International Conference on Chemical, Environmental Science and Engineering (ICEEBS'2012) July 28-29, 2012 Pattaya (Thailand) Biological denitrification of wastewater-A Mini Review on Carbon Source Srinu Naik. S and Y. Pydi Setty cancer [1-4], and blue baby syndrome [5-6]. Denitrifiers may Abstract—A comparative study on the use of carbon sources as play an important part in the breakdown of various supplementary source to enhance denitrification in primary and hydrocarbon compounds. In agricultural areas denitrification secondary anoxic zones is reported. Denitrification is a process in leads to a loss of fertilizer effiency [7], and incomplete which the oxidized nitrogen substances, i.e. nitrates and nitrites are denitrification can lead to the release of NO and /or N2O, both reduced to nitrogen gas, such as N2O and N2, when a proton donor (energy source) is available. Nitrate contamination is one of the of which are known to be involved in the formation of acid major problems in groundwater, which is increasingly becoming a rain, ozone depletion and global warming. threat to groundwater supplies. Nitrate in drinking water for animal To address this problem, specific rules have been and human consumption is not recommended for health reasons. The established globally. The European and the USA World Health Organization has set a limit of 10 mg/L NO3- for - Environmental Protection Agency, set 5.6 mg (NO3-N)/L and human consumption and 100 mg/L NO3 for animals. Nitrogen in 10 mg (NO3-N)/L) respectively [8-10]. Therefore, the nitrates groundwater results from human excreta, ground garbage and industrial effluents, particularly from food processing plants. The removal is essential to successful wastewater treatment. addition of external carbon source helps in increasing denitrification Biological denitrification is an attractive treatment option, in rates and enhances the nitrogen removal. Nitrates cause cancer, which the denitrifying bacteria converts nitrate to inert methemoglobinemia (blue baby syndrome), hypertension and thyroid nitrogen gas and the waster product usually contains only hypertrophy and eutrophication in plants. In most biological biological solids. Biological removal of nitrate is widely used denitrification systems, the nitrate polluted wastewater (e.g. domestic in the treatment of domestic and complex industrial sewage) contains sufficient carbon source to provide the energy source for the conversion of nitrate to nitrogen gas by the wastewaters [11-17]. denitrifying bacteria. The groundwater, in which the nitrate contents Organic carbon is often the limiting substrate for biological may be as high as 100 mg/L with low dissolved carbon content, an denitrification and many wastewater treatment plants add additional proton acceptor is required. The review addresses the state extra carbon for denitrification to balance the processes [18]. of the art of each electron donor candidate for its potential Among the various process techniques available for nitrate application to the treatment of industrial wastewater containing removal like ion exchange ion exchange [19], reverse osmosis nitrate. [20], electro dialysis [21], chemical and catalytic Keywords—Biological treatment, Carbon source, denitrification [22]. Ion exchange and adsorption processes Denitrification, Wastewater. have been developed mainly for the production of high purity water and generate concentrated brine that needs additional I. INTRODUCTION treatment, often involving adsorption resins whose capacity is rapidly reached by these concentrates, thus requiring frequent EMOVAL of nitrate from wastewater through biological R treatment in enforced in most developed countries to replacement or regeneration. Membrane technology is a protect local waterways from eutrophication. In biological concentrating method which needs a post-treatment of the denitrification the nitrate reduction reactions involve the brine before discharging out of the process, and chemical following pathway, in this process microorganisms first treatment is inefficient in treating the low nitrate reduce nitrates to nitrites and then produce nitric oxide, concentrations of wastewater and requires a continuous and nitrous oxide and nitrogen gas. large supply of chemicals [23-26]. Each of them has its own advantages and disadvantages, →→→→ 3 2 2 NONNONONO 2 from which biological method is found to be the most The beneficial aspects of denitrification include control or commonly used and effective method as shown in Table 1, bioremediation of NO3-contaminated waters, which can cause biological denitrification is a well developed and widely used eutrophication. However, it has also been linked to stomach method, one technically and economically superior compared to other competing methods [27] and involves a microbial Sapavatu Srinu Naik is with the National Institute of Technology, respiration which uses nitrate as a final electron acceptor Warangal, Andhra Pradesh-506 004, INDIA (phone: 09885 165 333 under anoxic conditions. ; fax: 0091-870-2459547; e-mail: [email protected]). Several studies has been carried on denitrification [28-36] Yelamarthi Pydi Setty is with National Institute of Technology, Warangal, Andhra Pradesh- 506 004, INDIA.(e-mail: [email protected]). 47 International Conference on Chemical, Environmental Science and Engineering (ICEEBS'2012) July 28-29, 2012 Pattaya (Thailand) A source of organic carbon is an important component of Wilawan et al. (2010) made a comparison report on the denitrification process; generally most of wastewater biological nitrate removal using three different carbon sources treatment plants use Methanol as external carbon source due like acetate, ethanol and hydrolyzed rice. The results to economic reasons in denitrification process. Other suggested that acetate is the most appropriate carbon source compounds can be used in denitrification process includes for nitrogen removal and hydrolyzed rice is a satisfactory acetate, ethanol, methane, glucose, peptone, saw dust, alternative carbon source [37]. glycerol, lactic acid, molasses, etc. A number of Sunil et al. (2010) has been also compared with acetate, investigations are reported in the literature on the nitrate methanol and ethanol as supplementary carbon sources in the removal from wastewater using carbon sources. nitrite denitrification process using bio-granules as shown in Fig. 1, 2, and 3. The bio-granules degraded 200 mg/L and Carbon source studies suggested that the capacity of supplementary carbon source on enhancing nitrate removal enzyme activity is TABLE I methanol>acetate>ethanol on molar basis or COMPARISON OF DENITRIFICATION PROCESSESS acetate>ethanol>methanol on an added weight basis [38]. Process Merits Demerits Biological Economically superior Electron donor is required, Jae Yeon Park et al. (2009) made a review on biological method process among available Shedding suspended solid. nitrate removal in industrial wastewater treatment and processes, transformation of compared the estimated costs of substrates for nitrate removal. nitrate into harmless nitrogen gas, stable and continuous Denitrification rate of different electron donors like methanol, removal of nitrate, No need ethanol, acetic acid, acetate, cotton, sulfur and hydrogen was for intermittent pause of compared. According to the review methanol has the low reactor for maintenance and price 0.7 $/kg substrate and nitrate removal rate nearly 1000- repairing. -3 -1 27,000 g-N m day compared to other sources [39]. Reverse Used for high concentration High capital, operation and Guven Didem (2009) have reported an overview on the Osmosis of nitrate, insensitive to maintenance costs due to effect of carbon sources (acetate, propionate, ethanol and temperatures variations, No power consumption, post treatment required, Requires pre-treatment of glucose) on denitrification efficiency associated with culture Relatively easy to operate, water to remove organic adaptation and C/N ratio and suggested that methanol has Removes virtually all matter, suspended and shown to select for a denitrifying population consisting of contaminants, Producing the colloidal particles and other highest quality of water. contaminants, Sensitive to Paracoccus and Hyphomicrobium vulgare genera, when used PH and pressure variation as only external carbon source [40]. and chlorine content. Ginige et al. (2009) made a comparative study on methanol Ion High efficiency when other Disposal of spent regenerate as carbon source using three lab-scale sequencing batch exchange ions do not exist, effluent brine solution, Sensitive to reactors (SBR) as shown in Fig. 4. The experiments were concentration can be lowered high levels of TDS, Large conducted by adding more methanols to the first reactor, a under very low level, can teat volumes of salt required for small amount of methanol to the second and no methanol was high concentration of nitrate, regeneration, Water may relatively insensitive to flow require pre-treatment due to added to the third reactor. The addition of methanol stimulates and temperature variations, suspended solids, organic the growth of specific methanol-utilizing denitrifiers, which low capital cost. matter, iron and other improves the capability of sludge to use methanol and ethanol oxidizing agents, to prevent anion resin fouling, Requires for denitrification, but reduces its capability to use wastewater post treatment due to COD for denitrification [41]. corrosive to product water, Shao et al. (2008) reported on water