TREATABILITY STUDIES OF BLACK LIQUOR BY UASBR

Abstract

Treatability Studies of Black Liquor by UASBR The pulp and paper industry is one of the most important sectors of the Indian Economy, and is one of the major polluting industries as it consumes large quantity of water and discharges huge quantity of highly polluted wastewater . The wastewater generated from the digestors of the pulp and paper industry is known as Black Liquor (BL). It contains high organic and inorganic dissolved and suspended solids. Lignin and its derivatives present in BL are responsible for the brown colour. These are not readily biodegradable and thus constitute recalcitrant pollutants. The activated sludge process conventionally used, has been able to remove the sugars from pulp and paper mill wastewater. Lignin, however, has not been removed by this process, leaving a large portion of the COD and nearly all of the colour to be discharged in treated effluents. Anaerobic processes have always been an integral part of the wastewater treatment scheme. There are currently 900 full-scale high rate anaerobic biological reactor installations throughout the world , and they are distributed as follows: Upflow Anaerobic Sludge Blanket (UASB) 67% ; Continuously Stirred Tank Reactor (CSTR) 12% ; Anaerobic Filter (AF) 7% ; and other like Anaerobic Digestor , Anaerobic Lagoon , etc. 14%. Anaerobic treatment of pulp and paper mill wastewater by UASB process has been studied in recent times . In the present study, investigations have been carried out for treating the black liquor along with readily biodegradable wastewater i.e. sugar mill wastewater. Treatability studies were carried out (in alaboratory scale UASB reactor) in four phases of operation. In the first phase, synthetic wastewater (SWW) prepared from molasses and supplemented with required nutrients was used. In the second phase, 10% of the SWW COD was replaced by BL COD. In the third and fourth phases, the SWW COD was replaced by 20% &30% BL COD respectively. ORGANIZATION OFTHE THESIS : The thesis has been organized in six chapters. The first chapter presents a brief background information on the pulp and paper mill, wastewater produced by the process ofpulping and the need for the present study. In the second chapter, a brief description of the wastewater characteristics from pulp and paper mill and its treatability by physicochemical and biological process have been presented. Application of high rate anaerobic systems for the treatment of pulp and paper mill waste water has also been incorporated. UASB process and its modifications are presented in the third chapter. Chapter four describes the materials and methods employed in the present experimental work. The data obtained from the studies, and the results and discussions pertaining to the ii evaluation of UASB process for treating different concentrations of black liquor mixed with synthetic waste water (SWW) have been presented in chapter five. Chapter six deals with conclusions drawn from the present study and recommendations for future work. EXPERIMENTAL WORK : Continuous flow experiments were conducted at a constant liquid temperature of 30±2°C in a bench scale UASB reactor of 12.5 lit capacity, equipped with gas solids separator and gas holder. Initially digested sludge from a sewage treatment plant was used for seeding the reactor, followed by granulated acclimatized sludge from another lab-scale UASB reactor. Feasibility to treat black liquor using UASB reactor was investigated by gradually replacing the synthetic waste water (SWW) COD with the black liquor (BL) COD. The experimental work was done in four phases of operation. In each of the operation phase, the composition of the feed was not changed until COD reduction and gas production were stabilized. The data pertaining to the operational parameters and the performance evaluation of the reactor were recorded for 509 days. REACTOR OPERATION: Phase I : Reactor -Start-Up (day 1-276) The reactor was started in accordance with the guidelines given by Lettinga et al. [57]. The sludge acclimatized to 30±2°C was fed m continuously with SWW using peristaltic pump. Urea and ammonium phosphate were added to the feed as sources of nitrogen and phosphorus. The reactor was started with OLR of 2Kg CODmV. SLR of 0.19 KgCODkg-'VSSd-i and HRT of 12 Iks. It took along time for reactor to stabilize. The sludge granules, however, did not appear even after 50 days of continuous operation of the reactor. To facilitate granulation, around 1.5 lit of granular sludge from another reactor was added. On the 78th day the SWW COD ranged from 1230-1430 mg/1 and the COD removal efficiency reached to 80%. The OLR at this stage was between 2.4-2.9gCODr,d-1. On the 79th day from the reactor start-up, an attempt was made to granulate by replacing the 10% of SWW COD with BL COD. The operation continued for two months. The reactor performed well for nearly 40 days. The COD removal of 85% was attained. Afterwards the sludge started to disperse, causing lifting of the sludge. This culminatedmthe loss of most of the sludge along with the effluent. Therefore, the feed composition was again changed to 100% SWW-COD only. The operation continued for nearly four and a half months. Phase II: (day 277-325) In this phase, the reactor was fed with SWW containing 10% BL. The sludge responded, and 82-84% COD removal efficiency was recorded. IV The organic loading rate towards the end of this phase was 12 gCODl" d" . Small granules were noticed at this stage on day 288. Phase III : (day 327-440) In phase III, the feed composition was changed and fed with SWW containing 20% BL. At HRT of 6 hrs., the average OLR & SLR of the reactor was 12.9 gCODl'M"1 and 0.85 gCODg'VSSd"1 respectively were recorded. The COD removal between 65%-79% was attained in 33 days, which decreased to about 45% at the end of second month of operation in this phase. To reactivate the bacterial growth, the feed composition was again changed to 100% SWW COD and later on experiments with 10% BL & 20%o BL were conducted. Phase IV : (day 441-509) In this phase, the reactor was fed with SWW containing 30% BL. The operation continued for five weeks with feed concentration ranging between 3100-3500 mg/1. The average OLR & SLR values were 13.29 gCODfM"1 & 1.6 gCODg'VSSd-1 respectively. The COD removal reduced from 68% to 46%> on (day 475), the HRT was changed from 6 hrs. to 8 hrs. and again to 12 hrs. on day 491. The operation continued for 20 days more (upto day 509), when the COD removal efficiency reduced to 28%. The operation of the reactor ended at this stage with an organic loading rate of 6.6. gCODl'M"1. RESULTS: (1) Reaction Operation : Results obtained from different phases of reactor operation are summarized in the table given below : Waste Type Biogas Methane Content % COD Removal Efficiency (%) Maxi OLR KgCOD m^d"1 Maxi. SLR KgCODkg" VsSd"1 Colour Removal (%) rnVd"1 m3kg-' COD fed 100% SWW 0.81 0.331 70 73.5 6 1.4 - 10%BL + 90% SWW 0.96 0.224 65 78.9 12 1.0 20 20% BL + 80% SWW 0.36 0.53 60 61.9 14 1.7 9 30%BL + 70% SWW 0.5 0.38 60 49.9 14 1.3 4 (2) Gas Production : The effect of organic loading on the biogas production has been analysed and established. An empirical relation between gas produced and COD fed has been given by the equation of the type Y = AxB (3) The COD mass balance has been analysed for all sets of experiment. (4) The sludge concentration (profile) along the height of reactor has been measured during phase two and compared with existing model. (5) Specific methogenic activity (SMA) of the accumulated biomass/sludge was also measured and was found very low indicating slow degradability of the black liquor. VI