BIOTRANSFORMATION AND BIODEGRADATION OF PHARMACEUTICAL COMPUNDS IN WASTEWATER

1. Introduction

Pharmaceuticals compounds are ubiquitous in the aquatic environment because wastewater treatment plants are not designed to remove them completely. Various biotic and abiotic processes in the environment can transform this compounds in to transformation products (TPs). This TP’s can sometimes be more toxic than the parent compound.

The first transformation usually is initiated in the human body by a variety of metabolizing enzymes.

When this compounds are excreted from human body enter in wastewater treatment plants (WWTP’s) where the non removed part of them go to clean water sources. Here we will focus our attention in the biotic processes of biotransformation and biodegradation of these pharmaceutical compounds.

2. Biotic transformations

2.1. Human metabolism

The most important organ for biotransformation of farmaceutical compounds is the liver because is a rich source of enzymes like CYP450 which are directly involved in oxidation reactions (Asha and Vidyavathi 2010). This reactions of oxidation and reduction introduce reactive functional groups like hydroxyl (-OH), amino(-NH2,-NH), carboxy (-COOH) or thiols (-SH) in the molecule creating more hydrophilic compounds.

Sometimes, the resulting metabolites are bioactivated and present more pharmacology activity than the initial drug, in this cases the pharmaceutical compound is called prodrug.

Then, this metabolites and the initial drug are transformed to more hidrophilic molecules by conjugation with endogenous compounds as glucuronic acid, sulfate or glycine.  The final metabolites are more polars so they are easily excreted by the urine and be further transfomed by microorganisms.

However, there are many pharmaceuticals compounds that are not metabolized at all.

2.2. Microbial degradation

Most part of biotransformation of pharmaceutical compounds take part in the wastewater treatment plants (WWTP) biological reactors filled with activated sludge.

The biodegradation occurs because of microorganisms metabolism. They can use a compound as carbon or energy source (used for the growth of biomass) and they can also transform the compound but don't use it as an energy source.

Although some pharmaceuticals are degraded during anaerobic sludge digestion, the biodegradation under aerobic conditions is considered as the main removal process (Fent et al. 2006).

2.2.3 Activated sludge

Activated sludge is a biological treatment method in which microorganisms help degrade the organic compounds in the wastewater (Ternes 2004). Most pharmaceuticals have low sorption potential, which also implies a low removal from activated sludge. So the removal of pharmaceuticals in WWTP’s is generally poor. The designs of WWTPs are based on the removal of nitrogen, phosphorus and particulate matter. Nevertheless, activated sludge treatment has the ability to do parcial degradation on certain pharmaceuticals.

Ilustration 1: Representation of the main units in the biological stage of the activated sludge system ( M.Von Sperling, 2017).

The biomass is separated in the secondary sedimentation tank due to its property of flocculating and settling. This is due to the production of a gelatinous matrix, which allows agluttination of the bacteria, protozoa and other microorganisms responsible for the removal of the organic matter, into macroscopic flocs. The flocs infividually are much larger than microorganisms, which facilittes their sedimentation (M.Von Sperling, 2017).

Ilustration 2: Schematic representation of an activated sludge floc. (M.Von Sperling, 2017)

This process has a small biotransformation potential that was studied with some acidic pharmaceutical compounds (José, Stefan and Thorsten 2005). This study shows that some of this compounds like ketoprofen can be partially mineralized by activated sludge, Others like ibuprofen or  naproxen can also be partially degraded but not as a source of carbon. Some others, for example diclofenac are not degraded at all by activated sludge. This means that some bacterias has a metabolic activity that can degrade some pharmaceutical compounds.

2.2.4 Amonnia oxidizing bacterias

Pharmaceuticals are usually present in the level of μg/L or ng/1 in wastewater systems, with some of them being toxic to the microorganisms.

Therefore, these pharmaceutical active compounds can't be utilized as carbon source through metabolic biodegradation by microorganisms. So the way used by microorganisms for their removal is via cometabolic biodegradation.

Illustration 3: Metabolic and cometabolic cycle of pharmaceuticals, degradation by AMO (ammonia monooxygenase) present in ammonia oxidizing bacterias, and the biotransformation of the products by heterotrophic bacterias. (Yifeng, Zhiguo, Bing-je. 2016).

Ammonia oxidizing bacteria has a protein called ammonia monooxygenase (AMO). This protein can work with differents substrates and can degrade some pharmaceutical compounds with high efficiencies but not using them as a source of carbon. The products of the degradation of this compounds can be metabolized by heterotrophic bacteria to compounds that are in nature and are not problematics like CO2,H2O,etc.

AMO uses NH4+ as a primary substrate as it’s shown in illustration 3 to growth the bacteria. This reaction helps to degrade some compounds like aromatic compounds (most pharmaceuticals are aromatic).

Furthermore, this protein can biotransform the pharmaceutical compounds by oxidation. In an active side the protein has metal ions such as Cu+ that in aerobic conditions can react with oxygen to produce Cu2+, and also contains the oxygen used. This oxygen is used to oxidize the pharmaceuticals (Yifeng, Zhiguo, Bing-je. 2016).

Some compounds can only be biotransformed when the primary via is active, but some others can be degraded without this nitrification activity.

The removal efficiency is clearly related with the AOB presence and with AMO activity and it’s also higher than in non enriched with AOB activated sludge.

2.3 Biodegradation  after the WWTP treatment.

Biodegradation of pharmaceuticals is not limited in the WWT. After the treatment, there are lots of TP's and parent drugs still in the water which natural ecosystems can also degrade reducing his impact.

The same biotransformation pathways, and others, that are shown in this blog can occur in natural ecosystems if bacterias that do it are present in this ecosystems. Also other abiotic methods like photodegradation by sunlight, sedimentation... can reduce the presence of pharmaceuticals in natural ecosystems.

3. Conclusion

Pharmaceutical compounds can be dangerous for some ecosystems specially for water ecosystems. The high use by humans or veterinary of this compounds has been reported as a problem for human health and some ecosystems for the last few years.

In this situation, biotransformation of this compounds to non-dangerous compounds has been studied by various investigation groups.

This process appears in different situations. Biotransformation happens in human body, in WWTP, in concrete bacterias metabolism that are been studied and on natural ecosystems.

In this different pathways TP’s are formed and sometimes are more dangerous than the parent compounds.

The removal efficiencies in wastewater treatment plants has been reported as not effective because removal efficiencies can go from a insignificant value to 95% approximately.

The removal of this compounds in activated sludge enriched with AOB’s with a high range of AMO activity has been studied and reported as a better way to biotransform this pharmaceutical compounds with highly efficiencies.

It’s necessary for conservation to study how to apply the different techniques of biotransformation pharmaceutical compounds on WWTP to clean better the wastewater and reducing the impact of pharmaceuticals.

4. Bibliography

• Boix, C., Ibáñez, M., Sancho, J. V., Parsons, J. R., de Voogt, P., & Hernández, F. (2016). Biotransformation of pharmaceuticals in surface water and during waste water treatment: Identification and occurrence of transformation products. Journal of hazardous materials, 302, 175-187.

• Fent, K., Weston, A. A., & Caminada, D. (2006). Ecotoxicology of human pharmaceuticals. Aquatic toxicology, 76(2), 122-159

• Quintana, JB. Weiss, S. Reemtsma, T. (2005). Pathways and metabolites of microbial degradation of selected acidic pharmaceutical and their occurrence in municipal wastewater treated by a membrane bioreactor. Elsevier.

• Von Sperling, M. (2017). Activated sludge and aerobic biofilm reactors. IWA publishing.

• Yifeng, Xu. Zhiguo, Yuan. Bing-Jie, Ni. (2016). Biotransformation of pharmaceuticals by ammonia oxidizing bacteria in wastewater treatment processes. Elsevier.

• Žonja, B. (2017). Identification and Fate of Known and Unknown Transformation Products of Pharmaceuticals in the Aquatic System.