Challenge

The River Mountain Water Treatment Facility (WTF) is one of two plants that provide drinking water to the Las Vegas Valley and is situated a quarter-mile from a residential neighborhood in the city of Henderson. The facility was designed to produce 150 million gallons per day (mgd) and included provisions for subsequent 150 mgd expansions, up to 600 mgd if more supply was needed. In 2006, the capacity of the River Mountain WTF was 300 mgd.

Owned and operated by the Southern Nevada Water Authority (SNWA), the facility treats surface water and includes secondary chlorination to maintain a residual in the distribution system. The SNWA decided to install an on-site hypochlorite solution system to alleviate safety concerns associated with using chlorine gas or commercial sodium hypochlorite solutions. The Authority's consultant Montgomery Watson, in a joint venture with CH2M Hill, recommended installing the OSEC® On-site Electrolytic Chlorination System from Siemens Water Technologies. Three 2000 ppd OSEC hypochlorite generators started up in November 2003, and another six started up in March 2006. The nine systems comprise the largest brine-based hypochlorite generators in North America, with an installed capacity of 18,000 lb/d of equivalent chlorine.

Solution

MW/Hill’s project design manager reported that, “Siemens supplied a reliable system that met the Authority’s special control system requirements and engineering challenges. With a treatment facility this size and located so close to the neighbors, safety was a paramount concern.”

Equipment layout constraints, special seismic considerations, control system requirements and customization, documentation and quality control also proved challenging.  In order to reduce the building size, the power rectifiers were installed one floor above the generator room. The bus connections for the rectifier and generator needed to go through the concrete floor in the rectifier room and join the generators below.

SNWA required the plant to be recessed into the hillside as much as possible. The large concrete OSEC storage tanks and brine tanks were constructed below grade. The delivery of salt is facilitated by this construction arrangement because trucks drive over the salt pits and gravity dump their load of salt.

SNWA also required Modicon hot redundant standby PLCs with fiber optic communications as well as complete Process Control and Instrumentation Systems documentation, which included loop drawings, Full Control Loop Validation Testing and System Loop Validation Testing prior to commissioning.

Results

The OSEC system's performance has exceeded the Authority's expectations. The system consumes only 3.1 lb of salt/ lb of chlorine and less than 2 kWH/lb of chlorine. Since the initial installation at the River Mountains facility, the SNWA gave authorization to expand the facility's capacity to 600 mgd.