By Paul Turgeon and Tonya Chandler
Wastewater systems have long been subject to issues with odour and corrosion, which is understandable given the nature of what they convey. While odour certainly needs controlling for public health and aesthetic reasons, corrosion creates the greatest potential for environmental harm and real systemic and economic damage.
This damage can arise in the form of burst pipes and other equipment and system failures. Failures of this type require the repair and replacement of system materials and equipment, and they have the potential to expose the environment to unpredictable releases of hazardous waste that are difficult, if not impossible, to contain or recover.
Corrosion caused by hydrogen sulfide
A major contributor to odour and corrosion in industrial systems is hydrogen sulfide (H₂S) and its associated compounds. Some industrial wastewater contains sulfur compounds, which provide the molecular basis for the generation of H₂S. It arises from the combination of anaerobic conditions and the presence of sulfites and sulfates in conjunction with colonies of microorganisms present on the inner walls of all collection systems, referred to as the slime layer.
Sulfate reducing bacteria (SRB) will use these compounds in the absence of free oxygen (O₂) for metabolism. These bacteria do not use the sulfur component, and it is available to react with water, specifically free protons (H+), which results in the generation of H2S. Following its generation, H2S can be released into the atmosphere and find its way to receptors through junctions of the atmosphere and collection system, at which point it is an odour concern.
H₂S is a colorless gas that has a characteristic rotten egg odour. It is highly toxic and corrosive to certain metals and is heavier than air, meaning it can accumulate in wells, manholes and other similar locations that lack ventilation. The effect it can have on humans, at varying concentrations relative to ambient air, is shown in Table 1.
| Concentration (ppm) | Physiological Effect |
|---|---|
| 0.1 to 3 | Odour threshold |
| 3 to 100 | Offensive odour |
| 10 to 50 | • Headache • Nausea • Throat and eye irritation |
| 50 to 100 | Eye injury |
| 100 to 300 | • Conjunctivitis • Respiratory tract irritation • Olfactory paralysis |
| 300 to 500 | • Pulmonary edema • Imminent threat to life |
| 500 to 1,000 | • Strong nervous system stimulation • Apnea |
| 1,000 to 2,000 | • Immediate collapse with respiratory paralysis • Risk of death |
H₂S becomes a corrosion issue when it contacts moist concrete or steel, among other metals, in the presence of oxygen, even at very low gaseous concentrations. Conditions such as these are common in the headspace of some pipes and other areas where the collection system has easy access to atmospheric oxygen. Bacteria in these areas convert H₂S into sulfuric acid, which then begins a destructive reaction with the infrastructure.
Historically, control of odour and/or corrosion has been implemented through either vapour phase techniques, where the headspace of a system is treated, or liquid phase techniques, where treatments target the liquid flow. Vapour phase treatments like scrubbers do not provide corrosion control. Some of the liquid phase techniques offer corrosion control.
The most common method of inducing liquid phase treatment, or directly treating wastewater inside the collection system, has been by dosing chemicals into the system. A constant and continuous dose of chemical is fed from a large reservoir with a small pump into the collection system, typically at a manhole or pump station. These chemicals are meant to react with the odour-causing compounds present in the wastewater or cease their formation and/or release from solution.
Conventional control options
Chemical oxidation of H₂S is accomplished through use of a compound with a high oxidation potential, called an oxidant, such as hydrogen peroxide or sodium hypochlorite (bleach).
Chemicals that interact with H₂S and sequester, or scavenge the sulfur into a relatively insoluble form, such as ferric chloride and ferrous chloride, can be used to remove sulfur from the cycle entirely. Because of the way that its ions dissociate in the aqueous phase, the release of H₂S from wastewater will not occur if the pH is 9 or higher.
Alternative oxygen source/sulfate substitute
In an anaerobic environment, the microbiology in a collection system will use oxygen from a nitrate (NO₃) molecule more readily than from a sulfate (SO₄) molecule. As a result, benign nitrogen is released rather than H₂S. Chemicals like calcium or sodium nitrate are commercially available and can be used for this purpose. However, they can be expensive, and they feed and grow the SRB layer, potentially requiring a higher volume for treatment over time.
Upon cessation of treatment, the amount of H₂S can be even worse than before. Excess wet well build-up requiring increased clean-out cycles (because of the waxes used to stabilize the nitrate molecules) can be encountered downstream in the collection system. In addition, emerging regulations are beginning to include nitrate concentrations on discharge limitations.
Real-time, active monitoring of wastewater H₂S levels is seldom carried out, so enough chemical to control peak values is typically added on a constant basis. By treating for peak values with chemicals such as these, the likelihood is very high that excess nitrate will be present and actively added to the wastewater. This can require additional denitrification processes or incur fines, both of which can be very expensive.
An issue with all chemicals is that, to introduce them to a collection system, a bulk quantity must be stored nearby. To ensure that chemicals are always available for treatment, continued deliveries to the bulk storage tank must be made. To avoid adverse effects to the environment, engineered controls, such as secondary containment and leak monitoring, must be designed, implemented and maintained. Ideally, a successful treatment of wastewater odour and corrosion would:
- End sulfide production.
- Quickly eliminate sulfides that are present.
- Bring about no additional hazard to life or the environment.
- Do no harm to the collection system.
- Create no additional challenges downstream.
In addition, the treatment solution must be cost-effective. One answer is introducing ozone and oxygen into wastewater systems to control odour and corrosion.
H₂S control with ozone
Ozone has long been used in water treatment, dating back to at least the late 19th century, primarily for the disinfection and polishing of drinking water. In Europe, ozone treatment of water is a common process. Its environmental sustainability and relative safety versus chemical systems have established it as a favoured current and future technology. The controlled use of ozone as a treatment does not produce harmful byproducts that could contaminate or damage the environment or ecology. Typically, the only byproducts from its reaction are O₂ and inert oxides.
In recent years, interest in its use to treat wastewater has led to the development of new and sustainable (green) technology for odour and corrosion control in wastewater collection systems.
Ozone is a special, naturally occurring form of atmospheric oxygen. Instead of two oxygen atoms it has three, represented by its chemical formula O3. This third oxygen atom makes it a highly reactive molecule with very high oxidation potential. In fact, it has the highest oxidation potential of any commercially available molecule and the fourth highest overall with an oxidation potential of 2.07 volts (V). Above it, in terms of oxidation potential, are atomic fluorine (F•, 2.87 V), the hydroxyl radical (•OH, 2.86 V) and atomic oxygen (O•, 2.42 V).
Ozone can be generated by exciting a flow of oxygen with sufficient electrical or optical energy. This will cause a certain amount of oxygen atoms to split and re-combine with other O₂ molecules nearby.
Under typical treatment conditions, using a relatively pure oxygen stream and a corona discharge chamber that uses a high-voltage electrical arc, this reaction can produce up to 9 to 12% by weight (wt%) ozone, although typically output is 1 to 9 wt% ozone. The remainder of the stream is left as oxygen.
As ozone concentrations rise above this concentration, the destruction reaction becomes more frequent, returning greater quantities to O₂ and maintaining this equilibrium. This instability is also the reason ozone cannot be stored and must be generated immediately before application.
Because of its extreme instability and high oxidation potential, ozone is powerful and indiscriminate in terms of reactivity with other chemical species. Ozone has been shown as an effective treatment for the destruction of volatile organic compounds, removal of metals, total suspended solids and organic carbon, and significant reductions to chemical oxygen demand.
In freshwater, the half-life of ozone is typically 10 to 20 minutes, but in wastewater, ozone has been documented as being entirely consumed within 8.6 seconds. This is because of the extreme amount of potential reactants present in wastewater, including H₂S.
The simple structure of H₂S makes it an easy target for oxidation by ozone. In addition to its high oxidation potential, ozone’s unique structure tends to create free radicals, chemical species that have unbonded electrons making them highly reactive, especially in water.
Not only is the benefit of ozone’s direct reaction with different chemical species realized, but also, as part of these reactions, additional free radicals, which can be even more reactive than ozone, can form. Additionally, radicals tend to create additional radicals as they react, in what is termed a free radical chain reaction.
These additional reactions are indirect effects of ozone. With the source of ozone generation being ambient air, it is the ultimate in sustainable and green chemical treatment. The current technology for producing ozone has benefited from more than 45 years of ongoing development, resulting in cost-effective and robust operation. Using little more than an oxygen separator, a corona discharge chamber and some compressors and other electrical components, on-site generation of ozone is relatively simple and safe. This is in sharp contrast to other treatments that are currently commercially available.
Because of the way ozone is produced, oxygen is necessarily going to be part of the treatment gas cocktail when using ozone. This is beneficial because oxygen is also an oxidizer. With an oxidation potential of 1.23 V, oxygen reacts slower than ozone but is an excellent complement.
Aside from its ability to assist in oxidation, its primary benefit is increasing the dissolved oxygen concentration of the wastewater, encouraging the growth of aerobic bacteria, which do not create compounds that are odorous, corrosive or otherwise harmful to collection systems. It also eliminates the ability of SRB to produce sulfides, either by removing the SRB entirely, or promoting the growth of aerobic species that will oxidize any sulfides before they are able to enter the wastewater stream.
Combined use of oxygen and ozone for treatment
In terms of a robust and green method of treatment and prevention of odour and corrosion in collections systems, the combined forces of oxygen and ozone are at the top of the list. Oxygen is widely available, making up roughly 21% of the atmosphere, and is easily converted to ozone. The generation and infusion of these two gases into wastewater collection systems has proven to be a clean, safe and cost-effective treatment.
The first method of action is the powerful destructive effects of ozone on H2S, quickly converting it to sulfites and sulfates on contact. In addition, ozone’s antimicrobial properties can help to reduce the presence of SRB and other microorganisms present on pipe walls. As a product of its reaction, oxygen is generated. This in turn adds more power to the oxygen portion of the treatment gas cocktail, which provides secondary treatment by significantly increasing dissolved oxygen, and allows for more complete utilization of infused treatment gases.
Because of these indiscriminate and powerful oxidizing characteristics, the concern is sometimes raised regarding the possibility of ozone attacking the wastewater infrastructure itself. This is unlikely to occur in applications, especially in wastewater where liquid phase infusion is implemented. This is due to the high ratio of liquid volume compared to pipe surface area per unit pipe length and the extreme availability of reactants in the liquid portion.
Paul Turgeon and Tonya Chandler are with Anue Water Technologies Inc. This article appears in ES&E Magazine’s December 2019 issue.