Maintenance Backwashing

A typical physical-chemical treatment system incorporates three "dual" medial (sand anthracite) filters connected in parallel in its treatment train. The major maintenance consideration with granular medial filtration is the handling of the backwash. The backwash will generally contain a high concentration of contaminants and require subsequent treatment. 

Clearly, the efficiency of the EMS is dependent on the magnetic character of the transported iron oxide and other particles, and if they are not magnetic, then no filtration occurs. Unfortunately, it is not unknown for installed EMS units to be ignored or their true function to be in doubt. Consequently, they may have to operate for long periods without any effective backwash or maintenance program being provided. 

At the Environmental Protection Agency s (EPA s) Perham Arsenic Superfund Site in Perham, Minnesota, a continuous-backwash filtration unit is used to treat groundwater contaminated with arsenic. In 1994, according to the EPA Record of Decision (ROD) at the site, the present worth costs of the remediation were 2,548,776. The annual operations and maintenance (O M) costs were projected to be 217,805 (D17114C, pp. 2-3). 

Model 4 system described contains steel pipes. Model 10 system described is backwashable. Systems include aU piping and manual valves to comprise a complete adsorption system, enabling all operations. Equipment costs include drawings, technical submittals, and provision of an operation and maintenance manual. Freight cost may need to be added for some models. 

The location, operation, and maintenance of the BSC should be carefully planned. Similar to LFCB, the BSC should be located in an area free of steady or intermittent air currents, which might defeat the laminar airstream by a backwash effect. The BSC should be connected to an adequate power source, having the minimum possible current fluctuation. The effects of voltage variation on cabinet performance can be pronounced (Fig. 10A), causing a variation in intake and supply velocities sufficient to result in an unacceptable unit performance (Fig. lOB), thereby compromising personnel and product protection design features... 

Water consumption of the jets as a percentage of process effluent decreases from 5% at 15 psi to 2% at 50 psi. In general, backwash systems are operated at as low a pressure as possible consistent with successful cleaning. High-pressure operation incurs added system maintenance, particularly jet replacement, and is used only as needed. 

An innovative potable filtration plant with a design capacity of 1.2 MGD has been reliably serving 10,000 residents and tourists in the town of Lenox, Massachusetts, USA, since July 1982. Its process system consists of chemical flocculation, dissolved air flotation, and automatic backwash and sand filtration. It substantially improves upon the conventional flocculation, sedimentation, and filtration system in performance, capability, operation, maintenance, and energy use (19 2). 

In the case of the water supply systems, the type of actions which cause costs are maintenance works, replacement of failed equipment, technological activities (filter backwashing). To find the total cost of the operational phase, costs related to materials (spare parts, water for backwashing), labour and energy was calculated. 

The granular media filters are backwashed using filtered water or, if acceptable, concentrate from the SWRO membrane system. The backwash rate is typically designed to provide 30-50% media bed expansion. The number of individual filter units is usually selected such that the filtration system can operate at fiiU production capacity with one filter out of service in backwash and one out of service for maintenance. 

Cieaning Maintenance cleaning by backwash or chemically enhanced backwash is an option for contained and submerged modules and easier for hollow fibers. Clean-in-place may be easier for contained systems and smaller volumes of chemicals may be required. 

Backwashable cartridge filters have low solids-loading limits therefore, they have potentially short intervals between backwash cycles. It is important not to expose backwashable filters to differential pressures over approximately 170 kPa because the particles may become too deeply imbedded in the pores to be removed by backwashing. With proper maintenance and repeated backwashing, this type of filter may last up to 2 years. 

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