What is a reasonable recovery rate for a reverse osmosis plant system?

What is a reasonable recovery rate for a reverse osmosis plant system?

The utilization rate of the reverse osmosis system is usually based on the performance of the membrane modules and working experience, and the utilization rate can generally be expressed as follows:
Primary RO system software: Utilization rates for small to medium sized systems (less than 6 membrane modules) are set at 50% to 75%.
A reverse osmosis system: small, medium and large comprehensive utilization rate is set at 75%.
Secondary reverse osmosis system software: utilization rate is set at 85% or more.

I. Utilization rate, extraction factor and electrode polarization factor briefly look back

Utilization of the formula: R = (Fp / Ff) x 100%
In this formula:
Ff means ro reverse osmosis membrane (system software) water intake (enterprise: meters / clock).
Fp ---- ro reverse osmosis membrane (system software) water production rate (meters / bell)
R ---- ro reverse osmosis membrane (system software) utilization rate
Extraction Factor Formula: CF = 1 ÷ (1 - R)
The CF in the formula calculation indicates the extraction factor.
R ---- ro reverse osmosis membrane (system software) utilization.
Concentration Value Electrification Factor
β (one-stage RO) should be less than or equal to 1.2; β (two-stage RO) should be less than or equal to 1.4.
Remark name: The utilization rate of traditional type RO reverse osmosis (RO) membrane alone membrane module is up to 15%, while the limit value utilization rate is up to 15% * 1.2 = 18%. Within the RO software development system, generally 15% as a design specification, when the value is exceeded will issue an alarm.

The total number of membrane components less than 6 small RO system software, can appropriately relax the above larger utilization limits.

Second, what are the factors affecting the utilization rate of RO system.
RO system utilization factors can be divided into four main parts: insoluble salt buildup, electrode polarization, tail-end component concentrated water flow and balanced membrane flux. The actual factors associated with each part are shown below:

The insoluble salt buildup is related to the source water quality and its recycling percentage.
Electrode polarization is the increase in salt concentration on the concentrated water side of the membrane module surface.
The concentrated water output of the tail end device should be sufficient to effectively drain the concentrated water with rising salts and reduce the risk of scale buildup on the concentrated water side of the tail end.
Membrane flux: A balanced membrane flux ensures that the efficiency of the front and back membrane modules is more proportional, preventing the utilization rate or membrane flux of a certain membrane module from being too high.

The superposition effect ① ② ③ occurs in the surface layer of the concentrated water side of the membrane module at the end of the plant (the final membrane module), where the risk of fouling by insoluble salts is the greatest. The solubility product constant Ksp for insoluble salts corresponds to the square meter multiplied by the extraction factor of the source water and the polarization factor of the electrode (for AB-type insoluble salts, A2B and AB2 are the third power). Taking 75% calcium bicarbonate utilization as an example, the extraction factor is calculated as 1/(1-0.75)=4, and the electrode polarization factor β is 1.2, resulting in CF*β=4.8, and the Ksp is 23.04 times the Ksp of the source water (feed water).
Considering that ② can be solved by short-time low-voltage cleaning, and ③ and ④ can be adjusted by effective sequencing of the membrane module, the utilization rate of RO in general system is mainly affected by the problem of the accumulation of insoluble salts. The limit is affected by the superposition of ①, ②, and ③, while the limit is only affected by the extraction ratio effect of the system software. Taking the above example as an example, its maximum is about 23.04 times, and the low limit is 16 times.

When we assume that the sorting of the system software membrane module is in a saturated state, the main factor that jeopardizes the recycling ratio is the quality of the source water, especially the buildup of calcium and aluminum silicates. Calcium is mainly in the form of calcium carbonate and calcium bicarbonate, with calcium bicarbonate buildup usually decisive when the sulfate concentration is significantly higher than the pH (which is very common in coal and chemical wastewater), but otherwise calcium bicarbonate buildup is predominant. Aluminosilicate buildup, on the other hand, suffers from the obvious direct effects of temperature and scale and corrosion inhibitors, and tends not to have a significant impact on the system's ultimate value utilization (except for MgCl2-rich water resources).
The following is my purely mathematical calculations (LSI and Ksp) based on the different source water bodies of insoluble salt build-up on the utilization of the limit of the statement (taking into account the effect of electrode polarization, the specific utilization limit is likely to be slightly higher than the table in the value of the label).
From the above statements, it is easy to see that in many cases the main factor limiting the utilization of the system software is the buildup of calcium carbonate. Only in the lower temperatures and higher magnesium chloride content under the premise of the precipitation of aluminum silicate will become relevant factors, and in such cases, the effect of scale and corrosion inhibitors also play an important role. Thus, in the data pretreatment link if you can not use water softening equipment, full utilization rate is difficult to reach 75% (even with scale and corrosion inhibitors). But when a reasonable layout of water softening equipment (Ca2+ <0.03mmol/L), calcium scale risk is substantially reduced, when the full utilization limit generally exceeds 75%.


III. Conclusions and recommendations (primary reverse osmosis system)

A. Small and medium-sized reverse osmosis system (the total number of membrane modules is less than 6, and the utilization rate suffers from a relatively large system structure limitation):

Source water body is good (such as drinking water, etc.): proposed utilization rate of 60-66.7% (extraction factor in the middle of 2.5-3), if necessary, can be upgraded to soften and anti-scaling machinery and equipment.
When the source water body is weak (such as water and iron and carbon filler, etc.), the proposed utilization rate is set at 50%-60%; if softening and scale inhibiting machinery and equipment are installed, the utilization rate can be increased to 60%-66.7%.

B. Small, medium and large RO system software (membrane module size between 6 and 20 both, the utilization rate to a certain extent suffers from system structure limitations):

Source water body is good (such as drinking water, etc.): proposed utilization rate of 66.7% -75% (extraction factor of 3-4), if necessary, can be added to soften and scale inhibition equipment.
Source water body is weak (such as water and iron and carbon filler, etc.): proposed utilization rate is set at 60-75%, and called for the assembly of softening and scale inhibition equipment.

C. Small and medium-sized and above RO system software (the number of membrane modules up to 20, the utilization rate is basically not limited by the structure of the system, unless the place is limited).
Excellent source water (e.g. drinking water, etc.): It is proposed that the utilization rate be set at 75% (with an extraction factor of 4) and called for the assembly of softening and scale inhibiting equipment.
The source water body is weak (such as water and iron and carbon filler, etc.), it is proposed to install the utilization rate in the middle of 60% to 75%, and it is necessary to assemble softening and scale prevention equipment.