1. Water sample collection and storage (most easily ignored)
1. The sampling bottle is not completely filled, leaving bubbles
When air is brought in, the initial DO is high; shaking during transportation will further reoxygenate the product, resulting in a high result.
✅ The bottle must be full, no headspace, and sealed.
2. Not refrigerated in time or refrigerated for more than 24 hours
Microorganisms continue to metabolize at normal temperature, and DO is consumed in advance, resulting in low results.
✅Refrigerate at 2-5℃, start pretreatment within 6-12 hours, maximum ≤24h.
3. Residual chlorine/disinfectant has not been removed
Residual chlorine directly sterilizes bacteria, and the BOD is low or even 0.
✅ Add sodium thiosulfate for removal (add 0.1mL 10% solution for every 0.1mg/L residual chlorine).
4. The pH is not strictly adjusted to 6.5–7.5
Peracidity and alkali inhibit microorganisms, resulting in low results.
✅ Must be adjusted to the optimal range of 7.0–7.2.
2. Culture bottles and assembly (high-frequency problems at Lianhua site)
1. The absorbent (NaOH particles) is directly injected into the sample
Strong alkali kills microorganisms, changes the pH drastically, and results in scrapping.
✅ Must be placed in a special medicine cup, and water samples are strictly prohibited from falling into it.
2. The bottle cap is leaking and not tightly sealed.
The pressure method (such as LH-BOD601) leaks, the pressure changes are inaccurate, and the data drifts/has no curve.
✅ The bottle cap is evenly tightened and the sealing ring is intact; the medicine cup is full but does not overflow.
3. Residual oil stains/old bacterial film on the bottle wall
New samples are contaminated or inhibited and results are erratic.
✅ Clean thoroughly with dilute acid + brush, and finally rinse with distilled water, no water hanging, no residue.
3. Culture conditions and time
1. The temperature is not 20±1℃
For every 1°C rise in temperature, microbial activity increases by about 10%; the fluctuations are large, and the parallel differences in results are large.
✅ The incubator is calibrated to 20°C and the temperature is constant throughout.
2. Direct sunlight
Algae produce oxygen through photosynthesis, but the DO is high and the result is low.
✅Cultivation in the dark (dark or low light).
3. The culture time is not 5 days (120h)
Short time → incomplete decomposition → low; long time → endogenous respiration → low.
✅Strictly 5 days ±1h.
4. Dilution factor
Lianhua recommends: measure COD first and estimate the dilution factor based on BOD≈COD×0.5–0.7.
5. Instrument operation details (differential pressure method)
1. Data recording is not timely and the cover is opened midway
Pressure interruption, gas exchange, data breakage/distortion.
✅ Keep it fully sealed, do not move or open the lid for 5 days.
Quick self-check checklist (check 30 seconds before experiment)
1. The sampling bottle must be full without bubbles, refrigerated at 2–5°C, and within 24 hours.
2. No residual chlorine, pH=6.5–7.5
3. Aerate the dilution water for ≥2h, DO 8–9mg/L, add nutrients, and blank BOD <0.2
4. Inoculation is correct (industrial wastewater must be inoculated)
5. The absorbent is in the medicine cup and no sample is added.
7. Incubator at 20±1℃, protected from light, for 5 days
Post time: Sep-17-2026
