Imagine you are the official in charge of sewers in a particular area. As "sewer commissioner," you are responsible for maintaining, repairing and replacing the miles and miles of sewer pipe under your authority.
Sounds like a powerful position to have, right? Not so fast.
Your colleague is the commissioner of water. His job is to provide citizens with fresh water for drinking, filling their swimming pools, washing their cars, etc. One thing he believes is vital for his job is keep the price of water as low as possible — it's a political thing.
To keep costs down, the commissioner of water insists on using a chemical called aluminum sulfate as the coagulant that aids water purification. However, a side effect of aluminum sulfate is hastening the corrosion of the sewer commissioner's concrete sewer pipes.
These two commissioners have different driving interests:
- The sewer commissioner wants his wastewater concrete pipes to last as long as possible.
- The water commissioner wants the cost of drinking water as cheap as possible; aluminum sulfate is the cheapest coagulant on the market.
But this scenario is no mere pipe dream. This give and take is happening all across the globe.
The science
The process begins at the water treatment facility where aluminum sulfate is added to the water supply.
Aluminum sulfate has positively charged aluminum molecules that attract organic and solid particles found in source water. When the molecules meet and attach, they weigh more and settle out, making them easier for removal during the filtration process.
However, the sulfate ions stay in the water. When they arrive at the end of their trip to the wastewater treatment plant, the paucity of oxygen in sewage causes toxic hydrogen sulfide gas (H2S). Expose the hydrogen sulfide gas to microbes on the walls of the sewers, and it becomes highly corrosive sulfuric acid (H2SO4).
The ongoing corrosion, cuts the life of a sewer pipe by as much as 90 percent. This means that a pipe with an anticipated life of 100 might only last 10 years.
An article in the August 15 edition of Science entitled "Reducing sewer corrosion through integrated urban water management" makes a strong case based on evidence-based research that sewer infrastructure life can extend by the surprising simple solution of using a sulfate-free coagulant.
The solution
The present system of most cities keeps water segregated into two components: potable water and wastewater. The purposes that each bureaucracy serves have different end goals. Combining their functions, or at least insisting on joint planning between the two, would likely result in movements to replace the contamination and not the concrete pipes.
After all, sewer systems rank as one of the most important infrastructure components of advanced urban society. They are indispensable for protecting the health of people. The costs of replacing corroded and broken sewer pipes are in the billions of dollars annually.
Changing from aluminum sulphite to another clumping medium will not end sewer corrosion. But it will delay it enough to extend sewer life. The money saved can go to added infrastructure needs. If the water system is an integrated one, the reduced cost of pipe replacement and emergency maintenance could be used to partly offset the minor increase in costs for potable water treatment.
It simply makes no sense that one end of the water system is managed by a different agency than the other end.
As shown in Australia, a switch to nonsulfate coagulants can be done easily. In only 10 hours, concrete corrosion decreases by 35 percent. If a different coagulate is continually used over time, the corrosion decreases by 60 percent.
The effect on industry
Many industries use city water for industrial purposes. However, they treat wastewater before they discharge it. Sometimes the runs are short and easy to get to, sometimes not so much. In many areas of the United States, property owners are responsible for the sewer pipe from the property to the main. In other words, this problem affects manufacturers, and not just homeowners.
To date, there has been little done toward urban water system integration. The potential savings is enormous. Unfortunately, it seems too technical for getting the public worked up about, so for now, it will likely be the same old, same old.
