The disadvantages of conventional MMF based pretreatment
Conventional pretreatment is based on multimedia filters (MMF) implemented in large scale desalination plants. Although this filtration method is well proven and provides reliable filtration efficiency, it conceals an inherent drawback, the disadvantages of which relate mainly to the backwashing mechanism.
The pretreatment includes several filters that are backwashed in sequence, and additional filters are required to compensate for those in backwash. The division of the filtration area for several filters means more concrete walls and backwash channels.
The hydraulic resistance of each filter changes continuously during the filtration cycle, necessitating instrumentation to measurement and control the flow for each filter.
Backwash (BW) of one MMF filter requires large amounts of water and air, as described in the following example. The area of a typical chamber in a large scale desalination plant is approximately 120 m2 . The flow rate of the BW water is approximately 6600 m3 /hr, and the required volume for each BW is approximately 1000 m3 . Taking availability requirements into consideration, the BW water tank size can reach 2000 m3 and more. Each pretreatment requires two air blowers (one standby) that provide air at a flow rate of about 6000 m3 /hr. Water and air backwashing systems require large diameter pipes and large valves, as well as measurement instruments. A large volume of backwash water (approximately 1000 m3 per BW) has to be evacuated and stored, prior to treatment or discharge to the sea.
The standard MMF underdrain is designed to collect the filtrated water, as well as to withstand the BW water coming from the BW water tank and the air coming from the blower during the backwash procedure. As the BW flow is five times stronger than the normal filtration flow, and the air is distributed equally to the entire filter area, the underdrain structure is larger and more complicated.
The height of water above the media in MMF includes height for media expansion during BW. This additional height is a ratio of 40% of the MMF filtration height. An additional 20% water height is taken as a safety margin.
The driving force for filtration is the height of the water above media. In conventional gravity MMF minimum 2.5 meter of water required above media for all pressure losses till filtrated water pumps.
Meeting all the requirements related to the method of BW and driving force brings the height of the standard pretreatment unit to approximately 5 m. This height requires thick concrete walls and complicated foundations.