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UF System Design

UF System Design

UF municipal application example (direct filtration, no coagulation)

Design a pressurized Ultrafiltration (UF) system capable of supplying 20 MLD (5.28 MGD) of drinking water (net permeate) from a lake water source characterized by low levels of dissolved organics (< 2 mg/L) and low turbidity (1 NTU typically, with spikes up to 10 NTU).

The system should have a minimum of two (2) trains. One (1) of the trains shall be redundant.

Minimum interval between CIP events shall be 30 days (monthly frequency).

Minimum interval between Maintenance Clean (MC) events shall be 2 days.

The first step is to select a suitable flux rate given the raw water quality and cleaning frequencies at design conditions.

Based on experience, piloting and full scale applications, on similar water quality, a typical flux rate (instantaneous) at design conditions is 50 gfd (85 lmh).

Assuming a UF module with a filtration area of 500 ft^2, each module can process 50 gfd x 500 ft^2 = 25,000 gpd (gallons per day) which is equivalent to 25,000 gpd : 1440 minutes/day = 17.36 usgpm/module (US gallons per minute per module).

In order to supply the desired net capacity of 20 MLD or 5.28 MGD, at least 212 modules are required, per the following:

5,280,000 gpd : 1440 mins/day : 17.36 usgpm/module = 3,666.6 usgpm : 17.36 usgpm/module = 211.2 ~ 212 modules.

The above calculation assumes the system is online 24 hours per day and no additional water has to be processed (filtered) for cleaning (backwash and maintenance clean).

In order to estimate the actual online time (OLT), the following assumptions are made:

a) backwash frequency is once every 30 minutes and takes 2 minutes b) maintenance clean frequency is once every two days and takes approximately 1 hour c) CIP downtime is not included in calculations, as one extra train is provided for redundancy d) 20 minutes are required for the daily integrity test (IT).

The online time can be calculated as follows: OLT = 1,440 minutes – 60 minutes (MC) – 2 x 2  x 23 (BW) – 20 (IT) = 1,440 – 60 – 92 – 20 = 1,440 – 172 = 1,268 minutes per day (approximately 21 out of 24 hours).

The online time percentage (OLT%) is therefore: 1,268 : 1,440 = 88%.

Assuming a recovery rate of 95%, approximately 2.5% (half of the reject or backwash water) represents permeate used for backwash and maintenance clean (MC) operations.

The number of modules would have to increase in order to maintain the same design flux rate (50 gfd), while producing 2.5% more permeate in a shorter amount of time, 1,268 minutes, rather than 1,440.

N_required = 212 x 1.025 : 0.88% = 247 modules (a 16.5% increase).

Next, the above number of modules would have to be divided over the number of duty trains, minimum 2.

The goal is to reduce the number of trains to a minimum, keeping in mind that minimizing the number of duty trains would result in a larger redundant train (more modules overall and higher cost).

My preference would be for a 3 duty + 1 redundant (4 x 33%) design to minimize the number of modules and the flow fluctuations when trains is taken offline for cleaning.

Since it is preferable to choose the number of modules per train in multiples of four, one could select either 80 modules per train (3 x 80 = 240, resulting a 3% higher flux rate), or 84 modules per train (3 x 84 = 252, 2% lower flux).

Considering N_selected84 modules per train, the total number of modules, including those installed on the redundant train, would be: 4 x 84 = 336 modules. Should we have selected only two (2) duty trains, the total number of modules would have been: 252 : 2 x 3 = 378 (42 extra modules).

In actual production, all four trains are online producing permeate (drinking water). Based on an average flow rate, usually half of the design capacity (10 MLD or 2.64 MGD), the average flux rate becomes approximately 18 gfd (36% of our original design flux of 50 gfd).

In the next post, we will evaluate the capital and operating cost of such system, along with the life cycle analysis over a 20-year horizon.

Should you have any questions regarding UF sizing and design, please feel free to drop me a line at:

cornell.evans@watertechnologiescanada.com

 

 

Water Technologies Canada Inc.

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