A Systematic Review on Anaerobic Textile Industrial Wastewater Treatment:Influence of Processes,Microbial Communities and Bioreactors

Innocent Tayari Mwizerwa,Yu Wang,Xiaoguang Chen

1 College of Environmental Science and Engineering,Donghua University,Shanghai 201620,China

2 School of Mechanical Engineering,Sichuan Provincial Key Lab of Process Equipment and Control,Sichuan University of Science&Engineering,Zigong,643000,China

3 Textile Pollution Control Engineering Center of Ministry of Environmental Protection,Donghua University,Shanghai 201620,China

Keywords Textile wastewater Anaerobic bioreactor Microbial communities

Abstract

1 Introduction

Textile industries have spread worldwide,and their processes generate a lot of wastewater that requires treatment before discharge into the receiving environment(Yang et al.,2018). Textile industries and their wastewaters have increased exponentially, thus make it major environmental pollution threat in all economies(Doble and Kumar,2005;Mullai et al.,2017). Textile industrial wastewater and its processes have increased because of the growth in the consumption and production (Pandey et al.,2007; Kant,2012). This is by the crude disposal of effluents into the environment which is not lawful, undesirable because of the components they possess that can cause mutations and harm to flora and fauna residual effect to a life of exposure(van der Zee,2002). This is because most of its constituents are non-biodegradable therefore,persist in the environment for quite a long time. (Doble and Kumar, 2005) Research has shown that the half-life of hydrolyzed-Reactive Blue 19 (RB19) is about 46 years at pH 7 and 250c (Shah et al., 2013). This has also rendered some countries water scarcity opting for recycling and reuse to sustain life.

Textile wastewater has a lot of environmental destructions as well as human ill health impacts(Delee et al.,1998). Nearly 40%of the worldwide colors in used have organic components of chlorine which are carcinogenic(Doble and Kumar,2005). Therefore,these components from textile industries require close attention since they are reactive with most disinfectants including chlorine. These compounds (Fig.2) can freely mix with the air used by human beings for breathing. The results from such exposure is harmful to humanity especially pregnant mothers and children. The most source of textile wastewater comes from(table 1)desizing,scouring,bleaching,mercerizing and dyeing processes(dos Santos et al.,2007).

The prominent colors,together with oil scum,affect the water quality through increasing turbulence,thus off odour and as well as distressed look. This shades off sunlight vital for the manufacturing of food to the aquatic flora Process(Kant,2012). Such contents result in the disruption of free Oxygen exchange processes at air-water mixing points. Exhaustion of dissolved Oxygen in water is the worst of textile wastewater because it reduces oxygen vital for aquatic fauna. The self-treatment of water can also be significantly affected. Furthermore,the discharge flow on the soil can result into blockage of soil pore spaces rendering it unproductive since the roughness or smoothness is tightened,making it hard for root systems to advance further.

Anaerobic industrial wastewater treatment has picked interest so far because of benefits attached it. These include resource recovery and excellent pollution abatement at a cheaper user-friendly technology. Anaerobic biodegradation is the process that incorporates microorganisms to reduce the complex compounds (Fig.5) into smaller structures without oxygen. This reduces or eliminates pollutants contained in the wastewater (Nigam,Banat et al. 1996). Presently anaerobic treatment alternatives have been thought and have promising results in the degradation of complex organic compound(Nigam et al.,1995). Anaerobic digestion,in relation with aerobic,has gained attention because of low-energy requirements as well as less sludge output. Thus the anaerobic process is highly preferred for the industrial wastewater with a lot of toxins(Kartal et al.,2010). The capacity to conserve,protect and preserve the environmental resources,anaerobic treatment process,microbial communities and anaerobic bioreactors have gained a lot of popularity.

Application of anaerobic reactors started way back in 1859 as anaerobic digester established in Bombay,India, a leper (Lettinga et al., 1980). This was later improved in 1895 by incorporating a septic tank for gas generation in the 1930’s, thus gained interest by researchers (Chen et al., 2012). Subsequently, UASB was invented in the 1970s by Dr Gatze Lettinga presently considered as the second generation of anaerobic bioreactor(Chen et al.,2010). Later,a high-rate bioreactor has come up with a higher treatment capacity of organic load rate and biogas production,and recently,a super high bioreactor SSSAB researched by our team.

2 Characters of textile industrial wastewater

2.1 Resource of textile industrial wastewater

Textile industries use variety of materials ranging from dyes, fibers (cotton, woolen and synthetics) with dry and wet processing operation (Arslan et al., 2016). The desizing, scouring, bleaching, mercerizing, dyeing,printing and finishing stages are included in wet fabric processing industry. During fabric formation, the water utilization and wastewater generation from a wet processing (Holkar et al., 2016). The effluent wastewater contain chemicals like acids, alkalis, dyes, hydrogen peroxide, and starch; surfactants dispersing agents and soaps of metals, therefore, have an environmental impact. The textile industry has been estimated to use more water than any other sector worldwide, and its water has the highest contented of pollutants to the ecosystem flora and fauna. An average sized textiles mills consume water about 200L per kg of fabric processed per day(Wang et al.,2009). Textile processing processes emit huge millions of volumes of effluent as hazardous toxic waste,color and complex compounds from dyeing and finishing salts. The contents of sulphurnaphthol,fat dyes,nitrates,acetic acid,soaps,chromium compounds and heavy metals of copper,arsenic,lead,cadmium,mercury,nickel, and cobalt and certain auxiliary chemicals turn the effluent super toxic. They may as well contain formaldehyde-based dye fixing agents, hydrocarbon-based softeners and non-biodegradable dyeing chemicals.The wastewaters always have increased parameters of temperature and pH,which are detrimental to various life forms(Doble and Kumar,2005).

Table 1 Characterization of the cotton wet processing wastewaters.

Wastewaters from Textile industries have features of significant changes of various parameters like Chemical Oxygen Demand(COD),Biochemical Oxygen Demand(BOD),pH,Color,Salinity,Turbidity,Total Dissolved Solids(TDS),Metals,Nutrients,Fats and Salts(Xiang et al.,2016). However,this is dependent on the content of wastewater based on the different organic-based compounds,chemicals and dyes used in the industrial dry,and wet-processing operations (Trgay, 2010). Anaerobic treatment is amicrobial centered process that is carried out in the absence of oxygen. The process is accomplished by microorganisms that don’t require oxygen for their activity through the production of enzymes that facilitate biodegradability. Temperature conditions in which sludge is fermented in tanks at a temperature of 55°C or mesophilic at a temperature of around 36°C is vital(Almeida Streitwieser, 2017). Though allowing shorter retention time makes thermophilic digestion more expensive regarding energy consumption for heating,under aerobic conditions,bacteria rapidly consume organic matter and convert it into carbon dioxide. The operating costs used is characteristically much more significant for aerobic digestion because of the energy used by the blowers, pumps and motors needed to add oxygen to the process (Abdelgadir et al., 2014). Thus anaerobic wastewater treatment has now become promising in the textile industrial wastewater treatment.

2.2 Anaerobic degradation characters of textile industrial wastewater

With anaerobic conditions,azo dyes can be quickly degraded in form of electron reducing manner at link points,thus resulting in the aromatic amines (Carliell et al., 1995). These electrons are donated by carbon-containing constitutions like starch, volatile fatty acids. More so these bacteria of methanogenic and acetogenic origin have enzymes adapted to reducing the wastes into simpler forms. Examples of such enzymes may include F430 and vitamin B12 (Manu and Chaudhari 2002). The action of these enzymes degrade colour of dyes although they don’t complete mineralization of aromatic amines created in the system. Therefore are not denatured(Chinwetkitvanich et al., 2000), unlike some few special exceptions because the carcinogens and mutagens contained in the wastewater that cannot be degraded by such reactors to meet the environmental discharge standard,incorporation of aerobic processes are called for to degrade aromatic amines(Razo-Flores et al.,1996).Examples of such complex dyes are chromospheres,auxochromes and anthraquinones as shown below;

2.3 Anaerobic digestion characters of textile industrial wastewater

Anaerobic digestion is a biological process that involves microorganisms to degrade both organic and inorganic matter in wastewater. Nowadays, anaerobic wastewater treatment has become an outstanding, cost-effective and resource recovery option among others for industrial liquid waste treatment (Fu et al., 2017). Anaerobic digestion is the most common treatment process for such wastes,which usually retains the sewage from one day to two days(Connaughton et al.,2006). The sludge is fed into large tanks and held for a minimum of 12 days to allow the digestion process to perform the four stages. These include hydrolysis,acidogenesis,acetogenesis and methanogens necessary to digest the sludge where complex proteins and sugars are broken down to form more simple compounds of water,carbon dioxide and methane(van Lier et al.,2015).

Fig.1 Schematic representation of dye auxochromes and chromospheres for azo dyes and anthraquinones dyes.

Fig.2 Schematic representation of operations involved in textile industry and their main pollutants.

3 Process of textile wastewater treatment

3.1 Process principle of textile wastewater treatment

The initial degradation starts with the making of anaerobic granules, which are similar to that criterion for the growth of Bacteria biofilm on solid surfaces (Montaez Hernandez et al., 2017). Therefore, the internal attachment materials are of great importance in the formation of granules in the system. Investigations have come up with Methanosaeta concilii as the main Organism that is responsible for granule formation(Abdelgadir et al.,2014).

During ideal conditions,the methanogen bacteria decompose the acidic intermediates as soon as they can be available. Although, when Methane bacteria are insufficient or not conducive, the bacteria are unable to utilise acids as fast as it would be made through the acid formers,thus higher values of volatile acids. Therefore,higher costs portray more acid values hence methane formation imbalance(Lettinga et al.,1980). Therefore bacteria are categorized based on temperature classifications like mesophiles bacteria as mesophilic temperature, whereas thermophiles bacteria are thermophilic temperature (Shah, 2013). The process stages of anaerobic digestion include the following;

Hydrolysis

Hydrolysis is a vital stage of anaerobic digestion. It is where high strength wastewater is reduced in content to allow smooth subsequent stages. Otherwise, the treatment system cannot easily degrade such wastes due to complexity(Chen et al.,2016). This is because the constitutes of cellulose need to be changed to less complex states such as sugars,amino acids as well as fatty acids as expressed in Equation 1.This is through hydrolisation by microorganisms from cellulose to sugars or alcohols and proteins to peptides or amino acids, by hydrolytic enzymes (lipases, proteases, celluloses, amylases, among others by specialized microorganisms thus suitable substrate for alternative bacteria. Below is the equation for degradation of complex sugars(polysaccharide) to a simpler form(glucose).

Acidogenesis

Acidogenesis is a stage in which acidogenic bacteria change the sugars and amino acids into carbon dioxide,hydrogen, ammonia and organic acids. When the system is in a steady state,organic matter is transformed into substrates for the methanogens like the acetate,hydrogen,and carbon dioxide as expressed in Equation 2. However,some of it is converted into fatty acids or alcohols. Such products will shift to methanogenesis,as indicated in Equation 3 and can, therefore, be utilised by methanogen bacteria in the final stage. This transformation expressed in equation 4 explains why the acid-producing phase(acidogenesis)is followed by hydrolysis. In this process, acetate and hydrogen are liberated in the process of acidification, and acetogenic reactions and all are substrates of methanogenic bacteria.

The Species that are isolated from anaerobic digesters depending on the substrate include; Clostridium,Peptococcus, Bifidobacterium, Desulfovibrio, Corynebacterium, Lactobacillus, Actinomyces, Staphylococcus,Streptococcus, Micrococcus, Bacillus, Pseudomonas, Selemonas, Veillonella, Sarcina, Desulfobacter, Desulfomonas and Escherichia coli (Kosaric and Blaszczyk, 1992). Therefore, the characteristic of wastewater will always influence the bacterial predominance.

The representation of acidogenesis reactions involves the conversion to ethanol, propionate and acetic acid in their orders below;

Acetogenesis

This is a stage where bacteria convert the organic acids resulting from the above reactions to form acetic acid,hydrogen, and carbon dioxide. Propionate shown in Equation 5 is converted to acetate strictly at low hydrogen pressure, glucose in Equation 6 and ethanol according to Equation 7 and much more are produced (Duarte et al., 2015). The products here are due to the activity of diverse microorganisms including syntrophobacter wolinii, propionate decomposer, and sytrophomonas spp Actinomyces lactobacillus, Peptococcusanaerobius,and butyrate decomposer(Abdelgadir et al.,2014)as below;

Methanogenesis

Methanogenesis is through either cleavage of acetic acid molecules to generate carbon dioxide and hydrogen with minimal hydrogen concentration in the digesters which leads to initial acetate reaction to produce methane (Abdelgadir et al., 2014) as indicated in Equation 8. In the final reaction, methane is produced by methanogen bacteria Eqn. 9. These Bacteria are capable of metabolizing formic acid, acetic acid, methanol,carbon monoxide, and carbon dioxide and hydrogen to produce methane as expressed in Equation 10. Some of the notable species that have been classified are Methanobacteriumformicicum, M.Bryant and M.thermoautotrophicum; Methanobrevibacterruminantium, M. arboriphilus and M. smithii; Methanococcusvannielli and M.voltae; Methanomicrobium mobile; Methanogeniumcariaci and M. marinsnigri, Methanospirilumhungatei and Methanosarcina (Kosaric and Blaszczyk,1992).

Methanogenesis is,therefore, a crucial phase regarding the whole process of anaerobic digestion; however,compared to other stages, it has the most sluggish biochemical reactions (Bajpai 2017). The other division is called the methyltrophic bacteria that can easily form ethane from methanol (Chen et al., 2008). Examples include Methanobacterium, Methanobacillus, Methanococcus, and Methanosarcina. They can as well be categorized as acetate and H2or CO2consumers. Also,Methanosaeta is regarded to be vital in Anaerobic Digester as both acetate and H2or CO2consumers(Yu et al.,2017). Methanogenesis reactions occur as below;

3.2 Process parameters in anaerobic textile wastewater treatment

Decomposition of textile constituents with the anaerobic system is dependent of various factors (Mahmoud et al.,2003). The critical process parameters in Table 1 may include temperature,pH,and OLR,up flow velocity),SRT as well as HRT and particle size distribution in the reactor(Chen et al.,2012).

Temperature

Temperature is an essential parameter for it determines the biodegradability of the wastewaters as well its viscosity(Xiao et al. 2018). The anaerobic bacteria classes of mesophilic (300c-400c) and thermophilic (500c-650c)influences the performance of the treatment process (Abdelgadir et al. 2014). Studies on the relatively high concentrations of dissolved methane in the effluent containing 28-75%of the total methane produced 11-26 C.Dissolved CH4was oversaturated during treatment of low-strength wastewaters,the degree of saturation ranged from 1.9 to 6.9 based on COD mass balance calculation (Chen and Zhang, 2013; Dalkilic and Ugurlu, 2015).Investigatigations in the dissolved CH4in an AnMBR when treating low-strength wastewater has been carried out on this phenomena. Glucose as carbon source with OLR of 0.39 to 1.1 g COD/L-d at 24-26 0 C and HRT of 1 d were observed. The dissolved CH4was 2.2-2.5 folds higher than saturation values according to Henry’s law accounting for 76%of total CH4production at OLR of 0.39 g COD/L-d. Mass transfer was minimal as a result of low gas production rate in the AnMBR because it was working at low OLRs and low influent COD,which lead to oversaturation in the reactor.

pH

PH is the measure of acidity or alkalinity of a solution. It is a vital parameter influencing performance and efficiency. Methanogen bacteria in anaerobic wastewater treatment systems majorly are most active in the neutral pH range 7.0 (Duarte, Silva et al., 2015). However, the concentration range suitable for most organisms is 6.0-9.0 (Almeida, 2017); beyond this, digestion could proceed although at minimal efficiency. The biomass inhibited at pH 9 was able to regain activities after adjusting the pH to neutrality,but that inhibited at pH 5 was not(Abdelgadir et al.,2014). At acidic conditions produced can become quite toxic to the methane bacteria(Liu et al. 2013). It is therefore essential that the pH is not does not drop below 6.2 for a significant period. Because this parameter is crucial, thus the system needs to control the ph. When the methane gas production stabilizes,the pH remains between 7.2 and 8.2. (Almeida,2017;Abdelgadir, Chen et al.,2014)reported that an optimum pH range of anaerobic treatment is about 7.0 to 7.2,but it can proceed quite well with a pH varying from about 6.6 to 7.6.

Hydraulic retention time(HRT)

HRT is the measure of the average length of time that a soluble compound stays in a constructed bioreactor. Hydraulic retention time is the volume of the aeration tank divided by the influent flow rate is expressed according to equation 11.

Where HRT(d)and usually expressed in hours or days,the V is the volume of aeration tank or reactor volume(m3),and Q is the influent flow rate(m3/d).

In principal, HRT is an excellent operational parameter that is easy to control and also a macro conceptual time for the organic material to stay in the reactor. In bioreactor engineering studies, the reverse of HRT is defined as dilution rate for which if it is bigger than the growth rate of microbial cells in the reactor. The microbes will be washed out, and otherwise, be accumulated in the reactor. Either of these situations result in the breakdown of the biological process happening in the reactor.

Organic loading rate

At the industrial scale arrangement of high-rate Anaerobic Fluidized-Bed(AFB)reactors such as UASB,USSB),EGSB), IC, and Inverse Anaerobic Fluidized Bed (IAFB) reactors can bear very high loading rates, up to 40 kg COD/(m3/d) (Abdelgadir et al., 2014).The OLR and Volumetric Biogas Production (VBP) of the Spiral Automatic Circulation (SPAC) reactor could reach up to 306 kg COD/(m3/d) (Robles et al., 2013). Several authors reported that up to a certain limit,the treatment efficiency of complex wastewaters,for example,potato,maize,slaughterhouse,in high rate anaerobic reactors increases with an increase in OLR.Also,accumulation of biogas in the sludge bed was noticed,forming stable gas pockets that lead to the incidental lifting of parts of the bed and a pulse-like eruption of the gas from this zone(Duarte et al. 2015). However,OLR could be varied by changing the influent concentration and due to changes in the flow rate. Thus,it implies changing the HRT and by changing the flow rate,under these conditions. OLR is expressed as;

Where OLR is organic loading rate(kg COD/m3/d),Q is flow rate(m3/d),COD is chemical oxygen demand(kg COD/m3),and V is reactor volume(m3). By using equation10,the OLR is expressed as;

Table 2 Key process parameters in anaerobic textile wastewater treatment.

Whenever the solid separation capacity from up-flow reactors is related to the OLR, it essential to differentiate these factors. Therefore, OLR is not enough to design parameter for efficient working of anaerobic treatment.

Sludge retention time

SRT is known to be the critical parameter affecting biochemical and physical properties of sludge. The success of UASB reactors is mainly dependent on the SRT(Chan and Chong,2012)which is the crucial factor determining the ultimate amount of hydrolysis and methanogenesis in UASB system at certain temperature conditions(Chan and Chong, 2012). Therefore, SRT should be long enough to provide sufficient methanogenic activity at the prevailing conditions.

The SRT is determined by the loading rate,the fraction of Suspended Solid(SS)in the influent,the removal of SS in the sludge bed,and the characteristics of the SS(Biodegradability, composition,and many more.) (dos Santos et al.,2007). Methanogenesis starts at SRT between 5 and 15 days at 250C and between 30 and 50 days at 150C,the maximum methanogenesis found at 250 C amounted to 51%and 25%at 150C.Maximum hydrolysis occurred at 75 days SRT and amounted to 50%at 25 0C,and 24%at 150 C(Chan and Chong,2012). The SRT and temperature have a significant influence on the hydrolysis of Proteins,carbohydrates, and lipids. The most substantial portion of the digestion of proteins, carbohydrates, and lipids occurs within the first 15 and 10 days at process temperatures of 25 0C and 35 0C respectively(Wang et al.,2009).

Up flow velocity

The up-flow velocity is one of the main factors affecting the efficiency of up flow reactors. An increase in up-flow velocity from 1.6 to 3.2 m/h can result in a relatively small loss in SS removal efficiency,from 55%to nearly 50%,which indicates the key role of adsorption and entrapment. (dos Santos,Cervantes et al.,2007)

Particle size distribution

The Particle-Size Distribution (PSD)of a powder, granular material, or particles dispersed in the fluid is a list of values or a mathematical function that defines the relative amount, typically by mass, of particles present according to size. The effluent quality from standard filters is highly related to the specific size of the filtering media. Most studies indicate that smaller media size gives more efficient removal(dos Santos,Cervantes et al.,2007).

3.3 Microbial communities for textile wastewater treatment

Literature on anaerobic reactor have revealed a variety of microorganisms involved in the decomposition of the compounds that is fermentative bacteria; acidate bacteria methanogenic syntrophic bacteria, methanogenic archaea and many more. However, the variations in the process parameters as well as the difference in compounds can lead to microbial structure change (Bae et al., 2016). In contradiction, another researchers (Yu et al., 2012) pointed out that microbial communities don’t vary in a UASB reactor when accompanied by of carbonaceous materials while(Liu et al.,2017)reported a change in an up-flow anaerobic bioreactor after attaching Anthraquinone-2-sulfonate Polyurethane foam.

Studies on Microbial communities involved in textile industrial water treatment have grouped them as archaea and bacteria (Meerbergen et al., 2017). Bacteria act on the complex substrates and decompose them into COD, CO2and H2and archaea are responsible for the production of methane, therefore, referred to as methanogens(Pearce et al.,2003). The study carried out using high thorough put sequencing detected variations in the microbial communities and means of maximising their output at different stages revealed that fungi could adhere its metabolism to changing environmental conditions. The intra and extracellular enzymes play a significant role in metabolic activity(Dellamatrice et al.,2017). These enzymes can degrade various dyes present in the textile wastewater (Saratale et al., 2009) reported 92% degradation of anthraquinone-based Reactive black five within 120 h under anaerobic conditions. Therefore, the kinetics of degradation of anthraquinone-based dyes facilitates the bacterial processes in the waste treatment.

Due to these enzymes, fungal cultures are suitable for the degradation of dyes in textile wastewater. Such enzymes include lignin peroxidase(LP),manganese peroxidase(MnP)and laccase(Holkar et al.,2016). Studies on White rot fungi Coriolopsis species (Yunnen, Xiaoyan et al. 2015),Penicillium simplicissimum (Yunnen et al., 2015) and white rot fungus Pleurotuseryngii (Hadibarata et al., 2013) showed degradation along with the COD removal. However, degradation of dyes in the textile wastewater by white-rot fungi has some intrinsic disadvantages like the long growth phase and the requirement of restrictive nitrogen environments,less enzyme production together with large reactor size requirement because of the long holding time for complete degradation(Hadibarata et al.,2013). The main problem with using fungi alone is that the system is not stable and after 20 to 30 days’ bacteria starts growing, and the fungi will not dominate the system and degrade the dyes (Toh et al.,2003). This is evidence that the presence of Algae is vital and is taking heights in the textile wastewater treatment(Bae et al.,2016).

The literature recommends dye degradation especially through the mechanisms of consumption of dyes for their growth, the transformation of dyes to non-colored intermediates or CO2and H2O and chromospheres adsorption on algae (Hayat et al., 2015). Research further recommends Green macroalgae Cladophora species(Mendez-Paz et al.,2005) for higher ability to decompose dyes like azo dyes due to the presence of azoreductase enzyme. (Meng, Liu et al. 2014) Studied azo dye (acid red 27) decolourisation shown in table 5 shows Shewanella algae(SAL)in the presence of high concentrations of NaCl and different quinones or humic acids which revealed mediated decolourisation of acid red 27 results into less phytotoxic aromatic amines. The author further reported the biodegradation of C.I.Basic Red 46(BR46)solution using the green microalgae nteromorpha sp. under optimum conditions with a reaction time of 5 h, a temperature of 250C, alga biomass of 2g and initial dye concentration of 15 mg/L.Thus,algal biomass plays a significant part in the elimination of azo dyes in the textile wastewater by biodegradation.

There has been recent study on biological treatment of model dyes and textile wastewaters using STATISTICA software. The scale ranging from 0.0 (non-desirable) to 1.0 (high desirable) to obtain a global function(D)as the highest value are achieved according to an efficient selection and optimization of the designed variables (Paz et al. 2017). The study revealed the maximum removal of up to (100%) with the desirability of 1 under the conditions of 37 0C,304.09 rpm and salt concentration of 19.204 g/L.These results were validated by repeating the experiment under these optimal conditions in triplicate, obtaining a decolorization percentage of 99.16±0.080%,near to the predicted value(100%),this implied the success of the empirical model. An additional experiment was carried out to increase the initial amount of CBB up to 180 mg/L under these operational conditions, achieving a considerable percentage of decolourisation of 98.31±0.082%. The color of the culture was brilliant blue at 0 h,drastically vanished after 48 h of cultivation,and it had almost transparent at the end of the process(72 h).

Meanwhile, the visible spectra showed maximum absorbance at 580 nm and a clear, sharp decrease with time of cultivation until complete removal after 72 h, demonstrating the effectiveness of the decolorization process (Fanchiang and Tseng,2009). Additionally, there was no appearance of other absorption peaks during the cultivation,indicating that no products with different colors are generated during the process.

Further studies on the optimal conditions achieved by the desirability option were also assayed with the other two dyes. Bacterial decolorization of Remazol Brilliant Blue R (RBBR)was 23.728 ± 0.806%, similar to optimization; however, Indigo Carmine (IC) was degraded in a percentage up to 99.55 ± 0.057%. Finally,an experiment raising the initial concentration of IC to 180 mg/L also provided a complete removal of this dye(100%) (Mendez-Paz et al., 2005; Wang et al., 2009). Removal of dyes from textile effluents in wastewaters using anaerobic biological process have previously shown resistance to decomposition, but anaerobic microbes have reducing capability(Li et al.,2015).

Further studies on texa revealed Bacteroidetes,Firmicutes and Proteobacteria prominent in the phyla,Clostridia and β-Proteobacteria are the dominant classes in most biological processes,and so are expected to play vital roles in the fermentation process(Shi et al.,2017). On the other hand,class Methanomicrobia,Thermoplasmata and Methanobacteria were reported in large numbers archaeal groups, members that show high activity by converting the substrate into methane and reducing their lethal capacity(Dai et al.,2018).

Studies on Cyanobacterium Phormidium revealed high decolourisation of indigo dye with (91%), unlike sulphur black and RBBR dyes. There was mild discolouration by Synechococcus in the dyes whereas Anabaena was able to degrade the indigo, but the discolouration percentage was lower than for Phormidium. The sulphur black was decolorised by Anabaena, which was produced at a partial degradation. Three strains removed the colour, whereas Synechococcus reduced due to the absorption measurement which affected by the green colour of the cultures. Phormidium led to improved colour reduction compared to Anabaena. Synechococcus,visual discolouration was observed to be more to the detection techniques (Talaiekhozani and Rezania, 2017).Anaerobic was first used rather than of anaerobic-aerobic system; it was reported that at least 17% and 33%efficiency was attained which was followed by the amalgamation of the two processes of anaerobic treatment.The anaerobic process led to the colour removal and produced wastewater with increased turbulence that was later treated with the aerobic process (van der Zee and Villaverde, 2005). Therefore, the complexity of such wastes requires the efforts of both anaerobic and aerobic processes for removal of color in anaerobic reactor and COD in the aerobic reactor (van der Zee and Villaverde, 2005). The study (Table 2) further revealed that the Cyanobacterium Phormidium decomposed the indigo dye extensively with at least(91%)than it could for sulfur black and RBBR dyes. In the same study, fewer colors were degraded with Synechococcus in the three dyes while Anabaena reduced indigo with less content compared with Phormidium results while Anabaena deteriorated black sulphur with little efficiency(Yang et al.,2012)three strains used efficiency attained colour removal,unlike only Synechococcus, showed a reduction in the rate of uptake. Phormidium had higher degradation efficiency compared to Anabaena, whereas Synechococcus, visual removal of colour was more compared to the technical ability to spot them(Dellamatrice et al.,2017).

Redox mediator has also been adopted to improve the biodegradability in anaerobic bioreactors of textile wastewater(van der Zee et al.,2003). Therefore combining an insoluble/immobilized Redox Mediator in table 3 to an anaerobic bioreactor is likely to influence on microbial structure(van der Zee et al.,2001). (a)The insoluble/immobilized Redox Mediators have quinones that change rapidly when interacted with quinone-reducing bacteria.

The continuous RM reaction would accelerate the growth of quinone-reducing bacteria. (b) Attached by Redox Mediator additional electrons originating from other constitutes could be moved to targeted contaminants(Dai et al., 2016). Therefore, it can result in the delayed methanogenic archaea. (c) Sulfate, which is one of the anions in textile wastewater,was degraded to sulfide by sulfate-reducing bacteria(SRB).The sulfide would then reduce targeted contaminants through a chemical reaction,which are easily speeded up by Radix Mediator(Singh et al., 2012). The accelerated consumption of sulfide may stimulate the metabolism of SRB,and thus,the SRB grows faster.

In another study of microbial accumulation in biological sulphate reduction in the inverted fluidized bed bioreactor. Chemical Oxygen Demand without sulphate in the influent, sulphite was liberated. This is the evidence that microbial accumulation in the bioprocess can donate electrons for resource recovery in biological treatment of wastewater(Cassidy et al.,2017). This innovation is vital for process performance in the treatment of wastewater. A recent study shows that removal of dyes like the reactive orange 16 can be efficiently bio decolurised(Mishra and Maiti,2018). This is an excellent achievement in bioreactor performance maximisation.

4 Anaerobic reactors for textile wastewater treatment

4.1 Pilot anaerobic reactors for textile wastewater treatment

A variety of high rate performing anaerobic bioreactor fabrications have been in existence for removal of pollutants in the wastewater at hydraulic retention time that is comparatively minimal. Among them is the anaerobic fluidized reactor which has proved a higher technology horizon and have been used in various wastewater strengths of low and strength where textile wastewater is among them. A research study about the anaerobic treatment of such wastewater with high rate anaerobic reactors like UASB and ABR are in use in addition to fluidised bed reactors(Stronach et al.,1987).

For considerable time anaerobic wastewater treatment has gained profound attention for its outstanding green resources like biogas, thus accredited technology (Kleerebezem et al., 2015). From its inception; it has been preferred mainly because of user-friendliness which requires less expertise to operate, less space requirement,minimal sludge release as well as better energy conversion about traditional aerobic treatment systems(van Lier,2008). Comparatively,the use of anaerobic treatment than activated sludge,almost 1 kWh of fossil energy kg-1COD liberated is much less. However, this is related to the system being used to aerate the activated sludge.Although during the anaerobic reaction the wastes are transformed to gaseous resource CH4,production of an estimate of 13.5 MJ CH4energy kg-1COD emitted thus 1.5 k Electric(this considers 40%electric conversion efficiency). In most developed nations of full-scale application, more than 90 %less is in sludge output; thus,great advantages have been reached at by countries where the systems are used. While the increased loading capacities of anaerobic high-rate reactors led to 90%decrease of the operation area,all related difficulties with old activated sludge are solved. Because of such merits,anaerobic high rate technology has been able to advance to date. The great researcher professor Gatze Lettinga reached at all these advancements(Lettinga et al.,1980).Nowadays there is an estimate of more than 4000 anaerobic high rate reactors, and more are getting adopted from the chemical wastewater treatment because it can readily decompose wastes of complex toxins(van Lier,van der Zee et al. 2016). As far as the technological advancements, wastewater high rate reactor technologieshave been upgraded. Resource recovery in the form of biogas was realised and again used in the production process that shows resource efficiency in the production which every industry would like to experience in the production(Quek et al.,2017). Also,the outcomes of anaerobic technologies have remained promising because of the tangible results entailed to it like less sludge, production of granular sludge, which are paramount in the production matrices. On a more significant note, the granular market value has been boosted for its one of the inoculants used in the reactors like UASB,which a great innovation in the reactor processes. To date,it has been used in the treatment of textile wastewater for its great benefits. The advancement has accrued to more than 90% of the full-scale applications in which anaerobic sludge bed technology is employed where the existence of granular sludge is vital(Show and Lee,2017).

Table 3 Effect of parameters on bacterial degradation.

Table 4 Recent reports on fungal cultures capable of dye degradation.

Table 5 Report on algae for dye removal.

4.2 Anaerobic reactors development trend of textile wastewater treatment

A variety of high rate performing anaerobic bioreactor fabrications are in existence for removal of pollutants in the wastewater at hydraulic retention time that is comparatively minimal. Among them is the anaerobic fluidized reactor which has proved a higher technology horizon and has been used in various wastewater strengths,including textile wastewaters. This has been evidenced the studies about the anaerobic treatment of such wastewater with high rate anaerobic reactors like UASB,and ABR are in use in addition to fluidised bed reactors(Stronach et al.,1987).

With the development of high-rate anaerobic reactor technology for textile wastewater treatment, a variety of reactor modifications have come up and are used in the anaerobic wastewater treatment as revealed by the ancestors of environmental biotechnology (Bajpai, 2017). The design of the first continuous flow anaerobic reactors in far back 1905 was by Karl. This was by the use of one tank flow through to settle the enhanced wastewater treatment. The works of this great scientist yielded a lot in the treatment of wastewater at a global level, and it’s still expanding boundaries more especially in the temperate regions of the world(van Lier et al.,2015). In another study,the biochemical oxidation and reduction activities during the oxygen-devoid digestion were discovered,which improved the understanding of the system(Buswell and Mueller,1952). In the invented reactors, hydraulic retention time and solid retention time were in the same compartment. In this context, the bacterial growth rate would determine the anaerobic conversion capacity of the reactor.

Until the early years of the 1960s completely stirred tank reactors were dominantly used for the anaerobic treatment of wastewater. However large volume requirement for biomass concentration remains their main problem. Unlike today, way back systems relied on the biocatalysts to boost the treatment capacity, especially the use of methane containing sludge in the bioreactors. High rate reactors are known to be the systems where the sludge retention time and hydraulic retention time are in different chambers. The come up of the innovations in the high rate reactors,has reduced capital costs and more for application in the industrial wastewater treatment.Therefore,high rate anaerobic reactors have been categorized based on the Sludge retention time being separated from the hydraulic retention time. The unattached anaerobic sludge through granule and biofilm formation is the only way the biomass was obtained in ancient technology that facilitated the biomass retention in the bioreactor;thus the high loading volume rates(Manyi-Loh et al.,2013). However, in cases where the biofilm and granule formation are delayed,the use of for physical retention could be the best alternative. This condition is common when the wastewater being treated contains too much suspended solids,salinity or high temperatures. Secondary clarifiers are the most appropriate whenever retention of sludge is required with waste activated sludge, more so filtration barriers or membrane can serve the same purpose. A variety of modifications high rate anaerobic reactors have been made since the come up of the technology about the nature of sludge retention method used.These include the following; Anaerobic Contact Process (ACP),anaerobic fluidized (AF),UASB,FB,EGSB,IC, ABR, membrane coupled high-rate (UASB/EGSB/FB) reactors, and membrane coupled CSTR systems.However, membrane coupled CSBR can as well be known as AnMBR. More so, various designs have been developed to full-scale applications.

At present, advanced high-rate sludge bed reactors include UASB and EGSB reactors and their derivatives that are currently being used in the treatment of industrial wastewater with a great market coverage of 90%being used at the moment(van Lier,2008). Presently;there has been an advancement in technology whereby high rate anaerobic reactors are manipulated due to their knowledge familiar with aerobic MBR operations as well as applications which are shared by the systems (van Lier et al., 2016). Microorganisms which are necessary for treatment of toxic as well as refractory wastewaters are availed by the sludge retention facilitated by the membrane expressed in table 4. In such a condition, the compactness of the biomass has no value for substrate destruction and washing out of the cell is not possible. Another advantage of combining the membrane is that it’s cheap and can offer effluent with a high level of nutrient resources for farming and application as in agriculture for soil water compensation(Yang,Ji et al.,2017).

There has been a breakthrough in anaerobic bioreactors by our research group with a patent(Patent number:ZL201, 210, 054, 218.6, which is SSSAB(Chen et al., 2016). SSSAB is partitioned into three compartments of elliptic plates that maintain the bacteria biomass and enhances material balance, as well as collection of methane gas(Chen et al. 2015). In this reactor, every compartment has got a pipe meant to evacuate the biogas generated thus abating the menace of obstruction of the bubble stream which boosts SSSAB’s ability to retain anaerobic glandular sludge. This enables the easy treatment of high strength textile wastewater since its confined,ease to use as well as excellent volume loading rates at minimal hydraulic retention time (Ji et al. 2012). A Simulation with computational fluid dynamics (CFD) of a high efficient anaerobic reactor and the structural parameters of SSSAB optimization has also been achieved(Zhang et al. 2018). This breakthrough has furthered the contribution to process performance of bioreactors for the current generation of the bioreactors. Increased performance creates solutions to high refractory textile wastewater treatment.

5 Conclusion

Anaerobic textile industrial wastewater treatment is a function of microbial activity in an anaerobic environment, bioreactor process and performance. This is important in achieving higher textile wastewater treatment efficiency. Then growing trends in new technologies is the best option to treat high strength textile wastewater. Therefore, the level of anaerobic wastewater treatment will always be dependent on process parameters,bioreactors, the nature of the compounds and character of microbes involved in the degradation.

The future of anaerobic textile wastewater treatment should focus on the new treatment technologies with attention to processes, microbial communities, bioreactors to effectively treat the high strength textile wastewater. The investigation should focus on how the combination of such factors discussed above for the textile wastewater treatment efficiency under anaerobic conditions in the new developed technologies. This would give a snapshot of the progress in textile wastewater anaerobic treatment and innovativeness.

Conflict of interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

The authors would like to thank the collaboration and contribution of our research group. Special thanks go to International Cultural Exchange School,ICES-Donghua university for the funding.