Most retrofit budgets start and end with the media. A plant needs more capacity, the operator prices new MBBR carriers or sponge media, the order goes in — and then the media arrives and the real list appears. The retention screens do not fit the new media. The air supply cannot fluidise the extra volume. There is no spare media for top-up, and nobody ordered it. The retrofit that was supposed to take a weekend takes a month, in stages, with the tank drained twice.
This is the normal way retrofits go wrong, and it is avoidable. The media is the headline purchase, but it is rarely more than half of what a retrofit actually needs. The rest is a short list of supporting items, and every one of them is cheaper and easier to buy before the media ships than after it is in the tank.
The stakes are highest where changeout is hardest. One US operator we are working with runs rotating drums with 5 m³ of media, and replacing that media is a difficult job — which is exactly why his questions were not only about the media itself, but about size, floating versus sinking, lifespan and drum volume. When you cannot easily open the reactor again, the first order has to be the complete order.
Start With the Tank, Not the Catalogue
Before ordering anything, measure what you have. The retrofit list follows from the tank, not from the product page.
Write down the tank or drum volume, the existing media type and size, how the media is kept inside, and how air reaches it. In the case above, the existing media is 120 mm suspended-ball media in rotating drums, and the replacement must be at least 120 mm — a 50 mm bio ball, however good on paper, would simply escape through the drum openings. That single measurement rules out a whole category of media before a single price is discussed.
Check the outlet arrangement too: what keeps the media in the tank today, and will it still work with the new media’s size and buoyancy? A screen that retains 120 mm balls may not retain smaller carriers, and a screen that worked for sinking media may not suit media that floats. Measure the openings. Do not assume.
The List, Item by Item
1. Retention screens
This is the item most often forgotten and the hardest to add later, because it usually means draining the tank. If the new media is a different size or buoyancy from the old, the screens must be re-checked or replaced. Order them with the media, to the opening size the new media needs, and confirm the material — stainless steel is the usual choice where the screens stay submerged. A retrofit that saves money on media and then loses media through the outlet has saved nothing.
2. Aeration check — diffusers and air supply
More media, or a different media, changes what the air system must do. MBBR carriers need enough air to stay in motion; sponge media needs air for both mixing and oxygen transfer. Before ordering, check the existing diffusers: are they the right type and condition for the new fill, and can the blower deliver the air the fuller tank needs?
If diffusers are fouled or at the end of their life, replace them while the tank is open — the labour is already being spent. See our aeration system overview for what the air side of the plant looks like as a system, and treat the blower, the diffusers and the media as one purchase decision, not three separate ones.
3. Media volume — and the fill ratio
Order the volume the tank needs at the right fill ratio, not the volume that looks tidy on a quotation. HNS MBBR media is dosed at 15–65% of the tank volume, and the right point in that range depends on the load and the tank — which is a technical question, and the capacity data behind it is covered on our technical site, Hydronovus. What the buyer needs to get right is the arithmetic: the volume ordered must match the tank volume at the agreed fill, with the packing understood. MBBR media packs at 0.1 m³ per bag, 70 m³ to a 40HQ container; sponge media ships vacuum-compressed at 500 m³ per 40HQ and expands to 1.7 times its compressed volume when it takes up water. Order by the volume in the tank, not by the volume in the container.
4. Spares and top-up media
Media is lost, damaged and topped up over the life of a plant. Order spares with the main order, while freight is already being paid. How much spare you need depends on the media’s life: MBBR carriers are 100% virgin HDPE with a catalogue life of more than 15 years, so top-up is a small allowance; our hydrophilic PU sponge runs about 3 years in normal use, so a retrofit built on sponge should budget for replacement on a shorter cycle; modified bio balls sit between the two at 5 years of normal use with a 1-year warranty. These are different purchasing patterns, not better or worse ones — but the spares order must match the media you chose.
5. Inlet, outlet and flow distribution
A retrofit changes how water moves through the tank. Check the inlet and outlet arrangements and any distribution piping while the tank is accessible. This is not a shopping item in the usual sense — it is a check that costs nothing if done before the order and a shutdown if discovered after. Add it to the list as a line to verify, not a line to buy.
6. A trial quantity, where the media is new to you
If the media type is new to the plant — sponge where plastic ran before, or a different carrier size — ask whether a small trial quantity is possible before the full order. A trial in one tank or one drum answers the questions no catalogue can: does it fluidise on your air supply, does it stay where it should, does it suit your water? It is the cheapest insurance on the list.
| Item | Check before ordering | Why it matters |
|---|---|---|
| Retention screens | Opening size vs new media size and buoyancy | Media lost through the outlet cannot be recovered |
| Aeration | Diffuser condition, blower capacity for the new fill | Media that does not move does not work |
| Media volume | Tank volume × fill ratio (MBBR 15–65%); packing understood | Wrong volume means a second order and a second shutdown |
| Spares and top-up | Allowance matched to media life (MBBR >15 yr; sponge ~3 yr; bio balls 5 yr) | Top-up freight alone can cost more than the media |
| Inlet and outlet | Flow distribution verified while the tank is open | Free to check now, a shutdown to fix later |
| Trial quantity | Small batch first if the media type is new to the plant | Answers fluidisation and retention on your water, not on paper |
The Media Choice Changes the List
The three media types on our MBBR system overview do not produce the same shopping list. MBBR carriers — see the MBBR media range — are rigid, long-lived and packed by the bag, so the list is dominated by screens and air. HPU sponge media ships compressed and expands on wetting, so the list must include the replacement cycle and the handling of a much larger delivered volume from a single container. Modified bio balls are small — 50 mm — so the retention screens must be sized to 50 mm, and in a drum built for 120 mm media they are simply the wrong choice unless they are carried inside a larger shell.
This is why the list comes before the order. Choose the media, then write the list the media implies, then order both together.
What to Send HNS for a Retrofit Quotation
Send us the tank or drum volume, the existing media type and size, how the media is retained today, the condition of the aeration, and what you want the retrofit to achieve — more capacity, a different effluent, or replacing media that has failed. We will reply with the media suggestion, the volume at the right fill, and the supporting items — screens, aeration checks and spares — so the order you place is the complete order.
A note on scope, stated plainly: HNS manufactures its MBBR and sponge media and the full range of wastewater treatment equipment at our own factory in Yixing. We do not take on whole-plant process design, civil works, installation, commissioning or operation. The retrofit list above is a purchasing list, not a design service — and a supplier who pretends otherwise is selling you a promise they cannot keep.
Then contact us with your tank details. Order the list, not just the media, and the retrofit will be finished when the media goes in — not a month later, when the missing items finally arrive.
