Abstract
Municipal wastewater sludge was produced by chemical coagulation of synthetic wastewater (sww) based on Synthene Scarlet P3GL disperse dye and real municipal wastewater (nww), coagulated by commercial coagulants PAX (prepolymerised aluminum coagulant) and PIX (a ferric coagulant based on Fe2(SO4)3). An attempt was made to correlate the sludge’s dewatering capacity (in terms of capillary suction time—CST) with operation parameters for wastewater treatment, size distribution and specific surface area of the sludge particles. It was found that the presence of phosphate ions in the system facilitates the removal efficiency of the above-mentioned dye (L) due to the interaction between the dye molecules and H2PO4− ions. Unlike sww, negatively charged organic substances (sorg) in nww are directly adsorbed on the surface of colloidal particles {Fe(OH)3} and {Al(OH)3} (prtc). It was also discovered that an increase in the dose of a coagulant led to an increase of CST for sww sludge and to a decrease of CST for nww sludge. It has been suggested that flocs composed of spherical {Al(OH)3} units possessed more internal space for water than aggregates consisting of rod-shaped {Fe(OH)3} units and, consequently, it is more difficult to remove water from Al-sww sludge than from Fe-sww. The results obtained showed that smaller particles dominate in sww sludge, while larger particles are prevalent in nww sludge. To explain this distinct difference in the size distribution of particles in sludge obtained with the use of Al3+ and Fe3+, simple models of aggregation and agglomeration-flocculation processes (aaf) of treated wastewater have been proposed. Except for PIX in nww, the analyzed particles of the investigated types of sludge were characterized by similar specific surface area (Sps), regardless of the kind of sludge or the applied coagulant. Slightly larger, negatively-charged sorg bridges, anchored directly on the surface of positive prtc are more effective in closing the structure of nww sludge than small L bridges of the dye molecules anchored on the surface of prtc via H2PO4−. All the discovered aspects could lead to improved performance of wastewater treatment plants (WWTP) by increasing the efficiency of sludge dewatering.
Highlights
Chemical coagulation is the second step in wastewater treatment [1,2]
Molecules of dye are adsorbed to prtc by bridging, with the help of H2 PO4 − ions previously adsorbed to these surfaces
Negatively charged sorg in nww are directly adsorbed on the surface of prtc
Summary
Chemical coagulation is the second step in wastewater treatment [1,2] This process has a significant influence on the properties and structure of the resulting sludge [3,4,5,6,7]. Positively-charged, colloidal particles {Fe(OH)3 } and/or {Al(OH)3 }, described further as prtc (colloidal particles), are formed during the subsequent reactions which occur in wastewater [1,8]. These particles act as adsorbents [9] in the processes of aggregation and agglomeration-flocculation (aaf ) of pollutants from wastewater. The removal of neutral or positively-charged pollutants takes place during the so-called flocculation [1,5,10]
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