Dewatering of the Ash By-Product from the
Wet Oxidation Process
WILLIAM R. WALTERS,  Chemical Engineer
GREGORY ETTELT,  Chemical Engineer
The Metropolitan Sanitary District of Greater Chicago
Chicago, Illinois
INTRODUCTION
Sludge Disposal Considerations
The Chicago Sanitary District treatment plants must process approximately
900 tons of sludge per day from its preliminary settling and its activated-sludge
operations. At present, the major portion of the activated sludge is converted to
fertilizer by dewatering and drying. Recently installed high rate digesters have an
estimated capacity of 120 tons per day when operating on a 20:80 mixture of primary and activated sludge. The Zimmermann wet air oxidation process presently
treats between 15 and 18 per cent of the total sludge. Realization of design capacity would increase this figure to about 22 per cent. All sludge is ultimately
disposed of as fertilizer, digested sludge, and oxidized sludge from the wet air ox -
■ idation process (hereafter referred to as Zimpro). Present practice is to lagoon the
Zimpro and digester effluents. The supernatant which has a high BOD is returned
to the activated sludge process.
The lagoon system of ultimate disposal is a good one providing that lagoons
are available and reasonably close to the treatment site. However, particularly
for the Zimpro effluent, it may be desirable in future plant locations to thicken
and dewater the oxidized sludge prior to transportation to the ultimate disposal
site. Sedimentation followed by sand bed drying does not appear to be economically feasible for large scale Zimpro plants because of the restrictions imposed by
by winter weather. Consequently, the remainder of this paper will be concerned
with two of the unit operations for solids-- liquid separation: filtration and centri-
fugatlon.   Most of the work, to date, has been on filtration.
Properties of the Zimpro Slurry
The existing plant at the West-Southwest Treatment Works in Chicago has
been described by Goldstein and Lokatz (1). Dewatering properties of the product
slurry have been shown to be a function both of the degree of oxidation and the nature of the feed material. Data reported herein is relevant to an average COD reduction of 70 per cent of a feed material with an approximate one to one ratio of
primary to activated solids. Feed solids concentration ranges from three to 3. 5
per cent. Effluent liquid carries in suspension about 1. 5 per cent of solids of which
about 15 per cent is volatile. The liquid contains considerable BOD and must be
returned to the activated sludge process.
Little has yet been attempted in chemical and physico-chemical analyses of
the Zimpro slurry. Some data in this regard has been reported by Hurwitz and
Dundas (2).
A batch settling test of a 10. 3 per centslurry diluted to 1.5 per cent with water was employed to estimate the minimum particle size.    Based on the initial
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