Waste Water Treatment Plant (WWTP) used to be located at the outer edge of a town so that the municipality could treat the water at a safe distance while minimizing wastewater transportation or channelling costs. However, rapid urbanization and commercialization of land have meant that a lot of the land surrounding WWTPs has become prime real estate for residential blocks, high-rise apartments and office buildings due to their proximity and easy access to the town centre.
This encroachment of WWTPs has influenced investment into odour control solutions. Wastewater treatment facilities are now extremely sensitive to the odour released into the vicinity of the treatment plant and are putting significant efforts into eliminating foul smells. Odour Control has become a major cause of opposition and political debate over the continued operation of several wastewater treatment plants located near commercial and residential areas.
Causes of Wastewater Odours
There are many factors that contribute to the formation and emission of foul smells from wastewater treatment plants such as the source or type of wastewater, methods of treatment, the structure of the WWTP and wastewater delivery methods. Nevertheless, the main factor contributing to the presence of foul odours at WWTPs is the formation of Hydrogen Sulphide (H2S) gas which has a characteristic pungent, rotten-egg smell.
Hydrogen Sulphide is formed when wastewater has a limited supply of oxygen circulating in it, effectively creating anaerobic conditions. These conditions are ideal for the growth of sulphur-reducing-bacteria (SRB) that thrive by extracting oxygen molecules from the abundant supply sulphate ions (SO4-) in the wastewater. This oxygen extraction results in the formation of H2S. Additionally, H2S has low solubility in wastewater thus easily gets released into the atmosphere. There are other compounds that contribute to the bad smells at the treatment plants but they are not as significant as H2S.
Wastewater Odour Control Measures
There are several technologies in use today to eliminate odour from wastewater treatment facilities and can be classified into two categories based on their mode of action. Vapour-Phase technologies target the point sources of the odour problem and treat the air released, while Liquid-Phase technologies treat the wastewater directly to prevent the release of odours into the atmosphere.
Vapour-Phase Odour Control Technologies
Vapour-Phase technologies work by treating the air expelled from the wastewater. The technology uses ventilation systems with negative pressure designed to isolate the air stream and direct it to the odour control systems for treatment. These systems are well designed to prevent fugitive emissions from getting into the atmosphere.
- Carbon Adsorption is the simplest method of odour control because it doesn’t use any chemicals or biological processes. In the adsorption process, the vapour is simply passed over a bed of carbon adsorbent and the molecules and odour-causing compounds adhere to the surface of the adsorbent. Although this method is very effective for H2S and sulphur-based compounds, it doesn’t work well on nitrogen-based compounds such as ammonia.
- Wet Air Scrubbing works by passing the vapour phase through an aqueous chemical solution. The chemical balance in the system is maintained automatically with set responses for a variety of loading conditions. Multi-stage scrubbers make it possible to deal with a wide range of contaminants because you can use different chemical solutions in each stage.
Liquid-Phase systems tackle the odour-causing compounds while they are still in the wastewater, effectively preventing the release of foul odours into the atmosphere. This is achieved primarily by adding a chemical to the wastewater and is usually done into wastewater collection systems rather than at the treatment plant. By preventing the formation and release of H2S into the vapour, liquid-phase treatment controls effectively provide both odour and corrosion control.
- Iron salts are used to either oxidize or precipitate dissolved sulphide. The ferrous salts such as ferrous sulphate are good for precipitating sulphide as ferrous sulphide. This ferrous sulphide is then carried downstream and is dissociated during the aeration process of the wastewater treatment plant. The Iron salts are basically employed to prevent H2S formation and are not effective in treating other odorous compounds.
- Oxidation Method is the application of strong chemical oxidizing agents, such as Hydrogen Peroxide, to wastewater in order to trigger a chemical reaction with dissolved sulphide. This reaction normally results in the conversion of sulphide to sulphates or sulphur. Large amounts of the chemical agent are needed because they usually react with and oxidize many other non-odorous compounds and can be depleted in the process.
Many WWTPs have been successful in dealing with bad odour at the wastewater facilities effectively raising employee job satisfaction ratings as well as the quality of life in the vicinity of the plant. Other very effective odour control solutions include liquid redox technology, biocide process, biofiltration and anthraquinone.