PERFORMANCE OF AN UASB REACTOR FOR THE TREATMENT OF CATECHOL AND RESORCINOL

Abstract

Hydroxy aromatic compounds, such as catechol and resorcinol are derivatives of benzene and belong to phenol family. These chemicals are widely found in the effluents of many industries such as dyes, plastics, cosmetics, textile, leather, steel, rubber, pharmaceuticals, pesticides, photography, petrochemical, petroleum refinery, pulp and paper etc. and in coal conversion process wastewaters. The effluents from the synthetic coal fuel conversion processes may contain catechol and resorcinol in a wide concentration range of 10-2000 mg 1" . These chemicals are highly toxic to flora and fauna. It is, therefore, necessary that these chemicals, even when present in relatively low concentration, must be removed in order to improve the biological treatment of other organics and dissolved matter contributing to the COD in the effluents. According to IS: 2490 (Part 3)—1974, the concentration of phenols in the industrial effluents should not exceed 1.0 mg 1 for their safe discharge into the surface water. Anaerobic treatment processes have attracted worldwide attention because of the advantage of the recovery of inherent energy of the effluents in the form of methane gas, low energy consumption and low sludge formation. The upflow anaerobic sludge blanket (UASB) reactor has the advantage over other anaerobic reactors because of the formation and retention of high quality settleable sludge in the form of granules without any support materials. The present thesis deals with the treatment of catechol, and the binary mixture of catechol and resorcinol bearing synthetic wastewater (SWW) in a laboratory UASB reactor. The concentration of catechol was varied between 100 and 1500 mg l'1. The binary mixtures had different concentrations of catechol and resorcinol with the total concentration of the two chemicals maintained at 1000 mg I"1. A 9.75 litre laboratory scale UASB reactor (100 mm diameter x 1200 mm height) was fabricated from the transparent plexiglass pipes and sheets. It had sampling ports along its height and a 600 mm high settler of 150 mm diameter, attached to the reactor at the top. Granular characteristics such as size, settling velocity, specific gravity, sludge volume index, chemical composition and morphology of granules were assessed at the end of each stage of treatment period. The biomethanation potential of the UASB reactor sludge in utilizing the glucose, catechol and the binary mixture of catechol and resorcinol containing substrates was assessed through the specific methanogenic activity (SMA) of the sludge at different periods. The start-up of the laboratory scale UASB reactor consisted of three phases. During the first phase, the reactor was seeded with anaerobic digested sludge from a wastewater treatment plant after washing and sieving it to remove debris. The reactor was fed with SWW containing glucose at a COD concentration of 500 mg 1" at an organic loading rate (OLR) of 1.5 kg COD m"3 d"'. Thereafter, the OLR was increased in steps after certain period of operation at each step. The aim was to facilitate the growth of the seed sludge in the form of granules and to acclimatize it to take high organic loading. The feed was supplemented with macro-and micro-nutrients. The hydraulic retention time (HRT) was kept at 8 h with an upflow velocity of glucose solution at 0.15 m h"1. Sufficient amount of NaHC03 was added to the feed so as to maintain the reactor effluent pH at 6.8±0.2. The reactor was housed in a temperature controlled chamber at 35 ± 2 °C. The overall COD removal efficiency of the system was about 75 to 90% during the operation time of 120 days up to an OLR of 15 kg COD m"3 d"1. The reactor reached steady-state in 3-4 days at every increment in OLR indicating the stability of the reactor and the adaptability of the reactor biomass. The reactor showed good performance and it could sustain an OLR of 15 kg COD m"3 d"1 with a COD removal efficiency of 75% and a gas production of 3.92 m3 gas (m3 reactor volume)"1 d"1 with a methane content of 73%. The sludge loading rate (SLR) at this stage was 1.35 kg COD (kg VSS)"1 d'1. The reactor had the robustness and resilience when the influent pH and temperature were maintained at 6.8 ± 0.2 and 35 ± 2 °C, respectively. Batch studies were conducted using the sludge withdrawn on days 80-85 and inoculating the catechol containing solutions (of varying concentrations upto 1500 mg l"1) in the serum bottles. These studies provided the information on the optimal dosage of glucose as a cosubstrate for enhanced catechol degradation under anaerobic conditions. On the basis of batch tests glucose was used as a cosubstrate (iii) (1000 mg 1") in the aqueous catechol solution to study the degradation characteristics of catechol in the UASB reactor. The treatment of catechol (2-hydroxy phenol) bearing SWW was studied from day 121 over a period of 205 days (i.e. total period of reactor operation of 325 days); split in three phases viz., Phase IV, V and VI. The treatment of SWW was carried out with a progressive step-up (ramp) change in the OLR. The feed had the catechol concentration (and COD values) of 100(189), 250(472), 424(801), 500(945), 650(1228), 800(1511), 1000(1889), 1250(2361), 1500(2834) mg l"1 applied on days 121, 136, 154,172,187, 201, 214, 229, 243, 255, 268, 283, 297 and 312, respectively. Glucose was also fed along with SWW at a concentration of 1000 mg 1"' from day 121 to 171 and 243 to 254; 1500 mg 1"' from day 172 to 242 and 500 mg 1"' from day 255 to 267. Catechol (feed concentration 100 mg 1 ) and glucose removal efficiency were observed to be 10% and 83.22%, respectively on day 121. The biomass in the reactor was slowly acclimatized to catechol and the catechol degradation increased day by day. On day 325, the reactor fed with SWW containing 1000 mg l"1 catechol (OLR=5.7 kg COD m"3 d"1) showed -100% degradation of catechol and produced biogas at the rate of 2.2 m gas (m3 reactor volume)"1 d"1 and the sludge loading rate (SLR) was 0.627 kgCOD (kg VSS)"1 d"1. Co-degradation of catechol and resorcinol (in different concentrations with a total concentration being 1000 mg l"1) bearing SWW was carried out over a period of 145 days starting from day 326 to day 470. On day 326 (i.e. 1st day) the catechol feed was substituted with the binary feed of 50 mg l"1 (OLR = 0.285 kg COD m"3 d"1) resorcinol and 950 mg l"1 (OLR = 5.415 kg COD m"3 d"1) catechol. COD removal efficiency suddenly decreased from -100% to 52% and the biogas production decreased from 2.2 m3 m"3 d"1 to 0.76 m3 m"3 d"1 on day 327. The resorcinol substitution created toxic environment and completely disturbed the degradation pathway of catechol. Gradually, the biomass got acclimated and adapted to resorcinol resulting in an increase in the COD removal efficiency. The experimental results clearly indicated that the catechol acclimated UASB reactor has a threshold of 200 mg l"1 of resorcinol concentration in a total concentration of 1000 mg l"1 of catechol and resorcinol for the optimumCOD removal and biogas production. (iv) The sludge granules were found to be in the size range of 0.5-2.5 mm; settling velocity in the range of30-75 mh"1; specific gravity between 1.0045 and 1.0085; SVI of 18-20 ml g"1; had an ash content of 11.7 %. The granules had the dominance of Na, K, Mg, and Ca. Phosphorous and cobalt content of the granules increased by -12% and 18.3% mg (g dry sludge)"1, respectively after the catechol treatment. SEM EDAX results indicated that sulphur content increased by 288% after catechol treatment on average atom % basis. Electron microscopic analysis showed the presence of such microbial species as those resembling Methanobrevibactor ruminantium,' Methanobacterium formicicum, Methanospirilium, Methanothrix and Methanosaeta dominating on the granule surface. The ICP-MS analysis of granule constituents showed that the concentration of most of the metals decreased drastically after the treatment of the binary feed mixture. However, Ca increased by -51% from 12.36 to 18.66 mg (g dry sludge)"1 and iron increased by 2891% from 0.677 to 20.25 mg (g dry sludge)"1. The EDXA results showed that the mineral content (Si, Al, S, Fe, Mg, Na) of the sludge increased and that Ca, P, Kdecreased after the treatment. The analysis ofthe FTIR spectra confirmed the reduction of ash content, accumulation of amine salts and biodegradation of aliphatic, aromatic ethers and polysaccharides in the granules. Scanning electron microscopy revealed the predominance of Methanosaeta like species on the granule surface. The studies showed that an UASB reactor which has grown microbial granules on glucose as a feed, can be used to treat catechol bearing wastewater upto a concentration of 1000 mg l"1 showing a catechol degradation efficiency being 97% and a mixed feed of resorcinol and catechol in the ratio of 1:4 upto a total concentration of 1000 mg l"1 with an HRT of 8hand an OLR of 5.7 kg COD m"3 d"1 and showing the total degradation efficiency of-92%.