An existing sewer treatment plant is experiencing maintenance issues with their first stage (bar screens) where grit which is just barely larger than the apperture (bar spacing) of the bar screens gets jammed in between the bars.

I have been tasked to solve the problem. It was suggested to install a grit separator upstream from the bar screens. The conventional approach is to provide grit separation downstream however.

I could not find a solution in my normal literature (Metcalf & Eddy & MWH) and google, so am asking the question here.

If grit separation can be done upstream, what type would be used? I assume that it would probably employ a vortex type system.

  • $\begingroup$ What's the bar spacing? What are the flow condition upstream? The grit is like sand, stones or is it lighter materials? profile of the individual bars? How is the bar screen cleaned? rake screen? Filter step rake? Different? Is there only one screen, or is there some redundancy? What approx. is low flow, typical flow and high flow? $\endgroup$ – mart Jun 14 '18 at 8:59
  • $\begingroup$ Thank you mart, that information is partly available at this stage, but I am looking for concept stage proposals which should be considered, not a final design. Do you have any proposed solutions? The bar screen is a Huber with 7mm spacing. The grit particles thus are >7mm in one dimension at least. $\endgroup$ – SlydeRule Jun 14 '18 at 13:04

The OP wrote they use a Huber screen. Huber often uses bars that are hydraulically optimized bars with a profile like the left one in the picture:

enter image description here

These have a slight funnel effect, so if d = 7mm, a particle with ~8mm size in one direction can get stuck in the part between the rounded tips. I suggest talking to Huber if you can replace the bars with a different form, the wedge shape on the right. This removes the funnel. I have not seen the hydraulic coefficients for these wedges, but according to the what I'Ve read the downstream end is the biggest influence on flow resistance of the bar so the wedges could be okay too. I don't know how constrained you are hydraulically.

I would also check the rakes: It should be so that one rake clears every other space (say space 1, 3, 5), alternating with the following rake (which cleans 2, 4 ,6 ...) so the bars can bend slightly. Also if the rake gets stuck, it should reverse and to try to dislodge the blockage. All of this is, AFAIK, standard with every rake manufactuer and certainly Huber so it should be implemented on your site.

I beleive a modification to the screens should be the first thing to try, because - like you - I don't see a viable alternative. You can have a screen upstream, but then you might as well replace the screens you have. You can try to sediment the large particles, but then you build a second grit separator. I beleive cyclones to be too energy intensive for a wastewater stream and maybe not robust enough for unscreened wastewater - screen waste is mostly tissues (at least where I am), I could imagine those blocking a cyclone or its pump.

The other idea I might consider would be to have a calm, low flow channel upstream of the screen to separate large grit <7mm - maybe just two wide channel with a small weir at the end, to be cleared by with a Bobcat every year or so. Not knowing the site and hydraulic conditions, I don't know if this is viable.

  • $\begingroup$ The upstream, rounded part of the 'teardrop design' shape (what Huber calls it) is designed with the intent to avoid grit getting jammed between the bars. We will need to ask them why it is not functioning as intended $\endgroup$ – SlydeRule Jun 19 '18 at 8:39

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