The present want for water footprint enhancements and continuous cost reductions has made the reuse of course of and wastewater in energy plants more and more important. This recycling of water will proceed to be a significant matter as the industry moves into the future.
Cooling towers require probably the most water in power plants, accounting for about ninety% of the plant’s total water wants. A significant amount of blowdown water is generated when running the cooling towers at totally different cycles of concentration, normally exceeding four. In the past few years, the conventional remedy scheme has the blowdown water being handled by softening and Reverse Osmosis (RO) membranes to remove suspended and dissolved contaminants. The handled blowdown water (permeate or filtrate) is then reused as boiler feed or process water. A portion of the blowdown, the RO reject stream, is shipped to brine concentrators or evaporators. That is important to enable the idea of zero liquid discharge (ZLD).
The therapy of blowdown water from the cooling towers is an unlimited technical problem and new technologies equivalent to microfiltration (MF) as protection to RO treatment are increasingly being employed to meet the stringent specifications.
The Indiantown, FL, Cogeneration plant is a 360 MW coal fired power plant. It has 2600 psi boilers supplying steam continuously to a “steam host.” The cooling tower is run at four-7 cycles of focus. The plant is designated as a ZLD facility. Blowdown volumes treated from the cooling tower have been designed to be 650 gpm. A Spray Drier Absorber (SDA) is used to cut back SOx emissions and evaporate the reject stream. Two brine concentrators have been initially used to process cooling tower blowdown water and provide high-purity boiler feed water.
Filtered makeup water is sent as feed to the cooling tower. Blowdown from the cooling tower was originally sent to the 2 brine concentrators (with out membrane remedy), and the clean distillate from the brine concentrators was used as boiler feed water after mixed bed demineralizer therapy. The reject from the brine concentrators was used as feed water to the SDA and was evaporated by the heat from the gasoline gasoline, thus reaching the specified zero liquid discharge. Nevertheless, the brine concentrators suffered from in depth corrosion issues, requiring expensive alternative.
The make up water to the cooling tower comes from three completely different sources:
1. Floor water from Taylor Creek, which is high in organics and cannot be used throughout droughts. That is the primary water supply for the plant.
2. Extremely saline floor water from wells. That is used only during droughts and creates essentially the most troublesome processing case for the blowdown.
Three. Treated municipal wastewater. This is used in combination with surface water during normal operation or properly water throughout droughts.
Combos of these three water sources are filtered before feeding the cooling tower.
Foremost Problems Encountered
The Brine Concentrators endure from stainless steel pores and skin corrosion points. Elimination of these problems would require vital downtime and substitute prices within the hundreds of thousands of dollars.
The 2 brine concentrators consumed zero.7 MWH each to function and discount of this parasitic load was desirable. The very best final result could be an entire bypass of those models.
The plant required flexibility to use any mixture of make up water to the cooling tower. A membrane system needed to be designed to accommodate totally different water high quality within the blowdown and supply low TDS permeate to the blended mattress demineralizer.
Membrane System Pilot Testing
In mid-2009, an automatic Pall single MF module was used within the pilot to treat the blowdown water. An automated pilot scale RO with four 4-inch diameter RO parts was additionally used to deal with the MF filtrate. The principle goal was to show the stable operation of each MF and RO.
The principle findings from the pilot study have been that the MF/RO system confirmed to be a viable expertise for the treatment of cooling tower blowdown water. The pilot MF system was successfully operated at 35 gfd with recoveries >91%. The recorded inlet turbidities on the blowdown water ranged from 1-25 NTU. Filtrate Turbidity was >zero.02 NTU with the Silt Density Index (SDI) >2 on the feed to RO. Iron in the cooling tower blowdown was recognized as a significant concern. Organic/biological fouling was noticed within the RO units after prolonged shutdown and required excessive pH cleaning.
The standard clarifier/multimedia filters for the remedy of incoming fresh water into plants endure from several shortcomings, the primary one being the inability of those techniques to cope with sudden upset circumstances that might lead to increases in whole suspended solids within the feed water. This can be mirrored in an increase in SDI, conductivity, or in turbidity in the product water.
Technological improvements for the reason that 1990s have resulted in processes resembling Microfiltration (MF) and Ultrafiltration (UF) turning into each economical and well-liked. In a typical utility, the incoming water passes through a number of thousand spaghetti-like hollow fiber polymeric membranes that remove suspended solids and bacteria. The effluent efficiency for turbidity of MF and UF is similar.
For elimination of dissolved solids, the handled water from the MF unit passes by means of the spiral-wound RO membranes. This know-how is employed before the demineralizers. The pores within the RO membrane are only some angstroms in dimension and might take away a majority of the dissolved salts.
Modes of Operation
The MF filtration programs can be operated in the dead-end mode or in crossflow mode. The RO models operate within the crossflow mode. The MF unit described on this paper makes use of a hollow fiber membrane operated within the conventional useless-finish filtration mode whereby the feed water flows in from the surface to inside of the hollow fiber and the suspended particles and micro organism are captured throughout the filter and the permeate is sent to the RO unit. These filters have a unique air scrub/reverse flush cleaning methodology every 10-20 minutes and are also frequently cleaned with chemicals.
The RO unit operates within the crossflow mode, by which the feed water flows parallel to the membrane floor. The water that is filtered by way of the high-quality pores (permeate) is mostly devoid of dissolved salts.
The Pall AriaMF system was installed at Indiantown. The fully automated system options 6350 hollow fibers made from PVDF in a single module. It gives very good membrane integrity and operates within the lifeless-end mode. Feed water enters the bottom of the module and is distributed uniformly to the outside of the hollow fibers. It operates beneath low stress, usually 45 psig. The water, below stress, flows through to the membrane core and the permeate flows to the highest of the module, from the place it’s conveyed by a filtrate header to the subsequent unit – in this case, the RO membrane.
Cleansing and Maintenance
Flux upkeep is finished continuously to lower the TMP values. Air scrub includes the injection of air at low strain into the feed aspect of the module. Clean filtrate can also be pumped in a reverse direction by means of the hollow fibers to dislodge the foulants and deposits. Enhanced Flux Maintenance (EFM) uses hot water with mild chemical options and is completed periodically to lower TMP. As the TMP approaches 1.7-2.0 bar (25-30 psi), chemical clear-in-place (CIP) is performed. This can be a two-step protocol, first utilizing a caustic after which an oxidant resolution, to return the modules to “practically new” condition. This is finished tons of of times over the lifetime of the ingredient.
A two two-stage spiral RO system was put in at Indiantown. The RO system was then absolutely integrated with the MF system, having a typical management system and the same cleaning chemicals. The MF system acts because the pre-filter to the RO system.
Throughout the primary month of operation (Could 2011), high pressure drops were persistently recorded on the first stage RO. An autopsy indicated extreme microbiological fouling on the main RO elements. A biocide (DBNPA) was injected at 100 ppm into the blowdown (MF feed). This eradicated the biological fouling problem and the system ran easily.
The plant additionally experienced fouling as a result of aluminum in the second stage RO, particularly when surface water was fed to the cooling tower. This drawback was solved by reducing the pH in the feed from 6.5 to 5.5. Additionally, a particularly environment friendly cleaner was used to wash the RO modules during this period till the pH discount mitigated the issue.
Determine 2 illustrates the variation of Trans Membrane Pressure (TMP) of the Microfiltration Unit and the turbidity of the incoming water to the unit (i.e, the cooling tower blowdown). It’s proven over a seven-week period. The TMP rose slowly from the initial value of about 5 psig to around 25 psig during this interval. A CIP was carried out shortly after this and showed that the membrane was cleaned effectively and worked well. The inlet turbidity diverse from 7-20 NTU. The turbidity of the filtrate was persistently below 0.2 NTU, thereby meeting design targets.
Figure 3 illustrates the variation of SDI over a seven-month period from August 2011 – March 2012. The measurements were made on the filtrate from the MF system being despatched as feed to the RO system. The SDI value remained effectively under 2.0, which displays the excellent safety offered by the MF to the RO system. Usually acceptable business requirements are SDI<Three.Zero.
Figure four reveals the variation of strain drop in the primary RO train as a perform of time from September 1 – October 25, 2011. The strain drops on each stages of RO A are proven over this seven-week period and the cumulative stress drop can also be shown. The stress drop remained relatively flat over this interval, indicating consistently good performance with out the need for a cleansing of the RO modules. Related outcomes have been obtained on the second train (RO B).
The automated integrated membrane system has efficiently handled difficult cooling tower blowdown water, and has met performance objectives for greater than a 12 months now. The system allowed for full alternative of the brine concentrators, resulting in a reduction of parasitic load and substantial savings.
All preliminary operational challenges were successfully overcome, and zero liquid discharge was achieved.
The outcomes demonstrated that the Membrane System generated very prime quality permeate that may very well be used as boiler feed. The plant now has the flexibleness to deal with completely different quality feed waters with the membrane system.
The Microfiltration System provided successful protection and easy operation of the RO system. A return of funding can be achieved inside three years. This case examine illustrates how membranes can be used to achieve Zero Liquid Discharge, while effecting vital savings in energy plant working costs.
In regards to the Authors: Marvin Drake holds a B.S. in chemistry from the University of South Florida, and has greater than 30 years of expertise in industrial water treatment. Ram Venkatadri holds a Bachelor of Technology in chemical engineering from the University of Madras and a Ph.D. in chemical engineering from the College of Pittsburgh. Dr. Venkatadri has more than 20 years of experience in the power, polymer, olefin, and refining industries. Narasimha Charan holds a Bachelor of Engineer in Instrumentation Technology from Bangalore College, and has greater than 17 years of expertise in industrial water remedy. Spence Sensible is a gross sales market supervisor for Pall Company, Power Generation Division. With more than 15 years of ‘in-the-discipline’ experience, he focuses on applying integrated membrane systems in energy plants.