Microsieving in municipal wastewater treatment : Chemically enhanced primary and tertiary treatment

Sammanfattning: Municipal wastewater treatment plants are constantly forced to make improvements. The main objectives are to reduce energy consumption, to increase the treatment capacity and to improve the effluent water quality. Microsieves in combination with chemical pretreatment can potentially fulfil these objectives. In this work, experiments were mainly conducted at the large pilot scale, and supplementary experiments were conducted at the laboratory scale to establish criteria for chemically enhanced primary and tertiary treatment and microsieving. Full-scale follow-up experiments were also conducted at the first treatment plant in Scandinavia, utilizing coagulation/flocculation and disc filtration in advanced tertiary treatment. Microsieves constructed as disc or drum filters showed removal efficiencies for primary treatment in the range of 30-60% with sieve pore sizes of 30-100 μm without chemical pretreatment. By dosing with cationic polymers, a suspended solids removal > 80% was possible, and the effluent water quality generally contained approximately 20-50 mg SS/L; in addition, most of the particulate phosphorus and COD was also reduced. To further improve the removal to > 95%, dosing of the coagulant is necessary; then, effluent containing < 10 mg SS/L and 0.1-0.2 mg TP/L with mostly dissolved COD remaining was produced. Sieve pore sizes in the range of 30-100 μm had a minor influence on the removal with chemical pretreatment; however, the solids loading capacity differed. For chemically enhanced primary treatment and microsieving, common feedback PI automation with occasional support from the feed forward control can be used together with online turbidity measurements to control the removal of COD. A consistent effluent COD concentration could be maintained independent of the influent COD or flow variations. Phosphorus removal could be controlled by adjusting the coagulant dose in relation to the polymer dose. Pretreatment with chemically enhanced primary treatment and microsieving was also beneficial for the microfiltration of primary wastewater treatment. The effluent water quality from microfiltration was improved if pretreatment with polymer and coagulant prior to microsieving was conducted. Anionic polymers were the most applicable because they generated a high flux. During tertiary treatment, for the highest possible removal, a sieve with a pore size of 10 μm was needed. It is possible to consistently achieve < 0.1 mg/L for the effluent phosphorus concentration, but careful design of the dispersion/coagulation and flocculation stages is important, and control of the operation is necessary. Polymer addition is crucial for chemically enhanced treatment and microsieving. Polyacrylamide based synthetic anionic or cationic polymers with a high molecular weight and low-medium charge were shown to be suitable for both primary and tertiary treatment. A general polymer dose of 1-5 and 0.5-1.5 mg polymer/L can be expected for primary and tertiary treatment, respectively. Alternative starch based biopolymers were also applicable, but the required dose was higher. Both iron and aluminium coagulants were applicable, but aluminium based coagulants were shown to be an overall a better choice. During primary treatment, the coagulant dose was variable, depending on the demand. During tertiary treatment, a molar ratio of 5-7 mol Me3+/mol influent TP was sufficient to fulfil the effluent criteria. This corresponded to a dosing of approximately 1-4 mg Al3+/L for the ordinary secondary effluents. Screening of chemicals and doses was performed in the laboratory with modified jar tests. Comparable treatment results were achieved in the laboratory, at the pilot scale and at the full scale; however, the laboratory experiments overestimated the solids loading capacity.