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Green energy from combined treatment of liquid and solid waste from tanning industry using UASB reactor E Ravindranath, Chitra Kalyanaraman, S.V. Srinivasan, Subramanian Porselvam, and Suthanthararajan R Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/ef5022686 • Publication Date (Web): 09 Feb 2015 Downloaded from http://pubs.acs.org on February 18, 2015
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Green energy from combined treatment of liquid and solid waste from tanning industry using UASB reactor E. Ravindranath, K. Chitra, S. Porselvam, S.V. Srinivasan*†, R. Suthanthararajan 1Environmental
Technology Division, Central Leather Research Institute, Adyar, Chennai – 600 020, India
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Abstract : During leather processing about 30- 50 m3 of tannery effluent (TE) and 150 - 250 kg of limed fleshings (LF) are generated for every tonne of hides or skins processed. Currently, tannery effluent (TE) is treated in individual or common effluent treatment plant and solid waste are disposed without treatment, leading to emission of greenhouse gases. In the present study, an innovative approach of treating limed fleshings after liquefaction (LLF) and tannery effluent together in Upflow Anaerobic Sludge Blanket (UASB) reactor has been carried out for generation of green energy in the form of biogas with methane. Further, effect of Organic Loading Rate (OLR) on COD removal and methane yield has been investigated in UASB reactor. Based on the present study, the maximum COD removal efficiency of 75% with methane yield of 0.29 m3/kg of COD removed at an optimum OLR of 12 kg/m3.d and hydraulic retention time (HRT) of 24 h was obtained. This innovative approach of combined treatment resulted in additional methane yield of 37.5% when compared to the methane yield from treatment of tannery wastewater alone in UASB reactor. Certified Emission Reductions (CERs) generated has been estimated for combined treatment of TE& LLF in UASB reactor by replacing open dumping of fleshings for a typical tannery cluster.
Keywords:
Limed fleshings, Tannery effluent, Anaerobic treatment, Certified Emission
Reductions (CERs), UASB
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INTRODUCTION Leather industries in India, contribute to foreign exchange and also provide employment opportunities in rural areas of India. Most tanneries in India are of small and medium scale category. Every tonne of hides or skins processed into leather generates 30- 50 m3 of tannery effluent (TE) and 150- 250 kg of limed fleshings (LF)1,2,3. About 0.14 million metric tonnes of LF is generated per annum, with moisture content of 75 – 87% and 75-82% volatile solids in total solids. Tannery effluents generated during leather processing contain high levels of salinity, organic load, inorganic matter, dissolved and suspended solids, ammonia, organic nitrogen and chromium
1,4,5.
Tannery effluent is collected through underground pipeline and treated in
common effluent treatment plant (CETP). At present, tannery effluent is treated by several treatment technologies like physical, chemical
6,7
and biological treatment systems
8-10.
In some
cases, specialized treatment like Membrane Bioreactor11 and Reverse Osmosis are also implemented for recovery of water and reuse in leather processing as per regulatory requirement of meeting zero liquid discharge12. For secondary biological treatment, conventional treatment systems, both aerobic and anaerobic technologies are adopted in tannery clusters (Figure 1). Aerobic treatment processes such as Activated Sludge Process (ASP)13 and Sequential Batch reactor (SBR)14 are highly energy intensive and generates enormous volume of biosludge15,16. Anaerobic process is more advantageous than the aerobic process due to low energy requirement, low biomass production and simultaneous production of renewable and green energy17. Initially the treatment scheme consisting of anaerobic lagoon followed by aerobic biological treatment in ASP was selected considering the operating cost. Nowadays, due to the emission of GHGs (CH4) and odour nuisance from anaerobic lagoons, it is not used and has been replaced by Upflow Anaerobic Sludge Blanket (UASB) reactors or two stage biological aerobic treatment for treatment of tannery wastewater18. Energy recovered from UASB reactors are used for electrical energy generation and in some places it is just burnt reducing the global warming potential18. In all upcoming tannery clusters, UASB followed by extended aeration is used for treatment of tannery effluent. Process solid wastes generated during leather processing are raw trimming, limed fleshings, lime sludge, chrome shavings, buffing dust and finished trimmings. Among the solid waste, limed fleshings (LF) is a major waste which is disposed off in low-lying areas or disposed
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along with municipal waste causing contamination of land, water and air environment19. It may be noted that the dumping of LF similar to municipal solid waste will emit methane due to anaerobic condition to a maximum extent of about 16 m3 of methane for every ton of LF20. Methane is one of the GHG with Global Warming Potential (GWP) 21 times that of CO2. LF is not permitted to be disposed off in secure hazardous landfill facilities due to high volatile solids content, while the same disadvantage can be advantageous as it has high potential for bioenergy generation when treated anaerobically. Anaerobic digestion of LF in solid waste digester requires very long retention time of 35 – 45 days21. Anaerobic process is a sequence of metabolic reaction like hydrolysis, acidogenesis, acetogenesis and methanogenesis22. Hydrolysis is a ratelimiting step during anaerobic treatment of solid waste,23.
Size reduction of LF results in
increase in the specific surface available and improves the biological activity, which leads to improved gas production24,25. But mechanical size reduction is not effective in LF due to presence of inert material like grit particles, which comes from lime used during the liming process. Various pre-treatment i.e. chemical and biological liquefaction of limed fleshings, have been reported for reduction in particle size and solubilization of organic matter for effective biomethanization of digestate for bio-energy generation in batch reactor20. In this paper, performance of UASB reactor for treating tannery effluent along with liquefied limed fleshings (LLF) and generation of green energy from wastewater and solid wastes together has been reported. In the present study, bench scale studies were carried out using UASB reactor to assess the performance of the reactor in term of COD removal and methane production by varying organic loading rate (OLR) by changing the ratio of LLF and TE. In addition, potential reduction of GHGs in terms of Certified Emission Reductions (CERs) due to avoidance of methane emission from open dumping of limed fleshings is also estimated as per the guideline of United Nations Framework Convention on Climate Change (UNFCCC) compared with combined treatment of LLF and TE in UASB reactor.
MATERIALS AND METHODS Liquefied limed fleshings (LLF) Pre-treatment study was carried out, using anaerobic inoculum present in treated effluent from UASB reactor treating tannery effluent for 8 days for solubilization of limed fleshings (LFs). Liquefied limed fleshings obtained after pretreatment was characterized. The
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characteristics of LLF are pH : 7.5-8.8; COD : 35000- 50000 mg/L; VFA : 12,000 – 18,000 mg/L and TKN : 8,000 – 12, 000 mg/L. Tannery effluent Tannery effluent was collected from tannery cluster, and the characteristics are: pH : 7.5 - 8.9; BOD : 1070 – 3050 mg/L; COD : 2778 – 5853 mg/L, Sulphates: 1680 – 2344 mg/L
and
Total Solids : 12570 – 21630 mg/L. UASB feed (LLF & TE) The UASB reactor was fed with both LLF & TE. The characteristics of UASB feed after mixing with TE and LLF are pH : 7.2 to 8.2, which is in suitable range for anaerobic treatment26,27. BOD : 1120-5656 mg/L and COD : 3500-14364 mg/L. Total solids : 13200 - 22515 mg/L, sulphate : 1556-2294 mg/L and sulphide : 164-349 mg/L. For increasing the OLR in reactor, LLF portion was increased gradually keeping the TE volume constant. C/N ratio of the LF was in the range of 3.6-4.2 which is very low for anaerobic process, but after mixing LLF with TE, C/N ratio increased to a range of 7.6-8.9 and average C/N ratio was 8.1. UASB Reactor setup Bench scale experimental studies were carried out with TE and LLF in 5 L capacity UASB reactors with HRT of 24 hrs. A schematic diagram of the bench scale UASB reactor is shown in Figure 2. The UASB reactor consists of three main parts: bottom, middle and top sections. The bottom and middle sections are provided with a jacket to maintain the temperature. The reactor elements were connected onto each other with stainless steel clamps. The top section, without temperature control, was provided with a Gas- Liquid- Solid (GLS) phase separator. For an equal distribution of feed, a pipe was provided at bottom with holes of UASB reactor in a downward position to avoid clogging of holes. The reactor was fed using a peristaltic pump controlled by timers. Biogas was passed through soda lime pellets and then through wet gas flow meter to measure the methane production. Temperature of the reactors was maintained at 30°C±1 by water circulation through thermostat. Start up and operation of UASB reactor The UASB reactor was seeded with sludge from operating pilot anaerobic reactor for treating TE. TSS and VSS concentrations of seed sludge were 50,000 mg/L and 29,000 mg/L
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respectively. Initially, 35 g LF was liquefied and mixed with 5 L of TE and fed into the UASB reactor. OLR in the reactor was increased gradually by increasing quantity of LLF. Certified emission reductions (CERs) estimation Certified emission reduction (CERs) were estimated as per the guidelines of UNFCCC by replacing open dumping of fleshings with liquefaction and treatment of LLF in UASB reactor for a tannery cluster processing 150 tonnes of raw hide or skin per day. Baseline emission and project emission were estimated based on equation 1&2 respectively and emission reduction due to project activity was estimated based on equation 3. Baseline Emissions are estimated as follows: BE y = BECH4,y + BEEL, y + BEHG, y Where, BE y
(Eq. 1)
- Baseline emission in year, tonne of carbon dioxide equivalent per year
[tCO2e/yr]; BECH4 - CH4 emissions from baseline treatment [tCO2e/yr]; BE
EL,y
- CO2 emission
from electricity consumption in the absence of the project activity in year [tCO2e/yr]; BEHG,y CO2 emission from fossil fuel combustion for heating equipment that is displaced by the project in year [tCO2e/y]. Project emissions are estimated as follows: PEy = PE CH4,effluent,y+ PE CH4,digest,y+PEflare ,y+ PESludge, LA ,y+PEEC,y + PEFC, y Where, PEy - Project emissions in year [tCO2e/yr]; PE
CH4,effluent,y
(Eq. 2)
- Project methane
emissions from effluent from the reactor [tCO2e/yr]; PE CH4,digest,y - Project emissions related to physical leakage from the reactor [tCO2e/yr] ; PE flare, y - Project emissions from flare [tCO2e/yr]; PE
Sludge, LA ,y-
Project emissions from land application of sludge [tCO2e/yr]; PE
EC , y
- Project
emissions from electricity consumption [tCO2e/yr] ; PEFC, y - Project emissions from fossil fuel combustion [tCO2e/yr]. Emission Reduction are estimated as follows: ERy = BEy – PEy
(Eq. 3)
Where, ERy- Emissions reductions of the project activity in year [tCO2e/yr]; BEy Baseline emissions in year [tCO2e/yr]; and PEy - Project emissions in year [tCO2e/yr].
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CERs from treatment of tannery waste
Currently fleshings are disposed off in open dumping leading to ground water contamination, odor nuisance and emission of methane. Based on the investigation, it is possible to liquefy the fleshings and treat in UASB reactor which results in reduction of pollution load and GHG emission, in addition to avoidance of odor nuisance and ground water contamination. A typical existing tannery cluster processing 150 tonnes /day of leather, generating 5000 m3.d of TE and 30 tonnes/day of fleshings has been considered for estimating at CERs generated for calculating financial benefit through carbon trading (Clean Development Mechanism -CDM). For the CERs estimation, the process flow diagrams of baseline and project activities considered are shown in Figures 3 and 4 respectively. Analysis Characteristics of wastewater were carried out as per the standard methods and particle size analysis was carried out by using laser scattering particle size distribution analyzer Model LA - 950 for particle size distribution.
RESULTS Pre-treatment of LF using anaerobic inoculum ensured liquefaction and size reduction of LF before treatment in UASB, which is observed in particle size reduction analysis, given in figure 5. Particle size of the LLF was in the range of 2 µm to 200 µm, out of which about 70% of the particles were below 35µm. Startup of UASB Reactor Bench scale UASB reactor was fed initially with a mixture of 35 g of LF after liquefaction (liquefied LF) and 5L of TE. The reactor was initially operated with OLR of 1 kg/m3.d and the OLR was increased gradually by increasing the COD of influent concentration by addition of LLF with TE. The OLR was gradually increased to 4 kg/m3.d within 50 days28. During the initial period of 50 days, COD removal efficiency was about 40%. With increase in time, COD load was increased, gas production also increased simultaneously, indicating consistent methanogenic activity. Methane production of 0.24 m3/kg COD removed was observed at the end of 50 days which indicated the successful startup.
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Effect of Organic loading rate From figures 6 a &b after the startup period, the reactor was operated at varying OLRs 4.5, 8.5 and 12 kg/m3.d by increasing the COD concentration. At OLR of 4.5 kg/m3.d, the COD concentration of influent was maintained at 9600 mg/L after mixing with TE and LLF in the ratio of 6:1. After reaching the steady state condition, COD removal efficiency and HRT were in the range of 65-75% and 50-60 h respectively and methane production was about 0.25-0.27m3 / kg of COD removed. After reaching the steady state condition at OLR of 4.5 kg/m3.d, organic load rate was increased to 8.5 kg/m3.d by increasing the COD concentration of the influent to 11000 mg/L (mixing ratio of 4.5 : 1) and HRT during this period was maintained as 28-35 h. COD removal efficiency was unaffected and was in the range of 72 – 76 % and methane production was 0.29 m3/kg of COD removed. Similar observation was reported wherein OLR of 6.5 g COD/ L.d was found to be safe and could be increased to a maximum of 7.5 g COD/L.d without affecting the performance of UASB reactor29. Methane Production observed in the present study of 0.29 m3/kg of COD removed match the results reported during treatment of wastewater in UASB reactor30. From the results, it was seen that there was no adverse effect on the performance of the reactor by further addition of LLF. After obtaining successful performance of UASB reactor at OLR of 8.5 kg/m3.d, OLR was further increased to 12 kg/m3.d in the influent by increasing the COD concentration to 11600 mg/L (mixing ratio of TE and LLF was 4:1), During this period, HRT was gradually reduced and operated in the range of 22-27 h, but COD removal efficiency was stable around 75% and methane production was also consistent at the rate of 0.29 m3/kg of COD removed. It was observed that consistent performance at higher loading rate was due to the biological pretreatment of LF. From Figure 6 (a), the COD removal efficiency was consistent and the reactor was stable with reduction in HRT up to 21 h. It is observed that further increase of OLR to 14 kg/m3.d resulted in reduction of COD removal efficiency to 65%. During this period of operation at higher OLR, sludge wash out and scum formation were observed due to more methane gas production. Sludge wash out resulted in reduction of methane production from 0.29 to 0.23 m3 per kg of COD removed. Hence it was concluded that OLR more than 12 kg/m3.d could not be
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fed into the reactor and the reactor could be operated safely at OLR of 12. Similar observation and operation were reported during treatment of slaughter house wastewater, when increase in OLR to 20 kg/m3.d loading resulted in operational problems including sludge wash out and loss of microbial population31. Profile of Volatile Fatty Acids (VFA) during operation of UASB reactor During the performance of UASB reactor, VFA concentration in the influent of the LLF with TE reactor was in the range of 5000-6800 mg/L and VFA concentration in the effluent was in the range of 700-1350 mg/L, as shown in Figure. 7. From Figure. 7, it is observed that there is considerable reduction of VFA in outlet, indicating the effective functioning of methanogenic bacteria. At steady state condition, effluent VFA was in the range of 1000-1100 mg/L ensuring effective degradation of the substrate in the reactor. Non accumulation of VFA in the reactor also indicates an active methanogenic bacteria population in the sludge resulting in efficient conversion of VFA to methane. In the reactor treating TE and LLF, no accumulation of VFA was observed indicating the presence of active bacterial population. Alkalinity of the feed influent and effluent of the reactor was monitored (Figure. 8) to check the condition of buffering activity of reactor. The influent concentration of alkalinity ranged between 1000-1600 mg/L while the alkalinity of UASB treated effluent ranged from 2200-3900 mg/L. The increase in alkalinity in the effluent is due to the solubilization of part of CO2 during the generation of biogas. This increases the bicarbonate alkalinity which helps in maintaining the pH of the reactor in neutral range which is more conducive for methanogenic bacteria. VFA/alkalinity ratio of UASB reactor was in the range of 0.3-0.4 indicating that the reactor is in stable condition32,33. Anaerobic treatment of tannery wastewater is not recommended due to the presence of high concentration of sulphate in the wastewater. It was reported that at high COD/SO42- ratios (>6), methanogenic bacteria predominates while at lower COD/SO42- ratios (
