PRETREATMENT THROUGH CHEMICAL OXIDATION. GENERAL
CONSIDERATIONS AND A CASE STUDY INVOLVING A
THIOSULFATE/SULFIDE LADEN WASTESTREAM
John R. Walton, Applications Engineer
Jeffrey A. Rutz, Applications Chemist
Sheldon B. Magid, Group Leader
Interox America
Houston, Texas 77227
INTRODUCTION
With tighter controls being applied to industrial wastewater discharges, pollution engineers are
increasingly finding established biotreatment processes stressed and dangerously susceptible to shock
loading. This is particularly true in the refining industry where stricter effluent guidelines coincide
with the processing of higher sulfur feed stocks. The causes of sporadic treatment performance may
be many (e.g., hydraulic overloading, high BOD feed, and toxics in the influent) [1,2]. Although
properly designed equalization systems can deal with small infrequent excursions, recurring or lengthy
upsets can be costly in terms of process downtime. One low capital solution to the stressed biosystem
involves segregation and pretreatment of troublesome wastestreams. Particularly attractive are those
streams in which treatability can be improved through simple pH control, oil/water separation, settling/filtration, metals separation, or chemical oxidation [3J. Although much has been said of the
former, the integration of oxidative pretreatment technologies has only recently begun to receive
serious attention.
Chemical oxidation is most appropriate for those wastes containing oxidizable toxics (phenol and
sulfide) or substances which adversely affect subsequent processes through dissolved oxygen scavenging (sulfide, sulfite, and thiosulfate), or in certain cases general waste overloads (BOD,, COD).
One must be careful with the particular oxidant system chosen so as to minimize wasteful side reactions or over-oxidations, toxic by-product formation, and/or residual oxidant carry-over into
downstream processes. In general, chemical oxidation is more appropriate for: I) Relatively low flow,
concentrated wastestreams; 2) Highly variable waste loads of moderate flow; 3) Wastestreams of
such a nature to cause biosystem strain or upset; and/or 4) Wastestreams which contribute to unacceptable corrosion or odors.
In assessing the feasibility of oxidative pretreatment, it is important to evaluate both composite
and grab samples so that both relative pollutant loads and variations thereof are considered. This
paper outlines the considerations involved in instituting an oxidative pretreatment program using, as
an example, the application of hydrogen peroxide to destroy thiosulfate and sulfide in a petroleum
refining wastestream.
PROCESS DESCRIPTION
A schematic of the system involved in this case study is shown in Figure 1. The intent of the
process is to strip as much sulfide as feasible from the sour water stream and to oxidize that sulfide
remaining in the stripped water. In the air oxidizer, steam and air convert the sulfide to primarily
thiosulfate, and hydrogen peroxide completes the process by oxidizing the thiosulfate to sulfate. The
wastewater is then blended with a second stream, collected in a holding tank, checked for compliance,
and discharged into the municipal collection system. Typical compositions and excursion levels of
the air oxidizer effluent are presented in Table I. Pretreatment objectives are to reduce thiosulfate
to < 50 mg/1 and sulfide to <0.1 mg/1. Hydrogen peroxide feed rates are determined from thiosulfate
and sulfide levels entering the holding tank.
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