Biokinetic Studies in the Treatment of Tannery Effluent

The present investigation deals with the anaerobic treatment of composite tan liquor with varying organic loads. BOD of the influent and effluent and MLSS of reactor were determined at various detention times to generate data for kinetic coefficients. The kinetic coefficients k (maximum substrate utilization rate), Ks (half velocity constant), Y (cell yield coefficient) and Kd (decay coefficient) were found to be 1.66 day-1, 1132 mg/L, 0.05 and 0.22 day, respectively. Overall BOD removal rate constant ‘K’ was found to be 1.46 day. The studies showed that an optimum BOD influent load of 0.8 Kg BOD/m/day with 3 days retention time could be adopted to yield about 97 percent BOD reduction. The Bio-kinetic coefficients were evaluated using modified Monod’s equations to study the metabolic performance of the digestion process.


Introduction
Environmental pollution has become major concern in developing countries in the last few decades.Major sources of water pollution are the untreated or partially treated industrial effluents.Tanning industry is reputed globally as major industry which contributes to water pollution.The quality of discharged water from tanneries is far from the desired level of acceptance into water ways.Arumugam (1976) has reported on the recovery of chromium from spent chrome tan liquor by chemical precipitation using lime.pathe et al. (1995) have studied the properties of chromium sludge from chrome tan liquor and related the sludge volume, sludge settling rate, surface loading rate etc. Archana Shukla and Shukla (1994) have studied the treatment of tannery and electroplating effluents by using lime, NaOH and their mixture in the temperature range of 25 to 100°C.Guruswamy et al. (1995) conducted study on a laboratory scale completely mixed continuous flow activated sludge system to treat settled chrome tannery wastewater and observed that the BOD and COD removal ranged from 84 to 96%.Elangovan et al. (1995) have conducted experiments on the activated sludge treatment of vegetable tanning waste admixtured with 10, 25, and 50 settled sanitary sewage and obtained BOD removal from 87 to 96.
Laboratory scale tests indicated that Biological oxygen demand is generally reduced by reducing the organic material with sodium hypochlorite.However, Cr(VI) is formed in this last process.Hydrotalcite (HT) and its calcination product were applied to separate the Cr(VI) formed.The best material found was calcined hydrotalcite, which removed not only Cr(VI) from the wastewater but also other anions, such as Cl -, SO 4 2-, and HCO 3 - (Gallegos, Martnez, & Bulbulian, 2005).
Removal of hexavalent chromium by adsorption technique from aquous solution using neem bark (Azadirachta indica), subabhul charcoal (SC, Leucina leucocephala) has been studied and the results are compared with powdered activated charcoal (PAC).The adsorption efficiency of PAC and SC for Cr(VI) was maximum at pH 2.0 and for NB it was maximum at pH 6.0 (Kulkarni & Shrivastava, 2002).
The reduction of the level of chromium and chemical oxygen demand (COD) in waste chrome liquor from tannery industry has been investigated.The chromium reduction achieved by groundnut shell powder (GNSP) treatment is 63-66% at pH 7.70.The total chromium and COD of the effluent after treatment with GNSP using batch reactor and anaerobic baffled reactor has been reduced to 99.9% and 93% respectively (Kumar, Verma, & Rao, 2002).
Cupressus female cone (CFC), an inexpensive plant material is investigated as an adsorbent for the removal of Cr(VI) from synthetic solution as well as from industrial wastewater so as to help its safe disposal.Maximum sorption occurs at the acidic pH range of 0.2-0.5 but it decreases on increasing the pH.Increase in initial concentration of Cr(VI) and sorbent particle size are found to reduce the amount of sorption, demonstrating the role of surface effects on sorption (Murugan & Subramanian, 2003).
In order to assess the transformation process involved in the chemistry of chromium, studies on adsorption kinetics and sequential extraction were conducted on oxisol, andept and alfisol soils using K 2 Cr 2 O 7 as source of hexavalent Cr and indicated more adsorption in oxisol due to its high kaolinite clay content and also due to its acidic nature (pH: 4.2).Besides adsorption, the added, Cr(VI) was considerably reduced by the presence of organic matter (Thangavel, Rajannan, & Ramasamy, 2002).
Study deals with the characterization of tannery effluent polluted soil in Dindigul city.The pH of unpolluted soil was high, whereas that of polluted soil was low.The electrical conductivity and chloride content was high and it was unfit for the growth of plants.The heavy metals in all the samples were adequate for the growth of plants (Mariappan, Balamurugan, & Rajan, 2001).
Alkalotolerant/alkalophilic actinomycetes NCIM 5080 and NCIM 5142 produce alkaline protease in presence of chromium ions.These properties of isolates are suitable for treatment of tannery effluents which are alkaline and contain chromium and proteinaceous matter.Both the actinomycetes are able to grow in undiluted tannery effluents and remove chromium almost completely and reduce the COD by 70%-80% during growth as well as by pregrown biomass (More et al., 2001).
A laboratory scale experiment was conducted on aerobic digestion of tannery effluent using cowdung as the seed material.The BOD removal of 95.8 per cent was obtained at an optimum organic load of 0.6kg BOD/m 3 /d.Biokinetic coefficients were calculated for the data obtained to study the metabolic performance of the microorganisms (Prakash, 2001).
Adsorption technique has been applied for the removal of hexavalent chromium from aqueous solution using wheat straw dust, saw dust, and coconut jute, and the results are compared with the powdered activated carbon.
The high uptake of hexavalent chromium was observed with PAC at pH 2.0, and for the other adsorbents at pH 6.0 (Rao & Bhola, 2000).
An attempt has been made to assess vermiculite, a phyllosilicate mineral group with high cation exchange capacity, as an alternative for activated carbon.In this investigation, the adsorption potential of raw vermiculite grade 2 was evaluated using vermiculite clay mineral barrier system for reducing pollutant loads in specially designed treatment system, where the important parameters like flow rate, effluent loading rate required were optimized.Kinetics of chrome-tannery effluent treatment by the activated-sludge system was studied and reported (Jayabalakrishnan & Mahimai, 2007).
Anaerobic digestion of tannery waste in which semi-continuous and anaerobic sequencing batch reactor processes was reported (Zupancic & Jemec, 2010).
A tannery discharges from 21,500-21,950 liters a day, corresponding to 86-88 liters per kg of leather processed.
Chromium is known to be highly toxic to the living aquatic organism in the hexavalent state and somewhat less toxic in the trivalent form.The effluents from chrome tanning industry shall meet with the specific tolerance limits for chloride with 1000 mg/L, BOD (5 day at 20 °C) with 30 mg/L, hexavalent chromium with 0.1 mg/L and pH between 5.5 to 9.0.
Tanning of animal hides to convert them into leather is an important industrial activity.But the pollution from tanneries has a long-term negative impact on the environmental resources.The liquid waste from tanneries is a dangerous pollutant because it contains organic matter and inorganic pollutants in the solution, in suspension as well as in colloidal dispersion.Hence, there is a need to remove these pollutants before they are released to render them harmless.In the past ten years, a number of different anaerobic processes have been developed for the treatment of industrial wastes.
Anaerobic digestion is one of the oldest processes used for the stabilization of sludges.It involves the decomposition of organic and inorganic matter in the absence of molecular oxygen.In this process, the organic matter in the mixture of primary settled and biological sludges is converted biologically, under anaerobic conditions, to a variety of end products including methane and carbondioxide.The process is carried out in an airtight reactor.Sludge, introduced continuously or intermittently, is retained in the reactor for varying periods of time.The stabilized sludge, withdrawn continuously or intermittently from the reactor, is reduced in organic and pathogen content and is non-putrescible.

Materials and Method
Laboratory scale reactors are normally used to determine kinetic coefficients.In such a reactor, detention time () is equals to mean cell residence time (c).The procedure is to operate the unit over a range of effluent substrate concentrations.Hence, several different c (at least five) are selected for operation ranging from 1 to 6 days.
Using the data collected at steady state conditions, mean values are determined for influent BOD (So), effluent BOD (S), and mixed liquor suspended solids (MLSS) of the reactor (denoted by X) to find out the kinetic coefficients.
The experiment was designed and operated on the principle of an anaerobic digestion process to evaluate the bio-kinetic parameters, which could be used in the rational design and operation of large-scale anaerobic installations.The reactor was a wide mouthed Pyrex glass bottle of 5 liter capacity.The reactor has provision for adding wastes, for removing treated effluent and settled solids and for gas transfer.The gas collection apparatus consisted of a glass bottle of 2 liter capacity and another bottle of 1 liter capacity for the water displaced from the gas bottle.Care was taken to remove the air from the reactor as well as from the gas collection bottle at the beginning of the experiment and the entire set up was checked for gas leaks.Tubes were connected to the digester to facilitate feeding of the west and removal of the effluent.The digester was carried out at room temperature.Cow dung was used as the seed material and fed into the digester to start with.After establishing necessary biota from cow dung sludge, the chromium free composite liquor is fed into the digester daily.The pH of the influent sample was adjusted to pH 7 by adding alkali before feeding.After feeding, the contents in the digester were given thorough mixing by manual shaking.The BOD load was kept at 0.25 kg BOD/m 3 /day in the beginning.After several displacements of the digester contents and after establishing stable conditions of digestion the loading rate was gradually increased.Gas measurements were done once a day.The gas was burnt periodically to confirm the presence of methane which formed a major portion of a gas.
Samples from the influent to the reactor and effluent from the final clarifier were simultaneously collected to carry out BOD tests.Samples from the reactor were collected to find out MLSS, dissolved oxygen (DO), pH and temperature.Mean values of So, S and X at various c were used to find out kinetic coefficients while DO, pH and temperature tests were carried out to ensure favorable environmental conditions in the reactor for biological treatment.All the tests were carried out as per procedures laid down in the "Standard Methods".
The samples of effluents' drawn at various stages were analyzed for pH, influent BOD (So), effluent BOD (Se), mixed liquor volatile suspended solids (MLVSS) before sludge wasting, initial MLVSS and the net growth rate of microorganisms ΔX/Δt which was obtained from the difference of MLVSS before sludge wasting and initial MLVSS values.The pH was maintained within the optimum range of 6.8 to 7.4 which is favorable for anaerobic bacterial growth.Calculated amount of diammonium phosphate and urea were added to the feed solution as and when required in order to maintain the BOD: N:P ratio at 100:2.5:0.5 which is effective for anaerobic digestion.
In anaerobic digestion, biomass is formed having a molecular formula C 5 H 7 O 2 N. Cell synthesis requires Nitrogen (amino acid formation) for which Nitrogen (in the form of Urea) rich nutrient is supplied.During cell synthesis, energy in the form of ATP is released for which phosphorus acts sink.The tannery wastewater was filled up to a volume of 2 litres in the anaerobic reactor and the mixture was mixed daily at frequent intervals.Neither waste feeding nor withdrawal of mixed liquor was done until gas production was noticed.Regular wasting and feeding were continued until a steady state condition was reached.The daily BOD loading rate was kept constant at around 0.8 kg/m 3 /day.The daily gas production, the influent and effluent BOD, Mixed Liquor Volatile Suspended Solids (MLVSS) which indicates the concentration of microorganisms in the reactor, pH, volatile acids and alkalinity were recorded at the steady state condition at which the sludge growth and gas production remained constant.The mean cell residence time was varied by operating the reactor at several MLVSS concentrations.During the course of study, the reactor temperature fluctuated between 29 to 31 °C, which falls within the suitable temperature range for heterotrophs treating wastewater under aerobic conditions (Rao & Bhola, 2000).The pH of the reactor remained between 6.5 and 8.0 which is a suitable range for biological treatment. www.ccsen

Results
The genera Table 1.G

All Values
The result Figure 1.The results indicate that the effluent has to be treated for an effective removal of chromium before being subjected to biological treatment.Lime was used as the precipitating agent for chromium removal and effect of lime on chrome precipitation is presented in Table 2.It was observed that the chromium removal increased with increase in pH and the maximum chromium removal of 99.7% was observed at pH at 8.9 with a lime dose of 4.4 g/lt.Further increase in lime has resulted in the decrease of chromium removal due to redissolution of the mixture under such experimental conditions.The results of the anaerobic digestion of the chrome free composite liquor are presented in Table 3.The data consists of varying BOD loading rate changes in pH alkalinity, volatile acid, and percentage BOD reduction.It was observed that a maximum BOD reduction of 96.9% was obtained at the BOD loading rate of 0.80 kg BOD/m 3 /day and throughout different loading rates.The BOD reduction was more than 94% which could be due to the proper maintenance of alkalinity and volatile acids in the digester.In the beginning of the process, the pH of the effluent was 6.9.The analysis of parameters was done as per the standard procedure given in APHA.
As the loading increased gradually the pH increased to 7.6 up to the optimum loading and dropped down slightly to 7.4 at the maximum loading.The increase in alkalinity was steady as the loading increased gradually.Side by side there was a production of volatile acids but was not considerable.With the initial pH correction and with proper seeding of the waste, the process of digestion was taken place unhindered, without undue accumulation of intermediate products.There was no possibility for the formation of free volatile acids.The results of the treated liquor with maximum and optimum removal efficiency of pollutants are represented in Table 4.
Total Kjeldahl Nitrogen (TKN) and Phosphorous (P) tests were carried out on the settled wastewater collected to check the level of these nutrients for satisfactory biological treatment.The BOD:N:P for the wastewater was found to be 100:21:0.19as against a recommended value of 100:5:1 (Kulkarni & Shrivastava, 2002).The phosphorous was thus deficient and this deficiency could hamper a satisfactory biological treatment.Therefore, the deficiency was met by adding calculated amount of Potassium Dihydrogen Phosphate (KH 2 PO 4 ) in the wastewater."k value is employed to find out the volume of biological reactors.Greater is the value of k, smaller will be the size of reactor.Ks has no direct application in process design.It only gives an idea about the change in the specific growth rate of bacteria with a change in the concentration of the growth limiting substrate.Y is used to estimate the total amount of sludge produced as a result of wastewater treatment.Kd is used to find out the net amount of sludge to be handled and hence the size and cost of the sludge handling facilities can be found out from this information.
Data to determine above coefficients were generated by operating the bench scale reactor at different c.Mean values of So, S and X corresponding to each are presented in Table 5.The following linearized equation was used to find k and Ks (Gallegos, Martnez, & Bulbulian, 2005).

Table 4 .
Results of the treated liquor k = Maximum rate of substrate utilization per unit mass of microorganisms, time-1, Kd = Endogenous decay coefficient, time-1, Ks = Half velocity constant, substrate concentration at one-half of the maximum growth rate, mass/unit volume, Y = Cell yield coefficient, mg/mg (defined as the ratio of the mass of cells formed to the mass of substrate consumed.

Table 5 .
Mean values of data for k and Ks