Carriers do not wear out the way settling media does. What changes is the load arriving at the plant. This page is about the diagnosis that has to happen before anyone quotes you a second batch of media.
Also searched as: MBBR media top up · when to replace biofilm carriers · moving bed biofilm reactor media fill fraction · mbbr carrier replacement
A moving bed plant that stops meeting its ammonia consent has four possible constraints: dissolved oxygen, alkalinity, temperature, and biofilm inventory. Only the fourth is a media question, and it is the one buyers reach for first.
| Check dissolved oxygen | If the basin cannot hold the setpoint at the new load, more carriers will not fix it - the carriers are oxygen-limited, not area-limited |
| Check alkalinity | Nitrification consumes it. A plant that has lost alkalinity margin shows the same symptom as a plant that is short of media |
| Check temperature | Published working range for our carriers is 5–60 °C; removal rates fall towards the low end, and winter data must not be compared with summer data |
| Then check fill fraction | Published design band is 15–67 % of basin volume. If the plant sits at the bottom of that band and the load has risen, that is a media answer |
New carriers are added to the same basin, dry, at a rate the plant can accommodate. The new media is colonised from the existing biomass, which is why a top-up is far less disruptive than a restart: published biofilm development on our carriers is 3–15 days, and the new carriers pick up film from the bulk liquid rather than starting from nothing.
Three things to confirm before ordering a top-up:
Same geometry. Mixing two carrier designs in one basin gives two different fluidisation behaviours. The published bulk density of our K-series is 0.96, and a carrier from another supplier with a different density will stratify or float differently. If the existing carriers are not ours, ask for the piece count and the bulk density before mixing.
Screen capacity. Every added cubic metre increases the solids loading on the outlet screens. Screens sized for 30 % fill may not pass the flow at 45 %.
Aeration distribution. Carriers are moved by the air. A top-up changes the mixing pattern in the basin and occasionally exposes dead zones at the ends. Expect to adjust the grid, and budget for it.
| Mechanical loss | Carriers leave through damaged screens or over the weir. Count what is missing before ordering the same volume again |
| Wrong carrier for the duty | A plant at high organic load running a small-protected-area carrier, or a nitrification duty run on a carrier with too little protected surface. Changing the type is a design decision, not a purchase |
| Contamination or scaling | Heavy mineral scaling inside the protected area reduces the effective surface. Confirm by weighing a sample against the published figure |
| Obsolescence | Published service life exceeds 15 years, so a plant replacing carriers at year six is usually replacing a design, not a material |
Our K1–K5 series carry published specific surfaces from over 800 to over 1000 m²/m³ and void fractions of 85–90 %, with nitrification rates of 400–1200 gN/m³·d and organic oxidation of 2000–15000 g/m³·d depending on the grade. Choosing between the grades is a question of protected surface against clogging risk, and we will say which one we would fit rather than quote both.
Full parameter tables: MBBR media and technical details.
Take the basin volume from the drawings, then estimate the carrier volume from the delivered quantity (pieces per cubic metre are published per grade) and any records of losses. If the plant was designed before the current load, the design fill fraction is usually in the commissioning file and is the number to start from.
Only after checking bulk density and geometry. Two carriers with different densities fluidise differently, and in a single basin they will separate. If the existing media is not ours, send us the piece count and density and we will tell you whether mixing is safe.
Published biofilm development is 3-15 days, faster in summer and slower at low temperature. In a top-up the new carriers are colonised from the existing biomass, so the plant usually sees the effect within two to three weeks of the media being fluidised.
Carriers are almost always the lower capital cost and the shorter outage. The reason to add tank volume is oxygen transfer and mixing headroom, not media cost - and that limit should be checked before either option is chosen.
Basin volume, current fill fraction, design and current ammonia load are enough for a written recommendation on top-up volume or grade change.
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