1. Introduction
Hydrogen sulfide,
H2S, is a gas generated in wastewater due to the reduction of sulfate to sulfide, as a result of the sulfate reducing bacteria (SRB) in anaerobic sewer biofilms. In the sulfate reduction process sulfides are formed, consisting of
H2S,
HS−, and
S2−, where the distribution between these sulfide species depends on the pH [
1,
2]. Hydrogen sulfide has a low solubility in water, and turbulence causes it to be released into the gas phase of the pipes at concentrations that may be harmful to people working in the sewage systems. In addition, this gas causes significant complaints of bad odors when it reaches street level. Another important problem is due to the formation of sulfuric acid, which favors the degradation of concrete pipes by neutralizing the alkalinity of the concrete and by the formation of expansive salts with no binding properties, which can cause internal cracking at the concrete interface. Moreover, iron corrosion occurs in reinforced concrete pipes.
Figure 1 shows a pipe suffering from this degradation phenomenon in the city of Murcia. Once anaerobic conditions (<0.1 mg/L of dissolved oxygen) are reached inside the pipe, some of the main factors influencing the formation of sulfides in wastewater as it passes through the sewer networks are the following: (1) organic load transported by the water in dry weather,
BOD5; (2) wastewater temperature; (3) hydraulic retention time (HRT); and (4) hydraulic conditions of flow, such as velocity, wetted perimeter, or percentage of filling of the pipe [
3,
4,
5].
Hydrogen sulfide generated in the sewer networks induces corrosion, which is the main cause of deterioration of concrete pipes today [
1,
6,
7]. The costs associated with the replacement of corroded pipes and the application of corrosion inhibitors amount to billions of dollars annually [
6]. In March 2000, the annual total cost attributed to corrosion in sewer systems of the United States was estimated to be
$13.75 billion [
6,
8]. The water and sanitation administrations and utilities are aware and concerned about the deterioration of these assets and are committed to intensifying inspections in the sanitation networks and in establishing prioritization in the renewal of pipes based on their state of corrosion and mechanical deterioration [
9,
10,
11,
12]. Techniques to protect pipes through inner coatings of the in-sewers are particularly important [
13]. In the case of the Municipal Sanitation Utility of Murcia (EMUASA), maps are prepared based on CCTV inspections of the pipes where mechanical deterioration is observed. Such information is used in the process of prioritizing pipeline renewal (
Figure 2).
Mechanical deterioration refers to the loss of wall thickness, due to biochemical corrosion, to any crack that could be due to external traffic load or any other cause of deterioration that could be observed thorough visual inspection.
Thus, knowing the concentration of
H2S inside sewerage networks, as well as its control and reduction, has become one of the main concerns in sewerage system management nowadays [
14]. Online and continuous monitoring of
H2S concentrations in sewers has been demonstrated as a useful technique in understanding and slowing the deterioration of concrete pipes [
15,
16,
17]. The dosing of chemicals into the sewerage network to reduce the
H2S concentration is a common practice, especially during the summer periods and in coastal areas. Continuous monitoring has shown the possibility of optimizing and reducing the chemicals used in sewers [
18].
Modeling sulfide dynamics over time in the water and air phases in sewer networks is of great importance to understand corrosion processes. Deterministic simulation models are used in order to predict
H2S concentrations, dissolved and in the gas phase inside the pipe. Such models can be based on the mass balance equations considering kinetics and stoichiometric of the processes, and require knowledge of a high number of parameters, such as the WATS model (2013, HV-Consult ApS, Aalborg, Denmark) [
3,
19,
20]. However, empirical models, with a lower number of parameters, also propose a balance based on various factors, such as the organic load and the hydraulic conditions of flow, reaching, in some cases, a sufficient approximation to values measured in the field [
21,
22,
23,
24,
25,
26,
27,
28,
29]. Three principal processes occur in the sewers and must be simulated: (1) sulfide generated in the biofilm in the contact area between the wastewater and the concrete pipe; (2) emission to the sewer gas phase inside the pipe; and (3) absorption of hydrogen sulfide through the concrete pipe.
Regarding empirical models, Pomeroy and Parkhurst [
22] presented an empirical equation for sulfide generation based on biochemical oxygen demand at five days. Boon and Lister [
23] proposed a similar generation equation based on the chemical organic demand (COD). Elmaleh et al. [
24] showed the production of sulfide in the pipeline from an installation of 61 km in length that transported recycled urban water by gravity and in full section. Yongsiri et al. [
25,
26] proposed a model of
H2S emissions in gravity pipelines considering two phases (liquid and air) with generation and emission. Lahav et al. [
27] proposed an approximation of
H2S emissions through a first order kinetic equation based on laboratory tests, using flocculation equipment to provide different degrees of turbulence to the experiments. Other authors such as [
28,
29] considered an extension of the WATS model, proposing different equations for each of the processes that generate the sulfur cycle in a sanitation system. Nielsen et al. [
30] presented a hydraulic model that analyzes the generation and emission from equations previously developed by [
22] and [
3] to study the influence of chemical dosing. Recently, studies of
H2S gas modeling with WATS have been presented by Vollertsen et al., 2015 [
31], for a large area of the city of San Francisco. Matias et al. [
32] modeled the Ericeira sewer system in Portugal, using WATS and AEROSEPT models, and adequately predicted the overall behavior of the system. Nowadays, several commercial software packages are available for modeling sulfide concentrations [
19,
33,
34].
Several researchers have studied the influence of sulfide corrosion on the life-cycle of the pipes [
35]. For instance, Teplý et al. [
36] proposed an approach to calculate a service-life prognosis that considers the influence of microbiologically induced corrosion (MIC) on bearing capacity. When corrosion is sufficiently advanced, it leads to diminished service life or structural failures. Zamanian et al. [
37] used computational numerical models to study the effects of a number of key modeling and construction factors on the leakage susceptibility of buried concrete sewer pipes. These factors included the loss of pipe wall due to the corrosion process as a function of time. The sensitivity analysis carried out by Mahmoodian and Li [
38] showed that the supplied concentration of sulfide and the relative depth of the fluid have significant effects on the service life of concrete sewer pipes. Ahammed and Melchers [
39] proposed a corrosion model to calculate the loss of pipe wall thickness with time. They used 20 independent random variables that included the geometric characteristics of pipes (including the radius and the wall thickness, among others), mechanical and load characteristics (such as the modulus of elasticity Poisson’s ratio and traffic loads), ditch construction characteristics (external soil and width, etc.), and temperature differential among others. Once the concentration of hydrogen sulfide in a sewer network can be predicted and the loss of wall thickness through corrosion can be calculated, this will lead to being able to establish the level of service of the pipes and this would make it possible to plan the renewal and management of assets in a more sustainable way.
In the present work, a dynamic model (called EMU-SANETSUL) was developed to calculate the
H2S concentration in both the water and air phases of the main sewers of the city of Murcia. This wastewater network mainly contains domestic and commercial sanitary sewage. The model is calibrated with data from field measurements. One-dimensional time-variation hydraulic variables and
H2S reactions are solved using an explicit discretization technique called the Discrete Volume Element Method (DVEM) [
40,
41,
42]. That scheme is based on a mass-balance relation within pipes that considers both advective transport as well as reaction kinetics of sulfides in the water and air phases. The reaction uses empirical equations included in [
21]. For this purpose, we start from an existing calibrated hydraulic model of the Municipal Water and Sanitation Company of Murcia (EMUASA) [
43,
44]. From the simulated
H2S concentrations in the gas phase inside the pipe of the main sewer network of the city of Murcia, this study provides a map showing the sections with higher sulfide concentrations. This information, together with the mechanical wear inventory, provides valuable information for the asset management of sewers, helping in the planning and prioritization of pipe renovation and rehabilitation. The objective of the present work is to provide information on the
H2S concentration in the air phase of the sewers. This information may serve to predict the pipes with high risk of corrosion and it will help in decision-making in asset management as one more variable.
4. Conclusions
A dynamic model (EMU-SANETSUL) to calculate hydrogen sulfide (
H2S) concentration in both the water and the gas phases of the main sewers of the city of Murcia was proposed. The model was calibrated with data from field measurements from both the gas phase and wastewater along the network. Samples were taken from the network from 2016 to 2019. The code was written in MATLAB
® and uses an explicit discretization technique called the Discrete Volume Element Method (DVEM) [
40,
41,
42], where reactions are calculated using empirical equations as proposed by [
21] expressed in a finite difference scheme. Results from an existing calibrated hydraulic model available from the Municipal Water and Sanitation Company of Murcia (EMUASA) [
43,
44] were used as starting information. Daily
BOD5 and temperature curves of the network were also input data for the code. The calibration process consisted in the variation of parameters
M and
fp, where
m was set to 0.7. Since
fp was in the range from 0.96 to 0.99, the sulfide generation flow coefficient due to the biofilm
M was the principal calibration parameter adopting values in the range from 1 to 6 × 10
−3 with an average value of 3 × 10
−3. Simulations were undertaken adopting a constant value of the parameters for all the pipes. In the seven calibration nodes and during nine different time periods, acceptable fit to the observed data was achieved with accuracy indexes such as
AI and
Er in the ranges of 40% and 15%, respectively. This model is considered as the starting point work for the modeling of
H2S in the gas phase inside a sewer network. For future work, it is recommended to increase the amount of observed data to increase the number of calibrated nodes. Furthermore, future work could use different calibration parameters for each of the links, according to different pipeline conditions, such as their age or mechanical wear. Although several previous works in the literature provide the range of the parameters with some approximation and enable simulations to be performed, achieving an accurate model of the network requires a field campaign to measure the calibration parameters.
The pipes with the higher values of H2S—100–200 ppm—are found in areas presenting some of these features:
Pipes located downstream of a pump station;
Overloaded pipes where the gas phase represents a low surface;
Higher values of slope or a combination of these.
From the simulation results, a map of the annual average
H2S concentration in the gas phase of each pipe was created as a tool that can contribute to asset management. Information on sections with the highest annual average concentrations were obtained, which could be considered in a prioritizing matrix for the decision of the rehabilitation, coating, or renewal of pipes. The calculated annual concentrations were compared with the map of mechanical deterioration of pipes obtained from CCTV. Mechanical deterioration of the pipes has its origin in diverse causes, not only corrosion from
H2S, although sections where high sulfide concentrations coincide with a high degree of mechanical wear could be prioritized for rehabilitation. The sulfide modeling provides valuable information that can be used for better asset management of the sewer network. The
H2S concentration enables the loss of thickness of the wall’s pipes along the time to be calculated, which is a factor to include in any life-cycle analysis of the pipes in conjunction with some other factors [
38,
39].