Industrial RO, water softener

Commercial RO


Zero Liquid Discharge (ZLD) System

Zero liquid discharge is a process that is beneficial to industrial and municipal organizations as well as the environment because it saves money and no effluent, or discharge, is left over.

Everyone needs water. Supplies of water are vital for agriculture, industry, recreation and human consumption. One problem that the water industry faces is disposal of concentrate from advanced water treatment processes.

Zero liquid discharge definition

Zero-liquid discharge (ZLD) is a water treatment process in which all wastewater is purified and recycled; therefore, leaving zero discharge at the end of the treatment cycle.

The water recovered in the process is 90-95% of the inlet water to the plant. It means only 5-10% water is the waste water which is treated further for evaporation.

The remaining wastewater is treated using different technology for converting it to solid waste which can be used for land filling activities.

Forbes Pro Water Projects also uses latest technologies such as Membrane Bio Reactor to recycle the sewage water and industrial wastewater from the plant to recycle back to process us

Process used for this are:

  1. Solar Pond Evaporation
  2. Mechanical Evaporation using solar pond
  3. Thermal Evaporation

The principle of “zero discharge” is recycling of all industrial wastewater. The zero discharge system essentially ensures no discharge of pollutants into the environment and recovery of water. There are different processes used to recycle wastewater for reuse.

Solar Pond Evaporation

The objective of the system is to evaporate the waste water using solar energy for solid wastes disposal option. The high TDS (total dissolved solids) water will be pumped into the solar evaporation pond. Due to natural evaporation process the solids concentration takes place.

Once the concentration is reached the form of salt solids gets separated out and the salt is taken out by scrapping the solar pond.

Solar Pond with Mechanical Evaporation

The objective of the system is to concentrate the rejects or high TDS streams and make it in the solids/slurry form. The slurry/solids can be disposed off by the Hazardous Solid wastes Disposal option.

The high TDS water will be pumped into the solar evaporation pond. In the first phase of the solar ponds, the pumps re-circulate the rejects a number and increase in the solids concentration takes place.

Once the concentration is reached where the liquid cannot be pumped or recirculated, the concentrated liquor The water recovered in the process is 90-95% of the inlet water to the plant. It means only 5-10% water is the waste water which is treated further for evaporation. is taken to the Auxiliary ponds and allowed to dry naturally. In these ponds, no recirculation takes place.

Thermal Evaporation

Normally RO reject water contains minerals & salts in huge quantity. So we cannot directly let this water into the environment. It needs to be treated and made suitable as per environmental conditions. For that we use Thermal evaporation method to recycle the RO reject water.

Reject water is collected into balance tank and fed into pre-heaters (type of shell & tube heat exchangers) where it is heated up to 70-80oC with calendria chest vapours to provide the initial preheating at cheaper cost.

After evaporation, liquid is separated into vapours & concentrated liquid. Vapours are collected in vapour separators and concentrated liquid is again fed into next calendria for heating. This process is repeated until final concentrated slurry is left. This slurry could be used for land filling

Financial Advantage

Zero liquid discharge minimizes the consumption of freshwater as the cost of treated water is 50% less than the fresh water intake; therefore by reuse of wastewater it helps relieve freshwater availability limitations in places where it is scarce or expensive thus considerable savings are realized and that resulted in a moderate payback period. In addition, elimination of liquid discharge also helps towards the need to comply with increasingly stringent environmental restrictions.

Environmental Impact

Purchased water, wastewater treatment and disposal costs are significant; thus, savings associated with minimized new water requirement and wastewater flows can justify capital expenditures to minimize. In the case of new constructions, zero liquid discharge can save money on real estate costs, since location near a suitable water resource would not be necessary.

ZLD SPECIFIC TECHNOLOGY

The ZLD System removes dissolved solids from the wastewater and returns distilled water to the process (source). Reverse osmosis (membrane filtration) may be used to concentrate a portion of the waste stream and return the clean permeate to the process. In this case, a much smaller volume (the reject) will require evaporation, thus enhancing performance and reducing power consumption. In many cases, falling film evaporation is used to further concentrate the brine prior to crystallization.

Falling film evaporation is an energy efficient method of evaporation, typically to concentrate the water up to the initial crystallization point. The resultant brine then enters a forced-circulation crystallizer where the water concentrates beyond the solubility of the contaminants and crystals are formed. The crystal-laden brine is dewatered in a filter press or centrifuge and the filtrate or centrate (also called “mother liquor”) is returned to the crystallizer. The collected condensate from the membranes, falling film evaporator and forced-circulation crystallizer is returned to the process eliminating the discharge of liquids. If any organics are present, condensate polishing may be required for final cleanup prior to reuse.

Zero Liquid Discharge (ZLD) systems, one possible solution to concentrate disposal. ZLD disposal is the only option currently available in many inland regions where surface water, sewer, and deep well injection disposal are prohibited.

A ZLD-system can produce a clean stream from industrial wastewater suitable for reuse in the plant and a concentrate stream that can be disposed, or further reduced to a solid. Furthermore, the prevalent technologies used for ZLD-systems and different types of components in a ZLD-system are being described as well as the possibility of integrating Xzero’s MD-technology into a ZLD-system.

Every ZLD-system is unique and has to be custom made each time. Today most of the ZLD facilities are primarily industrial and power plant applications. Typical waste streams that produce large volumes of wastewater include cooling tower blowdown, gas scrubbler blowdown, ion-exchange regeneration effluent and rinses, plant washdown and rain water runoff, and process wastes.

Typical industries producing these wastewaters are listed below:

  • Semiconductor manufacturing
  • Power industry
  • Printed circuit board manufacturing
  • Plating and metal finishing
  • Food and beverage

As an example one can mention that a typical semiconductor fab consumes 1-2 million litres ultra pure water (UPW) per day. And the feed water flow to be treated in a power plant can be 1000 gpm (gallons per minute). These large quantities of water demands advanced water treatment technology. Different ZLD-systems For over 30 years vapor compression evaporation has been the most useful technology to achieve zero liquid discharge.

Evaporation recovers about 95 % of a wastewater as distillate for reuse. Waste brine can then be reduced to solids in a crystallizer/dewatering device. However, evaporation alone can be an expensive option when flow rates are considerable. One way to solve this problem is to integrate membrane processes with evaporation. These technologies are nowadays often combined to provide complete ZLD-systems. The most common membrane processes used so far are reverse osmosis (RO) and electrodialysis reversal (EDR). By combining these technologies with evaporation and crystallization ZLDsystems have become less expensive.

They are however combined differently depending on the circumstances, see chapter general guidelines. Together with these components, a variety of other well-known water treatment technologies are used in ZLD-systems for pre-treatment and polishing treatment. These treatments are:

  • PH adjustment
  • Degasifier
  • Mixed/separate bed
  • Oil/water separator
  • Neutralization
  • 4 Oxidation (UV, ozone, sodium hypochlorite)
  • Dissolved air flotation (DAF)
  • Carbon adsorption
  • Anaerobic or aerobic digestion the variation of ZLD-systems are, as previously mentioned, endless.

Step 1. Reaction step. Influent waste water is pH adjusted, then mixed with organic and inorganic coagulent additives. The polymers react with the contaminants to form spheres. The reaction is complete in a few minutes.

Step 2. Filtration. The filtration is accomplished through low pressure membranes. The clean water then exits the top of the filter while solids are retained on the filter membrane. Operating pressures remain below 15 psi (1 bar).

Step 3. Backflush. The membranes are pulsed to remove solids and then solids are pumped to a settling tank.

Step 4. Solids formation/De-watering. Solids are pumped to a holding tank for further settling.

Conventional filter presses can be used to further separate and de-water solids. Overflow filter press water is returned to the reaction tank. 7 Tenergys’ plating waste water recovery system clean water effluent pre-treatment polishing plating Flash evaporator Ultra filtration Carbon filter Separate bed demineralizers Pumps with UVsterilizer Figure 3 Tenergys’ ZLD-system for the plating industry. This ZLD-system is used for the Ni and Cr plating industry. Filtration is combined with separated bed and an evaporator. The system can handle flow rates of 50 gpm. In the pre-treatment step ultrafiltration (UF) is combined with carbon filter. UF is used for removal of volatile organics, virus and bacteria and suspended solids.

The carbon filter contains granular activated carbon media that adsorb impurities within molecule-sized pores. Oxidants such as chlorine are also removed during their interaction with the carbon surface. After pre-treatment the water passes through the polishing step, which consists of separate bed demineralizers, where the salts in the water are separated into positively charged cations and negatively charged anions. The process begins when the water is passed through cation exchange resin. The cation resin is in the hydrogen form (H+) and exchanges all the positively charged ions for hydrogen, thus converting all the impurities in the water into acids.

The water from the cation exchange is then passed through anion exchange resin. Separate bed means that there are two tanks, one containing cation resin, the other containing anion resin. The anion resin is in the hydroxyl form (OH-) and exchanges all the negatively charged ions into the hydroxyl form, completing the conversion of all impurities into water (H+ + OH- --> H2O), thus providing pure demineralized water. The concentrates finally feed an evaporator providing clean water for plating , thus completing the closed ZLD-system. Description of components RO Reverse osmosis is a process where water is pressurized so that it passes through a semipermeable membrane, leaving dissolved inorganic salts and silica behind.

ZLD-system Membranes are expected to play critical roles in future ZLD solutions. Membrane technologies are available in a range of configurations and operating modes, and can be pressure or vacuum-driven, or use electrical potential as the driving force as in the case of EDR membranes. The membrane technologies commercially available today consist of the following:

  • Microfiltration (MF) membranes – used for removal of suspended solids and bacteria
  • Ultrafiltration (UF) membranes – used for volatile organics and virus removal, as well as the removal capabilities listed for MF membranes.
  • Nanofiltration (NF) membranes – used for water softening and sulfate removal.
  • Reverse osmosis (RO) membranes – used for salt removal for brackish and seawater.
  • Electrodialysis reversal (EDR) membranes – used for salt removal for brackish water.

The prevalent membrane technologies used to day in ZLD-systems are RO and EDR. These technologies have as described in chapter ‘description of components’ both advantages and disadvantages. One possible solution instead of RO and EDR is MD technology. The advantages with MD are as follows:

  • Less complicated than its competitors.
  • Not pH or chemical sensitive leading to fewer steps for pre-treatment.
  • Not so sensitive for changes in the feed chemistry.
  • The purity and quality of the water is constantly higher. One possible disadvantage:
  • Big costs. Shows a possible integration of the MD technology into a ZLD-system.
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