Bringing Wastewater Back to Life

July 13, 2010 7:18 am

Can raw sewage really be processed into pure water? Will industries realize that it is cost-effective and environmentally sound to use it in their processes? The water reuse industry is growing, and technology is growing along with it. Now it’s taken another step forward with Alfa Laval’s latest advance in membrane technology.
 
Alfa Laval has been in the wastewater treatment business for more than 50 years. Products such as decanter centrifuges, drum thickeners and spiral heat exchanges are being used in wastewater and sludge treatment for municipal and industrial applications involving more than 250 million users. Recently another technology has been added to the portfolio.
 
This latest technology, the Hollow Sheet from Alfa Laval, has been developed under the leadership of Nicolas Heinen, who has extensive experience in membrane filtration and wastewater treatment. Alfa Laval is now entering the MBR market with the Hollow Sheet Membrane Filtration Module, challenging existing products on the market. MBR is a growing technology for wastewater treatment driven by improved treatment processes, stronger effluent requirements and water reuse.
 
What makes the Alfa Laval solution unique?
“We have combined the best from two membrane technologies – hollow fibre technology and flat sheet technology, which are both currently used in MBRs installed in wastewater treatment facilities – and turned them into one membrane configuration,” says Ivar Madsen, Manager, MBR Unit, Alfa Laval.
 
Hollow fibre and flat sheet membranes each offer advantages, but until the advent of the Hollow Sheet technology, no membrane has been able to combine the advantages of both products into one. “With the Alfa Laval Hollow Sheet in the MBR, you get complete utilization of the whole membrane area,” explains Madsen. “This results in a much greater filtration capacity and at the same time lower energy consumption – 10 to 25 percent lower air consumption per membrane area than what is achieved by the flat sheet and hollow fibre membranes currently in use. Using the Hollow Sheet instead of the other technologies therefore translates into substantial energy savings.”
A Hollow Sheet MBR is designed with a cross-flow velocity of water and waste materials flowing upwards between the membrane elements while the water (permeate) passes through the membrane sheet. To ensure that this mixed liquid circulates effectively, air bubbles are used to create this cross-flow velocity while providing a scouring effect. The Hollow Sheet membranes are placed in a stainless steel frame within the Alfa Laval Membrane Filtration Module.
 
The Hollow Sheet features taller and wider membranes than anything else currently available, producing an optimized packing density. This design (see illustration) ensures that cleaned effluent is drained from the entire surface of the membrane, exiting through connectors at the top of the unit. This means that the pressure drop over the membrane is close to zero due to an open permeate system.
 
Since the Hollow Sheet has a low transmembrane pressure (TMP) while flowing through the membrane, an MBR plant will be simpler and easier to operate because it doesn’t require complicated vacuum systems that are present in MBRs today. This extremely low TMP (a factor of 10 lower) means that membranes are operated by gravity.
 
This low-pressure operation reduces fouling on the membranes, thus reducing the need for cleaning. As urban populations grow, municipal and industrial wastewater treatment will become a priority, Madsen says, noting that within the next 20 years, water reuse will become an everyday technology mainly because municipalities and industries will be forced to use it.
 
Madsen foresees opportunities in municipal wastewater for protection of sensitive rivers, as well as for water re-injection underground or irrigation. The Alfa Laval system was recently approved by the Californian authorities for water recycling purposes (California Title 22 Approval). Water reuse can also be an opportunity for industries, depending on processes and water criteria.
 
The opportunity for companies and organizations to improve wastewater treatment processes and make the reuse of water possible has a positive effect on the environment and in protecting water resources. “The higher the price for raw water and wastewater treatment, the better the return on investment will be for the MBR,” says Madsen.
 
Alfa Laval is looking for ways to improve even further, and it is currently participating in the Danish research programme Membio, where leading universities and institutes in wastewater technology are focusing on system cost, performance, packing density and energy consumption.