The COD in the end oxidation tank is higher than that in the front oxidation tank
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The process used in our plant is hydrolysis acidification plus two-stage aerobic treatment. The COD level in the second-stage aerobic tank is even higher than that in the first-stage aerobic tank (this has been the case after 20 days of monitoring). There has only been a slight reduction in the sludge volume in the second-stage aerobic tank – by 200 mg/L – and no significant decrease at all. We seek an explanation for this phenomenon.
To address this issue, it is necessary to provide your hydraulic retention time. If the retention time is 20 hours or more, stage 1 is usually sufficient; using stage 2 makes control more difficult.
The phenomenon of higher effluent quality in the second stage compared to the first stage is something that is common in two-stage biological treatment processes. This is because the SS level in the effluent from the second stage is higher, which leads to an increase in COD; the rise in COD is a consequence of the breakdown of activated sludge, resulting in sludge suspension in the water.
The load at stage two is generally low, and the B/C ratio is also low. It is advisable to test the B/C ratio of the water entering stage two as soon as possible; if it is below 0.3, a portion of the water from stage one can be directed to stage two.
The nutrient level at stage two is relatively low; at this time it is particularly important to ensure that the reaction depth is not too great, as this can easily lead to the degradation of the sludge. Once such degradation occurs, the COD level becomes low, even before the process has started.
F/M is a crucial indicator, and dissolved oxygen is the main factor used for control. If the organic load in the influent water is low, it is necessary to reduce the sludge concentration and strictly keep the dissolved oxygen level low – ideally not exceeding 1 mg/L. If there is no need to remove ammonia nitrogen or if its level is already within acceptable limits, the level can be kept even lower.
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