The Bow Reactor is the first complete water resource recovery technology developed by New Limits for advanced treatment and nutrient removal. The typical configuration consists of (from left to right in the figure below) a pre-anoxic selector zone, an aerobic reaction zone, and a solids separation zone.
The key to the high level of performance is the flow pattern created between the aerobic reaction zone and the solids settling zone by the Bow Pumps (depicted as red in the figure above). This flow pattern is termed Pump Forward Flow Return (PFFR) and is what makes the Bow Reactor innovative.
PFFR creates a short-circuiting effect between flow to the solids separation zone and return solids flow back to the aerobic reactor. This flow pattern is created through the use of air conveyance, which reduces floc sheer and increases floc size. The result is excellent liquid/solids separation combined with sludge flocs that are ideally suited for simultaneous nitrification/denitrification. The table below shows the results of a one-year pilot study conducted with American Water, where the Bow Reactor was tested against a recirculating sand filter.
The table demonstrates the effectiveness of the Bow Reactor and the PFFR concept. Not only did the Bow Reactor achieve a much higher level of ammonia removal than the sand filter – which was the purpose of the demonstration – but it also outperformed the sand filter in effluent BOD and TSS concentrations. This was wholly unexpected but, again, demonstrates the effectiveness of the treatment concept. There was no noticeable change in treatment or washout during the high flow stress test of the Bow Reactor (flow increased from 1,200 gpd to ~15,000 gpd). It should be noted that the plant receives influent from homes that have septic tanks as a pre-treatment step. The influent has a low carbon-to-nitrogen ratio, but the Bow Reactor was still able to produce water with a lower pH than the sand filter even though it provided more complete ammonia oxidation – suggesting a significant amount of denitrification and total nitrogen removal under less than favorable conditions.
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