Sodium Toxicity control by the use of Magnesium in an Anaerobic Reactor

This study investigated the effectiveness of magnesium in reducing sodium toxicity in mesophilic (35oC) completely mixed anaerobic digesters (CMADs). The CMADs were operated at a chemical oxygen demand (COD) loading of 0.5 g/l/day initially which was gradually increased to 1.8 g/l/day. To evaluate the effect of sodium concentration on methanogens, the biomass in one of the CMADs was acclimated to an increasing concentration of sodium while the feed to the second CMAD was not supplemented with any additional sodium. The COD removal efficiency and methane production decreased by nearly 30% and 20% respectively at a sodium concentration of 9.0 g/l. Similarly, the total volatile fatty acids (VFAs) concentration increased from a mere 200 mg/l to 3000 mg/l. At this point, magnesium was added gradually to one of the reactors. The COD removal efficiency and methane production returned to the original level of 92% and 62% respectively, and the VFA concentration became negligible. It can be concluded that magnesium is very effective in reducing sodium toxicity to methanogens. @JASEM Complete-Mix anaerobic digester is an efficient anaerobic system that is suitable for wastes with extremely high dissolved organic concentrations or high concentrations of solids. Since the hydraulic retention time (HRT) and solids retention time (SRT) are equal, it is more practical to use where thickening the effluent solids is difficult (Metcalf and Eddy 2003). The HRT may be in the range of 15 to 30 days to provide sufficient safety factors for operation and process stability (Parkin and Owen 1986). Anaerobic systems operated at thermophilic conditions (55oC) achieve higher biological reaction rates compared to mesophilic systems conducted at 35oC. Operation at thermophilic temperatures also enhances the hydrolysis rate of several recalcitrant organics, thereby improving the overall efficiency of the sludge stabilization process (Han et al. 1997). However, the process has failed to gain widespread acceptance due to potential drawbacks such as slow startup, higher energy requirements, organic loading variance and slow accommodation to toxic chemicals (Speece 1996). The last factor might also be a cause of concern for several mesophilic systems. It is in this context that the occurrence of sodium, which is commonly used in wastewater treatment plants (as sodium hydroxide) to supplement alkalinity, could prove detrimental to the operation of either system (Soto et al. 1993). Under anaerobic conditions, higher concentrations of sodium could readily affect the activity of microbes and interfere with their metabolism. McCarty (1964) has reported sodium concentrations in the range of 100-200 mg/l to be beneficial for the growth of mesophilic anaerobes. According to Kugelman and Chin (1971), the optimal sodium concentration for mesophilic aceticlastic methanogens in waste treatment processes was 230 mg/l. The optimal growth conditions, with respect to sodium, for mesophilic hydrogenotrophic methanogens reportedly occur at 350 mg/l (Patel and Roth 1977). However, an early study reported sodium concentrations ranging from 3,500 to 5,500 mg/l to be moderately and 8,000 mg/l to be strongly inhibitory to methanogens at mesophilic temperatures (McCarty 1964). For anaerobic granular biomass at mesophilic temperatures, sodium concentrations of 5, 10, and 14 g/l caused 10, 50 and 100% inhibition of methanogens, respectively, at neutral pH (Rinzema et al. 1988). A great deal of research has been done to study sodium inhibition on methanogens, but little work has been done to reduce or control sodium toxicity. In our previous studies, cations (potassium and calcium) were found to be very effective in reducing the inhibitory effects of sodium on bacteria. This particular work investigates the potential of yet another cation (magnesium) in minimizing sodium toxicity. MATERIALS AND METHODS The completely mixed anaerobic digester system consisted of five-liter capacity anaerobic digestion vessels. Each vessel had a ground glass flange and a multisocket flange lid held in place with a wire clip. The central socket of the flange lid accommodated a glass stirring rod with an air tight seal, while the four other sockets housed the feed funnel, sludge outlet pipe, gas outlet tube and the self sealing rubber JASEM ISSN 1119-8362 All rights reserved J. Appl. Sci. Environ. Mgt. 2004 Vol. 8 (1) 17 21 Full-text Available Online at http:// www.bioline.org.br/ja *Corresponding author:: E-mail: [email protected] Abstract available Online at http:// www.ajol.infoavailable Online at http:// www.ajol.info septum respectively. These units were then placed in a water bath, which was maintained at 35-37oC and flushed with nitrogen to ensure an anaerobic environment inside the digesters. The gas outlet from each vessel was connected to a 10-litre glass aspirator filled with water. Wet gas meters thereafter replaced this arrangement. Once they were air tight, gas production was measured by the downward displacement of water in the aspirators. The reactors were also seeded with digesting sludge from a local Wastewater Treatment Plant. Approximately 100-150 ml of mixed liquor was removed daily from the completely mixed anaerobic digesters. The digesters were designated “E” and “F”. Digester “E” was maintained as control while digester “F” was the experimental one. The mixed liquor removed from the CMADs was used for a range of experimental purposes. A portion of it was used for pH, volatile suspended solids and ammonia-nitrogen analyses, while the remainder was centrifuged for 30 minutes at 3000 rpm in a centrifuge. The sludge, after centrifuging was made up to volume with an appropriate amount of synthetic feed (Table 1) and returned to the reactor. The supernatant was used for further chemical analysis (Table 2). The organic loading rate at the start of the experiment was 0.5 g/l/day, gradually increased to 1.8 g/l/day and maintained at this level for the duration of the project in all of the CMADs. This organic loading rate was achieved after a period of six months. The control units were supplied with feed containing a constant concentration of sodium. The feed supplied to experimental reactors was supplemented with increasing amounts of sodium. These reactors were allowed to ‘acclimatize’ to each stepwise increase in sodium concentration. Sludge return was continued in order to maintain a suspended solids level of 15-18 mg/l within each reactor. Table 1. Synthetic Feed Composition Constituent Concentration (mg/l) Constituent Concentration (mg/l ) NH4HCO3 3000 MgSO4.7H2O 5 KH2PO4 3000 FeCl2.4H2O 15 NaHCO3 3000 CaCl2.6H2O 15 Whey powder 90,000 KCl 5 CuSO4 0.2 CoCl2.6H2O 5 ZnSO4 0.2 Na2MoO4.2H2O 0.2 Table 2. Digester Analysis Parameter Frequency Method Instrument brand Gas Production Daily Water displacement/ gas meter Becker 403 Gas composition Daily Gas chromatography Becker 403 pH Daily pH meter Corning Volatile fatty acids Daily Gas liquid chromatography Pye Unicam 304 Suspended solids Weekly Gravimetric Standard methods 1989 Volatile suspended solids Weekly Gravimetric -doAmmonia-Nitrogen 3 times per week Distillation/titration -doAlkalinity -doTitration -doInfluent COD -doDichromate reflux -doEffluent COD -do-do-doSodium Daily Flame photometer -doPotassium -do-do-doCalcium -doAtomic Absorption Spectrophotometer -doMagnesium -do-do-doMagnesium was added to the experimental reactors when the inhibition due to cation toxicity was at its peak i.e. when COD reduction and percent methane production were at their minimum, and volatile fatty acids were at their maximum concentrations inside the reactors. Different concentrations of magnesium were used starting from very low values. The optimum concentration (at which the inhibition is minimum) was recorded by taking into account the percent COD reduction, percent methane production and volatile fatty acids concentration in the digesters. Metal Analysis: Total, filtered, acid extractable and strongly bound metal concentrations were measured for each of the metals. The metal analyses were carried out using a Pye Unicam Atomic Absorption Spectrophotometer fitted with a computer for data processing. BASHARAT H. BASHIR; ASIF MATIN 18