The main goal of proper wastewater treatment is to keep sewage sludge disposal safe for human health and ensure minimal negative effect on the environment. Wastewater odour control was not an issue until communities started developing near treatment facilities. Complaints started flooding wastewater treatment plant managers who were pressured to quickly address the problem. Thus, various technologies have been developed to control these nuisance odours. Here is a list of a few of these wastewater odour control techniques deployed by treatment facilities.
Facility Design
Because it is less expensive to mitigate odour in the liquid phase compared to when its already dispersed in the air, engineers designed a wastewater odour control treatment process that prevents hydrogen sulphides from developing. Hydrogen sulphide (H2S) is a dense, colourless, malodorous toxic gas that can oxidize and damage wastewater treatment infrastructures. The formation of hydrogen sulphide results from the anaerobic decay of organic matter and typically smells like rotten eggs. These “dead zones” where H2S is likely to develop can be avoided by mindfully designing pipe slopes, transition structures, tunnels, manholes, siphons, wet wells and force mains. Adding vegetation can also increase local air turbulence.
Oxidizing Agents
Chlorine dioxide (ClO2) is a potent disinfectant and bactericide in wastewater treatment. It is effective in mitigating H2S odours by oxidizing sulphides without the formation of colloidal sulfur. It also provides safe and environmentally friendly wastewater treatment applications since it does not react with inert compounds, thus reducing the dose rates required.
Sodium nitrate (NaNO3) can be utilized to slow bacterial action and encourage the oxidation of H2S odours in systems with a retention time greater than 4 hours. Sodium nitrate is a biological solution to managing odours in sewage sludge. Using anaerobic bacteria to process other sources of bound oxygen results in the production of intermediate gases like nitrite, nitric oxide, and nitrous oxide instead of sulphide.
When applying these chemicals into wastewater, side reactions will occur aside from the desired reaction. In estimating dosing rates, experts should include an allowance to account for these side reactions. Pilot-testing should be employed to determine correct chemical dosages.
Precipitants
Iron and zinc salts can be used to precipitate sulphides. These salts are proven to be efficient in controlling dissolved hydrogen sulphide which stops the creation of H2S gas.
Iron salts binds to H2S in a precipitation reaction that results in the formation of ferrous sulphide. Iron salts can also effectively remove phosphorous in sewage sludge. Ferrous and ferric chloride have been used in collection systems, force mains, and at sewage treatment plants. However, they are hazardous compounds and usually require double-wall tanks and piping systems.
Water soluble zinc salts such as zinc chloride, zinc nitrate, zinc sulfate or zinc acetate are also used as oxidizing agents. However, it has been discovered that free zinc ion is quickly used up when wastes are added to the system.
Bioxide Treatment
Bioxide is a natural nitrate solution that also addresses foul odours associated with hydrogen sulphide. It is a patented process owned by USFilter. Calcium nitrate is the only leading method of treating hydrogen sulphides, which is not on the EPA CERCLA list of hazardous substances. It can also solve issues concerning other sulfur compounds such as mercaptans and organic sulphides.
Anthraquinone
Anthraquinone is a non-hazardous chemical compound that hinders bacteria from completing their metabolic processes. It is poorly soluble in water but soluble in some organic solvents. To be effective, it must settle with the slime layer of the wasterwater sludge. Anthraquinone causes bacteria in the slime layer to be inactive thereby stopping the production of sulphides. This occurs by upsetting the mitochondrial electron transport chain in bacteria, inhibiting oxidative phosphorylation (respiration). The chemical can be effective for a period of several days up to six weeks. But because it is only semi-soluble, it is only partly effective in force main applications and fast gravity main flows.
If you are a municipality in Ontario and in need of a biosolids management solution, please feel free to contact us at 1 (877) 479-1388.
Source:
https://www.ontario.ca