1. The purpose of corrosion control is to prevent:
- Health problems - corrosion can result in lead and cadmium leaking into the drinking water. These elements have M.C.L.'S (Maximum Contaminant Level).
- Aesthetic problems - corrosion can lead to rusty water, the staining of clothes when doing laundry, unpleasant metallic taste and odor (iron and copper).
- Corrosion of pipes, etc which lead to Main Breaks.
2.) The Lead and Copper Rule is the regulatory driving force for Corrosion Control measures.
- Pipe Rack studies conducted by Planning and Research and Water Treatment to find optimum treatment methods.
- Coupon Studies in the distribution system monitored to measure performance of corrosion control methods.
- Home sampling study lead to defining "mixed-zones" are problem areas. Fixed zones are areas where waters from different treatment plants blended, i.e. East Park Reservoir district. Baxter was using high pH for corrosion control and, Queen Lane was using zinc ofthophosphate at a pH of 7.3. Blending the two waters negated the corrosion inhibiting characteristics of the water. That is why the decision was made to standardize corrosion control measures at all three plants. Mixed zones would no longer be a problem.
3.) Zinc Orthophosphate
- he goal is for all three plants to dose zinc orthophosphate to obtain a phosphate residual of 0,72 mg/l at a pH of 7.4. We are currently using a 1:5 zinc to phosphate ratio product.
- Previous zinc orthophosphate-.; used were 1: 1, and 1: 3 ratios.
- Future usage may include 1:10 product or phosphoric acid, Phosphoric acid is cheap but has no zinc in it...Recent studies show zinc may play an important role in corrosion inhibition.
- We don't dose at the manufacturers recommended dosage because we are trying to keep zinc concentrations below 0.12 mg/l. Higher zinc levels cause problems with the composted sludge at the biosolids Recycle Center that limit its land application in certain areas.
4.) Calcium Carbonate Saturation
- This method was previously used at Baxter for corrosion control. The objective is to lay down a thin protective coating of calcium carbonate on the pipe wall, This is accomplished by a high pH in the finished water of approximately 8.5. Under-saturation leads to a loss of protective coating and over-saturation can lead to excessive scale buildup.
5.) Application Points
- Zinc orthophosphate is applied after the filters at all three of our water treatment plants. One uses an injector while the other two use metering pumps.
- Outages in chemical feed are not considered critical, but repairs should be made as soon as possible.
6.) Monitoring
- The Chemists measure phosphorous in the plant lab once per day, These locations include filter effluent, pre-clear well, and plant effluent.
- Once a week, the river and raw water basin effluent is monitored.
- Special procedures are used when alum is used a coagulant.. Background phosphorous is not completely removed by alum but it is by ferric chloride. When using alum, we dose the target value in addition to any background phosphorous.
7.) Treatment Guidelines
- The target is to achieve 0.23 mg/I of phosphorous. The phosphate residual is 3.06 times the phosphorous level, or 0.72 mg/l.
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