Sewer pipes party: Sulfate is not invited

sewer pipes

Editor's Introduction

Reducing sewer corrosion through integrated urban water management

annotated by
Benay Akyon

Why are so many sewer pipes being replaced? Sewer corrosion is one of the main reasons and primarily, sulfate is to blame. In this study, scientists investigated the main source of sulfate in our sewer systems. Does the majority of sulfate come from the nature or is it caused by human interference?

Paper Details

Original title
Reducing sewer corrosion through integrated urban water management
Authors
Ilje Pikaar Keshab R. Sharma Shihu Hu Wolfgang Gernjak Jürg Keller Zhiguo Yuan
Original publication date
Reference
Vol. 345 no. 6198 pp. 812-814
Issue name
Science
DOI
10.1126/science.1251418

Abstract

Sewer systems are among the most critical infrastructure assets for modern urban societies and provide essential human health protection. Sulfide-induced concrete sewer corrosion costs billions of dollars annually and has been identified as a main cause of global sewer deterioration. We performed a 2-year sampling campaign in South East Queensland (Australia), an extensive industry survey across Australia, and a comprehensive model-based scenario analysis of the various sources of sulfide. Aluminum sulfate addition during drinking water production contributes substantially to the sulfate load in sewage and indirectly serves as the primary source of sulfide. This unintended consequence of urban water management structures could be avoided by switching to sulfate-free coagulants, with no or only marginal additional expenses compared with the large potential savings in sewer corrosion costs.

Report

Urban sewer networks collect and transport domestic and industrial wastewaters through underground pipelines to wastewater treatment plants for pollutant removal before environmental discharge. They protect our urban society against sewage-borne diseases, unhygienic conditions, and noxious odors and so allow us to live in ever larger and more densely populated cities. Today's underground sewer infrastructure is the result of an enormous investment over the last 100+ years with, for example, an estimated asset value of one trillion dollars in the USA (1). This equates to ~7% of its current gross domestic product. However, these assets are under serious threat with an estimated annual asset loss of around $14 billion in the United States alone (1). Sulfide-induced concrete corrosion is recognized as a main cause of sewer deterioration in most cases (2).

Under the anaerobic conditions common in sewage, sulfate is reduced to hydrogen sulfide by sulfate-reducing bacteria (3). The emission of hydrogen sulfide in gravity sewer sections, sewage pumping stations, and inlet structures of wastewater treatment plants induces sulfide oxidation to form corrosive sulfuric acid on concrete surfaces exposed to air (45). The presence of sulfate in wastewater and its conversion to sulfide in anaerobic sewers are generally considered unavoidable, and water utilities around the world have been focusing on the removal of sulfide after its formation (35-7), incurring mitigation costs comparable to the value of asset losses (3).

Sulfate in sewage can originate from three potential sources, namely, sulfate in source water used for drinking water production, sulfate added as the counter ions of aluminum or iron salts used as coagulants in the water treatment processes, and human and/or industrial wastes discharged. Coagulation plays an important role in drinking water treatment (89). It removes (colloidal) solids and natural organic matter (NOM), which can compromise disinfection and act as precursors for disinfection by-products. Sulfate- and chloride-based aluminum (Al) or iron (Fe) salts are most commonly used.

To reveal the relative contribution of each of the three sources, we conducted an extensive measurement campaign over 2 years (January 2009 to December 2010), during which we monitored sulfate levels in raw source water, drinking water, and sewage in a suburban area in South East Queensland, Australia (10), where aluminum sulfate is used as coagulant in the drinking water production. Sulfate concentrations in the source water supplying a water treatment plant, produced water from the plant, and sewage from a sewage pumping station collecting freshly discharged sewage from ~2900 households supplied by the plant were analyzed weekly. We subsequently conducted an extensive industry survey in Australia, comparing sulfate data in drinking water with and without sulfate addition during water treatment.

As much as 52% of the sulfate present in the sewage of the monitored area is contributed by the addition of aluminum sulfate as coagulant in the drinking water production, with a net contribution of 9.2 mg S/liter (Fig. 1 and fig. S1). This result is supported by the national industry survey (table S1). Among the 77 plants surveyed, 51 plants provided sulfate data in both the source and produced waters. The sulfate levels in the source and product waters are comparable in the 17 plants that do not use sulfate-containing coagulants (Fig. 2). In the remaining 34 plants, where sulfate-based coagulants are added, the average sulfate concentration in the produced drinking water is approximately four times that in the source water (Fig. 2). The average sulfate addition in the 34 plants is 10.1 mg S/liter, comparable to the 9.2 mg S/liter increase measured in our sampling campaign (Fig. 1). These results reveal the substantial contribution of sulfate-based coagulants to sulfate levels in drinking water. The dosing rates (1.9 to 20.3 mg S/liter) are in agreement with the practice in other countries (table S2) and are also consistent with the theoretical demand. The coagulant dosing rate is typically determined by the NOM concentration in the source water, with a theoretical dosing ratio of 1.0 mg Al/mg C (11). The NOM levels in source waters typically range between ~3 and 15 mg C/liter (12-14), which would lead to a sulfate addition of 5 to 27 mg S/liter.

g1.jpg
Fig. 1.  Sulfate concentrations in source water, drinking water, and sewage in the sampled suburban area in 2009-2010.  The sample sizes for the source water, drinking water, and sewage are 101, 104, and 92, respectively. Presented data are means ± SEM. The inset shows the relative contributions of the three sources.
Sulfate concentrations

Figure 1 is composed of a bar chart and an inset pie chart. It shows two different representation/information from the same data collection process when they monitored the sulfate concentrations in source water, drinking water, and sewage.

y-axis

On the y-axis, sulfate concentration is shown. 

x-axis

The x-axis contains the three data sources: source water, drinking water, and sewage.

Ideal water cycle

Figure 1 draws the ideal water cycle by assuming a fixed sulfate concentration (no loss in the process) during the water cycle. It shows the increase in the sulfate concentration as water is collected from the source until its disposal as sewage.

If the total sulfate concentration in sewage (approximately 17.5 mgS/L as seen from the Figure) is 100%, the contribution of coagulant was calculated as 52% as in the inset pie chart.

g2.jpg
Fig. 2.  Sulfate concentrations in source water and drinking water. (Inset) Average sulfate concentrations in source and drinking waters with and without aluminum sulfate addition in the treatment process. Presented data are means ± SEM, except for those under the asterisk (*), which show the mean, maximum, and minimum values. In the cases marked by the asterisk, data were provided in the form of mean, maximum, and minimum values.
x- and y-axis

Figure 2 is a bar graph. It shows sulfate concentration on the y-axis and two different scenarios on the x-axis:

1) Without aluminum sulfate dosing during drinking water treatment.

2) With aluminum sulfate dosing during drinking water treatment.

Inset figure

The inset figure is drawn by taking the averages of source and drinking water values in two different scenarios. 

Forty-three out of the 77 plants surveyed (56%) use aluminum sulfate as the coagulant, which confirmed its widespread application. This result is consistent with reports from other countries such as the United States, China, India, and the United Kingdom (table S2).

There is evidence that sewers are more seriously corroded in cities and regions with higher sulfate concentrations in drinking water (fig. S2). To assess the impact that sulfate-based coagulants can have on sewer corrosion, we performed extensive simulation studies using a virtual sewer network with average characteristics (1015). To enable a robust and equitable comparison between the different simulation cases, we implemented chemical dosing in the model (16). Through the addition of ferric chloride, a commonly used chemical for corrosion mitigation (7), the dissolved sulfide concentration was maintained at low levels (mostly below 0.5 mg S/liter) in all cases. With the addition of sulfate at 5 to 15 mg S/liter in the drinking treatment, the predicted costs for sulfide mitigation in sewers increased by ~30 to 50% compared with the case without sulfate addition (Fig. 3). We performed 72 additional simulation runs with the source water sulfate concentration, hydraulic retention time, rising main fraction, and sewage temperature systematically varied within their typical ranges (10). The results reveal that substantial cost savings can be achieved by avoiding the use of sulfate-based coagulants in all cases except when the source water already contains a high level of sulfate (e.g., >10 mg S/liter) (fig. S4A). Sulfate concentration in source waters is dependent on geological conditions and varies substantially between regions. It varies between ~1 and 14 mg S/liter with a mean value of 3.4 mg S/liter and a median value of 2.2 mg S/liter in the surveyed Australian cities (Fig. 2). Wide concentration ranges have also been reported for other parts of the world from <1 to >300 mg S/liter, with typical values of 5 to 7 mg S/liter (1718). These results imply that the majority of source waters have a sulfate concentration that is below the threshold values (10 to 15 mg S/liter) revealed by our simulation studies.

g3.jpg
Fig. 3. Predicted impact of the use of sulfate-based coagulants in drinking water treatment on sewer corrosion mitigation for average sewer and sewage conditions. The relative sulfide mitigation cost increases were calculated by using the case without sulfate addition as the reference.
Two y-axes

Figure 3 is composed of two y-axes:

1) The left one shows the cost as $ per m3 of water treated and is to be used for the blue-colored data.

2) The right one shows the % increase in cost and is to be used for the red-colored data.

Reference for increase in cost

Because the point of reference for increase in cost is "no alum dosing," % increase in cost starts from zero even though it in fact costs a certain amount of money.

In other words, the authors assume the increase in cost for "no alum dosing" is zero and calculate the rest according to:

(X-Y)/Y*100

where X=cost with alum dosing, Y=cost of no alum 

Our results highlight the benefits that could be attained by replacing sulfate-based coagulants with alternative, non-sulfate-based coagulants. Ferric chloride and polyaluminum chloride (PAC) are similarly effective coagulants and readily available and are indeed already used by some water utilities worldwide (19). The changeover costs are generally very low compared with the downstream saving potential, which may help water utilities, who are facing large, and still escalating, expenditures for the protection and rehabilitation of sewer assets (2021).

In cases where source waters contain high levels of sulfate (e.g., > 10 to 15 mg S/liter), proactive sulfate removal could be considered. Nanofiltration or reverse osmosis, for example, typically removes 95% to >99% of sulfate but is associated with high capital and operational costs. Simulation results show, however, that substantial savings can be achieved in corrosion management by removing sulfate from source water (fig. S5), which could largely offset the cost for membrane filtration. For example, the addition of a nanofiltration step in a French water treatment plant resulted in an increase of only €0.045/m3 in its operational costs (22). The filtration process also removes NOM, which would generate additional benefits such as the reduction of disinfection by-products, micropollutants, and microbial hazards, all of which are key considerations for public health protection (2324). Similar benefits can be achieved by the use of "climate-independent" water sources, such as desalination or potable water reuse that typically incorporate similar membrane filtration processes. However, such advanced treatment methods are more energy- and cost-intensive compared with conventional drinking water treatment (25). Their potential application should be carefully assessed from a health, economic, and environmental perspective.

Many water utilities will need to upgrade both their water supply and wastewater service infrastructure over the next 10 to 15 years, which will require enormous capital investments (212326). Our findings show that there are critically important connections between these two seemingly independent systems. Although integrated urban water management and total water-cycle planning are firmly anchored in many policies developed by various levels of governments, in reality, individual subsystems—such as drinking water production and sewer and/or wastewater management—are often considered separately and optimized to generate locally maximized benefits (or least cost) without taking into account the existing connections across the water cycle. Our results show that such a disconnected management strategy can generate negative impacts in other parts of the urban water infrastructure. Although the technical challenges of reducing sulfate in sewage seem simple, institutional barriers may prevent a shift toward a whole-of-water-cycle optimization strategy. Given the increasing complexity and interdependence of the urban water systems, including recent trends in local water reuse, water-sensitive urban design, and low-impact development, the development and consistent application of novel, system-wide integration tools are essential to an overall optimization strategy. Such advances will be crucial to encourage the water industry to fully embrace integrated urban water management processes and to reap the benefits of this valuable concept.

Supplementary Materials

www.sciencemag.org/content/345/6198/812/suppl/DC1

Materials and Methods

Figs. S1 to S5

Tables S1 and S2

References (27-70)

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