<\/span><\/h2>\nOptical suspended solids sensors work by measuring scattered or absorbed light in the sample. Two technologies dominate:<\/p>\n
Backscatter Sensors<\/strong><\/p>\n\n- Ideal for concentrations from 100 mg\/L to 30,000 mg\/L.<\/li>\n
- Robust against color changes.<\/li>\n
- Well suited to reject lines where solid content is high and variable.<\/li>\n<\/ul>\n
Transmission Sensors<\/strong><\/p>\n\n- Better for lower ranges, 0\u20131,000 mg\/L.<\/li>\n
- More sensitive to color and dissolved solids interference.<\/li>\n
- Useful on final polishing steps within the recirculation loop.<\/li>\n<\/ul>\n
Shanghai ChiMay’s suspended solids sensor line offers both technologies with the same transmitter platform, so operators can mix sensor types without complicating training.<\/p>\n
Fouling Countermeasures<\/strong><\/p>\n\n- Automatic mechanical wiper on the optical window, cycling every 15\u201360 minutes.<\/li>\n
- Air-blast cleaning where wiper wear is a concern.<\/li>\n
- Retractable assembly on the toughest lines, enabling manual cleaning without shutdown.<\/li>\n<\/ul>\n
<\/span>COD Measurement: What UV-Vis Actually Sees<\/span><\/h2>\nReal-time COD sensors typically use UV-Vis absorption in the 200\u2013400 nm range. Absorption at 254 nm correlates strongly with organic carbon, and multi-wavelength instruments improve robustness in complex brines. Practical considerations:<\/p>\n
\n- Range<\/strong>: from 10 mg\/L to 4,000 mg\/L for reject-loop duty.<\/li>\n
- Reference wavelength<\/strong>: to compensate for turbidity, since suspended solids also absorb light.<\/li>\n
- Matrix calibration<\/strong>: the correlation between UV absorption and lab COD depends on the mix of organics, and it should be validated locally.<\/li>\n<\/ul>\n
Fouling Countermeasures<\/strong><\/p>\n\n- Automatic wiper and periodic acid clean, matched to fouling profile.<\/li>\n
- Sample bypass loop with slow return to reduce fouling of the main line.<\/li>\n
- Insertion assembly with sealing wear plate for high-solids duty.<\/li>\n<\/ul>\n
<\/span>Placement in the Reject Recirculation Loop<\/span><\/h2>\nSensor placement is often more important than sensor selection. Recommended positions:<\/p>\n
\n- Reject header<\/strong> immediately downstream of the RO or nanofiltration module, capturing composition before further concentration.<\/li>\n
- Return line to feed tank<\/strong>, so operators see what the recirculation actually contains after residence time in the tank.<\/li>\n
- Concentrator or evaporator feed<\/strong>, where sudden SS or COD spikes translate directly to downstream fouling risk.<\/li>\n<\/ol>\n
Shanghai ChiMay’s multi-parameter transmitters can host SS, COD, conductivity, and temperature inputs on a single head, allowing tight cross-referencing between measurements.<\/p>\n
<\/span>Comparative Snapshot: Sensor Choices by Range<\/span><\/h2>\n\n\n\n| Range<\/th>\n | Recommended SS Sensor<\/th>\n | Recommended COD Sensor<\/th>\n | Typical Use<\/th>\n<\/tr>\n<\/thead>\n |
\n\n| 0\u2013500 mg\/L SS, 0\u2013200 mg\/L COD<\/td>\n | Transmission optical<\/td>\n | UV-Vis, single wavelength<\/td>\n | Polishing loops<\/td>\n<\/tr>\n |
\n| 100\u20135,000 mg\/L SS, 200\u20131,000 mg\/L COD<\/td>\n | Backscatter, digital output<\/td>\n | UV-Vis with turbidity compensation<\/td>\n | Standard ZLD reject<\/td>\n<\/tr>\n |
\n| 500\u201330,000 mg\/L SS, 500\u20134,000 mg\/L COD<\/td>\n | Ruggedized backscatter<\/td>\n | Multi-wavelength UV-Vis<\/td>\n | Late-stage concentrated brine<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n<\/span>Diagnostic Signals Worth Trending<\/span><\/h2>\n\n- SS drift<\/strong> with fixed calibration: indicates fouling of the optical window.<\/li>\n
- COD baseline shift<\/strong>: usually reflects long-term chemistry drift or reference wavelength degradation.<\/li>\n
- SS\/COD ratio<\/strong>: sudden changes flag organic upsets vs. particulate excursions.<\/li>\n
- Sensor wiper duty cycle<\/strong>: rising wiper activation frequency indicates increasing fouling load and predicts maintenance need.<\/li>\n<\/ul>\n
<\/span>Real Data From Operating Sites<\/span><\/h2>\nIn audited case data from a coastal petrochemical ZLD facility, continuous SS and COD monitoring on the reject recirculation loop delivered:<\/p>\n |