INTRODUCTION:

Growing industrialization is producing numerous products and also lots of waste. Most of this waste causes risk of hazard to humans and the environment [1]. As per USEPA, “Hazardous waste is waste that is dangerous or potentially harmful to our health or the environment. The degree of hazard may vary in addition to the quantity of the waste produced [2].By nature, hazardous waste complicates the process of collection, handling, treatment and disposal, and, of course expensive and risk involved. Typical example of such wastes include heavy metals such as Chromium, Mercury, Nickel, Cyanide, etc and oil and grease laden wastes with toxic metals [3].

Textile industry is one of the oldest industries in India and has a major presence in the Indian economy, contributing to about 14% of manufacturing value addition and 1/3rd of the India’s gross export earnings [4].According to [5], annual hazardous waste generated is estimated to be around 6.23 million tonnes. Out of this, 49.55 % is recyclable, 6.67 % incinerable and remaining 43.78 % is disposable in secured landfills. Twelve states of the country (Maharashtra, Gujarat, Andhra Pradesh, Tamil Nadu, Odisha, Madhya Pradesh, Assam, Uttar Pradesh, West Bengal, Kerala, Karnataka and Rajasthan) account for 97 % of the total waste generation. In Tamil Nadu, large quantities of chemical sludge is generated i.e. 55.76 tonnes/day from Balotra CETP, 130.58 tonnes/day from Pali CETP, 184.60 tonnes/day from Tirupur CETP and 90.40 tonnes/day from Karur CETP are generated and are lying in CETP premises awaiting disposal to landfill. Awareness has been created for the safe management of hazardous waste. However, it is not yet put in practice in most of the states of India due to various issues. In spite of adoption of most expensive and advanced methods, the resultant residue may or may not be hazardous but it has to be treated again through a suitable technology. One also needs to work in the direction of developing various environment–friendly and effective economic technologies for the management of hazardous waste in India. Solidification/Stabilization (S/S) is a technology used for treating industrial solid wastes containing toxic constituents to prevent their dissolution and release to the environment [6]. It had been used for decades as a final treatment step prior to the disposal of both radioactive and chemical hazardous waste. Several investigators have attempted S/S technology for the treatment of different types of industrial wastes, mostly heavy metals containing inorganic/organic sludge. It is being used for the immobilization of Arsenic containing wastes by many researchers [7], [8],[9], [10], stainless steel factory sludge containing Cr and Ni [11]and[12], electroplating sludge containing Cr, Pb, Ni and Zn [13]and[14], foundry sludge [15], steel foundry electric arc dust [16], [17],[18],[19], nickel hydroxide sludge [20], oil refinery sludge [21] and asbestos waste [22]. The treated S/S waste can safely be disposed to landfill or can also be reused in construction materials depending upon its physical, chemical and engineering properties. The demand for construction materials is always escalating due to increasing urbanization. Therefore, utilization of waste materials in construction is the need of hour. It is not only saves money but also reduces burden on the dwindling natural resources. Several researchers have attempted to reuse waste materials into various types of construction materials such as blocks from sewage sludge using fly ash, lime and gypsum [23]and[24], Arsenic and Iron sludge in brick making [25], sewage and sludge ash in brick and tile making [26],[27] and sludge ash for light weight aggregates [28]and[29].Few literature of characterization and reuse of chemical sludge from textile CETPs in India are available. [30],[31],[32] have all attempted to characterize the chemical sludge and reuse it in construction materials with varying degrees of success. Still the efforts are on to find out a suitable economic and environment friendly solution for the safe reuse of chemical sludge. Besides, there are no comprehensive studies available on chemical sludge from textile wastewater covering all the aspects including characterization and micro-structural examination of sludge, treatability studies using different binder systems such as OPC and PPC containing fly ash, leach ability studies. Attempt has been made here to carry out such a comprehensive study covering many of the aspects such as detailed physico-chemical characterization and micro-structural examination of chemical sludge, solidification/stabilization of sludge using OPC and PPC (cement containing fly ash), leachabiity studies for heavy metals. Therefore, this research work is an attempt towards stabilization of chemical sludge which results in a matrix where mobilization of pollutants of concern is considerably reduced so as to render it harmless so that it can be safely reused.

MATERIALS AND METHODOLOGY:

Collection of Sludge:

Karur town (Latitude-10.95oN and Longitude-78.08oE) in Tamil Nadu located on the bank of Amaravathi river, a tributary of and confluences with famous river Cauvery about 12 km downstream of Karur[33]. Over last four decades the town emerged as a major textile centre with as big as 1000 units consisting of various processes along a 17 km stretch on the banks of Amaravathi, which undertake bleaching, dyeing, weaving, tailoring, knitting, knotting, packing, transporting and trading. Out of which 487 are bleaching and dyeing units[34]. Chemical sludge samples were collected from the CETP sampling site i.e. Sengunthapuram CETP, Karur. This site receives wastewater from 391 dyeing and printing units [35]. The total flow discharge from these industries is 9,825 K lit/day from which chemical sludge of 38.76 tonnes/day is generated [36].

Characterization of Sludge:

The physico-chemical treatment of textile effluent leads to the generation of large quantity of chemical sludge which is inorganic and non-biodegradable in nature. The dye stuff present in the wastewater gets precipitated in the form of settable sludge slurry in the presence of coagulants such as Lime and Ferrous Sulphate. The chemical sludge contains inert solids, polymer solids, precipitated dye products, metal salts and other chemicals. The sludge is considered hazardous as per Hazardous Waste Management Amendment Rules (2008) in schedule I under category no. 34. The amount and properties of sludge depends upon the many factors such as the composition of wastewater, chemicals used and the treatment process. The characterization of the sludge helps us to know about the nature of sludge so that appropriate treatment option(s) can be selected. On an average, the textile effluent is characterized by high alkalinity, high BOD and COD, intense color, high solids, high conductivity and other pollutants depending upon the processes used. Complete characterization of the sludge is tabulated in [35]&[32]. Due to limitations associated with other waste management options like land filling and incineration, S/S option was selected for treatment/reuse of sludge.

Solidification Methodology:

Solidification refers to techniques that encapsulate the waste in a monolithic mass of high structural integrity does not necessarily involve a chemical reaction between the waste and solidifying agents. Contaminant migration is restricted by decreasing the surface area exposed to leaching and/or isolating the surface within the impervious capsule. Stabilization refers to a technique which reduce the hazard potential of waste by converting the contaminants into their least soluble, mobile or toxic form. It is used extensively for the treatment of heavy metal containing inorganic sludges and contaminated soils containing toxic metals and organic constituents, incinerator residues and bulk wastes from power plants [37]&[10]. It is used widely because of its simple operation, cost effectiveness and their desirable characteristics [38]and[39]. The most common binder systems used in S/S systems are Alkaline matrices such as Lime and Ordinary Portland Cement. They incorporate wet wastes and their alkalinity greatly reduces the solubility of many inorganic toxic or hazardous metals. The S/S matrix can also be modified using additives such as fly ash, soluble silicates, slag and clay. In S/S process using cement, water reacts chemically with cement to form hydrated silicates and aluminates resulting in a solid monolithic mass. Hydration of Declaim Silicates and Tricalcium Silicates forms C-S-H gel also known as tobermorite and crystalline calcium hydroxide. The C-S-H gel is responsible for the strength development after the initial setting of the mixture. Real wastes are complex mixtures with contrasting physical and chemical properties that are difficult to characterize. The synergistic or antagonistic effects of multiple component mixtures can influence both hydraulic or pozzolonic activity. Several heavy metal salts have been implicated in the inhibition of hydration process with consequent retardation in the setting and curing of cement/waste matrix and thus reduction in strength development [40]and[41]. The S/S systems are evaluated using physical, chemical and micro-structural methods [42]. The setting and strength development are often used as indicators of solidification and leaching test is often used to assess the extent of fixation [43],[44] and [45]. The sludge was dried in a hot air oven for 24 h at 100 ºC. After drying, the sludge was powdered and then was passed through 212 μ BSS sieve. Mortar cube steel moulds of 70.6mm x 70.6 mm x 70.6 mm size were used to make the blocks. The blocks were prepared as per the BIS standard IS4031(6). Mixture of dry sludge, fly ash and cement were made with a trowel for one minute and then water was poured until the mixture was uniform in color and then poured this mixture in mould and immediately after casting each layer, tamp with a rod for 20 times in about 8 seconds to ensure elimination of entrained air. The filled moulds were kept in moist conditions for 24 hours. At the end of this period, the blocks were removed from the mould and put blocks in clean fresh water, maintained at 27ºC for curing. The sludge was used in ratio of0 % (control), 10%, 20%, 30%, 40%, 50% and 60 % as a partial replacement of cement. The water content in the cement sludge paste was varying from 21% to 35 %. Three sample of each ratio was prepared. The samples were subjected to 7 days, 14 days and 28 days water curing. The samples were cured at ambient temperature and Relative Humidity (RH) of 60%.

Physical Engineering Tests:

Hardening Time:

Hardening time was determined by visual observation and hand-pressing the specimen every 6 hour. A Standard Vicat apparatus was used to determine the consistency limits (BIS, IS: 4031(part 4) – 1980) of cement and cement with various percentages of sludge. Results given in table 1 and shown in figure 1 and 2.

Table1:Preliminary tests on cement with different percentage sludge

Percentage of sludge	Consistency (%)	Setting time (minutes)
Percentage of sludge	Consistency (%)	Initial	Final
10%	33	90	190
20%	39	125	220
30%	41	156	266
40%	47	178	308
50%	52	205	325
60%	55	236	356

Fig. 1: Consistency test on cement with different percentage of sludge Fig 2: Initial and Final setting time of the cement with different sludge percentages

Compression:

Compressive strength values of cement-sludge paste cubes were measured by compression testing machine as per BIS standard IS 4031(6). Total maximum loads were recorded at the point of fracture. For various percentages of sludge, three samples were made and subjected to a compressive strength test after the required curing period, and the average strengths were obtained. The compressive strength was determined using the formula.

fm = P/A

Where fm is the compressive strength (in N/mm2), P is the total maximum load (in N)and A is the area of loaded surface (in mm2). Results given in Table 2 and shown in figure 3.

Fig 3: Compressive Strength of cement-sludge mortar cubes with different sludge percentage

Table2: pH and Electrical Conductivity of the cured water of 7, 14 and 28 days and Compressive Strength of mortar of 7, 14 and 28 days curing

Parameter Tested	Curing time	Percentage of sludge
Parameter Tested	Curing time	10%	20%	30%	40%	50%	60%
pH	7 days	9.45	9.56	9.70	9.82	10	10.10
	14 days	8.66	8.87	8.90	9.04	9.10	9.18
	28 days	8.01	8.30	8.57	8.89	8.95	9.10
Electrical Conductivity(s/m)	7 days	12.63	12.32	12.40	15.48	15.88	15.93
	14 days	12.34	12.96	12.61	13.57	14.08	14.18
	28 days	11.95	12.25	12.37	13.35	13.76	13.95
Compressive Strength (KN) of mortar	7 days	142	120	102	54	34	28
	14 days	166	132	101	55	31	22
	28 days	152	154	97	57	35	23

Chemical Engineering Tests:

These tests are conducted on cured water of sludge cubes.

pH:

The cured water could be acidic, alkaline or neutral. The water after 7, 14 and 28 days curing was tested for testing the pH values. pH was measured using glass electrode pH meter of Systronics361. The results were given in table 2 and shown in figure 4.

Fig 4: pH of cured water in 7, 14 and 28 days with different sludge percentages

Electrical Conductivity:

Electrical Conductivity of proton solution is a measure of its ability to pass current. The SI unit of conductivity is Siemens/meter (s/m). Conductivity measurements are used routinely in many industrial and environmental applications as reliable way of measuring ionic content in solution. The water after 7, 14 and 28 days curing was tested for electrical conductivity values as per the method in CPCB solid waste manual [46]. The results were given in table 2 and shown in figure 5.

Fig 5: Electrical conductivity of cured water in 7,14 and 28 days with different sludge percentages

Hardness:

The hardness test is conducted to test the presence of Calcium and Magnesium in cured water which may cause corrosion or forms scale on steel reinforcement. The Total Hardness of the 28 days cured water only tested because it confirms that other lesser days cured water will be having lesser amount of Total Hardness. Total Hardness of the cured water is determined based on APHA standard method 2340-C. The results were given in table 3.

Table 3:Total Hardness and Heavy metals present in 28days cured water

Percentage of sludge	Total Hardness (ppm)	Copper (mg/l)	Chromium (mg/l)	Lead (mg/l)	Nickel (mg/l)
10%	162	0.768	0.048	0.143	0.090
20%	112	0.948	0.041	0.202	0.124
30%	68	4.549	0.021	0.331	0.105
40%	66	2.478	0.030	0.285	0.131
50%	58	0.856	0.013	0.170	0.058
60%	56	5.453	0.067	0.381	2.789

Heavy Metals:

This test is conducted to find out the leaching of heavy metals from the sludge cubes to the cured water by Atomic Absorption Spectrophotometer. This test is used in many of industrial and environmental applications to find the presence of heavy metals and to compare with the permissible limit of heavy metals as per BIS. 5 g of dried sludge was pretreated by nitric acid– perchloric acid digestion as prescribed in section 3030 H and further metal analysis was done by flame atomic absorption spectroscopy by the method prescribed in section 3111 A of APHA (1998). In this study, Copper (Cu), Chromium (Cr), Lead (Pd) and Nickel (Ni) were analyzed using AAS 7000 series of LABINDIA. The results were given in table 3.

RESULTS:

(1) From the physico-chemical properties given in the literature, it is evident that sludge is hazardous in nature with high electrical conductivity values. The volatile solids of sludge is such that it cannot be used for incineration as it would result in very high ash content. Among the heavy metals analyzed, Cu, Ni, Pb and Cr are found to be in high concentrations as compared to other heavy metals. Therefore, treatability studies using solidification/stabilization were conducted.

(2) Hardening time ranged between 30 to 45 h. The control samples, containing 100 % cement (with no sludge replacement) were cured within 24 hours. The hardening time of the sludge is not as good as cement.

(3) In case of compressive strength measurement, it has been reduced as the percentage replacement of cement with sludge was increased. The maximum compressive strength was found in case of control samples with only cement. The compressive strength in cement-sludge blocks after7 days, 14 days and 28 days of curing ranged from 28 to 142 KN,22 to 166 KN and 23 to 152 KN for10%,20%, 30%, 40%, 50% and 60% sludge replacement in cement. The control gave strength of 53.46 and 65.81 N/mm² after 14 and 28 days of water curing. The longer the material was cured, the stronger the solidified matrix became for all treatments including control. The blocks after 14 days of curing had achieved 40 to 90% of the strength achieved after 28 days of curing. When compared with standards for different types of construction materials, i.e. in case of brick which is available in the range of3.5 to 35 N/mm². There are other building materials also like hollow and solid concrete blocks having requiring strength 5 and 4 N/mm² and hollow and solid light weight concrete blocks having strength 10.8 and 7.0 N/mm², soil based blocks requiring compressive strength of 2 N/mm², lime- pozzolona concrete blocks for paving requiring compressive strength of 3.5 N/mm². Therefore considering the strength requirements for these construction materials, the cement-sludge blocks fulfill the requirements for most of the materials for non-structural application purposes.

CONCLUSIONS:

The attempt to study the possibility of recycling textile sludge by replacement of cement in construction industry can be concluded as following.

(i) The consistency of the mortar increase as the replacement of sludge is increasing. The initial and final setting time of the mortar are increasing as the replacement of sludge is increasing.

(ii) In case of compressive strength measurement, it has been reduced as the percentage substitute of sludge for cement was increased.

(iii) The pH of the curing water is increasing as the substitute of sludge is increasing.

(iv) The Electrical conductivity of the curing water is found to vary after 7 days curing while it does not show much variation after 14 days.

(v) The hardness of the curing water is decreasing as the substitute of sludge is increasing.

(vi) Generally, the heavy metal concentration of the curing water is found to be increase as the replacement of sludge is increase and it is found to be negligible.

Therefore the chemical sludge from textile wastewater treatment plants has a potential to be reused up to 20% replacement of cement and can be used as construction materials of different applications.

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Received on 18.11.2015 Accepted on 20.12.2015

Int. J. Tech. 5(2): July-Dec., 2015; Page 219-224

DOI: 10.5958/2231-3915.2015.00024.3