process, and makes MBR attractive where quality must be consistent without operator intervention. Where reuse is a clearance condition, that condition, not the general norm, is what the selection is judged against.