Phosphorus and organic matter removal from synthetic wastewater using alum and aluminum hydroxide

At present, chemical processes are widely used to remove phosphorus and organic matter
from water or wastewater either as the only advanced treatment method or as a pre-treatment
stage to biological treatment, and aluminium salts have been traditionally used as coagulants.
In the present study removal of orthophosphate or dissolved organic matter with the use of
two aluminium sources: alum [Al2(SO4)3•18H2O] and aluminium hydroxide [Al(OH)3(s)] is
examined for the wastewater treatment process. Amorphous aluminium hydroxide was
chosen because it is the hydrolyzed (olated) product of alum and an important adsorbent of
inorganic and organic substances in soils since bayerite, gibbsite and boehmite contain a
large portion of aluminium hydroxides. Laboratory jar test studies were carried out using
either an orthophosphate solution (10 mg l-1) or a tannic acid solution (50 mgC l-1) as
simulated wastewater and the effect of parameters such as coagulant dose (up to 15 mg Al l-1
in the case of alum and up to 90 mg Al l-1 in the case of Al(OH)3) and pH (2-12) is
investigated. The values of these parameters were based upon measurements on municipal
wastewaters from the input of the primary treatment of a wastewater plant in Athens.
Orthophosphates have been chosen as a P surrogate as they are the major portion of the
total P found in wastewater and tannic acid solution was used as a surrogate for soluble
organic matter. It was proven that alum is much more efficient in phosphorus and tannic acid
removal than aluminium hydroxide. The optimal pH values are 5-6 in both cases, alum and
aluminium hydroxide, although alum is efficient in a wider pH range (4-7) and a mechanism
was proposed to interpret these results. In pH values less than 6 the mechanism proposed
was chemical bonding between Al species and tannic acid or phosphates creating insoluble
complexes while in bigger pH values (6-8) adsorption on solid Al(OH)3. Freundlich isotherm
was proven to fit satisfactorily the experimental data for aluminium hydroxide and
orthophosphate at 250C suggesting heterogeneous sorption, with KF and N values 49,1 and
0,19 respectively. The findings of this work may not only contribute to a better understanding
of the chemistry of chemical wastewater treatment and therefore to an improvement of the
process but also on phosphorus and organics fixation in soils that contain a large portion of
aluminium hydroxides.