Abstracts Category : Other

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

by Janne Vnnen

Municipal wastewater treatment plants areconstantly forced to make improvements. The main objectives are toreduce energy consumption, to increase the treatment capacity andto improve the effluent water quality. Microsieves in combinationwith chemical pretreatment can potentially fulfil these objectives.In this work, experiments were mainly conducted at the large pilotscale, and supplementary experiments were conducted at thelaboratory scale to establish criteria for chemically enhancedprimary and tertiary treatment and microsieving. Full-scalefollow-up experiments were also conducted at the first treatmentplant in Scandinavia, utilizing coagulation/flocculation and discfiltration in advanced tertiary treatment. Microsieves constructedas disc or drum filters showed removal efficiencies for primarytreatment in the range of 30-60% with sieve pore sizes of 30-100 mwithout chemical pretreatment. By dosing with cationic polymers, asuspended solids removal > 80% was possible, and the effluentwater quality generally contained approximately 20-50 mg SS/L; inaddition, most of the particulate phosphorus and COD was alsoreduced. To further improve the removal to > 95%, dosing of thecoagulant is necessary; then, effluent containing < 10 mg SS/Land 0.1-0.2 mg TP/L with mostly dissolved COD remaining wasproduced. Sieve pore sizes in the range of 30-100 m had a minorinfluence on the removal with chemical pretreatment; however, thesolids loading capacity differed. For chemically enhanced primarytreatment and microsieving, common feedback PI automation withoccasional support from the feed forward control can be usedtogether with online turbidity measurements to control the removalof COD. A consistent effluent COD concentration could be maintainedindependent of the influent COD or flow variations. Phosphorusremoval could be controlled by adjusting the coagulant dose inrelation to the polymer dose. Pretreatment with chemically enhancedprimary treatment and microsieving was also beneficial for themicrofiltration of primary wastewater treatment. The effluent waterquality from microfiltration was improved if pretreatment withpolymer and coagulant prior to microsieving was conducted. Anionicpolymers were the most applicable because they generated a highflux. During tertiary treatment, for the highest possible removal,a sieve with a pore size of 10 m was needed. It is possible toconsistently achieve < 0.1 mg/L for the effluent phosphorusconcentration, but careful design of the dispersion/coagulation andflocculation stages is important, and control of the operation isnecessary. Polymer addition is crucial for chemically enhancedtreatment and microsieving. Polyacrylamide based synthetic anionicor cationic polymers with a high molecular weight and low-mediumcharge were shown to be suitable for both primary and tertiarytreatment. A general polymer dose of 1-5 and 0.5-1.5 mg polymer/Lcan be expected for primary and tertiary treatment, respectively.Alternative starch based biopolymers were also