Water Treatment Design

Water treatment system overview: 

The project involves the design, construction, installation, and operation of three wellhead water treatment systems in three small community sites in Salinas Valley, CA.  The water treatment systems are based on reverse osmosis water treatment for nitrate removal and salinity reduction, The treatment systems have a maximum treated water production capacity in the range of 2,500 – 4,000 gallons/day and include pre- and post-treatment units, feed and product storage capacity, and hardware and software for autonomous operation and remote monitoring and management.  In order to reduce the water treatment footprint, the RO portion of the water treatment system was designed to operate with a partial RO concentrate recycle back to the RO feed. The water treatment train consists of well water filtration through a multimedia filter, followed by a series of cartridge filters to remove suspended solids. Subsequently, the filtered water is dosed with a biodegradable antiscalant to reduce the potential for mineral scaling.

The nitrate concentration in the RO system permeate (i.e., product water) is continuously monitored via an online spectroscopic nitrate sensor. The salinity of the feed and product water streams is determined via online conductivity sensors. In order to avoid potential spikes of high nitrate concentration in the product water, a regulatory safety margin is set whereby product water was automatically diverted to the community septic tank should the permeate nitrate level exceeds 8 mg/L NO₃-N. Prior to being delivered to the product water tank, the produced permeate (product) water is re-mineralized (by passing the product water through a limestone bed contactor) and then chlorinated to a level of ~0.2-0.4 mg/L residual chlorine. Remineralization serves to both add calcium to the product water and stabilize the pH. Water from the product water tank is then delivered (via a delivery pump) to the community bladder (pressure) tank. The pressure tank serves as the source of pressurized potable water delivered to the residents through the community distribution piping network. The delivered Water quality is monitored based on both periodic grab sampling (by a CA State Certified Analytical Laboratory) and online system monitoring of nitrate and salinity levels. Given the hourly and daily variability in water demand in each of the project communities, the water treatment system operates intermittently (typically 5-10 hours per operational period) during the day for a period sufficient to ensure that the volume of treated water in the product water tank is sufficient to provide 1-3 days of peak water consumption.

The RO residual concentrate stream, water from the permeate flushing step (to avoid fouling of the RO membrane), and all community wastewater were discharged to the community septic tank. Given the daily water volume utilization in the small communities and the size of the septic tanks, the hydraulic retention time in the septic tank hydraulic retention time is typically in the range of 2-4 days which allows for effective sedimentation of solids and biological degradation (WADOH, 2005). Moreover, under the anaerobic conditions in the septic tank the influent nitrate is identified as has been verified via monitoring of the nitrate levels in the septic tank’s influent and effluent streams.

Reference

On the Feasibility of Small Communities Wellhead Treatment for Nitrate Removal and Salinity Reduction

Management and operation of the water treatment systems:

The cyberinfrastructure for the water treatment systems was developed for the project allowing for monitoring and management of the geographically separate water treatment systems as a “Virtual Water District.”  Online sensors and advanced system monitoring, controller, and data acquisition system enable transmission of the water treatment system operational data to a remote server such that the entirety of the system’s real-time information is accessible via a web interface.  Moreover, all system functions can be adjusted remotely. At the local level, process data from system sensors are utilized to actuate local control actions to initiate self-adjustments of system operation (e.g., for maintaining the needed treated water production volume and permeate quality). Information is relayed to a remote server and then processed to assess system performance and thus provide needed information for decision support regarding needed control and maintenance actions (e.g., sensor calibrations), replenishment of consumables (e.g., including chlorine, antiscalants and calcite), and possible membrane replacement. Although monitoring is accomplished remotely, periodic maintenance onsite visits are scheduled for routine maintenance functions such as replenishment of chlorine, antiscalant, and periodic calibrations of the conductivity, nitrate sensors, flow rate, and pressure sensors.

Design, construction, deployment and operation of the water treatment systems:

The design, construction, installation, and operation of the water treatment systems were guided by a series of initial studies that were carried out as summarized below:

1. The needed water treatment capacity and operational strategy were determined based on data on water use provided via wireless water meters installed in each of the project community sites. Water-use data, collected (in real-time) over a period of nearly two years served to determine community water use patterns which then provided the information needed to size the water treatment systems. Data are continuously provided to assess community water use patterns and assist the project communities in understanding the impact of various activities on water use and how to reduce their water footprint;
2. In order to enable water treatment at a sufficient capacity with a relatively small footprint, the UCLA team developed a membrane-based water treatment approach based on its patented Flexible Reverse Osmosis (RO) (FLERO) water treatment system. Initial water process treatment simulations were performed to arrive at the appropriate water system design specifications; and
3. Design and construction of a mobile small membrane-based water treatment system to carry out field testing of the water treatment technology:
   • Field tests were conducted with the UCLA mobile water treatment system for well-water nitrate removal and salinity reduction. Field tests were carried out in four different community sites.
   • Results from process simulations and field tests provided the needed data for the detailed design of the water treatment systems for the project community sites.
   • The detailed system design was completed for each of the three communities, and the systems were constructed based on a detailed P&ID system
4. A water system infrastructure upgrade was necessary in each community in order to provide for piping connectivity between the community water well and the water treatment system, between the water treatment and the community pressure tank, and between the water treatment system and the community septic tank.
5. The water treatment systems are housed in a secure shed which along with the treatment system ancillary equipment (water tanks) are fenced.
6. The water treatment systems are monitored remotely via their array of system sensors and webcams within the treatment system facility.