Any place or a process in which waste water is collected, conveyed or treated has the potential to generate and release nuisance odours to the surrounding area. However, most odour problem occurs in the collection system, in primary treatment facilities and in solids handling facilities. In most instances, the odours associated with collection systems and primary treatment facilities are generated as a result of an anaerobic or “septic” condition. This paper consists of an evaluation of the qualitative characteristics and the economic properties of the various technologies available for waste water control. There are a virtually limitless number of unique odour control problems and challenges and it would be impossible to identify one technology as the most applicable for all situations. It is hoped that this paper could be used as a guide to select the most appropriate technology or technologies based on the unique characteristics of a given application.
2. ODOUR PROBLEMS
Any place or process in which waste water is collected, conveyed or treated will release nuisance odours to the surrounding area. Most of the odour problems occur in the collection system, in primary treatment facilities and in solid handling facilities and these are generated as a result of an anaerobic or septic condition. This occurs when the transfer of oxygen for the microbes to survive, they become sulphate reducing bacteria to thrive. These bacteria utilize the sulphate ion, which is naturally abundant in most waters as an oxygen source for respiration. The byproduct of this activity is Hydrogen sulphide, which has a low solubility in the waste water and produces a rotten egg odour. It can also cause severe corrosion.
Solid handling facilities are another significant odour problem area. In bio-solids dewatering and treatment processes, the bio-solids commonly undergo extreme turbulence, pH adjustment and thermal treatment. Depending on the nature of the nature of the bio-solids stream and the treatment used, the odour compounds released can consists of ammonia, amines, hydrogen sulphide, mercaptans, organic sulphides etc. Here also, due to the anaerobic digestion of sludge, sulphate-reducing bacteria thrive, and hydrogen sulphide is formed.
3. ODOUR CONTROLLING TECHNIQUES
The various technologies can be split into min groups as:-
1. Vapour-phase technologies
2. Liquid-phase technologies.
3.1 Vapour-Phase Technology
It is used to control odourous compounds in the air or gas. This technology is used in point source applications such as waste water treatment plants and pump stations or for the treatment of biogas.
Vapour phase technologies ventilate the point sources of odour problems. For air treatment, the ventilation system is designed to maintain a negative pressure on the area at all times, which will prevent the “leakage” of odourous air from vents, manways etc. The discharge from the ventilation system is the only route for the odourous air to escape, therefore it must be treated. This system is very effective in preventing the odour problems. Even if large amount of air are ventilated, the system can be effective by providing adequate ventilation or by reducing the contaminant concentration. Biogas treatment systems are concerned primarily with the removal of hydrogen sulphide to prevent the formation of sulphur dioxide. The removal of hydrogen sulphide is required to meet the emission standards or to prevent corrosion in the combustion equipment. The technologies used in treating the ventilation air or biogas are wet air scrubbing, liquid redox technology, bio-filtration, solid scavengers and carbon adsorption.
3.1.1 Wet Air Scrubbing
It is the most flexible and reliable technology for vapour-phase waste water odour control. This can be used to treat virtually any water soluble contaminant. In addition to hydrogen sulphide and organic odours, wet air scrubbing is very effective for ammonia removal. In this, the odour contaminants are solubilised from the vapour phase into an aqueous chemical solution. The removal mechanism is purely chemical. The chemical balance in the waste water is automatically continuously maintained, even under changing loading conditions, minimizing the chance for odour break-through.
Fig. 1 Wet Air Scrubber
For reducing the chemical use, the design used is multi state scrubbing system. The most prevalent contaminant is hydrogen sulphide. Hydrogen sulphide can be solubilized with a solution of sodium hydroxide. The other odour causing compounds are treated by sodium hypochlorite.
In a single stage scrubber system, sodium hydroxide and sodium hypochlorite are used in a re-circulating chemical solution. Sodium hypochlorite is a strong oxidizer; it readily reacts with sulphide that is solubilized by the sodium hydroxide. Therefore in a single stage system, sodium hypochlorite must be added in sufficient quantities to oxidize the hydrogen sulphide and maintain a residual to break the other odour compounds.
· Reliability and flexibility provided by the use of chemicals and chemical reactions.
· Minimization of chemical use and cost while maintaining complete, flexible and reliable treatment.
3.1.2 Liquid Redox Technology
Liquid redox units use a chelated metal dissolved in a water solution to remove hydrogen sulphide from a gas stream and convert it catalytically to solid, elemental sulphur. The metal in the solution, which is held by organic chelating agents, acts as catalyst, seeping up the naturally occurring reaction.
The metal ions in the solution removes electron from a solution ion to form sulphur and in turn transfer the electrons to oxygen (O2) in the regeneration process. Even if many metals are there for performing these functions, iron (Fe) is the most commercially used as it is inexpensive and non toxic.
Fig. 2 Liquid Redox Unit
It is not widely used in the odour control market. Most liquid redox processes are used in the petro chemical and natural gas industries and are considered too complicated and costly for odour controlling applications. This has limited applications in waste water higher capital cost. Liquid redox units have operating costs that can be less than 10% of other vapour phase treatment operations because of the regeneration of the active catalyst, the iron solution. Liquid redox units mainly focus on digester gas treatment and ventilation air applications with extremely high loadings of hydrogen sulphide. Since they remove only H2S, the units used for ventilation air odour control in waste water facilities require polish with hypochlorite or carbon to remove other odour compounds present.
It can be used to treat a variety of biodegradable, water soluble contaminants. In a bio-filter, the odour contaminants are solubilized from the vapour phase to aqueous phase on the surface of an organic medium such as compost or peat. Then these compounds are degraded by bacteriological population on this media. Bio-filters are very effective in removing sulphur based odour compounds like hydrogen sulphide, organic sulphur and mercaptans. Two major challenges in bio-filtration are stability of media and control of bio-filtration process is not effective in removing nitrogen based compounds like ammonia and amines.
The media used in bio-filter can be prone to breakdown. When this occurs, the bed settles and compacts increasing the head loss through the filter, which causes a decrease in airflow and fugitive odour emissions. Control of the bio-filtration process is also an important concern. If the media are exposed to wide swings in environmental conditions, upset of bacteria population may result and break-through odours will occur.
3.1.4 Solid Scavengers
Scavengers are solid or liquid materials that remove sulphur compounds from gas or liquid streams, reacting with them and converting them to stable compounds. Solid scavengers differ from carbon absorbers in that the sulphur compounds actually react to form stable compounds. As a result, spent media from scavengers are easier to handle than spent carbon media. But the scavengers cannot be regenerated. It cannot be consumed in the removal of H2S.
Solid scavengers are of two types
1. Those that use an organic substrate (generally called an iron sponge)
2. Those that use an inorganic or ceramic substrate
These media have seen widespread use in the natural gas market and have spread into the treatment of anaerobic biogas. It has low capital costs. Operating costs for scavenger systems are scavengers are comparable to single stage wet air scrubbers, but have added the benefit of not absorbing carbon dioxide, making them ideal for treating biogas with low to moderate levels of H2S.
3.1.5 Carbon Adsorption
In this system, the air stream is passed over a bed of adsorbent (carbon) and the ofour causing compounds are attracted to and adhere to the surface of the adsorbent. This is the simplest method. There is no ongoing chemical supply to the system, and there are no biological processes to be upset. Sulphur based compounds such as H2S are removed effectively by carbon adsorption systems. But ammonia and other nitrogen based compounds are not effectively treated.
Fig. 3 Carbon Adsorption
3.2 Liquid-phase technologies
The objective of the liquid phase technology is multiple point odour control. This method is used to control odourous compounds in the liquid wastewater itself. These are typically used in collection systems, where both odourous and corrosion can be controlled. Most liquid phase technologies involve the addition of a chemical to wastewater or to control the formation of odourous compounds or react with those compounds once they are formed.
With liquid phase treatment, H2S is prevented from escaping the liquid into vapour, thereby corrosion is prevented. Through proper application of liquid phase treatment in a collection system, multiple odour release points such as manholes, air relief valves and re-pump stations can be controlled through one chemical application point.
Some of the liquid phase technologies are listed below:
3.2.1 Iron Salts
These are applied to wastewater to oxidize or precipitate dissolved sulphide. Ferrous salts such as ferrous sulphate precipitate. Sulphur as ferrous sulphide. Ferrous sulphide is a light precipitate, black in colour; within the aeration process of treatment plant, ferrous sulphide is dissociated. Ferric salts such as ferric chloride oxidize some of the sulphide to sulphur while reducing ferric ions to ferrous ions. Remaining ferrous ion reacts with dissolved sulphide to form the ferrous sulphide precipitate. Ferric ion can be used for chemical precipitation of phosphorous. Other odourous compounds are not treated with iron salts. Iron salt solutions are hazardous compounds and require wall tankage and piping systems.
3.2.2 Bioxide process
It involves the application of a nitrate solution to waste water. It use naturally occurring bacteria to biochemically oxidize dissolved sulphide in the presence of nitrate. This mechanism takes place when the solution is applied in wastewater that contains dissolved sulphide. Nitrate is injected in sufficient quantities to oxidize sulphide (biochemically) by the reaction;
3.2.3 Oxidizing agents
Oxidation for wastewater odour control involves the application of a strong chemical oxidizing agent to the wastewater. The oxidizing agent is used to chemically react with dissolved sulphide, converting it to sulphate or sulphur. In addition to dissolved sulphide, many other odourous compounds as well as non odourous compounds are treated by the oxidizing agent. Hydrogen peroxide is the most commonly used oxidizer.
It is a chemical compound that interrupts the sulphate reduction process carried out by sulphate reducing bacteria (SRB) in wastewater under anaerobic conditions. This process occurs in cytoplasmic membrane of the SRB at the cytochrome C3 location. Anthraquinone is a non hazardous compound. When used alone, anthraqinone is specific to the control of sulphide. It can be used in the conjunction with bioxide process.
3.2.5 Digester gas treatment
Because of the difficulty of comparing applications for biogas treatment, comparisons of the capital and operating costs of liquid redox systems and solid scavengers for the treatment of biogas are not included.
In any evaluation for the treatment of biogas, many factors need to be evaluated together to determine the best system to use
· Process conditions
As expected, this review and analysis does not provide a conclusive determination of the “best” odour control technology. Rather it supports the contention that the existing conditions, treatment objectives and economic restraints must be considered in selection and design of any odour control strategy.
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