Reverse osmosis systems with direct production, some construction choices
In the domestic environment, the need of water treated with osmosis without accumulation has led to the development of direct production plants. The water is treated at the moment without accumulation. The choice of project must take into account various aspects linked to each other.
- The effectiveness
- The duration in time
- System costs
- Operating costs
Finding the right balance point is not so obvious
Membranes in series or in parallel?
In the past, it was common to put the membranes in series. The waste from the first membrane was sent to the second, with the advantage of saving water. But the water saving was not sufficient to justify replacing the last membrane with a high frequency. This is because the wasted water left a large amount of deposit on the last membrane. The clogging of the membranes created: poor performances, a regulation of the conductivity that was not easy and the duration of the consumable parts was limited in time.
Even using three membranes, of which the last received the waste of the first two, did not resolve the issues, (even if the production of permeate was much higher)
The membranes in parallel meet the requirement of working in a homogeneous way, with the replacement of both in the same period.
Today are used 1812 or 2012 membranes, their duration depends on the use, the quality of the incoming water and the construction choices. Membranes must always remain as much as possible without surface deposit. To work well and last over time, the membrane is “water hungry”. A ratio of 1 to 2, i.e. one liter of permeate every two discarded, is the threshold beyond which it is good not to go.
The operating pressure that regulates the flow rate cannot exceed for two reasons: if we slow down the waste, the deposits increase, and at the same time we strain the system components, the vessels
Single membrane or two membranes in parallel?
Today the plants work with membranes for a capacity that varies between 300 and 400 GPD (4bar 20 ° C), with an hourly production that varies between 90 and 110 liters per hour (always in the absence of recirculation).
The difference between the use of a single membrane 3012 of 300, 400 or even 500 GPD and the pair of two membranes of 150-200 GPD lies in two factors: the surface of the closure cap of the vessel and the internal flow rate.
At 9 bar the thrust on the cap of the 1812 is about 190 kg while at the same pressure the thrust on the cap of the 3012 is more than double 420 kg. The component is much more stressed and at risk of breaking.
Another aspect that should not be underestimated is the internal speed of the water on the membrane, it is inversely proportional to the square of the radius. The more the diameter increases, the slower the water flows. In membranes 3012, the operating pressure cannot be pushed upwards and at the same time the concentrated waste ratio must drop to ensure a sufficient speed for surface self-cleaning. The more membranes increase in diameter, the more they are ‘hungry for water’ for the same production.