sustainability

Article Evaluation of Hydraulic Retention Time on Treatment of Processing Wastewater (CPWW) in EGSB Bioreactor

Abumalé Cruz-Salomón 1,* ID , Edna Ríos-Valdovinos 1,*, Francisco Pola-Albores 2, Selene Lagunas-Rivera 3 ID , Rocío Meza-Gordillo 4 and Víctor M. Ruíz-Valdiviezo 4

1 Faculty of Engineering, University of Science and Arts of Chiapas, Libramiento Norte Poniente No. 1150, Lajas Maciel, Tuxtla Gutiérrez C.P. 29039, Mexico 2 Center for Research and Technological Development in Renewal Energies, University of Science and Arts of Chiapas, Libramiento Norte Poniente 1150, Lajas Maciel, Tuxtla Gutiérrez C.P. 29039, Mexico; [email protected] 3 Professor CONACYT, Department of Chemical and Biochemistry Engineering, National Institute of Technology of Mexico-Tuxtla Gutiérrez Institute of Technology, Carretera Panamericana Km 1080, Tuxtla Gutiérrez C.P. 29050, Mexico; [email protected] 4 Department of Chemical and Biochemistry Engineering, National Institute of Technology of Mexico-Tuxtla Gutiérrez Institute of Technology, Carretera Panamericana Km 1080, Tuxtla Gutiérrez C.P. 29050, Mexico; [email protected] (R.M.-G.); [email protected] (V.M.R.-V.) * Correspondence: [email protected] (A.C.-S.); [email protected] (E.R.-V.); Tel.: +52-961-617-0440 (ext. 4272) (A.C.-S. & E.R.-V.)

Received: 11 December 2017; Accepted: 23 December 2017; Published: 30 December 2017

Abstract: The coffee processing agro-industry generates large quantities of wastewater requiring systematic treatment prior to disposal. For this reason, the aim of this study was to evaluate the hydraulic retention times (HRT) in treatment of coffee processing wastewater (CPWW) using a laboratory scale Expanded Granular Sludge Bed (EGSB) bioreactor at different HRT (3 to 9 days). The EGSB was evaluated in mesophilic condition (26 ± 2 ◦C) with an average pH of 7.5 ± 0.2 to determine the chemical oxygen demand (COD) removal efficiency. According to the results, the COD removal efficiency increases from 94 to 98% when the HRT increase from 3 to 9 days; the α factor remained stable (0.98) throughout the evolution of the bioreactor. The HRT´s between 7–9 days generated effluents capable to be dischargeable into bodies with a permitted COD concentration according to World Health Organization (WHO) and Official Mexican Environmental Regulations permissible limits. Results evidenced that the HRT of 9 days was the one that greater COD removal generated, so the EGSB bioreactor can be a sustainable alternative to solve the environmental problems, compared to other conventional methods to CPWW treatment.

Keywords: anaerobic EGSB reactor; coffee processing wastewater; HRT; chemical oxygen demand; wastewater treatment

1. Introduction Coffee, which belongs to the genus of rubiaceae family, is one of the most popular beverages consumed throughout the world. Mexico ranks ninth in the world in with 2.4 × 103 Ton year−1. Both varieties of coffee Arabica () and Robusta () are cultivated mainly in the hilly tracts of the Sierra Madre del Sur and Sierra Madre Oriental mountain, being the state of Chiapas is one of the major and famous coffee producing region within Mexico with a contribution of 36.9% (74,538 Ton year−1)[1,2].

Sustainability 2018, 10, 83; doi:10.3390/su10010083 www.mdpi.com/journal/sustainability Sustainability 2018, 10, 83 2 of 11 Sustainability 2018, 10, 83 2 of 11

ThisThis regionregion has a number of of small small-scale-scale coffee coffee processing processing industries industries situated situated along along the the banks banks of ofrivers rivers and/or and/or streams streams to reduce to reduce production production costs. costs. The coff Theee coffeeis processed is processed either by either wet byor dry wet method or dry method(Figure (Figure 1). Wet1 ). method Wet method of processing of processing results results in a coffee in a coffee of superior of superior quality quality compared compared to the to the dry drymethod. method. Presently Presently in Mexico, in Mexico, small small-scale-scale industries industries use use the the wet wet method method to process to process around around 86– 86–90%90% of ofcoffee coffee [3]. [3 ].

FigureFigure 1.1. General diagramdiagram ofof thethe coffeecoffee processing.processing. ** CoffeeCoffee processingprocessing wastewater.wastewater.

TheThe processprocess byby wetwet methodmethod beginsbegins byby bringingbringing thethe coffeecoffee cherriescherries toto aa reservoir.reservoir. FromFrom herehere onon thethe cherriescherries areare transportedtransported byby gravitygravity toto thethe de-pulpingde-pulping machines.machines. InIn Chiapas,Chiapas, thethe transporttransport toto thethe de-pulpingde-pulping machinesmachines isis broughtbrought forthforth byby waterwater andand gravity.gravity. InIn thethe de-pulpingde-pulping machines,machines, thethe cherriescherries areare de-pulpedde-pulped (process in in which which the the pulp pulp and and the the outer outer skin skin are areremoved) removed) and andselected selected based based on their on theirsize. size.Nevertheless, Nevertheless, a slimy a slimy layer layer around around the thecoffee bean with with a avarying varying thickness thickness of of 0.5 0.5 to 2 mmmm remains,remains, whichwhich willwill bebe laterlater eliminatedeliminated inin thethe fermentation.fermentation. InIn the the fermentation fermentation process, process, the grains the grains remain remain between between 12 and 36 12 h, and depending 36 h, on depending the temperature, on the altitude,temperature, the thickness altitude,of the the thickness mucilage of layer the and muc theilage concentration layer and the of enzymes. concentration The mucilage of enzymes. layer The is fermentedmucilage layer through is fermented a combination through of microbiala combination activity. of microbial activity. TheThe processprocess isis finishedfinished afterafter thethe grainsgrains areare washedwashed toto eliminateeliminate thethe lastlast remnantsremnants ofof decomposeddecomposed mucilage.mucilage. Afterwards,Afterwards, thethe grainsgrains inin pergaminopergamino statestate areare dried.dried. TheThe wetwet methodmethod toto coffeecoffee processingprocessing producesproduces aa veryvery highhigh loadload ofof pollutedpolluted wastewaterwastewater (CPWW(CPWW isis obtained obtained from from de-pulping, de-pulping, generated generated in the in pulp the removal pulp removal process process of coffee of cherries, coffee which cherries, represent which morerepresent than more 50% than of the 50% water of the used water in used the wholein the whole process process [4]; and [4]; ofand the of washingthe washing process process which which is generatedis generated after after the the fermentation fermentation of of the the coffee coffee bean bean [5 [5]);]); this this is is the the reason reason whywhy coffeecoffee processingprocessing industriesindustries areare oneone ofof thethe significantsignificant consumersconsumers ofof water.water. TheyThey produceproduce largelarge amountsamounts ofof wastewaterwastewater (between(between 8–10 8–10 L ofL CPWWof CPWW are generated are generated for each for kilogram each kilogram of coffee of produced). coffee produced). The CPWW The generated CPWW fromgenerated this agro-industry from this agro presents-industry an acid presents pH (3–5); an acid additionally, pH (3–5 it); contains additionally, high concentrations it contains high of organicconcentrations matter (1185–32,459of organic matter mg L −(11851 as– COD,32,459 3450–12,100 mg L−1 as COD, mg L −34501 as– biochemical12,100 mg L− oxygen1 as biochemical demand −1 −1 −1 −1 (BODoxygen5), 7000–10,900demand (BOD mg5), L 7000as–10 total,900 suspended mg L as total solids suspended (TSS)), nutrients solids ( (4.4–70TSS)), nutrients mg L as (4.4 phosphorus,–70 mg L 37–279as phosphorus, mg L−1 as 37 nitrogen)–279 mg and L− suspended1 as nitrogen matter) and [6 – suspended8]. In addition matter to these [6–8]. compounds, In addition the to CPWW these containscompounds, other the chemicals CPWW contains such as phenolsother chemicals (10–2904.7 such mg as L phenols−1) and (10 tannins–2904.7 (0.05–23 mg L−1 mg) and L− tannins1) that are(0.05 converted–23 mg L− into1) that recalcitrant are converted compounds into recalcitrant [9–11]. Thus,compounds considering [9–11]. the Thus, volume considering generated the andvolume the generated and the pollutants released through the wastewater, the coffee processing agro-industry represents one of the main contributors to the severe water problems (as presented in a Sustainability 2018, 10, 83 3 of 11

Sustainability 2018, 10, 83 3 of 11 pollutants released through the wastewater, the coffee processing agro-industry represents one of the conceptualmain contributors model toFigure the severe 2). Despite water pollutionthe severe problems pollution (as problems, presented init was a conceptual found that model none Figure of the2). coffeeDespite processing the severe agro pollution-industries problems, (small it was-scale) found have that any none effl ofuent the treatmentcoffee processing plants. agro-industries They directly discharge(small-scale) untreated have any colored effluent and treatment acidic effluent plants. into They the directly nearby discharge water bodies, untreated streams colored and open and acidic land [12]effluent. into the nearby water bodies, streams and open land [12]. Furthermore,Furthermore, in the the literature, literature, it it has has been been found found that that the the CPWW CPWW is is very very harmful harmful for for surrounding surrounding waterwater bodies bodies and and aquatic aquatic life life if if discharged discharged directly directly into the surface surface waters [13 [13–15]15] as as well well as as in in human human healthhealth (causing (causing many many severe severe health health problems problems,, such such as as dizziness, dizziness, eye, eye, ear ear and and skin skin irritation, irritation, stomach stomach pain,pain, nausea nausea and and breathing problems) of all the residents of nearbynearby areas [[16].16].

FigureFigure 2.2. AA conceptual conceptual model model of the of environmentalthe environmental pollution pollution impact impact causing causing by the disposing by the disposing untreated untreatedcoffee processing coffee processing wastewater wastewater to the nearby to the rivers nearby adapted rivers from adapted Elliott from & de Elliott Jonge & [ 17de]. Jonge [17].

Several anaerobic processes have been developed in order to treat CPWW. The diagram Several anaerobic processes have been developed in order to treat CPWW. The diagram presented presented below (Figure 3) gives an indication of the classification of methods applied. Even when below (Figure3) gives an indication of the classification of methods applied. Even when the EGSB the EGSB bioreactors do not appear in the diagram, many researchers, including this research group, bioreactors do not appear in the diagram, many researchers, including this research group, have done have done a lot of studies about it as the flow pattern, kinetics, toxicity inhibition, start up, a lot of studies about it as the flow pattern, kinetics, toxicity inhibition, start up, optimization and optimization and operation characteristics, so that the HRT is one of the most important factors for operation characteristics, so that the HRT is one of the most important factors for the control of this the control of this kind of anaerobic bioreactor, because it affects some factors such as the hydrogen kind of anaerobic bioreactor, because it affects some factors such as the hydrogen transfer, the anaerobic transfer, the anaerobic digestion processes and even can produce the cell washing phenomenon. digestion processes and even can produce the cell washing phenomenon. Bringing as a result the Bringing as a result the decrease in the percentage of COD removal and accumulation of volatile fatty decrease in the percentage of COD removal and accumulation of volatile fatty acids (VFA) in EGSB acids (VFA) in EGSB bioreactor, this because of the short contact time between the biomass and the bioreactor, this because of the short contact time between the biomass and the substrate resulting from substrate resulting from the decrease in HRT [18]. the decrease in HRT [18]. It has been found that this type of bioreactor is an excellent system for treating the high organic It has been found that this type of bioreactor is an excellent system for treating the high organic load wastewater but it does not remove the nutrients (N and P). However, in spite of the EGSB load wastewater but it does not remove the nutrients (N and P). However, in spite of the EGSB bioreactor has the capacity to treat high organic load wastewater, there is not literature that reports bioreactor has the capacity to treat high organic load wastewater, there is not literature that reports about the use of EGSB bioreactors in treating CPWW; For this reason, the aim of this research was to about the use of EGSB bioreactors in treating CPWW; For this reason, the aim of this research was to study the EGSB bioreactor to evaluate the HRT in the CPWW treatment. study the EGSB bioreactor to evaluate the HRT in the CPWW treatment. Sustainability 2018, 10, 83 4 of 11 Sustainability 2018, 10, 83 4 of 11

FigureFigure 3. DiagramDiagram of of CPWW CPWW treatment treatment syste systemsms applied applied..

2. Materials Materials and and Methods Methods

2.12.1.. CPWW CPWW Characterization Characterization The coffee processing wastewater was was obtained obtained from from a a producing producing coffee coffee farm farm from from Chiapas, Chiapas, ◦ Mexico. The The samples samples were stored at −2020 °CC until until they they were were used. used. The CPWW characterization waswas realizedrealized according according to to the the Standard Standard Methods Methods for for Examination Examination of ofWater Water and and Wastewater Wastewater [19], in [19], order in to order determine to determine pH, chemical pH, oxygen chemical demand oxygen (COD), demand biochemical (COD), biochemicaloxygen demand oxygen (BOD demand5), total (BOD solids5), (TS), total total solids volatile (TS), solids total volatile (TVS), sedimented solids (TVS), solids, sedimented color, turbidity, solids, color,acidity, tur alkalinity,bidity, acidity, total nitrogen, alkalinity, total total phosphorus nitrogen, total and sulfates.phosphorus The totaland sulfates. organic carbonThe total (TOC) organic was carbondetermined (TOC) using was the determined method by Walkleyusing the and method Black [20 by] and Walkley density and was Black measured [20] using and density a viscometer was measured(Anton Paar using SVM a 300)(Antonviscometer Paar,(Anton Graz, Paar Austria). SVM 300 All)(Anton the tests Paar, were Graz, analyzed Austria in) triplicate.. All the tests were analyzed in triplicate. 2.2. Bioreactor Set up and Operation 2.2. BioreactorThis study Set employed up and Operation an EGSB bioreactor, it was made in fiberglass with 3.3 L working volume and a 15.8 height/diameter ratio. Figure4 shows the schematic diagram bioreactor. It was inoculated This study employed an EGSB bioreactor, it was made in fiberglass with 3.3 L working volume with 1000 mL (30% of bioreactor working volume) of mesophilic inoculum (spherical in shape (∅, and a 15.8 height/diameter ratio. Figure 4 shows the schematic diagram bioreactor. It was inoculated c. 0.5–1 mm) and a grey-green color with a content of TS of 49.75 g L−1 and TVS of 29.5 g L−1). with 1000 mL (30% of bioreactor working volume) of mesophilic inoculum (spherical in shape (∅, c. The EGSB was operated at seven different HRT of 3 to 9 days and organic loading rate (OLR) of 4 to 0.5–1 mm) and a grey-green color with a content of TS of 49.75 g L−1 and TVS of 29.5 g L−1). The EGSB 1.5 kg COD m−3day−1. It was automatically fed using a peristaltic pump (Master Flex model 7534-04) was operated at seven different HRT of 3 to 9 days and organic loading rate (OLR) of 4 to 1.5 kg COD (Cole-Parmer, Vernon Hills, IL, USA) connected to a tank of 1.5 L that stored the CPWW and the m−3day−1. It was automatically fed using a peristaltic pump (Master Flex model 7534-04) (Cole- influent was buffered with NaHCO . Parmer, Illinois, U.S.A.). connected3 to a tank of 1.5 L that stored the CPWW and the influent was The HRT was calculated by following Equation (1). buffered with NaHCO3. The HRT was calculated by following Equation (1).V HRT = (1) 푉Q 퐻푅푇 = (1) 푄 where Q is the flow rate (m3 day−1) and V is the total volume of the bioreactor (m3). where Q is the flow rate (m3 day−1) and V is the total volume of the bioreactor (m3). Sustainability 2018, 10, 83 5 of 11 Sustainability 2018, 10, 83 5 of 11

Figure 4. Schematic diagram of EGSB bioreactor.

2.3.2.3. EffluentEffluent AnalysesAnalyses SamplesSamples from from bioreactors bioreactors effluents effluents were were routinely routinely taken taken for for COD, COD, pH pH and and temperature temperature measurementmeasurement according according to to Standard Standard Methods Methods for for Examination Examination of Water of Water and andWastewater Wastewater [19]. [The19]. Thealkalinity alkalinity factor factor index index (α) was (α) obtained was obtained according according to the tofollowing the following procedure procedure [21]: 10 [ 21mL]: of 10 sample mL of samplewas taken was and taken acidified and acidified with 0.1N with HCl 0.1N to HCla pH to of a 5.75. pH of The 5.75. required The required HCl (V HCl1) is (Vcorresponded1) is corresponded to the topart the alkali partnity alkalinity (PA). (PA). Subsequently, Subsequently, this this sample sample was was brought brought to to pH pH 4.3 4.3 (V (V22)) corresponding to to intermediateintermediate alkalinityalkalinity (IA)(IA) andand thethe totaltotal alkalinityalkalinity isis thethe sumsum ofof bothboth ((TATA == PAPA ++ IAIA).). TheThe α indexindex waswas calculatedcalculated byby thethe followingfollowing EquationEquation (2).(2). PA α = 푃퐴 (2) 훼 =TA (2) 푇퐴 The VFA (meq L−1) was determinate according to Iñiguez-Covarrubias and Camacho-López [22]. The VFA (meq L−1) was determinate according to Iñiguez-Covarrubias and Camacho-López [22]. The COD removal in the EGSB bioreactor refers to the difference between the influent concentration The COD removal in the EGSB bioreactor refers to the difference between the influent concentration (Cinf) and the effluent concentration (Ceff). Thus, the COD removal rate (Rr) is expressed by the (Cinf) and the effluent concentration (Ceff). Thus, the COD removal rate (Rr) is expressed by the following Equation (3). following Equation (3). Cin f − Ce f f Rr(%) = × 100 (3) 퐶푖푛푓C−in f퐶푒푓푓 푅푟(%) = × 100 (3) 퐶 This equation can also be used to calculate푖푛푓 the efficiency of the reactors in regards to otherThis nutrients. equation can also be used to calculate the efficiency of the reactors in regards to other nutrients.

Sustainability 2018, 10, 83 6 of 11

3. Results and Discussion

3.1. CPWW Physicochemical Characterization Table1 shows the physicochemical composition of CPWW. The majority of the values found for coffee pulp were similar to those reported by other authors [12,15,16,23,24]. The acidic pH is due to the presence of organic acids in berry skin and pulp. According to the findings of Hue et al. [23], they reported that the pH ranged from 3.5 to 4.5 in wastewater from the coffee fruits processing. The CPWW contained appreciable amounts of suspended, dissolved and total solids. The higher amount of suspended solids present in CPWW might be due to the presence of pectin, protein and sugars, which are biodegradable in nature. The concentrations of these organics vary with quantity of water used for processing of coffee berries [15,24]. Various researchers reported high pollution from wet processing [6,12,16,24]. As a result, the polluting potential of the coffee processing factories is enormous as shown by the BOD5 and COD content of coffee effluent reaching up to 37,944 and 45,955 mg L−1 respectively, this high level of BOD5 and COD in the CPWW might be due to the presence of high amount of organic substances and to the slowly degrading compounds present. The CPWW contained considerable amounts of nitrogen (700 mg L−1) and phosphorus (36.43 mg L−1), which might be due to the presence of pectin, protein and sugars in coffee berries.

Table 1. Physicochemical characterization of CPWW.

Permissible Limits Parameter Values WHO NOM-001-ECOL-1996 * pH 3.95 6.5–8.5 5–10 Color (Pt-Co) 17,966.7 - - Turbidity (NTU) 1481.7 5 - Density (g mL−1) 1.1075 - - −1 Alkalinity (mg CaCO3 L ) ND - - −1 Acidity (mg CaCO3 L ) 3360 - - −1 COD (mgO2 L ) 45,955 300 - −1 BOD5 (mgO2 L ) 37,944 100 150 BOD5/COD ratio 0.82 - - Sedimented solids (mL L−1) 380 - 2 TS (mg L−1) 19,593 650 - TSS (mg L−1) - 200 125 TVS (mg L−1) 8208 - - TOC (mg L−1) 11,400 - - Phosphorus (mg L−1) 36.43 - 30 Phosphate (mg L−1) - 5 - Nitrogen (mg L−1) 700 - 60 Nitrate (mg L−1) - 5 - Total sulfate (mg L−1) 10 250 - ND = not detected, * water discharged in rivers.

It was evident from Table1 that the wastewater was heavily polluted with high organic load, nutrients and suspended matter. Organic load was measured in terms of COD and BOD5 and nutrients in the term of phosphorus and nitrogen. Comparing values of pH, BOD5, COD, phosphorus and nitrogen for the CPWW with World Health Organization (WHO) [25] and Official Mexican Environmental Regulations [26] permissible limits for discharging of effluent purpose (Table1). The values of pH, BOD, COD, phosphorus and nitrogen were 3.95, 37,944 mg L−1, 45,955 mg L−1, 36.43 mg L−1 and 700 mg L−1, respectively; it was found that the concentration of all these parameters were very high. If this CPWW were directly discharged to the water bodies without prior treatment it could produce a serious environmental pollution impact. Among these impacts are anoxia, eutrophication, death of aquatic life and many severe human health problems (like dizziness, eye, ear Sustainability 2018, 10, 83 7 of 11 Sustainability 2018,, 10,, 83 7 of 11 andirritation,irritation, skin irritation, stomachstomach stomach pain,pain, nauseanausea pain, andand nausea breathingbreathing and breathingproblems)problems) problems)asas waswas reportedreported as was byby reported HaddisHaddis and by HaddisDevi [16] and Deviand [ Padmapriya16] and Padmapriya et al. [27]. et al. [27].

3.2.3.2 Reactor.. Reactor Operationr Operation Analysis Analysis TheThe EGSB EGSB bioreactor bioreactor was was operated operated over over 180 180 days at mesophilic condition. condition. The The average average temperaturetemperaturetemperature of ofof thethethe influents influentsinfluents waswas was 21.321.3 21.3 ±± 0.50.5± 0.5°C.°C. FigureFigure◦C. Figure 55 showsshows5 shows thethe temperaturetemperature the temperature andand pHpH and profileprofile pH ofof profile thethe effluents from the EGSB bioreactor in at different HRT. The pH is an essential factor to control during ofeffluents the effluents from fromthe EGSB the EGSBbioreactor bioreactor in at different in at different HRT. The HRT. pH Theis an pHessential is an essentialfactor to control factor during to control anaerobic digestion. The methanogens microorganisms have optimum growth in the pH range duringanaerobic anaerobic digestion. digestion. The methanogens The methanogens microorganisms microorganisms have have optimum optimum growth growth in the in the pH pH range range between 6.8 and 7.6 [28,29], although stability may be achieved in the formation of methane over a betweenbetween 6.8 6.8 and and 7.6 7.6 [ 28[28,29],,29], althoughalthough stability may may be be achieved achieved in in the the formation formation of ofmet methanehane over over a a wider pH range (6.0(6.0–8.0). Values below 6.0 and above 8.3 should be avoided, as they can inhibit the wider pH range (6.0–8.0). Values below 6.0 and above 8.3 should be avoided, as they can inhibit the methanogens microorganisms [30]. The pH levels were generally stable (pH 7.2.2–7.8) throughout the methanogens microorganisms [30]. The pH levels were generally stable (pH 7.2–7.8) throughout the evaluation (HRT from 3 to 9 days). According to Rittmann and McCarty [29], a pH between 6.5–8.2 evaluation (HRT from 3 to 9 days). According to Rittmann and McCarty [29], a pH between 6.5–8.2 is isis notnot detrimentaldetrimental toto anaerobicanaerobic processes.processes. InIn thisthis study,study, itit cancan bebe assumedassumed thatthat thethe bacteriabacteria adaptedadapted not detrimental to anaerobic processes. In this study, it can be assumed that the bacteria adapted well well to the HRT change and were not adversely affected by itsits increment.increment. to the HRT change and were not adversely affected by its increment.

3030 1414

2525 1212 26.426.4 ±±11ooCC 20 10

C) 20 10

C) o o StartStart--upup // StabilizingStabilizing 15 7.57.5 ±±0.30.3 8

15 8 pH pH

1010 66 HRT = 3 HRT = 4 HRT = 5 HRT = 6 HRT = 7 HRT = 8 HRT = 9

55 HRT = 3 HRT = 4 HRT = 5 HRT = 6 HRT = 7 HRT = 8 HRT = 9 44

Temperature Temperature ( Temperature Temperature (

00 22 00 2020 4040 6060 8080 100100 120120 140140 160160 180180 Time (days) TemperatureTemperature pHpH

FigureFigure 5. 5.Monitoring Monitoring of of pHpH andand temperaturetemperature in in the the effluents effluents of of EGSB EGSB bioreactors, bioreactors, the the temperatures temperatures shownshownshown are areare the thethe media mediamedia values valuesvalueswith withwith theirtheirtheir standardstandard deviations.deviations.

An aspect related to pH is the α index. This index indicates the buffering capacity of the system AnAn aspect aspect related related to to pH pH is is the theα αindex. index. ThisThis indexindex indicatesindicates the buffering capacity of of the the system system to toto any any abrupt abrupt pH pH change change in in the the bioreactor; bioreactor; therefor therefore, to carry out a continuous monitoring of any abrupt pH change in the bioreactor; therefore, to carry out a continuous monitoring of anaerobic anaerobic processes, and even to make decisions, the alpha index is a better option, for this reason processes, and even to make decisions, the alpha index is a better option, for this reason some authors some authors have suggested that the larger the α index value the better the buffering capacity of the have suggested that the larger the α index value the better the buffering capacity of the system. system. Jenkins et al. [21] recommended that the alpha index values should be larger than 0.5 during Jenkins et al. [21] recommended that the alpha index values should be larger than 0.5 during start-up start--up and reaching stable conditions in the system with values greater to 0.7 (as a rule of thumb). and reachingAs it can stable be seen conditions in Figure in 6, the the system α index with reached values an average greater to value 0.7 (asoptimal a rule during of thumb). the start--up α of 0.59As it± 0.16, can be after seen the in third Figure week6, the of the indexstart--up reached stage the an bioreactor average value reached optimal stability during with the an start-upα index of 0.59of 0.98± 0.16, ± 0.01, after which the third indicates week a ofgood the performance start-up stage of the the bioreactor bioreactor reached in the different stability HRT with evaluated. an α index of 0.98 ± 0.01, which indicates a good performance of the bioreactor in the different HRT evaluated.

1010 11 99

) 8 ) 0.80.8 8 77 0.60.6 66 5 0.4 5 (days) HRT 0.4 (days) HRT

Alpha Index Alpha Index (α Start up / Stabilizing Alpha Index

Alpha Index Alpha Index (α Start up / Stabilizing Alpha Index 44 0.20.2 HRT 33 00 22 00 2020 4040 6060 8080 100100 120120 140140 160160 180180 Time (days) Time (days) Figure 6. Behavior of the α index during the operation of the EGSB bioreactor. FigureFigure 6.6. BehaviorBehavior of the α indexindex during during the the operation operation of of the the EGSB EGSB bioreactor. bioreactor. Sustainability 2018, 10, 83 8 of 11 Sustainability 2018, 10, 83 8 of 11

OneOne ofof thethe mainmain problems problems of of the the anaerobic anaerobic biological biological degradation degradation of thisof this type type of wastewaterof wastewater is the is highthe high content content of easily of fermentableeasily fermentable organic organicmatter. Organic matter. matter Organic compounds matter compounds cause a fast causeacidification a fast ofacidification the wastewater of the that wastewater results in that a high results production in a high of production VFA and, therefore, of VFA and, it is necessarytherefore, toit is monitor necessary the contentto monitor of VFA the for content the good of VFA performance for the good of the performance anaerobic bioreactor of the anaerobic [31,32]. Thebioreactor increase [31, of32]. VFA The in theincrease effluents of VFA implies in the that effluents the acidogenic implies bacteria that the produce acidogenic more bacteria VFA than produce what more can be VFA utilized than by what the acetogeniccan be utilized and methanogenic by the acetogenic bacteria. and This methanogenic pH decreases bacteria. could affect This the pH bioreactor decreasesperformance could affect [33 the]. However,bioreactor in performance this study the [33]. EGSB However, bioreactor does in this not presentstudy the accumulation EGSB bioreactor of VFAs (Figure does not7) although present theaccumulation HRT was changed of VFAs 3(Figure to 9 days. 7) although The bioreactor the HRT presented was changed a good 3 to buffering 9 days. The capacity bioreactor (α index presented above toa good 0.7) maintaining buffering capacity a pH near (α index to 7.5. above to 0.7) maintaining a pH near to 7.5.

2 10

9 1.6

8

)

)

1

1 - - 7 1.2 7 6 Start-up/Stabilizing VFA 0.8

5 (days) HRT

5 (days) HRT VFA (meq VFA(meq L VFA (meq VFA(meq L HRT 4 0.4 3

0 2 0 20 40 60 80 100 120 140 160 180 Time (days) Time (days) Figure 7.7. Monitoring of VFA in the effluentseffluents ofof EGSBEGSB bioreactors.bioreactors.

The COD removal rate and average concentrations of COD in the influent and effluent of the The COD removal rate and average concentrations of COD in the influent and effluent of the EGSB bioreactor are shown in Figure 8. As it can be seen, the COD removal rate at 3 and 4 days HRT EGSB bioreactor are shown in Figure8. As it can be seen, the COD removal rate at 3 and 4 days HRT was around 94%. In addition, there is a slight increase at 5 and 6 days HRT (average 97%) and this was around 94%. In addition, there is a slight increase at 5 and 6 days HRT (average 97%) and this was increase further (average 98%) at a HRT of 7, 8 and 9 days. This indicates that COD removal rate was increase further (average 98%) at a HRT of 7, 8 and 9 days. This indicates that COD removal rate became more efficient and less variable with the HRT increase. However, the best HRT was 9 days. became more efficient and less variable with the HRT increase. However, the best HRT was 9 days.

3. OLR (Kg /m3.day) 3.5 2.5 2 6 6 3 2.5 2.5 2 1.5

24,000 100%

)

)

1 1 - 20,000

- 20,000 L

L 80%

2 2

16,000

60%

(%)

(%) r

12,000 r

R

R COD COD (mg O COD COD (mg O HRT = 6 40% Start up / Stabilizing HRT = 3 HRT = 4 HRT = 5 HRT = 7 HRT = 8 HRT = 9 40% 8,000

20% 4,000

0 0% 0 20 40 60 80 100 120 140 160 180 Time (days) COD Inf. COD Eff. COD Removal

FigureFigure 8. CChangeshanges in COD the influent influent and effluent effluent during the experimental period and COD removal raterate (%).(%).

In Figure 9, it can be observed that the HRT 9 days is the optimum value, because from this HRT In Figure9, it can be observed that the HRT 9 days is the optimum value, because from this HRT the bioreactor effluent lies below with the maximum permissible limit (300 mg L−1) and they could be the bioreactor effluent lies below with the maximum permissible limit (300 mg L−1) and they could be discharged to a surface water in accordance to reported by WHO and Official Mexican Environmental discharged to a surface water in accordance to reported by WHO and Official Mexican Environmental Regulations. Although there is no statistically significant difference with HRT 8 and 7 days, the HRT Regulations. Although there is no statistically significant difference with HRT 8 and 7 days, the = 7 days allowing a greater volume of CPWW treatment and thus reducing operating costs. HRT = 7 days allowing a greater volume of CPWW treatment and thus reducing operating costs. SustainabilitySustainability2018 2018,,10 10,, 83 83 99 ofof 1111

1200

1000

)

1

- L

2 2 800

600

400 COD COD Effluent (mg O

200 Permissible Limit WHO, (1995)

0 3 4 5 6 7 8 9 HRT (days) FigureFigure 9.9. HRTHRT Optimization.Optimization.

4.4. ConclusionsConclusions AnaerobicAnaerobic digestion digestion of of CPWW CPWW was was carried carried out out in in an an EGSB EGSB bioreactor. bioreactor. The The bioreactor bioreactor was was operatedoperated atat 26 °C◦C for for 180 180 days. days. OLR OLR was was diminished diminished step step by by step step from from 6 to 6 1.5 to 1.5kg COD kg COD m−3d may−3−1day while−1 whileHRT was HRT increased was increased from 3 from to 9 3days to 9. The days. bioreactor The bioreactor exhibited exhibited high efficie highncy efficiency in treating in treating of CPWW of CPWWwith a COD with removal a COD removalof 98% was of 98%achieved was achievedat HRT greater at HRT than greater 7 days than and 7 OLR days less and than OLR 2.5 less kg CO thanD 2.5m−3 kgday COD−1. These m−3 dayeffluents−1. These meet effluents the maximum meet the permissible maximum limit permissible established limit by established the WHO by and the Official WHO andMexican Official Environmental Mexican Environmental Regulations. Regulations. TheThe EGSBEGSB bioreactorbioreactor waswas stablestable inin termsterms ofof pH,pH, VFAsVFAs andand αα indexindex duringduring allall thethe operationoperation time, soso thethe EGSBEGSB bioreactorbioreactor cancan bebe anan efficientefficient andand sustainablesustainable alternativealternative forfor CPWWCPWW treatmenttreatment comparedcompared toto otherother conventionalconventional methods.methods. Acknowledgments: The authors are grateful for the access granted to the research facilities of the Faculty of Acknowledgments: The authors are grateful for the access granted to the research facilities of the Faculty of Environmental Engineering and the Center for Research and Technological Development in Renewable Energies Environmental Engineering and the Center for Research and Technological Development in Renewable Energies ofof thethe UniversityUniversity of of Sciences Sciences and and Arts Arts of of Chiapas. Chiapas. As As well well as as the the Polo Polo Tecnol Tecnológicoógico Nacional Nacional para para el Desarrollo el Desarrollo de Investigacide Investigaciónón y Pruebas y Pruebas Analíticas Analíticas en Biocombustibles en Biocombustibles of Tecnol ó ofgico Tecnológico Nacional de Nacional México/Instituto de México/Instituto Tecnológico deTe Tuxtlacnológico Guti deérrez. Tuxtla Gutiérrez.

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