Boiler Feed Make-Up Water

Surface water is water from above ground sources such as rivers, lakes, wetlands or the ocean and acquires its characteristics from the environment with which it comes into contact. It typically contains various contaminants, such as silts and clays, dissolved minerals and salts, organic material from vegetation and wildlife, algae, bacteria, protozoans, viruses and man-made pollutants.

Surface waters provide a convenient and reliable water source for both municipal and industrial uses. Steam boilers used in industrial and power plants require high purity feed water to ensure proper operation. By removing a high percentage of solids and contaminants from the feed, membrane filtration allows for the usage of treated surface water as an economic alternative to municipal water.

Boiler Feed Water Treatment Systems

Steam boilers require consistent, high quality water for operation despite changes in raw water qualities to ensure an efficient process and quality steam generation. A boiler feed water treatment system is typically made up of several individual technologies that address the specific boiler feed water treatment needs. The most appropriate boiler feed water treatment system will help avoid costly plant downtime, expensive maintenance and boiler failure as a result of scaling, corrosion and fouling of the boiler and downstream equipment. For example, low pressure boilers may only need reverse osmosis or ion exchange to achieve the appropriate level of water purity, while others may require an entire water treatment system, including coagulation and flocculation, screens and filters, ultrafiltration, ion exchange, dealkalization, reverse osmosis or nanofiltration and degasification to produce ultra-pure water.

To produce such water, all bacteria and organic material, suspended solids (TSS) and dissolved solids (TDS) such as hardness, iron, silica, sodium and chlorides, must be removed. MICRODYN-NADIR offers both ultrafiltration (UF) and reverse osmosis (RO) products to produce the high quality water required for steam boilers.

Ultrafiltration

Ultrafiltration membrane modules are typically used to remove suspended solids, viruses, silica and other particles that may have passed through the pretreatment step (i.e. coarse or fine screens, automatic or sand filters or even coagulants and flocculants). The reduction in particulate matter, suspended solids and total organic carbon (TOC) will prevent RO membranes from scaling and fouling, but will also enhance turbine and boiler efficiency.

MICRODYN AQUADYN® FZ50 UF modules are made of cellulose triacetate (CTA) membranes. These are inside-out hollow fiber membrane modules that can handle up to 50 mg/L suspended solids and 100 NTU turbidity. A system using these AQUADYN UF modules is typically operated as a feed & bleed system with a yield of 90%.

MICRODYN iSep™ 500 UF modules or MICRODYN SpiraSep™ 960 UF modules may be used for feed waters with higher concentrations of suspended solids. These modules are vacuum-driven, backwashable, spiral-wound membrane modules designed to handle up to 1,000 mg/L suspended solids. Both modules are available with either a 0.03 micron PVDF membrane or a 0.03 micron PES membrane.

In one case study, MICRODYN SpiraSep™ 900-PES UF modules were used to remove TSS from canal water. The high effluent quality was fit to send directly to the plant's RO system for further treatment for boiler feed make-up water.

Reverse Osmosis

Reverse osmosis is typically used after ultrafiltration to produce high purity feed water for the steam boilers. By removing a high percentage of dissolved solids, salts, silica and contaminants from the feed, RO reduces scale build-up and corrosion rates, reduces the use of boiler chemicals due to less frequent blowdown requirements, and reduces the frequency of ion exchange regeneration downstream resulting in less downtime and lower costs.

TRISEP® X-20™ low-fouling RO elements are highly recommended if organics are still present in the UF effluent or RO feed (often due to seasonal upsets such as high or low rainfall). The X-20 membrane is a fouling-resistant membrane with a unique, proprietary formulation that results in low-fouling characteristics. The unique barrier layer chemistry does not degrade over time like some competitive “fouling resistant” membranes that are simply “modified” or “coated” standard membranes. Excellent for wastewater and other high fouling applications, X-20 membrane elements are extremely durable and offer consistent high salt rejection while lowering cleaning frequency and extending membrane life.

MICRODYN RO elements may also be used so long as most of the organics have been removed prior to the RO system. MICRODYN BW RO elements have a high rejection brackish water RO membrane that is suitable for water purification where high solute rejection is required. MICRODYN LE RO elements are ideal for water purification applications where reduced energy consumption is required. These elements feature our low energy membrane for directly replacing competitive RO products and operate at low pressure conditions.

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Cooling Tower Blowdown

Water and wastewater reclamation and reuse in industrial applications has become a growing trend in the past decade due to rising water demand and scarcity. In fact, water treatment including the use of RO is estimated at more than 30 percent of all industrial water treatment sales globally. RO is now used to treat recycled cooling tower blowdown in zero-liquid discharge (ZLD) applications as well as to purify boiler feed water.

Many industrial plants use cooling towers as an efficient means to remove large amounts of heat from hot equipment. Cooling these plants requires a considerable amount of water—typically 80% or even up to 90% of the plant’s total water needs. To treat and reuse this water instead of using fresh water, membrane technology was introduced.

Cooling Tower Blowdown Process

Cool water circulating through a plant absorbs heat from the equipment and becomes warmer. When this now-hot water is circulated back to the cooling tower, some of the water is lost due to evaporation or the misting effect (when water is lost by drifting into the air). As pure water evaporates from the system, some of the dissolved solids (TDS) are left behind, causing the remaining TDS in the water to become more concentrated. If this process is continued, the dissolved minerals concentrate to a point where they no longer remain in solution. Less soluble minerals such as calcium, magnesium and silica may precipitate to form insoluble salts that may scale in the tower or within plant equipment. For these reasons, cooling towers must be operated in ways that keep dissolved minerals in solution. Often, this is accomplished by bleeding a small amount of concentrated cooling tower water to drain while replacing the water lost by evaporation and bleed with fresh cooling tower make-up water.

Most cooling towers use potable water as their make-up supply. However, as fresh water supplies dwindle and the cost per gallon rises, plants are increasingly forced to use alternative sources such as reclaimed water from municipal or industrial wastewater treatment plants or, instead of discharging water and replacing it with new fresh water, the cooling tower blowdown may be treated for reuse. Many industries are relying on reusing cooling tower blowdown to reduce their operating costs. Although using reclaimed water reduces the use of fresh water, treatment of the wastewater may be required to make it suitable for use in the cooling tower.

Ultrafiltration

Whether municipal water, industrial wastewater or reclaimed cooling tower blowdown is used as make-up supply, ultrafiltration membrane modules may be used to remove suspended solids, viruses and bacteria, silica and any large organic molecules before sending the water to reverse osmosis (RO) membranes for further treatment. MICRODYN iSep™ 500 UF modules and MICRODYN SpiraSep™ 960 UF modules may be used for feed waters with high concentrations of suspended solids (up to 1,000 mg/L TSS). These modules are vacuum-driven, backwashable, spiral-wound membrane modules designed to operate at high yields and provide a more consistent quality effluent despite changes in raw water quality. Both modules are available with a durable 0.03 micron PVDF membrane.

In one case study, a power plant aimed to reduce the amount of cooling tower blowdown water discharged to their evaporation ponds without implementing expensive brine concentrating systems such as evaporators or crystallizers. Instead, the power plant implemented electric coagulation (EC) to precipitate sparingly soluble salts, particularly silica, alkalinity and hardness. MICRODYN SpiraSep™ 960 UF modules followed the EC process and removed all total suspended solids prior to the RO system. This treatment approach enabled the power plant to recover close to 90% of the cooling tower blowdown water.

Reverse Osmosis

Reverse osmosis is typically used after ultrafiltration to produce high purity water for the cooling towers. By removing a high percentage of dissolved solids including calcium, magnesium and silica from the cooling tower blowdown, RO reduces scale build-up within the cooling tower and scaling of the heat exchanger equipment may result in a loss in productivity on the process side of the facility. By removing these ions with reverse osmosis (RO) membrane elements, the cooling tower is able to run at higher cycles of concentration, and ultimately reduces the amount of blowdown and fresh make-up water required.

TRISEP® X-20™ low-fouling RO elements are highly recommended if organics are still present in the UF effluent or RO feed. The X-20 membrane is a fouling-resistant membrane with a unique, proprietary formulation that results in low-fouling characteristics. The unique barrier layer chemistry does not degrade over time like some competitive “fouling resistant” membranes that are simply “modified” or “coated” standard membranes. X-20 membrane elements are extremely durable and offer consistent high salt rejection while lowering cleaning frequency and extending membrane life.

MICRODYN RO elements may be used if most of the organics have been removed prior to the RO system. MICRODYN BW RO elements have a high rejection brackish water RO membrane suitable for water purification where high solute rejection is required. MICRODYN LE RO elements are ideal for water purification applications where reduced energy consumption is required. These elements feature our low energy membrane for directly replacing competitive RO products and operate at low pressure conditions.

Zero-Liquid Discharge

Zero-liquid discharge (ZLD) systems often use primary and secondary RO systems to treat wastewater for make-up water. Often times, wastewater from cooling tower and boiler blow downs is first sent to a pretreatment system or an ultrafiltration system using MICRODYN iSep™ 500 UF or MICRODYN SpiraSep™ 960 UF modules. The effluent from the pretreatment or ultrafiltration system is then sent to a primary RO system (which may consist of MICRODYN RO elements). The permeate from the primary RO is sent to the cooling tower for use while the reject is sent to a secondary RO system for further treatment and water recovery.

Because reject from the primary RO system is typically very high in TDS concentration, the secondary RO is made up of seawater RO elements. The reject from the secondary RO is then sent to a brine concentrator and crystallizer. The secondary RO system reduces the load on the brine concentrator significantly, meaning a smaller brine concentrator may be used. By using a smaller brine concentrator, capital costs are dramatically reduced.

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Digestate Treatment

Digestate is the material remaining after the anaerobic digestion of a biodegradable feedstock. Anaerobic digestion is a complex biochemical process of biologically mediated reactions by various microorganisms (without the presence of oxygen) to convert organic compounds into digestate and biogas.

 Anaerobic digestion is a well-established process for the treatment of organic waste and the generation of renewable energy. Historically, the digestate produced from the process is a nutrient-rich substance and has been applied to land as a fertilizer or soil conditioner.

 Due to the high concentrations of nutrients such as nitrogen, ammonium, phosphorus and potassium, digestate is both a versatile commodity and an environmental challenge. These nutrients are useful in limited amounts, but over application of a fertilizer can lead to high nitrate values in the ground water and can be harmful to humans or animals if consumed.

 With an increase in the number and capacity of anaerobic digestion plants to treat a variety of organic waste streams, it is necessary to treat digestate to prevent consumption of harmful nitrates.

DigestATE Treatment Process

Digestate from the anaerobic digestion process is often dewatered by a conventional centrifuge or mechanical screen. Centrifugal treatment can retain up to 40% of present solids. The digestate fiber can then be used for land restoration, fertilizer or energy recovery.

 Biological oxidation may be used to reduce the loading of organics (BOD) and ammonia in the digestate if necessary. The process is most commonly used to treat the digestate liquor prior to discharge, but can also be used as a pretreatment stage or used to treat the whole digestate (wet composting). Typically, the digestate is aerated in the presence of bacteria which oxidize the BOD and ammonia. This is can be done using a MBR system using MICRODYN BIO-CEL® MBR modules. This process produces a biological sludge as a byproduct which can be returned to the feed of the digester.

 The filtrate from the MBR system can then be further treated with reverse osmosis (using either MICRODYN RO or TRISEP® X-20™ elements depending on the organics load in the MBR effluent). The concentrate of the reverse osmosis system may contain approximately 10-15% volume of the total input and can be dried out and sold/used as fertilizer. The permeate of the RO system may be further polished using an ion exchange for discharge or reuse.

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