36    SELENIUM REMOVAL FROM PETROLEUM REFINERY
WASTEWATER
Yakup Nurdogan, Senior Process Engineer
Robert P. Schroeder, Engineering Supervisor
Bechtel Environmental, Incorporated
San Francisco, California 94119
Charlie L. Meyer, Senior Research Engineer
Shell Development Company
Houston, Texas 77001
INTRODUCTION
Selenium is an essential trace element for animals and humans, but it can cause toxicity at elevated
levels in dietary intake. Parts of the San Francisco Bay estuary, including Suisun Bay, San Pablo Bay,
and the Carquinez Strait, have been classified by the U.S. Environmental Protection Agency as
impaired due to the impact of selenium on waterfowl.1 As a result of this classification, the San
Francisco Bay Regional Water Quality Control Board (RWQCB) amended the NPDES permits for
the six refineries discharging into the estuary to include specific permit limits on total selenium. Each
refinery was given a mass discharge limit for total selenium based on the refinery's average annual
flow and an effluent concentration of 50 ppb.2 The effluent concentration limit was based on a 5 ppb
EPA water quality criterion for selenium and a dilution ratio of 10:1 for deep water discharges.
Each of the six refineries has been working independently to develop treatment processes to reduce
the selenium in their effluent.2"7 Most of the work has been done by the four refineries owned by
Shell, Unocal, Chevron, and Exxon. Table I summarizes the findings of studies conducted by or for
these refineries.8
The best results were obtained by the ferric chloride treatment of biotreated stripped sour water (SSW)
and final effluent at the Shell Martinez Manufacturing Complex (MMC). The efficiency of selenium
removal was greater than 95% in laboratory studies and 90% in the pilot study. Both bench- and pilot-
scale tests demonstrated that the selenium concentration could be lowered to less than 50 ppb using a
ferric chloride dosage of 50 ppm as Fe. This paper will summarize the results of the bench- and pilot-
scale iron adsorption/coprecipitation tests conducted on the Shell MMC wastewater using ferric chloride
reagent. The study was conducted by Bechtel Environmental, Inc. (BEI) of San Francisco, California,
with oversight by Shell's Westhollow Research Center (WRC) in Houston, Texas.6,7
Merrill et al.9 explained the mechanism of iron adsorption/coprecipitation for removing selenium
from the ash pond effluent of a coal-fired power plant. Ferric iron (e.g., as FeCl3), when added to
water, produces a ferric hydroxide precipitate. If the reaction occurs at pH levels around 6, the
precipitate can remove selenium from water by adsorbing or coprecipitating the oxyanion forms (e.g.,
selenate and selenite). Competition from sulfate, which is present in significant amounts in refinery
effluent, significantly reduces the effectiveness of the process for selenate removal, but selenite can
still be removed.
MATERIALS AND METHODS
Bench-Scale Setup
A "gang" stirrer with six paddles was used to conduct one-liter "jar" tests. These jar tests were
conducted on the wastewater samples from sour water stripper (SWS) bottoms, dissolved air flotation
(DAF) unit effluent, corrugated plate interceptor (CPI) effluent, and biotreater effluent. Most of the
tests were designed to determine the efficiency of iron adsorption/coprecipitation as a method of
selenium removal. The tests were conducted according to a standardized procedure described in an
earlier report.6
49th Purdue Industrial Waste Conference Proceedings, 1994 Lewis Publishers, Chelsea, Michigan 48118. Printed in
U.S.A.
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