New Water Treatment System Increases Reliability, Capacity
Tiffin, IA, is a growing community with 1,700 residents. Like many Midwest communities, Tiffin uses groundwater for its drinking water supply.
by Daniel L. Willers
Tiffin, IA, is a growing community with 1,700 residents. Like many Midwest communities, Tiffin uses groundwater for its drinking water supply. After many years of useful service, the water treatment system had become unreliable and obsolete due to age, corrosion, and the inability to provide adequate treatment for radionuclides in the raw water.
The raw water was supplied from two wells, both having gross alpha that sometimes exceeded the MCL of 15 pCi/l. In addition, the total hardness was in the range of 480-500 mg/l as CaCO3, and the presence of hydrogen sulfide caused objectionable odors and taste in the water. The existing iron removal filters were not sufficient to meet the treatment objectives. When one of the city wells stopped working, the city decided it was time to construct a new water treatment system.
The new drinking water treatment plant was designed by Hart-Frederick Consultants P.C. (Tiffin, IA) to meet the city’s treatment needs. Hart-Frederick worked with Siemens Water Technologies, General Filter Products group, on the design of the major process equipment. The system was designed for a capacity of 800 gallons per minute (gpm) to allow for future growth in the community.
Aluminum forced draft aerator for dissolved gas reduction and iron oxidation.
The source water is supplied from two wells. A Jordan aquifer well is 1,400 feet deep, while a Solarium aquifer well is 350 feet deep. The combined raw water has a total hardness of 480-500 mg/l as calcium carbonate, iron concentrations of 0.25-0.30 mg/l, almost 2.0 pCi/l of radium on average with a gross alpha of 13 pCi/l, which sometimes spikes above MCL of 15 pCi/l. Hydrogen sulfide concentrations are significant enough to cause unacceptable odors. While the blended water has iron concentrations of 0.25-0.3 mg/l, the Jordon well has concentrations of about 1.0 mg/l. Since the system is capable of operating only on the Jordan well for significant periods of time, iron removal was incorporated in the overall system design.
Raw water pumped from the wells is chlorinated before entering the first stage of treatment. The chlorinated water enters a forced draft aerator located outdoors next to the treatment plant. Once inside the unit, the water is distributed across the aerator cross section. As the water cascades through the unit, it is spread into a thin film on a series of PVC pipe internals arranged on 6" vertical centers to maximize oxygen transfer into the water and gas transfer out of the water. A forced draft blower located at the bottom of the unit sends a counter-current flow of air upward through the aerator to enhance gas transfer. The forced draft design was selected because of the hydrogen sulfide concentrations. In this arrangement, the off-gas does not pass by the blower, thus minimizing corrosion of the blower components. In addition, the aerator is constructed of aluminum to provide corrosion resistance. If iron precipitates on the PVC internals, the PVC pipes may be individually removed for cleaning by access through a hinged door on the aerator.
After the aeration step, the water enters the detention tank where the additional reaction time allows for complete iron oxidation and the formation of a filterable floc. Two high service pumps are included for duty and stand-by operation. These pumps send the water through the remaining treatment processes and into the distribution system.
Four vertical pressure filters provide iron removal at the facility. These allow the water to be treated through the system without the need for additional pumping. The filters contain a 30-inch depth of anthracite and sand media to capture the iron floc as well as any other solids from the wells.
The face piping was designed to allow the operator to use filtered water from three operating pressure vessels to backwash any other vessel. This simple arrangement eliminates the need for backwash pumps. Since filter backwashing would be infrequent, the design called for manual valves, which saved on costs. The operators backwash the filters every two weeks or when the headloss across the filters exceeds 3.5 psig differential. A special open-ended orifice plate and pressure gauge are used to determine the appropriate backwash flow rate. Backwashing a single filter typically takes five minutes with each of the filters being washed in sequence during a wash cycle.
Vertical pressure filters (left) treat the water prior to the cation exchange units (right).
After pressure filtration, a portion of the water flows through two cation exchange softeners. The units are designed to treat the amount of water needed to achieve the required hardness and radium reduction. An adjustable by-pass loop around the softeners allows the operators to control the effluent hardness and radium concentration. The cation exchange softeners use a resin bed as the site where sodium is exchanged for the radium and hardness.
When the resin exchange capacity is reached, the unit is regenerated using sodium chloride brine. The regeneration operation begins by backwashing the resin to remove solids and expand the bed. Next, brine is introduced at the top of the bed, reversing the exchange process and restoring the sodium content on the resin. The final step during regeneration is to operate the unit to waste, which ensures that all brine is removed from the unit before placing it back into operation. The regeneration waste is sent to the wastewater treatment plant. This operation is automated with an automatic control system to ensure a consistent regeneration process.
After the cation exchange process, the water is chlorinated. Polyphosphate and fluoride are added before the water enters the distribution system.
The vertical pressure filters and cation exchange softeners were built at the Siemens manufacturing facility in Ames, IA. The units were designed and constructed using standard pressure vessel design guidelines and manufacturing processes. The underdrain systems in the vessels were factory installed to minimize onsite installation costs.
The plant began operation in October 2005, within a year after the installation bid, and has met the city’s requirements for reliability, capacity, and all water quality standards.
The radium and gross alpha concentrations have been reduced below drinking water standards, ensuring safe water at all times for Tiffin residents. An additional benefit is the reduction of iron to less than 0.08 mg/l and total hardness to 230 mg/l, or roughly half of the raw water concentration.
The system has exceeded the expectations of Brett Mehmen, City of Tiffin Water Treatment Supervisor and Public Works Director.
“We are able to produce excellent finished water quality with the system,” Mehmen said. “On top of that, the system has been easy to operate and maintain.”
Since the plant started up, the city has averaged 130,000 gpd of production in the summer and 90,000-100,000 gpd in the winter. Due to the construction of two new subdivisions, the demand is expected to continually increase. Throughout the seasonal variations, the plant has consistently met all water quality standards with a minimum of operator attention.
About the Author
Daniel L. Willers is a Sales Support Engineer at Siemens Water Technologies, Microfloc and General Filter Products, in Ames, IA. During his career, Willers has designed water treatment, wastewater treatment and remediation systems in North America. He holds a B.S. in Chemical Engineering from Iowa State University. He can be contacted at Daniel.Willers@Siemens.com.