Water demand is constantly rising to meet the needs of expanding industrial production, food and energy consumption. For many decades water resources have been withdrawn, wasted and polluted at elevated rates. In order to ensure an environmentally sustainable future, water use practices need to be changed. A more responsible individual approach to water saving and conservation is crucial, but at a global scale it is not enough. As usual, the solution is offered by the technologies of water treatment that allow its reuse in various fields: from industrial applications to drinking water supplies.
The evolution of microfiltration
In the last 30 years the evolution of water filtration, especially in the tertiary treatment section of the WWTP, received a remarkable impulse from agriculture, industrial activities, urban services and facilities. In these fields the main requirements are removal of suspended solids and hygienic quality. The interest in the prospects of water recycling and reuse, fueled research and development and brought on some important technological shifts.
At the beginning the filtration processes were carried out by “depth filtration” systems that were later replaced by “surface filtration” applications. Substantially the change consisted in passing from sand filter plants, that needed large spaces for installation, elevated amounts of back washing water and electrical power, to the filtration performed by micro-screening, which is comparable to mechanical microfiltration using very thin elements.
From drums to disks
At first, following the decision to put aside sand filtration technology, the microfiltration systems were composed of differently structured parts, mostly of rotating drums with horizontal axle immersed into the water to be filtered. Most of these machines, apart from few exceptions, were characterized by internal feeding. The microfiltration units were installed before the disinfection tanks or directly inside them, as secondary clarifiers had limited height.
In the course of the years the drum systems changed into disk systems with an intermittent operating, that allowed to increase the filtration surface while maintaining the same overall dimensions. The working principle has remained, as the machines are constructed using rotating disks with horizontal axle that maintain the “orthogonal” filtration towards the filtering media.
The new approach
The demand for water reuse on the global scale and, in particular, in the geographical areas with low water resources, have led to the development of a new approach to tertiary treatment. Today the microfiltration systems are designed to last for long periods of time, even more than twenty years, and the emphasis is on high quality and economical maintenance.
The microfilters of the new generation are installed exclusively above the ground, in order to ensure a quick and economical installation and a complete and easy maintenance of all components. This avoids the construction of complex plants and allows direct access to the system for repairing and maintenance operations.
The material that ensures high resistance of the microfilters for tertiary treatment is stainless steel. Unlike plastic materials, that were previously used in this field for manufacturing the disks structure and the filtering media, stainless steel demonstrates superior performance in the long term. Machines entirely realized in stainless steel can be exposed to severe weather conditions and do not require the construction of expensive structures, to be covered and protected. So the disk wheels and the removable filtering sectors are manufactured using stainless steel filtering media and frames. Compared to polyester or other plastic materials, the advantages of stainless steel are beyond dispute.
The new frontier of microfiltration
In the past 10 years a new technology has reshaped the world of water microfiltration - the “dynamic-tangential filtration."
How does it work? This kind of microfilter is equipped with filtering disk wheels in continuous rotation and the feeding of the water to be filtered is performed directly between the disk pairs. Solid particles pass through the meshes in oblique (tangential) directions, with a high speed. The filtering surface changes continuously as the filtering disks rotate. This leads to an increase of the filtration efficiency and prevents up to 10 microns diameter solid particles from passing through the filtering media. This new technology treats flow rates three times higher than the conventional filtering systems with immersed disks and intermittent operating. Even with the same installed filtering surface, the “dynamic-tangential filtration” working principle considerably reduces the machine’s footprint and maintenance and operating costs.
Thus, microfiltration technologies develop continuously in order to ensure the availability of water resources for the needs of agriculture, urban areas, industrial production and drinking water supply. Water reuse is to become a norm on a large scale as water recycling and reuse is the only way for a sustainable management of the world’s water.
Donato Massignani is an environmental mechanical engineer and president of Nuove Energie srl, the leading company in the field of environmental and energy technologies. He is a Certified Energy Manager, designer and constructor of plants for water treatment, water recycling and reuse with more than 35 years of experience, and owns numerous Italian and international patents among which the Ultrascreen microfilter.
