|M.Sc Student||Avishag Yaacobi|
|Subject||Study of Chemically Enhanced Primary Sedimentation of Raw|
|Department||Department of Civil and Environmental Engineering||Supervisor||Professor Emeritus Galil Noah (Deceased)|
The preliminary settling of wastewater
is an important stage in the treatment sequence. Previous studies reported
improved removals of suspended solids and organic matter, following the
addition of coagulation-flocculation before primary sedimentation.
The objectives of this research were to investigate the potential of coagulation-flocculation as pretreatment to primary sedimentation, to evaluate the primary sludge produced and the influence on the overall sludge handling.
The experimental work was based on two sources of raw wastewater, from the municipal treatment plants of Haifa and of Cramiel. The first phase was to study the removal efficiency of various contaminants based on standard Jar Test procedure using either ferric chloride or polyaluminum chloride in dose ranges of 0 to 180 mg/L.
Settling column tests were carried out to evaluate the settling rate of flocculated material with no chemical addition and with addition of 50 mg/L FeCl3 or PACl. The third stage in the experimental work focused on the treatment of two different mixtures of sludge that were exposed separately to anaerobic digestion in batch conditions, for 21 days at 35oC
The experimental results indicate that mixing of raw wastewater in slow conditions could create aggregation and improved suspended solids removal by 5 to 30 percent. The rate of suspended solids settling due to chemical coagulation-flocculation using 50 mg/L of FeCl3 was higher by 1.6-2.8 in Haifa wastewater and by 3.2-4.0 in Carmiel wastewater compared to plain settling. The removal of suspended solids was 81-88% for FeCl3 and 83-95% in case of PACl, both coagulants being used in doses of 50-180 mg/L. Turbidity reductions of 83-98% proved efficient reduction of colloids due to the chemically-enhanced process. Removal performance of organic load expressed as CODT indicated removal efficiencies of 75% at Haifa wastewater, and 60-80% in Carmiel wastewater. Removal of phosphorus compounds was achieved at 50 mg/L of coagulant, in the range of 40-48% at Haifa wastewater and 71-87% at Carmiel wastewater. Higher removal of 87-98% could be achieved only at higher doses of 180 mg/L. Removal performance of heavy metals achieved: Zinc 62-95% Copper 94-98%, Barium 75-86%, Aluminum 92-94%, Strontium 25-27%. The experimental results did not reveal any preference of one coagulant on the other.
Anaerobic digestion of mixtures containing raw primary and biological sludge in ratios of 40 to 60% accordingly indicated better performance as compared to chemically enhanced primary and biological sludge in ratios of 60 and 40% accordingly.