6    HEAVY METAL SCREENING CONCENTRATIONS FOR
HAULED SEPTIC WASTES
M. Philip Lo, Senior Engineer
Paul C. Martyn, Section Head
Industrial Waste Section
County Sanitation Districts of Los Angeles County
Whittier, California 90607
NEED FOR GUIDANCE IN HAULED SEPTAGE SCREENING
Liquid wastes from septic tanks, cesspools and chemical toilets need to be disposed of properly to
protect public health and water quality. Many sanitation agencies provide this disposal service by
accepting these wastes into their sewerage systems. Some agencies have disposal stations at their
wastewater treatment plants, while others have designated dumping manholes located near trunk
sewers. However, there is a concern that liquid industrial and hazardous wastes may also be brought
illegally to the stations for disposal. In response to this concern, sanitation agencies have set up
control programs to regulate septic wastes. Some agencies require the haulers to take out permits and
abide by local ordinances prohibiting industrial waste disposal. Others use manifest systems and
sample septage wastes before disposal. Some others also take samples periodically for detailed chemical analyses to check for possible industrial and hazardous waste contamination. The desired outcome
is to deter the illegal disposal of industrial and hazardous wastes at septage disposal stations while
providing for legitimate disposal.
Guidance in the interpretation of laboratory results for septage samples is needed to detect industrial waste contamination. Septage samples have a wide range of solids concentrations. Some of the
samples are fairly dilute, while others have high solids concentrations like those of sludge. The
concentrations of heavy metal in septage samples may also vary considerably, especially for copper
and zinc. These variations may be due to the type of plumbing system used, the age of the house and
the time of accumulation of the waste in the septage holding tank. Comparison of the heavy metal
concentrations for septage to those for domestic sewage is not always appropriate. Heavy metals like
copper and zinc may accumulate in septage sludge as part of the anaerobic degradation of solids in the
holding tanks, resulting in concentrations of heavy metals in septage being much higher than that for
normal sewage. Guidance has been needed on what constitutes a normal versus an abnormal septage
load for proper industrial waste control.
A METHODOLOGY FOR DEVELOPING SCREENING CONCENTRATIONS FOR SEPTAGE
An empirical approach has been used to develop screening concentrations for heavy metals in
septage. A study was initiated using septage samples from the County Sanitation Districts' Pomona
dumping manhole station for the year 1992. A total of 138 samples was taken in 1992, including 117
samples of septic tank and cesspool wastes, 18 samples of chemical toilet wastes, one sample of
regular sewage and two samples of shampoo brine wastes. Eight of the septic tank waste samples were
discarded as being unrepresentative, leaving a total of 109 samples usable for the study. Some of the
discarded samples had extremely low suspended solids concentrations that might have been the result
of unrepresentative sampling. Others had extremely high metal concentrations and were investigated
for possible contamination with industrial wastes containing heavy metals.
The heavy metal data were normalized for solids content to minimize the effect of solids on the
metal concentrations. The normalization was done by dividing the metal concentrations of the
samples by the corresponding suspended solids concentrations. High solids concentrations in the
samples usually result in high metal concentrations. The normalized data were found to vary by
several orders of magnitude, but were nevertheless found to fit a log-normal distribution. A statistical
analysis technique used by Thomas' for coliform data was employed to develop screening concentrations for heavy metals in septage.
49th Purdue Industrial Waste Conference Proceedings, 1994 Lewis Publishers, Chelsea, Michigan 48118. Printed in
U.S.A.
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