Effects of Tanning on Seasonal Variation in the Physiochemical Quality of Surface and Groundwater and Including an Analysis of Trace Metals in Hazaribagh

The leather sector is one of the promising sectors of Bangladesh and one of the most important foreign exchange income in Bangladesh. The Government of Bangladesh (GoB), and the Ministry of Industry (MoI), have designated it as a priority sector due to the fact that it is a value-added sector. In Bangladesh, tannery industries occupy 3% of all industries [1]. Hazaribagh is the largest tannery hub in Bangladesh, located in the western part of Dhaha city, the capital of Bangladesh. Although 185 leather-processing plants [2] are located here, it is still considered as a residential area as most leather-processing workers and staff live there. This particular region is well-known for its tannery businesses and is ranked fifth among the world’s 10 most polluted locations [3]. More than 70% of tanneries in Bangladesh are located at Hazaribag, while the rest of the tanneries are scattered all over the country [4].

Leather tanning is a completely wet process that needs a lot of water. Almost 90% of the water used in the process turns into wastewater [5]. The process of tanning involves transforming hides and skins into leathers with specific physical, chemical, and biological qualities so that they can be used for everyday use and by industries. A number of orderly steps, including soaking, liming, deliming, bating, pickling, tanning, neutralization, fat-liquoring, finishing, etc. are used to turn this operation. The water becomes murky when lime, hair, skin, etc. are present.

There are two types of tanning processes: vegetable tanning and chrome tanning. However, the most common tanning process in Bangladesh is chrome tanning. Most tanneries generate chrome-tanned leather, which results in the discharge of a substantial amount of Cr via the effluents. In chrome tanning, a full tannage requires 0.2% or more of the hide’s weight in chromium, and the fixing rate of the Cr is about 60-70%. Cr is used to tan 80-90% of the world’s leather today.

Huq [6] claims that several chemicals are employed in the chrome tanning process of hides and skins for soaking, tanning, and post-tanning. Na₂SO₃, Cr₂(SO₄)₃, Na₂CO₃, CaO, NH₄NO₃, NH₄Cl, and enzymes are the principal compounds used. Other chemicals include CH₂O, Na₂S, NaCl, H₂SO₄, CH₂O₂, HCOONa, and NaHCO₃ as well as vegetable tannins and resins. Leather only absorbs about 20% of the massive quantities of chemicals required in the tanning process; the remainder is discharged as waste [7].

The production of tanned and untanned leather uses a lot of water, and the associated solid waste and liquid effluents are a substantial source of organic load, salts, and Cr (40-25,000 mg/L) that are released into the environment. Suspended solids (SS), NO₃^- sulfate, sulfide, and Cr are common pollutants found in tannery effluent that are directly discharged into an aqueous system in most developing countries without any treatment [8]. Water systems with high concentrations of total dissolved solids (TDS), biological oxygen demand (BOD), and chemical oxygen demand (COD) place aquatic species under stress [9]. Dissolved organic matter breaks down into cations and anions and changes the basic composition of water [10].

In Hazaribagh, the majority of liquid sewage is discharged into natural water bodies directly or indirectly, with little or no treatment. The Hazaribag tannery effluents discharged into the canal (from the southeast) are ultimately discharged into the Buriganga river, located at the northeastern corner of the study area [11]. Through open drains, the leather processing plants discharge solid and liquid waste either directly or indirectly to low-lying areas, rivers, and natural canals without sufficient treatment. The water in the low-lying regions close to Hazaribagh, which has a direct connection to the Buriganga river, is so contaminated that it is no longer fit for public use for general purposes [2]. Along with animal flesh and dissolved hair and lipids, this water also contains lime, hydrogen sulfide, chromium sulfate, sulfuric and formic acids, colors, oils, and a variety of heavy metals utilized in the tanning process. This water eventually flows into the Buriganga River and eventually settles in the low area of Hazaribagh, causing severe deterioration of the surface and groundwater quality [12].

Direct disposal of these pollutants has contaminated ground and surface water with Pb, Cd, As, and dangerously high levels of Cr. Because of pollution in rivers, these chemicals can bioaccumulate in fish and shellfish, which are important sources of food for people in the area [11]. In addition, the BOD in the river tends to increase, thus depriving aquatic life of oxygen [13]. Not only are the water, soil, and plants in this area at high risk of heavy metal pollution, but Mondol et al. [14] found that soil and plant samples have concentrations of heavy metals that are over the maximum acceptable concentration. At a greater distance from the primary disposal point, the concentration was found to decrease steadily. The main disposal point at the Buriganga river has an exceptionally high quantity of Cr. A similar pattern of results was observed with plant specimens. When contaminated sewage is used to irrigate agricultural land, the metals may accumulate there and then bio-concentrate in the edible plants and feed [12]. Moreover, tiny pieces of leather, hair, flesh, and liquid and solid wastes generated in various stages of the tanning process are thrown and piled up all over the Hazaribagh area, and are often used as fish feed ingredients [15].

Living in the Hazaribagh area is challenging due to the unpleasant visual and olfactory effects of tanning activities. The tanneries produce a great deal of waste, both solid and liquid. Every day, the tanneries in Hazaribagh make about 75 metric tons of solid waste, mostly salt, bones, and leather shavings and trimmings, and about 21,600 cubic meters of liquid waste that is bad for the environment. According to the WHO in 2001, more than 8,000 Hazaribag tannery workers have gastrointestinal, dermatological, and other illnesses, and 90% of them passed away before the age of 50.

Our study’s primary objective was to determine the impact of the tanning process in the quality of the surface water and groundwater of this tannery processing area as well as to evaluate the seasonal of different physical, chemical, and physio-chemical parameters of surface and groundwater due to the tanning process.

At different sampling locations, the turbidity during the wet season ranged from 2.4 to 530 FTU, while during the dry season, the turbidity ranged from 3.68 to 4096 FTU. In the context of water, turbidity is caused by the presence of suspended particles or sediment in the water. These particles scatter and absorb light, making the water appear cloudy or opaque [22]. Turbidity reached its highest point during the dry season at sampling site I (4,096 FTU), which then began to steadily fall from the source all the way downstream and beyond the barrier. Groundwater had the highest clarity, followed by surface water in the order of decreasing clarity (3.68 FTU). During the months with more precipitation, the same pattern was seen, with the greatest turbidity value (530 FTU) being found close to the sampling point of origin, also known as sampling site I. Turbidity above the regulatory limit was observed at sampling spots II, III, IV, and I in the dry season [17].

3.2 Heavy metals

We measured the concentration of Cr in both dry and wet-season wastewater samples. During the dry months, the highest Cr content (8995.0 mg/L) (Table 3) was found at the primary tannery disposal point. Sampling point I may be contaminated by Cr salts used in tanneries or wastewater, and effluents from other industries are constantly being added [21]. There was a large gap between sample locations II, III, IV, and V, VI. As expected, Cr levels dropped during the wetter months. Cr was found to be 17.86 and 863.03 mg/L at sampling points V and I, respectively. Dilution by rainwater may be responsible for this reduction. The Cr in the effluent from Hazaribag was measured at 0.28 ppm by Imamul-Huq [6]. This finding is consistent with that of Ullal et al. [21]. All samples had Cr. Cr concentration across the MAC (Maximum Allowed Concentration) for surface water 0.5 mg/kg [18] and WHO drinking water standard 0.05 mg/kg during dry and rainy seasons [19].

During the dry seasons, the Cd contents in wastewater samples ranged from 0.006 to 0.035 mg/L, whereas during the rainy seasons, these values were ranged from 0.005 to 0.035 mg/L (Table 4 and Figure 3). There were significant differences present in groundwater samples as well as VI samples. During the wet season, the exact sampling location had significantly lower levels of cadmium. The rain was the main responsible for lowering the Cd concentration. Maximum concentration of Cd that the WHO considers safe for drinking water is 0.005 mg/kg. However, all sampling sites had higher levels of Cd than the national guidelines (0.05 mg/kg) and WHO for inland water [18].

Table 3. Cr mg/L in wastewater at sampling points of study area both in dry and rainy season

Table 4. Cd mg/L in wastewater at sampling points of study area both in dry and rainy season

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Figure 3. Change of Cd (ppm) at sampling points in dry and wet season

The highest concentration of Zn in water samples collected during the dry season was found at point I (3.10 mg/L), followed by concentrations of 2.61 mg/L, 2.21 mg/L, 1.76 mg/L, 1.73 mg/L, 0.601 mg/L, and groundwater concentrations of 0.601 mg/L (Table 5 and Figure 4). There were major deviations only at points II and III. Ullah et al. [21] found only 0.12 ppm in the wastewater of Hazaribag. Inland and drinking water samples all had MAC concentrations higher than 5 mg/kg [18, 19].

Sampling site I, the main tannery disposal point inside the embankment, had Mn concentrations of 1.8 mg/L (Table 6 and Figure 4). Tannery manganese salts could contaminate sampling site I [23]. Both the sample and the groundwater were statistically significant (5% level of significance). During the monsoon months, concentrations of Mn 0.03 and 1.73 mg/L were found at sampling point I and groundwater, respectively. On the other hand, Ullal et al. [21] measured 0.014 ppm of Mn in the wastewater of the Hazaribag tannery. For Mn, national and international standards (by WHO) is for both inland water (5 mg/kg) [18] and drinking water (0.05 mg/kg) [19]. From this study, we observed Mn in all collected water samples was higher than the national and WHO standards.

The concentration of Fe ranged from 38.93 to 275.01 mg/L in the dry and wet seasons, respectively (Table 7 and Figure 5). In case of Fe concentration, sampling point I and II was different from other sampling points. During the wet season, Fe was lower at the same sampling point compared to the dry season. Observed Fe concentration at all sampling points (sampling points I to VI and GW) was higher than the WHO and national standards.

Table 5. Zn mg/L in wastewater at sampling points of the study area both in dry and rainy seasons

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Figure 4. Change of Zn & Mn (ppm) at sampling points in dry and wet season

Table 6. Mn mg/L in wastewater at sampling points of study area both in dry and rainy season

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Figure 5. Change of Pb & Ni (ppm) at sampling points in dry and wet season

Pb concentrations during the dry season were 1.53 and 3.15 mg/L at sites I and III, respectively (Table 8 and Figure 5). There were no noticeable variations in any of 5 sampling locations. Total Pb concentrations at both sampling locations I and II were between 0.366 and 0.94 mg/L respectively. The typical effluent standard for inland surface water was 0.1 mg/kg, however all tests exceeded this standard [18].

Table 7. Fe mg/L in wastewater at sampling point of study area both in dry and rainy season

Table 8. Pb mg/L in wastewater at sampling points of study area both in dry and rainy season

Table 9. Nickel mg/L in wastewater at sampling point of study area both in dry and rainy season

The Ni concentrations in wastewater measured from sampling sites I through VI, as well as groundwater, are shown in Table 9 for the dry season. There was no noticeable variation across samples. However, observations were different during the rainy season. Ni concentration was lower in the rainy season compared to dry season. Ni levels in both drinking water (0.1 mg/kg) and inland water (1 mg/kg) (Table 9) were above the maximum contaminant level (MAC) [18, 19].

The research was funded by the Ministry of Science and Technology in Bangladesh through the National Science and Technology (NST) Fellowship (grant number 164). Laboratory work was done at the University of Dhaka’s Soil, Water, and Environment lab as well as at the Bangladesh Council for Scientific and Industrial Research (BCSIR). We appreciate the guidance of the SWED head and the BCSIR chairman. We would like to extend our sincere appreciation to the Hazaribag area’s tannery employees, producers, and traders for their helpful suggestions, insights, and support in collecting samples.