51

4. PERFORMANCE EVALUATION OF EFFLUENT TREATMENT

PLANT IN A TANNERY AT TIRUCHIRAPPALLI

4.1 INTRODUCTION

Tannery effluent is one of the most polluting industrial wastes. Leather

production requires large amount of water (i.e.) 35 litre/kg of water is consumed

per kg of raw material processed. Leather production consists of three main

processes. They are:

a) Beamhouse process in which salt, dirt and hair are removed. The process

involves the following:

i) Desalting and soaking the hides to remove salt (which is used to

preserve skins). The process uses a large amount of water (up to 20

cubic meter water per ton of hide). The most significant pollutants

produced by the soaking process include salt, hide surface impurities,

dirt and globular protein substances dissolved in water.

ii) Unhairing and liming. Conventionally, unhairing is done by treating

soaked hides in a bath containing sodium sulphide/hydrosulphide and

lime. The effluent from this process is the most polluted effluent of the

tanning process. The pollutants include suspended solids, sulphides

and nitrogenous material.

iii) Deliming and Bating. In this pelt is processed in a bath of ammonium

salt and proteolytic enzymes. The pollutants from the process include

calcium salts, sulphide residues, degraded proteins and residual

proteolytic enzymatic agents.

52

b) Tanning under which the hide is treated with chemicals to produce

leather. Chrome is the most common tanning agent used in the world.

Conventionally, chrome tanning consists of pickling, tanning and basifying. The

main pollutants of the tanning process are: chrome, chlorides and sulphates.

c) Post tanning (wet finishing) which includes neutralization, retanning,

dying and fat liquoring. The pollutants from the process include chrome, salt,

dyestuff residues, falt liquoring agents and vegetable tannins.

d) Finishing in which the leather is given desired properties. The main

pollutants produced during finishing are suspended solids and chrome.

In addition to the above mentioned pollutants, which are discharged in the

effluent, leather production also produces emissions. These include: ammonia

during deliming and unhairing; sulphide during liming; chrome during chromate

reduction and from the buffing process. Also, alkaline sulphide may be

converted to hydrogen sulphide if the pH is less than 8.0. Further, particulate

emission may occur during shaving, drying and buffing.

In short, tannery effluents have high concentration of proteins, chlorides,

trivalent chromium, nitrogen, sulphate, sulphides, COD, BOD and suspended

solids. Volume and characteristics of wastewater discharged vary from process

to process, tannery to tannery and from time to time. The operations in a tannery

are done in batches and discharge of wastewater is also intermittent.

53

Wastewater from beam house operations like soaking, liming, deliming,

bathing etc., is alkaline and contains decomposable organic matter, hair, lime,

TSS, TDS, Sulphides and BOD. This is mainly due to the poor quality of

calcium hydroxide and other chemicals used in excess without proper control.

Wastewater from tannery yard operations namely pickling, vegetable tanning,

chrome tanning, fat liquoring etc. is acidic and coloured. Wastewater from

vegetable tanning has high organic matter and wastewater from chrome tanning

has large quantity of chromium, since only 50 to 70% of ‘Basic Chromium

Sulphate’ (BCS) is taken up by leather and the balance is discharged in the

waste. It is estimated that 40,000 tonnes of BCS is used in Indian tanneries per

year and about 15,000 tonnes of BCS is discharged as waste in the effluent

(Rajamani and Ragavan, 1995).

Total chromium, sulphide, TDS, TSS, BOD, COD are the important

characteristics of tannery effluent. The suspended solid components of effluent

are the quantity of insoluble matter in wastewater. These insoluble materials are

produced from all parts of leather making, comprising of leather particles,

residues from chemical discharges etc. Very large quantity is generated from

beam house process. Solids are also protein residues from beam houses. TDS is

due to the usage of large quantities of sodium chloride used in the pickling

process (Sahasranaman and Buljan, 2000). BOD and COD refer to the high

oxygen demand by the effluent through biological organisms and chemicals.

Ammonium results from deliming process. Sulphides is the result of using

sodium sulphide and sodium hydrosulphide.

54

According to present pollution control regulations of India, the treated

effluent shall meet the pollution control standards i.e. pH 5.5 - 9.0, temperature

40°C, BOD 30 mg/L, COD 250 mg/L, TDS 2100 mg/L, Chloride 1000 mg/L,

chromium 2 mg/L, TSS 100 mg/L, for discharge into inland surface water

(Rajamani, 1995). The treatment of tannery effluent with higher BOD, COD,

dissolved solids, chlorides is a major task and it needs various types of treatment

like primary treatment (physical and chemical treatment), secondary treatment

(biological treatment) and tertiary treatment (special treatment).

The primary treatment process of an ETP includes pretreatment chamber

for the removal of suspended solids, equalization tank to minimize the wide

fluctuation in effluent flow rate and variation in the tanning process, flash

mixture tank where the effluent is quickly mixed with alum to get better

coagulation and clariflocculator where it is mixed with polyelectrolyte to

flocculate and settle the suspended particles. Secondary treatment system consist

of aeration tank 1 and 2, clarifiers 1 and 2. The aeration tank system brings out

decomposition of organic matter by microorganisms which utilize complex

organics as a food source and multiply at a high rate. In clarifiers 1 and 2,

microorganisms come in contact with both soluble and insoluble organic

materials. The soluble materials pass through the bacterial cell wall and the solid

material sticks to the surface of the cells. The biological solids subsequently

settle out in the clarifier. Tertiary treatment system involves pressure sand filter

followed by activated carbon filter and it removes solids, colour, odour, and

other impurities.

55

The treatment of tannery effluent squarely depends on the efficiency with

which the ETP functions. Hence, continuous evaluation is a pre-requisite for

upkeeping the efficiency of ETP. The present study has been undertaken for a

period of twelve months (January 2008 to December 2008) to evaluate the

performance efficiency of each treatment unit and seasonal changes affecting

the treatment of an individual ETP functioning at N.M. Tanning Industry,

Sempattu, Tiruchirappalli district.

4.2 MATERIALS AND METHODS

4.2.1. Study Area

N.M. Tanning Industry is situated at Sempattu 6 km south of

Tiruchirappalli Junction. The tannery produces semifinished vegetable tanned

leathers. Ground water is the main source for various leather processing

operations. About 400 M3/day waste water could be generated from different

processing units of the industry. The tannery has installed an Effluent Treatment

Plant at the cost of 1.5 crores to treat the waste water. This plant is designed to

treat the effluent at the rate of 400 M3/day. The effluent treatment plant has

primary, secondary and tertiary treatment systems. The primary treatment

system includes Pretreatment Chamber, Equalisation Tank, Flash Mixture Tank

and Clariflocculator and the secondary treatment system consisting of Aeration

tank - I, Clarifier - I, Aeration Tank - II and Clarifier - II. The tertiary treatment

is carried out in two units, namely Pressure Sand Filter and Activated Carbon

Filter.

56

4.2.2. Waste Water Generation from Tannery

The waste water is generated from various tanning operations such as

soaking, liming, deliming and tanning. The first one is the soaking process in

which the raw skins and hides arc soaked in water in order to remove the

common salt used in the skins for preservation purpose. The second one is the

liming process. In this process the skins are soaked in 35% lime solution for a

period of 24 hrs and this facilitates the removal of flesh and hairs. After

deliming with ammonium salts the material is pickled with sulphuric acid and

common salt. The final process is tanning with tree extract in which the skin is

treated with extract of cinchona tree bark and myrobalan nut extract. In this

stage, the material is called semifinished leather. The applied tanning process

to convert the raw material into semifinished leather is- called East Indian

Tanning Method. Most of the chemicals used in the process are organic in

nature, extracted from tree bark and nuts. Hence, the effluent is discharged into

solar evaporation pans. The waste water generated from the other units of

processing is diverted in to the ETP consisting of primary, secondary and

tertiary treatment units. Total waste water generated at the maximum

processing capacity (13000 kg skins) is 400 M3/day. The quantity of water

utilized at each stage of processing was monitored carefully in the present

study.

4.2.3. Design of Effluent Treatment Plant

N.M. Tannery Effluent Treatment Plant at Sembattu, Tiruchirappalli is

designed to treat 400 M3 waste water per day. The plant is constructed in about

1 acre of land. The plant has different units such as pretreatment chamber,

57

equalization tank, clariflocculator. aeration tanks, clarifiers, filters, sludge

thickener, sludge collection sump and sludge drying beds.

The treatment plant design involves a combination of physical,

chemical and biological processes. The raw waste water flows by gravity and

passes through several screens before it is collected in pretreatment chamber.

Pretreatment chamber is constructed in the shape of Imhoff cone for the

removal of suspended solids. The screened effluent flows into equalisation

tank for achieving uniform water characteristics. The equalised effluent is

transferred continuously to the flash mixture tank where alum solution is

added to coagulate the suspended particles. The raw effluent after flash

mixture flows to the clariflocculator, where polyelectrolytes are added to

flocculate the suspended particles. The clear effluent flows to the biological

system (secondary treatment).

A two stage aerobic biological process is employed in the aeration tank

for the degradation of the organic pollutant load based on the activated sludge

process. Activated sludge generally consists of microorganisms like bacteria,

protozoa, rotifiers, etc. in the presence of dissolved oxygen. The process

involves the degradation of organic matter by the action of various

microorganisms. The activated sludge process equation is given below:

Organic Waste ± Micro Organisms + Oxygen + Nutrients =

CO2 + Ammonia + Energy + New Micro Organisms

58

The first stage aeration system is designed with a food to micro

organisms ratio of 0.15 (F/M), and mixed liquid - suspended solids (MLSS)

concentration of 4000 mg/L. Required oxygen is transferred to the system

through mechanical surface aerators. The desired F/M ratio and MLSS levels

are maintained by the recirculation of the sludge settled in the first stage

clarifier. A portion of sludge that settles in the first stage clarifier is

continuously pumped back to the first stage aeration tank, and the excess

sludge to the sludge thickener.

The second stage aeration system is designed with F/M ratio of 0.15 and

MLSS concentration of 3000 mg/L. Two mechanical surface aerators provide

the oxygen necessary for the process. The effluent from the second stage

aeration flows by gravity to the second stage clarifier. A portion of sludge that

settles down on the bottom of clarifier is continuously pumped back to the

second stage aeration tank and the excess sludge is pumped to sludge thickener.

Clarifier II overflow is collected in filter- feed sump. The bio-treated water is

pumped to pressure sand-filter followed by activated carbon filter. The clear

water from filters flows to the final disposal sump. The filtrate water of sludge

drying beds, over flow of sludge thickener and back wash water from filter are

routed back to the equalisation tank.

4.2.4. Efficiency of Effluent Treatment Plant

To evaluate the performance of the effluent treatment plant of the

Tanning Industry, monthly samples were collected for a period of one year

(January 2008 to December 2008) from the outlets of six units namely raw

effluent, Pretreatment Chamber, Clariflocculator, Clarifier-I, Clarifier-II and

59

Tertiary Treatment. The samples were analyzed for the water quality

parameters such as pH, temperature, total suspended solids, total dissolved

solids, chlorides, oil & grease, chemical oxygen demand and biochemical

oxygen demand. The waste water analyses were carried out following the

methods described by APHA (1989) and Trivedi (1984). A brief description of

the methods is given below.

4.2.4.1. pH

pH of effluent samples was estimated using a digital pH meter (Elico

Model No. LI 120) to the nearest 0.01 pH.

4.2.4.2. Total Suspended Solids

Fifty ml of effluent sample was taken and filtered through preweighed

Whatman No. 1 filter paper. After filtration, the filter paper along with the

residue was dried in a hot air oven at 80 °C till weight constancy was achieved.

After complete drying the filter paper was weighed again. The difference

between the final and initial weight of the filter paper was the amount of

suspended solids present in 50 ml of water sample. This value was converted for

one litre of water sample and expressed as mg/L.

4.2.4.3. Total Dissolved Solids

Fifty ml of effluent sample was transferred to a clean and dry

evaporating dish (made of silica) whose weight was already determined. The

water in the dish was evaporated at 80 °C till weight constancy was achieved.

After, complete evaporation, the evaporated dish was weighed again. The

difference between the final and initial weight gave the amount of dissolved

60

solids for the samples taken and the value was finally converted for a litre of

effluent to be expressed as mg/L.

4.2.4.4. Chlorides

Silver nitrate reacts with chloride to form very slightly soluble white

precipitate of AgCl. At the end point when all the chlorides get precipitated,

free silver ions react with chromate to form silver chromate of redish brown

colour.

Procedure

Fifty ml of sample was taken in a conical flask and added 2 ml of

potassium chromate (5%) solution. The content was stirred with a magnetic

stirrer and titrated against 0.02N AgNO3 till the achievement of brick red colour.

The amount of chlorides was calculated as mg/L of effluent.

4.2.4.5. Oil & Grease

Oil and grease present in effluent can be extracted in petroleum ether,

which is immiscible with water and can be separated by a separating funnel.

The residue, after evaporation of this petroleum ether will yield the oil and

grease. Sample for determination of oil and grease was collected in a wide

mouth glass bottle (200 - 250 ml).

Procedure

250 ml of sample was taken in a separating funnel and added 10 ml of

H2SO4 (1: 2) and 50 ml of petroleum ether were added. The content was shaken

well up to the separation of two distinct layers. The lower level of the solution

61

was discarded and upper layer of the solution was collected in a preweighed

beaker. The solution in the beaker was evaporated by placing on a water bath.

After complete evaporation, the evaporated dish was weighed again. The

difference between the final and initial weight gave be the amount of oil and

grease.

4.2.4.6. Biochemical Oxygen Demand (BOD)

Principle

Biochemical Oxygen Demand (BOD) is the measure of degradable

organic material present in the water sample and can be defined as the amount

of oxygen required by the microorganisms in stabilizing the biologically

degradable organic matter under aerobic conditions. The principle of the

method involves measuring the difference of oxygen concentrations between

the samples and after incubating them for 5 days at 20 °C. The amount of

oxygen depleted is calculated as BOD.

Procedure

The sample was aerated by bubbling compressed air for about 30

minutes. 1 ml each of phosphate buffer, magnesium sulphate, calcium chloride

and ferric chloride solutions were added. The sample was neutralized to pH

7.0, using 0.1 N NaOH or HCl. The contents were mixed thoroughly and filled

in 2 sets of the BOD bottles. One set of the bottle was kept in a BOD incubator

at 20 °C for about 5 days, and the dissolved oxygen content in the other set was

determined immediately. Dissolved oxygen in the sample bottle was tested

after the completion of 5 days. A blank was performed with distilled water.

62

4.2.4.7. Chemical Oxygen Demand (COD)

Principle

Chemical oxygen demand (COD) is the measure of oxygen consumed

during the oxidation of the oxidizable organic matter by a strong oxidizing

agent. Potassium dichromate in the presence of sulphuric acid is used as an

oxidizing agent in the determination of COD. The sample is refluxed with

K2Cr2O7 and KMNO4 in presence of mercuric sulphate to neutralize the effect

of chlorides, and silver sulphate (catalyst). The excess of potassium dichromate

is titrated against ferrous ammonium sulphate using ferroin as indicator. The

amount of K2Cr207 used is proportional to the oxidizable organic matter present

in the sample.

Procedure

20ml of effluent sample was taken in a 500 ml COD flask and added 10

ml of 0.2 N K2Cr2O7. A pinch of Ag2SO4, MgSO4 and 30 ml of H2SO4 was

added. The content was refluxed for 2 hours and then cooled. 2 drops of ferroin

indicator was added and titrated with 0. 1 N ferrous ammonium sulphate till the

colour changed from green to dark red. A blank was performed with distilled

water using the same quantity of chemicals added to it.

4.3 RESULTS

The mean quantity of water used to produce 1 kg of semi finished

leathers and the mean quantity of waste water generated per day at N.M.

Tannery during different stages of processing are given in Table 4.1 and 4.2

respectively. To assess the treatment Efficiency of ETP, effluent quality

parameters were analyzed and its results are presented in Table 4.3. The mean

63

values of effluent quality parameters from January 2008 to December 2008 are

presented in table 4.4. Seasonal changes in the BOD and COD after secondary

treatment of effluents are presented in the Table 4.5.

Table 4.1 shows the mean quantity of water used for the production of

1 kg of semifinished leather at different stages are soaking 6.0 L, liming 3.0 L,

fleshing 2.0 L, deliming and bating 5.0 L, pickling 2.0 L, pretanning and

tanning 4.0 L and washing 3 L. The percentage of water utilized with regard to

above stages were 24%, 12%, 8%, 20%, 8%, 16% and 12% respectively. These

values were deduced from the total quantity of water used in one day for all the

treatment processes and the production of semi-finished leather on a daily

basis.

Table 4.2 provides data of mean quantity of waste water generated per

day at N.M. Tannery during different stages. The soaking process generated

50 M3/day, liming 30 M3/day, fleshing 20 M3/day, deliming and bating

50 M3/day, pickling 40 M3/day, pretanning and tanning 30 M3/day and

washing 30 M3/day. The percentage of water utilized with regard to above

stages were 23.2%, 11.5%, 7.7%, 19.2%, 15.4%. 11.5% and 11.5%

respectively. The trend closely following the water utilization ratios given in

the table 4.1.

Table 4.3 presents the water quality parameters through the different

processing stages during January 2008 to December 2008. Gradual reduction in

the quantitative parameters was evident, as the effluent passed through the

successive treatment stages.

64

The maximum pH level 6.6 was noticed in raw effluent during the

month of November 2008 and the minimum pH level was 5.8 during April

2008. The pH level after teritaty treatment ranged between 8.8 and 10.8.

The temperature in the raw effluent ranged from 27°C to 30°C. After

tertiary treatment the effluent temperature ranged between 28°C to 30°C

during the period of study.

The level of oil & grease in the raw effluent was between 50 mg/L and

80 mg/L. The level of oil & grease after tertiary treatment was nil during all

the twelve months from January 2008 to December 2008.

The level of TSS was high (1795 mg/L) during the month of October

2008 and it was reduced to 110 mg/L after tertiary treatment. The minimum

level of TSS (1650mg/L) was noticed during March 2008 and it was reduced

to 520 mg/L after teritary treatment.

The TDS level in the raw tannery effluent was 3190 mg/L in the month

of August and the minimum level of TDS after tertiary treatment was 2490

mg/L during the month of January 2008.

The maximum chloride level was 1760 mg/L in the raw effluent during

the month of May and the minimum level of chloride after tertiary treatment

was 1360 mg/L during the month of January 2008.

65

The maximum reduction of BOD level from 940 to 30 mg/L was

noticed during the month of March 2008 from raw effluent to tertiary

treatment. The minimum reduction was during October from 900 to 80 mg/L.

The COD level was 2500 mg/L during September and it was reduced

to 128 after teritary treatment. The minimum level of COD was 2380 mg/L

during the month of Febuary 2008 and it was reduced to 40 mg/L after tertiary

treatment.

Table 4.4 represents the mean values of water quality parameters of raw

effluent at primary, secondary and tertiary treatments from January 2008 to

December 2008. The pH ranged from 5.8 to 10.8 and temperature from

28.62±0.88 to 29.42±0.67: TSS got reduced from 1718±42.70 to 65.83±27.46;

TDS from 3076±101 to 2551.67±41.52; Chlorides from 1695±48.71 to

1519.58±81.95; Oil and Grease from 60±9.5 to Nil; BOD from 883.2±29.6 to

53.42±13.55 and COD from 2444±37.3 to 145.42±20.25. The percentage of

efficiency achieved at the end of tertiary treatment with respect to the parameters

such as TSS, TDS, Chlorides, Oil and Grease, COD and BOD were 98.5%,

3.5%, 2.5%, 100%, 92% and 91% respectively. All the parameters were within

the permissible limit prescribed by the TNPCB.

Table 4.5 presents the seasonal changes of COD and BOD at the end of

secondary treatment during January 2008 to December 2008. The temperature

ranged from 30 to 27oC. COD from 2180 to 180 mg/L and BOD was reduced

from 790 to 95 mg/L. The percentage reductions of BOD with respect to pre-

monsoon, monsoon and post-monsoon were 86.15±0.96, 87.33±1.03 and

66

88±1.41 respectively. The percentage of reductions of COD with respect to pre-

monsoon, monsoon and post-monsoon were 90.75±0.5, 90.67±0.52 and 91±1.41

respectively.

Effect of primary, secondary and tertiary treatments on effluents and the

comparison with TNPCB standards are depicted in the figures 4.1 (pH,

temperature and oil & grease), 4.2 (TSS, TDS and Chlorides) and 4.3 (BOD and

COD). Pronounced influence of secondary and tertiary treatments on the

reduction of oil and grease, BOD and COD was evident in these figures.

4.4 DISCUSSION

Effluents generated by tanneries are the major source of pollution in

Tiruchirappalli. The direct discharge of effluents from the tanneries into soil and

bodies of water has become a growing environmental problem. These effluents

are extremely complex mixtures containing inorganic and organic compounds

(Fu et al., 1994). Tannery effluent is highly polluted with high concentrations of

protein, chlorides, chromium, sulphate, COD, BOD, TSS, TDS etc (More et al.,

2001). Processing of skins and hides require large amount of water and generate

huge quantities of tannery effluent which is discharged indiscriminately into

nearby fields either treated or untreated. Water and land pollution problems

related to tannery effluent have been reported as a serious problem in many

countries (Sahasranaman and Buljan, 2000).

Provision of effluent treatment plants for individual industries or common

effluent treatment plants mainly for a cluster of small scale industrial units in the

various industrial estates in India was intended to produce the effluent of desired

67

quality. The design of the Effluent Treatment Plant under study, involved a

combination of primary, secondary and tertiary treatment processes. The primary

treatment system consisted of pretreatment chamber, equalization tank, flash

mixture and clariflocculator. The raw effluent flowed by gravity and passed

through several screens before it was collected in pretreatment chamber. The

pretreatment chamber was constructed in the shape of Imhoff cone and provided

with screens, bend-flow and break points for the removal of suspended solids.

Before the water reached the equalisation tank, it was subjected to the above

treatment which was more effective in reducing the suspended solids, and

thereby BOD and COD. The effluent was discharged into equalisation tank at

different flow rates and was subjected to aeromixer to facilitate the bacterial

breakdown of oxygen demanding wastes.

Curtis et al. (1985) reported that the purpose of equalisation is to

minimize the wide fluctuation in effluent flow rate and variation of the tanning

process. No treatment is achieved in equalisation itself. However, the uniformity

of effluent produced by this process improves the consistency in the

performance of subsequent treatment. For proper homogenization of the effluent,

two aeromixers were provided in the equalisation tank. These aeromixers

absorbed air from atmospheric medium and passed into the tank-water, which

facilitated non-settling and the reduction of BOD and COD in subsequent

treatment units. The effluent from equalisation tank is pumped to the flash mixer

tank where it was quickly mixed with alum. The effluent when reached the

clariflocculator tank, it was mixed with polyelectrolytes. In this unit the

suspended solids first became coagulated on reaction with alum. Extensive

studies carried out in the laboratory and full scale plant level have shown that

68

alum, can serve as a excellent coagulant (Dhabadgaonkar et al.(1990), and hence

it is the most commonly used.

Peters et al. (1990) reported that polyelectrolytes themselves arc poor

coagulants. However, maximum settling rates are achieved when alum and

polyelectrolytes work together at an optimum pH of 8.5. According to

Kaussen (1989) linking solid particles with active groups of polyelectrolyte

is responsible for fusing of suspended particles and joining of large macro

molecules. In the present study, the total suspended solids were drastically

reduced from 1869 mg/L (raw effluent) to 214 mg/L in clariflocculator,

achieving thus an efficiency of 88.5%. Tables 4.3 and 4.4 indicates that all

parameters including oil & grease could drastically be reduced during the

treatment process. The TSS, TDS, Chlorides, Oil & Grease, COD and BOD

of raw effluent were observed to be 1869 ± 229 mg/L; 2403 ± 142 mg/L;

4277 ± 172 mg/L; 1523 ±59 mg/L; 59 ± 8 mg/L; 2207 ± 99 mg/L and 844

±77 mg/L respectively, while the percentage of efficiency achieved at the end

of primary treatment for the same parameters, were 88.5%, 0.95%, 0.92%,

72.7%, 35.5% and 35.4% respectively.

The effluent from Clariflocculator was subjected to secondary treatment.

The secondary treatment system consisted of aeration tank 1 & 2 and clarifier 1

& 2. In the aeration tank-I, COD and BOD values were greatly reduced from

1421.53 ± 75.28 to 545.07 ± 54.70 mg/L and from 665.23 ± 117.02 to 222.15 ±

22.97 mg/L, respectively. Peavey et al (1986) reported that mechanical aerators

produce turbulance at the air and liquid interface and this turbulence entrains air

69

into the liquid. BOD and COD are reduced at shorter period by the aeration

process.

Activated sludge was introduced in the aeration tank 1 and 2 for the

digestion of organic wastes present in the effluent. Activated sludge is

biologically active and can oxidize organic matter (Rajamani and Krishna,

1983). It is obtained by settling sewage that contains numerous aerobic bacteria

and other forms of micro organisms which facilitate the digestion of organic

matter present in the waste water. These micro organisms are capable of

aerobically decomposing organic matter into CO2 and H2O. Sulphur containing

compounds are oxidised into sulphates and nitrogen containing components into

nitrates (Klemenc and Gantar, 1995).

Generally in the clarifier tank, the micro organisms come in contact with

both soluble and insoluble organic materials, the soluble materials pass through

the bacterial cell walls and the solid material sticks to the surface of the cells.

The biological solids subsequently settle out in the clarifier. A portion of the

clarifier sludge returns to the aeration tanks for proper maintenance of MLSS

concentration in the aeration tank. If the sludge is much thickened it will be

transferred to sludge collection sump. A high reduction of solids, BOD, COD,

could obtained in the present study. It was due to settling of organic solids in the

clarifiers. The percentage of reduction of TSS, TDS, Chlorides, Oil & Grease,

COD and BOD at the end of secondary treatment was found to be 81.2%, 0.36%,

0.86%, 100%, 87.6% and 92.8% respectively.

70

The tertiary treatment system consists of pressure sand filter and activated

carbon filter. The pressure sand filter is filled with pebbles and sand and effluent

is allowed to pass through under pressure. The application of pressure over the

sand facilitates the settlement of solids and organic matter in the sand then the

effluent passes through adsorbing medium like activated carbon filter. Activated

carbon has the ability to reduce the level of total organic matter as well as levels

of specific trace organics (Metcalf and Eddy. 1995). Hence, a considerable

amount of organic matter was removed from the effluent when it was passed

through the activated carbon filter in the present study.

Comparison of data on water quality parameters of untreated and treated

effluent clearly indicated that the ETP under study fulfills the norms prescribed

by TNPCB. Present study has generated scientific data to ascertain the

efficiency of ETP at N.M. Tanning industry. Secondary and tertiary treatments

of the effluent could completely eliminate the presence of oil and grease, and

also reduce the TSS, COD and BOD limits within the acceptance levels.

However the above treatments had no appreciable effects on the TDS and

chlorides of the effluents.

Table 4.1. Mean Quantity of water used at different stages of Tanning For the

Production of 1 Kg of Semi finished Leather.

S.No. Two Stage Aerobic

System

Utilised water

1 kg

Percentage of utilised water (%)

1

2

3

4

5

6

7

Soaking

Liming

Fleshing

Deliming and Bating

Pickling

Pretanning & Tanning

Washing

6.0

3.0

2.0

5.0

2.0

4.0

3.0

24%

12%

8%

20%

8%

16%

12%

Total 25.0 100%

Table 4.2. Mean Quantity of Waste Water Generated Per Day by N.M. Tannery at Different Stages of Tanning Process

S.No. Two Stage Aerobic

System Water utilised

(M3 / day) Percentage of

utilised water (%)

1

2

3

4

5

6

7

Soaking

Liming

Fleshing

Deliming and Bating

Pickling

Pretanning & Tanning

Washing

60.0

30.0

20.0

50.0

40.0

30.0

30.0

23.2%

11.5%

7.7%

19.2%

15.4%

11.5%

11.5%

Total 260.0 100%

Table 4.3. Quality of Tannery effluents at different units of ETP during January 2008 to December 2008

Months Parameters Primary Treatment Secondary Treatment

Tertiary Treatment Raw effluent

Pre Treatment Chamber

Clari floculator

Clarifier - I Clarifier - II

January

pH 6.2 6.6 6.9 8.5 10.1 10.4 Water Temp(oC) 27 27 28 28.5 28.5 28 TSS 1720 840 460 280 80 20 TDS 3100 2560 2540 2510 2500 2490 Chloride 1660 1580 1410 1390 1370 1360 Oil & Grease 80 70 50 30 Nil Nil BOD 880 778 652 318 85 56 COD 2439 2180 1850 1280 180 168

February

pH 6.4 6.5 6.7 7.1 9.8 10.6 Water Temp(oC) 28 28 28.5 28 29 28.5 TSS 1690 910 520 210 100 50 TDS 2990 2810 2645 2590 2580 2500 Chloride 1620 1590 1570 1550 1520 1510 Oil & Grease 60 50 20 10 Nil Nil BOD 830 780 520 210 66 40 COD 2380 2100 1972 1310 200 176

March

pH 6.3 6.5 6.7 7.3 8.9 9.9 Water Temp(oC) 28 28.5 28 29 29.5 29.5 TSS 1650 960 480 290 90 20 TDS 2910 2800 2690 2670 2585 2550 Chloride 1670 1610 1590 1570 1530 1520 Oil & Grease 50 40 30 10 Nil Nil BOD 940 845 660 215 80 30 COD 2400 2150 1800 1180 185 153

Months Parameters Primary Treatment Secondary Treatment

Tertiary Treatment Raw effluent

Pre Treatment Chamber

Clari floculator

Clarifier - I Clarifier - II

April

pH 5.8 6.2 6.4 8.2 9.3 10.1 Water Temp(oC) 29 29.5 29 30 30 29.5 TSS 1650 1110 620 280 140 50 TDS 2900 2880 2700 2690 2660 2610 Chloride 1720 1700 1645 1640 1630 1605 Oil & Grease 60 40 20 10 Nil Nil BOD 910 888 676 280 90 40 COD 2420 1834 1520 1120 130 110

May

pH 6.5 6.7 6.8 8.1 8.8 9.1 Water Temp(oC) 30 30 29.5 29 30 30 TSS 1730 1380 780 310 120 70 TDS 3160 2930 2680 2610 2600 2580 Chloride 1760 1710 1700 1645 1640 1610 Oil & Grease 70 60 40 20 Nil Nil BOD 886 728 695 300 95 50 COD 2470 2185 1678 1220 150 38

June

pH 6.2 6.4 7.2 8.5 8.6 8.8 Water Temp(oC) 29 29.5 29 30 30 29.5 TSS 1750 1400 950 480 210 90 TDS 3170 2880 2725 1610 2600 2590 Chloride 1710 1640 1620 1615 1610 1600 Oil & Grease 60 40 30 20 Nil Nil BOD 840 790 556 323 80 60 COD 2450 2100 1890 1310 170 143

All the values are in mg/L except pH

Months Parameters Primary Treatment Secondary Treatment

Tertiary Treatment Raw effluent

Pre Treatment Chamber

Clari floculator

Clarifier - I Clarifier - II

July

pH 5.9 6.3 6.7 8.2 9.2 10.2 Water Temp(oC) 28 29 29.5 30 29 30 TSS 1736 1390 880 490 210 80 TDS 3140 2800 2730 2620 2610 2560 Chloride 1690 1600 1575 1560 1515 1490 Oil & Grease 50 40 20 10 Nil Nil BOD 870 750 610 245 95 55 COD 2480 2215 1778 1100 160 123

August

pH 6.1 6.8 7.5 7.9 8.8 9.7 Water Temp(oC) 29 29.5 29 30 29 30 TSS 1695 1445 985 450 225 90 TDS 3190 2900 2880 2690 2660 2590 Chloride 1680 1593 1535 1500 1495 1475 Oil & Grease 60 50 30 20 Nil Nil BOD 900 685 546 320 65 50 COD 2400 2240 1960 1300 190 140

September

pH 6.4 6.9 7.2 8.5 8.8 9.5 Water Temp(oC) 28.5 29.5 30 29 30.5 30 TSS 1700 1615 1080 490 225 80 TDS 3100 2700 2600 2590 2560 2550 Chloride 1700 1640 1620 1600 1590 1580 Oil & Grease 50 30 20 10 Nil Nil BOD 890 765 585 325 70 60 COD 2500 2235 1965 1310 125 128

All the values are in mg/L except pH

Months Parameters Primary Treatment Secondary Treatment

Tertiary Treatment Raw effluent

Pre Treatment Chamber

Clari floculator

Clarifier - I Clarifier - II

October

pH 6.5 6.7 8.2 9.3 9.9 10.8 Water Temp(oC) 28 29 28 30 29 29 TSS 1795 1385 980 455 220 110 TDS 2995 2930 2680 2530 2500 2510 Chloride 1620 1590 1575 1510 1500 1495 Oil & Grease 50 30 30 20 Nil Nil BOD 900 880 690 350 95 80 COD 2480 2185 1925 1215 185 165

November

pH 6.6 7.1 7.8 8.5 9.3 10.1 Water Temp(oC) 29 29 30 30 29 29 TSS 1740 1650 1018 495 240 60 TDS 3150 2995 2650 2610 2590 2560 Chloride 1640 1620 1615 1610 1600 1590 Oil & Grease 60 50 40 20 Nil Nil BOD 882 798 651 353 80 70 COD 2460 2285 1980 1350 174 136

December

pH 6.5 7.5 7.9 8.2 9.5 10.8 Water Temp(oC) 28 28.5 29 29.5 30 30 TSS 1770 1750 1110 450 300 70 TDS 3110 2910 2800 2655 2580 2500 Chloride 1650 1510 1480 1430 1410 1400 Oil & Grease 70 50 40 20 Nil Nil BOD 870 685 415 348 60 50 COD 2450 2222 2100 1240 190 165

All the values are in mg/L except pH

Table 4.4. Mean values of effluent quality at different units of ETP from January 2008 to December 2008 (mean±SD)

Seasons Months & Year

2007 Temp(°c)

Primary Treatment

Secondary Treatment Percentage of Reduction

BOD in Mg/L

COD in Mg/L

BOD in Mg/L

COD in Mg/L

BOD COD

% of Reduction

Mean ± SD

% of Reduction

Mean ± SD

Post Monsoon

January

February

27

28

778

500

2180

1972

85

66

180

200

89%

87% 88±1.41

92

90 91±1.41

Pre monsoon (summer)

March

April

May

June

28

29

30

29

660

676

695

556

1800

1520

1678

1890

80

90

95

80

185

132

150

170

88%

87%

86%

86%

86.75±0.96

90

91

91

91

90.75±0.5

Monsoon

July

August

September

October

November

December

28

29

28.5

28

29

28

610

546

585

790

651

415

1778

1960

1965

1925

1980

2100

85

65

70

95

80

60

160

190

175

185

174

190

86%

88%

88%

88%

88%

86%

87.33±1.03

91

90

91

90

91

91

90.67±0.52

Table 4.5. Seasonal changes of BOD and COD after secondary treatment

Parameters

Primary Treatment Secondary Treatment Tertiary

Treatment Raw Effluent

Pre Treatment Chamber

Clariflocculator Clarifier - I Clarifier - II

PH 5.8 – 6.6 6.1 - 7.5 6.6 - 8.2 7.1 – 9.3 8.6 – 10.1 8.8 – 10.8

Water Temperature ( °C )

28.62±0.88 29±0.83 28.95±0.72 29.42±0.70 29.46±0.62 29.42±0.67

TSS mg/L 1718±42.70 1319±299.7 821.91±241.4 390±105.98 180±70.81 65.83±27.46

TDS mg/L 3076±101 2842.5±118.32 2693.33±88.12 2613±66.09 2585.42±49.98 2551.67±41.52

Chlorides mg/L 1695±48.71 1615±53.13 1577.92±76.38 1551.66±80.91 1534.17±84.88 1519.58±81.95

Oil & grease 60±9.5 47.5±10.55 30.83±9.96 16.67±6.51 NIL NIL

BOD mg/L 883±29.6 778.1±62.10 603±83.86 298.92±150.66 80.08±12.51 53.42±13.55

COD mg/L 2444±37.3 2160.92±116.67 1868.27±156.4 1244.58±79.98 174.08±19.52 145.42±20.25

Fig. 4.1. Changes in the level of pH, temperature and Oil & grease in

different stages of effluent treatment plant

0

10

20

30

40

50

60

RawEffluent

Primarytreatment

Secondarytreatment

TertiaryTreatment

TNPCBStandard

PH Water Temperature Oil & Grease

Fig. 4.2. Changes in the level of TSS, TDS and Chlorides in different stages

of effluent treatment plant

0

500

1000

1500

2000

2500

3000

RawEffluent

Primarytreatment

Secondarytreatment

TertiaryTreatment

TNPCBStandard

TSS TDS Chlorides

Fig. 4.3. Changes in the level of COD & BOD in different stages of effluent

treatment plant

0

500

1000

1500

2000

2500

RawEffluent

Primarytreatment

Secondarytreatment

TertiaryTreatment

TNPCBStandard

COD BOD