CONTROLLING SEWER CROWN CORROSION USING THE CROWN SPRAY PROCESS WITH MAGNESIUM HYDROXIDE

The County Sanitation Districts of Los Angeles County (Districts) developed the crown spray process in 1989 to neutralize the acid formed by sulfur oxidizing bacteria (SOB) and control sulfide corrosion. The purpose of crown spraying the sewer is to neutralize the acid formed on the crown with a high pH material and to leave a residual alkalinity on the sewer to neutralize the acid produced as the SOB recolonize the sewer. By neutralizing the sulfuric acid the crown spray process is able to eliminate the corrosion of concrete pipe. Many alkaline solutions were evaluated for application in the crown spray process in the early 1990s, with particular interest in non-hazardous chemicals. Magnesium hydroxide slurry has been the most effective chemical used thus far for concrete pipe corrosion control.Depending on sewer size and depth of flow, various methods must be employed to deliver the magnesium hydroxide to the sewer crown. The following is a brief description of the Districts' procedure for crown spraying. A modified combination jetvac sewer cleaning machine is used to spray the sewer crown. A floating jet nozzle is put on the end of the hose and water is used to send the nozzle and hose downstream. At the downstream manhole the jet nozzle and hose are pulled from the line and the crown spray float is then attached to the end of the hose. The float is equipped with a spray nozzle that allows the magnesium hydroxide to be sprayed 180 degrees at the sewer crown. The boat and hose are pulled upstream while the magnesium hydroxide is sprayed on the sewer crown.Concrete sewer pipes are selected for the crown spray program based on surface pH and general condition. If the pH of the sewer crown is measured at 5 or below the sewer is placed on the crown spray program. If visible corrosion is found during CCTV inspection the sewer will also be placed on the crown spray program. Generally a sewer which has suffered visible corrosion will also have a pH lower than 6. Currently the Districts has 1.74 million feet (330 miles) of sewer requiring treatment annually. To meet treatment requirements, the Districts has supplemented their crown spray production with the services of a contractor since 1999. Currently the Districts' crown spray crew sprays an average of approximately 3,700 feet per workday at an average unit cost of 0.81 per foot and on the 2002 Contract the Contractors' crown spray crew sprayed an average of approximately 3,480 feet per workday at an average unit cost of 1.29 per foot.The Districts has found that the crown spray process with magnesium hydroxide is an economical corrosion control technology. The present worth to rehabilitate or replace everything on the crown spray program is approximately 700 million. The present worth to crown spray for the next 50 years is approximately 43 million and for the next 100 years is approximately 47 million.