(Continued from May 2004, U.S. Water News)
By Chris Fox
Carter & Burgess, Inc.
U.S. Water News Online
"Oh the times, they are a changing," sang Bob Dylan. And change is
the only real constant we all face.
During the past quarter century, technology in general has changed
more rapidly than any other time in history. In the personal computer
industry, for example, purchase a state-of-the-art computer and
within weeks it is replaced by new technology that is faster and
smarter. Within industries and across industries, technology has
almost outpaced itself.
The wastewater industry is no different in the need to advance
existing technologies and embrace new technologies that make the
systems work more effectively and efficiently.
"The need for ever-changing technology in the wastewater industry
stems from past biological systems that did a good job at removing
particulate matter that we knew existed. But instrumentation just
kept getting better and better at detecting more and more trace
compounds that we didn't know existed or couldn't detect in earlier
years," said Robert McMillon, former president of the Water
Environment Federation (WEF). "Now, as the need to protect ever
shrinking water supplies increase and stricter permit limits that
require higher water quality standards occur, the technology needs to
rise to the challenge."
Some solutions may come from the increasing use of membrane
treatment technology and biological system enhancements such as
biological nutrient removal (BNR) and sequencing batch reactors (SBR)
coupled with energy efficiency innovations.
Biological nutrient removal
Biological nutrient removal (BNR) was talked about in other
industries in the 1980s but did not become of interest in the
wastewater industry until the late 1990s. In earlier years, the
technology was not very refined, not very reliable, and not very well
understood. Today BNR technology has resurfaced because it is now
well understood in wastewater applications with regard to design
criteria, process, operation, maintenance, and computer controls,
says Al Petrasek, Carter & Burgess principal and senior project
BNR uses naturally occurring micro-organisms, with oxygen rather
than chemicals, to remove such nutrients as nitrogen and phosphorus
from the wastewater stream.
Raw or untreated wastewater contains ammonia, which is toxic to
fish. Ammonia degrades to nitrates, which removes the oxygen from the
stream, therefore, killing animal and plant life. Nitrates also
become fertilizers promoting algae growth. As algae die and
decompose, algae create a high oxygen demand, which creates low
dissolved oxygen in the water. The BNR denitrification process can
convert nitrate into nitrogen gas bubbles that are harmless and the
wastewater effluent has no deleterious effects on receiving waters,
One of the driving forces behind BNR technology is tightening
permit limits, says Ray Hamilton, public works unit manager in
Denver. If permit limits for phosphorous are 1mgd/L, BNR alone can
very effectively meet these parameters. If the permit limit is .5
mgd/L or less, then it is most economical to use BNR to treat
phosphorous down to the 1 mgd/L limit and then use chemical
precipitation to get to the lower limits, Hamilton continues. There
can still be a significant chemical cost reduction using the BNR
"In plants with low alkalinity in the wastewater, BNR is
advantageous in the denitrification step because it reduces oxygen
required for treatment and restores alkalinity previously destroyed
in the nitrification stage," says Hamilton.
Using BNR, fewer chemicals are added to the treatment process,
which reduces the amount of sludge produced and increases sludge
quality, therefore positively impacting sludge disposal or reuse
Sequencing batch reactors
According to the United States Environmental Protection Agency
(EPA), "fill-and-draw batch processes" like sequencing batch reactors
(SBR) were introduced in the 1920s. The SBR and conventional
activated sludge systems are almost the same. A 1983 EPA report
describes the SBR as "no more than an activated sludge system which
operates in time rather than space."
SBR technology advances and equipment improvements with regard to
the aeration devices and control systems, have recently re-ignited
interest in the technology. In one unit, an SBR can undertake
equalization, biological treatment, and secondary clarification with
a timed, controlled sequence. A single-unit SBR can also include
primary clarification. Each of these processes occurs in separate
units of a conventional activated sludge system.
In a single chamber SBR, there are multiple biological
environments that can be created and maintained to nurture each
specific type of bacteria for optimum performance at the right time
and right stage of the process, Hamilton says. For example, "one type
of bacteria absorbs excess phosphorous in the aerobic condition when
aeration occurs, but the bacteria do nothing at all in the absence of
oxygen or in the anaerobic stage."
Bacteria quality and quantity is key. With conventional systems,
the bacteria are settled in the clarifier and pumped back to the head
of the plant. The bacteria are recycled to maintain sufficient
quantities. In SBRs, the bacteria continue to live and work in the
unit as needed, when needed. These bacteria stay in the system longer
and, therefore, are stronger and more effective.
According to the EPA, an SBR can include "1) idle, 2) fill, 3)
react, 4) settle, and 5) draw" steps. A major technology improvement
in the SRB, according to Hamilton, allows effluent to be drawn out of
the system even when the effluent or water level is at a variable, or
During the "fill step," static fill, mixed fill, or aerated fill
can be used, depending on the facility and its goals. "Static fill"
is when influent is mixed with existing biomass and "no mixing or
aeration" has occurred when the mixing begins. "Static fill" can be
compared to using 'selector' compartments in a conventional activated
sludge system," according to the EPA. "Mixed fill" is mixing influent
organics and biomass. "Aerated Fill" consisted of aerating the SBR
contents to start an aerobic reaction.
During the "react stage," the biological reactions are finished so
that nitrification or denitrification and phosphorous removal can be
During the settling stage, no influent or effluent can get in to
disturb settling. Gentle mixing and settling time can produce a
clearer effluent than a conventional system.
In the "draw step," a decanter is used to remove treated effluent
out of the top of the tank while the bacteria settle at the bottom.
SBR have been used successfully in both municipal and industrial
wastewater applications. SBRs can provide possible facility cost
savings due to omission of clarifier construction and other
conventional process equipment purchase. But SBRs also require
operator training to set up and maintain SBR unit timing and control
systems, which can become complicated.
Membrane treatment technology
Basic membrane technology first began about 30 years ago in water
systems. At that time, membranes did not provide quality or
reliability for wastewater applications, according to Al Petrasek,
Carter & Burgess principal and senior project manager. Costs
associated with initial membrane technology were extremely high. The
combination of low quality performance and high cost made the initial
As membrane technology advanced, however, it became much more
appealing. Advances dramatically improved performance and reliability
in wastewater applications that require a very high quality effluent
while dramatically reducing utilization costs. Other improvements
include membranes with reduced thickness, improved stability with pH
and temperature extremes, and better compatibility with chlorine and
Membranes provide a separation process using a wide variety of
molecular sizes or pores to fit specific filtering or
demineralization needs. Membranes not only filter out most suspended
solids but can also remove potentially dangerous bacteria. In a
membrane plant, ultraviolet light can be used to effectively kill
bacteria for disinfection. Membrane treatment technology completely
eliminates the need for secondary clarifiers.
Membranes, as first designed, worked like a "soda straw" with
pressure applied to the inside to force fluids outside, says
Hamilton. Pressure was applied to the inlet side of the membrane and
caused the liquids to flow outside, but solids would "plug up the
straw" making it an operation and maintenance nightmare in a
wastewater application, Hamilton says.
"Membranes were an easy design in clean water applications but
because of the 'inside-outside' operation, small particles in solids
would plug up the 'soda straw' making it unworkable in wastewater
treatment. The advent of 'outside-inside' technology made the
wastewater application attractive," Hamilton says. "The membrane
works better because pressure is applied to the outside of the
'straw' and pushes the liquid from the outside to the inside through
the membrane so the solids stay outside. If solids plug up the
outside of the membrane and reduce the flux rate, a backwash
technique can refresh the membrane for continued use." When
considering membrane treatment technology, it is important to note
that membrane technology is geared at specifics and not standardized;
equipment can be specifically designed to meet a facility's specific
effluent quality needs.
Currently, membrane treatment technology can be a more costly
design feature than other filtration processes, but as more
manufacturers enter the marketplace, the competition will reduce
costs, says Hamilton. But in the design process, "It is important to
consider the membrane's design life, which is usually between seven
to ten years, as part of the process economics," Petrasek contends.
With that said, "membranes for wastewater treatment are a
technology advance that is getting the most attention right now as
the most promising addition to increasing effluent quality," Petrasek
says. McMillon predicts that "membrane use will increase and cost
will decrease, especially as the need for direct reuse and blended
water supplies increase."
Energy efficient technology
Anoxic/Oxic System Design. Anoxic/oxic systems are biological
nitrogen removal processes. These systems can lower power costs
because bacteria is oxidized in the nitrate molecule and used or
rather re-used. Therefore, there is no need for a wastewater facility
to purchase oxygen to put into the wastewater process.
During this anoxic/oxic process, basically a wastewater treatment
facility is using the oxygen that is available as a byproduct of the
treatment process itself. Without anoxic facilities, or in a
conventional process, the oxygen produced is discharged with the
The energy savings and cost savings will vary by facility.
Vertical Loop Reactors. A vertical loop reactor is a variation of
the BNR process that incorporates an anoxic zone into the process.
The mixing efficiency of the vertical loop reactor lowers energy
costs. Similar design criteria, advantages, and disadvantages exist
with the addition of a vertical loop reactor as with the basic BNR
Vertical Tube Reactors. "Vertical tube reactors are a very new
alternative technology, using very sound engineering theory for
sludge treatment," says Petrasek. "Engineering evaluations show the
technology does work and holds promise as an up-and-coming method for
A vertical tube reactor begins with a hole drilled straight into
the ground about 6,000 feet deep. A pipe is inserted into the center
of the hole and sludge is pumped in to the pipe. Over time, the
sludge flows back up around the pipe, but still remains in the hole.
While the sludge achieves a "critical pressure and temperature" all
the organic material is burned up without the use of chemicals,
energy or horizontal surface space.
Energy demand and usage
About 70 to 80 percent of energy used in wastewater treatment is
in the aeration process. If energy use can be lowered in this
process, costs can be dramatically impacted overall.
In new facility design, for example, changing from coarse bubble
aeration to fine bubble aeration and increasing the tank depth can
lower power consumption in this process by about 50 percent or more.
Also, if improvements can be made to the dissolved oxygen control
system so that it supplies only the amount of air really needed to
effectively and efficiently operated the aerators, this fine-tuning
can reduce energy usage and costs, Petrasek said.
With regard to energy efficiency overall, it is very important to
examine the "control demand charge" of equipment with an electric
service review. For example, it is financially beneficial to make
sure that all equipment is operating at 100 percent efficiency
because any less operation efficiency requires additional power to be
used. "A good example is a 10 horse power (hp) motor that is
operating at 100 percent efficiency will use only x amount of energy.
If that same motor is operating at only 50 percent efficiency, then
it may require twice as much energy to get it to function at the
rated 10 hp," says Petrasek.
Wastewater technology today
Today's wastewater treatment challenges are to improve or develop
technologies that address the changing issues of society, while
keeping an eye on the advances these technologies may lead to in the
future. Some technology advances will lead away from chemical
additives and back to using naturally occurring bacteria as well as
recycling what is produced naturally in the process, such as oxygen
and methane. Therefore, technologies like membranes and BNR will
thrive. Other technology changes will lead toward more complicated
systems controls and computers, as is the case with SBRs, which means
the industry will need to keep personnel trained to operate and
maintain the more complicated systems. As the philosopher Heraclitus
said "Nothing endures but change."
For more information, please contact Chris Fox, Carter&
Burgess, Inc., 777 Main St., Fort Worth, Texas 76102-5304,
817)735-6212, fax (817)735-2890, firstname.lastname@example.org, www.c-b.com.
The Peninsula Water Reclamation Plant Dallas,
Carter & Burgess designed innovative processes for high
operational flexibility and increased ability to meet strict effluent
limits for the Peninsula Water Reclamation Plant in Dallas, Texas.
Located in a residential neighborhood, stringent effluent water
quality criteria including nitrogen and phosphorus limits, and
sensitivity to odors were special considerations. To address the
nitrogen and phosphorus limits, innovative biological nutrient
removal (BNR) with anoxic/oxic and anaerobic cells were added. BNR is
an environmentally friendly process with no chemical additives. The
plant discharges into a receiving stream that merges with a major
drinking water source, so high quality effluent was required, which
BNR can provide. Because of the plant's proximity to a residential
neighborhood, ultraviolet (UV) disinfection is used instead of
conventional gaseous chlorine as a risk management strategy, Tertiary
filters are used to achieve a very high clarity effluent. Belt press
dewatering equipment is planned to minimize odor and to render
biosolids safe for disposal at an approved landfill.
Hot Sulphur Springs Wastewater Treatment Plant
When the existing treatment plant at Hot Sulphur Springs
encountered compliance issues for over accumulation of solids in the
aerated lagoons, they commissioned Carter & Burgess to design an
advanced facultative pond system for the Hot Sulphur Springs
Wastewater Treatment Plant. The technology utilized is an accelerated
treatment process from the conventional lagoon system. There are five
cells in the process. The first cell is anaerobic, called a
fermentation cell, which acts as a primary clarifier and digester to
reduce organic. The next three cells are aerobic cells that provide
aerated treatment. The fifth cell is a deep, still cell for settling
biological solids before discharging the effluent. A portion of the
highly oxidated water from cell 3 is recirculated back to cell 1 to
provide an aerobic cap in the fermentation cell to prevent odor
issues. A PVC liner was used in this pond system because the
substrate material is a very permeable river rock.
For more information, please contact Chris Fox, Carter&
Burgess, Inc., 777 Main St., Fort Worth, TX 76102, 817)735-6212, fax
(817)735-2890, email@example.com, www.c-b.com.
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