Pre-aeration and Transversal Baffling Effect on Aerated Grit Chamber Performance of Kufa Wastewater Treatment Plant

This paper investigates solutions for a problem of heavy loads of grits and grease encountered in a 50, 000 m3/day capacity municipal wastewater treatment plant of Kufa city located at 160 Km south of Baghdad in Iraq. Total suspended solids TSS concentrations in the influent were 410 780 ppm causing about 1 2 ton/day of grits to deposit in a reducing flow velocity basin located in between the fine screen and the aerated grit removal chamber. Oil and grease O & G were found with high concentrations 145 255 ppm in the raw sewage causing operational problems in secondary treatment. The solutions investigated were, using pre-aeration in the reducing flow velocity basin, installing effluent transversal baffle at the last 35.5 cm length section before end wall of the aerated grit chamber of 14.8m total length, and longitudinal diffused aeration condition near baffle location. Six operational cases were investigated with various installations of baffle submerged depth, pre-aeration and longitudinal chamber aeration conditions. For each of the six cases studied, three daily samples for each of influent, effluent and settled grits were collected and tested. Sampling was carried out for seven days period for each of the six cases at 900am, 1200pm and 200 pm in order to cover quality possible variation. Influent and effluent samples were tested for TSS, and O & G, while grit samples were tested for total organic carbon TOC. Flow rate of the chamber was observed three times daily at the mean time of sampling by means of a Parshal Flume installed downstream the chamber. Longitudinal and transversal local flow velocity measurements were carried out by means of a flow velocity meter specially developed and calibrated for the field work. Three transversal sections were adopted along the chamber. First section was at 3.24m behind the inlet wall, second section was at 8.24m from inlet wall, corresponding to the center of the middle air diffuser, while third one was at 13.24m from inlet wall. Each of the three transversal sections was subdivided into five successive bays of 0.5m width each for the main channel of the chamber. At centerline of every bay, point (local) flow velocity was measured at water depths 0.25, 0.75, 1.25, 1.75, 2.25, 2.75 and 3.25m for both longitudinal (main) and transversal directions. Both longitudinal and transversal local velocities measured were divided by the main average flow velocity to get dimensionless relative flow velocity distribution along the chamber in both directions. The results showed that pre-aeration of 7.5 m3 air/ hr. m3 water increased TSS and O & G removal efficiencies of the grit chamber with 43 % and 134 %, respectively. Installing the transversal baffle with a depth corresponds to 50 % of water entire depth beside the same pre-aeration have together greater effect to increase O & G removal efficiency with 209 %. However, the best dual removal efficiency for both TSS and O & G was through installing the transversal baffle with submerged depth of 24 % of entire water depth and the same pre-aeration. Such conditions increased TSS removal efficiency by 44.5 % and increased O & G removal efficiency by 170 %, and neither adversely effected the helical flow velocity distribution pattern nor the longitudinal flow. TOC results indicated relatively long detention time adopted in the chamber causing considerable deposition of organic matters with the grits removed.