Des Moines Water Works is an independently operated public utility providing drinking water to
a population in excess of 300,000 in Des Moines, Iowa and the surrounding metropolitan area. The utility was originally established as a private company in 1871 and became a customer-owned utility in 1919. Today, DMWW is the largest water utility in Iowa, one of the largest 100 utilities in the country and nationally recognized as a water industry leader.
Des Moines Water Works' Fleur Drive facility is a surface water treatment facility rated for 100 million gallons per day (mgd) and pumps on average, 43 mgd. It draws water from three sources: the Raccoon River, the Des Moines River and an infiltration gallery. The infiltration gallery is a large horizontal well that lies in sand and gravel sediment adjacent to the Raccoon River. This source yields approximately 15 mgd of clean, naturally filtered river and ground water. The remaining demand is obtained from either the Raccoon or Des Moines rivers. The selection of river water is based primarily on source water quality and the ability to treat substances in the water. Treatment strategy and design must accommodate rapid changes in river quality and water demand.
To best understand the treatment process, refer to the illustration on the next page as you read how the water is treated.
Powdered activated carbon is first fed into the selected river water for removal of man-made and naturally occurring organic chemicals. The water is then treated to remove dirt and debris and combined with water from the infiltration gallery system. The combined water then flows into softening basins. The pH of the water is then adjusted before the final filtering process. The water is passed through layers of sand and various sizes of gravel to remove any remaining particles. During periods of possible increases in nitrate levels, Des Moines Water Works activates its nitrate removal facility to remove this contaminant. Next, fluoride is added to aid in the prevention of tooth decay and then chlorine (in the form of solution sodium hypo-chlorite) is added as a disinfectant to kill bacteria. Finally, the now clean water is stored in a clear-well until pumped into the pipes of the distribution system.
DMWW Fleur Drive Water Plant Treatment Process
Klorigen™ On-Site Sodium Hypochlorite System Installation
Prior to selection, DMWW performed an economic evaluation of the various technology alternatives that were available to replace their liquid chlorine system, which then consisted of multiple one-ton cylinders of compressed elemental chlorine gas. Alternatives that were evaluated included:
· Traditional high strength (12.5%) sodium hypochlorite (“bleach”) storage system dependent on frequent deliveries from third party suppliers using large tank trucks,
· On-site electrochemical generation of commercial strength (12.5%) bleach using salt, and
· On-site generation of low strength bleach (0.8%) that also used salt.
DMWW made their decision in favor of a Klorigen™ on-site electrochemical system manufac-tured by Electrolytic Technologies Corporation, for the following reasons:
a. production cost was originally projected (and continues) to be significantly less than the prevailing market price of commercially-supplied bulk hypo,
b. on-site generated bleach produced by the Klorigen™ system was compatible with conventional commercial strength monitoring, measurement, pumping and storage systems,
c. the Klorigen™ produced hypo was to be of equal - if not better quality than that commercially available and provide a consistent high concentration (at least 12.5%),
d. compared to the low strength (0.8%) alternative, storage requirements for the high strength solution were was less - by a factor of 15,
e. On-site production removed the risk of an accident from transporting hypo through the local community, and
f. The Klorigen™ hypo product met the requirements for NSF/ANSI 60 certification.
The final selection comprised a Klorigen™ system rated at 1,500 gpd of 12.5 trade % sodium hypochlorite and two 5,000 gallon storage tanks. Due to Klorigen’s unique modular design, DMWW was able to design their own OSG facility and managed the installation with internal personnel at minimal cost. The Klorigen™ on-site sodium hypochlorite generating system at DMWW was commissioned in the 1st quarter of 2004.
Data depicting the performance of the Klorigen™ system was compiled using DMWW’s SCADA system. The system data collection began in the 3rd quarter of 2004 and the results in this study
have been compiled through the 4th quarter of 2006.
DMWW Monthly Sodium Hypochlorite Production
Table 1 shows the DMWW hypochlorite system production for a period of 30 months. The water plant sodium hypochlorite usage varies according to the time of year, with the highest sodium hypochlorite production requirement occurring during the summer months.
Table1. DMWW Klorigen™ System hypochlorite production graph (30 month period)
DMWW 12.5% Sodium Hypochlorite Generation Costs
Table 1. DMWW Klorigen™ system quarterly operating costs for producing sodium hypochlorite versus the price for local sodium hypochlorite delivery.
Table 1 above and Figure 1 below show the combined chemical and power operating costs per gallon of the DMWW Klorigen™ System for 12.5 trade% sodium hypochlorite over a ten calendar quarter (30 month) period. The chemical raw material pricing increased by about 20% in 2006 (salt, NaOH, HCl, and bisulfite) and is depicted as an increase in $/gal (2006) column.
Figure 1. DMWW Klorigen™ system quarterly operating costs showing a base operating cost (2004-2005 pricing) and the impact of the raw material price increases in 2006.
DMWW On-Site Sodium Hypochlorite Generation - Operating Cost Reductions
Table 2 depicts the calculated quarterly Klorigen™ System operating costs and Figure 2 shows the cost savings over purchased sodium hypochlorite that the DMWW facility has had over ten calendar quarters of operation. The cost savings after only ten operating quarters have been nearly $360,000 and are already approaching the original capital cost for the system.
Table 2. DMWW Klorigen™ System quarterly operating costs and calculated cost savings over purchased sodium hypochlorite.
Figure 2. DMWW Klorigen™ System quarterly hypochlorite operating costs and savings in comparison to delivered purchased hypochlorite over ten quarters (30 months) of operation.