Flow reversal retrofit boosts city’s water self-sufficiency ambitions

Working to become a self-sufficient city

Santa Monica, a city in the US state of California, is on an ambitious journey to becoming 99 per cent self-sufficient when it comes to water supply. To achieve this goal, it wants to increase water production while avoiding, where possible, the need to build new facilities or expand existing ones.

Like the rest of California, Santa Monica is experiencing the ongoing problems caused by climate change and the effects of prolonged periods of drought. As a city, it is working on a ‘one water’ programme worth approximately €170m, which promotes water recycling and the renewal of groundwater supplies as solutions to its future water security.

 

Desalination and the One Water approach

One Water was adopted as a phrase in 2011 by the City of Santa Monica, following a request by the Metropolitan Water District of Southern California, which supplies the city with imported water. This followed an earthquake, which disrupted the systems that were being used to import water into the city. 

As a result, the city developed its One Water self-sufficiency plan to reduce its reliance on imported water. Key to this plan is the Arcadia brackish water desalination facility, which was initially built in the 1960s and provided the city with 50 per cent of its water supply. 

One Water aims to increase Arcadia’s output to 60-65 per cent, which will be combined with alternative supplies (19 per cent), conservation efforts (20 per cent), and imported water (only one per cent) to secure the city’s water supply.

 

Retrofitting Arcadia

To achieve the stated output aims of 60-65 per cent from the Arcadia desalination facility, the city employed engineering consultants Brown & Caldwell to lead the project,  after securing a Prop 1 Desal Grant from the California Department of Water Resources.

The company led on the overall design of the project and chose ROTEC’s Flow Reversal technology as offering the best chance of the required recovery rates. Walsh Group was chosen to implement the design. 

ROTEC managed the design and procurement of the membranes, built the electrical cabinets and had a team on-site for installation, startup and commissioning.

 

What is flow reversal technology and how does it work?

In essence, FR-RO is a reverse osmosis system in which the direction of the saline stream is periodically switched.

ROTEC’s Max Finder explained the process to Aquatech Online in more detail: “It’s a smart operation methodology for deionisation systems in which the flow direction of the saline stream in RO pressure vessel arrays is periodically switched.”

The reason for this switch is so that scale does not have time to form on the membranes’ surfaces, prior to being dissolved by undersaturated feed solution conditions.

The frequency of switching is dictated by the time it takes for a supersaturated solution in the concentrate to grow a population of scale particles that can allow continued scale growth. This is called the induction time.

 

This approach affords operating at much higher recovery rates than can be achieved with anti-scalant alone - Finder

 

Finder continued: “By using the effectively under-saturated feed to dissolve the scale particles in the concentrate before they exceed a critical size, extensive precipitation is prevented. This approach affords operating at much higher recovery rates than can be achieved with anti-scalant alone.”

ROTEC’s block rotation technology (patented) enables its FR-RO system to be used in large systems, in tapered flow arrangements.

Finder explained: “This allows us to isolate a group of pressure vessels into a block using pipes and valves. Each block can then operate as either the first or last stage of the system, allowing the pressure vessels to spend the majority of their time as the first stage and only a small part of their time as the last stage.”

He added: “This ensures the block finishing its role as the last stage can recuperate and be cleaned from the scaling it may have endured, enabling the system to operate at ultra-high recovery rates.”

 

Why was FR-RO chosen for this project?

One of the benefits of the FR-RO system was its ability to be retrofitted to the existing facility without causing any significant delays to operation.

Finder explained: “We used the existing high-pressure pumps, valves, pressure vessels, instrumentation, and then by applying our FR-RO process, we were able to improve the recovery rate from 82 to 90 per cent, which is a dramatic improvement.”

That ability to use the existing equipment significantly reduced cost and implementation time. While the original plan involved phased commissioning, city officials opted for a full system shutdown to streamline installation. 

 

By applying our FR-RO process, we were able to improve the recovery rate from 82 to 90 per cent, which is a dramatic improvement - Finder

 

“Thanks to the design flexibility of FR-RO, individual treatment systems were brought in as they were completed,” Finder continued,  “initially operating in conventional RO mode to meet production needs without regulatory delays.”

He added: “This is a unique attribute of FR-RO technology – if a plant doesn’t actuate the valves, they are running a conventional RO system. This allowed water production to begin promptly, while FR-RO testing and validation took place in parallel.”

 

How does FR-RO achieve such high recovery rates? And why is it so energy efficient?

Anti-scaling and fouling enable the FR-RO system to operate at higher rates than the conventional RO membranes that were being used in the Arcadia plant. These properties also reduce the required cleaning frequency, which helps to extend the life of the membranes.

Finder explained: “If a conventional system were to operate at these recovery rates, it would likely scale up immediately and be forced to shut down and clean the membranes. These cleanings use aggressive chemicals, which shorten membrane life.

Another bonus for the One Water ambitions of the city is that the FR-RO system is producing more potable water using less energy than the plant before it was retrofitted. According to Finder, this was achieved through ‘optimised design and balancing hydraulics across the full system’.

 

The power of FR-RO and retrofitting

While every facility is different, retrofitting the Arcadia facility has shown that FR-RO has the potential to produce more water at higher recovery rates using less energy. This will offer options for other cities exploring options to become more self-sufficient in water production, especially where importing water is becoming more expensive and subject to the same climate change and increased demand pressures evident in many places, not just in the US, but around the world.