The preservation methods of polyurethane immobilized nitrifying bacteria pellets which had been enriched in laboratory were provided. Factors such as temperature, pH and light, which affect the nitrification activity of polyurethane immobilized pellets, were investigated. The result showed that dark, deionized water and low temperature is suitable for polyurethane immobilized nitrifying bacteria pellets’ long term preservation.
The ammonia degradation and preservation ability of nitrifying bacteria products were two important factors affecting its commercialization [
Activated sludge was obtained from the aeration tank of Minhang municipal wastewater treatment plant, Shanghai, which was utilized as seed sludge for acclimating nitrifying bacteria and acclimated in laboratory for 2 months. The suspensions of nitrifying bacteria were concentrated to the density of 20 g/L and mixed with a polyurethane prepolymer emulsion. Then N, N, N’, N’-tetramethylenediamine as a promoter and potassium persulfate as initiator were added to the beaker. Consequently; the polyurethane immobilized nitrifying bacteria in the form of an elastic gel were obtained. The resulting polymerized gel carrier was cut into 3 × 3 × 3 mm cubes by the special cutting machine and then washed thoroughly with distilled water [
Nitrifying bacteria are oxygen-consuming bacteria. The change of nitrifying bacteria pellets activity during the preservation process was measured by oxygen uptake rate. The respiration rate was obtained by measuring the oxygen uptake of pellets with a dissolved oxygen meter at 25˚C. The change of nitrifying bacteria particles activity in the preservation process was tested using the following method. Inorganic synthetic wastewater saturated with oxygen was placed in a 100 mL incubation flask along with 20 mL of pellets. They were then stirred. The respiration rate was obtained by measuring the decrease in dissolved oxygen concentration. The relative activity of preservation pellets was calculated by Equation (1).
R r = ( R 1 / R 0 ) × 100 (1)
where Rr is relative activity (%), R1 is the respiration rate of pellets preservated for a certain time (mg-O2/L-pellet・h) and R0 is the respiration rate of the pellets before preservation (mg-O2/L-pellet・h).
After determination of polyurethane immobilized nitrifying bacteria pellets’ respiratory activity interval a certain time, the decay rate (Km) and half-life (t1/2) fitting model was calculated according to Equation (2).
R r = exp ( − K m × t ) . (2)
where Rr is relative activity (%), Km is the decay rate (day−1) and t is the preservation time (day).
Light have a negative impact on the nitrifying bacteria [
Nitrifying bacteria on is very sensitive to the changes of environmental pH value. Bacteria grows well in slightly alkaline environment [
Temperature has a greater impact on the growth and preservation of nitrifying bacteria. Generally believed that the suitable temperature range of nitrifying bacteria growth of 20˚C - 30˚C [
In order to extend the preservation time of polyurethane immobilized nitrifying bacteria pellets as long as possible, the storage temperature was further reduced below 0˚C to −20˚C. The experimental sample particles divided into two parts, which half were directly frozen, the other half added 10% (w/w) glycerol as a
protective agent. The purpose of adding glycerin is to prevent extreme low temperature cause irreversible damage of nitrifying bacteria. As can be seen from
Preservation conditions | Km (day−1) | R2 | t1/2 (day) | |
---|---|---|---|---|
Light | Sunlight | 0.0464 | 0.9988 | 15 |
Dark | 0.0123 | 0.9725 | 56 | |
pH | 6.0 | 0.0797 | 0.9985 | 9 |
7.5 | 0.0149 | 0.9937 | 47 | |
8.0 | 0.0210 | 0.9728 | 33 | |
8.5 | 0.0251 | 0.9978 | 28 | |
Temperature | 4 | 0.0056 | 0.9681 | 124 |
25 | 0.0123 | 0.9725 | 56 | |
30 | 0.0333 | 0.9981 | 21 | |
35 | 0.0677 | 0.9976 | 10 | |
40 | 0.2255 | 0.9937 | 3 | |
freeze (−20˚C) | Glycerol | 0.0015 | 0.9611 | 462 |
- | 0.2308 | 0.9903 | 3 |
half-life of nitrifying activity is 462 days, which is suitable for nitrifying bacteria pellets’ long term (1 - 2 years) preservation.
The result showed that dark, deionized water and low temperature is suitable for polyurethane immobilized nitrifying bacteria pellets’ long term preservation. Though this way, we can significantly reduce the decay index and prolong half-life of nitrifying bacteria pellets by changing conditions. The results also provided theoretical basis and implementation for nitrifying bacteria products’ storage and transportation.
This project was sponsored by Shanghai Rising-Star Program (16QB1402200).
Dong, Y.M. (2018) Preservation Property and Decay Kinetic of Polyurethane Immobilized Nitrifying Bacteria Pellets. Journal of Geoscience and Environment Protection, 6, 93-100. https://doi.org/10.4236/gep.2018.66007