Sequestering and Ammonia/Chloramination Case History
Central Michigan Groundwater System
This discussion is about a Central Michigan municipality that was experiencing dirty water complaints and unknowingly, was affected by ammonia in its aquifer. A competitive phosphate sequestering product had been used in the past but was not working. Water Solutions Unlimited was asked to look at the problem. Their knowledge of chlorine demand, ammonia, and how to quickly evaluate a problem, quickly helped the city alleviate its problem.
Background
The Central Michigan municipality produces approximately 500,000 gallons per day. The water production process starts with two wells followed by chlorination, only. Approximately 0.81 mg/L of iron, and 0.023 mg/L of manganese are in the water being pumped. A phosphate sequestrant was being applied at an approximate feed rate of 3.0 mg/L as PO4. A significant number of customer complaints for dirty water were being received by the water office. Water department personnel were “spot flushing” fire hydrants at different locations, several times a week. This resulted in wasted water and operator time that could be used on more productive projects.
Discussion
Water Solutions Unlimited was contacted by the Michigan Rural Water circuit rider who was attempting to assist the municipality with their problem. We contacted the water superintendent who gave us permission to stop by to assess the problem.
The first thing we did was check the operator log sheets to determine current phosphate feed rate and the applied chlorine feed rate. The phosphate was being fed at approximately 3.0 mg/L as PO4, which is an adequate feed rate for the amount of iron and manganese present in the well. The applied chlorine feed rate was 7.0 – 7.5 mg/L. This was excessive, and the cause of the problem. The municipality was using free chlorine to disinfect. For some reason their chlorine demand was so high that it took this much chlorine to reach breakpoint and establish a free chlorine residual. Our question was- What caused such a high chlorine demand?
We immediately had the raw water tested for ammonia and found approximately 0.54 mg/L of ammonia in both wells. This accounted for 5.4 mg/L of the chlorine demand. The other 1.5 mg/L of chlorine was due to the oxidation of the iron and manganese.
Solution
WSU’s solution was to use the natural ammonia which was sufficient to go to chloramination. We recommended that the chlorine applied feed rate be reduced to 2.0 – 2.25 mg/L. This allowed the chlorine and ammonia to be fed at a 4:1 ratio, which formed mono-chloramines. This is the correct form of chloramine for disinfection without taste and odor problems.
We asked the municipality to flush the system to get the dirty water out of the system. After the flush the system remained clean and a 1.0 mg/L of combined chlorine was maintained throughout the system. It was recommended that the system be flushed twice a year. For two weeks prior to each flush, the system is put on free chlorine for 10-14 days to “burn off” and potential nitrate buildup in the system.
Over the past few years the municipality has experienced minimal “dirty water” complaints. The reduction in chlorine from 7.5 to 2.0 mg/L has saved the municipality $5,860 in chlorine costs per day. This savings paid for the yearly phosphate treatment costs.
Please contact your Water Solutions Unlimited for more details and answers to any questions you might have.
Summary
This case points out several important distinctions about Water Solutions Unlimited:
We have an excellent understanding of:
1. Sequestering and the external conditions that negatively affect results (high chlorine).
2. Disinfection options (free chlorine and chloramination) and how to implement them.
3. How to evaluate and analyze a problem.