EcoMat, Inc. of Hayward, California (EcoMat)
has developed a 2-stage ex situ anoxic biofilter biodenitrification
process. The process is a fixed film bioremediation, using biocarriers
and specific bacteria to treat nitrate-contaminated water. Unique
to EcoMat's process is a patented mixed bed reactor that retains the
biocarrier within the system, thus minimizing solids carryover. Fixed
film treatment allows rapid and compact treatment of nitrate with
minimal byproducts. Methanol is added as a source of carbon for cell
growth and for metabolic processes that remove free oxygen. The resulting
oxygen-deficient environment encourages the bacteria to consume nitrate.
Methanol is also important to assure that conversion of nitrate proceeds
to the production of nitrogen gas rather than to the more toxic nitrite
intermediate.
The mechanism for anoxic biodegradation of nitrate
consists of two sequential denitrification reactions. Oxygen must
be consumed to a dissolved oxygen concentration of <1 mg/L. In
the first denitrification step, the bacteria are forced to substitute
the nitrate as the electron acceptor and the nitrate is reduced to
nitrite. In the second step, the nitrite is further reduced to nitrogen
gas. Nitrite production is an intermediate step and there is no a
priori reason to assume that the second reaction is at least as fast
and/or favored as the first reaction in the presence of a specific
bacterial population. Consequently, any evaluation scheme must establish
that there is no buildup of nitrite, particularly since the nitrite-nitrogen
maximum contaminant level (MCL) is only 1 mg/L, one tenth that of
nitrate. High concentrations of nitrate and high nitrate/methanol
ratios may also affect the concentration of residual nitrite in a
particular process configuration.
A simplified process diagram of the EcoMat treatment
system used during the demonstration is shown in Figure 1. The system
is composed of two major components: a biodenitrification system and
a polishing or post-treatment system. The biodenitrification system
is intended to convert nitrates in the groundwater to nitrogen, thus
reducing nitrate concentrations. The post-treatment system destroys
or removes intermediate compounds generated during the biological
breakdown of nitrate and removes bacteria and suspended solids that
are not attached to the biocarrier. The post-treatment system can
also incorporate treatment for other contaminants, such as VOCs, that
may be present in the influent.
Biodenitrification is conducted in two reactors,
identified as R1 and R2 on Figure 1. The majority of the oxygen removal
step is conducted in R1 where aerobic bacteria reduce dissolved oxygen
levels of the influent. Methanol is metered to the tank to encourage
the bacteria to begin consuming nitrate. The resulting oxygen-deficient
water is pumped from the bottom of R1 to the bottom of R2, which is
densely packed with biocarrier media ( 1 cm) which have the appearance
of small foam cubes. A patented mixing apparatus within R2 directs
the incoming water into a circular motion, thus assuring intimate
contact with the biocarrier. Within R2, the majority of denitrification
occurs by anaerobic bacteria that are continually fed methanol and
populate on the large mass of biocarrier media. After a sufficient
retention time, depending on concentration and goal, denitrified water
drains by gravity to an overflow tank, which allows for a continuous
and smooth transfer to the post-treatment system and removal of entrained
bacteria and media.
Depending on the presence of other contaminants,
the post-treatment system consists of a series of varying sized filters
downstream of one or more contaminant-specific treatment units. For
instance, ozonation may be used to oxidize any residual nitrite to
nitrate and to deactivate/destroy all residual biological materials
leaving the biodenitrification unit. If VOCs are present, an air stripper
and/or carbon adsorption unit can be used. If state regulations require
chlorination of drinking water, then chlorine can be added as a post-treatment,
or directly to the overflow tank immediately following denitrification.
Figure 1. Simplified Flow Diagram.
Waste Applicability: Anoxic biodenitrification using one or more biocarriers
should be applicable to industrial wastewaters and leachate from commercial,
industrial and hazardous waste sites containing various nitrate concentrations,
as well as for treatment of groundwater (the medium treated during
the demonstration). The presence of other contaminants could play
a significant role in the effectiveness and viability of the overall
treatment system. For example, if volatile chlorinated hydrocarbons
are present along with nitrate, a post-nitrate treatment system (e.g.
carbon filters) may be necessary to remove those compounds to acceptable
levels.
Demonstration Results: A SITE demonstration of the EcoMat biodenitrification
system was conducted at the location of a former public water supply
well in Bendena, Kansas. This study, which occurred from May until
December of 1999, was conducted in cooperation with the Kansas Department
of Health and Environment (KDHE). The KDHE provided a small building
and necessary utilities for the EcoMat systems. In addition, the state
is analyzing water samples independently.
The demonstration focused on treating contaminated
water from the Bendena Rural Water District No. 2 Public Water Supply
(PWS) Well No.1. This former railroad well, constructed in the early
1900s, was at one time the sole source of water for the town of Bendena.
The primary contaminant in the water is nitrate from uncertain sources
ranging from 20 to 130 mg/L. Low concentrations of VOCs, particularly
carbon tetrachloride (CCl4), in the groundwater is a secondary problem
ranging from 2 to 31 g/L.
EcoMat's main goal of the study was to demonstrate
that its biodenitrification system could reduce incoming nitrate-N
in excess of 20 mg/L to a combined nitrate plus nitrite concentration
below 10 mg/L. A second goal of the study was to demonstrate that
the post-treatment system used would produce treated water that would
meet applicable drinking water standards with respect to nitrate-N
and nitrite-N; and that the final effluent would not contain turbidity
of greater than 1 NTU, detectable levels of methanol (1 mg/L), increased
levels of biological material or suspended solids, and will have a
pH in the acceptable 6.5 to 8.5 range.
To evaluate both the biodenitrification system
and the post-treatment system adequately, water samples were collected
from four specific points along the entire process. These were: 1)
an influent sample point between PWS #1 and R1; 2) a partial treatment
sample point between R1 and R2; 3) an intermediate effluent sample
point between the biodenitrification system and post-treatment system;
and 4) a final effluent sample point downstream of the post-treatment
system. To assure a statistically adequate number of samples, an average
of 30 influent and 30 effluent samples were collected for each of
four separate sampling episodes. Over an approximate seven and one-half
month period, EcoMat operated its system at a flow between three and
eight gallons per minute.
Results from the EcoMat biodenitrification process
were encouraging when the entire system was operating at optimal performance.
In those instances where the final combined nitrate-nitrite effluent
concentration was above the regulatory limit, operational problems
(mostly mechanical) were suspected as the primary cause.
