By T.A. ‘Ted’ Kuepper
POU reverse osmosis (RO) drinking water systems and POE ion exchange water softeners are the two most universal water treatment technologies used today for residential and commercial locations. These systems have operational characteristics that limit their use, however, if installed in water-short regions of the world.
Wastewater associated with membrane-based systems, such as RO and nanofiltration (NF), is relatively high and this inefficiency alone limits POU membrane
applications. Salt brine created by ion exchange water softeners increases salt concentration of domestic wastewater, making future water reuse options for municipalities more expensive and complex.
Concern over water wastage, in particular, has kept membrane-based processes from being considered for many POU applications in water-short locations. The increase of salinity concentration in wastewater, however, is now also a growing concern in communities that are considering future water recycling on a municipal level.
Removal of salinity corresponds to a considerable cost and a membrane-based water softener that does not increase domestic wastewater’s salt content or cost is highly desirable. It does offer the potential for significant cost savings for those municipalities that wish to implement water reuse options in the future.
Membrane-based demineralization systems do an excellent job of softening water without the addition of salt. As previously mentioned, however, RO and NF membrane systems are not routinely used for water softening applications today. This is specifically because of the quantity of water wasted by conventional membrane processes.
But a membrane process design that can reduce, and in some cases eliminate wastewater; and it therefore , has the potential to provide a real alternative to conventional salt regenerated water softening equipment. In addition, this low-waste (and in some cases no-waste) design also saves significant amounts of water when it is used in residential, commercial and industrial drinking water purification applications.
Why recycle at the point of use?
Water reuse is one of the conservation tools that water users and water purveyors have to reduce water usage. One such method that has proven to be very useful in order to conserve water is called an open loop recirculation flow pattern. It allows water to be recycled at the POU and is applicable for all membrane processes.
Recycling primarily saves water resources. How much water is saved depends on how much water is being used with a membrane process to demineralize or to soften it. How much water you want to treat will then dictate how much wastewater you create by the treatment process itself. Depending on the application, for RO/NF membranes operating on a municipal water source, the ratio of wastewater production to treated water ranges from two to ten times the amount of water you treat.
Another reason to recycle is because membrane concentrate is potable, generally of a very high quality and can be easily reused at a location, depending on the feed water TDS and percent recovery of the system. A characteristic of all membrane processes is the fact that they do not add anything to a municipal water source, but rather separates very pure H2O from that source. Therefore, if reused, concentrate created during a membrane process has an increased concentration only of those constituents already present in the water source and nothing more.
There are some caveats to this, of course. If the feedwater contains high concentrations of some element which, when concentrated, exceed a recommended maximum density, concentration that can be an issue. But this is not a problem when operating on municipal water in many parts of the world.
If a location is treating a portion of the water to a high level with a membrane, the implication is that the rest of the water supply is to be used for lesser quality functions such as washing hands, bathing, washing clothes and, of course, toilet flushing.
Another reason for recycling is that no plumbing changes are required to recycle membrane concentrate. It is of such a high quality it can generally be reused throughout a residential and commercial plumbing system with no further treatment.
Also, recycling at the POU is very efficient because a high quality water is created for use specifically for highest quality demands. This generally means water for drinking, cooking and ice making.A slightly lesser quality of water can be used for those needs that allow for a lesser quality as previously noted. [Is this paragraph necessary?]
Simply put, without recycling at the POU, a large percentage of our municipally distributed water is used for applications that do not require the absolute highest quality. [I have difficulty understanding this paragraph. It would make more sense to strike “without recycling at the POU”.]
Open- and Closed-Loop Recirculation flow patterns
A conventional, spiral wound membrane element operates with feedwater coming into the element and flowing across the surface, exiting as a concentrate waste stream. H2O and a small percentage of dissolved ions are transported through the membrane that exit as a demineralized product water (permeate). In this way, feed-flow is split into two streams – a demineralized product water flow that we want to use and a concentrate flow that is normally wasted.
A healthy flow of feedwater is necessary to keep its non-dissolved constituents from fouling a membrane and its dissolved constituents from concentrating too much and precipitating on the membrane surface to form a solid. All membrane systems must be designed to prevent a solid precipitate and non-dissolved particulate matter from forming on its membrane surfaces where it can result in blinding of the membrane and premature fouling.
In large commercial systems, fouling and scaling result in chemical cleaning. In small residential and light commercial systems, fouling and scaling generally result in membrane replacement.
A method to keep feed-flows high inside a membrane element without sacrificing recovery led to the creation of the closed loop recirculation flow pattern. With a closed loop recirculation flow pattern, as shown in Figure 1, a portion of the concentrate flow is recirculated back to the inlet of the element in order to increase flow and consequently flow velocities across the membrane without the necessity to increase feedwater.
This process is primarily used routinely in commercial systems and is a very good way to maintain high flow-rates inside membrane elements in order to minimize fouling without sacrificing recovery. As concentrate/brine is recycled back to the feedwater of the RO elements, less new feedwater is required and consequently overall system recovery increases. The recovery of an RO system is defined as:
Membrane System Recovery = Product Water Flow Rate x 100% ÷ Feedwater Flow Rate
This process design is an evolution of the conventional closed loop recirculation flow pattern and it has been created to produce a membrane-based system that can have as much as a 100 percent recovery capability. It has the potential to operate most efficiently to purify a municipal water source in residential, commercial and industrial locations where system efficiencies are low and consequently water wastage is high. Furthermore, this is accomplished while solving scaling and fouling problems that are a valid concern with concentrate recycling.
We call this new design, also shown in Figure 1, an open loop recirculation flow pattern; it has the same advantage of the closed loop system by keeping flow velocities up inside an element with the added characteristic that now a tank is used to hold membrane concentrate as it forms.
The tank volume itself provides a buffer and dilutes membrane concentrate to initially prevent precipitation of salts and minerals. In addition, the tank is connected to the normal plumbing in a building where its contents can be diluted periodically whenever water is used at a location. This prevents water-borne constituents from concentrating to the point of precipitation.
Low- (or no-) waste membrane design
In a low-waste membrane design, using faucets and flushing toilets inside a building permits periodic dilution of concentrated salts and minerals that build up during all membrane processes. In this way, effluent from a membrane system does not go directly to drain, but instead is diluted and reused throughout a location’s plumbing system.
This again usually means membrane product water that is used for drinking, cooking, ice making and dishwashing, while water containing diluted concentrate is used for sink faucets, showers, ice making equipment cooling, toilets and landscape irrigation. With the open loop recirculation flow pattern process, this separation of water usage is inherent in the design and is performed without making plumbing changes to a building’s piping system.
The only plumbing connection required is a standard water softener connection (with product water tubing installed wherever product water is desired).This membrane water-saving feature can be retrofitted quite easily at many residential, commercial and industrial locations.
In addition, the open loop recirculation flow pattern design creates a membrane system with features that are extremely flexible and can accommodate a wide variety of applications and product water flow requirements. This is demonstrated by the fact that this low-waste design can create small capacity ‘under a sink’ drinking water systems for residential use, as well as large capacity water softeners and purified water systems suitable for commercial businesses, such as restaurants, office buildings and hotels.
Figure 2 shows an installation of a light commercial membrane treatment unit. All concentrate from this unit is reused in the plumbing at the installed location.
The only ‘waste’ is the flow from the flush-tank that flushes the membrane periodically and even this modest amount of water is recycled to water outside plants. In this configuration, this unit reuses and recycles 100 percent of its concentrate – a true no-waste membrane unit.
Design limitations
Of course, all water treatment designs have limitations and a membrane system that reduces and in some cases eliminates wastewater is certainly no exception. The limitation of this low-waste design lies in the fact that as a membrane is producing water for the highest quality applications (drinking, cooking, etc.), the quantity of water used for lower quality applications (sink faucets, toilets, etc.) should be sufficient to dilute a membrane’s concentrate stream at a particular location in the long-term.
In practical terms, for residential and light commercial drinking water systems, virtually no limitations exist in many locations because of the relatively small amount of purified drinking, cooking and ice making water often required at many locations compared to the total quantity of water used at that location (so the dilution factor is relatively high). But for water softening applications, a low-waste system can generally be created by softening only the hot water used in a building and sizing a recirculation tank to accommodate short-term concentrate dilution requirements.
Implementing the open loop recirculation flow pattern can allow reuse membrane concentrate at the POU while creating the highest quality water for drinking, cooking, dishwashing and ice making and reducing (and in some cases eliminating) water wastage normally inherent in membrane processes. This should be an important value-added feature for membrane systems in all water-short regions of the world as well as all water-conscious regions.
About the Author
Ted Kuepper is Executive Director of Global Water (www.globalwater.org), an international non-profit, humanitarian organization committed to the development of safe water supplies, sanitation and hygiene facilities in rural areas of developing countries. A Registered Environmental Manager, Kuepper holds US and international patents for the open loop recirculation flow pattern design. He formerly served as Director of the Seawater Desalination Test Facility in California, where he managed equipment development, testing and evaluation projects for the US Government and private industry. Kuepper invites questions by companies looking for a water conservation design for residential, commercial and industrial membrane applications, please contact him via email at [email protected].