Improving biological treatment of textile wastewater

Textile industries are among of primary contributors of water pollution. Treatment of textile wastewater is very important before discharging it to the environment. In the present study Laboratory scale anaerobic batch reactors were used for co-treatment of a mixture of textile and domestic wastewater at 37 °C. The objective of this work was to investigate optimum conditions for the anaerobic co-digestion of textile wastewater and domestic wastewater. Domestic wastewater as a carbon source to enhance treatment of textile wastewater in color and other pollutants removal was examined. Textile and domestic wastewater were mixed at different proportions to make a total volume of 500 mL. Proportions of domestic wastewater and retention time were two main factors studied in influencing pollutants removal efficiency. Optimum conditions for removal of pollutants were 18 days residence time at 60 and 40% textile and domestic wastewater respectively. The removal efficiencies were 52.8, 58.3 and 51.6% for Color, BOD and COD, respectively. Phosphorus (PO4 3 ), Ammonium (NH3-N) and Nitrate (NO3-) increased at 78.5, 49 and 87% respectively. However, the concentration levels were above Tanzania Bureau of Standards (TBS) discharge limits. Post treatment is suggested to achieve standard discharge limits.


INTRODUCTION
Increasing industrial activities and urbanization lead to the increasing discharge of organic and inorganic contaminants to the environment. Textile industry is the primary contributor of water pollution among industries (Punzi 2015). Textile wastewater consist of colors, pigments, surfactants, grease, oil, metals, sulfate, chloride, and starch; all of these have impact on water quality. Dyes in water impede the passage of light; eventually reduce photosynthetic activities and oxygen level in water (Bafana et al. 2009). Heavy metals particularly, Lead (Pb), Chromium (Cr), Cadmium (Cd) and Copper (Cu) are widely used for the production of color pigments of textile dyes. High concentrations of heavy metals affect microbial activity in the system and impede biological wastewater process (Kumar & Mudhoo 2013). However, the recent study reported that, the concentration of heavy metals in textile wastewater from textile companies in Tanzania are below the recommended limit by Tanzania Bureau of standards (Bidu et al. 2021). Additionally, one of the impacts of textile effluents is that it has carcinogenic and mutagenic effects of which it is toxic to all forms of life. Textile industries consume huge quantities of water and yields huge amount of wastewater from different stages of production processes. For example, it is estimated that about 0.08-0.15 m 3 of water is used to produce 1 kg of fabrics (Ghaly et al. 2014). Therefore, wastewater from textile industries is a global problem which need appropriate treatment before discharging to the environment. There are several technologies available for the treatment of wastewater from textile industries. The selection of a suitable type of technology depends on various factors such as production process, chemical used, constituents of effluent, discharge standards, location, capital, operating costs, availability of land area, options of reusing/recycling the treated wastewater, skills and expertise available (Jegatheesan et al. 2016). During the past two decades, different treatment technologies have been studied to evaluate the sustainable treatment of textile wastewater. The studies showed that there are various wastewater treatments technologies but most of them are expensive, not environmentally friendly, require large space and produce much of biological sludge (Wei et al. 2020). One of the alternative approaches for the treatment of textile wastewater is by means of anaerobic treatment. Anaerobic treatment uses less energy, produces less biological sludge and require no aeration as compared to aerobic treatment. In addition to that, anaerobic biological treatments are considered as cheap and environmentally friend (Tapsoba et al. 2020). Among of the best anaerobic treatment of textile wastewater is codigestion of textile and domestic wastewater. The term co-digestion means the anaerobic digestion of two effluents, whereby a readily biodegradable stream is mixed with a more recalcitrant one in order to enhance the digestibility of the latter. Co-digestion of textile and domestic wastewater is used to reduce toxicity, increase dilution during treatment and provide carbon source to enhance dye degradation (Tapsoba et al. 2020). The use of bacterial methods can be useful in degrading dyes, including azo dyes. The use of microorganisms for biodegradation is suitable because it is multipurpose, has active metabolisms, and has potential machinery of enzymes. The use of microbes does not only ensure a non-toxic process but also have the capability to decolorize very complex synthetic dyes (Ferraz et al. 2014).
The method is among of an efficient way of degrading textile dyes and most preferred due to its effectiveness in improving quality of wastewater (i.e. BOD, COD, pH, and suspended solids) (Ferraz et al. 2014). Domestic wastewater is used as a carbon source which normally improves de-nitrification and microbial mediated processes causing degradation of wastewater pollutants (Ferraz et al. 2014).The composition and amount of dissolved organic carbon entering anaerobic treatment system may significantly affect the level of enzyme activity.
As it was noted that textile wastewaters are often rich in color and chemicals therefore need suitable handling before being released to the environment (Khatri et al. 2015). Even though Co-digestion is considered as among of the best method in treating textile wastewater, but due to the contents of pollutants such as toxic compounds in which some have low biodegradability therefore treatment of textile wastewater is still very challenging (Punzi 2015). Toxicity of textile wastewater to essential micro-organisms is another challenge in anaerobic treatment. Mercury, Antimony, Lead, and Arsenic, are some metals present in textile wastewater which are very toxic and limit micro-organisms activities (Kumar & Mudhoo 2013).
Several researchers investigated optimum condition by using various techniques in improving anaerobic treatment of textile wastewater. For example, Gnanapragasama et al. (2011) used tapioca sago wastewater rich in starch to treat textile wastewater. 30/70 was the optimum ratio of textile and starch wastewaters to reduce color and COD by 87.3 and 81%, respectively. Another method was the use of anaerobic biofilm reactor in 3 days retention time the optimum condition was able to reduce COD by 70% (Punzi 2015).
C/N ratio of the substrates is a crucial factor in the decolorization process because an appropriate nutrient balance is required by anaerobic microorganism for their growth as well as for maintaining a stable environment. Generally, a C/N ratio of 20-30 is considered optimal for bacterial growth in an anaerobic digestion system .
C/N ratio, pretreatment, retention time and pH variation are some of the factors regulating efficiency of codigestion. Therefore, this study was conducted with an objective of investigating optimum conditions for the treatment of textile wastewater through anaerobic co-digestion in which textile and domestic wastewaters inoculated with cow dung. Cow dung has several group of microorganisms such as Acinetobacter, Pseudomonas, Serratia, Bacillus and Alcaligenes spp. which make them fit for microbial degradation of pollutant (Gupta et al. 2016). The study used domestic wastewater at different ratio as source of nutrients to improve micro-organisms activities. Even though this study was conducted at laboratory scale, identified optimum condition can easily be applied at industrial scale since domestic wastewater can cheaply be obtained and used.

MATERIAL AND METHODS
The experimental setup was done in tropical region at the Nelson Mandela African Institution of Science and Technology (NM-AIST)'s laboratory located at À3.400905 latitude and 36.795659 longitude, Arusha-Tanzania.
Textile wastewater was collected from equalization tank of A-to-Z textile mill located at À3.38442 latitude and 36.59465 longitude. Domestic wastewater was collected from Nelson Mandela African Institution of Science and Technology (NM-AIST). Physiochemical parameters were determined in-situ for both textile and domestic wastewaters.
A laboratory-scale reactor was set up as follows; Four bottles containing domestic and textile wastewater at different ratios of 4:1, 3:2, 2:3 and 1:4 together with two bottles of pure textile and domestic wastewaters which were used as control experiments (Table 1). It should be noted that 100 and 0% textile wastewater represent pure textile and pure domestic wastewater, respectively. A total of six bottles were sealed and incubated in a water bath, which was maintained at a constant temperature of 37°C. This is the temperature at which microbial enzymes are most active, when enzymes are active the rate of biological reaction increases (Hance 2020). The reactor was constructed using an Erlenmeyer flask bottle (0.5 L) connected to a measuring cylinder (1 L). A plastic pipe was used to connect the reactor to the gas collector and parafilm was used to seal the Erlenmeyer flask's outlet to prevent gas leakage. The water displacement method was used to collect the biogas produced. According to Sharma (2017) for better inoculation, 1 g of cow dung needs to be added in 50 mL of wastewater.
The present study used 10 g of cow dung in 500 mL of wastewater, the initial parameters were determined after introducing a cow dung. Cow dung was used to introduce microorganisms in the mixture. Anaerobic treatment was conducted in batch for 3, 6, 12 and 18 days. According to Passos et al. (2015) the average time that the impact of anaerobic digestion can be observed or noticed is 15-20 days .The pH of the wastewater during system operation was adjusted to be 7.0. It is reported by that pH in the range of 6.5-7.5 is effective and favorable for microbial growth rate involved in anaerobic digestion. Physiochemical parameters were measured before and after mixing the two wastewaters and displayed in Table 2. Thereafter, physiochemical parameters were measured after treatment of 3, 6, 12, and 18 days and the results were compared with the initials readings.

Analytical procedures
Wastewater samples were collected as per standard method for water and wastewater sampling procedures (APHA 2012). The samples were stored tightly in 20 liters plastic cans of high-density polyethylene for further analysis. Parameter measured in-situ by Multiparameter (HI 9,829) were Temperature, Electrical Conductivity (EC), Total Dissolved Solids (TDS), Dissolved Oxygen (DO) and pH.
Parameters analyzed in laboratory by Hach spectrophotometer DR2800 according to standard methods described in (APHA 2012) were; Chemical Oxygen Demand (COD), nutrients (Nitrate (NO 3 -N), Ammonia (NH 3 -N), Phosphate (PO 4 -P), color and Suspended Solids (SS). Oxitop pressure was used for determination of Biological Oxygen Demand (BOD) while turbidity was measured by using turbidity meter.
To determine color, samples were Centrifuged (4,000 r/min) for 15 min then measured. Measurement of each parameter was done before mixing, after mixing and after anaerobic reaction of 3, 6, 12, and 18 days. To ensure quality of data measured, the equipments were calibrated before use. Measurements were also done twice for quality control of the values obtained in each measurment.

Data analysis
Origin pro version 9.0 (Origin lab 2012) and Microsoft Excel were used for data analysis. The removal efficiency in each parameter were obtained through Equation (1) except for the Nitrate, Nitrate -Ammonium and Ammonium which were calculated by Equation (2). C/N ratio was determined by Equation (3)

Characteristics of wastewaters before treatment
Characteristics of wastewater after mixing textile and domestic wastewaters at different proportional are displayed in Table 2. Pure textile wastewater had higher concentration of almost all measured parameters compared to that of domestic wastewater. High nutrients, TDS, TSS in pure textile wastewater possibly contributed to decrease amount of dissolved Oxygen. Concentration of dissolved Oxygen was lower in textile wastewater (1.9 mg/L) than domestic wastewater (2.98 mg/L). Color was higher in pure textile wastewater and the amount decreased with the increasing amount of domestic wastewater. For example, color was 2,800 Ptco in pure textile wastewater but additional of 80% domestic wastewater color decreased to 710 Ptco. Both textile and domestic wastewaters had basic pH, but textile wastewater was more basic than domestic wastewater. Generally, in this co-digestion treatment, domestic wastewater diluted textile wastewater Table 2.

Uncorrected Proof
Level of almost all pollutants in the mixture before treatment were above all available Tanzania Bureau of Standards (TBS) recommended level for discharging wastewater except nitrate and turbidity ( Table 2).

Characteristics of wastewater after treatment
Color removal Figure 2 shows that color removal varied when the experiment was set for different retention times and at different ratios of textile wastewater. As can be seen some of the values obtained were positive indicating color removal while others were negative indicating color increase. In general, color changed based on duration of the treatment and the composition of the mixture. There was increase in color for samples with 0% to 20% textile wastewater over all the days of anaerobic treatment. This might be caused by the addition of inoculum to the system which contains a number of natural dissolved organics matters. These organic carbons include humic acid, fulvic acids and natural tannins (Yap et al. 2018). For the 0-20% color increase was significantly affected by natural dissolved organic Carbon, as the number of days increased it continues to decay and the natural tannins tend to increase color to the domestic wastewater. For the case of 40-100% the effect was not seen due to strong color in textile wastewater. However, color reduction was observed for samples with 40-100% of textile wastewater. This is because at these proportions the contribution of the color of textile wastewater was high bearing in mind that textile wastewater was rich in color (Table 2). Therefore, the effect of color due to natural colorants from cow dung became negligible. In this case availability of domestic wastewater played part in reducing toxicity and provide carbon source to enhance dye degradation (Ferraz et al. 2014).
For all textile water proportions above 20% the removal efficiency increased significantly with textile wastewater proportion to 60% above which the increase became insignificant for all retention times tested. There was insignificant difference between graphs of retention time of 12 and 18 days especially for the textile  Uncorrected Proof wastewater proportion of 60%. At 60, 80 and 100% of textile wastewater for 18-and 12-days color removal was fairly ranging from 42 to 54%. The optimum removal occurred after 18 days of anaerobic treatment with a mixture consisting of 60% textile wastewater and 40% domestic wastewater in which removal efficiency was 52.8%. This improvement was due to the addition of external Carbon source, which helps to establish a reducing environment and possibly increased the concentration of enzyme co-factors, such as F430 and Vitamin B12 in the reactor that could also potentially reduce azo bonds and hence resulted in better color removal (Razoflores et al. 1997). Color removal increased with an increasing HRT in anaerobic process (Figure 2). Another factor influencing anaerobic reaction was the amount of domestic wastewater for example at 80% of textile wastewater, removal efficiency was 42.5 and 45% for 12 and 18 days, respectively. In this case color removal from additional of 20% domestic wastewater was lower than additional of 40% domestic wastewater. Domestic wastewater not only helped for dilution and dye degradation but also acted as electron donor for azo bond reductive cleavage (Ferraz et al. 2014). At optimum condition (60% textile wastewater in 18 days) amount of color decreased from 1,840 to 868 Ptco but still the final amount of color was higher than recommended level for discharging wastewater (Table 2). Post treatment such as constructed wetland can be used to remove color to the recommended level for discharging wastewater.

Chemical oxygen demand (COD) and biological oxygen demand (BOD 5 ) removal
The efficiency of anaerobic digestion can also be evaluated by using COD and BOD removal which reflects the amount of degradation taking place (Van Lier et al. 2008). Figure 4(a) shows that COD removal varied when the experiment was set for different number of days and at different ratios of textile wastewater. There was high removal of COD at pure domestic wastewater (0%) for 18 and 12 days which was 82.6 and 60%, respectively (Figure 4(a)) This removal was due to high microbial degradation of organic matter found in domestic wastewater. Since the aim of this experiment was the treatment of textile wastewater, even though removal of COD was maximum the conditions were not considered as optimum because no textile wastewater was included. Furthermore, COD removal decreased with the increasing textile wastewater. Maximum COD removal efficiency of 51.6% occurred after 18 days of treatment at 60% textile wastewater. Therefore, the results suggest that optimum condition for COD and color removal occurred at 60% of textile wastewater (Figures 2 and 4). Generally, time has significant effect on COD removal efficiency, as the number of days increased in the anaerobic system the more COD will be removed in the system. This was because as the time increased, more microorganism were produced and more organic matter were consumed which decreased COD.
BOD removal decreased with increasing textile wastewater. The optimum condition for BOD removal which coincides with COD and color removal was at 60% of textile wastewater after 18 days of treatment (Figures 2 and  4). As it was observed in COD removal, time has also significant effect on BOD removal, as the number of days increased in the anaerobic system the more BOD was removed (Figure 4(b)). High BOD in the untreated textile Uncorrected Proof wastewater can cause rapid depletion of dissolved Oxygen if it is directly discharged into the surface water sources. Thus, as the residence time increased the more BOD removal was observed but the removal was low when the ratio of textile wastewater was 80 and 100% this is because textile effluents are highly toxic containing heavy metals and most of them are non-degradable (Aslam et al. 2010).
After treatment in optimum condition COD and BOD decreased from 1,357 mg/L to 706 mg/L and 600 mg/L to 350 mg/L, respectively. The level of COD and BOD after treatment were above recommended TBS level for effluents discharge which is 60 mg/L for COD and 30 mg/L for BOD 5 (Table 2). In order to meet the TBS discharge standards this study recommends post treatment such as aerobic process by means of constructed wetlands.
There was production of biogas during anaerobic digestion when microorganisms break down (eat) organic materials in the absence of air (Tapsoba et al. 2020). Methane yield was efficient, and its production increased exponentially during the first 12 days of the experiments. Biogas is mostly methane (CH 4 ) and carbon dioxide (CO 2 ), with very small amounts of water vapor and other gases (Tapsoba et al. 2020).

CN ratio
There was a gradual decrease in the COD and BOD 5 removal with a decrease in C/N ratio and retention time whereas opposite was the case for color removal ( Figure 5). The results concur with ) who suggested that the low C/N ratio (,20) imbalances the anaerobic digestion and the releases of excess Ammonia in the digester and also inhibits the growth of methanogens. Maximum removal for both COD and BOD occurred after 18 days of anaerobic treatment with a C/N ratio of 11.92 in which removal efficiency was 82.6 and 66.5% respectively (Figure 5(a) and 5(b)). This removal was due to high amount of Carbon, low Nitrogen content and high microbial degradation of organic matter found in domestic wastewater. The function of Carbon in microbial processes are substrates for microbial enzyme synthesis and enzyme action.
At C/N ratio of 9.52, 9.12 and 8.46, color removal was 52.8%, 46.5% and 54.3% respectively for 18 days and 48.8%, 42% and 40.5% respectively for 12 days. Color removal increased with increasing retention time. Minimum color removal occurred at CN ratio of 11.92 in which the removal efficient was À67% (this means, the color of domestic wastewater has increased at 67 percent). Color increase was significantly affected by natural dissolved organic matter, as the number of days increased it continues to decay and the natural tannins tend to increase color to the domestic wastewater. (Yap et al. 2018).
From the Figure 5(a)-5(c), for both COD, BOD 5 and color optimum removal occurred after 18 days of anaerobic treatment with a C/N ratio of 9.52. These results indicated that decolorization of textile wastewater and the biological toxicity of textile wastewater significantly decreased after mixing with domestic wastewater due to availability of easily biodegradable Carbon Therefore, the anaerobic co-digestion of textile wastewater and domestic wastewater at certain ratios producing appropriate C/N ratio is possible for increasing the efficiency of color, causing also a remarkable BOD 5 and COD removal. The pH of the reaction mixture initially was 7.0 when the experiment was set for 18, 12, 6, and 3 days of anaerobic reaction. The pH of the mixture increased from pure domestic wastewater to pure textile wastewater ( Figure 6). The pH of Textile effluent was generally high because of the use of many alkaline substances in Textile processing (Khatmode et al. 2015). Anaerobic digestion of the organic substances produces Ammonia, the increase in Ammonia to the system will lead to the increasing of pH, most anaerobic bacteria grow best around neutral pH (6.5-7.5). (Dai et al. 2017). The pH at optimum condition (60% of textile wastewater in 18 days of treatment) was 8.3 which was under recommended TBS level for discharging wastewater (Table 2). Extreme's value of pH has to be avoided in order to maintain good reactor performance in biological systems, thus pH adjustment necessary.

Removal of total dissolved solids (TDS) and conductivity
For 3 days of treatment TDS and Conductivity was observed to increase. When graphs deviate to the negative side means that there was an increase in TDS and Conductivity to the system and vice versa on positive side. However, the increase of TDS and conductivity decreased with increasing textile wastewater and increase in retention time (Figure 7) Therefore, the increase in TDS and conductivity in 3 days might be caused by the addition of inoculum to the system and low retention time which limit time for reaction (Peng et al. 2020). The increase in TDS and EC might also arise from mineralization, i.e., the conversion of organic Carbon into smaller and simpler organic compounds. TDS and Conductivity decreased with increased retention time, that's why in 3 days Uncorrected Proof TDS and EC increased (Peng et al. 2020). Maximum increase of TDS and conductivity occurred at pure domestic wastewater whereas optimum removal occurred at 60% textile wastewater when the treatment was conducted for 18 days. Level of TDS at 60% textile wastewater before treatment was 3,511 mg/L whereas after 18 days of treatment level of TDS decreased to 1,305 mg/L. The concentration of TDS after treatment was still higher than 1,000 mg/L recommended WHO for discharging wastewater therefore post treatment is highly recommended such as activated Carbon, reverse osmosis, that can further improve effluents to attain WHO standard for discharging wastewater.

Phosphorus
Phosphorus in all forms increased with increasing textile wastewater for 18, 12, 6 and 3 days (Figure 8). In general, Phosphorus increased with increasing domestic wastewater and increase in retention time. This is because Phosphorus enters the wastewater stream primarily in the form of excreted human metabolic products (urine, feces), food residues, and detergents (Egle et al. 2015).
The driving forces for the increase in Phosphorus might also be caused by the presence of Phosphorus Accumulating Organisms (PAOs). This organisms can keep excess amounts of Phosphorus as polyphosphates in their cell under aerobic conditions, and secondly they can accumulate organic material in the anaerobic stage (Nieminen  Uncorrected Proof 2010). When exposed to anaerobic conditions, PAOs start using their intracellular Polyphosphates as an energy source to assimilate fermentation products from water. As retention time increase the energy supply for the PAOs will become depleted and lead to secondary release of phosphorus (Metcalf and Eddy 2003).
It should be noted that anaerobic treatment can be effective in the removal of organic compounds, i.e. COD, but it is unable to remove mineralized compounds such PO 4 3À (Van Lier et al. 2008). Concentration of PO 4 3À after treatment at 60% textile wastewater was 31.7 mg/L compared to 13 mg/L before treatment at 60% textile wastewater (Table 2). After treatment, level of Phosphorus was above recommended TBS level for effluents discharge (6 mg/L). Post treatment is recommended such as aerobic treatment (microbial biofilms of PAOs), that can further remove Phosphorus from wastewater to attain TBS standards.

Ammonium and nitrate
Both Nitrate-Nitrogen and Nitrate increased with increased textile and with increase in residence time (Figure 9(a) and 9(b)). Possibly there was a presence of iron in textile wastewater which might cause a feammox reaction. In feammox reaction, ferric iron [Fe(III)] is reduced coupled with anaerobic Ammonium oxidation. Fe(III) is reduced to Fe(II), accompanied with oxidation of NH 4 þ to N 2 , NO 2 À and NO 3 À (Yang et al. 2018).
This may led to increase in nitrate which might be further denitrified to form Nitrogen gas (N 2 ) (Yang et al. 2018). It was observed that, Ammonium in all forms increased with textile wastewater and with increased in retention time (Figure 10(a)-10(c)). The increase is due to anaerobic degradation of organic matter in the anaerobic environment which increases the release of Ammonium (Mahenge & Malabeja 2018). Also, Nitrate can be converted into Ammonium in a dissimilatory process called Nitrate ammonification. Domestic wastewater urea or uric acid in their Nitrogen-containing urine, along with diverse organic Nitrogen compounds in their feces (Bidu et al. 2021). The urea, uric acid, and organic Nitrogen of feces are all substrates for ammonification. It was observed that, there was increase in Ammonia with increase in textile wastewater this might be due to the presence of urea as dying auxiliaries in dying step (Bidu et al. 2021).

Uncorrected Proof
Various processes enable the removal of NH 4 þ in anoxic sediments, but any potential interaction with Fe(III) the removal remains largely unknown (Clement et al. 2005). If there was presence of Iron in wastewater may led to increase in Ammonia when treated in anaerobic system. Fe (III) may accelerate the release of Ammonia from the decomposition of protein which is the major organic components of wastewater sludge (Yang et al. 2018). Biological Nitrogen removal involves two successive processes, i.e., nitrification and denitrification. The nitrification transforms Ammonia to a more oxidized Nitrogen compound such as Nitrite or Nitrate, which is then converted to Nitrogen gas in the subsequent denitrification process (Ahmed et al. 2005). Without the availability of a ready source of biodegradable Carbon, denitrification will not occur, or will occur at a slow rate this is because of the absence of a readily biodegradable Carbon source that can be used as an effective substrate by denitrifying bacteria during the denitrification process (EPA 2013). The concentration of Nitrate (NO 3 À ) at 60% textile and 40% domestic wastewater in 18 days increased from 55 mg/L before treatment to 108 mg/L after treatment. Concentration of Nitrate after treatment was above recommended TBS levels for effluents discharge (Table 2). Post treatment is recommended that can further improve effluents to attain TBS standards. For the case of Ammonium, the trend was as follows;

CONCLUSION
This study investigated how domestic wastewater and cow dung can be used to improve treatment of textile wastewater. Domestic and textile wastewater were mixed at different proportions and allowed the co-digestion to take place in different days so as to identify optimum condition for the treatment of textile wastewater. It was observed that mixture of 60% textile and 40% domestic wastewater in 18 days gave optimum results for the treatment of Color, COD and BOD. Though the level of Color, COD, and BOD 5 after treatment were above recommended TBS level for effluents discharge. Post treatment such as constructed wetland can be used to remove Color, COD and BOD 5 to the recommended level for discharging wastewater. The importance of domestic wastewater was a co-substrate in removing Color, COD and BOD 5 by means of anaerobic system, which seems to be of great importance due to its effectiveness in treatment of those mentioned pollutants. The outcomes evidently displayed that small addition of domestic wastewater as Carbon source and 10 g of cow dung as inoculate are the appropriate co-substrates for bacterial dye decolorization process. Meanwhile, level of Phosphorus, Nitrate and Ammonium increased after treatment suggesting that there was a need to come up with an integrated technology that can remove Phosphorus, Nitrate and Ammonium from the system. From this study it is concluded that the use of domestic wastewater in treating textile wastewater is a promising technology in removing color, COD and BOD 5 from textile wastewater.

DATA AVAILABILITY STATEMENT
All relevant data are included in the paper or its Supplementary Information.