Pharmaceuticals and personal care products (PPCPs) are a common chemical groups serving as human, veterinary, diagnostic agents, nutraceuticals, consumer chemical as well as inert ingredients or excipients used in PPCP formulations and manufacture. Pharmaceuticals and personal care products (PPCPs) have increasingly attracted attention since the past few years due to their universal consumption as well as indiscriminate discharge to the aquatic environment. Furthermore, most wastewater treatment plants (WWTPs) did not have adequate function to remove PPCPs, many of them are released into surface water.
In order to cope with the problems, recent technologies including ozonation, reverse osmosis and advanced oxidation process, as well as process optimization (e.g., increasing sludge residence time) have been taken into account to eliminate level of pharmaceutical compounds in water. However, cost effectiveness is said to be the main drawback for the wide spread using of these technologies. Thus, an alternative which fulfill the demand of high PPCPs removal efficiency and cost effectiveness becomes urgently need.
Constructed wetlands (CWs) have been proved to be a possible solution as low cost technology and operational alternatives to conventional WWTPs in eliminating such contaminants including PPCPs in some extent. However, the mechanism and contribution of each mechanism is poorly understood. Pharmaceutical substances were removed by various mechanisms in CWs, namely photolytic degradation, sorption, plant uptake and microbial degradation.
This paper will systematically cover the removal performance by these technologies while focus on the detail and contribution of degradation processes in CWs.
Generally, all the articles are closely related to PPCPs removal; however conducting by different removal processes and scale. The two below articles were the most critical in this review:
Zhang, D.Q., Gersberg, R.M., Hua, T., Zhu, J., Goyal, M.K., Ng, W.J. & Tan, S.K. 2013, ‘Fate of pharmaceutical compounds in hydroponic mesocosms planted with Scirpus validus’, Environmental Pollution, vol. 181, no. 0,pp. 98-106. This publication is a peer reviewer article which published in Environmental Pollution journal. The journal is a high reputation one with IF 4.839. The research investigated the performance of constructed wetland (no chemcail required) in the removal process of 5 PPCPs compounds. The research tried to evaluate the contribution of photodegradation and microbial degradation in PPCPs removal. However, it is limited at lab scale level.
Ternes, Stüber, J., Herrmann, N., McDowell, D., Ried, A., Kampmann, M. & Teiser, B. 2003, ‘Ozonation: a tool for removal of pharmaceuticals, contrast media and musk fragrances from wastewater?’, Water Research, vol. 37, no. 8,pp. 1976-82. It is published in the Water Research journal (IF 5.991), the top distinct journal in this major. This research focus on the advanced oxidation process which using chemical for waste removal. Group of PPCPs which more than 10 substances were tested for removal efficiency under ozonation process. The pilot plant with real municipal wastewater was used as well.
3. Literature Review
Technologies including ozonation, reverse osmosis as well as process optimization do exist to reduce level of pharmaceutical compounds in wastewater.
Ozonation is one of the most common processes using for non-biodegradable waste removal including PPCPs. Previous investigations by Akmehmet Balcıoğlu & Ötker (2003), Ternes et al. (2003) and Andreozzi et al. (2005) have reported that ozone is efficient in the degradation of any pharmaceutical agents.
Akmehmet Balcıoğlu & Ötker (2003) conducted an ozonation experiments of three PPCPs compounds, namely ceftriaxone sodium, penicillin in group of antibiotic and enrofloxacin in group of veterinary, in order to examine the increase of their biodegradability. The influence of pH, initial chemical oxygen demand (COD) concentration and hydrogen peroxide on ozonation process were investigated. The experiments used synthesis wastewater and subsequently compared with selected wastewater on the degradability of chosen PPCPs.
In the different research, Ternes et al. (2003) examined the PPCPs removal efficiency by using a pilot plant for ozonation process. Wastewater from municipal sewage treatment plant (STP) was collected for experiments with the original concentration as 5 antibiotics (0.34 – 0.63mgl-1), 5 betablockers (0.18 – 1.7mgl-1), 4 antiphlogistics (0.10–1.3mgl-1), 2 lipid regulator metabolites (0.12 – 0.13mgl-1), the antiepileptic drug carbamazepine (2.1mgl-1). The result stated that the ozone dose of 10-15 mg/l with contact time 18 minutes might could have reduced all pharmaceuticals, musk fragrances as well as estrogen in wastewater below detectable level.
However, the above research have not mentioned about the kinetic constant or oxidation intermediates and products. Andreozzi et al. (2005) focused on amoxicillin with the concentration up to 120 ngl-1 and its reaction kinetic as well as first stage ozonation intermediates. The kinetic constant of the reaction is strongly dependent on pH value of solutions.
Pharmaceuticals in wastewater have been proven to be effectively removed high pressure membranes in form of nano-filtration (NF) and reverse osmosis (RO).
Membrane bioreactors (MBRs) increasingly attract societal attention as technical feasibility and significant cost reductions characteristics. The high biomass concentrations and long sludge retention times benefit the biodegradation of organic pollutants as well as micro contaminants as PPCPs (Sipma et al. 2010).
Clara et al. (2005) examined the PPCPs treating efficiency of MBR by verifying sludge retention time. Group of pharmaceuticals, polycyclic musk fragrances and endocrine disrupting chemicals were detected in many waste water treatment plants (WWTPs). A pilot scale membrane bioreactor was run at various solid retention times (SRTs) and the results were compared to conventional activated sludge plants (CASP). Consequently, the different PPCPs compounds behave differently. The antiepileptic drug (e.g.carbamazepine) was not removed in any of the sampled treatment facilities. Other compounds (e.g.bisphenol A), the analgesic (e.g.ibuprofen) or the lipid regulator (e.g.bezafibrate) were considerably treated with removal efficiency more than 90%. Thus, the author concluded that there was no significant difference between two techniques.
In another concept, Dutta et al. (2014) applied two-stage anaerobic fluidized membrane bioreactor (AFMBR) treating municipal wastewater. The granular activated carbon (GAC) as carrier medium were used in both stages. The two-stage AFMBR was operated at total HRT of 5 h in which include 2 h for AFBR and 3 h for AFMBR. The influent COD concentration of 250 mg/L was used as well. Regarding PPCPs removal, the removal efficiency of carbamazepine was reported to reach 73% in AFBR as well as AFMBR 86.6%. This removal efficiency give a significant results in comparison of other techniques which carbamazepine is rarely treated beyond 50% (Zhang et al. 2014).
Carballa et al. (2007) examined the behavior of 13 PPCPs during anaerobic digestion (AD) of sewage sludge including:
- Two musks (galaxolide and tonalide)
- One tranquilliser (diazepam)
- One anti-epileptic (carbamazepine)
- Three antiphlogistics (Ibuprofen, naproxen and diclofenac)
- Two antibiotics (sulfamethoxazole and roxithromycin)
- One X-ray contrast medium (iopromide)
- And three oestrogens (estrone, 17b-oestradiol and 17a-ethinyloestradiol)
In the model, two parallel processes have been conducted as one in mesophilic range (37oC) and the other in thermophilic range (550C). The variables in form of temperature and sludge retention time (SRT) were chosen. The results illustrated that antibiotics, natural oestrogens, musks and naproxen were remove more efficiently. Regarding other PPCPs, the removal efficiency fluctuated from 20% to 60%, except for carbamazepine, which illustrated insignificant elimination. In general, no influence of SRT and temperature on PPCPs removal was observed but the distribution efficiency and solid concentration.
In different method, Suarez, Lema & Omil (2010) studied the removal efficiency of 16 PPCPs in two lab scale conventional activated sludge reactors which operated in nitrifying (aerobic) and denitrifying (anoxic) conditions in the period of 1.5 years. It is observed that the nitrifying reactor demonstrated the higher PPCPs removal kinetic in comparison with denitrifying system.
As compared to CWs, the above techniques demonstrate advantages in construction cost and land requirement. However, the operation and maintenance cost raise a significant concern as well as labored-skill demand. The O/M cost of a conventional wastewater treatment is 14 to 30 times larger than the O/M cost of CWs. The cost of treating per m3 wastewater by AOP process is 208 times more than CWs.
3.2 Degradation processes in CWs
3.2.1 Photolytic degradation process
Photolytic degradation was considered to be the most important and predominant mechanism in PPCPs removal. However, the treatment efficiencies largely depended on several factors as seasonal variation, light intensity and light attenuation by water depth. There have been few studies about photo-degradation of pharmaceutical removal in aquatic plant-based systems compared to surface water (e.g. lake, river, sea).
Llorens et al. (2009) investigated PPCPs removal in form of diclofenac and ketoprofen from a secondary effluent by FWS CWs. The high treatment efficiency was indicated related to high HRT (one month) and sunlight exposure. So that the most probable mechanism was photodegradation in FWS CWs in order to deal with recalcitrant substances as diclofenac and ketoprofen. Matamoros, García & Bayona (2008 also determined the removal of 12 PPCPs contaminants by 1 ha FWS CWs. The treatment efficiency of ketoprofen was recorded as 97-99% which might be attributed to its fast photolytic degradation.
However, Zhang, Hua, et al. (2013) found that photolytic degradation played an minor part in carbamazepine treatment while naproxen demonstrated a potential for both photolytic degradation. The removal of carbamazepine in the dark ranged from 56 to 67% as same as removal efficiency in sunlight condition with 57 to 69%. In contrast, the removal of naproxen under dark condition stayed at 48 to 60% and it climbed beyond 90% with the support of sunlight. Furthermore, Zhang, Gersberg, et al. (2013) also stated the photolytic degradation is not potential for the removal of clofibric acid and caffeine as well. The experiments with sunlight condition could treat clofibric acid, caffeine at 29-73% and 75-99% while the dark condition one also could remove 28-69% and 72-93%, respectively.
The sorption of dissolved organic compounds as well as micro contaminants on soil, organic carbon, and mineral surface of media bed might have been an important removal pathways.
Verlicchi, Al Aukidy & Zambello (2012) stated that the sorption might have occurred due to the hydrophobic interactions of the aliphatic, aromatic groups of contaminants with lipophilic cell membrane of the microorganism or the lipid fragment of suspended solids. Recently researches have examined the close relation of PPCPs sorbed into substrate with their hydrophobic characteristics as galaxolide and tonalide process. However, compounds with high water solubility and low hydrophobicity as caffeine might not suitable for adsorption process. Hijosa-Valsero et al. (2010) stated that pharmaceuticals with moderately hydrophilic did not attached sufficiently into organic matter.
Lin et al. (2010) in the lab scale aqueous environment found that caffeine, propranolol and acetabutolol is highly attracted by sorption while acetaminophen was not. The partition coefficiency Kd of acetaminophen, caffeine, propranolol and acebutolol was 5, 250, 270 and 1900, respectively. This implied the substantial potential for adsorption, even in low organic content as it ranged from 240 to 1900 L/kg, except acetaminophen. It was also founded that the sorption activity fit the Freundlich isotherm with high affinity.
Yet, the hydrophobicity of PPCPs compound might not been fully responsible for the adsorption and distribution characteristics of PPCPs as the adsorption also much depended on the chemical structure of targeted compounds (Bui & Choi 2010).
The removal of micro-contaminant by plant uptake, accumulation and translocation was proved to be major pathways for phytoremediation process (Collins, Fryer & Grosso 2006). However, there have been moderately quantitative studies for details of mechanism as well as rare pharmaceutical compounds and plant species have been taken into account (Redshaw, Wootton & Rowland 2008).
In details, the ability of plant uptake was calculated commonly based on its physical-chemical characteristics, e.g. log Kow (Burken & Schnoor 1998). It was proven that there was no specific transporter for pharmaceutical compounds in plant cells, so that the diffusion mechanism might have been mainly responsible for the consumption of these compounds. These diffusion pathways, however, largely depend on the physical-chemical characteristics, especially their hydrophobicity (Stottmeister et al. 2003). The log Kow range of pharmaceutical from 0.5 to 3 was proved to be substantially efficient in removing by plant uptake mechanism (Dietz & Schnoor 2001).
There have been conflicts regarding the dependence of pharmaceutical uptake with hydrophobicity characteristics. Briggs, Bromilow & Evans (1982) indicated that the pharmaceutical uptake of plants root was proportional to log Kow in contrast with Reinhold et al. (2010) who stated that hydrophobicity and log Kow were independent to the plant uptake mechanism leading to the unclear predictive value. Meagher (2000) also concluded that certain plants might have appropriate efficiency in micro-contaminant removal.
Biodegradation was considered an important mechanism for pharmaceutical removal in CWs which consist of three sub processes as mineralization, transformation to more hydrophobic compounds and transformation to more hydrophilic compounds (Kümmerer 2003). Biotransformation and degradation can lower toxicity and increase water solubility of pharmaceutical (Kümmerer 2009). Recently, there were moderate studies in the aspect of PPCPs removal by microbial degradation as well as the use of indirect method for quantitative evaluation (Onesios, Yu & Bouwer 2009).
Ávila et al. (2013) studied the degradation in SSF CWs of diclofenac (DCF) metabolites, namely 4-hydroxy-DCF, and found that the concentration of intermediate was relative low. Moreover, the effect of pharmaceutical compounds on the growth and stability of microbial communities in CWs also attracted much concerns. Weber et al. (2011) investigated that the presence of ciprofloxacin in the CW planted with Phragmites australisenriched inhibited the microbial communities. However, the bacterial communities were found recover after 2 – 5 weeks adaptation periods.
It was worthwhile noticing that the micro-organism alone was unable to eliminate pharmaceutical substances due to the variety of microbial communities in wastewater. First, the low concentration of pharmaceutical in wastewater was insufficient to induce enzymes that responsible for the degradation of pharmaceuticals. Second, the growth and activities of microbial communities in wastewater might have been deactivated due to the presence of pharmaceutical. Joss et al. (2005) examined the effect of micro-biodegradation in the removal of pharmaceutical compound in WWTPs. Author found that, among 35 compounds, only 4 (ibuprofen, acetaminophen, 17b-estradiol, and estrone) was removed efficiently as above 90%, the others was treated below 50% in a biological process.
This review showed the relatively high-importance of various techniques in PPCPs removal. It is worth noting that phytoremediation is considered particularly important for those pharmaceuticals which are relatively recalcitrant to biodegradation (e.g., clofibric acid), or are highly polar/soluble (e.g., caffeine). On the other hand, highly hydrophobic persistent PPCPs (e.g., galaxolide, tonalide and carbamazepine) may have great potential to be adsorbed in CWs. Furthermore, the design of CWs might have certain impact on the removal process and suitable for specific PPCPs. Other techniques such as ozonation and membrane process showed the considerable removal efficiency, although having high chemical cost and require labor skill.
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Ternes, Stüber, J., Herrmann, N., McDowell, D., Ried, A., Kampmann, M. & Teiser, B. 2003, ‘Ozonation: a tool for removal of pharmaceuticals, contrast media and musk fragrances from wastewater?’, Water Research, vol. 37, no. 8,pp. 1976-82.
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Zhang, D., Gersberg, R.M., Ng, W.J. & Tan, S.K. 2014, ‘Removal of pharmaceuticals and personal care products in aquatic plant-based systems: A review’, Environmental Pollution, vol. 184, no. 0,pp. 620-39.
Zhang, D.Q., Gersberg, R.M., Hua, T., Zhu, J., Goyal, M.K., Ng, W.J. & Tan, S.K. 2013, ‘Fate of pharmaceutical compounds in hydroponic mesocosms planted with Scirpus validus’, Environmental Pollution, vol. 181, no. 0,pp. 98-106.
Zhang, D.Q., Hua, T., Gersberg, R.M., Zhu, J., Ng, W.J. & Tan, S.K. 2013, ‘Carbamazepine and naproxen: Fate in wetland mesocosms planted with Scirpus validus’, Chemosphere, vol. 91, no. 1,pp. 14-21.
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