Bed Expansion monitoring During Filter Washing
ATi supplies interface detection equipment for a wide range of public utility and industrial process industries. This equipment provides reliable, repeatable and accurate measurement of sediment and suspended solids bed levels in liquid filled vessels. Recent evaluations at water treatment plants have demonstrated the effectiveness of this equipment in providing accurate measurement of media expansion during filter backwashing.
The principal means of treating water from a surface water source is filtration through large filters that trap impurities as they pass through sand, anthracite, carbon or other filter media. Filters are periodically cleaned by backwashing to waste (or return to the head of the plant) once head pressure through the filter reaches a stipulated point. Particle "breakthrough" (detected by turbidity or particle count measurement) may also prompt backwashing. Some plants also utilize a maximum filter "run time" between backwashes.
Typical Backwash Routine
A typical backwash routine involves. 1) stop the flow of water through the filter and lower water below the level of the backwash trough, 2) initiate an air, water or combined air and water scour to loosen and dislodge compacted particles on the surface of the filter media, 3) reverse water flow through the filter and into the backwash trough, and 4) return the filter to service after a "curing" period.
During the backwash process, the filter media is "fluidized" and expands as a result of reversed water flow. The rate of flow is controlled such that the relatively heavy media is not washed away with the fine impurities. When properly expanded, the media retains a distinct bed-like character, but the level of the bed rises or expands an amount that is determined by the rate of reverse water flow.
The importance of securing adequate media expansion during backwash is significant and the need is well documented. Adequate expansion loosens media particles and promotes particle collision that is essential to breaking loose "sticky floc" that attaches to the filter media. Media that is not sufficiently expanded tends to hold particles in place and can promote mud-ball formation and other problems. Studies and filter design specifications suggest that media should be expanded in the range of 30-50% of the total media depth for effective cleaning.
Prior to the development of the ATi Expansion Pro Analyzer Ô (EPA 2000), surface water plants did not have the ability to accurately measure media expansion other than by crude hand measurements with infrequent and spurious results. Current practice at most plants is to perform complex calculations to estimate the required flow rate. These calculations, when made, are flawed by inaccurate assumptions regarding media size, uniformity, depth and other variables. And, calculations must be corrected for changes in water temperature and other variables. Additionally, problems with backwash pumps, valves and other equipment may result in less than expected expansion that goes undetected and uncorrected.
While municipal water plants may often find these calculations adequate to achieve acceptable levels of filter cleaning, they do not offer the potential to optimise filter cleaning by direct, confirmed measurement of actual expansion.
EPA 200O Advantage
EPA 2000 is an excellent tool to optimise filter cleaning. It is successfully employed to provide highly accurate and repeatable measurement of media expansion without the requirement of complex and questionable calculations that become erroneous over time. Reliable and continuous media expansion measurement is available at every filter and for every backwash cycle. Convenient 4-2OmA output and contact relays provide the opportunity for efficient control of pumps and valves and for historic trending of important process control information.
Optimize Filter Cleaning
The sole purpose of backwashing is to clean the filter, and adequate media expansion is essential to this purpose. Inadequate expansion produces insufficient cleaning and all of the associated problems discussed below.
Extend Filter Run Time
In most cases, this is the most significant benefit to be achieved by optimising filter backwashing. A typical plant wastes 2-5% of its annual processed water product in backwashing. Extending filter run time directly and immediately reduces this cost.
Reduce Wash Water Usage
Optimizing media expansion allows shorter wash period and reduces the volume of water spent for this purpose.
Reduce Media Loss
Plants may employ a reverse flow rate that is excessive or is not "stepped up" properly. The result is loss of media with the backwash water. There is a direct cost associated with the replacement or addition of media and in filter inefficiency if media is not promptly restored. EPA 2000 provides information that is highly useful in gaining a better understanding of media behavior during backwash and providing insight for correcting and improving process control.
Reduce processed water turbidity, reduce mud-ball formation, promote even media re-stratification and reduce filter bed cracks and tunnels.
For further information please email ATi UK Limited