New Option for Ultrafiltration Pretreatment
By Gregg Poppe, Dow Water & Process Solutions

Depending on its intended use, water is treated to various degrees of purity. And depending on the raw water source, whether it is seawater, surface water, ground water, or wastewater, it typically requires multiple steps to effectively treat the water. Each step progressively removes smaller and smaller contaminants. Sure, the fine filtration provided by reverse osmosis (RO), which specializes in removing even dissolved contaminants, can also reject bigger particles such as colloids and suspended particles, and larger. But the most cost-effective treatment system does not rely on the reverse osmosis for this duty because RO elements are not optimally designed for that task, and require more frequent cleaning or replacement when employed in that manner.

The solution, then, in this example is to pretreat the water in advance of RO to remove most of the suspended solids using some type of water treatment technology that is more economically viable for suspended solids removal. For centuries, media filtration has been used for this purpose. Over the last couple decades, ultrafiltration (UF) is being recognized as a cost-effective and more reliable pretreatment for RO (more details available in “The Basics of Ultrafiltration and Reverse Osmosis” in the November/December 2010 issue of International Filtration News).

Analogously, for a robust water treatment system, successive pretreatment steps are advisable all the way back to the water source. Each step backwards specializes in cost-effectively removing larger solids, commonly with less technologically-advanced techniques, but some of these steps can still be expensive considering the size of equipment, the amount of land, or the operating expense (i.e., chemicals, electricity and air) that can be involved.

For example, typical pretreatment steps to ultrafiltration might be clarifiers, dissolved air flotation (DAF), settling ponds, and hydrocyclones. For the highest UF operating flux and recovery, resulting in the smallest capital and operating expense for the UF plant, the feed water into the UF is commonly recommended to have a turbidity of 5 NTU or less. So the pretreatment for UF should be able to achieve this level of filtration consistently, and ideally along with other attributes such as very high water recovery, low operating and maintenance expense, and a small footprint.

To compete against this list of common low-tech, large-footprint options such as clarifiers, DAF, and settling ponds for UF pretreatment, some more compact, self-cleaning filtration devices have entered the market. One of the latest entries is the patented CFT Turboclone™ Filter supplied by Clean Filtration Technologies, Inc.

From the outside, the CFT Turboclone resembles a traditional hydrocyclone (Figure 1), but on the inside it is completely different. It has been designed and optimized with computational fluid dynamics to not only remove large particles by centrifugal force, but also to remove smaller particles by cross-flow filtration through a proprietary, durable metal filtration membrane. The smallest particles (those less than the pore size of the metal membrane) are allowed to pass easily through the specially-engineered filter material. The particles that do not pass through the filter are all captured in the lower part of the housing and eliminated through the drain valve.

Also unique to the device is a water-propelled, self-cleaning brush mechanism that sweeps the surface, keeping particles away from the metal filter membrane to optimize performance (Figure 2). This cleaning assembly moves with the flow of the feed so no additional motor is required.

Unlike conventional dead-end filtration systems that operate at a high differential pressure, the CFT Turboclone operates consistently at a very low differential pressure up to 1.5 psi because, unlike dead-end filters that produce a “cake layer” requiring increasing amounts of pressure to produce the same amount of flow, the Turboclone is designed to keep the particles away from the filter screen and prevent caking, which results in a consistently low differential pressure and uniform filtrate flow. Actual flow rate through the Turboclone can vary depending on the quality of water and the filter pore size selected, but approximate flow rates are 5-15 gpm for the smallest unit, 20-60 gpm for the mid-size unit, and 80-240 gpm for the largest unit.
The CFT Turboclone has some key product features:

•Well-controlled filtration down to 10-µm cut-off (15-µm and 20-µm filters are also available)
• Operates independent of solids specific gravity; can filter solids with a specific gravity close to water
• Filters a wide dynamic range of particles from 10-µm up to coarse particles such as sand and even larger
• Flexibility to handle varying inlet water quality
• Able to handle feed water with high turbidity (> 100 NTU) and high total suspended solids (TSS > 100 mg/L)
• Very high water recovery up to 99.5% or even 99.9% by recirculation
• Easily scalable by installing parallel units with a range of unit sizes
• Small footprint, high capacity
• Chemical-free
• Virtually maintenance-free
• Low power consumption, with an operating pressure drop up to only 1.5 psi

Field Trials
The associated benefits of these features have been demonstrated by several completed field trials.

In one trial, the Bear Valley Water District (BVWD) in Alpine, California, tested the CFT Turboclone as an alternative to dissolved air flotation in the pretreatment step prior to ultrafiltration in order to ultimately produce California Title 22 reuse water. The specific objective was to reduce the turbidity in the feed source from > 15 NTU to ultrafiltration feed quality of < 5 NTU. For this trial, a 20-µm membrane was used in the Turboclone.

BVWD judged the trial to successfully meet their objectives. The pressure drop across the unit was very low over the entire test period, using 20 psi of feed pressure. In a 3-month period, the feed turbidity typically ranged from 6-10 NTU with an extreme of 14 NTU, but the CFT Turboclone filtrate consistently met the < 5 NTU goal, ranging from about 3-5 NTU, allowing the downstream UF to consistently produce water at only 0.05 NTU. The Turboclone filtrate quality was so consistent that the initial plan to install a 5-µm cartridge filter between it and the UF was deemed unnecessary, saving additional expense. As the trial concluded, the CFT Turboclone + UF system accomplished the treatment goals at less than half of the projected capital cost of the proposed DAF + UF system.

In a second trial, the Sewer Authority Mid-Coastside (SAM) Wastewater Treatment Plant in Half Moon Bay, California, evaluated the effectiveness of the CFT Turboclone as pretreatment to a UF membrane plant in producing California Title 22 reuse water for irrigation customers. The specific goal of the trial was to reduce the turbidity in the feed from > 10 NTU to ultrafiltration feed quality of < 5 NTU.

The pilot trial was considered successful by SAM management. As observed in the previous trial, the pressure drop across the test unit was very low over the entire test period, with 20 psi of feed pressure. Over a 1-month period, the feed turbidity usually ranged from 7-13 NTU, with a spike as high as 19 NTU, but the CFT Turboclone filtrate quality was always < 5 NTU, averaging about 3.5 NTU, allowing the UF to consistently produce filtrate of very low turbidity. Throughout the same period, the flow rate through the unit was consistently 35 gpm and its membrane never fouled, nor required any maintanance or operator intervention; not even a single backwash was required during the trial period. And as seen before, the quality of the water from the Turboclone was so consistent that it eliminated the need for the cartride filter, which is generally recommended by the UF system manufacturer.

As the popularity of ultrafiltration continues to grow, innovative cost-effective pretreatment solutions that offer consistent feed water to the UF while also delivering high recovery in a small footprint with low maintenance requirement, such as this option with the CFT Turboclone, will continue to be more and more interesting in order to optimize the overall operation of the water treatment plant.

In addition to pretreatment for ultrafiltration, CFT currently has installations in the oil & gas industry for salt water disposal prior to injection and on offshore oil platforms as pretreatment to RO for potable water. CFT is also evaluating the technology for water treatment in the pulp and paper industry. The Dow Chemical Company is studying the CFT Turboclone in seawater, surface water, ballast water, and wastewater applications. Dow is also an investor in Clean Filtration Technologies, Inc..

Gregg Poppe is a global application development specialist at Dow Water & Process Solutions, focusing on the industrial water and power generation markets. Mr. Poppe also serves on International Filtration News’ Editorial Board.
Tel: 1-952-897-4317 | Email: poppeg@dow.com
Website: www.dowwaterandprocess.com