Ferric chloride for odour control: studies from wastewater treatment plants in India

Wastewater treatment plants (WWTPs) are integral in wastewater management and maintaining urban sanitation. A major issue encountered in WWTPs is foul odour emissions (Al-Shammiri 2004). Anaerobic microbial reactions in WWTPs and odorous volatile organic compounds (VOCs) produce hydrogen sulphide (Carrera et al. 2014), the target of this study. Hydrogen sulphide has a distinct, rotten egg smell and concentrations as low as 0.00047 mg/L can be detected (Crites & Tchobanoglous 1998) and it is produced by sulphate-reducing bacteria.

Hydrogen sulphide emissions at WWTPs can: (i) emanate foul odour; (ii) corrode exposed metal surfaces; (iii) lead to health issues among workers (Esswein et al. 2016), and (iv) cause fires if the gas comes into contact with strong oxidising agents (Nielsen et al. 1992). Strategies must be devised, therefore, to mitigate hydrogen sulphide emissions at WWTPs for both public health and safety. Odour neutralizers or masking agents are often used to neutralize odours, and industrial grade covers are used, in some cases, to prevent odour diffusion into the atmosphere (Ando 1980). These methods only mask the problem and do not treat the odour's source. Other technologies are available but are not economically feasible for use in developing countries. In addition to higher maintenance costs, biological odour control methods such as biofilters and bio-trickling filters have a large footprint and high water consumption, respectively (Estrada et al. 2011).

Previous studies have analysed the efficacy of chemical oxidants such as hydrogen peroxide, sodium hypochlorite and potassium permanganate in wastewater samples from WWTPs. Studies with hydrogen peroxide showed that 20–30 minutes of reaction time was required for H₂S control at 50% (v/v) concentration. However, there are safety issues with handling hydrogen peroxide at the concentration used (Thomas 2007). The concentration of H₂S in WWTP influents is directly related to the sulphide concentration in the wastewater (Al-Shammiri 2004), so the use of strategies to reduce the influent sulphide concentration at WWTPs is expected to lower hydrogen sulphide emissions proportionally.

Iron salts are commonly used both for phosphorus removal and as coagulants for removing suspended solids (Metcalf & Eddy 2003). The potential of sludge containing ferric and alum salts for hydrogen sulphide removal has been explored by Wang & Pei (2012). The sludge that they used contained twice as much iron as aluminium (w/w) and demonstrated significant H₂S removal at neutral pH. Previous studies have shown the use of ferric salts in sewer systems for sulphide and phosphate removal from wastewater (Gutierrez et al. 2010). However, this model might not be feasible in all developing countries since not all areas have a centralized sewer system. For example, raw sewage in most unsewered areas in Delhi is currently trapped from open drains, via the Interceptor Sewer Project, and diverted to the nearest WWTP.

The effectiveness of ferric chloride dosing on sewage sludge in controlling malodourous sulphur gas emissions was also reported by Devai & Delaune (2002). As far as is known, however, there are no current studies on the application of ferric chloride as a pre-treatment step in WWTPs for potential odour control in relation to sulphurous gases.

The major objective of this study was to explore the potential of ferric chloride dosing in WWTP inlets to reduce sulphide concentrations substantially in the presence of TSS and phosphorus. Significant sulphide and TSS removal at WWTP inlets can potentially mitigate odour nuisance due to hydrogen sulphide emission and reduce oxygen demand during further treatment in the plant.

1,000 mg of ACS reagent grade Iron (III) chloride hexahydrate (Sigma-Aldrich, Assay 97%, CAS No. 10025-77-1), molecular weight 270.30, was dissolved in 1,000 ml of deionized water (Milli-Q) to make the stock ferric chloride solution. Different concentrations of ferric chloride were prepared by further dilution of aliquots. Deionized water from Milli-Q was used throughout.

Samples of influent wastewater at WWTPs in Delhi were withdrawn and the sulphide concentrations determined by the iodometric method (APHA 1999). Influent wastewater samples were collected from the WWTPs at Kondli (phases II and IV), Chilla, Coronation Pillar (phases I, II and III), Yamuna Vihar (phases I, II and III), Okhla (phases I, II and III), Keshopur (phases I and II), Najafgarh, Nilothi (phases I and II), Pappankalan (phases I and II), Rithala (phase I and II) and Rohini.

The generation and emission of hydrogen sulphide in WWTPs depend on the wastewater's physical, chemical and biological constituents (Park et al. 2014). In addition to the problem of corrosion, hydrogen sulphide is a serious health and environmental hazard with high concentrations being fatal (Hvitved-Jacobsen et al. 2002; Nielsen et al. 2008). H₂S emission is related to the wastewater's free sulphide concentration (Al-Shammiri 2004). Typically, total sulphide concentrations in wastewater are between 0.1 and 10 mg/L (Henze & Comeau 2008), while sulphate ranges between 20 and 50 mg/L (Metcalf & Eddy 2003). Anaerobic regions in WWTPs, etc, facilitate the generation and emission of H₂S with the help of sulphate-reducing bacteria (SRBs). Maintaining the pH above the neutral range (the optimum pH for SRB growth is between 6 and 8) (Lopes 2007; Ayangbenro et al. 2018) and dissolved oxygen levels above 1 mg/L, can inhibit SRB growth and reduce foul odours at WWTPs.

A study undertaken at 11 WWTPs in Delhi, all of which used the activated sludge process, showed that most had odour nuisance problems due to H₂S emission. A few plants also reported equipment corrosion thought to be due to high sulphide levels. Influent sulphide levels were measured (June 2020) in the WWTP inlets, therefore, and the results are shown in Table 1. The sulphide levels varied between 1.1 and 14.8 mg/L. Total sulphide concentrations can vary through the year depending on both the influent wastewater's characteristics and external factors like temperature. Studies at the Ardiyah WWTP, Kuwait, which reported odour emissions, with measured annual median total sulphide levels in the raw influent of about 10 mg/L (Al-Shammiri 2004), consistent with the results obtained for this study.

If the influent wastewater sulphide can be precipitated at the WWTP inlet by pre-treatment, further wastewater treatment will be eased, H₂S emission during treatment reduced, and, potentially, treatment costs reduced if the installation of odour control equipment is not needed. Previous studies have shown the ability of soluble sulphides to react with Fe salts and precipitate as insoluble iron sulphide (Grady et al. 1999), and the use of Fe salts to minimize sulphide generation in gravity sewers was studied by Cao et al. (2019). The latter used Fe salt concentrations in the range 30–60 mg/L, with different dosing frequencies, in their study and found that the microbial diversity and SRB function in the sewer were altered.

Jar tests were carried out using raw wastewater from the Najafgarh drain, one of the major drains discharging into the River Yamuna. The sulphide concentration of the wastewater was 3.61 mg/L, and its TSS and BOD concentrations were 105 and 74 mg/L respectively (Table 2). Initial studies were carried out with 5, 10 and 15 mg/L FeCl₃ doses. While 25% of the phosphate – from 2 mg/L – was removed at all dosing concentrations, however, there was no significant removal of TSS, sulphide or BOD. Because of this, the dosing concentration was increased and 20, 30, 40 and 50 mg/L concentrations were used and determined.

Table 2 shows that increasing the ferric chloride concentration produces an increase in the proportional removal of sulphide – e.g. from 64 to 76% as the dosing concentration was raised from 20 to 50 mg/L respectively. The elimination of hydrogen sulphide from sewer systems using iron salts depends on the precipitation of the sulphide present, thus lowering the soluble sulphide concentration (Park et al. 2014). The presence of FeCl₃ has also been shown to inhibit SRB activity significantly in anaerobic zones (Zhang et al. 2009) and so reduce hydrogen sulphide emissions.

FeCl₃ dosing had almost no effect on removing ammoniacal nitrogen removal until 40 mg-FeCl₃/L was achieved. A further increase to 50 mg-FeCl₃/L removed 25% of the raw wastewater's ammonia. Increases in the ferric chloride dose led to increased TSS removal, nearly 70% TSS removal being observed at 50 mg-FeCl₃/L and natural pH. Other studies have shown that ferric chloride is most effective as a coagulant between pH 4 and 12, with maximum TSS removal at pH 4.9 (Reynolds & Richards 1996; Amokrane et al. 1997; Selcuk 2005; Aziz et al. 2007). Sarparastzadeh et al. (2007) showed a 48% reduction in TSS levels when raw wastewater was treated with an optimum ferric chloride dose of 70 mg/L. COD concentrations were determined and ∼35% removal was observed at 40 mg-FeCl₃/L dosing. Precipitation of soluble organic species with ferric chloride can be attributed as a factor in COD removal (Tebbutt 1998).

Increases in ferric chloride dosing also led to increased phosphorus removal from the wastewater. Proportional removal increased from 40 to 52% as dosing increased from 20 to 40 mg-FeCl₃/L. Park et al. (2014) reported precipitated phosphorus using iron salts. Iron salt addition to wastewater causes sulphide to precipitate as iron sulphide and the larger aggregates settle in the primary settling tank. Gutierrez et al. (2010) indicate the need for ferric chloride dosing at locations close to WWTPs to maximize phosphate precipitation in the aeration tanks. Similar trends were observed in BOD removal and are consistent with reports by Mostafa & Peters (2016) indicating a gradual improvement in BOD removal with increased ferric chloride dosing. The lowering of BOD concentrations may be due to the corresponding precipitation of phosphorus together with the coagulation-flocculation of suspended particles. The dosing concentration and frequency required depend on the wastewater's characteristics as the proteinaceous components of the organic matter affect iron species precipitation from wastewater (Kiilerich et al. 2017).

The jar test results indicate that a dosing concentration of 40 mg-FeCl₃/L can remove 76% of sulphide in addition to significant reductions in BOD and TSS levels. The main objective of this study was to remove sulphide from wastewater to reduce WWTP odour emission arising from hydrogen sulphide. Lowering of BOD in TSS concentrations would potentially simplify downstream aeration requirements during treatment as well as being cost-effective for odour control. Use of 30 mg-FeCl₃/L removes around 74% of influent sulphide and a 25% increase in dosing rate from 30 to 40 mg/L yields a 25% operating cost (chemical purchases) increase but only 2–3% sulphide removal improvement. Assuming that 40 mg-FeCl₃/L were used at a 10 MLD WWTP, 0.4 tonne/day of ferric chloride would be required at a cost of 10,000 INR/day (about USD 140/day). Dosing at 30 mg/L would lead to a saving, against 40 mg/L, of around 2,500 INR/day and can be taken into consideration as it can reduce the number of haulage journeys, materials handling required and so on. The long-term environmental impact must also be included when considering cost and benefits. Ferric chloride addition as a pre-treatment step is highly efficient in reducing odour and has low environmental impact. It can also improve the health of workers and local residents significantly.

• 1.

  Foul odours from WWTPs arise mainly from hydrogen sulphide emissions. Many WWTPs use odour masking agents or covers to deal with the issue.

• 2.

  Since hydrogen sulphide emissions relate directly to the presence of sulphide in wastewater, the feasibility of using ferric chloride to lower the wastewater sulphide concentration was studied.

• 3.

  Influent sulphide concentrations at 11 WWTPs in Delhi were measured and jar test studies performed on raw wastewaters. The results indicated that dosing 40 mg-FeCl₃/L led to significant proportional removal of sulphide, in addition to removal of TSS, BOD, phosphate and COD.

• 4.

  Ferric chloride dosing as a pre-treatment step at WWTP inlets can lower sulphide and other contaminant concentrations substantially, reducing downstream aeration requirements in treatment while providing cost-effective odour control.

All relevant data are included in the paper or its Supplementary Information.