Zinc in Relation to Activated Sludge
and Anaerobic Digestion Processes
GERALD N. McDERMOTT, Senior Sanitary Engineer
EDWIN F. BARTH, Chemist
VINCENT SALOTTO, Chemist
M. B. ETTINGER, Chief, Chemistry and Physics
Robert A. Taft Sanitary Engineering Center
Cincinnati 26, Ohio
The efficiency of treatment of sewage containing zinc was studied by
operation of pilot activated-sludge plants. Sewage from a common source
was fed to three replicate plants. Zinc solutions were introduced continuously
to the feed of two of the plants to produce selected constant concentrations;
one plant was operated with no metal addition to the feed. Differences in
effluent quality, as measured by BOD, COD, suspended solids, and turbidity,
between the zinc fed units and the unit receiving the unaltered sewage were
attributed to the presence of zinc in the feed.
The digestion of the zinc-bearing sludges was studied by operation of
bench-scale digesters on sludge feeds obtained from the activated sludge
plants. Differences in gas production between the digesters receiving control
sludge and those receiving the zinc-bearing sludges were attributed to the
zinc in the sludge.
The objectives of the research were to determine the level of zinc that
can be tolerated in waste waters without reducing the efficiency of biological
processes in removing the organic matter or in stabilizing the sludges, and to
determine the efficiency of the process in removing zinc.
PLANT DESCRIPTION AND OPERATION
The pilot activated-sludge plants were designed to simulate standard
activated-sludge plants of the spiral flow type. The activated-sludge plant
included: primary settling, aeration with continuous sludge return, and secondary settling. Thus opportunity existed for precipitation, reduction, and
complexing such as might occur during primary settling in an actual situation
before a biological process is reached. Effects were related to metals added
to the incoming sewage rather than to metals added to some specific plant
component. The units are illustrated in a previous article concerning heavy
metals and se>vage treatment (1). Capacity and loading factors for the plant
are given in Table I. Sewage was fed to the units at a constant rate. Sludge
from the final settler was pumped continuously to the head of the aerator at
a rate of about 35 per cent of the sewage feed flow. An automatic device,
activated once per minute, diverted the return sludge about 5 per cent of
each minute to a collecting vessel. This procedure wasted about 25 per cent
of the suspended matter in the aeration tank each day.
The sewage available in the area where the pilot plant was located,
while of essentially domestic origin, had an average BOD of about 75 mg/1
because of high ground water infiltration. Fortification of this sewage with
dog food produced a sewage showing characteristic behavior and adequate
strength.     Dry  granular  dog  food  was ground,   soaked in water overnight,
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