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How to Judge the Bioavailability of Waste Water by B / C Ratio

Qian Cheng Technology 2024-02-14 07:00

Determination of Bioavailability of Waste Water by B / C Ratio
Are these six things wrong?

The biodegradability of wastewater, also known as the biodegradability of wastewater, indicates the degree of difficulty in biodegradation of organic pollutants in wastewater.

Therefore, it is very important to determine the biodegradability of wastewater for the selection of treatment methods, determination of influent volume and organic load of biochemical treatment section.

The methods used to evaluate biodegradability includemicrobial respiration curve, adenosine triphosphate (ATP) index, CO2 production and BOD5 / CODCr ratio.

Among them, the B / C method is the most used and the most simple method at present. It is generally believed that the greater the B / C ratio, the higher the evaluation of the bioavailability of waste water.

But in practical application, the B / C method can only be used as a preliminary judgment, and it is easy to cause misjudgment and has certain defects.

6 Ways to Make B / C Ratio Go Wrong!

Although under normal circumstances, B / C ratio can play a certain role inmonitoring allegations, it is not necessary to over-demand its accuracy. However, if the data deviated seriously, it is likely to lead to the failure of the entire waste water treatment program.

Therefore, understanding the B / C ratio in advance and timely warning can correct the judgment of the evaluation results of the biochemistry of waste water to a certain extent.

Water dilution

Can cause a decrease in the concentration of substances that inhibit microorganisms

Since the waste water often contains some substances that have a suppressive effect on the microbe, when determining BOD5, if the CD concentration of the waste water is high, it is generally necessary to dilute the water sample.

However, the diluting operation will cover up the effect of substances in the waste water on the microbe, resulting in the determination of BOD5 value higher than the actual waste water BOD5 value, and then cause the B / C to be larger.

It is worth noting that there are two types of substances that have an effect on the microbe and are easy to cause misjudgment of the biochemistry of the B / C method:

The organic matter itself is the main COD contribution, the B / C is higher at low concentration, and the biochemistry is better, but when the concentration reached a certain degree, it will have a negative effect on the microbe;

It has no contribution to BOD5 and CO DCr, and it is common for salt.

Introduced into water samples

Inoculated with undomesticated microbes

Although it is stipulated in the "Determination of Biochemical Oxygen Demand (BOD5) of Water Quality" (HJ505-2009): "When there is organic substance in wastewater which is difficult to be degraded by microorganisms in general domestic sewage at a normal rate or contains highly toxic substance, Acclimated microorganisms should be introduced into water samples for inoculation. "

However, it is often difficult to adopt "acclimated" microbial inoculation in the actual water sample detection, and the measured BOD5 value is usually small, which leads to a lower B / C ratio.

It should be noted that in some practical projects, the degradation effect of the bacteria is not bad after a long time of domestication.

Some organic pollutants

Difficult to degrade under aerobic conditions

Because BOD5 is determined under oxygenation, andsome organic pollutants are difficult to be degraded under aerobic condition, the B / C ratio is small.

In the actual project, the biochemical method can be expanded to the anaerobic range, and the degradation effect of this kind of wastewater under anaerobic conditions is satisfactory. After anaerobic treatment, the B / C ratio was increased to a certain extent, so thebiochemical process (anaerobic + aerobic process) wasconsidered to

When degraded

Changes in pH to prevent the reaction

When determining BOD5, if the pH value of raw water is not in the range of 6-8, the operators oftenuse somehydrochloric acid or sodium hydroxide solution to adjust its pH to 6-8- while some organic compounds will be accompanied by microbial degradation- -so that the pH of wastewater changes greatly- -Finally reach the degree of inhibition of microbial reaction.

Ignoring the effect of the wastewater's own temperature

As we all know, non-self-collected samples, data analysis is often easy to ignore the "temperature" this indicator. Temperature is a very important indicator for biochemical treatment, which has a great impact on the treatment effect.

It is generally considered that the biochemical suitable temperature:

Anaerobic temperature 10 ~ 30 °C, medium temperature 35 ~ 38 °C, high temperature 50 ~ 55 °C;
Aerobic room temperature 15 ~ 30 °C, medium temperature 30 ~ 40 °C, high temperature 50 ~ 60 °C.

In the determination of BOD5, thetemperature is 20C and the B / C ratio is higher than the normaltemperature (if the actual water temperature is too high or too low).

Reaction is limited by the physical form of the dissolved oxygen bottle

In the determination of BOD5,because the organic suspended matter is limited by the physical form of dissolved oxygen bottle and can not contact and react with microorganism sufficiently, the value of BOD 5 is often too small and the result is that B / C is too small- it is easy to misjudge.

In the practical engineering, the reaction conditions in the DO bottle can be greatly improved.The engineering will adopt the methods of baffle 、 mechanical stirring 、 air stirring to optimize. At the same time, after the stage running and the biological flocculation function play, part of the organic suspended substance is adsorbed on the surface of microorganism and then enter the cell by the extracellular enzyme hydrolysis. So the measuredB / C ratio is not bad but biodegradability is not poor.

Comparison of Methods for Determining Biodegradability of Wastewater

1, microbial respiration curve method

Compared with other methods, this evaluation method has the advantages of simple operation, short experimental period, and can meet the needs of large quantities of data.

However, to evaluate the biodegradability of wastewater by this method, strict regulations must be made on the source, concentration, acclimation of microorganisms and the concentration of organic pollutants and reaction time.

The microbial respiration curve is a curve drawn with time as abscissa and oxygen consumption as ordinate. The characteristic of the curve mainly depends on the nature of organic matter in wastewater. The apparatus for determining the rate of oxygen consumption includes Warburg respirometer and electrode type dissolved oxygen meter.

2, the amount of CO 2 generation method

The evaluation method requires special instruments and methods, and is complicated to operate. It is only used in laboratory research and rarely used in actual production.

3 、 The ATP index method

Although there are some mature methods for the determination of ATP, the microbial physiological index method is mainly used for the determination of biodegradability and ecotoxicity of single organic pollutants because of the high requirements for instruments and drugs and the complicated operation.

4. Methylene blue toxicity assay

Using methylene blue as indicator, the toxicity of wastewater and some wastewater to microorganisms can be judged by observing the fading time of methylene blue.

Set t1 as the time when methylene blue was discolored by waste water and t2 as the time when methylene blue was discolored by domestic sewage. According to the comparison of fading time, it can be concluded that:

When t1 = t2, there is no substance that can prevent the growth of microbe in the waste water;

When t1 < t2, the biochemistry of the waste water is good and it is easy to be treated by biochemistry.

When t1 > t2, the waste water is toxic to the active sludges and will prevent the growth of microbe.