Odour control: A successful experience in sorocaba city – Brazil

A number of countries have reportedan increase in nuisance complaints related to wastewater sources. In many areas, this may be the result of increased residential developments near wastewater treatment facilities. This has led to increased interest in the development of effective odours regulations or policies and regulating odours can be a difficult task because of their subjective nature.

Odours can be released at all phases of wastewater collection, treatment and disposal. While past odour control efforts by municipalities have typically focused on wastewater treatment plants, there has recently been more attention directed toward odour emissions from collection systems and pumping stations.

The city of Sorocaba, located in São Paulo state, has a population of 652,500 inhabitants (2016 Census) and SAAE is the municipal sanitation Company responsible for treating water and wastewater for the city and surrounding areas. Currently Sorocaba city has around 1,030 kilometres of collection system, 10 wastewater treatment plants and 22 pumping stations, achieving 98% of collected and treated sewage.Due to a significant increase of the collection system, in 2012 some complaints about bad odours were reported that led company to seek a new solution.

Odours from wastewater and its residuals become significantly more intense and develop much higher concentrations of odorous compounds when oxygen in the waste is consumed and anaerobic conditions develop. For this reason, much of discussions and issues related to odour generation focus on anaerobic conditions that can be developed in collection system upstream ofthe wastewater treatment plant and unit process such as primary clarifiers, gravity thickeners and sand removal chambers.

In the case of domestic sewage, the anaerobic decomposition of the protein content in organic matter is the main cause of the generation of odours (Chernicharo, Stuetz Souza & Melo 2010).

In sewage collection and interception systems, most of the generated sulfide occurs in the biofilm layer attached to the pipe walls or to sludge deposits in the lower part of the pipes (WEF 2004). Table 1 shows typical sulfide levels for different domestic wastewater treatment units.

In collection systems, most of the sulfide generation occurs in the slime layer on the pipe wall or in sludge deposits on the pipe invert. The contribution of sulfide from the liquid is relatively small. Hydrogen sulfide is often released into the gas phase when dissolved sulfide is present. sulfide is generated within the slime layer attached to sewer pipe walls because of anaerobicbiofilm activity. The bacteria responsible primarily for sulfide formation, ‘sulfate-reducing bacteria,’ use sulfate (SO₄⁻²) as their terminal electron acceptor, reducing it to sulfide (S⁻²). The sulfide then diffuses through the biofilm to the bulk liquid phase. Once in the liquidstream, H₂S can volatilize into the gas phase, where it can become an odour and potentialcorrosion problem. Figure 1 shows a schematic drawing of such processes (WERF 1995).

The key parameters that affect sulfide generation are:

• Concentration of organic material and nutrients;

• Temperature;

• Retention time;

• Sulfate concentration;

• Dissolved oxygen.

The pH of wastewater has an important role in determining the amount of molecular H₂S gas available to be released to the sewer atmosphere. It is only molecular H₂S that will lead to odour problems. Figure 2 shows this relationship. At pH 7.0, approximately 50% of the H₂S is in this form. At pH 6.0, over 90% of dissolved sulfide is present as dissolved gas. At pH 8.0, less than 10% is available as gas for release from wastewater. Therefore, a decrease of one pH unit in wastewater can significantly increase the release of H₂S gas, potentially causing odour and corrosion problems. So, acidic conditions will enhance H₂S odour problems, and basic conditions will suppress them (Gostelow et al. 2001).

Another major factor affecting H₂S release is turbulence. High levels of turbulence can dramatically increase H₂S emissions in drop manholes, junction stations, etc. Figure 3 shows the partial pressure of H₂S gas as a function of temperature for a range of concentrations of dissolved H₂S. At 20 °C, 3.0 mg/I of dissolved H₂S will be in equilibrium with approximately 780 ppm by volume of gaseous H₂S. Increasing the temperature decreases the solubility of the gas, and consequently gas more will be present in the atmosphere.

There are several different types of controlmeasures that can be applied to treat sulfide and other odorous compounds in the liquid phase before they can become a nuisance. One of the best cost benefit measures consists in the use of chemicals to control sulfides in either of two ways:

• 1.

  By reacting with any sulfide already present to the stream to prevent the escape of H₂S into the air. A chemical applied for this purpose may function in one of these three ways:

  • a.

    Oxidize the sulphide to sulphate (or other intermediate oxidation products);

  • b.

    Convert dissolved sulfide to an inert metallic sulfide;

  • c.

    Convert H₂S to HS⁻.

• 2.

  By killing the sulfide-producing bacteria or by so altering the environment to which the slime layer on the pipe wall is exposed so that it will not produce sulfide. The ways to do this include the adding of an oxidizing agent that raises the oxidation-reduction potential to a level where sulfate reduction is inhibited, or by adding a toxic substance that either destroys the slime layer or temporarily suppresses the activity of the sulfide-producing bacteria (USEPA 1974).

A disadvantage of such process is that when chlorine reacts with certain organic components in water or wastewater, chlorinated organics are formed, like chloroform and chlorophenols, considering potentially toxic and carcinogenic. Many of these compounds are also strictly regulated in many countries.

Peroxide is fast acting, which makes it useful for addition immediately upstream of problematic locations. However, it is also quickly consumed, so multiple injection sites and relatively high doses are required to treat long reaches of collection system.

Several other reactions in between these two may take place to produce elemental sulfur, sulfate and other compounds, depending on the local wastewater chemistry. Manganese dioxide (MnO₂) is produced as a byproduct, which is a fluffy, brown floc that is practically nonreactive and settles as a chemical solid and will slightly increase solids production.

Ozone is an extremely powerful oxidant that can oxidize H₂S to elemental sulfur. It is also an effective disinfectant when bacteria levels are low. Although ozone reacts with practically everything in wastewater, including dissolved sulfide, its principal usage has been to treat odorous gas stream. Ozone is unstable and must be generated on site immediately prior to its application. It is also potentially toxic to humans if inhaled continuously for extended time periods in closed rooms at concentrations of 1.0 ppm or greater in air.

Iron metals can chemically combine with dissolved sulfide to form relatively insoluble precipitates. The iron salt precipitates are in the form of blacked or reddish-brown flocs that do not deposit in the collection system, but readily settle with other solids at the treatment plant. Iron salts are in widespread use in collection system throughout the United States and Brazil.

Iron salts act fast and are often applied just upstream of a treatment plant to remove sulfide before the headworks facilities. Greater benefits in odour and corrosion control are obtained when iron is added upstream in a collection system. Iron salts do not react with organic material in wastewater, so they can be overdosed at one upstream location to treat long reaches of collection system. The iron precipitate settles rapidly in a quiescent basin, but in the collection system it remains suspended in the flow and does not form deposits. The iron precipitates add to the overall solids production at the treatment plant and the volume is dependent on the amount of sulfide treated. Even in systems with high sulfide concentrations, the added solids are typically less than 5% of the overall solids. Iron salts can also be dosed at wastewater treatment plant before UASB (Upflow Anaerobic Sludge Blanket) reactors, achieving good results in terms of H₂S level reductions without any damage to the methanogenic activity. This is a great advantage when comparing such approach with using oxidizing agents, for instance.

The identification of the H₂S levels and odours complaints was carried out in a full-scale pumping station (PS #08) located in the East Region of Sorocaba (SP, Brazil). Through this pumping station the average wastewater flow is 1,440 m³/h, collecting sewage from approximately 200,000 inhabitants.

The main avenue of the city is located next to this pumping station, as well as a cycle path where many people pass daily.

In order to identify and quantify the H₂S levels before start dosing any kind of chemical, an electronic sensor (‘nose’) was installed.

H₂S levels were monitoring during the period March-April 2013 using an electronic sensor whosemeasurement interval is fixed at every five minutes and all data were transmitted to a server (using a GPRS SIM card) every two hours, which was able to create a good monitoring system. All technical data of the electronic device are described below in Table 2.

After monitored H₂S levels during the period March-April 2013, it was possible to evaluate and understand its behavior throughout the day. Other important factors were also considered to define the best solution for this application, such as the possibility of local manufacture of the application product and a good cost-benefit ratio. A blend of iron salt – chloride based – was selected and produced according to basic specifications, shown in Table 3.

Figure 4 shows that during the first period (March-April, 2013) of full scale trials, H₂S levels were reduced in 60% on an average, considering both the variation in the dosage and the operating time were variable.

The dosage was changed from 10 mg/L to 40 mg/L (as Fe) during one week (7 days) and best results were achieved with 30 mg/L (Refer to Figure 5).

Based on the results achieved, new trials were conducted during a long period, thus optimizing the application of iron salts along 12 hours per day.

At this time H₂S levels were reduced by 83%, which was considered adequate for mitigating complaints around the target pump station area.

Sulfide precipitation using iron salts has several advantages, such as: very fast reaction, high residualscan be maintained to precipitate sulfide as it is generated, resulting in sulfide control oververy long collection system reaches; iron salts can be used to treat sludge; reaction byproducts are innocuous; and precipitates are beneficial to downstream treatment processes.

However, iron salt solutions are corrosive and have a low pH, so special precautions must be taken in handling and storing iron salt solutions. As iron salt applications in general demand relatively high dosages – considering long pipingreaches to be treated – there should be an adequate wastewater flow at all times at the injection point to dilute the chemical and avoid corrosion problems.

Results obtained by two full scale trials showed the high efficiency of an iron salt blend in greatly reducing H₂S levels in the air and consequently greatly reducing complaints about bad odours. After that a permanent installation (storage tank, hose pumps and a complete dosing system) was implemented. According to our client, odour control has been effective with it.