| Cracks and weeping chemical from a tank containing sodium hypochlorite are two things no one wants to see. Even though these problems were severe, for the Neenah Wastewater Treatment Plant, the worst was yet to come.
 At the Neenah Wastewater Treatment Plant in Menasha, WI, Maintenance Manager, Jim Peichl, noticed that three 7,000 gallon high-density linear-polyethylene tanks installed a year before were starting to crack and weep sodium hypochlorite through the tank wall.
The Neenah Wastewater Treatment Plant has been treating water since 1935, and serves about 45,000 people living in the towns of Neenah and Menasha, WI and some outlying districts around them. The plant treats about 10 million gallons of water a day, water that eventually flows into a local river, after being treated with sodium hypochlorite.
About 400 to 500 gallons of sodium hypochlorite is used to treat the wastewater on a daily basis. Peichl says, “We take salt brine and mix it with city water. Then it’s run through a cell similar to a large auto battery that uses electrodes and an inverter to treat the water. Our sodium hypochlorite is only a 1% solution. The liquid bought in a store for cleaning is about 5%.” Since these tanks are filled automatically on a daily basis, cracks and weeping was a concern. Not only were they creating a hazardous work environment for the employees, they were losing chemical.
Pfeiffer felt that uniform wall, crosslink polyethylene tanks made by Assmann would stand up better to the chemical attack of the sodium hypochlorite. |
Peichl knew that in time the sodium hypochlorite would eventually attack the tanks, and they would need replacing, but usually this happens after many years. These types of problems should not have happened within one year. The installation contractor and manufacturer were asked to replace the tanks under warranty. After repeated attempts to get the contractor and manufacturer to do this, the problem was not resolved.
Peichl couldn’t wait to resolve the problem because of the environmental issues they imposed. He tried plastic welding the tanks without success. Next, Peichl asked John Pfeiffer, owner of Fiberglass Technologies, a representative of Assmann Corporation of America, to review the problem. Pfeiffer recommended cutting out the area around the fitting, where the cracks were, and installing a larger fitting. The tanks cracked again beyond the larger fittings.
Since this didn’t work, his next recommendation was to replace the tanks. Pfeiffer felt that uniform wall, crosslink polyethylene tanks made by Assmann Corporation of America would stand up better to the chemical attack of the sodium hypochlorite. Neenah Wastewater Treatment Plant purchased three 5,200 gallon tanks from Assmann Corporation to replace the 7,000 gallon tanks.
Why crosslink polyethylene?
 Pfeiffer says, “If you have a tank failure with sodium hypochlorite using a linear polyethylene tank, there’s a possibility that the tank could catastrophically fail. The reason for this is molecules in linear high-density polyethylene link up in long strands. Crosslink polyethylene has a molecular structure where all molecules are linked together making a stronger tank for harsh chemical services. Sodium hypochlorite is a chemical that hardens the tank wall and attacks the UV stabilizer compounded into the raw material. Using high-density linear polyethylene that is already a hard material may cause the tank to prematurely fail. Since all of the molecules in crosslink polyethylene are linked together, the tank will better withstand abuse from impact or chemical attack. Therefore, it’s a safer more reliable tank.
From a safety standpoint, crosslink polyethylene is a stronger, safer material."
| “These tanks should easily last at least three years, but I have customers that have been using them for 12 years.” |
The industry standard for a polyethylene tank in a sodium hypochlorite service is typically one year full warranty. Assmann Corporation offered Neenah Wastewater Treatment Plant a full two-year warranty. Pfeiffer said, “These tanks should easily last at least three years, but I have customers that have been using them for 12 years with sodium hypochlorite. Sodium hypochlorite gets more aggressive as the chemical reduces in concentration.”
Peichl said there are other types of tanks available for this application. Even a fiberglass tank would only give roughly the same longevity, but would be about 30-40% more expensive.
Pfeiffer said, “Crosslink polyethylene tanks can take an incredible amount of physical abuse while a fiberglass tank needs more care. You need to be careful when installing fiberglass tanks because if you bump them, you stand the chance of cracking the corrosion barrier.”
Peichl was looking for a cost effective way of storing sodium hypochlorite. The best solution to his problem was crosslink polyethylene tanks supplied by Assmann Corporation. Assmann Corporation had the tanks delivered within four weeks resolving Neenah Wastewater Treatment Plant's safety issues.
 Another point that Pfeiffer made is some manufacturers produce tanks with tapered wall thickness. "During the manufacturing process, this generates a thinner tank wall at the top and a thicker tank wall at the bottom. Assmann Corporation calculates the wall thickness needed for maximum hoop stress at the bottom of the tank and produces a tank that has a uniform wall from top to bottom. This also has other benefits; the tank has a uniform cure throughout making it able to withstand harsh chemical environments such as sodium hypochlorite and strong acid concentrations. In the case of a stress-cracking chemical agent, you depend upon the wall thickness and the cure of the tank for its quality.” Peichl has said that the new tanks have performed well with no signs of weeping or cracks after a year’s use. |
