STUDY OF THE EFFECT OF BORON TOXICITY
ON AN ACTIVATED SLUDGE SYSTEM
Wayne G. Webber, .Project Engineer
Donald W. Kemp, Chemist
Samuel E. Rice, Project Engineer
Metcalf and Eddy, Inc.
Boston, Massachusetts 02114
INTRODUCTION
Boron levels in aqueous systems can vary widely depending on the water source and
the effluent that is discharged to the receiving water.   For surface waters the observed
boron concentration ranges from 0.1 mg/l to 5 mg/l in lakes and streams and is relatively
constant at 4.5 mg/l in sea water [1],   Generally, boron levels in ground water exceed
those of surface water but there is a wide variability due in part to the presence or absence of boron-containing minerals.   The concentration of boron in the influents to
municipal systems will vary depending on the boron levels contributed by industrial
sources.
The boron in aqueous systems originates from the leaching of naturally occurring
boron deposits, effluents from industrial processing that utilizes boron compounds as
raw material and wastes from boron-containing consumer products such as borate-based
detergents.   An increase in the consumption of these consumer products is expected to
result in increased levels of boron in some receiving waters.
The presence of boron in the water has both beneficial and detrimental effects, depending upon its relative concentration.   For example, boron is a necessary micronutrient
for some plants and algae species; however, toxic effects can result if certain minimum
concentration levels are exceeded.   These concentration limits vary with different plant
systems.   For example, plant crops have been categorized into three different groups with
respect to toxicity levels:   tolerant, 2 to 4 mg/l; semitolerant, 1 to 2 mg/l; and sensitive,
< 1 mg/l [ 1 ].   The tolerant classification includes most of the root species, such as turnip,
beet, and carrot, and some leaf species, such as cabbage and lettuce.   The semitolerant
toxic level includes the grains and most of the stalk vegetables, such as tomatoes, beans
and peppers.   Included in the sensitive class are citrus and fruit trees.
Algae species show variable behavior with respect to boron.   Some species, such as
blue-green algae, absorb boron and show a growth response to boron, whereas some of
the green algae species were observed to be neither stimulated nor inhibited by boron
levels up to 10 mg/l [2,3].   Animal or human life has not been observed to be adversely
affected by boron concentrations that one normally observes in water systems.
Toxic effects, however, have been observed in certain biological processes.   One study
using a synthetic activated sludge derived from a yeast extract observed a toxic or inhibiting effect when the sludge was contacted with 1 mg/l solution of boron [4].
Based on these observed effects EPA has defined concentration limitations for the
usage of boron-contaminated water sources [4J.   Different standards ranging from
0.75 mg/l to 2 mg/l have been imposed for irrigation water depending upon which crop
species are being irrigated.   The standard given for raw water used as a public water supply is a maximum acceptable concentration of 1 mg/l [5].   The rationale given for this
low limit is not because of the toxicity to humans, but because of the potential toxic effects that might result from watering home gardens, fruit trees and ornamental plants
with concentrations of boron higher than 1 mg/l.   No standard has been given for aquatic
life or wildlife in fresh water, but in marine water the maximum concentration of boron
has been set at 5 mg/l [6].
743