ELECTROCHEMICAL TREATMENT OF WASTEWATER FROM AGRI-BASED PULP MILL

Abstract

Electrochemical treatment (ECT) technology has shown considerable interest during recent years as a very effective treatment method for wastewaters containing refractory and non-biodegradable materials. It uses sacrificial electrodes in an electric field, which can directly or indirectly degrade the pollutants/contaminants at ambient temperatures. In India, small scale agri-based pulp mills consume large amount ofwater (200- 300 m3) and generates 150-250 m3 of highly colored and potentially toxic wastewater per tonne of paper produced. Most mills do not have chemical recovery units and they discharge black liquor without or with partial treatment only along with other wastewaters leading to adverse environmental impacts. The wastewater discharge from a30 TPD agri-based small paper mill is equivalent to the pollution load of a 150 TPD mill with chemical recovery facility. Since the pulp produced corresponds to only 45-50% ofthe original weight ofthe raw material, the effluents are heavily loaded with large amounts of organic compounds, measured as BOD, COD and color, etc. The ECT of agri-based mill wastewater has not been reported in the open literature or in the patented material. In the present study, black liquor (BL) emanating from the cooking-washing section ofa local agri-based pulp mill has been treated using ECT technology with or without additives followed by coagulation either alone or in combination. Objectives of the Work The present study has been undertaken with the following objectives: 1. To carry out laboratory scale investigations on the ECT of diluted black liquor (obtained from the cooking-washing section of an agri-based pulp mill using wheat straw as the raw material) in a batch and a continuous electrochemical reactor using parallel plate electrodes. 2. To study the effect of number of electrode plates, current density, cell voltage, initial pH, electrolysis time, and the concentration of additives li Abstract viz, polyacrylamide (PAA) and sodium chloride (NaCl) on the removal of COD, color and solids by ECT. 3. To study the performance of the ECR on a continuous mode for the parameters optimized in the objective (2) on the removal of COD, color and solids without any additives. 4. To study the effect of polarity reversal (PR) on the degradation characteristics (COD and color) of the effluent in batch and continuous ECRs. 5. To study the settleability of the solid residue and the filterability characteristics of the treated effluent from batch and continuous ECRs. 6. To study the physico-chemical characteristics of the solid residue obtained from ECT of the BL. To fulfill the above objectives, the concentrated BL as obtained from the pulp mill, having a very low transparency with dark yellow-brown color and COD -30,000 mg dm"3 was diluted with double distilled water (DDW) to have workable wastewater. This BL had the following characteristics : BOD -615-670 mg dm"3, COD -2000 mg dm"3, chlorides -48-62 mg dm"3, total solids -2200 mg dm"3, pH -6.86-7.12, total alkalinity -380-410 mg dm"3, color -1750 platinum cobalt unit (PCU), AOX ~540ug dm'3, and a turbidity value of182 NTU. The ECTin both batch and continuous mode were conducted in a 2 dm3 ECR of cubical shape using iron plates as electrodes. The electrodes were arranged in parallel and connected to the respective terminals of the DC rectifier. In each batch experimental run, 2dm3 of wastewater was fed into the electrochemical reactor (ECR), and stirred continuously to maintain good mixing in the reactor. The coagulant additives used in the ECR during the batch studies were polyacrylamide (PAA), polyaluminium chloride (PAC) and sodium chloride (NAC1) while no chemical additives/coagulant aids were added during continuous ECT of BL. The coagulant and coagulant aid used for the posttreatment of the batch effluent were commercial alum and PAA, respectively. Of the 4, 6 and 8 plate configurations, a current density (CD) of 55.56 A m"2 with an active electrode surface area to volume ratio (SA/V) of54 m2 m"3 at neutral pH in Abstract with a 6plate arrangement was found optimal achieving a maximum COD and color removal of 80% and 90% respectively. Increase in salinity reduced the treatment time significantly and the sludge settling characteristics also improved. The addition of PAA (10 mg dm'3) to the ECR enhanced the COD removal rate with a very short treatment time with excellent sludge settleability. The post-treatment ofelectrochemically treated wastewater from the batch ECR by chemical coagulation using commercial alum (360 mg dm"3) along with 20 mg dm"3 PAA further reduced COD values to < 180 mg dm"3 and a near 100% color (< 5 PCU) removal. An overall 91% COD removal and near 100 % color removal could be achieved by the two stage treatment viz., ECT followed by coagulation/flocculation from its initial COD value of 2000 mg dm"3. In the batch experimental runs, the pH changed from its initial value of7.08 to its steady value of above 11.5 for the CD in the range of 44.44 - 66.67 Am'2. At pH -11.65, the COD and color removal ceases, although the COD removal at this pH after about 60 min was found to be the highest. The chemical dissolution of iron was strongly influenced by pH0. ECT at higher pH0 (pH0 > 9) increases the dissolution of 9 1 • iron electrodes by an order of magnitude. At the optimal SA/V of 54 m m", the iron electrode consumed is 31.27 g/m2.h giving a maximum COD removal. SVI of the sludge obtained from the treatment at optimal conditions without any additive and with the addition ofPAA (10 mg dm"3) and NaCl (625 mg dm"3) were found to be 0.312, 0.165 and 0.183 dm3 g"1, respectively. The sludges obtained from the ECT of the BL at its natural pH without any additive and with 625 mg dm" NaCl or 10 mg dm"3 PAA during EC process had very good settling characteristics. PAA addition hastened the EC process and the sludge settling rate improved considerably. The settling velocity of the sludge obtained from the ECT of BL without any additives at the optimal conditions could be satisfactorily correlated with the Richardson and Zaki's model, V = K (1 - nX)4 65 with K (the free falling velocity of individual particles as obtained from the fitting of the experimental data) value of 2.408 and n (the factor which converts the initial mass concentration into fractional volumetric concentration) value of 0.365. The results of the gravity filtration of the treated BL showed that the addition of NaCl (625 mg dm"3) improved the filtration characteristics and reduced the IV Abstract values of the specific cake resistance (a). The values of the specific cake resistance (a) and the resistance of the filter medium (Rm) were in the range of 3.25 - 4.67 x10" mkg"1 and 2.37 - 2.98 x109 m"1, respectively. The treatment of BL in a continuous ECR at near neutral pH without any additive showed good potential in reducing COD, TS, turbidity and color at flow rates 0.5-2 dm3 h"1 at the CD of 55.56 Am"2 and the residence time (x) in the range of 2-4 h. ACOD removal of 78.2 and 82.15 %was achieved at flow rates of 1and 0.5 dm3 h"1, respectively, with a corresponding color removal of -90 and 79%. ECT proved its stability at spiked (stepped-up) flow rates during ECT. After ET = 180 min during continuous treatment, the treated BL slurry had a TS concentration of 0.763 gdm"3 reduced from its initial value of- 2.10 gdm"3 while that of the reactor content had 6.682 gdm"3, i.e., the TS concentration reduced by - 65% before and after EC treatment at the flow rate of 1dm3 h'1. During the batch study at the optimal current density of 55.56 Am"2, the SEC at 60 and 75 min ET was found to be 5.313 and 6.641 kWh kg"1 of COD removed. In the continuous system under similar operating conditions, the SEC obtained at 60 min ET was found to be 5.4 kWh kg"1 of COD removed for x=2h. Also, at 75 min ET, SEC was 6.9 kWh kg"1 of COD removed for t - 1h. In the batch study, a charge of 3.75 Ah dm"3 was required to remove COD by -80 %at 75-80 min ET at a CD of 55.56 Am'2. In the continuous experimental runs, at similar operating conditions by supplying the same amount of charge, COD removal was -68 %at 90 min ET for x= 1h (reduces by 17.5% on continuous mode) ascribing to the fact that the micro floes flow away along with the treated BL from the outlet ofthe ECR until the ECR matures (-90 -100 min ET). The current efficiencies (CE) of batch and continuous studies were compared. The CE values for both the batch and the continuous studies coincide between acharge of 3- 4Ah dm'3 and 75-90 min ET. On supplying acharge of 3.125 Adm'3 at 75 min ET, CE values were 171.5 %for batch, and 167.23 %for the continuous ECR at x=4h. Further, at a charge of3.75 Adm'3 at 90 min ET, CEs were 142.93% for batch, and 143.12 %for the continuous ECR for x=2h, respectively. Prolonged use of iron electrodes as anodes produced large number ofdents on their surface. The ECT efficiency showed deterioration on long time runs ofthe anode Abstract plates as the nucleated dents are swarmed with micro floes on the plates thus prompting polarity reversal. Pulse Polarity reversal (PPR) was resorted to, to reduce flake deposition onthe anode plates during continuous ECT of BL. The thermal analysis of the sludge showed the completion of the oxidation process at 382, 396 and 523 °C for the top scum, settled sludge after ECT ofBL and the sludge obtained after the post-treatment of the supernatant with alum and PAA. The heating value ofthe sludge was found to be 11.33 MJ kg"' which is - 55% that ofthe Indian coal. The chemical analyses showed -15 % of insoluble residue and - 76 % Fe203 and A1203. The settled sludge is a little leaner in carbon and hydrogen content as compared to that of the top scum. The carbon content in the EC generated sludge approximates 50 % that of Indian coal. XRD scans of the top scum and the settled precipitate complex obtained after EC treatment showed the amorphous nature of the sludge. The zero value of ApH was at the pH0 value of 8.4, which is considered as the pHpzc of the untreated dried sludge obtained after the ECT of BL. The sludge had - 42% of volatile matter and low ash content (-18 %). The presence of volatiles in the EC generated sludge showed the possibility of generating/exploiting bio-oil from the sludge. The pH of the EC generated sludge was - 8.96 with a bulk density of -688 kg m"3. The rate equation for COD removal was represented by the power law equation and the ECT of BL shows first order kinetics. Cost analysis showed the operating cost ofECT as INR 12.19 per m3 of BL treated. The sludge can be easily dewatered and dried to make fuel briquettes which could be fired in the furnaces/incinerators for recovering its energy value. Water reclaimed after ECT can be used for secondary purposes. The iron slag obtained after burning in boilers can fetch iron salvage value. The bottom ash obtained after incineration/combustion may be used for blending with organic manure for use in agricultural/horticultural fields or may be blended with clay/coal fly ash to make bricks/ceramic tiles for the building industry. The results from the present study suggest a scheme for the treatment of the pulp mill effluent. This scheme includes electro-coagulation followed by simple coagulation at ambient conditions.