Particle size distribution (PSD) obtained in effluents from an advanced primary treatment process using different coagulants

The particle size distribution (PSD) obtained in effluents treated with two types of coagulants was compared. The objective was to maximize the removal of particles with a size between 0.75 and 1.5 mum, 1.5 and 5.0 mum, and 20 and 80 mum, corresponding to the sizes of fecal coliforms, salmonella and helminth ova, respectively. The coagulant doses tested were 20, 30, 40 and 50 mg/L for both anhydrous aluminum sulfate and PAX.XL-60. The raw wastewater had a PSD between 0.04 and 310 mum, of which 4.4% of the particles were found between 0.75 and 1.5 mum (by volume), 12.7% between 1.6 and 5.0 mum, and 39.4% of the particles were between 20 and 80 mum. It was determined that with 50 mg/L of aluminum sulfate 91% of the particles ;in the influent were eliminated, whereas with 30 mg/L of PAX XL-60 a 93% removal was achieved. The percentage of remaining particles with diameters between 0.7 and 5.0 mum was 48.9 and 53.8%, respectively, for the two coagulants. Particles between 20 and 80 mum were completely removed by the treatment system in both cases. Under these conditions the effluents had total suspended solids (TSS) concentrations between 39 and 37 mg/L, turbidity between 30 and 39 NTU, and COD between 218 and 233 mg/L, respectively. In terms of the removal of microorganisms, both coagulants removed helminth ova to values below 1 ova/L. Fecal coliforms were reduced from 2.82.10(8) to 5.45.10(6) with aluminum sulfate and to 4.85.10(6) NMP/100 mL with PAX XL-60; salmonella were reduced from 5.43.10(6) to 9.85.10(5) and to 4.85.10(6) NMP/100 mt with the respective coagulants. These levels would require an additional disinfection step to eliminate these organisms to acceptable levels for irrigation. Under the conditions in this study, the change in the PSD made it possible to determine the coagulant dose required to produce suitable water for agricultural reuse by an advanced primary treatment.