Fast Growing City Extends Life of Sewer System

Phoenix, the second fastest growing city in the United States, lies in Arizona?s Sonora Desert with mild winters and hot summers. Rapid growth and urban sprawl over the past two decades have created a textbook scenario for sewer pipe corrosion and odors.

Recognizing the problems that lay ahead, city engineers contracted Montgomery Watson Engineering to conduct an ?Unlined Concrete Sewer Condition Assessment.? This assessment studied the condition of all unlined concrete pipes 24?90 inches in diameter. The objective was to locate, identify, and categorize by diameter the unlined system and make recommendations as to the best method of saving and extending the life of the pipe.

In its final report, Montgomery Watson recommended that the city begin an annual crown spray program using magnesium hydroxide slurry as the medium to control the increasing corrosion found in the system. The study concluded that the city of Phoenix and SROG (Sub-Regional Operating Group), which jointly own and operate the major trunk collection lines from five cities feeding into a jointly owned treatment plant, would save approximately $16.9 million over the next 20 years. This savings compares the cost of the entire crown spray program to rehabilitation cost of the unlined system.

Phoenix engineering contacted Premier Chemicals for information on the company?s Thioguard? system, a magnesium hydroxide based slurry, and to obtain a list of licensed applicators for the product.

The first contract was awarded to Hoffman Southwest ProPipe in June 1996. It was a one-year contract with two, one-year extensions possible. Subsequent contracts have been let each year. City project engineers from the Water Services Department have tracked the program on an annual basis.

?Corrosion stabilization is by far one of the most cost effective means of maintaining your concrete system,? said Robert Webb, Project Manager for Water Services. ?Saving the investment (an in-ground piping system) of decades ago with O&M dollars makes much more sense than investing capital improvement dollars through bond monies today. If you can stave off the expenditure of rehabilitation dollars for another 10 to 20 years or until a specific line segment is placed into a scheduled project you are way head of the money game.?

Premier Chemicals began its patented process of spraying the crown of concrete pipe and concrete structures in the early 1990s. More than 300 miles of pipelines are sprayed annually in the western states. Licensed applicators or licensed municipalities apply Thioguard via converted combination cleaning trucks. It is common for a crew of three or four men to spray more than 4,000 linear feet of pipe in a single workday. Most lines are sprayed during normal daytime flow conditions with the above water line being coated.

Thiobacillus (the bacteria that converts hydrogen sulfide gas to sulfuric acid) will colonize on a surface with a pH of approximately 8 or less. The sulfuric acid generated by this bacterium attacks the lime in concrete and corrodes the concrete surface or structure.

Thioguard, an alkaline slurry with a pH of 10 ? 10.5, has a proprietary additive that makes it adhere to the pipe surface. This alkaline coating neutralizes the acid present on the pipe surface and creates an environment on which Thiobacillus will not colonize.

The spraying device is pulled through the pipe on a flotation ?boat?. The spray heads are positioned so the slurry is applied from water line to water line. The pull rate and pumping rates are predetermined by pipe diameter to give a uniform desired thickness of applied material. Typically a 3,000-gallon, 10-wheel water truck is used to supply Thioguard to the spray truck. This keeps traffic control to a minimum and allows for easier movement through congested areas. The converted cleaning truck can be fitted with a smaller supply tank for easier application.

Licensed applicators have modified the spray trucks with a hydraulic hose reel capable of carrying 2,000 linear feet of 3/4-inch hose. The capability of long reaches being sprayed with one setup enables the crew to attain those 4,000-linear-foot spray days. A typical contract will consist of some hydro cleaning, CCTV, and crown spraying.

In most cases a single application will neutralize the acid attack for a full year, stopping corrosion and preventing further loss of pipe wall strength. Other benefits of using Thioguard in the collection system are the reduction of sulfides produced in the pipeline, which can greatly reduce odor problems.

Thioguard was used to control an H2S gas problem outside the newly constructed Bank One Ball Park home of the Arizona Diamond Backs in Phoenix.