N AND P IN ANAEROBIC BIOTRANSFORMATION: TREATMENT OF PHENOLIC WASTEWATER

Abstract

Phenols enter into water bodies from various sources such as phenol man ufacturing plants, pharmaceutical industry, wood processing industry, pesticide manufacturing plants, coal conversion, and oil-refineries etc. Phenol in the ef fluent ranges from 10 -17,500 mg/L (Veeresh et al, 2005). According to Indian standards the permissible limit of phenol is 1 mg/L for industrial effluents to be discharged into inland surface waters (IS: 2490-1981) and 5 mg/L into public sew ers (IS: 2296-1974). For many nutrient deficient industrial wastewaters, nutrient (N and P) addition is necessary in order to make it feasible for anaerobic wastew ater treatment (Hulshoff Pol et al., 1983; Speece ,1987). The nitrogen content of the bacterial cells represented by C5H7N03 is 11 %, though the nitrogen ranges from 6-15 %with an average of 12 % (Rittmann and McCarty, 2001). The phos phorous content of the cells has been reported to be ~ 14 %of nitrogen. This yields an empirical microbial formulation of C5H703NPom. The stoichiometric requirement of nitrogen and phosphorous thus depends on the cell composition and its yield. At a cell (C5H703N) yield of 5 % (i.e. 5 % incoming COD is converted to cell COD) , the nitrogen requirement is ~6 kg/1000 kg ofCOD or 1 kg/60 m3 methane produced (Speece and McCarty, 1964). The bioconversion of phenol to methane in batch tests has been reported under a wide range of vari ation in the COD: N: P. For example, Healy and Young (1979) studied methane generation from phenol at - 300:45:3; Fedorak and Hrudey (1986) - 300:2.1:0.42 and Kennes et al. (1997) gave their observations at - 300:6.9:1.41.The cheese in ABSTRACT whey wastewater of high COD (55000 mg/L) has been treated at COD: N: P value of 300:0.6:0.48 (Erugder et al, 2002). Hendriksen and Ahring (1996), Kalyuzhrigi and Davlyatshina (1997) and Langenhoff et al. (2000) have used COD: P ratio more than that of COD: N ratio. Researchers like Berg et al. (1977), Manjunath et al. (1990), Fang et al. (1990), Fang et al. (1996), Gonzalez et al. (1998), Lay and Cheng (1998), Punal et al. (2000), Tay et al. (2001), Martnez et al. (2002), Luostarinen et al. (2005), Ammaray (2004) and Chou and Huang (2005) have worked on the bench-scale continuous fed UASB reactors utilizing different COD: N: P ranging from 300:0.75:4.4 to 300:45:12 . In most of the cases nutrients added were much more than required by the growing biomass. The effluent was found to bo rich in nutrients. When the COD: N: P of the influent was equal to 300:10:1, the ratio of effluent to influent ammonical nitrogen ranged between 0.85 and 0.95 (Manjunath et al., 1990), meaning there by that nitrogen added in the influent was much more than stoichiometrically required amount. Unnecessary addition of nutrients must, therefore, be avoided wherever possible (Fedorak and Hrudey, 1984). The optimal nutrient dosage for anaerobic wastewater treatment is not well precedented in the literature. Industrial wastes are often augmented with nutri ents. N and P more than the stoichiometric requirement for anaerobic degrada tion are subsequently released into water bodies and bring about changes in the ecology of the aquatic systems. The lakes are rendered productive and streams demand DO for nitrification. Keeping these in view, it became necessary to find out the effect of COD: N: P on methanogenesis of phenol, a representative of the industrial wastes. Therefore, as a first step the experiments were conducted in bench-scale batch reactors containing same phenol COD but varying amounts of N and P. Requirement of nitrogen and phosphorous depends on the nature of wastewater, SRT, cell yield coefficient and Vc X & P in the cell. Keeping the iv ABSTRACT above mentioned facts in view, an attempt has been made to explore the nutri ent requirement by varying the COD to nitrogen and COD to phosphorous ratio in the influent fed to UASB reactor. It has been aimed to study the feasibility of treating phenol COD at varying ratio of COD: N maintaining COD: P as constant and then varying COD: P ratio maintaining COD: N as constant in a UASBR. The scope of the work is given as under: Bench Scale Batch Studies: 1. Influence of substrate to inoculum ratio (F/M) on biodegradation of phenol as a sole substrate. 2. Methane generation rate and potential (Biochemical methane potential) of synthetic phenolic wastewater by varying COD: N and COD: P ratios. Bench Scale Continuous Studies 1. Start-up of UASB reactor 2. Variation of COD: N ratio keeping COD: P fixed 3. Variation of COD: P ratio keeping COD: N fixed 4. Specific methanogenic activity (SMA) of granular sludge at each step: (i) start-up, (ii) Pseudo steady-state at (a) different values of COD: N and COD: P, and (b) different values of hydraulic detention time (HRT). The research work has been organized into five Chapters: Chapter 1: Introduction Chapter 2: Literature Review Chapter 3: Materials and Methods Chapter 4: Results and Discussion Chapter 5: Conclusions Chapter 1: deals with the introduction, identification, justification and scope ABSTRACT of the problem as briefly mentioned above. Chapter 2: presents literature on the treatment of phenolic wastewater, role of nutrients, their importance and necessity in the anaerobic degradation. Themain focus is on nutrient requirement in anaerobic degradation of phenolic wastewater particularly in a UASB reactor. Chapter 3: deals with the materials and methods adopted during experimental investigations. Experiments were conducted concurrently in two parts: namely batch studies (Part-I) and continuous reactor studies (Part-II). In order to assess Table 1: Bench-scale batch experiments: operational variables Phase Sludge VSS g/L) Nature of the sludge used Phenol COD* (g/L) Phenol concentration (g/L) COD:N COD:P I* 3.4 Flocculant (Unacclimatized) 0.97-6.57 0.29 - 2.76 300:10 300:1 II 2.7 Flocculant (Unacclimatized) 2.02-2.05 0.84-0.86 300:10 to 300:1 300:1 III 3.4 Granular (acclimatized) 2.5-2.6 1.05-1.09 300:10 to 300:1 300:1 IV 4.01 Granular (acclimatized) 3.0 1.26 300:1 300:1 to 300:0.1 V 4.22 Granular (acclimatized) 3.15-3.18 1.32-1.34 300:1 to 300:0 300:0.1 VI 4.22 Granular (acclimatized) 3.15-3.18 1.32-1.34 300:1 300: 0.1 to 300: 0 VII 4.22 Granular (acclimatized) 3.15 1.32 300:0 300:0 * Phenol COD = 2.38 g/g phenol **Ratio of Food (phenol COD, F) to microorganism (VSS, M) (F/M varied from 0.25 to 2.00). the impacts of N and P on the anaerobic biodegradability of phenolic wastewater, batch experiments were performed in seven phases in accordance with the method proposed by Owen et al. (1979).Serum bottles of 125 mL with working volume of 110 mL (Phase I and II) and Oxitop bottles (WTW, Germany) of 312 mL with working volume of 250 mL (Phases III to VII) were used as anaerobic batch VI ABSTRACT reactors. The operational variables are given in Table-1. The bottles were incubated in a temperature controlled chamber maintained at 30±2°C. Gas produced in each serum bottle was measured daily or recorded through Oxitop® Control measuring system. Blanks (i.e. reactors without sub strate) were run parallel to these reactors in all phases of the study. pH, COD, microbial concentration in terms of volatile suspended solids (VSS), total sus pended solids (TSS), nitrogen andphosphorous were determined as per Standard Methods (APHA, 1998) .Biomass was digested to determine cell-N and P per centages. Methane collected at room temperature was normalized to standard temperature and pressure (STP). Methane content in the biogas was monitored by passing total biogas through NaOH solution.Experiments were carried out in triplicate. Values reported here in are the average values. The amount of methane or methane COD reported is after blank corrections. The UASBR of 10 L capacity was maintained in a temperature controlled chamber at 30±2°C. Digested sludge collected from the outlet of anaerobic di gester of 18 ML/d activated sludge process based sewage treatment plant lo cated at Kankhal, Haridwar; Uttaranchal, India was used as seed. The reactor was charged with 3 L of sludge having VSS concentration of 18 g/L. During start-up, reactor was fed with molasses based synthetic wastewater, which was subsequently replaced with phenol. Afterwards, the amount of N and P were changed. The operational details are given in Table-2. Day 48 onward the reac- Table 2: Bench-Scale Continuous Feed UASBR: Operating Condi tions/Studied Variables Phase Period of study 1 Day 0 - 75 II Day 76 - 452 III Day 453 - 675 IV Day 676 - 722 Operating conditions: continuous feed reactor Start-up of UASBR (Feed changed from 100% molasses based synthetic wastewater to 100%phenolic wastewater in 5 steps) COD:N - 300:10 to 300:1 (in 7 steps); COD:P - 300:1 (fixed) COD:P - 300:1 to 300:0 (in 10 stcps);COD:X - 300:1 (fixed) COD:N - 300:1 to 300:0 (in 6 steps);COD:P - 300:0.1 (fixed) vn ABSTRACT tor was fed with 100% phenol based wastewater. Throughout the startup phase, the COD and COD: N: P of the synthetic feed was maintained 1000 mg/L and 300:10:1 respectively. During studies of phases II to IV, phenolic COD of the wastewater was maintained at 1000 mg/L and only amounts of N and P were varied to get wastewater of a desired COD:N ratio (Phase II and IV) or COD:P ratio (Phase III). At every change, reactor was run till it attained steady-state. Values obtained at steady-state only were used to compare reactor performance. Chapter 4: Results are organized according to the sequence of experiments reported in Chapter 3. Batch Studies: Optimum F/M was found to be 0.75. The degradation profiles of phenolic COD conform to logistic growth model (Eq. 1): 4~ [C0 + (Cmax-C0)e-kt] Where, Cmax = the maximum methane produced after 80 to 120 days of incubation (mL), k = rate constant (d_1), C0 = initial methane concentration at time t=0(mL), Ct = concentration of methane at time t (mL). A lag in general of 10 - 30 days followed by accelerated rate of CH^ generation was noticed. The Cmax was found close (97 - 98 %) to the stoichiometric methane upto COD: N: P equal to 300:1:0.1. At values less than 300:1:0.1, the Cmax or ultimate methane generation is reduced drastically. Methane generation rate constant (k) has been found to vary from 0.12 to 0.21 d_1. A definite decreasing trend in k was noticed at COD: N: P < 300:1:0.1. The reducing amounts of N and P in COD: N: P from 300:10:1 to 300:1:0.1 also do not seem to have any effect on biochemical methane potential expressed as //{,, (ratio of CH4 - COD viii ABSTRACT generated to COD fed). However, on further lowering the N and P from 1 and 0.1 mg/ L (for a phenol COD of 300 mg/L) respectively, the fib at 90 days (^90) was reduced from 0.93 to 0.12 . Gas generation completely ceased at COD: N: P equal to 300:0:0. The trend of sludge activity [mL CH4 or g of CH4 -COD formed g~l VSS (sludge)] was also found to be similar to that of k and /2b90. Optimum COD: N: P ratio was judged to be 300:1:0.1 based on the results of batch studies. Conditions in a batch reactor are different than continuous feed reactor. In a batch reactor the substrate is fed once while in a continuous feed reactor feeding is maintained on continuous basis. Therefore, a UASB reactor was operated by feeding synthetic wastewater of fixed phenolic COD, and COD: N and COD: P were varied on the same lines as in batch experiments. II. Continuous Reactor Studies Reactor start-up was accomplished in five stages. The biomass got acclima tized to 100 %phenol COD by day 75 when COD removal > 90 %was achieved. Thereafter, reactor was operated at different values of either COD: N ( keeping COD:P fixed ) or COD:P ( keeping COD:N fixed ). Numbers of parameters were studied. Effect of variation in COD : N: P was evaluated in terms of changes in the following: (a) Performance of the reactor, changes in pH, COD removal and biogas / CH4 production. (b) Solids retention time(SRT). (c) COD mass-balance. (d) Sludge growth rate. (e) Substrate removal rate. (f) Specific methanogenic activity. (g) N mass-balance, (h) P mass-balance. ix ABSTRACT COD removal efficiency remained > 90%when COD:N was reduced from 300:10 to 300:1 in steps. However, when it was further reduced to 300:0, COD removal dropped to ~ 47 %. Similarly from COD:P value of 300:1 to 300:0.1, COD removal observed was > 90 % and on further reducing it to 300:0 , COD re moval reduced to ~ 32 %. COD:N:P ratio of 300:1:0.1, therefore, appears to be optimum for anaerobic biodegradation of phenol in UASBR. (a) 4-7 %of influent COD was transformed to biomass upto COD:N of 300:1. It reduced to 1.3 % on reducing COD:N to 300:0.1, (b) COD in reactor effluent ranged from 3 to 6%only upto COD:N of 300:1 .It increased to as much as 50 % on reducing COD:N to 300:0.1. COD mass balance when COD:P was lowered gradually from 300:1 to 300:0 indicated that (i) > 91 %ofinfluent COD is converted to C#4-COD, upto COD:P value of 300:1 and reduced to as low as 34 % on further reduction of COD:P to 300:0, (ii) Similarly COD consumed for biomass growth reduced to 1.3 %from a value of 4 %, (iii) Effluent COD increased to a value as high as 55 %from 2 -4 %. It could be concluded that COD mass-balance remain unchanged upto COD:N and COD:P values of 300:1 and 300:0.1 respectively. Lower values of COD:N and COD:P bring significant changes in COD mass-balance. In general, conversion of COD to Ci^-COD and biomass is reduced and its presence in effluent is increased. Biomass yield coefficient, Y (kg VSS produced / kg COD consumed) was worked out at every step. It was found that at COD: N:P of 300:0:0.1 and 300:1:0, Y reduced by a 50 %. ABSTRACT The nature i.e. specific methanogenic activity (SMA) of the sludge grown with substrates having varying N and P amounts has been evaluated using ac etate and benzoate as substrates. The sludge adapted to phenol degradation responds to benzoate more favorably than acetate. The SMA values with ben zoate ( 0.26 -0.74 g Cif4-COD/ g VSS/d) were higher than with acetate ( 0.15 - 0.66 g C#4-COD/ g VSS/d). SMA values decreased significantly when COD:N and COD:P values were lowered below optimum values. Optimum COD:N:P appears to be 300:1:0.1, less than the stoichiometric requirement i.e. 300:1.8:0.36 suggested by earlier workers. It could be con cluded that phenolic wastewaters (as sole substrate) could be treated effectively by UASB process operating at phenol concentration of 450 mg/L ( phenolic COD « 1000 mg/L) at COD:N:P ratio as low as 300:1:0.1. Chapter 5: The conclusions from the study are summarized in this chapter. XI