Ultrapure Water Systems for Manufacturing Solar Cells

Introduction

Regular surface water consists of a wide range of contaminants including bacterial proliferation, particulates, organic contaminants, metallic contaminants and anionic contaminants. Such contaminated water is deemed unsuitable for the manufacturing of semi-conductors that demands de-ionized and filtered water that is completely free from unwanted contaminants.

System

Hyflux’s solution for this client is the UltraPure Water II (UPW II) treatment plant designed to supply 15 cubic metres per hour to the point-of-use (POU). This plant comprises two systems:

1. A make-up water system which comprises of a 17.0 cubic metres per hour with two- pass, two-stage Reverse Osmosis (RO), and

2. A polishing loop system that consists of a 19 cubic metres per hour Electrodeionization (EDI) system and a standby primary mixed bed capable of handling 19 cubic metres per hour

The UPW II treatment plant includes :

    Multimedia filters
  • Activated carbon filters
  • Softener
  • 5 μm cartridge filter
  • 254 nm ultraviolet (UV) sterilizer
  • RO 1st pass
  • RO 2nd pass
  • Electrodeionization System
  • Standby primary mixed beds (regenerable) catering to outage of EDI System during cleaning and maintenance
  • UV Total Organic Carbon Reduction Unit
  • Polisher mixed bed
  • Cartridge filters, 1 μm (absolute)
  • Cartridge filters, 0.2 μm (absolute)
  • Main control panel and system instrumentation

Process

n the make-up water system, raw water is pumped through the multimedia filters (MMF) where bigger particles and suspended solids are removed. The filtered water is then collected in the MMF filtrate storage tank and subsequently pumped through the activated carbon filters (ACF).

ACF removes chlorine, low molecular weight organic and non-polar compounds as well as high molecular weight organic compounds. Removing free chlorine is critical for protecting polyamide RO membranes and strong base anion resins while eliminating organic compounds and suspended solids is necessary for reducing fouling of membranes.

After the ACF stage, the permanent water hardness is removed with a softener prior to RO separation and the softened water is micro-filtered using 5 μm cartridge filters. Bacterial growth is effectively controlled using a UV sterilizer unit.

Pre-treated feed water is then routed through the first RO pass via high pressure pumps. Permeate from the first RO pass is then pushed into second RO pass via inter-pass high pressure pump. The overall product water recovery of this RO system is 68%.

The second RO pass permeate pours into the Electrodeionization (EDI) feed tank which signals the first stage of the polishing loop system. From the EDI tank, the RO permeate flows into UV Total Organic Carbon Reduction (UV-TOC) unit before moving through the EDI system that holds 6 EDI stacks. The product water from the EDI system has a resistivity of 10 to 16 Mohm-cm and is stored in the UltraPure water storage tank. Taking into consideration of the outage of the EDI system during cleaning and maintenance, we recommended the client to include a regenerable primary mixed bed unit as a standby.

Deionised water from the storage tank is then pumped through another UV-TOC unit to photo-oxidise any organic species followed by non regenerable polisher mixed bed. Then, the resistivity of 16 – 17.5 Mega Ohm-cm is to be achieved. Before entering the distribution loop, water flows through final polishing steps which includes filtration with 1.0 m and 0.2 m absolute micro filters.

Conclusion

The water quality is continuously monitered and the plant is able to produce constant high quality ultrapure water for the manufacturing facility.

Water Quality Feed Permeate
Total Organic Carbon (ppm) 5 <0.02
Copper (mg/L) 0.05 0.001
Zinc (mg/L) 5 0.001