Evaluation of the performance of a rural municipal wastewater treatment plant in Nacogdoches, East Texas (USA)

The performance of a rural wastewater treatment facility, Nacogdoches Wastewater Treatment Plant (NWWTP), in East Texas, USA, was assessed from January 2013 through June 2014. The elemental concentrations (Na, Mg, Ca, Ni, Pb, Mn, Cr, Mo, and Cu, Al, As, B, Ba, Ag, Cd, Fe, Hg, K, Se, Zn, Co, P, and S) were measured using inductively coupled plasma optical emission spectrometry. The anion concentrations (Br−, , F−, Cl−, and ) were measured by ion chromatography. In general, the NWWTP was found efficient in removal to ≥ 96% for metals. The removal efficiency for anions was in the range 33–100% (33% for Cl−, 39% for F−, 84% for , and >96% for ). The mean concentrations for Cl−, were in the range 0–172.5, 0.0–0.4, 0.0–18.6, 0.0–98.0, 0.0–0.9, and 4.0–89.4 mg/L, respectively. The concentrations for most metals and anion concentrations, with the exception of phosphates, were found below USEPA maximum contaminant limits.


Introduction
The analysis of wastewater for trace metal contamination is important for ensuring human and environmental health. The provision of water quality free of contaminants presents a challenge to water resources management. The removal of pollutants, including pathogens, heavy toxic metals, organic, and pharmaceutical pollutants, [1][2][3] is important in the assessment of toxicological studies. The emerging pollutants, which are detrimental to the environment and human health [4,5] are increasing, while the problems caused by traditional pollutants pose special challenges. In many countries various agencies have established criteria to regulate water quality pollutant levels in effluents; in the USA sewage treatment plants are regulated and monitored by the US Environmental Protection Agency (USEPA). [6] The wastewater influent emanates from various sources including kitchens, restrooms, food and pharmaceutical companies, and industrial sources. [7] The continued demand for the scarce water resource for use in industries, agriculture, and drinking necessitates efficient treatment processes, reuse, and engineering treatment designs. Thus, in the future treated wastewater will *Corresponding author. Email: onchokekk@sfasu.edu increasingly be reused for drinking as is the case in countries such as Namibia, Singapore, and Australia. [8,9] Such reuse helps to reduce water scarcity. [9] A number of studies have previously investigated the efficiency of wastewater treatment plants (WWTPs) to remove metal, inorganic, and organic constituents from WWTPs within the USA, the Europe, and the rest of the world. [10] Simonich et al. [10] found that the removal of sorptive, nonbiodegradable fragrance materials (FMs) were correlated with the removal of total suspended solids (TSS), while nonsorptive biodegradable FMs were correlated to 5-day biological oxygen demand (BOD). In Ji et al.'s study [10] the ecological risk of three WWTPs in Wuxi, China, was assessed via metal concentrations. The removal efficiencies of metals (Cu, Pb, Zn, Cd, Cr, As, and Hg) were found to be greater than 70% for Cu and Cr and while that for As, Cd and Hg were less than other heavy metals. [11] In the USA, many studies have documented the removal efficiencies of physicochemical parameters (including chemical oxygen demand (COD), TSS, and BOD), anions, metals, and inorganics from WWTPs. [12][13][14] In a number of studies, the risk assessment of these systems has been investigated especially the effects of the metal and non-metal concentrations upon organisms such as fish. [15] It has clearly been established that the wastewater treatment wetlands remove hydrophobic organic contaminants, and the wastewater contaminants decreased 40-99% between the inlet and effluent. [15] Investigations by Zeiner et al. determined the removal efficiencies of Cr, Mn, and Co from textile wastewater by the horizontal rotating tubular bioreactor and the maximum removal rates for chromium, manganese, and cobalt were 100%, 94%, and 69%, respectively. [16] Using microalgae (Chlorella vulgaris, Spirulina maxima), Chan et al. [17] showed that the removal of heavy metals from a wastewater treatment plant discharge and the removal efficiency 'in the untreated secondary effluent trial, were removed up to 81.7% of the copper, and for Zn was reduced by up to 94.1%'. Such studies attest to various methods of removing pollutants in wastewater. Furthermore, it shows the role WWTPs play in cleaning up wastewater before discharge to the environment.
Given the many studies on wastewater treatment, we were motivated to evaluate the performance of a rural WWTPs. The Nacogdoches Wastewater Treatment Plant (NWWTP) is owned by the Nacogdoches Municipality, East Texas, with a capacity of 12.88 Million Gallons per day (MGD), and an average pumping capacity of 3-4 MGD. The NWWTP serves a population of about 33,000 residents, and is an activated wastewater sludge treatment plant (WWTP) which receives an average influent load of about six MGD throughout the year. To date little research to evaluate the performance removal efficiency of metals and anions of NWWTP facility is available. Therefore, this study is focused on the analysis of anions and metal cations using ion chromatography (IC) and inductively coupled plasma optical emission spectrometry (ICP-OES), respectively. The general objectives of this study are to evaluate the performance of the NWWTP. Specifically, this investigation was geared towards determining: (i) the physicalchemical parameters (dissolved oxygen (DO), COD, carbonaceous biological oxygen demand (CBOD), TSS, pH, temperature, and electrical conductivity) and (ii) the influent/effluent anion and metal concentrations of the NWWTP. The results obtained were compared with the maximum contaminant limits (MCLs) USEPA guidelines. [18] This study provides data useful for planning of water resource management by municipal government.

Study site
Nacogdoches is the oldest city in Texas located at 31°36 32 N 94°39 3 W. The city has a total area of 25.3 square miles (66 km 2 ); of which 25.2 square miles (65 km 2 ) is land and 0.1 square miles (0.26 km 2 , 0.24%) is water. NWWTP receives wastewater from all sources within the city, which is treated in the following four stages: aeration chamber, clarifier, chlorination chamber, and the sulphur dioxide chamber. Figure 1 depicts the map and location of the treatment plant.

Sampling techniques
Samples were collected in triplicates, biweekly from each of the four treatment stages between January 2013-June 2014 (for physical chemical parameter, Table 1) and September 2013-June 2014 (for metal and anion concentrations see Tables 3-6). Polyethylene sample bottles were rinsed thoroughly with 10% HNO 3 acid and Millipore water. Bottles were flushed three times before filling.

On-site measurements
The Texas Commission on Environmental Quality (www.tceq.state.tx.us) requires the assessment of DO, COD, CBOD and TSS. The COD and BOD were determined by the HACH 2010 analyzer and the manometric method, [19,20] respectively, at the Nacogdoches Wastewater Environmental Laboratory from January 2013 through June 2014. The concentrations of various metals/anions examined are listed in Tables 1-6. The pH and temperature were recorded at the site. The electrical conductivity of the samples was measured before filtration and digestion. Samples for anion analysis were filtered through a 0.45 µm filter following Standard Methods. [20] For total concentration of elements, samples were digested using USEPA Method 2001.7 (1 mL of HNO 3 (1:1, v/v), and 0.5 mL of HCl (1:1 v/v)). [21]

Anion analysis
All chemicals and reagents used were of high-purity analytical grade. Hydrochloric acid, nitric acid (BDH chemicals, from VWR, USA), and 1000 mg/L or 10,000 ppm stock metal standards (BDH ® ARISTAR ® Multiple Element Sets, BDH chemicals, from VWR) were used for the analysis. All calibration standards were prepared using deionised water of 18.2 M cm resistivity. The anions were analysed using the Dionex ICS-2100 ion chromatograph (Thermo Fisher Scientific Inc., USA). A DionexIonPac AS19 analytical column (2 × 250 mm) thermostated at 30°C, guard column (IonPac AG19), and KOH eluent (ECG III KOH) were used with the following operating conditions: a sample flow rate of 0.25 mL/min, a suppressor column ASRS-2 mm, and pressure of 1200-2300 psi. pH, temperature, and electrical conductivity were measured with a

Data analysis, quality control, and quality assurance
To minimise background contamination during the procedure, all known sources of contamination including materials from the instruments and apparatus were removed. To validate the method, a National Institute of Science and Technology (NIST) standard reference material (SRM 1515, Apple leaves) [22] was analysed for 23 elements shown in Table 2. Comparative analytical results are in agreement with SRM 1515 to 82% (Cr), and ≥ 90% for all other elements was deemed useful analysis. The averages, standard deviations, and any significant differences were performed at the 95% confidence level. Table 1 shows the measured DO, TSS, CBOD, and COD values. The average DO were 1.1 ± 0.6 and 8.7 ± 0.8 mg/L, while average TSS were 225.2 ± 106.0 and 3.2 ± 0.8 mg/L  in the influent and effluent, respectively. Thus, an average DO increase of 634.5 ± 162.6 mg/L was found in the effluent vis-à-vis the influent. There was a 98.6% decrease in the TSS at the effluent before discharge (calculated from average effluent and influent values as ((TSS influent − TSS effluent )/(TSS ifluent )) × 100%). An average 95.1% reduction ( Table 1) in COD of the effluent/influent wastewater is evident. There is a marked decrease in CBOD (from 289.4 to 3.9 mg/L) and COD (from 622.8 to 30.3 mg/L) in the influent vis-à-vis the effluent. This is due to the presence of oxidisable organic matter at the influent stage. A demand for more oxygen for subsequent biodegradation and decay of vegetation leads to a higher consumption of oxygen in the aeration chamber. Table 3 lists the mean pH, temperature, and electrical conductivities of samples. The mean pH (between 6.5 and 6.8) of the aeration chamber and effluent during treatment falls within the USEPA standards in the range 6.5-8.5. Figure 2(a) shows that the pH of the aeration chamber and the sulphur dioxide contact chamber increased from 6.5 to 6.7 due to removal of hydrogen ions at each treatment stage. The mean temperatures were relatively constant in the range 17.3-23.7°C ( Figure S1 and Table 3) throughout the treatment stages. The electrical conductivity of samples was in the range 447.1-663.5 µS/cm (Table 3 and Figure 2(b)). Concomitant with higher TSS, highest electrical conductivity was observed at the aeration stage (Tables 1 and 3)

Analysis of major element concentrations in wastewater samples
Twenty-three elements were analysed in wastewater samples using ICP-OES. Reliability and precision of concentrations were gauged from measurement of a standard reference material (SRM 1515, Apple leaves). [22] For most elements (B, Ba, Ca, Cd, Co, Cu, Fe, Hg, K, Mg, Mn, Mo, Na, Ni, P, Pb, and Zn), laboratory concentration samples were comparable to known NIST SRM 1515 results to > 90% ( Table 2). The elemental concentrations of As, Cr, and Se in SRM 1515 were in agreement with 2921%, 82%, and 214% vis-à -vis known SRM 1515 values. The selenium concentrations from this analysis (0.1 ± 0.2) compares favourably with Xinbang et al.'s study [23] of < 0.1-< 0.2 ppm. Notably, Xinbang et al.'s study [22] showed higher recoveries for As (282-339%) and Ni (104 ± 5 to 146 ± 6%).     levels exceeding the USEPA standards ( Figure 3). This may be due to aluminium from soils, cans, and the use of alum in water treatment.

Concentrations of major elements
Average concentrations of magnesium and potassium in all samples were between 7.8-52.0 and 17.3-130.7 mg/L, respectively ( Table 4). The maximum contaminant concentrations for Mg and potassium WHO guidelines are 50 and 12 mg/L, respectively. [24] Figure 3 depicts the variation of Mg and K along the treatment stages. Magnesium concentrations in all samples were in agreement with WHO guidelines, [24] while potassium concentrations were higher. The high potassium levels may be due to the high solubility of potassium chloride, and its use in treatment devices such as water softeners. Iron and sodium levels varied from 0.0 to 72.2 and 40.8 to 123.1 mg/L (Table 4 and Figure 3). While sodium concentrations are below WHO limits of 200 mg Na/L, [24] Fe concentrations exceed USEPA limits of 0.3 Fe mg/L. Except for the aeration chamber, concentrations of iron amounts fall below USEPA standards.  Table 4). An increase in sulphur concentrations observed in the sulphur dioxide chamber is due to the addition of sulphur dioxide during treatment. Although sulphur levels were in compliance with the USEPA standard (250 mg /L), the total phosphorus concentration exceeded the USEPA standard (0.015 mg/L). This could be due to the use of detergents from homes, and fertiliser from farms and animal waste. [25] 3.

Analysis of trace metal concentrations
Trace elements such as copper, manganese, and zinc are needed in trace quantities by plants and animals. Table 5 shows concentrations of trace elements. Among trace elements, arsenic, chromium, cadmium, molybdenum, and cobalt were detectable in the aeration chamber between 0.00 and 0.09 mg/L ( Figure 4 and Table 5).
The concentrations of As, Cd, Co, Cr, and Mo meet USEPA standards. [24] In reference to these metals, the treated water may safely be discharged into the La Nana Creek.
The variation of barium, boron, and copper concentrations is depicted in Figure 4 and Table 5.  Table 5). This is lower than USEPA standard guidelines of 0.015 mg Pb/L and 0.05 mg Mn/L. Except for the aeration chamber, Pb and Mn concentrations were within USEPA limits ( Table 5). The concentration of Pb could be due to rusting of lead pipes into water, [26] whereas manganese may result from erosion and weathering of rocks. [27,28] Although the discharged water may be safely used for irrigation, continued long-term use of wastewater may pose health risks to plant and animal health.  Mercury and nickel mean concentrations (Figure 4 and Table 5) in the range 0.00-0.02 and 0.00-0.13 mg/L, respectively, were found below USEPA standards of 0.002 mg Hg /L and 1.0 mg Ni/L. The concentrations of mercury may be a result of run-off from landfills and croplands treated with pesticides. [29,30] The zinc concentrations (Figure 4) in the range 0.02-2.62 mg/L are below the USEPA standards (5 mg/L). Selenium and silver concentrations fell below detection limits 0.006 and 0.027 mg/L, respectively, in all samples. . Calibration curves show correlation coefficients (r 2 ) of 0.9892-1.000. Table 6 shows mean concentrations of chloride and fluoride in the range 0.0-172.5 and 0.0-1.3 mg/L, respectively. Figure 5 depicts the variation of these anions during treatment. The Cl − and F − concentrations were below USEPA MCLs of chloride 250 mg Cl − /L and 4.0 mg F − /L [18] at all stages of treatment. Figure 5 and Table 6 show average nitrates and nitrites' concentrations in the range < 0.0-18.6 and < 0.00-0.4 mg/L, respectively. The NO − 3 and NO − 2 concentrations were found below USEPA MCLs of 10 mg NO − 3 /L and 1 mg NO − 2 /L. [18] The mean phosphate and sulphate concentrations were in the range < 0.0-98.0 and 3.5-89.4 mg/L, respectively ( Figure 6 and Table 6). Whereas sulphate concentrations are below the maximum regulated 250 mg SO 2− 4 /L (USEPA), PO 3− 4 in all samples exceeded the USEPA limit of 0.2 mg PO 3− 4 /L. [18] 4

. Discussion
The current study shows the efficiency of NWWTP in removing anions, trace, and major metals and/or elements from influent wastewater. Ni, Pb, Mn, Cr, Mo, Cu, As, B, Ba, Ag, Cd, Hg, Se, Zn, and Co had low concentrations while Ag and Se concentrations were below the detection limits. The per cent removal of metals and non-metals from the effluent shows a decrease in trace elements in the order Zn > Mn > Ba > Ni > Cu > Pb > As > Co > Cd > Cr > Mo > B. The concentrations of major elements decreased in the order Na > S > K > Ca > Fe > Mg > P > Al. Although based on total metal concentrations rather than dissolved analytes, these studies are compared on a conservative basis for drinking/wastewater standards. The following discussion compares the current results to other studies from other WWTPs. The implications of the study are then discussed. Tables 3 and 4 show that NWWTP efficiently removes most toxic metals (As, Pb, Cr, Cd, and Hg) and anions in effluent wastewater. Table 5 shows the per cent removal at the effluent metal concentrations meets the USEPA guidelines, suggesting the safe discharging of treated water to the La Nana Creek. Results from this investigation compare favourably with various conventional treatment plants such as in Italy, Pakistan, Nigeria, South Africa, and Turkey. [31][32][33][34] Mansouri and Ebrahimpour [35] and Drozodova et al. [36] found Zn concentrations of 0.014-0.118 ppm and 0.12-0.33 mg/L, respectively, in treated wastewaters, while Shamuyarira and Gumbo [37] found effluent concentrations in the range 0.3-0.1 ppm in wastewater and river water samples. This is in agreement with the current study. The mean concentration for Pb (Table 5) was found to be lower than reported concentrations of 0.832 ppm by Muhammad et al. [32] In comparison with studies by Olujimi et al. [38] and Drozdova et al., [36] mercury had a higher mean concentration varying between 0.00 and 0.02 ppm. The mean concentration for Cd (0.0000-0.00468 ppm) was higher vis-à -vis than that published by Busetti et al. [31] (0.0003-0.0006 ppm). The Cr concentration in the range 0.00-0.09 ppm was lower than results published by Belhaj et al. [39] and Yayintas et al. [33] 2.27-6.97 and 0.01-0.23 ppm, respectively. The mean copper concentration in the range 0.00-1.32 ppm was found to be higher in comparison with published data of 0.00-0.09 ppm, [35] whereas iron mean concentrations (in the range 0.0-72.2 ppm) were higher than published by Muhammad [33] respectively. The average concentration of manganese in the range 0.04-1.74 ppm was higher in comparison with Ferrar et al. [41] in range 0.0-0.5 ppm. Aluminium, sodium, and sulphur had mean concentrations in ppm ranging from 0.0 to 23.3, 56.0 to 123.1 and 4.0 to 165.6 mg/L, respectively. Results published by Salihoglu [42] and Gondal and Hussain [43] were high for aluminium (0.43 and 13 ppm) and sodium (173 ppm) but low for sulphur (72 ppm). Molybdenum was measured at low concentrations with selenium and silver being below detection limits.

Metal/anion removal efficiencies from effluent after treatment
In comparison with a study in a surface-flow constructed wetland in Damyang, Korea, and a ternary tertiary treatment plant, [44,45] NO − 3 and SO 2− 4 concentrations in range 3.2-7.3 and 46.4-56.6 mg/L, respectively, were found to be higher. The fluoride and phosphate mean concentrations in the range 0.3-0.5 and 7.9-58.8 mg/L, respectively, are higher for both fluorides and phosphates vis-à -vis results published by Grzmil and Wronkowski [46] (8-18, and 5-10 mg/L, respectively). The mean chloride concentration (50.6-75.4 mg/L) was found to be lower when compared with a study by Iram et al. [47]This is attributable to the differences and sources of the wastewaters. The study by Iram et al. [47] was done in a large city, Rawalpindi, with a population of 1.5 million vis-à -vis Nacogdoches City with a population of ∼ 32,000. Figure 5 shows a decrease in the concentration of anions from the aeration chamber to the sulphur dioxide chamber; thus demonstrating the efficiency of the NWWTP treatment plant to remove anions. Although the aeration chamber sample recorded the highest concentration anions, most analyte anions concentrations fell below the maximum USEPA levels before discharge.

Implications of the study to water management
This study shows that the NWWTP performs excellently in the removal of most metals to ≥ 95%. This study enhances our understanding of the efficient wastewater treatment processes before discharge to river courses (in the current case to the La Nana Creek) and possible design of methods for recycling water for consumption in urban centres. In addition, anion concentrations such as levels of nitrates and phosphates were of 97% and 84% removal, respectively. These nutrients are cumulative in the environment and may encourage eutrophication in the water bodies where the wastewater is discharged. The NWWTP therefore performs well in the removal of these anions. It is recommended that future studies be focused on developing ways of removing some of these anions prior to discharge.
The chemical analyses undertaken in this study are useful for further designs of the toxicological studies; useful for comparative studies with other systems that may have complex treatment designs. In turn these can be used in future for systematic modelling analysis for best ways of wastewater treatment. Furthermore, the design and research on the toxicological studies on defined mixtures for comparison with the complex mixture are here pertinent. In conjunction with future toxicological studies, it will be necessary to assess the performance of this system in the removal of complex organic micropollutants and emerging pharmaceutical pollutants.

Conclusion
IC and ICP-OES were successfully used to assess the performance of the WWTPs. This study shows that the NWWTP is efficient in the removal of selected metals before discharge into the streams/environment. Silver and selenium were below detection limits in all samples. The concentrations of cations and anions were found above the maximum contaminant level at the aeration chamber. This is feasible since the influent wastewater is first received at the aeration stage. Although electrical conductivity did not change significantly, between the influent and effluent water in the range 470.7-663.5 μS/cm, the highest electrical conductivity was found at the aeration chamber (highest value of 663.5 μS/cm). Except for phosphates, potassium, mercury, and phosphorus most analyte concentrations were found within the required USEPA standard before discharge into the environment via La Nana Creek.