The standard texts are good at quoting that a trickling filter needs a minimum wetting rate to ensure even flow - typically around 0.5 m3/m2-h.
What is required if the average wetting rate, without recirculation, is 0.3 m3/m2-h, BUT the feed is pumped, and the pump operates for around two minutes in every six? Then the wetting rate when feed is applied is 0.9 m/h, while the wetting rate when there is no feed is 0.
Is there a need to recirculate? If the recirculation pump is set to provide 0.2 m3/m2-h, then the filter is inadequately wetted when the influent pump is not running, defeating the benefits of the recirculation. If the recirculation flow is increased to 0.5 m3/m2-h then we are potentially harming performance from the hydraulic overload when the influent pump does run.
What would happen if the recirculation pump was switched off, on the grounds that (i) the wetting rate is sufficient when the feed pump runs and (ii) the time from one pump operation to the next (4 minutes) is too short for significant drying out?
Dr Dudley,
As you are no doubt aware, the wetting rate of biological trickling filters is dependent on a range of factors not least being fixed jets or rotating distributors, wastewater strength, and the nature of the media (rock or plastic). The Royal Commission design criteria (eg 90 gal/yd3.d approx. 0.3 m3/m2.d) was for ADWF which equals 1.0 m3/m2.d for WWF or DWF plus 2x recirculation flow. Even then flows may not be continuous and maximum wetting rates would be in the order of 1.5 m3/m2.d. 70 years ago WPRL demonstrated optimised contact with biomass and best BOD removal was at about 3 m3/m2.d. Therefore, your proposal to cease recirculation and operate at an intermittent wetting rate of 0.9 m/h should be an improvement but why not try it? It will also save power and optimise clarifier settling time. Potential benefits of recirculation include dilution of strong influent wastes and inoculation with nitrates which encourage aerobic filter bed conditions and reduce odours.
Michael, Thank you.
I had the units wrong - I typed m/h when I should have typed m/d.
I think that the WPRL work was done with continuous gravity flow; the filters had been removed by the time I joined what had been WPRL. I could find no discussion about adjusting rules of thumb that use hourly or daily flows when the flow is strongly intermittent because of pumping to the sewage works.
With more time to look at the data, the influent is pumped and shows clear on/off signals, but the effluent shows the effect of the primary and humus tanks, and has a continuous profile - there is no clear on/off pattern. So, I think, at least with a primary tank to provide some hydraulic smoothing, I can use the text book approach of an average flow and assume continuous inflow, rather than worrying about the full nature of the intermittent flow.
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