Treatment of Waste Gases: 2 Methods | Microbiology

The following points highlight the top two methods used for the treatment of waste gases. The methods are: 1. Removal of Volatile Organic Compounds (VOCs) 2. Biological Removal of Sulfur and Nitrogen Compounds from Flue Oases.

Method # 1. Removal of Volatile Organic Compounds (VOCs):

Physico-chemical treatment of polluted industrial waste gases, such as combustion or adsorption on activated coal filters which are used conventionally tend to waste a lot of energy and result in secondary pollution. Pollutant concentrations in industrial emissions, for example, are of the order of 100 ml m^-3.

To burn these waste gases in an incinerator, at least 50 litres methane is required to be added per m³ in order, to ensure complete destruction. A bioreactor may, in most cases, achieve the same oxidation provided the Volatile Organic Compounds (VOCs) are brought in close contact with degradative microbes, O₂, H₂O and nutrients.

Biodegradation rates vary being dependent upon the pollutant degraded:

(i) Quickly biodegraded: alcohols, ketones, aldehydes, organic acids, organo-N;

(ii) Slowly biodegraded: phenols, hydrocarbons, solvents (e.g., chloroethene); and

(iii) Every slowly biodegraded: poly-halogenatcd and poly-aromatic hydrocarbons.

Despite the broad spectrum of air pollutants amenable to bio-filter treatment, the introduction of this new technology is slow. It is most probably because its low cost does not ensure high profit margins and because the physico-chemical air pollution control industry is well entrenched. Different types of reactor designs are used to treat air biologically (Fig. 32.10).

In bio-filters, contaminated air flows slowly through a wet porous medium-compost, peat, or wood chips—which support a degradative microbial population living in the thin water film coating the solid support material. The superficial gas flow varies from 1 to 15 cm s^-1.

This yields a contract time, for a typical bed height of 1-3 m, of 10-100 s. For normally biodegradable compounds, removal efficiencies of 90% can be expected at volumetric loading rates of 0.1-0.25 kg organics m^-3 reactor day.

The advantages of bio-filters are:

(i) Simple and cheap design (support material replaced every 2-4 years);

(ii) High internal surface area makes bio-filters ideally suited to remove poorly soluble pollutants, e.g., hydrocarbons; and

(iii) Possibility to inoculate with bacteria especially adapted for the breakdown of xenobiotic compounds, e.g., chloromethane.

The most difficult problem is the control of the pH in the bio-filter since H₂S is oxidised to H₂SO₄, NH₃ to HNO₃, and chloro-organics of HCl. For convenience, the removal efficiency of dimethyl sulphide in a compost bio-filter seeded with the bacterium Hyphomicrobium dropped within two months of operation, from 1 to 0.1 g m^-3 day due to a pH drop to 4. Repeated dosing of 25 kg limestone powder (CaCO₃) per m³ compost carrier eliminated the inhibition for a two-month period.

Disadvantages of bio-filters are as follows:

(i) Large floor space necessary;

(ii) Not possible to control the process conditions, e.g., pH; and

(iii) Support materials such as compost themselves generate odours.

The disadvantages of the bio-filter can be avoided in a bioscrubber (Fig. 32.10). A conventionally used scrubber transfers a substance present in a gaseous stream to a liquid stream by spraying a liquid in a chamber through which the gas is passed. In a bioscrubber, the sprayed liquid is a suspension of microorganisms which cycles back and forth between the spray chamber and a waste water treatment unit where biodegradation takes place.

The process parameters like as adequate nutrient supply and pH can be much more easily controlled (in the circulating liquid of a bioscrubber) than in a biofilter, leading to fast reaction rates. While biofilters require a large footprint since their height preferably should not exceed 1 m in order to avoid clogging, bioscrubbers require much less space because the tank where biodegradation takes place can be several metres high.

Bio-scrubbers are considered to be the best suited for large air flows because of their low back pressure and small size. They however can be employed only for the removal of waste gases which are sufficiently soluble because the mass transfer rate in a spray chamber is less than that attainable in a biofilter unit.

In case the obtained contaminant concentration in the outlet gas is too high, a second bioscrubber inoculated with microorganisms capable of degrading lower contaminant concentrations must be installed. This aspect requires further development.

At present, considerable effort has been done to develop such design of a system that can combine the adsorption of the gas onto a solid surface (e.g., activated carbon) and biodegradation of the sorbed compound. Bio-trickle filters are sheets of a plastic or other microbial support medium hung in the contaminated air stream.

The sheets are bathed continuously by a re-circulating stream of water possessing the nutrients required by the microorganisms. Bio-oxidation rates per unit volume in biofilters are high so that these filters can be as small as physico-chemical units. Being operated at higher loading rates, they are however more sensitive to peack loads and nutritional requirements need be monitored closely.

Method # 2. Biological Removal of Sulfur and Nitrogen Compounds from Flue Gases:

Nitrogen oxides (NO_x) and sulphur dioxide (SO₂) are considered to be major air pollutants formed during the combustion of coal and oil and released in flue gases. There is considerable interest in the development of an efficient and low-cost biotechnology for the simultaneous removal of these air pollutants, since conventional physico-chemical technologies are either very expensive or inefficient.

A new system has been currently proposed in which the flue gas is led through a scrubber in which > 95% SO₂ and >80% NO_x dissolve in a solution of NaHCO_v and Fe(II)-EDTA (the latter compound seems to raise the solubility of NO_x). The S- and N- laden solution is regenerated in three sequential biological steps (Fig. 32.11).

The first step consists of an anoxic reactor wherein NO is converted to inert N₂ gas via biological denitrification:

2Fe^II (EDTA) (NO) + electron donor → 2Fe” (EDTA) + N₂ + CO₂ + H₂O

An electron donor, (e.g., methanol or ethanol), are added to sustain the reaction.

In the two following steps, H₂SO₃ is sequentially reduced biologically to H₂S and finally partially re-oxidized to solid elemental sulphur:

H₂SO₃ + 3 H₂ → H₂S + 3H₂O

H₂S + 1/2 O₂ → S°+H₂O

The reduction of H₂SO₃ takes place in a UASB reactor containing sulphate-reducing bacteria. Flocculant polymers are supplemented, together with necessary nutrients and reducing equivalents (ethanol or H₂) to adjust the (BOD/H₂SO₃) molar ratio at a value of one.

In the third bioreactor, aerobic bacteria oxidise sulphide back to solid S° (end-product). The further oxidation of S° to H₂ SO₃ and H₂SO₄ is prevented by providing limiting amounts of O₂. The overall process is fully automated with about 120 parameters being continuously analysed, most of them on-line. The water is continuously recycled.

This process of bio-desulphurisation will undoubtedly also be applied in days to come to treat other waste streams. There is a growing interest in depolluting waste waters through the activity of sulphate reducing bacteria in sulphidogcnic USAB reactors.

Sulphate concentrations reach very high levels in effluents from the paper board industries (2 g 1^-1), in molasses-based fermentation industries (2-9 g 1^-1), and in edible oil refineries (up to 50 g 1^-1). Very large amounts of sulphate are also present in acidic mine drainage where pyrite rock is being processed. When heavy metals are present, these can be very efficiently removed (> 99%) via sulphide precipitation.