In any mechanical system, choosing the drive is important to the health of the system and the efficiency of the process. In Ultrafiltration membrane based plants, one of the fundamentally and profoundly important choices that an user makes is the selection of the right feed, backwash and dosing pumps. This article will discuss the factors to keep in mind will designing/choosing the pumps for UF based systems especially the ones using Theway Membranes.
1. Hours of operation - If the planned system is to be operated for more than 16 hours a day, it is important to ensure that the pumps purchased are of higher quality and from reputed manufacturers, who will have service personnel within 24 hours of a raise of complaint to the site of the UF plant. It is always prudent to ensure that there is a standby pump to ensure continuous operation of the UF plant.
2. Power rating - It may shock you to know that CAPEX in a UF plant is anywhere between 1/15th to 1/80th of the cost of OPEX over the lifetime of service of the UF plant and membrane. One of the main factors deciding OPEX is power of pump. While choosing an UF pump, care should be taken to not select a pump with a much higher HP or kW rating than required. This will have an adverse impact on OPEX. Also care is to taken to select pumps with the highest efficiency possible.
3. Pressure rating - Head selection is extremely important while selecting a pump for your UF membrane. During design, it is very easy to ignore the role the piping size and length of pipe between pump and membrane and height difference between pump and membrane. It is also important to look at the Transmembrane pressure that the membrane manufacturer suggests for successful operation of the UF membrane. It is prudent to aim to operate at a TMP of between 1 - 1.5 bar for regular water, wastewater applications. A higher feed pressure can increase chances of water hammers and breakage of fibers. It is also very important to factor in piping losses, head losses while selecting the pressure rating of your pump. Manufacturer suggested Max Feed Pressure and Max TMP are for ideal piping conditions. For any major deviation in UF plant configuration in terms of elevation differences between pump and membrane and extended distances between pump and membrane, the effects need to be considered and proper modifications in pressure rating selection of the pump needs to be done. One can say that this is the most important criteria in selection of pump, since this majorly defines permeate flow rate.
4. Flow rating - All pumps have curves that need to be studied properly. Each pump has a sweet spot range of flow rate for a given head. Head, generally is expressed in meters. 10.19 m is 1 bar. Considering that you would like to operate with a feed pressure of 1-1.5 bars and maximum feed pressure of 2 bars to the membrane, in the case of ideal piping (reasonable distances between pump and membrane, reasonable height difference between pump and membrane), it is prudent to select a pump that can provide the required feed flow rate at 2.0 - 2,5 bar. The same applies for a backwash pump, while using Theway UF membranes. To understand the required feed and backwash flow rate, refer to Membrane manufacturers projection (MEPSOR in the case of Theway Membranes). For your projection, please write to . Manually, flow rating required can also be reverse calculated based on permeate flow required, permeate flux recommended from manufacturer and recovery (Generally between 85-92%)
5. Material of pump - Feed parameters need to be understood for selection of pumps. Based on feed parameters, it is always better to get the recommendation of pump manufacturers while choosing the pump. For applications that have very low TDS (Total Dissolved Solids) and non corrosive components in the feed water, Cast Iron is the way to go considering ready availability, widespread use and low cost. Based on the increase in TDS and corrosive components SS 3o4, SS 316, SS 316L, Duplex Steel, Hastelloy can be chosen.
For chemical dosing pumps that will be used in chemically enhanced backwash, SS 316 is the minimum grade of material to be used due to the varied pH and TDS range that the pump will be exposed to.
6. Voltage and Phase - Three phase motors are to be preferred over single phase for UF membrane systems, since three phase transfers power more efficiently and has a lower operating cost compared to single phase mode. It is worth to shell out those few extra bucks for getting the three phase motor, since the ROI periods are so short. Only in facilities where three phase is not available, then single phase motors are acceptable.
7. Efficiency - Motors are classified on the basis of their efficiency levels as per the International Efficiency -30-1. On the basis of the efficiency levels, the induction motors are classified into 4 categories which are IE1, IE2, IE3, and IE4. The IE1 motors lie in the Standard Efficiency range. The IE2 motors lie in the High-Efficiency range. The IE3 motors lie in the Premium Efficiency range and the IE4 motors lie in the Super Premium Efficiency range. For UF systems by IE2 and IE3 motors are the most used.
8. Ingress Protection - In most UF membrane systems, Ingress protections or IP levels of IP 55 and above are preferred. This ensures dust resistance and resistance to splashes of water. Higher IPs are preferred, but cost escalation and actual environment of the motor should be kept in mind.
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One facet of technological advancement is attempting to mitigate the more glaring issues that consistently crop up due to the nature of a system process. Of course, even with decades of improvement nothing is infallible. In this article, we will discuss the common issues that can occur using UF filtration systems.
Ultrafiltration is a pressure driven membrane separation technology that is a compact and refined filtration method utilized in drinking water and tertiary wastewater reuse applications. Its semipermeable membrane can remove solids as small as 0.01 microns, including silt and viruses. However, membrane filtration technologies will have problems without proper care for appropriate pretreatment, operation and maintenance.
UF filter systems are typically affected by three main issues. For information on how to resolve these issues, see our upcoming article on how GWT solves problems with ultrafiltration in order to optimize the process.
Membrane Fouling
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UF filtration, like any other membrane separation technology including reverse osmosis, is susceptible to what is known as membrane fouling. In simple terms, fouling is what happens when particulate matter adheres to the surface of a membrane. The unchecked buildup will eventually cause reduced efficiency, a pressure drop, and increased energy consumption.
There are a few different types of fouling that can occur. Each has its own cause as well as some differencse in effects. Of these membrane foulants, some are reversible and others are irreversible.
Solids
Suspended solids and colloidal particles collect on the surface of the ultrafiltration membrane as well as within its pores, preventing the flow of water through the membrane. This fouling occurs more commonly in applications with high turbidity and suspended solids without appropriate pretreatment.
Scaling
Membrane scaling is not unlike what happens in pipes that carry water with high concentrations of hardness materials. When the concentration of these dissolved minerals is high enough to surpass the saturation limit of the solvent solution, they begin to precipitate out of solution onto the surface of the membrane. These minerals can crystalize, which makes them nearly impossible to remove without some sort of chemical cleaning or antiscalant pretreatment. Calcium and magnesium are two primary minerals that can cause scaling to occur on the UF filter systems membranes.
Microbiological
Biological contaminants like algae and microorganisms are often found in surface water sources. Provided with a warm environment and low flow rates, these contaminants will attach themselves to the surface of a membrane and begin multiplying. Over time, they can form a film that will prevent water from passing through the membrane and cause an increase in the trans membrane pressure differential. This increased pressure differential will put more strain on the pumps and increase the amount of energy they draw.
Waste Stream Disposal
This relates to the UF filter concentrate discharge. The filtration system did what it’s supposed to do and you have clean water that you can safely discharge into an outdoor stream without having to pay any environmental regulation fines. Or maybe you are going to reuse it somehow. Regardless, of what is going to happen to it, you have this water resource.
However, what about all that contaminants that were removed? Sadly this concentrate stream didn’t disappear into thin air, never to be dealt with again. Nope. It’s still there, whether it’s stuck to the membrane or sitting in a concentrate waste tank, and something needs to be done about it.
The problem is, you can’t just toss it out the window and call it a day. This reject wastewater is a concentrated form of whatever was in the feed water. Therefore, in some cases, it may be safe enough to discharge into the environment, however, in others, the facility would be charged a hefty fine if it contains harmful pollutants.
Increased Permeate Contamination
This point is pretty rare for systems that are well maintained and monitored. To reiterate, permeate refers to the water that has been separated from the contaminating solids. It’s the clean water that you get out of this filtration process. Therefore, it’s definitely an issue when you start noticing that the quality of your permeate water is getting worse. Either there are larger solids or bacteria that should have been retained by the membrane contaminating the water.
This decrease in removal efficiency is usually indicative of a compromised membrane. Polymeric membranes can get worn out over time. High temperature or pH levels can degrade them pretty quickly, and without a decent pretreatment regime, rough particles can damage the inner pores of the membrane. To state the obvious, membranes do not work very well if they are full of additional holes (other than their pores of course). And now the system isn’t meeting it’s designed specifications and you have to replace the membrane and recirculate the contaminated permeate.
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