Using hot water to backflush a sand filter 2

[jacquesgiraud]

We are currently involved in a design/build of two pressurized high rate sand filters for production of cooling water for use in our factory. We have procured a professionally designed underdrain lateral system and plan to use 8/12 garnet for base support media, 20/40 garnet for middle filtration layer and #1 anthracite for top layer. The underdrain will consist of a central hub with lateral pipes extending radially outward. Filter depth is 48" and filter area is 19.63 sqft. Design parameters are for 200 usgpm forward flow and 300 usgpm backwash flow. Filters will be supplied with river water at 45 psig and observed maximum 50 ppm TSS. Part of our process involves evaporation yielding very clean hot (200 degF) water in large volumes (>10 Mgal/day). We reuse this water in process as well as for floor washing. Would using this hot water for filter backwash have any ill effects? If so, what temperature would this water have to be cooled down to in order to make it suitable for this purpose? Our motives are to put this excess water to good use and to avoid the cost of installing a separate hold tank and pumping system for backwash service.

 

[sbush]

The clean, hot water is acceptable, provided that your vessel, coatings, instruments, vessel internals, exterior piping and valves can handle the temperature and additional backwash flow rate that is required to expand the media sufficiently for cleaning.

The first thing you should check is the size of your underdrain and overdrain systems to see if they can handle the backwash flow rate necessary. Because of the elevated temperature, you will need at least 30 gallons per square foot (GPM) or 589 GPM of backwash flow. For that rate, I would recommend at least a 6" diameter backwash piping system. Depending upon the design of your underdrain system, there could be other flow limiting factors such as laterals and orifice sizes or open area of screens, etc.

With a water only backwash method, you must expand the filter bed by at least 20% to release debris trapped in the lower parts of the bed (30% expansion is the preferred). Even with air scour prior to backwash, you still need to eventually expand the bed at least 20% during the subsequent backwash. The only other way to achieve sufficient bed expansion with less water velocity is by using a method known as simultaneous air and water wash. This process usually requires the use of a special overdrain design that will not allow media to be lost to drain in the process. These designs are proprietary to a select group of experienced filter manufacturers and you may not have access to them; however, you can check with your underdrain supplier. There is another option using level probes to accomplish a limited air/water wash, but I hesitate to lead you down that path at this time. If you have already purchased the vessel, then I doubt that a new overdrain design or new tank penetrations could even be installed at this point.

The next thing to check, is that anything coming into contact with the backwash water (i.e., vessel coatings, underdrain piping, overdrain piping, supply and drain piping, instruments, and valves) can withstand the temperature.

Finally, there are a couple of things that you need to be cautious of:

One is the "cold wall effect". Even if the coating itself can withstand the temperature, it can still separate from the metal substrate under certain conditions when the temperature on the inside wall is substantially different from the temperature on the outside wall of the vessel. When lamination occurs, water will soon find its way between the layers and rapid corrosion will ensue. Insulating the exterior of the tank will help to mitigate the potential for this problem, but may not eliminate the potential completely. In your case the contact time is not continuous and this is certainly beneficial.

Another problem can occur if or when the backwash water source is changed, for whatever reason, and the source becomes cold water. If subsequent changes haven't been made to correct the backwash rate due to a lower water temperature, then some of the media can be backwashed from the system.

And lastly, when the hot backwash water enters the cold vessel and the system is vented to atmosphere, there is a potential for flashing to occur.

S. Bush

http://www.water-eg.com