Investigation of functional performance of treatment systems for textile wastewater in selected textile industries in Tanzania

Textile industries are globally known for various industrial unit operations that are associated with large volumes of water consumption (Dasgupta et al. 2015; Oyetade et al. 2022). This high amount of water consumption accounts for the large volume of corresponding wastewater generated (Dasgupta et al. 2015). The quantitative properties of textile wastewater are in tandem with various unit processes such as resizing, scouring, bleaching, mercerization, dyeing, printing and pigmentation of textile substrates (Ghaly et al. 2014). An approximate value of 10,000 chemicals such as dyes, pigments and dyeing auxiliaries are industrially used in these unit processes and the resulting wastewater is characterized by 5–35% constituents of these chemicals on discharge (Khataee & Kasiri 2010; Verma et al. 2012). The wastewater discharged significantly holds an appreciable amount of these dyes of which <40% are biodegradable (Chacko & Subramaniam 2011). The chromophores, chromogens and auxochromes present in the dye structure in collaboration with dyeing axillaries account for the coloration and physicochemical properties of the wastewater (Chaube et al. 2010; Chung 2016; Amin et al. 2020). Among these dyes, the dominance of the azo group is rated 70% due to its unique fastness rating. However, poses great toxicity when discharged as effluent (Ventura-Camargo & Marin-Morales 2013). Also, the constituents of effluent from the process line of pigmentation, dyeing, printing and finishing are associated with extreme environmental toxicological impacts (Lu & Liu 2010; Abayomi et al. 2020). Effluent within this process line significantly alters the physicochemical parameters of waterbodies after indiscriminate discharge (Shindhal et al. 2021). Textile industries in Tanzania conventionally undergo treatment actions as remedial mitigation against environmental pollution resulting from discharged effluent (Bidu et al. 2021). These treatment processes are broadly classified as physical, chemical and biological with examples such as screening, neutralization, sedimentation, coagulation, flocculation, filtration, oxidation, adsorption, reverse osmosis, biological decolonization, ion exchange, phytoremediation (Bhatia et al. 2018; Roy et al. 2020; Oyetade et al. 2022). The physicochemical properties of generated wastewater are commonly measured in compliance with the standards set by the Tanzanian Bureau of Standards (TBS) and World Health Organization (WHO) before (influent) and after treatment (effluent). However, many industries seldom measure effluent from each treatment section to access its functionality and the efficiency of the system. Hence, it becomes imperative to quantitatively access the functionality of each treatment system through the quantitative physicochemical assay of the corresponding effluent. The study carried out a comprehensive assessment of the functional performance of various treatment systems for textile wastewater in selected textile industries in Tanzania and proffer logical scientific recommendations to achieve better treatment efficiency of textile wastewater.

Calibrated glassware was used for the experiment and all the chemicals for analysis were analytical grade. Chemicals such as HACH powdered pillows (HR) for determination of dissolved chemical compounds and HACH COD HR reagent vials for chemical oxygen demand (COD), respectively. Also, analytical grade deionized water was used throughout the experimental process. The instruments used include multiparameter kit HACH: HQ40d, Eppendorf Centurifuge:8810, portable UV spectrophotometer HACH: DR900, BOD machine: Oxitop^R IS 12 and COD reactor Hanna Instruments: H1839800.

The study was in Dar es Salam (−6.7924° S, 39.2083° E), Morogoro (−6.8278° S, 37.6591° E) and Arusha (−3.3869° S, 36.6830° E). Arusha region is geographically situated in the northeastern part of Tanzania, having 4.3% covered with lakes such as Eyasi, Manyara, Babati and Natron. Its land mass generally occupies 9.2% of Tanzania's mainland. Dar es Salaam is a low-level coastal elevation city having the highest localized industries and inhabiting the major port of Tanzania. It occupies about 0.2% of Tanzania's mainland and borders the Indian Ocean. Morogoro is the third largest region after Arusha and Tabora having an approximate value of 8.2% of the total area of Tanzania mainland. Four textile industries were selected for effluent sampling within this locale, with one in Dar es Salaam, one in Morogoro and two in Arusha. For the selected textile industries, a qualitative survey was carried out on the catalog of industrial dyes commonly used by these industries and the corresponding input and outputs of wastewater vis-à-vis the treatment system were monitored from 7 am to 9 am daily. Also, the process line involving the wet and dry treatment process was surveyed and the corresponding effluent from the respective treatment plant of each industry was quantitatively evaluated.

The textile wastewater sample was obtained from the influent (wastewater after the wet treatment process of textile material) and wastewater after each treatment system (effluent). The sample collection vial was labeled accordingly for the onsite quantitative determination of physicochemical parameters. Another analysis was carried out in the laboratory of Nelson Mandela African Institution of Science and Technology (NM-AIST) Arusha, Tanzania. The transported samples were kept in an ice bag and transferred into the refrigerator below 4 °C before analysis. The techniques and methods followed for collection, preservation, analysis and interpretation are those given by APHA (2012).

The quantitative determination of the physicochemical parameters such as pH, temperature, total dissolved solids (TDSs), total suspended solids (TSSs) turbidity, dissolved oxygen (DO) and conductivity of the influent and the effluent at each sampling point were done onsite using the multiparameter kit instrument, following APHA (2012). For the analysis of the color (Pt-Co) at 420 nm, an aliquot of the samples was centrifuged (4,000 r/min) for 15 min and then quantified with the multiparameter kit (Laizer et al. 2022).

The quantitative analysis of the COD and dissolved compounds such NO₃, NH₃, NH₃-N, PO₃, PO^4–, P₂O₅ and P was done using HACH pillow reagents for each compound and quantified via colorimetric technique at NM-AIST Arusha, Tanzania.

The data generated from the experimental processes were statistically analyzed using IBM SPSS version 16.0 (Chicago, USA) computer software programs. The experimental data are presented as the mean ± SE of the replicate experiment at *p ≤ 0.05.

Similar, observation was made for the TSS value in Figure 2 having the highest value of 960.67 ± 0.67 mg/L was recorded for the raw while 361 ± 0.58 mg/L (62.46%) was reported for the filtered stream, respectively. The high amount in the color and TSS values equally reflect in the turbidity of the raw effluent (518.33 ± 1.00 NTU) which is eventually below the tolerable limits (281.33 ± 0.88 NTU) after the filtration system. Generally, a high amount of TSS results in high turbidity which has deleterious effects on discharged water bodies and greatly maligns the porosity and texture of soil (Saini 2017; Slama et al. 2021). Although, the value of dissolved chemical species such as ⁠, ⁠, phosphorus in mg/L are within the tolerable limits after treatment (Figure 3(b)). However, the high amount of NH₃ (31.30 ± 0.00 and 21.80 ± 0.06 mg/L) and NH₃-N (23.14 ± 0.03 and 16.37 ± 0.1 mg/L) in raw and coagulated stream accounts for high BOD and COD values (Figure 3(b)). However, the slight increase 17.73 ± 0.03 mg/L in the value of NH₃– can be due to contamination of the effluent discharged by nomadic grazing and discharges of animal feces. The result in Figure 3(a) reported an extreme amount of COD and BOD of 4,025.73 ± 0.10 and 329 ± 0.00 mg/L, respectively, in raw effluent which is similar to a study carried out by Rajendran (2018) with appreciable amount of statistical significant difference from other respective point of discharge. However, significant reduction of 459.07 ± 0.03, 122.00 ± 0.00 mg/L and 371.2 ± 0.07, 75.00 ± 0.00 mg/L was recorded for the BOD and COD of filtered stream and final output, respectively.

Although 75.00 ± 0.00 mg/L was lower than the limits by WHO and TBS, values of other section based on TBS limits were beyond the tolerable limits. These high-reported values of COD and BOD pose great toxicological impacts on the environment and indicate a high amount of bio-resistant chemical compounds in the textile wastewater (Halim et al. 2018).

After treatment of the textile wastewater in the biological reactor, the BOD value of 108.00 ± 0.00 mg/L was recorded which was initially 155.00 ± 0.00 mg/L. However, 75.00 ± 0.00 mg/L was quantified with continuous treatment in non-aerated CWs, while 65.00 ± 0.00 mg/L was recorded for aerated CWs (lower than the limits). Similarly, the TSS value of 165.07 ± 0.06 mg/L was observed for the raw effluent which reduces significantly to 99.51 ± 0.82 mg/L after passing through the bioreactor system coupled with a aerated CW. This significant reduction in the aerated CW shows improved microbial growth that aids dye decolorization thereby reducing the suspended solutes in the effluent (Bidu et al. 2021). Furthermore, the vital importance of the integration of CW enhance the removal of heavy metals pollutants via phytoremediation alongside microbial decolorization of dye molecules (Masi et al. 2019; Tara et al. 2019). However, the situation of extreme bioaccumulation due to high organic loading from the effluent consequently lowers the functional efficiency of the treatment system (Wu et al. 2015). Technically, these CWs are engineered systems which uses soil, plants and microorganism to remediate toxic pollutants in wastewater. The textile wastewater treated via these technology uses the process of biodegradation and phytoremediation to naturally attenuate toxic pollutants for subsequent removal and mineralization (Truu et al. 2015).

The mechanism CWs involves the use of soil with large surface area and plant roots as biofilm layers which separate out the large, suspended solids from the effluent by the action of filtration, sedimentation, adsorption and precipitation (Qin & Chen 2016). The treatment process involves organic matter decomposition via aerobic or anerobic action with the end products of CO₂ and H₂O or CO₂ and CH₄, respectively, to achieve the reduction in effluent parameters (Qin & Chen 2016; Barik 2018).

However, the low absorption capacity of the treatment system may be due to the oversaturation of the binding site available during treatment processes (Zare et al. 2018; Khan et al. 2021). Also, the color of the raw effluent was reported higher (2,051.00 ± 0.58), however, it undergoes a significant reduction of 90% (237.33 ± 0.67) at Eff 6 (activated carbon filtration system), which is below the tolerable limits. Effective dye-adsorbent interaction account for the significant color reduction of the resulting effluent (Nassar et al. 2015). From Figure 7(a) and 7(b), the dissolved ions such as phosphorus (mg/L) reduces with continuous treatment below the tolerable limits.

However, a significant amount of ⁠, NH₃, NH₃-N observed was similar to the study carried out by Singh et al. (2013) on the remediation of textile wastewater. The highest COD reflected in Eff 2 having a value of 1,580.67 ± 0.67 mg/L but decreases considerably to the value of 267.33 ± 0.33mg/L (83%) and 375.67 ± 0.09 mg/L Eff5 and Eff 6, respectively. Furthermore, the sand filtration system (Eff 5) proves effective due to the resulting value of the BOD (40 ± 0.00 mg/L) which is within the tolerable limit of WHO. The BOD, COD and BOD/COD often give indications of the extent of the organic pollution and biodegradable index, respectively, in wastewater (Paul et al. 2012). It is noteworthy to add that increase in BOD causes depletion in oxygen levels which has an impaired effect on aquatic life (Tishmack & Jones 2003). The hydrolysis and reaction of azo dyes with other pollutants results in the generation of carcinogenic byproducts (Ventura-Camargo & Marin-morales 2013). Generally, one of the limitations of the conventional treatment for textile effluent is the challenges of removing pollutants by transferring them from one phase to another or to more toxic secondary pollutants (Martí et al. 2010). This is because only 45–47% of dyestuff has been reported biodegradable while others are characterized as recalcitrant to treatment (Rauf et al. 2011; Zubair et al. 2017).

However, the observed dispersity is due to the effluent constituents and different treatment technologies used for the generated wastewater (Aniyikaiye et al. 2019). Also, the TDS results in Figure 8 are extremely higher with a value of (11,950 ± 5,100mg/L) after coagulation with CaOCl and exhibit no statistical difference concerning continuous treatment. However, in Figure 6, it was 7,955.00 ± 2.89 mg/L in value with statistical significance with respect to continuous treatment. Aboulhassan et al. (2014) added that the high TDS value is a direct determinant of the conductivity value of the effluent. The TSS value of the effluent was reported to be 5,450.10 ± 0.06, 1,065.03 ± 0.33, 9,924.00 ± 0.58 1,239.33 ± 0.33 mg/L for S2, S3, S4, respectively, which is similar to the influent and effluent analysis of textile wastewater by Aniyikaiye et al. (2019). Similarly, the level of TSS indicates the level of turbidity, which was considerably high above the tolerable limits in Figure 8 (Alrumman et al. 2016). Furthermore, only was reported below the tolerable limits in Figure 9(a) and 9(b), but the value of other dissolved ions is considerably high. It is expected that high values of the quantified ions is consequent of the increase in COD and the BOD textile wastewater (Ghaly et al. 2014). Additionally, Figure 9(b) describes NH₃ and NH₃-N has the highest among compounds quantified. Their reported value was 100.13 ± 0.88 and 75.06 ± 0.07 mg/L, respectively, after coagulation treatment and their persistence through the treatment process were shown in Figure 9(b). Although the value of NH₃ increases to 132.5 ± 0.06 mg/L, the filtered stream reduces to 5.10 ± 0.06 mg/L. The high amount of the compounds with other ions is suggestive of the low DO reported in Figure 8. Generally, the value of DO is the main determinant in an aquatic ecosystem as it indicates the survival of aquatic life (Trick et al. 2008; Edokpayi et al. 2015; Aniyikaiye et al. 2019). Also, persistency of ammoniacal nitrogen and NH₃ is a characteristic feature of effluent laden with recalcitrant azo. The transformative behavior of these dyes and auxiliaries in effluent can result in the generation of excessive biomass and non-biodegradable substances on discharge (Asia et al. 2006; Smith et al. 2007).

The study shows that reactive, vat and dispersed dyes are the most commonly used dyes in these selected textile industries having a predominant class of the azo chromophoric system. The analyzed performance of the respective treatment system regarding their corresponding wastewater exhibited a high amount of quantified parameters when compared with the set standard by TBS and WHO. Among the measured parameters BOD, COD, Color, TSS, TDS, Turbidity and dissolved chemical compounds were significantly higher than the tolerable limits. Among the dissolved chemical species, the highest recorded were phosphorus, NH₃ and NH₃-N which were above the tolerable limits for all the sample effluents. However, the use of an activated carbon system and sand filtration proffer a significant remedial action for the color and corresponding turbidity of the effluent. Also, aerated CW exhibited significant functional performance in the remediation of NH₃, NH₃-N which was significantly persisting in the treated effluent. The study noted the recalcitrant behavior of these industrial synthetic dyes to continuous treatment and indicated susceptibility of land and many riverine areas to increasing algae blooms and consequent toxicological allergies in man.

Based on the reported findings of functional efficiency of the current treatment technologies within the selected textile industries, it is needful to recommend the following:

• The need for prompt and proactive measures on the general maintenance of the treatment plants of respective textile industries with strict monitoring of the quality of discharged effluent in compliance with set standards.

• The incorporation of the continuous integrated system with high selectivity to lower vital parameters such as color, TDS, TSS, COD and BOD which are considerably higher than the tolerable limits from the report.

• The used of modern design CWs such as the floating treatment wetlands (FTWs) and implementation of effective control of seepage and oversaturation of water during flood and heavy rain in coastal region like Morogoro and Dar es Salaam.

• The introduction of more effective bioorganisms such as Trametes versicolor and Aspergillus luchuensis into the CWs and bioreactor to completely remove persisting dissolved ions such as NH₃ and NH₃-N, phosphorus and PO₄³⁻ pollutants.

• Monitoring of influence concentration to reduce oversaturation and constant maintenance of the filter cartridges of the filtration system.

• Adoption of durable and highly selective nano textile filters used in modern sequencing batch reactor.

• Adoption of the flow rate and concentration control system to reduce oversaturation and overloading of the sorbent system.

• The use of improved and impregnated nanosorbents with effective selectivity in the removal of persisting and toxic pollutants from the wastewater.

This work was supported and funded by the Regional Scholarship for Innovation Fund (RSIF), a flagship program of the Partnership for Skills in Applied Sciences, Engineering and Technology (PASET).

The authors declare an absence of competing financial interests in personal relationships that could influence the work reported in this paper.

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

The authors declare there is no conflict.