Wastewater Treatment Plant Samples Research Articles

Stepwise impact of urban wastewater treatment on the bacterial community structure, antibiotic contents, and prevalence of antimicrobial resistance

Bacteria, antibiotics, and antibiotic resistance determinants are key biological pollutants in aquatic systems, which may lead to bacterial infections or prevent the cure of bacterial infections. In this study, we investigated how the wastewater treatment processes in wastewater treatment plants (WWTPs) affect these pollutants. We found that the addition of oxygen, polyaluminum chloride (PAC), and polyacrylamide (PAM), as well as ultraviolet (UV) disinfection could significantly alter the bacterial communities in the water samples. An overall shift from Gram-negative bacteria to Gram-positive bacteria was observed throughout the wastewater treatment steps, but the overall bacterial biomass was not reduced in the WWTP samples. The antibiotic contents were reduced by the WWTP, but the size of the reduction and the step when antibiotic degradation occurred differed among antibiotics. Ciprofloxacin, sulfamethoxazole and erythromycin could be removed completely by the WWTP, whereas cephalexin could not. The removal of ciprofloxacin, cephalexin, and erythromycin occurred in the anaerobic digester, whereas the removal of sulfamethoxazole occurred after the addition of PAC and PAM, and UV disinfection. Antimicrobial resistance determinants were highly prevalent in all of the samples analyzed, except for those targeting vancomycin and colistin. However, wastewater treatment was ineffective at removing antimicrobial resistance determinants from wastewater. There were strong correlations between intI1, floR, sul1, and ermB, thereby suggesting the importance of integrons for the spread of these antimicrobial resistance genes. In general, this study comprised a stepwise analysis of the impact of WWTPs on three biological pollutants: bacteria, antibiotics, and antimicrobial resistance determinants, where our results suggest that the design of WWTPs needs to be improved to address the threats due to these pollutants.

Detection, Occurrence, and Fate of Fluorotelomer Alcohols in Municipal Wastewater Treatment Plants.

Fluorotelomer alcohols (FTOHs) are the most well-known precursors of perfluoroalkyl carboxylic acids (PFCAs), but limited information is available on their occurrence and fate in municipal wastewater treatment plants (WWTPs). The occurrence of FTOHs was investigated in influent, secondary effluent, and sludge of 12 municipal WWTPs in nine cities of China. FTOHs were detected in all WWTPs, and 8:2 FTOH was the predominant congener, with concentrations of 2.10-11.0 ng/L, 3.05-12.4 ng/L, and 0.36-1.91 ng/g dry weight in the influent, secondary effluent, and sludge, respectively. Relatively high proportions of long-chain FTOHs (C10-16) were mainly detected in sludge samples. The mass balance of FTOHs and PFCAs in one of the WWTPs with an anaerobic-anoxic-oxic process was further explored. The decrease of mass loads was observed for 4:2 FTOH (mass change percentage: 21 ± 3.3%), 8:2 FTOH (22 ± 1.5%), and 10:2 FTOH (29 ± 7.3%) through aerobic treatment, while the increase of mass loads was observed for 12 PFCAs from 18 ± 16% (perfluorononanoic acid) to 165 ± 15% (perfluorobutyric acid)), suggesting the potential biotransformation of FTOHs to PFCAs in the aerobic unit. This work provides the first report on the occurrence of FTOHs in sludge samples of municipal WWTPs and their mass balance and highlights a new emission route to environment via WWTPs.

Assessing the efficiency of wastewater treatment plants: A double-bootstrap approach

Benchmarking the performance of wastewater treatment plants (WWTPs) is essential for promoting their long-term sustainability. Recent research has applied data envelopment analysis (DEA) models to evaluate the efficiency of WWTPs providing a synthetic index of their performance. However, the traditional DEA is a deterministic method; therefore, regression analysis cannot be used to explore the external factors influencing efficiency scores. To overcome this limitation, in this study, a double bootstrap DEA model was used for the first time to compute the efficiency scores for a sample of WWTPs. The confidence intervals for efficiency scores were estimated for each facility. Results evidenced that in average WWTPs assessed could save 30% of their costs. Moreover, at the second stage of the analysis, the volume of the treated wastewater and energy consumed were found to influence the efficiency of WWTPs. This study shows the importance of using a reliable and robust method to benchmark the WWTP performance. Information about factors affecting WWTPs efficiency is essential to develop policies for promoting sustainable wastewater treatment.

Photolytic degradation of the β-blocker nebivolol in aqueous solution

Nebivolol (NEB) is one of the top-sold prescription drugs belonging to the third generation of beta-blockers. However, so far, occurrence data in the environment are lacking. Within this study NEB has been found for the first time in effluent samples of wastewater treatment plants in Germany with an average concentration of 13 ng L−1.Its photodegradation behavior in the environment and in technical processes is largely unknown. To fill this gap, three different UV treatment procedures (UV-C at 254 nm, UV-B at 312 nm and UV-A at 365 nm) were investigated in three different matrices: pure water, pure water in presence of the hydroxyl radical (OH) scavenger tert.-butanol and real wastewater. No elimination was observed during UV-A treatment. In contrast, NEB degradation during UV-B and UV-C treatment followed pseudo first order reaction kinetics, with highest removal rate during UV-C treatment in pure water (k = 7.8 × 10−4 s−1). The rate constant for UV-C irradiation decreased to 2.9 × 10−4 s−1 in the presence of the OH scavenger and in the presence of the wastewater matrix. The rate constant for the UV-B lamp was 4.4 × 10−4 s−1, Three transformation products were identified after UV-B and UV-C photolytic degradation using high resolution mass spectrometry. The main photoreaction is the substitution of the fluorine atoms of NEB by hydroxyl groups. A photolytic cleavage of the CF bond can be excluded as the high bond dissociation energy of aromatic CF bonds (525 kJ mol−1), exceeds the energy of electromagnetic radiation applied in the present study (≥254 nm, i.e., max. 471 kJ E−1). The quantum yields for NEB degradation for the UV-C lamp achieved in pure water, the OH scavenged system and wastewater matrix were Φdeg = 0.53, 0.19 and 0.22, respectively. For UV-B Φdeg was 0.023 ± 0.003, noticeable differences in quantum yield were not found. The photooxidation involves reactive oxygen species such as superoxide and singlet oxygen. These oxidative species may be formed upon reaction of photo-excited NEB with oxygen.

An evaluation of operation and maintenance costs of wastewater treatment plants: Gebze wastewater treatment plant sample

An evaluation of operation and maintenance costs of wastewater treatment plants: Gebze wastewater treatment plant sample

Anthropogenic rare earth elements and their spatial distributions in the Han River, South Korea

Rare earth elements (REE) consist of lanthanides (from La to Lu), together with yttrium and scandium, in which anthropogenic REE, such as gadolinium (Gd), lanthanum (La), and samarium (Sm), has emerged as micro-contaminants in natural waters in highly developed countries. Here, we collected water samples in the Han River (HR) and its tributaries flowing through Seoul Capital Area, the world's second largest metropolitan area in order to examine how and to what extent anthropogenic REE anomalies may occur. Water samples show higher light REE concentrations than heavy REE concentrations, while wastewater treatment plant (WWTP) samples display much higher heavy REE concentrations due to high Gd concentration. The PAAS-normalized REE patterns indicate that WWTP samples display the pronounced positive Gd anomalies, in which anthropogenic Gd from magnetic resonance imaging (MRI) diagnostic system occurs as a form of Gd complexation with either Cl− or SO42−. Due to the WWTP, both the HR and tributaries show also positive Gd anomalies and the anthropogenic Gd concentrations increase as a function of the distance from the Paldang dam. This result indicates a positive correlation between populaton, number of MRI instruments, and positive Gd anomaly. Similarly, positive La and Sm anomalies exist in the HR, indicating that the HR is also affected by their point sources. Based on the discharge rate and anthropogenic REE concentrations, their fluxes are estimated to be 952 ± 319 kg/yr, suggesting that this amount of fluxes could disturb REE distribution in the Yellow Sea, and pose harmful effects on aquatic ecosystems.

New PCR primers targeting hydrazine synthase and cytochrome c biogenesis proteins in anammox bacteria.

PCR primers targeting genes encoding the two proteins of anammox bacteria, hydrazine synthase and cytochrome c biogenesis protein, were designed and tested in this study. Three different ecotypes of samples, namely ocean sediments, coastal wetland sediments, and wastewater treatment plant (WWTP) samples, were used to assess the primer efficiency and the community structures of anammox bacteria retrieved by 16S ribosomal RNA (rRNA) and the functional genes. Abundances of hzsB gene of anammox bacteria in South China Sea (SCS) samples were significantly correlated with 16S rRNA gene by qPCR method. And hzsB and hzsC gene primer pair hzsB364f-hzsB640r and hzsC745f-hzsC862r in combination with anammox bacterial 16S rRNA gene primers were recommended for quantifying anammox bacteria. Congruent with 16S rRNA gene-based community study, functional gene hzsB could also delineate the coastal-ocean distributing pattern, and seawater depth was positively associated with the diversity and abundance of anammox bacteria from shallow- to deep-sea. Both hzsC and ccsA genes could differentiate marine samples between deep and shallow groups of the Scalindua sp. clades. As for WWTP samples, non-Scalindua anammox bacteria reflected by hzsB, hzsC, ccsA, and ccsB gene-based libraries showed a similar distribution pattern with that by 16S rRNA gene. NH4+ and NH4+/Σ(NO3- + NO2-) positively correlated with anammox bacteria gene diversity, but organic matter contents correlated negatively with anammox bacteria gene diversity in SCS. Salinity was positively associated with diversity indices of hzsC and ccsB gene-harboring anammox bacteria communities and could potentially differentiate the distribution patterns between shallow- and deep-sea sediment samples. SCS surface sediments harbored considerably diverse community of Scalindua. A new Mai Po clade representing coastal estuary wetland anammox bacteria group based on 16S rRNA gene phylogeny is proposed. Existence of anammox bacteria within wider coverage of genera in Mai Po wetland indicates this unique niche is very complex, and species of anammox bacteria are niche-specific with different physiological properties towards substrates competing and chemical tolerance capability.

Determination of antipsychotic drugs in hospital and wastewater treatment plant samples by gas chromatography/tandem mass spectrometry.

The development and performance evaluation of a method for the simultaneous determination of six antipsychotic drugs in hospital effluents and wastewater treatment plants (WWTP) samples are herein presented. The method involves an off-line mixed mode (reversed-phase and strong cation exchange) solid phase extraction (SPE) with gas chromatography (GC) coupled to tandem mass spectrometry (MS/MS). The present methodology was validated following internationally accepted criteria, and the studied parameters included selectivity, linearity, limits of detection (LOD) and quantitation (LLOQ), instrumental limits, precision and accuracy, stability and recovery. The procedure was linear for concentrations ranging from 0.1 to 10μg/L (0.02 to 2μg/L for haloperidol), with determination coefficients higher than 0.99 for all analytes. Intra- and inter-day precision was lower than 15% for all analytes at the studied concentrations, while accuracy remained between a ±15% interval. Recoveries ranged from 31% to 83%. Low LODs were achieved, between 2 and 10ng/L, allowing a reliable identification of all analytes at trace levels, using only 50mL as sample volume. All studied parameters complied with the defined criteria and the method was successfully applied to gather preliminary results of the determination of antipsychotics on hospital effluents and on influent and effluent of WWTPs, opening perspectives for the study of their fate in the aquatic environment.

Steroids contents in waters of wastewater purification plants: determination with partial-filling micellar electrokinetic capillary chromatography and UV detection

ABSTRACTPartial-filling micellar electrokinetic capillary chromatography (PF-MEKC) with UV detection was applied for determination of human-based steroids in water samples of Finnish wastewater treatment plants. The samples were purified with solid-phase extraction (SPE) on octadecyl substituted polymer sorbents obtaining analyte enrichment of 20,000-fold. The steroids studied were androgens, estrogens, and progesterone. Three of the steroids could be quantified with the PF-MEKC method. The detection and quantification limits were 0.05–1.06 μg/mL and 0.15–3.2 μg/mL, meaning in the SPE concentrates as 2.5–53 pg/L and 7.5–160 pg/L, respectively. In the influent waters, the total amount of testosterone glucuronide, androstenedione, and progesterone was up to 350 ng/L. In effluent water samples the total steroid quantity was maximum at 320 ng/L. Remarkably high quantity of androstenedione was quantified in both influent and effluent water samples. The cleanest effluent waters were produced in Western Finland. Correspondingly, the highest quantities were located near the largest lake and river areas in South-Eastern Finland. The concentration variation in effluent waters was explained with differences in the purification materials and processes at the plants and with steroid adsorption on soil and organic material suspended into water.

Eco-efficiency assessment of wastewater treatment plants using a weighted Russell directional distance model

Improving the performance of wastewater treatment plants is essential to ensuring their long-term sustainability. Most of the previous studies on this topic have assessed the techno-economic efficiency of wastewater treatment plants and ignored the emission of greenhouse gases. For the first time, the weighted Russell directional distance model was applied to estimate the eco-efficiency of a sample of real wastewater treatment plants. Moreover, this approach allowed an inefficiency score to be obtained for each variable (cost factors, pollutant removal and greenhouse gases) involved in the model. Subsequently, a second stage of analysis was applied to identify factors influencing the previously computed inefficiency scores. The results illustrated that approximately half of the facilities assessed had significant room for improvement in their eco-efficiency. Moreover, the characteristics of over- and undersizing of the plants significantly affected their eco-efficiency. The methodology and results of this study are of great interest, not only for wastewater treatment plant managers and water authorities but also for citizens because there have been growing concerns regarding minimizing the ecological footprint in the urban water cycle.

Detection of pathogenic Campylobacter, E. coli O157:H7 and Salmonella spp. in wastewater by PCR assay.

The aim of this study was the evaluation of the occurrence of pathogenic Campylobacter, Escherichia coli O157:H7, E. coli virulence genes and Salmonella spp. in different wastewater treatment plants (WWTPs) using a method based on an enrichment step and PCR. This method was sensitive enough to detect low levels (∼2CFU100ml(-1) of raw sewage) of all the investigated pathogens. In the WWTP samples, E. coli O157:H7 DNA and the eae gene were never found, but 33% of influents and effluents exhibited amplicons corresponding to Shiga-like toxin I. Twenty-five percent of the influent and 8% of the effluent exhibited the presence of Shiga-like toxin II. Campylobacter jejuni and C. coli DNA were identified in 50 and 25% of the influents and in 8 and 25% of the effluents, respectively. Salmonella spp. DNA was present in all the samples. Considering the results obtained, the method tested here offers a reliable and expeditious tool for evaluating the efficiency of the effluent treatment in order to mitigate contamination risk. Influent contamination by Salmonella spp. and Campylobacter spp. provides indirect information about their circulation; moreover, their presence in effluents underlines the role of WWTPs in the contamination of the receiving surface waters, which affects public health directly or indirectly.

Efficiency assessment of wastewater treatment plants: A data envelopment analysis approach integrating technical, economic, and environmental issues

The assessment of the efficiency of wastewater treatment plants (WWTPs) is essential to compare their performance and consequently to identify the best operational practices that can contribute to the reduction of operational costs. Previous studies have evaluated the efficiency of WWTPs using conventional data envelopment analysis (DEA) models. Most of these studies have considered the operational costs of the WWTPs as inputs, while the pollutants removed from wastewater are treated as outputs. However, they have ignored the fact that each pollutant removed by a WWTP involves a different environmental impact. To overcome this limitation, this paper evaluates for the first time the efficiency of a sample of WWTPs by applying the weighted slacks-based measure model. It is a non-radial DEA model which allows assigning weights to the inputs and outputs according their importance. Thus, the assessment carried out integrates environmental issues with the traditional “techno-economic” efficiency assessment of WWTPs. Moreover, the potential economic savings for each cost item have been quantified at a plant level. It is illustrated that the WWTPs analyzed have significant room to save staff and energy costs. Several managerial implications to help WWTPs' operators make informed decisions were drawn from the methodology and empirical application carried out.

Sulphate production by Paracoccus pantotrophus ATCC 35512 from different sulphur substrates: sodium thiosulphate, sulphite and sulphide

ABSTRACTOne of the problems in waste water treatment plants (WWTPs) is the increase in emissions of hydrogen sulphide (H2S), which can cause damage to the health of human populations and ecosystems. To control emissions of this gas, sulphur-oxidizing bacteria can be used to convert H2S to sulphate. In this work, sulphate detection was performed by spectrophotometry, ion chromatography and atomic absorption spectrometry, using Paracoccus pantotrophus ATCC 35512 as a reference strain growing in an inorganic broth supplemented with sodium thiosulphate (Na2S2O3·5H2O), sodium sulphide (Na2S) or sodium sulphite (Na2SO3), separately. The strain was metabolically competent in sulphate production. However, it was only possible to observe significant differences in sulphate production compared to abiotic control when the inorganic medium was supplemented with sodium thiosulphate. The three methods for sulphate detection showed similar patterns, although the chromatographic method was the most sensitive for this study. This strain can be used as a reference for sulphate production in studies with sulphur-oxidizing bacteria originating from environmental samples of WWTPs.

Environmental friendly method for urban wastewater monitoring of micropollutants defined in the Directive 2013/39/EU and Decision 2015/495/EU

The fate and removal of organic micropollutants in the environment is a demanding issue evidenced by the recent European policy. This work presents an analytical method for the trace quantification of 37 micropollutants including Priority Substances (Directive 2013/39/EU), substances of the recent watch list (Decision 2015/495/EU) and contaminants of emerging concern: pesticides, multi-class pharmaceuticals and a metabolite, estrogens and one industrial compound. The analytical method was based on solid phase extraction (SPE) followed by ultra-high performance liquid chromatography with tandem mass spectrometry (UHPLC–MS/MS), using eco-friendly solvents (ethanol and ultrapure water) both in the SPE and UHPLC, according to green analytical chemistry principles. Briefly, Oasis® HLB cartridges were used to preconcentrate 100mL of water samples and the reconstituted extracts were analyzed with a Kinetex™ column under reversed mode, the linear ranges extended to 200ngL−1 (R2>0.99) for all the analytes. The method detection limits were between 0.04 and 2.26ngL−1 and the method quantification limits were between 0.13 and 6.85ngL−1. The identity of the compounds was confirmed using two MS/MS transitions and its ion ratios, according to Decision 2002/657/EC. The validated method was applied to wastewater treatment plant samples, assessing the concentration of micropollutants after secondary biological and tertiary UV treatments. Lab-scale photolysis and ozonation experiments were also performed with the secondary effluents, with ozonation showing the best performance for the removal of most of the determined micropollutants.

Ubiquitous Detection of Artificial Sweeteners and Iodinated X-ray Contrast Media in Aquatic Environmental and Wastewater Treatment Plant Samples from Vietnam, The Philippines, and Myanmar.

Water samples from Vietnam, The Philippines, and Myanmar were analyzed for artificial sweeteners (ASs) and iodinated X-ray contrast media (ICMs). High concentrations (low micrograms per liter) of ASs, including aspartame, saccharin, and sucralose, were found in wastewater treatment plant (WWTP) influents from Vietnam. Three ICMs, iohexol, iopamidol, and iopromide were detected in Vietnamese WWTP influents and effluents, suggesting that these ICMs are frequently used in Vietnam. ASs and ICMs were found in river water from downtown Hanoi at concentrations comparable to or lower than the concentrations in WWTP influents. The ASs and ICMs concentrations in WWTP influents and adjacent surface water significantly correlated (r (2)=0.99, p<0.001), suggesting that household wastewater is discharged directly into rivers in Vietnam. Acesulfame was frequently detected in northern Vietnamese groundwater, but the concentrations varied spatially by one order of magnitude even though the sampling points were very close together. This implies that poorly performing domestic septic tanks sporadically leak household wastewater into groundwater. High acesulfame, cyclamate, saccharin, and sucralose concentrations were found in surface water from Manila, The Philippines. The sucralose concentrations were one order of magnitude higher in the Manila samples than in the Vietnamese samples, indicating that more sucralose is used in The Philippines than in Vietnam. Acesulfame and cyclamate were found in surface water from Pathein (rural) and Yangon (urban) in Myanmar, but no ICMs were found in the samples. The ASs concentrations were two-three orders of magnitude lower in the samples from Myanmar than in the samples from Vietnam and The Philippines, suggesting that different amounts of ASs are used in these countries. We believe this is the first report of persistent ASs and ICMs having ubiquitous distributions in economically emerging South Asian countries.

Estrogenic activity of wastewater, bottled waters and tap water in Finland as assessed by a yeast bio-reporter assay.

Environmental pollutants appearing in wastewater, bottled mineral water, tap water, and bottled drinking water are potential, but yet poorly characterized, sources of human exposure to endocrine disrupting chemicals globally. Here, we investigated the current situation in the most densely populated region in Finland. Influent and effluent bi-monthly samples from a major wastewater treatment plant in Helsinki were obtained over a preceding 2-year period at two time-points (in 2011 and 2014). Equivalent samples from a household water purification plant (located in the same region) were also analyzed, together with various brands of bottled still and mineral water as well as tap water from residential buildings. Samples were obtained in one liter sterile containers, extracted by solid-phase extraction method, and their estrogenic potential determined by a yeast bioluminescent assay. The estrogenic activities of influent samples from the wastewater treatment plant in Helsinki were generally low (from less than limit of detection to 0.7 ng/L estrogen equivalent quantities (EEQ)), except in March and August 2011, when relatively high levels (14.0 and 7.8 ng/L EEQ, respectively) were obtained. Meanwhile, no estrogenic activity was recorded in any of the treated effluent samples from the wastewater treatment plant, influent and effluent samples from the drinking water plant, as well as tap water, bottled still, and mineral waters. These findings indicate that the purification method applied in Helsinki wastewater treatment plant, activated sludge with mechanical, chemical and biological purification steps, is effective in reducing estrogenic activity, and that tap or bottled waters are not a significant source of these compounds to the population in this region.

Analysis of Samples from Wastewater Treatment Plant and Receiving Waters Using EEM Fluorescence Spectroscopy

The samples from several treatment units in wastewater treatment plant (WWTP) and different sample sites of receiving waters were characterized using excitation-emission matrix (EEM) fluorescence spectroscopy. After analyzed by parallel factor analysis (PARAFAC) method, the EEM fluorescence spectra of the principal components and fluorescence intensity scores matrix were acquired. Results showed that protein-like and fulvic-like substances were the main components of the samples from the WWTP and the receiving water body. The intensity of protein-like fluorescence in the influent samples was rather strong, and those of the following samples decreased significantly. The protein-like fluorescence intensity of the samples was relatively weak from the upstream of receiving water body, while those of the samples increased significantly from the water body near the urban area. The protein-like fluorescence intensity of upstream samples of the WWTP disposal outlet was stronger than that of downstream in the receiving waters. The fluorescence intensity scores of protein-like substance could be correlated with the COD concentration of the samples and the correlation curves were established. The correlation coefficient of the WWTP samples was 0. 930 and that of receiving water body samples was 0.913. The protein-like fluorescence could be used to evaluate the organic pollution of the samples. This study will provide a new method to investigate the operation status of WWTP and corresponding effect on the receiving water body.

Optimization of a Solid-Phase Extraction Method for the Determination of 12 Aminoglycosides in Water Samples Using LC–ESI–MS/MS

A new and simple method has been developed for determination and quantification of 12 aminoglycosides in environmental water samples. The method is based on solid-phase extraction (SPE) followed by liquid chromatography coupled with tandem mass spectrometry (LC–ESI–MS/MS) for analysis. After optimization of LC–ESI–MS/MS conditions, two SPE cartridges, reversed-phase C18 with pentafluoropropionic acid (PFPA) and polymeric mixed mode cation exchange cartridges, have been tested to determine aminoglycoside residues in water. Parameters affecting the extraction efficiency for the two cartridges such as eluent type, percolation volume and percentage of ion pair PFPA were optimized. Extraction on C18 cartridges with PFPA reagent showed a better result in terms of recovery (65–115 %) and relative standard deviation (0.5–15 %) relative to strong cation exchange. This method was validated and their limits of quantification were in the range 5–50 ng L−1. The method was applied to evaluate the occurrence of these compounds in influent and effluent wastewater treatment plant and hospital water samples in Lebanon. Gentamycin was found in hospital wastewater samples at a concentration of 30 ng L−1.

Measuring the CO 2 shadow price for wastewater treatment: A directional distance function approach

Abstract The estimation of the value of carbon emissions has become a major research and policy topic since the establishment of the Kyoto Protocol. The shadow price of CO2 provides information about the marginal abatement cost of this pollutant. It is an essential element in guiding environmental policy issues, since the CO2 shadow price can be used when fixing carbon tax rates, in environmental cost-benefit analysis and in ascertaining an initial market price for a trading system. The water industry could play an important role in the reduction of greenhouse gas (GHG) emissions. This paper estimates the shadow price of CO2 for a sample of wastewater treatment plants (WWTPs), using a parametric quadratic directional distance function. Following this, in a sensitivity analysis, the paper evaluates the impact of different settings of directional vectors on the shadow prices. Applying the Mann–Whitney and Kruskal–Wallis non-parametric tests, factors affecting CO2 prices are investigated. The variation of CO2 shadow prices across the WWTPs evaluated argues in favour of a market-based approach to CO2 mitigation as opposed to command-and-control regulation. The paper argues that the estimation of the shadow price of CO2 for non-power enterprises can provide incentives for reducing GHG emissions.

Productivity growth of wastewater treatment plants – accounting for environmental impacts: a Malmquist-Luenberger index approach

As the number of wastewater treatment plants (WWTPs) has increased, the economic considerations associated with their management have become more relevant. Measuring the productivity of WWTPs allows the best practice to be identified and resource use to be optimized. Previous studies assessing the productivity change of WWTPs have ignored undesirable outputs; thus, wastewater treatment was considered to be free of environmental impacts. To overcome this limitation and for the first time, we assessed the productivity growth of a sample of Spanish WWTPs, while also accounting for the emission of greenhouse gases and sludge production. The Malmquist-Luenberger productivity index (MLPI) and its components, the efficiency and technological changes, were estimated. To explore the role of environmental impacts in the measurement of the productivity growth of WWTPs, the MLPI was contrasted with the conventional measurement of productivity change: namely, the Malmquist productivity index (MPI). The omission of undesirable outputs led to an underestimation of the productivity growth, with statistically significant differences between the MLPI and MPI results. The comparison of the MPI and MLPI illustrated that WWTP operators and water agencies have internalized some good environmental practices into their production process costs.