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POREX® Tubular Membrane Filter (TMF)™ Applied in a Copper Wastewater Reclaim System for a Printed Circuit Board Facility

Abstract Introduction

A Printed Circuit Board, (PCB), is used to mechanically support and electrically connect electronic components. It uses conductive pathways, tracks or signal traces etched from copper sheets which are then laminated onto a non-conductive substrate. The PCB manufacturing process is very complex, and many kinds of heavy metal wastewater and organic wastewater are drained from PCB facilities. Copper contaminated wastewater is one stream. The normal utilization of the copper foil in the manufacturing process is typically only 30-40% of the total amount used. The remainder of the copper is dissolved; with a large volume ending up in the used etch solution and wastewater. Reuse of the water and copper is a matter of great importance and cost saving to the manufacturing facility.

This specific manufacturer is one of the world’s largest PCB contract electronic manufacturing groups. They have built several copper wastewater reclaim systems with Porex Tubular Membrane Filter (TMF)™ systems. The combined capacity of the existing treatment systems using Porex TMF is more than 500m3/hr.

This case history gives a brief description of a newly built 150m3/hr copper wastewater reclaim system located in Qinhuangdao in northern China.

Background

In the PCB industry, there are several steps that may produce copper waste: copper precipitation, pattern plating of copper, etching and other pretreatment processes (chemical treatment, the brush plate before processing volcano ash grinding plate processing, etc). Wastewater from the above steps can be divided into two categories: chelate type and non-chelate type. Below is a simple description of these two kinds of wastewater: 

A. Chelated copper wastewater comes from the following processes:

  1. Electro-less copper precipitation which uses EDTA and sodium tartrate. 
  2. Alkaline etching and micro etching which use ammonia and ammonium. 
  3. Other steps such as acid degreasing and alkaline degreasing.

It is very difficult for copper to be precipitated due to complexants’ existence. It can be oxidized before precipitation or, a stronger precipitant can be used such as sulfide or a special heavy metal chelating agent. Sometimes resin adsorption or Ca displacement are other options.

B. Non-chelated copper wastewater comes from pattern plating copper, acid etching and rinse water from other steps. Normally precipitation, especially an alkali precipitation method, is effective on non-chelate copper wastewater treatment.

Wastewater Information

The wastewater in this facility comes from the rinse stream of the pattern plating copper process. Total system capacity is designed as 150m3/hr.

Traditionally this kind of wastewater has been treated by a simple alkali precipitation (with caustic soda, ferric chloride and polymer dosing) followed with a clarifier. However, the treated water quality is not good enough for water reuse. That is why the manufacturer decided to select the Porex TMF system as an alternative to a traditional clarifier (or sedimentation tank).

TMF Characteristics and Advantages

Several advantages of Porex TMF compared with a traditional clarifier process 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, a multi media filter, activated carbon filter or ultra filtration process are typically needed prior to sending through RO.
  3. Ferric chloride and polymer pre-treatment is not necessary in a TMF system. Only caustic soda is required. Ferric chloride results in more sludge cake volume and higher treated water TDS. Also, polymer will cause RO membranes to foul which is problematic and difficult to recover.
  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. Ease of maintenance. The system can be designed for automatic operation and can be placed into service mode from standby mode at any time.
  6. Compared with a traditional clarifier, the TMF skid frame requires less space. Also, the TMF skid is available for expansion meaning that the water capacity can be enlarged by simply adding more skids or modules.

Porex TMF System Information

The details below describe the installed Porex TMF system.

Water Capacity: 150 m3/hr 
Module Specification: Porex Tubular Membrane (module model # MME3S01613VP)
Tubes per Module: 13
Module Housing: PVC Housing
Membrane Tube Diameter: 1 inch
Membrane: 0.1 micron
Active Membrane Surface Area: 1.82m2 per module
Module Quantity: 12 modules per train, 4 trains per skid (3 skids) totaling 144 modules in this system

Process Schematic

Porex Filtration Process Schematic

 

Process Description

  1. Copper wastewater from the facility workshop is collected in a raw water tank for equalization. The solution is then pumped to a reaction tank for pH adjusting. Caustic soda is dosed into this tank during mixing to increase the feed water pH to above 9.0 so that copper hydroxide precipitate forms. A pH meter is installed in the reaction tank for pH monitoring and controlling caustic soda dosing via a pneumatic valve.
  2. The mixture of water and precipitate overflows to a concentration tank (also called a recirculation tank). A recirculation pump (also called a process pump) sends the process water into a series of Porex TMF modules for solid/liquid separation. In a cross flow mode, most of feed stream will recycle between the TMF modules and the recirculation tank, which returns suspended solids back to this tank. Filtered water (flow rate equal to system capacity) is sent to a separate product water tank (Permeate Tank) for short term storage before it is sent to the RO system for desalination.
  3. RO product water (with lower TDS) is sent back to the facility workshop for reuse as process water while RO concentrated water is sent out to the existing biological treatment system for further treatment before discharge.
  4. During operation, suspend solids concentrate in the recirculation tank and a portion of the concentrated water is transferred to a filter press system for dewatering. Sludge cake from the filter press, normally containing 70% water, is transferred out of the facility for copper recovery. Squeezed water is sent back to the raw water tank.

TMF System Specifications

Porex Filtration TMF System Specifications

There are 12 total trains in this system which are installed in 3 skids (4 trains per skid). Each train contains 12 modules connected in series, and each train has a separate process pump, filtered flow meter, back pulse unit and set of isolation valves. A common clean-in-place (CIP) unit (including 3 tanks and 1 air driven diaphragm pump) is set for every 4 trains in one skid. The system is designed to ensure that each train is back pulsed separately. Also, the operator can do CIP for each train separately.

Operation Status 

  • The system was commissioned in November 2011. System performance has met all design specifications.
  • Filtrate flow rate per train ranges from 25 m3/hr to 10m3/hr according to service time. The average flow rate is 15m3/hr. As operators do chemical cleaning for all 12 trains alternately, system capacity is always above 150m3/hr.
  • TMF filtrate turbidity is less than 1.0 NTU. Based on this, the guard filter cartridges used before reclaim RO do not need to be replaced frequently. (cartridge service life is approximately one month).
  • Typical copper ion concentration in the TMF feed water, RO permeate water and RO reject are 60 mg/l, 0.08 mg/l and 1~2 mg/l respectively. RO reject water is sent to a biological system for further treatment to remove remaining copper.
  • System feed water conductivity ranges from 800 to 2000 µs/cm while RO permeate conductivity is less than 50µs/cm.
  • Suspended solids in the concentration tank is about 20,000 mg/l (2%), Sludge Volume (SV) of 30 mL/L equal to 30~50% by volume.
  • Due to the low organic level (COD 10~20 mg/l) in the feed water, chemical cleaning with 2~3% NaOCl is executed every 15~20 days. Acid cleaning frequency is typically every 40 days.
  • The total system and RO recovery rate is 55%. The TMF system recovery rate is nearly 100%, with a small volume of water discharged with the sludge cake.

Summary

This case history is an example of “treat and reuse” water treatment technology applied for copper contaminated wastewater. With simple chemical dosing (only caustic soda), copper ions form copper hydroxide. The Porex TMF system has demonstrated excellent performance for this application because of barrier separation. The treated water can be sent to an RO system without any further treatment. 55~60% of the wastewater returns to the facility’s workshop instead of discharge, and the copper is concentrated in the sludge cake so that it can be treated and reused. The entire process is based on simple chemical and physical principles, resulting in significant economic and environmental benefits.

About Microfiltration

Microfiltration is a cross flow, pressure-driven membrane separation technology designed to remove submicron (and larger) suspended solids from water supplies. It differs from conventional (“dead-end”) filtration in that in a conventional process the entire water supply passes through the filter medium, whereas in the cross flow process, a portion passes through the membrane, becoming “permeate,” while the remainder exits the system as “concentrate,” carrying away almost all of the suspended solids.

The following illustration compares these two processes.

Porex Filtration Crossflow Filtration Diagram

 

The microfiltration membranes used in this application are POREX® TMF tubular membranes, depicted below.

Porex Filtration Microfiltration Membrane Diagram

 

The feed flow is down the center of the tube (lumen feed) with the permeate passing through the tubular wall and collecting in the area around the outside of the tubes inside the module housing.

The tubes in this application are 1” I.D., with a polyethylene substrate supporting a PVDF (polyvinylidene fluoride) membrane with 0.1µm pores. The membrane module is illustrated below.The tubes in this application are 1” I.D., with a polyethylene substrate supporting a PVDF (polyvinylidene fluoride) membrane with 0.1µm pores. The membrane module is illustrated below.

Each membrane module consists of thirteen 72” long tubes enclosed inside a PVC housing. Specifications on the modules and tubes are as follows:

Porex Filtration Microfiltration Modules Chart

 

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