Research Progress of Anaerobic Digestion Pretreatment of Antibiotic Waste

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Jilin Academy of Agricultural Sciences, Changchun 130033, China

1 Introduction

The current environmental pollution is getting worse. The pollution of water resources, as a kind of non-renewable resources, seriously endangers people’s normal life. Pharmaceutical waste contains a large amount of antibiotics, and the pollution is more serious, endangering human health[1-2]. At present, anaerobic biological digestion is an effective way to dispose of pharmaceutical waste. However, antibiotics in pharmaceutical waste severely inhibit the normal physiological activities of anaerobic microorganisms. Studies have found that the pretreatment of antibiotic waste can improve its biodegradability, and the pretreatment of anaerobic microorganisms can increase their anti-inhibitory effect on antibiotics. Both of these treatments provide a good basis for the subsequent process of anaerobic fermentation, thus improving the ability of anaerobic digestion to remove antibiotic waste[3-4]. This article summarized the currently available pretreatment methods and analyzed the advantages and disadvantages of different pretreatment methods to provide reference for future anaerobic biological digestion of antibiotic waste.

2 Definition of antibiotics

Antibiotics[5]are a type of secondary metabolites with anti-pathogen or other activity produced by microorganisms or higher animals and plants that can interfere with the development other cells. At present, commonly used clinical antibiotics are mainly microbial culture extracts and some compounds synthesized or semi-synthesized by chemical methods. Common clinical antibiotics include penicillin and its second- and third-generation derivatives.

3 Current status of antibiotic use in China

In China, the history of antibiotic production can be traced back to 70 years ago. After the founding of New China in 1949, in the first five-year plan, the antibiotic plant construction was clearly listed as a prior development project[6]. Subsequently, the antibiotic pharmaceutical industry developed rapidly. The production methods gradually industrialized, and the antibiotic industry developed steadily. After the 1970s, China gradually became the world’s leading producer of antibiotics[7].

The current abuse and pollution problems of antibiotics have made some microorganisms resistant to drugs, which is becoming more and more common. China’s antibiotic waste is characterized with high concentration, complex structure, difficult disposal and difficult purification, and it has caused serious damage to the environment[7-8]. Since the reform and opening up, China’s antibiotic industry has developed rapidly. It has gradually become one of the world’s leading producers. Up to now, there are as many as 1 000 antibiotic companies in China[9]. Product categories and production have increased year by year, and total waste emissions have also increased year by year. The environment has been seriously damaged. The problem of antibiotic waste pollution has become a serious problem of environmental pollution.

4 Problems in anaerobic digestion process

Anaerobic digestion refers to the process that organic matter degrades under the action of anaerobic microorganisms into CH4and CO2under anaerobic conditions. Antibiotic waste has characteristics of high concentration, complex structure, difficult disposal, stable performance and difficult purification. It is difficult to be biodegraded and the residue has a strong inhibitory effect on other strains. The advantages of anaerobic biological treatment are difficult to play, resulting in low efficiency and high cost. Therefore, the waste needs to be pre-treated to break down and degrade residual antibiotics and other active molecules to improve the biodegradability of the waste, facilitating the subsequent anaerobic bio-digestion process of the waste.

5 Pretreatment methods

5.1UltrasonicandmicrowavecombinedpretreatmentIn the study of Juan Tongetal.[10], by gradient heating of sewage sludge, the changes of antibiotic-resistant bacteria and antibiotic genes were used to show the effect of microwave combined pretreatment on antibiotic waste treatment. The combination of MW-H showed obvious pretreatment effect, with total ARB reduced by 35.5% and the absolute concentration of ARG reduced by 15.0%. The anaerobic biological treatment performed later showed a good antibiotic removal effect.

The results showed that MW-H pretreatment could effectively reduce the concentration of antibiotic-resistant bacteria, and most antibiotic resistance gene concentrations tend to decline during pretreatment. Subsequent anaerobic digestion showed significant antibiotic removal performance.

5.2PretreatmentmethodofoxidationofFe2+/K2S2O8After adjusting the pH to the corresponding range, clindamycin phosphate production wastewater was added with certain amounts of FeSO4and K2S2O8. After stirring for a period of time, the pH was adjusted again. Then, polyacrylamide (PAM) solution was added to perform the anaerobic gas production test[11].

The results showed that the cumulative gas production before oxidation was only 11 mL, and the cumulative gas production after oxidation increased to 151 mL. In addition, the COD removal rate was only 27.6% before oxidation. After 7 d of anaerobic gas production, the COD removal rate reached 75.4%, increased by nearly 50%. It suggested that the biodegradability of wastewater was significantly improved by pretreatment with Fe2+/K2S2O8oxidation, and Fe2+/K2S2O8can greatly improve the biological treatment efficiency.

5.3ElectrocatalyticoxidationpretreatmentThe electrocatalytic oxidation refers to adding the sample to be processed to the electrocatalytic oxidation device and controlling the compressed air flow after adjusting the voltage. After the reaction is completed, the sample is analyzed[12]. In the process of hydrolysis and acidification of un-pretreated wastewater, within the first 14 d, with the increase of sludge age, the removal rate of COD gradually increased, and when the COD of influent water increased to certain extent after 20 d, the COD removal rate dropped significantly. Even if the influent COD dropped to the initial value, the COD removal rate could not be recovered. It shows that the un-pretreated semi-synthetic antibiotic wastewater has serious inhibition on the hydrolysis and acidification process. For the samples pretreated with electrocatalytic oxidation, when the initial COD concentration was small, the removal rate of COD could reach 20%-40%. With the increase of the influent COD concentration, the COD removal rate decreased significantly. But after properly reducing the COD concentration in the influent water, the COD removal rate quickly increased and stabilized. It suggests that electrocatalytic oxidation pretreatment can greatly improve the biodegradability of antibiotic wastewater.

5.3.1Hydrolysis and acidification pretreatment. Anaerobic biochemical process can be summarized in three stages, namely hydrolysis, acidification and methanogenesis. The hydrolysis and acidification is a pretreatment measure that controls anaerobic biochemical process in the first two stages by human regulation[13]. In the study, trials were conducted on high-concentration lincomycin production waste. First, the anaerobic granular sludge was placed in a reactor with pH ranging from 5.8-6.2 to perform a semi-continuous culture. After the COD concentration in the reactor water was stabilized, the proportion of wastewater was increased gradually to acclimate. Until the inlet was all wastewater and the removal rate was stable, the acclimation ended. The results showed that in the stable operation phase, under the optimal pH condition, the average removal rate of COD was up to 11.65%; the volatile acid concentration in the effluent was controlled within 148.3-152.8 mmol/L; the acidification rate was stabilized at 10.74%-12.60%; the ORP was stabilized at -200 mV; and the B/C ratio increased from 0.34 to 0.60. Hydrolysis and acidification pretreatment produce large changes in the chemical structure and properties of the waste.

5.3.2Hydrothermal and alkali-heat pretreatment. Wangetal.[14]treated penicillin residue in the process of thermal hydrolysis. Under the optimum process conditions,i.e., residue to water ratio of 1∶3, hydrolysis temperature of 60℃ and reaction time of 30 min, the penicillin residue was less than 0.5 mg/kg, and the Kjeldahl’s nitrogen reduction rate reached more than 45%, providing a stable basis for future anaerobic digestion.

The research results of Li Chunxingetal.[15]showed that at the optimal temperature of 120℃, compared to simple heat treatment (TPT) [290 mL/(g·VS)], methane production decreased significantly [<200 mL/(g·VS)]. When the pretreatment temperature was reduced to 80℃ and appropriate amount of base was used (adjusting the pH of AMR to 12), TPT rebounded to 231 mL/(g·VS) at 80℃. The research of Tian Baokuo[16]showed that the alkali-heat pretreatment is the best pretreatment process. When the reaction temperature was controlled at 75℃, the addition amount of alkali was 0.08 g NaOH/(g·TS), and the treatment time was 2 h, the SCOD of the residue decreased from 6 066 mg/L to 42 674 mg/L, the VSS reduced from 37.56 g/L to 16.55 g/L, and the dissolution rate was all more than 40%. The methane yield increased by 14% after pretreatment. Li Zaixingetal.[17]conducted an alkali-heat study to improve the anaerobic digestion ability of streptomycin residue. Under the conditions of 0.10 g NaOH/(g·TS), 70℃ and 2 h, the methane production was increased by about 23%. Alexandre’s study[18]showed that under alkali-heat conditions (170℃), alkaline treatment significantly increased the solubility of COD and increased gas production by 45%.

5.3.3Other pretreatment method. The study of Li Zaixingetal.[17]showed that after ultrasonic/alkali joint pretreatment, the dissolution rate of COD in penicillin residue was increased by 2.08 times compared to the original under the optimum pretreatment conditions. The results of BMP test confirmed that under the optimum pretreatment conditions, the methane yield of residue pretreated with ultrasonic/alkali was increased by 2.2 times compared to the original.

6 Conclusions

Microwave pretreatment, as a new type of oxidation technology in each pretreatment method is still in the exploratory research stage, characterized with poor controllability, immature technology and difficult industrial application. The Fe2+/K2S2O8oxidation pretreatment method requires higher operating costs due to the addition of pharmaceuticals and harsh operating conditions, and is still in the experimental stage. The electrocatalytic oxidation technology is relatively mature, and has obvious advantages over other advanced oxidation technologies, so it has good development prospects. In the hydrolysis and acidification process, the dominant bacteria have strong adaptability to antibiotic waste and strong metabolic capacity. In addition, this process is simple to operate and can significantly improve the biodegradability of the waste, facilitating the subsequent anaerobic bio-digestion. Alkali-heat pretreatment is a better method for improving the biodegradability of waste than chemical oxidation, and is suitable for small- and medium-sized antibiotic waste treatment plants and companies that do not have high requirements for waste disposal for its low operating costs, strong operability, high feasibility and easy control.

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