Introduction

Phthalates have wide application in plastic manufacturing units as plasticizers. Their characteristics of imparting flexibility to polyvinyl chloride (PVC) products has intensified their production, particularly since 1950 (Kimber and Dearman 2010). Apart from this, a broad range of industries including manufacturers of personal care products are using phthalates in their processes (Simoneit et al. 2005). Phthalates can easily leach directly or indirectly, into the adjacent environment by leaching from their polymeric matrix (Cadogan et al. 1993). Phthalates are abundantly found in the environment including the atmosphere, wastewater, drinking water, sludge, and sediments (Xie et al. 2007; Dargnat et al. 2009; Gao et al. 2014). Among the group of phthalates, some have endocrine disruption potential also and have received remarkable attention to prevent the public health from their exposure. Consequently, six of them have been included in a priority list of pollutants by the European Union (EU) and the United State Environment protection agency (US EPA) (Net et al. 2015).

Among the main source of phthalates in freshwater bodies are effluents from wastewater treatment plants (Gao et al. 2014). Removal of phthalates in these treatment plants has been reported to range between 60 and 80%. (Fauser et al. 2003; Reyes-Contreras et al. 2011; Balabanic et al. 2012). However, fate and removal of phthalates in wastewater treatment plants that are based on up flow anaerobic sludge blanket (UASB) combined with polishing pond systems has not been investigated to date, and UASB combined with polishing pond systems represent a treatment scheme that is commonly used in developing countries like India because of its low cost and self-sustainable function. In terms of volume of sewage treated by UASB technology, India is among the leading countries worldwide (Sato et al. 2007). Overall, there are approximately 43 UASB-based wastewater treatment plants in India which were previously installed under different national plans of various river catchments (Khalil et al. 2008). For example, the government of India has implemented 16 full-scale UASB-based wastewater treatment plants with an aggregate capacity of 598,000 m3/day under the Yamuna Action Plan (CPCB 2003).

Discharge of the phthalate-laden treated wastewater into surface water bodies can cause risk to aquatic life and human beings, provided the surface water is being consumed for drinking. Net et al. (2015) reviewed that the presence of phthalates in surface water was associated with physiological changes in reproductive health of aquatic life. The risk to aquatic life and human beings is represented by hazard quotient (HQ) and exposure values (Matamoros et al. 2016). Selvaraj et al. (2015) reported the HQ of DEHP in Kaveri River (India) was greater than 1 while as HQ of DEP, DBP, and BBP was less than 1. In south Delhi, Das et al. (2014) estimated the phthalate exposure of humans to phthalates from drinking water, in which the contribution of DEHP was greatest. House-hold swimming pools, especially those used by kids for recreational activities, are made of PVC from which phthalates can leach out, and the leaching increases when the PVC is exposed to heat.

There is growing concern regarding the exposure of humans and wildlife to phthalates due to increasing evidences of their toxic effects in recent years. Visible inflammatory reaction was observed after exposure of rabbits to DEHP (Calley et al. 1966). Reduction in levels of testosterone and delay in puberty after exposure to DBP was reported by Salazar et al. (2004). Furthermore, DBP is classified as class D (not classifiable) carcinogen by US EPA based on data available in literature (US EPA 1990). Likewise, DEHP is classified as a possible carcinogen by US EPA and has been reported as a potential cause for a few breathing and lung disorders in children if taken up through drinking water or inhalation (Shneider et al. 1991). Wastewater treatment plants have an important role in reduction of the risk of micropollutants, such as phthalates (Matamoros et al. 2016). However, the contribution of UASB/pond system in reduction of aquatic and human risk has not been investigated so far.

With these concerns, the aim of this yearlong study was to examine fate and removal of four priority phthalates in UASB pond system together with seasonal impacts on its removal performance. Seasonal variations in a polishing pond are important because of its high surface area to volume ratio. The four phthalates evaluated in this study were DEP, DBP, BBP, and DEHP. Four phthalates were selected based on their presence in Indian surface waters reported in past studies (Selvaraj et al. 2015) and another consideration was to choose phthalates with different chain lengths which influence their fate. These four compounds are among priority phthalates as declared by the US EPA (Net et al. 2015). A risk assessment of phthalates in UASB pond treated effluents was also carried out. Finally, the ability of the treatment system was evaluated for the regulatory compliance of phthalate discharge.

Materials and methods

Chemicals

Standard solutions of phthalates were supplied by Sigma Aldrich (Germany). n-hexane, acetone, and dichloromethane which were used for extraction and sample cleanup were purchased from Merck (Mumbai, India). Anhydrous sodium sulfate was also procured from Merck (Mumbai, India).

UASB/pond system

The operational information of the investigated UASB /pond system located in Saharanpur city of Uttar Pradesh, India is presented in Table S2. The treatment plant was chosen based on its proximity to the laboratory (45-min drive) and its satisfactory wastewater treatment performance as per Indian discharge regulations (Mungray and Kumar 2008). Conventional wastewater parameters of its effluent were within the Indian regulations for discharge of effluents into water bodies Table S4 (CPHEEO 2012). The wastewater treatment process of the chosen treatment plant is shown in Fig. S1. The operating capacity of the UASB/pond system was 38,000 m3/day. Degritted sewage was treated in the UASB with a hydraulic retention time (HRT) of 10 h, followed by treatment in polishing ponds with a surface area of 12,700 m2 at an HRT of 24 h. The effluent of the WWTP was discharged into the river Dhamola, which discharges into the river Yamuna. The sludge of the UASB reactors was dried over sludge drying beds (20 in number) with a surface area of 350 m2 each.

Sampling strategy

Samples of wastewater were collected on monthly basis for a period of 1 year from October 2014 to September 2015. While as sludge samples were collected every 2 months, wastewater samples were collected from the UASB effluent and polishing pond effluent. The samples of UASB sludge were taken from the outlet of the sludge wastage line of the UASB tank. Glass bottles with amber color and Teflon caps were used for sample collection, to avoid exposure of light and contamination of plastic. Before taking samples, the cleaning of sample bottles was done with soap solution and vigorous washing with distill water and acetone. Collected samples were extracted within 24–48 h with preservation at 4 °C (Oliver et al. 2005; Gao et al. 2014).

Sample preparation

Extraction of phthalates was done by liquid extraction with hexane and dichloromethane (US EPA 2002; Gao et al. 2014; Das et al. 2014). To remove any potential prior contaminations, the glassware was baked at 350 °C for half an hour. The extraction was done thrice from 250 ml of unfiltered wastewater with 25 ml of a solvent mixture containing 80% hexane and 20% dichloromethane. Further drying of extracts was done with anhydrous sodium sulfate, then concentrated in a water bath to approximately 2 ml at 80 °C and cleaned up in a column of 2 cm internal diameter containing 10 g of alumina. The column was pre eluted with n-hexane (20 mL) followed by elution with 20 ml of acetone/hexane (80% hexane and 20% acetone) (Gani et al. 2016). Dry sludge (1 g) prepared by air drying was crushed to fine powder and extracted at room temperature (27–32 °C) in 80 ml of n-hexane/dichloromethane (3:1) solution on a rotary shaker. The speed of the shaker was maintained at 250 RPM and the samples were kept for 12 h in the shaker. After extract separation, the cleanup and concentrations of sludge samples were the same as that of wastewater samples.

Analysis of phthalates

Quantification of phthalates was done with a gas chromatogram (GC) with a mass spectrophotometer (MS) (Varian 450 GC; 240 MS, Agilent Technologies, USA). Helium (~ 98% purity) was used as a carrier gas and the flow rate was 0.8 mL/min. During data acquisition, temperature of transfer line and ion source was 280 °C and 230 °C, respectively. The temperature in the column was maintained at 60 °C at the beginning and increased to 280 °C at a rate of 6 °C/min (hold time 2 min). A five-point calibration ranging from 5 to 500 μg/L was used for measurement of concentration of phthalates. The recoveries of phthalates were in the range of 80–95.2% for all compounds (Table S3, supplementary data). The limit of detection (LOD) was DEP (0.071 μg/L), DBP (0.130 μg/L), BBP (0.216 μg/L), and DEHP (0.084 μg/L), and limit of quantification (LOQ) was 0.210 μg/L, 0.182 μg/L, 0.342 μg/L, and 0.241 μg/L. With every analysis regime, laboratory contaminations were adjusted by analysis of blank samples.

Conventional analysis

Conventional pollution parameters (COD, BOD, TSS, ammonia, and phosphate) of wastewater samples were quantified according to standard methods (APHA 2005).

Fate calculations

Annual mean concentrations observed in wastewater and sludge were used for the fate calculation of phthalates. Mass rate of phthalates that exit from the UASB with excess sludge was assumed as removal by adsorption (Msor). This rate was determined by multiplying observed mean concentration of phthalates in sludge and rate of solid removal (TSwas) (Joss et al. 2005). The bio-transformed portion (Mdeg) was calculated by subtraction of adsorbed portion from overall difference in influent (Mi) and effluent (Mo) phthalate loadings (Eqs. 1 and 2). The adsorbed as well as bio-transformed phthalate loadings were expressed as percentage of influent phthalate loading:

$$ {\mathrm{M}}_{\mathrm{deg}}=\left({\mathrm{M}}_{\mathrm{i}}-{\mathrm{M}}_{\mathrm{o}}\right)-{\mathrm{M}}_{\mathrm{sor}} $$
(1)
$$ {\mathrm{M}}_{\mathrm{deg}}=\left({\mathrm{C}}_{\mathrm{i}}-{\mathrm{C}}_{\mathrm{o}}\right)-\left({\mathrm{C}}_{\mathrm{sor}}\times {\mathrm{TS}}_{\mathrm{was}}\right) $$
(2)

where Q is influent wastewater flow rate; Ci and Co is the concentration of compound in raw wastewater (influent to UASB) and UASB-treated effluents respectively; Csor is the average compound concentration in UASB sludge; TSwas is the solid concentration in UASB sludge. Due to field scale limitations, fate of phthalates in polishing pond was not evaluated. Instead of using metered pumps, the sludge from ponds was drawn out manually due to which sludge wastage estimation in the polishing pond was unfeasible.

Calculations for risk assessment and phthalate exposure

The risk associated with discharge of phthalates in the surface water is represented by HQ, which is ratio of predicted environment concentration (PEC) of phthalates and predicted no effect concentration (PNEC) for fish (carp species). Since, the treated effluents are discharged into fresh water bodies where they get diluted. As per EU, TGD guidelines, a dilution factor of 10 was assumed to calculate PECwater from mean seasonal phthalate concentration in WWTP effluents (EU TGD 2003). To get the PNEC values, least lethal concentration (LC50) of the phthalate is divided by an assessment factor of 1000. These calculations were according to guidelines mentioned in EU TGD (2003). LC50 values of phthalates were derived from ECOSAR (v1.1) software (Table S5, supplementary data) (US EPA 2011). The species in deriving PNEC was chosen as fish (carp species) because it exists in the river in which WWTP effluent is discharged. The fish species commonly observed in this stretch (Yamunanagar stretch) of Yamuna river is carp (CIFRI 2014).

Exposure of phthalates to human beings from consumption of water of surface water bodies was according to Das et al. (2014) as rivers are sources of drinking water after treatment in a drinking water treatment plant.

In this study, it was assumed that an average adult of 60 kg will consume 2 l of water (ICMR 2010).

The exposure was therefore calculated according to Eq. 3 and 4.

$$ \mathrm{Phthalate}\ \mathrm{exposure}\ \left(\mathrm{ng}/\mathrm{kg}/\mathrm{day}\right)=\left({\mathrm{C}}_{\mathrm{water}}\times 2\right)/60 $$
(3)
$$ {\mathrm{C}}_{\mathrm{water}}={\mathrm{C}}_{\mathrm{eff}.\mathrm{mean}}/10 $$
(4)

where Cwater is the phthalate concentration in water bodies receiving effluent and Ceff.mean is the mean seasonal concentration of phthalates in effluents of WWTPs. Cwater is obtained by dividing Ceff.mean with the dilution factor of 10.

Results and discussion

Conventional sewage treatment performance

Table S4 shows the wastewater treatment performance of the investigated wastewater treatment plant. Percentage removal performance for COD and BOD in UASB and polishing pond was around 60%; however, the removal in the full treatment combination (UASB + polishing pond) was 71% and 82%, respectively (Table S4). The mean solid removal was 76% in the investigated treatment plant. Nitrification as well as total nitrogen removal was approximately 60%. The phosphorus removal in the whole treatment plant was relatively poor in the range of 39–75% with annual mean of 57%. The treatment performance of investigated treatment plant was satisfactory according to Indian regulations as the quality of the effluent from wastewater treatment plant was satisfying the standard discharge regulations of CPHEEO (2012) (Table S4).

Occurrence level of phthalate in sewage and sludge

The distribution of phthalates in untreated wastewater was DEHP > DBP > DEP > BBP (Table 1). DEHP had the highest mean concentration of 20.87 μg/L, which was in agreement with the previous studies by Gani et al. (2016) and Gao et al. (2014). Least concentration of BBP was observed in raw wastewater with 2.14 μg/L. This may be due to its comparatively less application in plastic and personal care products (CPSC 2010). The maximum concentration of phthalates was observed during the summer season. DEP and DBP were present in highest concentrations in July while maximum BBP and DEHP concentrations were measured in May and June, respectively. This can be explained by the atmospheric deposition of phthalates with rainfall during summer months (summer season in this study was inclusive of monsoon). Pollutants are removed from air during precipitation, and this occurs either by dissolving of vapor compounds in rain or washing down of pollutant containing atmospheric particles by raindrops (Gill and Graedel 1983).

Table 1 Concentration of priority phthalates in raw wastewater and sludge from UASB reactors (N = 12)
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Teil et al. (2006) has reported the occurrence of phthalates in atmosphere and their transfer by washing down of phthalate containing atmospheric particles by raindrops to urban runoff. At most of places in India, the urban runoff flows into the sewers, which may alter the concentration of phthalates in untreated wastewater. More use of personal care products in summer and increase in leaching of phthalates with temperature may be another reason for their high occurrence in summer (Rose et al. 2012; Gani et al. 2016).

In sludge from UASB reactor, greatest concentrations of DEHP were observed with an annual mean concentration of 45.31 mg/kg and a maximum concentration of 76.47 mg/kg. DEP and DBP, which are lower-molecular-weight phthalates, were present at concentrations < 5 mg/kg. The distribution of phthalates in sludge was DEHP > BBP > DBP > DEP which was in agreement with their octanol water partition coefficient (log Kow). Measured maximum concentration of DEHP in sludge was comparable to 67.18 mg/kg/dw reported by Roslev et al. (2007); however, Gao et al. (2014) and Dargnat et al. (2009) reported lower concentrations of 4.70 mg/kg and 0.50 mg/kg, respectively, in full-scale aerobic WWTPs. This variation may be anaerobic sludge characteristics of UASB sludge compared to aerobic sludge. Shi et al. (2011) and Alvarino et al. (2015) reported that intra-molecular diffusion of the compounds inside the anaerobic sludge granule can increase the concentration of micropollutants in it. DEHP concentrations did not exceeded 100 mg/kg, which is the recommended limit by European Union for safe disposal of dewatered sludge (EU 2000). This may indicate the safe application of dried sludge from the UASB for soil improvement.

Overall removal of phthalates in UASB/pond system

Higher removal of DEP and BBP was observed in the polishing pond compared to the UASB. The mean percentage removal of total phthalates (∑PAE) was 63 ± 5% in the pond and 53 ± 3% in the UASB (Fig. 1a). In the combined treatment scheme, the removal was 83 ± 2%. Chernicharo et al. (2010) has reported lower removal of 53% to 70% in a UASB/polishing pond system. Maximum removed phthalates were lower-molecular-weight ones (DEP in polishing pond and DBP in UASB), which may be due to their more degradation in the treatment systems, owed to their more bio availability and less susceptibility to sorption (Gao et al. 2014; Gani et al. 2016).

Fig. 1
figure 1

a Overall removal of phthalates in UASB, pond and in their combination with pond as post treatment of UASB. b Temporal variations in the removal of phthalates in UASB pond system. (Error bars represent standard deviation; Number of samples (n) = 12)

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Higher removal in polishing ponds may be due to different redox conditions (aerobic and anaerobic) prevailing in it compared to only anaerobic treatment in UASB. Depending on water depth, these redox conditions may facilitate co-existence of mixed removal pathways (aerobic–anaerobic) for degradation of phthalates (Reyes-Contreras et al. 2011). In lab-scale experiments of DEHP with A/A/O (anaerobic, anoxic and oxic) process, Huang et al. (2008) reported that degradation increased when DEHP was treated through different redox environments. Poor degradation of phthalates in anaerobic conditions like UASB was reported in previous studies as well (Huang et al. 2008, 2010; Semblante et al. 2015). Compared to aerobic degradation, Madsen et al. (1999) reported anaerobic degradation of DEHP as 4–5 times slower. However, Liang et al. (2007) reported dimethyl phthalate (DMP) removal as 99% in a pilot-scale UASB. DMP has lower chain length, which may have less steric hindrances to enzymatic binding as compared to large side chain of BBP and DEHP (Ejlertsson et al. 1997).

Seasonal influences on phthalate removal efficiency of the UASB/pond system

Average temperature in winter (November to March) was below 25 °C and it was > 25 °C in summer (April to October) (Fig. 2S) with mean temperature of 18 ± 4 °C in winter and 30 ± 2 °C in summer respectively. Pearson correlation coefficient between percentage removal of phthalates in wastewater and ambient temperature was positive in the range of 0.606–0.854. There were seasonal discrepancies in removal of phthalates in the UASB/pond treatment system with removals in summers (Fig. 1b). The seasonal variation in percentage removal of different phthalates was 4–30%. In the treatment scheme, seasonal discrepancies in removal of DEP and DBP were 15% and 11%, respectively. Maximum seasonal variation was observed in case of DEHP. The percentage removal was 57 ± 3% in winter and 75 ± 3% in summer (Fig. 2). This may be because there was a significant contribution of sorption as well as biotransformation in the removal of DEHP, which are both affected by temperature. Moreover, likewise roots of plants in constructed wetlands, algae in polishing pond also enhance the sorption capacity (Tanner and Sukias 1995). Therefore, the reduced algae during winters owed to less photosynthesis may also reduce the removal of organic contaminants (Matamoros and Salvadó 2012). These seasonal variations in removal of phthalates may have implications on risk and seasonal regulations associated with the discharge of phthalate-laden effluents in fresh water bodies.

Fig. 2
figure 2

Percentage removal of phthalates by sorption and biotransformation in UASB. ∑PAE represents total sum of four phthalates. (Error bars represent standard deviation; Number of samples (n) = 12)

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Biotransformation and sorption (fate) of phthalates in UASB

Overall, the removal by biotransformation of four phthalates was in the range of 30 to 61% with greatest biotransformation having occurred for DBP (Fig. 2). Phthalates with smaller alkyl side chains (DBP and DEP) were bio-transformed to a greater extent than large and aryl side chain phthalates (BBP and DEHP), which was in agreement with earlier studies of Zeng et al. (2004) and Gao et al. (2014). The order of biotransformation was DBP > DEP > BBP ≈ DEHP. Hydrophobicity of organic compounds is expressed by log octanol water partition coefficient (log Kow) and sorption of phthalates increases with the hydrophobicity. Sorption of phthalates in UASB was in accordance with their log Kow values (Fig. 2). Similar relationship for BBP and DEHP was reported by Clara et al. (2010) when mass fractions of BBP removed with sludge were 21–74% and 78–81%, respectively. The removal of DEP and DBP by sorption was almost negligible (0–3%) while a significant portion of BBP and DEHP in UASB was removed by sorption (15–24%). Previous full-scale studies have evaluated the fate of phthalates in selected aerobic processes such as conventional ASP and sequencing batch reactor (Marttinen et al. 2003; Dargnat et al. 2009).

Reduction in phthalate risk to aquatic environment

In this study, the contribution of the UASB/pond system in reducing the phthalate risk from raw sewage was assessed. This was important because the surface water body into which treated effluents are discharged get mixed with the river Yamuna, which serves as habitat for aquatic species such as fish. Furthermore, after treatment in drinking water treatment plants, it is an important source of drinking water for the local population. Ecological risk assessment was done based on US EPA (1998) guidelines in which a representative aquatic organism was chosen, and in this study, it was fish (species: carp). Figure 3 shows the HQ of phthalates in raw sewage and treated sewage along with percentage reduction in the risk after sewage is treated by the UASB/pond system. With exception of DEHP, HQ of other three phthalates in raw sewage and treated effluents was less than 1 showing there is negligible to small risk of DEP, DBP, and BBP (Fig. 3). However, HQ of DEHP in raw sewage as well as treated sewage was greater than 1 during both seasons, which was consistent with its greater concentrations in wastewater and its low lethal concentration (LC 50) for fish (0.01 μg/L from ECOSAR).

Fig. 3
figure 3

Hazard quotient (HQ) due to presence of phthalates in raw sewage and treated effluents and the reduction in risk by UASB pond system

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The reduction in risk due to treatment in UASB + pond system was greater than 81–87% in summer and 65–74% in winter (Fig. 3). This is in accordance with the greater removal of phthalates at higher temperatures. The reduction in HQ was least for DEHP in summer as well as in winter (81% and 65%, respectively) which was consistent with its comparatively more persistence to removal in the treatment plant. To reduce the HQ of DEHP in treated effluents below one, the increase in removal efficiency of DEHP in the UASB + pond system or the increase in dilution of the treated effluents is necessary. But during summer, removal of DEHP was already efficient (87 ± 7%); therefore, one of the possible options to reduce the risk of DEHP in effluents to safer limits is dilution of effluents. In winter, removal efficiency of DEHP was moderate (57 ± 3%) suggesting more dilution as well as improved removal efficiency of DEHP in treatment plant is required to reduce DEHP risk to safer limits.

Reduction in phthalate risk to human beings

Surface water is used as source of drinking water after filtration. Table 2 contains seasonal exposure level of phthalates and its reduction by UASB/Pond system. The cumulative exposure levels as well as exposure level of individual phthalates was well below the tolerable daily intake limits (TDIs) of European Food Safety Authority (EFSA) and reference doses of US EPA (Table S6, supplementary data). This implies that disposal of UASB/Pond system in river water was safe with respect to phthalate risk to human beings. Exposure level of DEHP was greatest, 101 ng/kg/day due to its presence in raw sewage and 19 ng/kg/day due to its presence in UASB/polishing pond-treated effluents (Table 2). This implies that there is need for systematic monitoring of DEHP risk at this wastewater treatment plant which includes the background evaluation of untreated wastewater to find solutions of DEHP contamination at source. In view of the reference dose and TDI (Table S6 supplementary data), the contribution to human risk from treated wastewater is still marginal. Least exposure was calculated for BBP (7 and 8 ng/kg/d in raw sewage in summer and winter, respectively and 1 and 2 ng/kg/day in UASB/polishing pond effluents). Individual and cumulative exposure of phthalates due to discharge of raw sewage and treated effluents was more in summer than winter. There was a seasonal variation of 94 ng/kg/day in cumulative exposure from raw sewage and 11 ng/kg/day from treated effluents. Higher exposure of phthalates from raw sewage in summers suggests better precautionary measures preferable in summer compared to winter.

Table 2 Exposure estimates of priority phthalates to a normal adult, if raw sewage is discharged in surface waters without treatment or after treatment by UASB pond system. The percentage reduction shows the contribution of UASB pond system in decreasing the exposure of phthalates (N = 12)
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Overall deduction in phthalate exposure by UASB/pond system was observed 65–87%. The reduction in exposure of phthalates by UASB/pond system was more in summer than winter which was apparently due to their better removal at higher temperature. The reduction in exposure was 81–87% in summers and 65–74% in winter. This implies that UASB/pond system can considerably minimize the risk associated with phthalates in raw sewage and thereby can limit the health risks associated with them.

Comparison with other WWT technologies

By and large, the efficacy of UASB/pond system for phthalate removal was comparable with that of other wastewater treatment technologies (Table 3). Moreover, the percentage removal of DEP in UASB/pond system was higher than in an UASB/constructed wetland system. It is also worth mentioning that the removal of DEP, DBP, and BBP in the UASB/pond system (80–84%) was approximately equivalent to that of MBR (77–87%) (Boonyaroj et al. 2012). Probably higher residence times in polishing ponds and co-existence of mixed removal pathways (aerobic–anaerobic) in the pond as well as in the whole treatment scheme are positive factors for effective removal of phthalates. Brandt et al. (2013) also compared pharmaceutical removal in a UASB/pond system with AS systems and found better or comparable removal in the former systems. Although Roslev et al. (2007) stated that activated sludge processes tended to have higher removal of DBP and BBP (91 and 90%, respectively) and Iran et al. (2015) reported higher removal of DEHP (93%) than UASB/pond system, there are operational and economic advantages of UASB/pond system (Tare et al. 1997), which renders it more sustainable and the comparably preferable phthalate removal system.

Table 3 Comparison of phthalate removal in UASB pond system with various wastewater treatment technologies
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Dilution requirement for achieving regulatory compliance

To check whether UASB/pond system is adequate to produce an effluent that meet regulatory limits such as environmental quality standards (EQS) of phthalates, dilution factors required in discharge water bodies were calculated (Fig. 4). EQS of DEP, DBP, and BBP was according to review work of Net et al. (2015) and EQS of DEHP was according to EU 2013. There was no dilution requirement for DEP, DBP, and BBP in effluent of UASB/pond system as their concentration in treated wastewater was well below the EQS limit (Table S7, supplementary data). The environmental compliance of effluent for these three phthalates was supported by their low occurrence in untreated sewage as well as their better removal by UASB/pond system. The concentration of DEP, DBP, and BBP in untreated sewage was also below the EQS limits (Table 1 and Table S7).

Fig. 4
figure 4

Dilution of UASB pond effluents required to ensure satisfaction to EQS limits of DEP, DBP, BBP, and DEHP. EQS of DEP, DBP, and BBP was according to review work of Net et al. (2015) (200, 8, 20 μg/L respectively) and EQS of DEHP was according to EU (1.3 μg/L). Default dilution factor of 10 was adopted, according to the European Commission (EU TGD 2003)

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A dilution factor of 4.4 in summer and 2.1 in winter was calculated for regulatory compliance of DEHP concentrations in effluents. Lower value of EQS of DEHP recommended by EU and its ubiquitous large environmental occurrence as compared to other phthalates necessitates this level much of dilution. This may also suggest need of enhanced removal of DEHP from untreated sewage. Even DEHP removal of 90% in the UASB/pond system cannot make effluent eligible to meet EQS limit (1.3 μg/L). Therefore, the dilution factor calculated may have an important impact on environmental compliance of UASB/pond system with respect to DEHP discharge.

Conclusion

Removal of emerging contaminants in wastewater is one of the main issues for which new solutions are being investigated. At the same time, existing wastewater treatment facilities are to be investigated for removal performance of these ECs so that the new solutions can be installed in the existing treatment stream or the existing facilities can be improved for their efficient removal. This study may provide similar inferences after UASB/pond system was investigated for the phthalate occurrence, fate, removal and risk. Four phthalates DEP, DBP, BBP, and DEHP being of priority concern were chosen for the investigation. Overall removal of sum of four compounds in the UASB/pond system was satisfactory (80%) and comparative to UASB; the polishing ponds seem efficient in total removal of phthalates, which may be due to different treatment environments across the depth of the pond. Future focus should be fate assessment in polishing ponds which was not possible in this study. Since field-scale polishing ponds have manual sludge withdrawal, scaling down to laboratory condition may derive insights into this. Aquatic and human risk assessment of phthalates was also carried out from which it seems that the risk associated with DEP, DBP, and BBP in effluents is well reduced by UASB/pond system but the risk associated with effluent DEHP can be reduced to safer limits along with dilution. The removal of phthalates was faintly lower than other processes such as conventional activated sludge process or MBR; however, the sustainable and economic benefits are more associated with UASB/pond system.