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

摘要

The County Sanitation Districts of Los Angeles County (Districts) developed the crownrnspray process in 1989 to neutralize the acid formed by sulfur oxidizing bacteria (SOB)rnand control sulfide corrosion. The purpose of crown spraying the sewer is to neutralizernthe acid formed on the crown with a high pH material and to leave a residual alkalinity onrnthe sewer to neutralize the acid produced as the SOB recolonize the sewer. Byrnneutralizing the sulfuric acid the crown spray process is able to eliminate the corrosion ofrnconcrete pipe. Many alkaline solutions were evaluated for application in the crown sprayrnprocess in the early 1990s, with particular interest in non-hazardous chemicals.rnMagnesium hydroxide slurry has been the most effective chemical used thus far forrnconcrete pipe corrosion control.rnDepending on sewer size and depth of flow, various methods must be employed torndeliver the magnesium hydroxide to the sewer crown. The following is a briefrndescription of the Districts' procedure for crown spraying. A modified combination jetvacrnsewer cleaning machine is used to spray the sewer crown. A floating jet nozzle is putrnon the end of the hose and water is used to send the nozzle and hose downstream. At therndownstream manhole the jet nozzle and hose are pulled from the line and the crown sprayrnfloat is then attached to the end of the hose. The float is equipped with a spray nozzlernthat allows the magnesium hydroxide to be sprayed 180 degrees at the sewer crown. Thernboat and hose are pulled upstream while the magnesium hydroxide is sprayed on thernsewer crown.rnConcrete sewer pipes are selected for the crown spray program based on surface pH andrngeneral condition. If the pH of the sewer crown is measured at 5 or below the sewer isrnplaced on the crown spray program. If visible corrosion is found during CCTVrninspection the sewer will also be placed on the crown spray program. Generally a sewerrnwhich has suffered visible corrosion will also have a pH lower than 6.rnCurrently the Districts has 1.74 million feet (330 miles) of sewer requiring treatmentrnannually. To meet treatment requirements, the Districts has supplemented their crownrnspray production with the services of a contractor since 1999. Currently the Districts'rncrown spray crew sprays an average of approximately 3,700 feet per workday at anrnaverage unit cost of $0.81 per foot and on the 2002 Contract the Contractors' crown sprayrncrew sprayed an average of approximately 3,480 feet per workday at an average unit costrnof $1.29 per foot.rnThe Districts has found that the crown spray process with magnesium hydroxide is anrneconomical corrosion control technology. The present worth to rehabilitate or replacerneverything on the crown spray program is approximately $700 million. The presentrnworth to crown spray for the next 50 years is approximately $43 million and for the nextrn100 years is approximately $47 million.