Background
Decades ago, I clearly recall the prognostications. At both high-purity water conferences and in company conference rooms, talk was of the forthcoming demise of service exchange deionization (#SDI) as a viable technology in high-purity water industries. The logic was sound, as advances in membranes technology had produced reverse osmosis (#RO) elements featuring higher rejections, lower energy consumption, lower fouling potential, and better thermally stability. Coupled with the advent of electrodeionization (#EDI) - which could produce high resistivity water at low operating costs, without the need for chemical regenerations - the demise of SDI seemed all but inevitable.
With these decrees self-engrained, we outright disregarded SDI as a technology we wanted to offer our customers when our company first entered the high-purity water service industry. Surely, the market was on course to be disrupted as new technologies were introduced – and the wouldn't the competiveness of SDI suffer as a result?
Ultimately, we realized that one could not be in the industrial water service industry without having a SDI offering. Commercially, SDI was not the ill-fated technology so many had predicted. So, several decades later, why does SDI still have such as strong foothold in the high-purity water industry?
Here are seven reasons why.
1. Flexibility/Convenience
SDI offers flow rate flexibility and expansion potential much more so than RO and EDI systems. If additional capacity is required, service exchange bottles or tanks can be installed with minimal mechanical or electrical work. Conversely, if water demand decreases, the system is easily downsized. A high-purity water system based on SDI can adapt and change to a plant’s water needs, which can be very convenient for multiple product facilities or contract manufacturers with variable water requirements.
2. Minimal Capital Investment
It can be very difficult to predict future operational demands in a quickly changing business environment. SDI systems are most often leased from a local service provider - and can be installed with minimal (or sometimes no) capital investment. SDI is particularly competitive for lower flow rate applications. However, leased RO and EDI systems may become commercially viable at high flow rates or for remote locations where transportation and logistics increase costs. SDI may also be a good option for start ups or R&D centers where capital investment may be limited.
3. Fixed Cost
SDI systems offer a predictable operating expense where the cost of the SDI is directly proportional to the volume of water treated for a specific feed water supply. RO and EDI system maintenance costs may be less predictable - and certainly require more pretreatment unit operations - which in turn, will require additional maintenance. The idea of having a fixed operating expense, with minimal maintenance costs or spare parts requirements, can be particularly attractive to smaller companies.
4. Simple Process with Minimal Maintenance
SDI is a very forgiving technology. Most mixed bed SDI systems can produce high-resistivity water when fed untreated tap water. Many only use activated carbon (often service exchange type) as pretreatment. There are rarely scaling or fouling concerns with SDI, like those that exist with RO or EDI systems. Off-site regeneration with acid and caustic regeneration chemicals provides adequate sanitization for the SDI beds, and generally in-line sanitization is not required.
There is no waste stream with SDI (like RO or EDI), and separation is achieved by an ion-exchange process that requires no high pressure pumps or power supplies to separate impurities from the water. Unlike traditional deionizers, on-site regeneration chemical handling, neutralization, and disposal are not required. Overall, the technology is very forgiving and sounds over a wide range of applications and feed waters.
5. Short Lead Times
While RO/EDI systems may be immediately available on a leased basis, SDI systems are easily installed, often without the need for any utility hook-up and with minimal mechanical work. Often times, they are installed directly to potable water lines with flexible hoses. It is a very short lead time to have entire systems installed and producing high resistivity water. Our company offers emergency deliveries and same same-day installations in our service regions.
6. Lucrative Service
Service deionization remains a lucrative business for water treatment service providers and is a prominent treatment offering for all of the major industrial water companies. As a significant cost of this service relates to delivery of the SDI bottles or tanks, a local service office and delivery trucks are essential.
There are also many hidden costs that make SDI even more lucrative for water treatment service companies, and customers should be aware of this. Most companies will charge for a SDI bottle or tank at a specified price. In addition, there are often rental fees and delivery fees. In addition, be aware of additional fees such as fuel and chemical surcharges. Ultimately the cost of SDI may be much higher than the base price quoted.
7. Easily Transferable
Although the quality of the regeneration and exchange processes are important, unfortunately, SDI is often considered a commodity. SDI bottles and tanks are easily exchangeable among various service providers. End-users can simply obtain competitive bids and switch providers without impacting operations. For high-purity applications, due diligence is critical - and service providers should be vetted to ensure that quality procedures are in place and documentation is available for all regeneration batches.
Future of SDI in High-Purity Applications
In summary, SDI simplifies everything for the end-user, and is a profitable service for water treatment companies. Inevitably, new technologies will eliminate SDI as the primary or polishing technique in high-purity water systems. However, having been told the same thing thirty years ago, we’ll continue to offer SDI as a technology option to our customers until market economics tell us otherwise.