Case study on R.O Reject recycle - First step is the biggest Step mission Zero Liquid Discharge

Published by

Process Schematic for Zero Liquid discharge from R.O. unit 

Process Description

Reject water from the 1st RO of DI water system is collected and sent to the 1st reaction tank; caustic soda is added to raise pH to more than 11.0 so that Magnesium ions convert to relevant insoluble matter (Magnesium Hydroxide).

Silica can also be precipitated by means of adsorption and reaction with Magnesium Hydroxide. Precipitated water overflows to the 2nd reaction tank in which more caustic soda and sodium carbonate is added.  pH is maintained at about 10.5, which is beneficial for Calcium converting to insoluble Calcium Carbonate.

After two stage chemical precipitation, the solid-liquid mixture overflows to the TMF concentration tank (also called a recirculation tank). A circulation pump (also called process pump) sends water into tubular membrane filter modules (connected in series) for solid/liquid separation.

In a cross flow mode, most of the feed stream will recycle between TMF modules and circulation tank; suspended solids return to the circulation tank from the membrane tubes.

Filtrated water (flow rate equal to system capacity) is sent to a pH adjusting tank for neutralization and then flows to a separate filtrate tank for short term storage before it is sent to reclaim RO system for desalination.

This volume is the amount of reclaimed water, which is sent back to DI water system for reuse. Meanwhile, an amount of final brine is discharged from the reclaim RO and is sent to the evaporator for crystallization. Condensate from this evaporator is also sent back to the main DI water system for reuse while crystal salt is carried out of the factory for resale.

During the filtration process, suspended solids are concentrated in the recirculation tank. To control the solids, a portion of concentrated water is sent to the filter press system for dewatering. Sludge cake produced from the filter press is also carried out of the factory, while squeezed water is returned to the 1st reaction tank.

In zero liquid discharge filtration, the most important concern is to reduce water volume as much as possible so that a smaller evaporator can be utilized---because evaporation is a costly approach, it can result in higher construction and operating costs. This unique pre-RO treatment system (chemical softening reaction+ TMF separation) can effectively remove hardness ions and silica so that reclaim RO can operate at a very high recovery rate.  Therefore, only a small-scale evaporator is required and may be selected for final crystallization. 

TMF Characteristics and Advantages

The initial purpose of developing tubular membrane filter, and the broadest application up to now, is as a replacement of conventional solid/liquid separation process, i.e. clarifier. There are several advantages of Porex TMF compared with a traditional clarifier process that include:

  1. The Porex TMF filtrate water quality is much better than clarifier-treated water. Due to the presence of the filtration membrane, all particles larger than the nominal pore size will be rejected. Treated water quality is equal to UF product water.
  2. Due to the excellent filtrate water quality, the Porex TMF product water can be fed directly into an RO system without any other treatment. In comparison, when the water comes from a clarifier, then a multimedia filter, activated carbon filter or ultrafiltration process is typically needed prior to sending through RO.
  3. Coagulant (PAC, FeCl3, FeCl2, FeSO4, etc.) is either not necessary in a TMF system, or the dosage is greatly reduced. Flocculent (PAM) is also not required in a TMF system. Only caustic soda is required. Eliminating coagulant results in much less sludge cake volume and reduced treated water TDS compared to a conventional clarifier system.
  4. The unique design of the cross flow Porex TMF system can easily handle a 2~5% suspended solids concentration. This produces less remaining slurry and results in better performance of the filter press.
  5. The Porex TMF system can be designed for automatic operation and can be placed into service mode from standby mode at any time.  This ensures for easier maintenance..
  6. Compared with a traditional clarifier, the TMF skid frame requires much less space. Also, the TMF skid is available for expansion meaning that the water capacity can be enlarged by simply adding more skids and TMF modules.

 

System Information

There are two RO units in the main DI water system. Either of them produces 75m3/hr RO permeate water and generate 25 m3/hr RO brine at the same time. So, capacity of the ZLD system is designed as 50m3/hr. The following table lists the feed and permeate rates for the RO systems along with the resulting recovery calculations (permeate rate/feed rate) for each. For the whole system, total recovery rate is 95% and the evaporator treats 10m3/hr brine water vs a system capacity of 200 m3/hr.

 Parameter Membrane System Flow and Recovery
  Main system R.O.  Reclaim R.O.                                          
 Feed      (m3/hr) 200   50                                            200
 Permeate (m3/hr) 150 40 190
                  Reject   (m3/hr) 50 10 10
            Recovery (%) 75% 80% 95%

 

 
  Operation Status  
 

System construction work was finished in July of 2013 and commissioning work started subsequently. System performance has met all design specifications: 

Most hardness ions (Ca, Mg, Sr, Ba) and silica have been removed from the influent water, which eliminates the risk of RO scaling making it possible to run each RO at a very high recovery rate (80%).

 

Tags