Food and beverage plants share a pattern: high organic load, a flow that follows the production shift rather than the clock, and a discharge consent that does not. Attached growth handles that pattern better than suspended growth, and the reason is arithmetic.
The load arrives in slugs. A washdown shift or a tank changeover can put several hours of load into the plant in twenty minutes. In an activated sludge plant that slug passes through the biology, and the biology takes days to recover. In a moving bed reactor the biomass sits on carriers that stay in the basin, so the inventory that meets the slug is the whole inventory, not whatever happened to be in the mixed liquor at that moment.
The biology you need grows slowly. Nitrifiers divide far more slowly than the heterotrophs that consume BOD. Attached growth holds them against washout, which is why plants that pass a carbon consent and fail an ammonia consent are often the same plants running a short sludge age.
You usually cannot build. Most food and beverage upgrades happen inside an existing aeration tank, during a shutdown window measured in days. Loading carriers into a basin that already exists is the intervention that fits that window.
| Published series | K1–K5 in HDPE, Φ10–25 mm, with a specific surface above 800–1000 m²/m³ depending on the series |
| Fill fraction | 15–67 % of reactor working volume — a design band, not a preference |
| Biofilm formation | 3–15 days, depending on temperature and load |
| Nitrification rate | 400–1200 gN/m³·day, published per series |
| Service range | 5–60 °C; published service life more than 15 years |
Mass removed per day = flow × (inlet − outlet) concentration. Carrier volume = that mass ÷ the removal rate for the series. Then divide by reactor working volume and check the answer lands inside 15–67 %.
Worked example: 800 m³/day with BOD falling from 900 to 40 mg/L is 688 kg/day. At 4,000 g/m³·day that is 172 m³ of carrier. In a 400 m³ reactor that is a 43 % fill — inside the band, so the design closes on BOD. Now run the same arithmetic for ammonia; if that answer is larger, the carrier volume follows the ammonia, not the BOD.
Fat, oil and grease. FOG coats carriers and reduces the effective surface until the biofilm cannot exchange substrate. That is a pre-treatment problem: a dissolved air flotation unit or a properly sized grease trap in front of the reactor protects the media far more cheaply than any media modification protects the plant.
pH swings. CIP chemicals and caustic washdown can push a reactor outside the range a biofilm tolerates. Equalisation ahead of the reactor is the fix; a carrier is not a buffer.
Screens and air. The two things that decide whether an MBBR works are the retention screens that keep carriers in the basin and the aeration that keeps them moving. Both are outside our scope and both are where retrofits fail. If the existing diffusers cannot move the full bed, that is the first item on the list, not the last.
Often yes, and that is the usual reason for the enquiry. The carriers go into an existing aeration tank, retention screens are fitted, and the air distribution is checked. What decides feasibility is whether the existing reactor volume lands the fill fraction inside 15–67 %; if it does not, the answer is a bigger reactor, and we will say so.
Biofilm forms in roughly 3 to 15 days depending on temperature and load, and the load is then raised gradually because a young biofilm washes out easily. Do not commission against the best-case number in winter.
Yes, through the surface-area-to-size trade. Smaller carriers give higher specific surface but blind faster in water carrying fibres and grease; larger carriers stay open. Send the analysis and we will propose a series rather than guess one.
They stay in the basin. Keep them wet and, if the shutdown is long, keep some air on to prevent the bed compacting. Restart is a matter of restoring air and flow, not of re-inoculating.
Flow, inlet and outlet concentrations for every limited parameter, reactor volume and the coldest sustained temperature. We return the carrier volume and the fill fraction.
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