MICROBIAL BIODEGRADATION OF POLYETHYLENE OF LOW DENSITY, UNDER CONTROLLED THERMAL CONDITIONS IN AIR LIFT BIO-REACTOR

The present investigation seeks to identify new mechanisms that serve as tools for the mitigation of plastic contamination through the biodegradation of low density polyethylene using microorganisms of the species Pseudomona aeruginosa (bacteria) and Aspergillus brazilensis (fungus) under controlled thermal conditions in an airlift bioreactor. The methods used were 2 samples of low density polyethylene with concentrations of 50 mg/L and 2 samples of 100 mg/L deposited in an airlift bioreactor under controlled thermal conditions with a duration of 7 days. As a result it was obtained that the species Pseudomona aeruginosa (bacteria) reduced the low density polyethylene sample by 2% with a concentration of 49 mg/L at a temperature of 21.8ºC with a pH of 6.5 and dissolved oxygen (OD) of 6.8 mg/L, likewise the species Aspergillus brazilensis (fungus) reduced the low density polyethylene sample by 7% reaching a concentration of 93 mg/L at 22.1ºC of temperature, 7.14 of pH and 7.45 of dissolved oxygen (OD).


INTRODUCTION
As we know, plastics are a big problem nowadays, since we live in a world where people do not have an adequate environmental conscience, unfortunately this generates a very negative impact to our planet. Plastics have a low economic value in the market and are the most used, therefore, the amount of plastic waste increases. In addition, he tells us that "In 2010, 8.07% of plastic waste was generated in our country and in 2011 9.85%, this indicates that every year there is an increase in plastic waste. Plastic bags (low-density polyethylene) take 150 years to degrade, which is why we have been looking for alternatives to reduce the life span of plastic, such as incineration, which causes health problems because it generates the famous greenhouse gases (GHGs), in addition to the release of dioxins and furans (POPs) that are highly carcinogenic according to the World Health Organization (WHO) (Barlow et al., 2019).
Plastic waste is a worldwide problem since people use the bags only once, throw them away and buy others again without realizing it; this is how pollution begins, causing damage mainly to marine ecosystems, since fauna often confuses them with food. A study conducted for MINAM in June 2012 indicates that Peruvians generate approximately 23,260 tons of solid waste, which gives us a per capita production of 800 grams (Córdova et al., 2020).
A study released in 2015 tells us that sea turtles have been ingesting 52% of garbage, with plastic waste such as bags being the most common (Inforegion, 2015).
Finally, according to the Peruvian newspaper El Comercio (2018), 79% of plastic waste globally is found in dumps or thrown on the roads; only 9% is recycled and 12% is incinerated.

PLACE OF STUDY AND SAMPLING
In this stage the place to work was selected and was an informal dump located in the town center of Santa Clara in the district of Ate in Lima in April, which presented a large amount of plastic in an apparent state of degradation (Catto et al., 2016).

ELABORATION OF AIR LIFT BIOREACTORS
The bioreactors are made in the laboratory, they have a cylindrical glass culture chamber of 100 mm diameter by 200 mm height with a small pipe to take the samples, a deflector tube or central distribution of 80 by 28 mm, a venting filter attached to a regulating valve for air output, control valves for air input/output and air flow connectors attached to the tank pumps; which are the main source of oxygen for the bioreactor (Paço et al., 2017). The culture chamber and deflector tube, the bioreactor accessories such as the top cover (airtight lid), air inlet/outlet control valves and air flow connectors are sterilized by immersion in 70% medicinal alcohol for 15 minutes and rinsed with purified water.

SOWING OF MICROBIAL CULTURES
The microorganisms chosen for the biodegradation process were a bacterium (speudomona aeruginosa) which was seeded on Trypticase Soy agar and incubated for 24 hours at a temperature of 30oc to 35oc and a fungus (Aspergillus brasiliensis) grown on Saburo Dextrose agar, incubated for 48 hours at a temperature of 20oc to 25oc (Giacomucci et al., 2019).
It should be considered that for the process to be more effective, the strains should be used within the first two hours after leaving the incubator.

BIOREACTOR OPERATION OF THE CULTURES
The microbial colonies kept inactive in refrigeration are reactivated in new plates with half Trypticase Soy and Saburo Dextrose forming in seed inocula for the bioreactors. The proportion is 15% of the total volume (1000ml). The operation starts with the ignition of the air pumps, the total opening of the vent valve for a period of at least 7 days (Soto, 2016).

CONCENTRATION OF LOW DENSITY POLYETHYLENE BAGS
The degradation of the low density polyethylene (LDPE) is determined with the decrease of concentrations placed in the bioreactor, for which we work with 50mg/L for the type A bags, 100 mg/L for the type B bags, as concentrations for the beginning of operation, these samples are measured again after three days. To find the final concentration, the whole culture medium is autoclaved for 15 minutes, then the low density polyethylene (LDPE) samples are extracted, rinsed with purified water and taken to the oven for 12 to 24 hours at 80°c. Finally, the weighing is done in the analytical balance.

DETERMINATION OF TEMPERATURE, DISSOLVED OXYGEN AND PH
The determination of the internal temperature was obtained at the beginning, after three days and at the end after 7 days by means of a thermometer, for dissolved oxygen in the same time interval it will be determined by an oximeter in the unit's mg/L and finally for the determination of the pH in the same time interval it was obtained by means of a potentiometer (Janczak et al., 2019).

RESULTS AND DISCUSSION
The results obtained in low density polyethylene (LDPE) type A using the concentration of 50 mg/L in relation to the determination of concentrations had a decrease of 1% for pseudomonas and 5% for aspergillus, taking into account the higher efficiency of the latter, according to the variation of the temperature of 1°c. influenced in the efficiency due to the fact that these temperatures are within the environmental thermal conditions.
However, the concentrations of dissolved oxygen increased by 4% pseudomonas and 20% for aspergillus during the days of experimentation, as opposed to a reduction of pH by 17% for pseudomonas and 13% for aspergillus (Sadhukhan et al., 2019). The results obtained in low density polyethylene (LDPE) type B using the concentration of 100 mg/L in relation to the determination of concentrations had a decrease of 2% for pseudomonas and 7% for aspergillus, taking into account the higher efficiency of the latter, according to the variation of the temperature of 1°c. influenced in the efficiency due to the fact that these temperatures are within the environmental thermal conditions. However, the concentrations of dissolved oxygen increased by 8% pseudomonas and 4% for aspergillus during the days of experimentation, as opposed to a reduction of pH by 19% for pseudomonas and 11% for aspergillus. with Aspergillus brazilensis (Perpetuo et al., 2020).
The chemical conditions are important for microbial biodegradation, the pH of the samples was determined during the process. The most efficient pH for the sample with Pseudomona aeruginosa was 6.49 and for the sample with Aspergillus brazilensis was 7.14. This value was the most efficient for the whole process. For the optimum pH for the polyethylene sample with bacteria such as Pseudomonas sp was 5.5 and for the sample with fungi (unidentified yeast) was 7 (Alvarado et al., 2020). For the low density polyethylene (LDPE) type A and B sample with Aspergillus brazilensis fungus are 5% and 7% respectively at controlled thermal conditions. The thermal condition favorable for microorganisms to degrade the low density polyethylene (LDPE) efficiently is 22.1ºC since a degradation of 7% was obtained for the sample of low density polyethylene (LDPE) type B with Aspergillus brazilensis.
The chemical conditions necessary for the microorganisms to degrade the low density polyethylene (LDPE) efficiently were pH and dissolved oxygen (OD), being the most efficient pH value for biodegradation 7.14 in 7 days of duration of the process and the most efficient dissolved oxygen (OD) value is 7.45mg/L for the sample 2 type B with Aspergillus brazilensis.