Level Measurement in a Contact Oxidation Tank: Biofilm, Foam and Moving Media
A contact oxidation tank (biological contact oxidation, MBBR or fixed-media aeration basin) is a hostile place for any instrument that has to sit in the water for months. The basin is in permanent turbulence from the blowers, the surface carries a layer of biological foam, the water is warm (15–35 °C) and loaded with suspended biomass, and in MBBR designs the moving carriers physically knock against anything placed in the tank. Yet the level in this tank drives aeration control, return-sludge and transfer pumping, and detects froth-over events that spill biomass onto the walkway.
Real working conditions
- Continuous aeration at the tank floor, so the water surface is never still and air bubbles rise through the whole column.
- Biological foam 50–300 mm thick, which is exactly what an ultrasonic or radar unit sees instead of the liquid surface.
- Biofilm growth on any wetted surface, including the transmitter diaphragm and the cable.
- Moving media (MBBR carriers or fixed bundles) that abrade and occasionally trap a cable.
- Headspace gas with H₂S and humid condensate, aggressive to junction boxes and cable glands.
- Rodents along the tank deck, which is the leading cause of cable failure in outdoor wastewater installations.
What the buyer is trying to solve
Searches in this niche are usually from operators replacing float switches that clog with rags and biofilm, or ultrasonic units that read the foam layer. The search intent is a flush-diaphragm hydrostatic transmitter that will not foul, with IP68 sealing, an oil-resistant PUR cable, and often a wireless uplink because running signal cable across the aeration deck of an existing tank is expensive and frequently damaged. In practice this points to a 316L submersible hydrostatic level transmitter with a 4–20 mA two-wire output, or to a LoRa wireless tank level unit with high/low alarm and remote control where no cable route exists.
Engineering selection
| Parameter | Recommended specification | Reason |
|---|---|---|
| Measuring principle | Hydrostatic, submersible | Reads true liquid head, ignores foam layer |
| Diaphragm | Flush or extended (open-face) diaphragm, 316L with PTFE or ceramic face | Less surface for biofilm to anchor on, easy to wipe clean |
| Range | 0–4 m or 0–6 m for typical 3–5 m basins | Matches tank depth with surge margin |
| Output | 4–20 mA + HART, or RS485 | Works with existing aeration control loops |
| Protection | IP68 double-sealed, PUR cable | Permanent immersion, oil and UV resistance |
| Options | Armoured or conduit-protected cable, LoRa/4G RTU | Rodent and mechanical protection, no trenching |
Installation logic
- Choose a quiet corner of the basin, away from the bubble plume of the diffusers — aerated water has a lower effective density and will bias a hydrostatic reading low.
- Mount the sensor inside a perforated stilling tube (typically DN100 PVC with 10 mm slots) fixed to the tank wall. The tube removes turbulence and keeps carriers and rags away from the diaphragm.
- Keep 200 mm bottom clearance so settled sludge does not bury the diaphragm, and so the tube can be lifted for cleaning.
- Use a guide rail or lifting bracket: in an aeration tank, the sensor must be removable from the deck without entering the basin. This single detail decides whether maintenance ever happens.
- Run the cable in conduit or armoured form across the deck; leave a drip loop and never let the cable hang from the entry gland.
- Set the cleaning interval from observation: in a warm MBBR basin, wiping the diaphragm every 4–8 weeks is typical; a flush diaphragm makes that a two-minute job.
Expected outcome
With a stilling-tube installation the level signal becomes stable enough to be used for aeration staging and transfer pump control, and a high-level alarm catches froth-over before biomass reaches the walkway. Because the measurement no longer depends on a clean reflective surface, heavy foam days stop producing runaway alarms.
Procurement checklist
- Is the diaphragm flush or recessed, and how is it cleaned in place?
- Is the cable PUR (oil and hydrolysis resistant) rather than plain PVC?
- Is a stilling tube, guide rail or mounting bracket supplied as an accessory, or must it be fabricated locally?
- Are H₂S-resistant wetted parts and a sealed junction box offered?
- For wireless versions: what is the realistic radio range through a concrete tank deck, and what is the battery or solar configuration?
