Total Waste Recycle System for Water Purification
Plants Using Alum as Primary Coagulant
LAWRENCE K. WANG, Assistant Professor
Bio-Environmental Engineering Division
Rensselaer Polytechnic Institute
Troy, New York 12181
JOHN Y. YANG, Principal Chemist
Environmental Systems Department
Calspan Corporation
Buffalo, New York 14221
INTRODUCTION
The general public as well as the responsible officials of governmental and industrial
sectors are becoming increasingly conscious of the need to safeguard our environment. The
development and implementation of pollution control techniques to minimize industrial
waste discharges have made significant gains during recent years. Further efforts for
environmental quality improvements by achieving "zero" waste discharges are being
suggested. It is recognized that a major difficulty in achieving a "zero" discharge objective
lies in the lack of satisfactory technologies for ultimate disposal of liquid and solid waste
residuals accumulated from pollution abatement controls. Inasmuch as any further
treatment of such residuals will introduce an endless cycle of air, water or land
contaminants, recovery and recycle of waste treatment reagents would have to be
implemented if "zero" waste discharge is to be achieved.
One of the most important water and waste treatment processes in which relatively
large quantities of chemical reagents are expended is coagulation-sedimentation. Alum is
generally accepted as the primary coagulant in a wide range of waste treatment practices.
Effective alum recycle would represent a major step in achieving "zero" waste discharge
objectives. This study is therefore directed toward evaluating potential alum recovery and
reuse techniques. Waste effluents from a local water purification plant were selected as
study materials to provide a study addressing to realistic industrial waste management
applications.
In a water purification plant, major waste components consist of sludges collected
from flocculation-sedimentation underflow and filter backwash water. When alum is
employed as the primary coagulant, the sludge may be characterized chemically as a
combination of inert silt and alum floe. Alum floe can be recovered from the sludge mixture
by solubilization with either acid or alkaline reagents. The relevant chemical reactions are
illustrated as Equations 1 and 2:
2A1(0H)3 + 3H2S04 - 2 Al4-1"1" + 3S04"" + 61^0 (1)
Al(OH)3 + NaOH ^Na+ + A102" + 2H20 (2)
The solubilized reagent must be subjected to pH adjustment to regenerate alum floe for
effective reuse as a coagulant. The systems approach employed in this study is based on
optimum design applications of acid and alkaline reagents so that the requirement for
added pH adjustment reagents would be minimized. A brief description of prevalent water
purification unit operations and waste disposal practices is included to clarify the waste
recycle and process compatibility requirements envisioned in this study. Waste
characterization and alum recovery experimental data are presented. The significance of
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