BACTERIAL DEGRADATION OF BLACK LIQUOR - A PULP MILL EFFLUENT

Abstract

With increasing industrialization and urbanization, the problems of pollution have manifested into disastrous proportions all over the world. In India, with the promulgation of Water and Air acts, it has become mandatory on the part of industries to treat their wastes to effluent standards before disposal. The pulp and paper industry in recent years have registered a boom, specially with the introduction of agricultural wastes as raw material. Water discharged from these industries is one of the major problems of river pollution in India. The most toxic waste emanating from these mills is "black liquor" generated from digesters. It has characteristically high BOD, COD, suspended solids, dissolved solids and colour apart from slowly biodegradable lignin compounds and its derivatives. The present day technology for the treatment of black liquor comprises of chemical precipitation or biological treatment in anaerobic lagoon/digesters or in activated sludge process when mixed with the other waste of the paper mill. In the first method large quantities of sludge generated further pose disposal problems, whereas the digesters require skilled operation and maintenance and lagoons are land intensive, requiring a detention period of 25 to 30 days. In case of activated sludge process, the problems of frothing and high power costs are common. As such a need for techno-economically feasible solution to the treatment of black liquor is of paramount importance A few species of fungi and bacteria are found to degrade lignin fast and easily. However, not much work has, so far, been reported on the degradation of black liquor by these microorganisms and more investigations in this direction are called for. Investigations presented in this dissertation are centered on the degradation of black liquor by three bacterial strains viz. Pseudomonas putida, Acromonas (i) formicans and Acinctohacter calcoaceticus obtained from the National Chemical Laboratory, Pune, India. Black liquor was collected from the effluent channels (at first extraction stage) of an agricultural residue based small pulp and paper mill. This plant is located in Himachal Pradesh, India and employs soda process for pulping. Sterilized black liquor was used throughout the study. In preliminary experiments conducted for research, the growth pattern for all the three strains were studied in basal medium and generation time was calculated for each strain. Effect of physico-chemical parameters such as pH and aeration on the growth of selected bacteria was investigated. Growth was measured by bacterial enumeration (7 days growth) method. The growth pattern ofbacteria indicate a well defined log period which extends upto the 10th day and then rounds off to a stationary phase. Generation time varied between 0.67 and 0.98 days. Various physico-chemical parameters e.g. black liquor concentration, pH, amount ofglucose, ammonium chloride and phosphate as extra carbon, nitrogen and phosphorus sources respectively were optimized for the best growth of bacterial strains in black liquor using bacterial cells suspension in distilled water (102 -104 cells per ml) as inocula. The degradation ofblack liquor was monitored as percent removal efficiency of COD, colour and lignin. Batch studies were conducted to degrade black liquor under optimized conditions for each strain and the reaction mixture was analysed per day for pH, viable cell, count, COD, colour and lignin up to 20 days of incubation. The results ofbatch studies show that the efficiency ofremoval ofCOD, colour and lignin ranged between 70 to 90 percent for each strain and most of the removal occurred in first 8to 10 days of the experiment. There is agradual fall in pH in initial stages andthis becomes constant in lastfew days, while the bacterial cell count increased exponentially initially and reached to aconstant level in last stage ofexperiment. The decrease in pH was suspected as a result ofacid formation after degradation of lignin present in black liquor. (ii) The uninoculated incubated black liquor used as a control and samples obtained after 20 days ofdegradation by individual bacterial strain in batch studies were analysed on gas chromatography to identify the products of lignin degradation. Ethyl acetate extractives were prepared from the supernatants obtained after filtration of black liquor in each case and analysed on gas chromatograph as trimethyl silyl derivatives. Various peaks were obtained due to unknown compounds present in the sample and some of these compounds were identified as phenolic acids by matchingtheir retention time with known standards. The identified compounds formed as aresult of interaction ofP.putida with lignin are p-hydroxy phenyl acetic acid (PHPA), p-hydroxy benzoic acid (PBA), vanillic acid (VA), protocatechuic acid (PCA), syringic acid (SA), p-coumaric acid (CA) and ferulic acid (FA). The identified products oflignin degradation by A.formicans include-PHB, VA, PCA, SA and FA and the same in the case ofA. calcoaceticus are PHPA, PHB, VA and FA. These compounds are lignin fragments can be reasonably justified on the basis of their structures. Phenolic acids were formed due to oxidative degradation of lignin by different bacterial strains. Continuous degradation of black liquor was carried out at laboratory scale and for this purpose completely mixed, continuous flow aerated reactors/lagoons were used. The reactors were fabricated using plexiglass having a capacity ofsix litre with an outlet at four litre. Black liquor was taken at optimized conditions and inoculated by tested strain ofbacteria. These lagoons were kept in batch for 8 days to mature and removal efficiencies of COD, colour and lignin were measured every day. On the 8th day, 500ml of black liquor was fed to the reactor from the reservoir ata rate of2.88 ml per minute to give adetention time ofaround 8 days. The effluent from the reactor was tested regularly for COD, colour and lignin removal as parameters oftreatment. The values of these parameters were in the range ofabout 70 to 90 percent for each strain ofbacteria. Experiments were continued for 27 days after continuous fedding of black liquor in the reactor. (iii) Various kinetic parameters were also calculated from the above results and these can be utilized in the design of reactors for COD, colour and lignin removal from black liquor by selected bacterial strains. The detention time can be found out with the help of these parameters for achieving the desired efficiency of removal.