Research review paperBioremediation of wastewaters with recalcitrant organic compounds and metals by aerobic granules
Introduction
Aerobic granulation is a highly complex process involving biological, chemical and physical factors (Liu and Tay, 2002, Liu and Tay, 2004). Aerobic granules are formed by self-aggregation of microorganisms in the absence of a support carrier. The granules are dense microbial aggregates assembled by consortia of microbes wherein the various species perform possibly different and specific roles in biodegradation of organics during wastewater treatment (Beun et al., 1999, Morgenroth et al., 1997). Close association among the microbial entities and degradation of organic pollutants via multiple steps result in the layered structures seen in aerobic granules (Tay et al., 2002, Li et al., 2008). This layered structure also create concentration gradients of target compounds inside the granule and these protect microorganisms from the impact of direct acute toxicity associated with the compounds. There is strong evidence aerobic granulation is potentially capable of handling a wide spectrum of wastewaters including high-strength recalcitrant industrial and low-strength municipal wastewaters (Maszenan and Liu, 2009).
Man-made organic compounds or xenobiotics are characterized by their complex ring structures or substituents that make them resistant to biodegradation and so persistent in the environment. Generally, such organics can be classified into 4 main types: (i) the saturated hydrocarbon compounds which has no double or triple bonds. The degradability of such compounds decreases as their molecular weight and degree of branching increase, (ii) the aromatic hydrocarbon compounds which contain conjugated double bonds. The aromatics with one or two rings degrade easily, while higher molecular weight aromatics are less biodegradable. An example of a relatively degradable aromatic is benzene which is the simplest of the aromatics, (iii) the asphaltenes which include the phenols, fatty acids, ketones, esters and phorphyrins, and (iv) the resins which include the pyridines, quinolines, carbozoles, sulfoxides and amides (Colwell and Walker, 1977, Leahy and Colwell, 1990). The biodegradation of these organics in terms of increasing difficulty is: n-alkanes < branched alkanes < low molecular weight aromatic < cyclic alkanes < aromatic alcohols and ester < nitrobenzene < chlorinated benzene (Perry, 1984).
Industrial wastewater can contain a variety of aromatic pollutants such as benzoate, phthalate, phenol and its derivatives, and various halogenated compounds. Some of these are toxic and have been listed as priority pollutants by the US Environmental Protection Agency (US EPA). These compounds can have been extraneously introduced into the environment in large quantities due to their widespread use as herbicides, insecticides, fungicides, solvents, plasticizers, cleaning agents, propellants, gasoline additives and degreasers (Bhatt et al., 2007). Some of these compounds can be degraded by acclimated microorganisms in wastewater treatment systems. However, others can remain undegraded and therefore require application of systems which support novel consortia of organisms to be effective.
The performance of a biological wastewater treatment system is dependant on appropriate active biomass density and its ability to degrade the organics present in the wastewater, the biodegradation kinetics, reactor configuration, and the availability of nutrients such as oxygen. The morphology of the biomass can impact on process efficiency and the latter can be enhanced by selecting an appropriate morphology such as granular sludge. Aerobic granular sludge can have a much higher conversion capacity and rate due to the higher biomass concentration. Additionally granular sludge has the advantage of enhanced liquid–solids separation in the mixed liquor due to superior settling properties. Granulation can, however, be compromised with loss of auto aggregating capability arising from low exopolysaccharides (EPS) protein. This can be associated with high organic loadings (Adav et al., 2010a). Other factors which can impact granulation include filamentous overgrowth, cells lysis in granules, and disintegration of aerobic granule due to anaerobic core formation. There are differences between the EPS in sludge flocs and aerobic granules and one of these is the existence of a reversible, pH dependent sol gel transition which only exists in EPS extracted from aerobic granules and not present in sludge floc EPS (Seviour et al., 2009). In granular EPS, EPS gel, exopolysaccharides or glycoside, are the important gelling agents for the granules (Seviour et al., 2009). In a study on starved aerobic granules, where the carbon substrate has been exhausted, α and β amylase were detected in the inner core of the aerobic granule, which suggested endogenous respiration occurred at the core of the granules. Amylase activities promote polysaccharide hydrolysis and the formation of cavities in the inner core. However, this internal polysaccharide hydrolysis would lead to eventual loss of granules integrity (Lee et al., 2009). The studies suggested EPS polysaccharide hydrolysis adversely impacted aerobic granules stability and eventually led to disintegration. This review discuss the application of aerobic granules for treating industrial wastewater, the mechanism involve in granule formation with microbial population selection and EPS types.
Section snippets
Degradation of phenol
Phenol can be inhibitory, even at low concentrations, and may consequently upset the conventional activated sludge process. Industrial wastewaters such as those from oil refineries, and pharmaceutical and pesticide plants can be major sources of phenolic wastewater. Aerobic granules used to treat such wastewaters have been found to be less susceptible to phenol inhibition due to the compact and dense granule structure, which then served as a phenol diffusion barrier. For instance, aerobic
Degradation of p-nitrophenol
p-Nitrophenol (PNP) is a widely used nitroaromatic compound that poses a significant risk to the environment and public health. PNP is used in the manufacture of the drug acetaminophen, pesticides such as methyl and ethyl parathion, and as a conditioner for leather treatment (Spain and Gibson, 1991). Due to its persistence, PNP is commonly found in industrial wastewater and has contaminated both subsurface water and groundwater (Labana et al., 2005). The risks associated with PNP include its
Degradation of chlorinated phenols
Chlorinated phenolic compounds are widely used as industrial chemicals for wood preservation, manufacturing of pulp and paper as well as in agricultural activities via application of pesticides. Due to their toxicity, even at low concentrations, chlorinated phenolic compounds have been classified as priority pollutants. The persistency of chlorinated phenolic compounds and their toxicity is mainly due to the strong C–Cl bonds, the degree of chlorination and its “lipid loving” properties (Loehr
Degradation of pentachlorophenol
Pentachlorophenol (PCP) is a highly recalcitrant compound with toxic and carcinogenic properties, and has been used extensively in herbicides, fungicides, and in preservatives for wood and wood products. It is listed on the Priority List of Pollutants by US EPA (1988). Under aerobic condition, PCP can be hydroxylated to tetrachloro-p-hydroxyquinone, which is converted further to trichlorohydroxyquinone and dichlorohydroxyquinone. However, the substituted chlorine in PCP inhibits the activity of
Degradation of pyridine
Pyridine is used widely in the manufacture of agrochemicals and pharmaceuticals. Pyridine and its byproducts are derived from coal gasification (Stuermer et al., 1982) and they have also been widely used as catalyst by the pharmaceutical industry (Leenheer et al., 1982). Pyridine is harmful if inhaled, swallowed or absorbed via the skin. Symptoms of acute intoxication by pyridine include dizziness, headache, nausea, anorexia, abdominal pain and pulmonary congestion (Gilchrist, 1997, Shimizu et
Degradation of phthalic acids and esters
Phthalic acid (PA) (IUPAC name: benzene-1,2 dicarboxylic acid) is an aromatic dicarbocylic acid. It has two isomers — isophthalic acid and terephthalic acid, and is used in the production of dyes, perfumes, saccharin, and phthalates. It is an eye, skin and respiratory irritant. Phthalic acid degrading granules have been developed in the SBR system using acclimated activated sludge as microbial inoculum within seven days of operation (Zeng et al., 2007). It was shown that the PA degrading
Degradation of tert-butyl alcohol
Tert-butyl alcohol (TBA) is directly added to fuels, and often combined with methyl tert-butyl ether (MTBE) as an octane index enhancer to reduce vehicle emissions (Piveteau et al., 2001) and is widely used as a solvent for the manufacturing of plastics, resin polymers, perfumes, paint remover, insecticides and pharmaceutical products. TBA is also a known potent toxin and carcinogen (Cirvello et al., 1995, Bradley et al., 2002, Schmidt et al., 2004). TBA is biodegradable even though the
Degradation of methyl tert-butyl ether
Methyl tertiary-butyl ether (MTBE) is added to gasoline as an octane enhancer and to reduce emissions of carbon monoxide and smog associated air pollutants. The wide use of MTBE in gasoline represents one of the largest chemical use in industrialized countries. MTBE is introduced in reformulated and reoxygenated gasoline in many countries worldwide (Depasquier et al., 2002). A serious issue arising from the wide use of MTBE is its leakage from underground storage tanks which can contaminate
Degradation of chloroanilines
Chloroanilines (CIA) are widely used as intermediates for the synthesis of organic chemicals and industrial polymers such as polyurethanes, rubber additives, dyes, pharmaceuticals, pesticides and herbicides. CIA widespread applications coupled with its toxicity and inherent recalcitrant nature meant they are also considered an important environmental pollutants which have attracted strict legislative control by environmental protection agencies worldwide. Biological treatment to remove CIA are
Degradation of metal-chelating agents
Synthetic chelating agents are widely used in many industrial applications for their metal binding and masking of metal ions. Aminopolycarboxylic acids (APCAs) are one such important group of chelating agents, as it can form stable and water-soluble complexes with many metal ions and radionuclides (Venugopalan et al., 2005, Nancharaiah et al., 2006a). Nitrilotriacetate (NTA) and ethylene diamine tetra acetate (EDTA) are two of the most widely used APCAs, and have been extensively used for
Bioaccumulation of pigments/dyes
Most industrial application such textile manufacturing, and paper and pulp processing involves the use of dyes and pigments which contribute to environmental pollution (Gupta et al., 2003, Wang et al., 2006). Furthermore, many dyes and pigments are known carcinogens and mutagens which can be trapped in the food chain and eventually pose human health risks (McKay et al., 1985, Gregory et al., 1991, Rahman et al., 2005). Technologies involving chemical coagulation/precipitation and adsorption
Bioaccumulation of heavy metals
Heavy metals are present in high concentration in a wide variety of industrial effluents such as textile, leather, tannery, electroplating, galvanizing, pigment and dyes, metallurgical, and paint industries (Zhang et al., 2005, Ahluwalia and Goyal, 2007). Stringent limits on heavy metal discharge concentrations have been imposed by environmental regulatory agencies worldwide due to their toxicity and non-biodegradability. The heavy metals can be trapped and biomagnified along the food chain via
Conclusions
Table 1 shows the aerobic granules so far developed which are suitable for treatment of wastewaters containing potentially inhibitory organic compounds, toxic heavy metals and radioactive material. In process start-up and initiation of granulation, gradual stepwise introduction of target chemicals and with these replacing a non-inhibitory carbon source has been a successful procedure. In some cases, in order to overcome slow formation of granules, especially for those which are exposed to
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