The arsenic levels at both Arrow Canyon and Baldwin Springs is 17 μg/L, low enough to allow for bypass/blending. But for the first several months of the new
systems’ operation, the District treated the entire flow through the systems’ vessels in order to achieve a non-detectable level of arsenic. This was done primarily to give operators time to become comfortable using the new system and learning the bypass/blending procedure.
At Arrow Canyon, the design flow rate per adsorber is 900 gpm; at Baldwin Springs, 875 gpm. Both systems utilize a media bed depth of 3.2 feet.
By September 2009, MVWD began bypassing a portion of the pre-treatment flow to achieve arsenic levels in the blended water of 7 μg/L. To achieve this level, the Arrow Canyon system can bypass 1,125 gpm, or 25% of the combined well flow rates; the Baldwin Springs system can bypass 470 gpm, or 27% of the combined springs flow rates.
Monitoring System, Media Performance
The Moapa Valley adsorbers are operated in parallel flow configuration. If there is variation in the flow rate to each of the adsorbers and balancing is desired, the
effluent butterfly valve on the higher flow adsorber(s) can be positioned to distribute more flow to the lower flow adsorber(s).
The pressure differential through each adsorber is monitored, and when the differential on either adsorber exceeds the high setpoint (normally 10 psi), an alarm sounds indicating high pressure differential on that adsorber. After a 15-minute backwash is conducted to reclassify the compacted media, the adsorber is returned to service.
The differential pressure gauge PDI measures the differential across each adsorber. A discrete signal is sent to the well’s SCADA system to notify the MVWD operators that an adsorber has a high differential value requiring attention.
Typically, every one to four months, depending upon the water’s quality, each adsorber is taken out of service for backwashing, or fluffing, to expand the compacted media bed and to remove solids that may have built up within the bed. Interestingly, MVWD has never exceeded 1 psi pressure differential and has not needed to perform routine backwashing. Aside from this, there is no other
non service action required until the media is exhausted.
Media life ranges from 6 months to 6 years depending upon the system’s utilization factor and on the influent water’s arsenic level and the presence and concentration of other ions in the water that could shorten the media’s arsenic adsorption capacity.
During the first six to seven months of operation, with the De Nora SORB 33® systems achieving non-detect arsenic levels, MVWD called the arsenic removal system’s performance “flawless.” According to the District, “The SORB 33® technology has allowed the District to achieve and maintain our system in compliance with the new arsenic standard in a very cost effective manner.
Because of the ease and reliability of operation, the District has been able to minimize rate increases and bolster consumer confidence in the water being delivered – both of which are positives for our customer base.”
MVWD was pleased to discover that the De Nora SORB 33® arsenic removal media lasted about four years, which was longer than expected. The District was able to easily conduct the replacement on its own and continue the operation seamlessly.