Most people who work with an apparatus unlike any in their previous experience approach it with some apprehension. This is common behavior and fits the way most of us approach unfamiliar things. Perhaps you deal with this kind of uncertainty about the reverse osmosis (RO) machine in your dialysis unit. This article is intended to increase your understanding of the machine and how it operates and will make you as comfortable as possible with the RO machine.
What Does 'RO' Mean?
RO stands for reverse osmosis or the reverse of energy's natural flow from higher to lower levels. In normal osmosis, pure water dilutes impure water due to pure water's higher energy level. It is through osmosis that water in the bloodstream flows into the dialysate of an artificial kidney machine. Reverse osmosis involves applying pressure (mechanical energy) to drive pure water out of impure solutions.
How Does it Work?
This applied energy is normally provided by a medium- to high-pressure water pump, which overcomes energy differences (osmotic pressure) and drives feedwater through a porous, semipermeable membrane made of polymeric material. In the case of an RO machine, semipermeable means the membrane will pass pure water through, but will reject most dissolved impurities, either by blockage due to physical size (organics and biologicals) or by repulsion due to electrical charge (inorganic salts). The membrane used in artificial kidneys is more porous, allowing water and almost all impurities to flow from the bloodstream into dialysate solution.
Although the RO membrane contains very small pores, which allow water passage, a slight amount of feedwater impurities exists in the purified water. The percentage passing depends on such things as membrane polymer type, membrane condition (e.g. age, cleanliness), and driving pressure (energy), but is normally less than 5% of feedwater content. These same characteristics, in addition to water temperature, affect the pure water flow through the membrane as well.
Not all feedwater passes through the membrane, however. Some is required to flow over the membrane, sweeping away rejected impurities in a crossflow filtration mode. Therefore, from one feedwater stream, an RO machine produces one purified water stream called permeate. It makes a second stream called concentrate, brine, or reject.
How is a Machine Constructed?
Feedwater enters the machine at fairly low pressure. Water flows through prefilter cartridges to remove suspended particles, such as silt. The prefilters provide an inexpensive way to assure that the membrane is kept clean. An automatic valve opens when the machine is turned on, permitting water flow to the pump, which provides the pressure necessary for operation. Stainless steel piping, which is suitable for high pressure, now transports a quantity of feedwater to fiberglass or stainless steel housings that contain the sepralators (membrane elements). Permeate exits the sepralator under low pressure once again, due to the pressure drop through the membrane. A mechanical orifice and / or control valve produces necessary pressure drop in the concentrate stream, allowing transport via low-pressure piping as well.
What´s Important to Know?
First, the machine is designed to run by itself. It requires only periodic operating data collection and routine maintenance. Most machines are equipped with protective alarms to shut the machine down or post an alarm condition in case of an undesirable situation.
Also, machines have a few simple instruments to measure performance. These may include permeate flow rate and conductivity, machine feed pressure and discharge pressure, feed pH, and the calculable percentage recovery. All measurements represent machine operating characteristics and are based on the machine's design. Consistency in each of the values indicates the machine is operating properly.
Furthermore, the machine purifies water using consumable products, which are safely disposed of after their useful life. Cartridge prefilters are typically replaced every two weeks. Sepralator life, however, is approximately three years. All other components of a machine are considered 'hardware' and should not require replacement other than through normal mechanical wear and tear.
One last item is perhaps the most important. As with any mechanical device, the RO machine's performance is optimized under a regular maintenance program. Activities within the program include adding grease or oil to the high-pressure pump, changing prefilters, sanitizing the machine, and cleaning the sepralators when they become fouled. Pretreatment equipment, such as softeners, requires attention as well.
Their performance and upkeep affects the RO machine. Centers may procure this type of work through a service contract with a manufacturer's local distributor who is trained to perform these activities efficiently and economically.
Is Something Wrong?
As noted earlier, the machine is equipped with a number of protective alarms to signal obvious problems. Alarms are provided, for example, to assure that feedwater has sufficient pressure and is within an acceptable pH and temperature range.
Daily log sheets are perhaps the most underutilized tool for detecting abnormal operation. With consistent recording of measurements like pre- and post filter pressures, pump discharge pressure, final pressure, permeate flow, recovery, percent rejection and pH, it is fairly easy to recognize a deviation from trends or expected values and quickly identify a potential problem. The increasing popularity of PLC's and solid-state controllers on newer machines is improving the situation through their ability to store, display, transmit, and/or print operating histories. On units without computers, however, once-per-shift recording can reduce a lot of uncertainty, especially when operating parameters change.
Sometimes an unusual condition is not signaled by alarms or trend changes, but is simply apparent to someone working with the machine. Examples include unusual noises and leaks. Though much rarer in occurrence, this kind of observation should be handled in the same way as the previous two.
Perhaps an analogy is appropriate before discussing how to handle problems. One analogy can be drawn to the human body. Like an RO machine, the body functions normally when given proper attention and maintenance through nutrition, exercise, and rest. Body temperature, blood pressure, and normal heart rate are measurements of proper functioning. Consistency and proximity to expected values indicate good health. Likewise, deviation from these values, or the presence of pain, are symptoms of possible problems.
If common medications or treatments do not help someone with those symptoms, that individual will probably contact their doctor. The RO machine operator should do likewise.
What if I Find Something Wrong?
Regardless of how the problem is discovered, recognition should lead to investigation of possible causes, similar to diagnosis. You may find and work through a substantial problem or simply have to make a small adjustment to return the machine to normal.
Thorough guides are provided in operating manuals manufacturers supply with machines. When this proves insufficient, we recommend contacting your local distributor for a technical 'talk-through.'
Does the Machine Work with Other Components?
Although we have focused on the RO machine, it is rather uncommon for a machine to operate in isolation. Machines are normally preceded by multimedia filters or softeners, which serve to prolong RO membrane life through prevention of fouling or membrane scaling. An ion exchanger may be used downstream to provide further purification of permeate, and final product water is sometimes stored.
Any of these components can be set up to work in concert with the RO, with on/off interlocks and feedbacks as required.
Operating an RO machine is much simpler than what you might expect when first introduced to it. The greater comfort level you obtain, the more you will optimize the machine's performance and gain maximum return on your investment.