Chemical precipitation and treatment control parameters in wastewater treatment

Abstract

The requirements for discharges from urban wastewater treatment plants are for BOD5 at 20 °C a maximum of 25 mg/l O2; for COD a maximum of 125 mg/l O2; and for Total Suspended Solids the maximum concentration allowed is 35 mg/l. To meet these requirements, and to treat the volume of water reaching treatment plants, it is common use to implement a chemical unit process. The use of chemicals is to increase particle size by means of coagulation and as a consequence the performance of physical unit processes downstream is improved. The removal of suspended solids is thereby enhanced along with the removal of associated pollutants. The second purpose of using chemicals is to precipitate soluble materials, aiding on their removal downstream. Ferric chloride is one of the most common coagulants used in wastewater treatment; one of its characteristics is its multivalence, which attracts strongly to colloidal particles. Also, it is relatively insoluble, assuring a high degree of removal.

Coagulation is the process whereby destabilization of a given suspension or solution is effected. The function of coagulation is to overcome the factors that promote the stability of a given system. Flocculation is the process whereby destabilized particles, or particles formed as a result of destabilization, are induced to come together, make contact, and thereby form large(r) agglomerates that can be removed by gravitational sedimentation.

Observations were made at the SNJ Wastewater Treatment Plant that the optimum pH and ferric chloride dose change during the day due to changes in the composition of the wastewater. The aim of the project is to investigate the cause of these observations in order to further understand which parameters affect the precipitation process and to improve the dosing of ferric chloride in wastewater treatment.

A series of jar tests were performed on 6 different dates, under diverse weather conditions. Alkalinity, pH, and total suspended solids were determined and analyzed with relation to different dosages of ferric chloride as coagulant. It was found that adding ferric chloride significantly increases the removal of suspended solids, but that excess coagulant is counterproductive since it leads to re-stabilization of the particles. Also, the removal rate of TSS decreases with higher doses of coagulant, which means higher costs for treatment. Higher alkalinity affects precipitation negatively thus requiring higher ferric chloride dosage. Lower alkalinity produced lower effluent TSS. When the pH drops significantly, the concentration of solids rises again. Precipitation occurs over a range of pH values, and no exact optimum pH could be determined.