Research review paperDevelopments in odour control and waste gas treatment biotechnology: a review
Introduction
Odours in wastewater treatment arise mainly from the biodegradation of sewage, especially anaerobic degradation. Other odours associated with wastewater treatment come either directly from industrial wastewater (solvents, volatile organic compounds, petroleum derivatives) or indirectly from warm, highly degradable, or sulphurous effluents (Vincent and Hobson, 1998). Odours are generated by a number of different wastewater components (Vincent and Hobson, 1998), the most significant being the sulphur compounds, hydrogen sulphide (H2S) and mercaptan. Domestic sewage contains 3–6 mg/L organic sulphur, mainly arising from proteinaceous materials, plus ∼4 mg/L from sulphonates contained in household detergents (Boon, 1995) and 30–60 mg/L inorganic sulphur (as sulphates) (Gostelow and Parsons, 2000). While nitrogen-based odour compounds and the organics associated with anaerobic wastewater treatment are important, hydrogen sulphide is toxic to microorganisms and corrosive to concrete and mild steel, and as the predominant odorant associated with sewers (Vincent and Hobson, 1998), it has been extensively studied Halkjaer-Nielsen et al., 1998, Gostelow and Parsons, 2000.
A range of technologies is available to treat odorous air emitted from wastewater treatment plants, sludge handling facilities, and industrial processes. There are three methods of odour treatment, biochemical (biofilters, bioscrubbers, activated sludge), chemical (chemical scrubbers, thermal oxidation, catalytic oxidation, ozonation), and physical [condensation, adsorption (activated carbon), absorption (clean water scrubbers)].
The selection of a particular technology or combination of technologies is dependent on such factors as: site characteristics including operation and maintenance capabilities, treatment objectives, foul air flowrates, contaminant loading patterns, and the characteristics and strength of odorous air. Waste gases from industry have traditionally been treated using physicochemical processes, such as scrubbing, adsorption, condensation, and oxidation (Table 1); biological treatment of waste gases has only gained support as an effective and economical option in the past few decades (Kennes and Thalasso, 1998). Biological methods of odour treatment gained much attention in Europe in the 1990s owing to their efficiency, cost-effectiveness, and environmental acceptability, and by 1994 accounted for 78% of odour treatment in Germany (Frechen, 1994). In all types of bioreactors for waste gas treatment, the pollutants diffuse into the liquid phase where microorganisms degrade them into products, such as CO2, H2O, and minerals WEF/ASCE, 1995, Kennes and Thalasso, 1998, Vincent and Hobson, 1998. Microorganisms are known to play an important role in geochemical and biogeochemical cycles by mineralising biopolymers and xenobiotic compounds (Lie et al., 1998), although this was recognised only 30 years ago, and current understanding of the mechanisms of biodegradation of such materials is still not complete (Tan and Field, 2000). According to Brauer (1986), the transformation process can be expressed simply by:When provided with an oxygen source, bacteria in wastewater consume ionic sulphide species and oxidise them to nonodorous sulphur species. This mechanism suggests an acclimated biological mixture, such as returned activated sludge, could convert dissolved sulphide into nonodorous sulphur species, if provided with an oxygen source (Joyce, 1995). It is believed that odour-producing compounds are removed by adsorption and/or dissolution in the liquid and a combination of chemical and biological oxidation (Ostojic et al., 1992). A summary of the removal methods is presented in Table 2. When Cho et al. (1992) carried out some work on H2S oxidation using a peat biofilter colonised by Xanthomonas sp. strain DY44, the same levels of H2S removal were achieved with viable cells and a γ-irradiated cell suspension. The importance of biological oxidation, therefore, comes into question. Contrary to Cho et al.'s findings, Kanagawa and Mikami (1989) found that in the experiments they conducted to remove H2S from contaminated air using Thiobacillus thioparus, H2S oxidation occurred in the sterilised medium, although the rate was very slow. For this reason, these workers concluded that H2S oxidation was carried out mainly by microorganisms. In a paper addressing the kinetics of chemical and biological sulphide oxidation, Buisman et al. (1994) discovered that at a sulphide concentration of around 150 mg/L the biological oxidation rate was seven times faster than the chemical oxidation rate. The biological oxidation rate below 10 mg of sulphide per liter is a factor 75 times faster than chemical oxidation.
The literature documents examples of the use of phototrophic (Cork et al., 1983), heterotrophic (Cho et al., 1992), and autotrophic Jensen and Webb, 1995, Sublette and Sylvester, 1987a, Sublette and Sylvester, 1987b, Kanagawa and Mikami, 1989 microorganisms for the desulphurisation of gas. Biological oxidation of sulphide to elemental sulphur has several advantages over other physicochemical processes (Buisman et al., 1991). The advantages presented include economic gain, as there is a comparably low capital outlay Gadre, 1988, Benedek et al., 1988 and money is saved on oxidants and catalysts, resulting in a reduction in operating cost Fox and Venkatasubbiah, 1996, Janssen et al., 1999, Li et al., 1998, Buisman et al., 1991, Comas et al., 1999, Jensen and Webb, 1995, Benedek et al., 1988, Kanagawa and Mikami, 1989. It is possible to recover sulphur from a biological process and this can be reused Fox and Venkatasubbiah, 1996, Buisman et al., 1991. No chemical sludge is produced Fox and Venkatasubbiah, 1996, Gadre, 1988, Buisman et al., 1991, and there is a reduction in the sulphate or thiosulphate discharge Fox and Venkatasubbiah, 1996, Buisman et al., 1991. In general, biological sulphur removal consumes less energy than physicochemical removal methods Fox and Venkatasubbiah, 1996, Gadre, 1988, Buisman et al., 1991, Jensen and Webb, 1995, Kasakura and Tatsukawa, 1995. According to Comas et al. (1999), chemical absorption is approximately 62% more expensive than biological processes.
The use of biotechnologies for foul air treatment has grown dramatically because of their ability to destroy the pollutants rather than simply transfer them from the gas to the liquid phase, although chemical odour treatment remains popular. Published performance data for odour compounds treated using similar process units vary widely Veiga et al., 1997, Kennes and Thalasso, 1998, suggesting that research into the parameters affecting the performance of biological air treatment would significantly improve our ability to use odour and air pollution control biotechnologies to their full potential. Three major types of bioreactor that currently dominate waste gas biotreatment are biofilters, trickling biofilters, and bioscrubbers Table 2, Table 4, Table 5. A recent review by Kennes and Thalasso (1998) presents the state of reactor design and factors for optimization. In addition to these three most widely used technologies, other alternatives have been proposed, such as an external loop airlift bioreactor (Ritchie and Hill, 1995), a spiral bioreactor (Shim et al., 1995), membrane bioreactors Reij et al., 1995, Reij et al., 1997, Reij et al., 1998, Parvatiyar et al., 1996a, Parvatiyar et al., 1996b, Ergas and McGrath, 1997, Ergas et al., 1999, and activated sludge diffusion Fukuyama and Honda, 1976, Fukuyama et al., 1979, Fukuyama et al., 1980, Fukuyama et al., 1981, Fukuyama et al., 1986, Ostojic et al., 1992, Ryckman-Siegwarth and Pincince, 1992, Frechen, 1994, Stillwell et al., 1994, Bentzen et al., 1995, Johnson et al., 1995, WEF/ASCE, 1995, Bielefeldt et al., 1997, Æsøy et al., 1998, Vincent and Hobson, 1998, Bowker, 1999, Bowker, 2000a, Oppelt et al., 1999.
Section snippets
Bulk media biofilters
The odorous compounds present are degraded as the contaminated air passes through a bed of naturally occurring microbes immobilised on a support media. The air stream and media bed are moistened to facilitate microbial activity. Odorous compounds in the air stream provide a source of carbon for the biomass, while the moisture added is used to supply other nutrients (e.g. nitrogen, phosphorus, potassium) to the biofilm. Because of the low density of microbes, which are present as a mixed
Trickling biofilters
The odour-laden air stream is passed over a microbial consortium immobilised on support media with a high surface area. Recirculating water maintains humidity in the media bed and allows nutrient supply. Odorants dissolve into the aqueous phase and are degraded by the biofilm present. Foul air can pass through a trickling biofilter either co- or counter-currently to the liquid that provides the biofilm with nutrients. Trickling filter media can be ceramic or plastic structures, activated
Bioscrubbers
Bioscrubbing has several advantages over media-based filtration (Table 6). The process is more easily controlled because the pH, temperature, nutrient balance, and removal of metabolic products can be altered in the water of the reactor (Smet and van Langenhove, 1998). Bioscrubbing is reliant on good gas dissolution, as it employs the absorption of pollutants into the aqueous phase in a gas/liquid exchange column, followed by degradation in a liquid phase bioreactor (Fig. 4). The liquid phase
Membrane bioreactors
In membrane bioreactors, the gaseous pollutants are transferred from the gas to the liquid phase (where they are degraded) via a membrane. Two membrane materials are available for treating odours: dense (e.g. silicone rubber) and hydrophobic microporous (e.g. polysulphone); dense materials are more selective and microporous materials more permeable (Reij et al., 1998). There are also two types of biomass available: fixed film cultures (biofilms) and suspended growth cultures. The membrane forms
Activated sludge
There are fewer examples of liquid-based odour control systems than media-based systems (WEF/ASCE, 1995). The comparative merits of liquid- and media-based systems differ, and so their suitability to the conditions in different wastewater treatment plants also differs (Table 8). Activated sludge diffusion is used as an alternative to more established bioreactors for waste gas treatment, such as biofilters, bioscrubbers, and biotrickling filters. Contaminant removal mechanisms in activated
Conclusions and research needs
Activated sludge treatment of odorous air has been increased over the last 25 years. Ryckman-Siegwarth and Pincince (1992) and Bowker, 1999, Bowker, 2000a have shown that aeration tanks can and have been successfully employed at wastewater treatment sites covering much of North America. The restrictions of corrosion of structures and deposition problems have been overcome. However, the longest trial reported in the literature ran for only 33 days (Fukuyama et al., 1986), and the longer term,
Acknowledgements
The authors would like to thank Anglian Water Services for financial support of this work.
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