This research was devoted to studying the improvement of removal efficiency of pollutants in terms of COD and SS from municipal wastewater by using a modified primary treatment system. The modified system involved recycling a portion of the settled primary sludge and mixing it with the incoming raw sewage in a flocculation tank to induce bioflocculation, biosorption and particle adsorption/enmeshment. In order to maximize biosorption, the recycled sludge was pre-aerated for a controlled period from 15 to 60 minutes to induce microbial growth. The flocculation time was 30 minutes at a mixing speed of 60 rpm, equivalent to a tip velocity of 0.126 m/s. The MLSS concentration was maintained at 6000 mg/L in the flocculator....[ Read more ]

This research was devoted to studying the improvement of removal efficiency of pollutants in terms of COD and SS from municipal wastewater by using a modified primary treatment system. The modified system involved recycling a portion of the settled primary sludge and mixing it with the incoming raw sewage in a flocculation tank to induce bioflocculation, biosorption and particle adsorption/enmeshment. In order to maximize biosorption, the recycled sludge was pre-aerated for a controlled period from 15 to 60 minutes to induce microbial growth. The flocculation time was 30 minutes at a mixing speed of 60 rpm, equivalent to a tip velocity of 0.126 m/s. The MLSS concentration was maintained at 6000 mg/L in the flocculator.

It was found that both COD and SS removal efficiencies in the modified system were consistently higher than those without sludge recycling. In this particular study, the best performance was obtained with 60-min sludge aeration. Under such a condition, the recycled sludge had developed sufficient microbial growth to achieve an SOUR (specific oxygen uptake rate) of 72 mg O₂/g-MLSS-hr. The average removal efficiencies for COD and SS were 48.1% and 61.6% respectively, while the corresponding efficiencies for the control unit (no sludge recycling) were only 35% and 52.0%. These values represented an improvement of 37.4% [i.e., (48.1%-35%)/35% = 37.4%] for COD and 18.5% for SS. When the sludge aeration was reduced to 30 minutes, its SOUR value was also decreased to 60 mg O₂/g-MLSS-hr. The treatment improvement were 34.8% for COD and 10.8% for SS. When the aeration was further reduced to 15 minutes, the corresponding improvement were 26.8% for COD and 17.7% for SS. Thus, appropriate aeration of the recycled sludge could provide proper bacterial growth to enhance the sorption capacity as well as to achieve partial oxidation of the biosorbed pollutants.

The experimental data also showed that without sludge aeration, some particulate pollutants could be hydrolyzed and released back to solution, thereby causing increases of both COD and SS in the treated effluent.