Closure to “Model Study of Jet-Circulated Grit Chamber” by Asher Brenner and Mordechai H. Diskin

Sarikaya raises several questions regarding the design and operation of
a prototype grit chamber based on the water jet model study. Our responses
to these questions are as follows. We chose the transverse bottom velocity as a basic index for the intensity of the rotating motion of the water and the scour velocity; we could correlate it nicely to the jets' power input with further guidance in regard to upscaling. This has enabled a rough comparison of hydraulic and energetic efficiencies of water-circulated and air-circulated grit-removal processes. The actual rotating velocity and the consequent power input (dictated by the flow rate of water jets) should be determined on-site with the real waste stream. The flow for the jets is to be withdrawn from the outlet of the chamber (i.e., the grit chamber effluent) in a way similar to the recycle employed in biological wastewater-treatment processes such as trickling filters. Indeed,
the flow through the chamber will increase due to the recycle. However, it
may not be necessary to design the chamber for larger flows for these
reasons: First, the recycle (jet's flow) does not contain grit; therefore, the
raw wastes are diluted and the selective settling characteristics of grit particles may be improved. Second, increased jet's flow may increase the turbulence and thus prevent flocculation of organic matter. Third, although the retention time decreases by R% (R being the recycle ratio), each grit particle may pass through the process more than one time (i.e., 1 + R times); furthermore, since the recommended retention time for aerated grit chambers is only a few minutes, the R% increase in chamber volume is not economically significant. The jet's flow is distributed along the chamber and thus its effect is less pronounced than if it were introduced at the main waste inlet. The longitudinal velocity is actually gradually increased from the inlet to the outlet zone. However, the change, even for a 60% recycle, is negligible (the mean axial velocity in the model chamber varied between 0.03 and 0.05 m/s). Based on three years of operational experience gained in the Soreq Treatment Plant, in Israel, employing full-scale jet-circulated grit chambers ("Soreq" 1991), we can shed light on some actual performance data. The plant's preliminary treatment stage includes four grit chambers following a barscreen system. The average wastewater flow rate during the reported period was 9,375 m3 /h and the effluent recycle rate employed in the grit units was approximately 30%. The average quantity of wet grit removed was 70 gm per cubic meter of wastewater influent to the plant, and the measured percentage of organic matter present in the removed grit was approximately 15%. The inside diameters of the jet nozzles were 3.80 cm, and no clogging problems were observed. These data demonstrate reliable and efficient performance of the full-scale jet-circulated grit-chamber system similar to the performance achieved by conventional aerated grit chambers (Morales and
Reinhart 1984).