REMOVAL OF TOXIC ORGA ICS FROM WASTEWATER A D E ERGY PRODUCTIO USI G EC A D MFC

Abstract

Organics present in water reduces dissolved oxygen and creates problem to the aquatic animals and plants for their survival, some toxic organics like phenols etc., are also carcinogenic in nature. Organics mostly enter into water stream through various industrial sources. Amongst various industries the distillery and pulp and paper industries produce wastewater containing very high amount of toxic organics and colour. COD value of these wastewaters are significantly high (> 30,000 mg/l) with considerable BOD to COD ratio (biodegradability index). Thus, the anaerobic digestion is a preferred route as the initial step, to be followed by aerobic treatment (activated sludge process) and tertiary treatment steps. After anaerobic digestion of these wastewaters the residual COD still remains high and in many cases the biodegradability index reduces significantly, which makes aerobic treatment inefficient. Further, various conventional tertiary treatment techniques such as adsorption, chemical oxidation, chemical coagulation etc., require chemicals and produce secondary pollutants. Under this backdrop, the present investigation has been undertaken to develop an alternate treatment method using electro-coagulation (EC), which produces in situ coagulants, removes all types of organics and increases biodegradability index, for the removal of toxic organics from wastewater followed by finishing stage treatment through microbial fuel cells (MFC). Potential of the scum and sludge, generated during EC process, as an energy source as well as the electricity generation capacity of the MFC process have also been explored. Pulp and paper industry wastewater and distillery anaerobic digester effluent collected from local industry have been considered as two important toxic organics containing wastewater. Synthetic solution of lignin has also been considered because of the fact that some wastewater streams of pulp and paper industry, particularly from delignification process, contain very high amount of lignin and its derivatives, which are not biodegradable. However, very less reports are available on the removal of pure lignin from wastewater. Further, the synthetic glucose-glutamic acid (1:1) solution has also been considered because of the facts that these are normally available in the distillery effluents and model compound as substrate is required to understand the MFC process, which is relatively less developed. ii Various technology options along with best available techniques for the treatment of pulp and paper industry wastewater as well as distillery effluents, which contain very high amount of COD and colour, have been reviewed to find out the scope for the EC and MFC processes. Literatures available on the removal of organics from paper and pulp industry wastewater and distillery effluents through EC and MFC has also been reviewed to find out the gap area and to finalize the domain of experiments for the present investigation. Experiments have been conducted to the study the removal of BOD, COD, TOC and colour from synthetic lignin solution, diluted real pulp and paper industry wastewater and diluted anaerobic bio-digester effluent of a distillery unit through EC to find out optimum process conditions and to develop empirical models between input and output variables. The scum and sludge generated during EC of diluted pulp and paper industry wastewater and diluted anaerobic bio-digester effluent of a distillery have been characterized to assess their potential for energy production. The EC treated pulp and paper industry wastewater and distillery anaerobic digester effluent, with improved biodegradability index, have further been treated as a polishing step through MFC, under the optimum conditions determined by the MFC study with synthetic glucose-glutamic acid solution in batch as well as column MFC. Removal of toxic organics in terms of BOD, COD, TOC and colour removal from synthetic lignin solution, pulp and paper industry wastewater and distillery anaerobic digester effluent have been carried out in an electro coagulation unit using aluminium as sacrificial electrode. Effects of current density, pH, run time, inter electrode distance, NaCl concentration on the removal of COD, and colour have been studied. The optimum conditions have been determined and empirical input output models have been developed by using design of experiment technique and Design Expert software. Optimum conditions for the removal of organics from synthetic lignin solution, pulp and paper industry wastewater and distillery anaerobic digester effluent, respectively, are found to be (current density 100 A/m2, pH 7, run time 75 min , NaCl concentration 1 g/l, inter electrode distance 1cm, initial COD 2500 mg/l); (current density 110 A/m2, pH 7, run time 75 min , NaCl concentration 1 g/l, inter electrode distance 1cm, initial COD 7000 mg/l) and (current density 125 A/m2, pH 3, run time 120 min, NaCl concentration 1 g/l, inter electrode distance 1cm, initial COD 5000 mg/l). Under optimum conditions removal of BOD, COD, TOC and colour have been found as 100 %, 93.86 %, 95.22% and 99.12 % for synthetic lignin solution, 29 %, 75.89 %, 75.12% and 97.25 % for pulp and iii paper industry wastewater as well as 90.73 %, 91.8 %, 63.3 % and 97.4 % for distillery anaerobic digester effluent. Biodegradability index has been improved from 0.108 to 0.31 for pulp and paper industry wastewater and from 0.38 to 0.43 for distillery anaerobic digester effluent. Sludge and scum generated during the EC of pulp and paper industry wastewater as well as distillery anaerobic digester effluent have been characterized through proximate and ultimate analysis, Fe-SEM, FTIR, TGA, DTGA, DTA etc. to understand the process as well as to assess the energy production capacity of these materials. The scum and sludge of pulp and paper industry wastewater have been found to contain more carbon content as well as higher heating values than those of distillery anaerobic digester effluent. The reasons for these have been explained on the basis of the nature of organics present in paper and pulp industry wastewater as well as distillery anaerobic digester effluent. Simultaneous production of electricity (expressed in terms of voltage, current density and power density) and organics removal (in terms of COD removal) has been studied in batch and continuous scale ML-MFC with synthetic glucose glutamic acid solution, as these are normally present in distillery effluent. Effect of process parameters such as pH, temperature, initial COD on the electricity generation and organics removal have been studied in batch MFC. Optimum conditions have been identified and the empirical input-output models have been developed. Efficiency of voltage generation in MFC has further been improved by addition of methyl blue mediator and metal ions. In continuous MFC the effects of flow rate/ empty bed contact time (EBCT), mediator concentration, inter electrode distance, area of anode, metal ions concentration and initial COD have been studied. Optimum temperature, pH and initial COD values for batch MFC are found to be as 35 oC, pH 7 and 1500 mg/l, respectively. Optimum EBCT was found to be ~ 4 days whereas the inter electrode distance was 20 cm in continuous MFC. Although optimum COD for batch MFC was 1500 mg/l, it was 500 mg/l for continuous MFC. EC treated pulp and paper industry wastewater and distillery anaerobic digester effluents were treated in batch and continuous MFC under the optimum conditions determined for synthetic glucose glutamic acid solution. Under these conditions, in batch MFC, the pulp and paper industry wastewater with initial COD value of 750 mg/l and distillery anaerobic digester effluent with initial COD value of 1000 mg/l are able to produce treated water containing COD iv below permissible limits. The EC treated pulp and pepr industry wastewater and distillery anaerobic digester containing initial COD of 750 mg/l and 1000 mg/l respectively, were able to produce treated water with COD and BOD values below permissible limits in continuous MFC. The capability of the combination of the EC and MFC process for the treatment of these two real industrial wastewater samples has also been studied. It has been observed that pulp and paper industry wastewater with initial COD value of 5500 mg/l and distillery anaerobic digester having initial COD value of 7200 mg/l can produce treated water with permissible COD and BOD values if these are treated through EC followed by MFC operation. The possibilities for the improvement in voltage generation and scale up of the MFC techniques have also been discussed.