mr.wilson
.Registered Member
That sounds like it will work. The slow flow shouldn't increase the dissolved oxygen level by much once it travels through a few inches of aerobic bacteria. You could use a plenum at the bottom of the box to avoid clogging, provide even distribution, and allow for a small aerobic zone to reduce dissolved oxygen.
I'm not sure how important a through put (flow through) is. A passive water exchange exists even in with a static set-up (water passing over stationary sand). Denitrifying bacteria require a long contact time, so there isn't much demand for flow. Conditions in nature don't provide flow through rock or sand, other than a natural migration.
A passive flow can be obtained with a slow release of air in the feeding/access tube. Such an airlift can be operated at a very slow rate, or it could be on a timer that cycles on and off throughout the day (two minutes on, 15 minutes off etc.). The perforated PVC tube would allow for even exposure throughout the depth of the sand.
Another method of passive flow is a heat source at the base of the column (thermal exchange). This can be achieved with a small 50 watt submersible heater, a refugium lamp located below, or the ballasts from a MHL lighting system. As the heat rises through the column, old water is brought to the surface and new water is introduced in a circular motion.
The difficulty here, is we don't know how much flow is optimal. Too much flow, and you lose the ideal conditions for bacteria; too little flow, and you may starve the bacteria, rendering the sand bed useless.
The only way to establish which system is best is to monitor the success and failure within this thread. The problem is, there are often too many variables. It's difficult to keep controls in place for an extended period of time. People aren't likely to report a failure, and success stories need to be qualified.
