Biological treatment of ground water

BIOLOGICAL
TREATMENT OF
GROUND WATER
December 1997
WHAT IS BIOLOGICAL
TREATMENT OF WATER?
Water used for drinking and household use, even water
from a ground water supply, should be treated before it is
used to ensure it is safe and aesthetically pleasing. One
innovative method of water treatment is biological water
treatment. Biological treatment has been used in Europe
since the early 1900s and is now receiving more interest
in North America.
Biological water treatment involves the use of naturally
occurring micro-organisms in ground water to improve
water quality. Under optimum conditions, including
enough oxygen, the organisms break down material in the
water such as dissolved organic carbon (DOC) and iron
and thus improve water quality. Sand or carbon filters are
used to provide a place on which these micro-organisms
grow.
One type of in-house biological system for treating ground
water was studied in two households in Saskatchewan as
part of research conducted under the CanadaSaskatchewan Agriculture Green Plan Agreement.
WHY USE BIOLOGICAL
TREATMENT OF WATER?
Biological water treatment processes are well suited to
treat some problems common to Prairie ground water
supplies, including iron, arsenic, and dissolved organic
carbon (DOC), as well as colour and turbidity (see the
Water Quality Matters publication “Prairie Water
Quality Problems”).
including a slow sand filter and biological activated
carbon (BAC) filter, effectively reduced DOC, colour and
turbidity, as well as iron and arsenic. The biological
system was not as effective in reducing manganese
concentrations as first anticipated.
HOW WILL BIOLOGICAL
TREATMENT IMPROVE
WATER QUALITY?
Biological treatment of ground water improves water
quality by reducing iron and arsenic, which are aesthetic
and health problems, respectively. The biological process
also reduces colour and turbidity as well as concentrations
of DOC, which can cause health problems in chlorinated
water (see the Water Quality Matters issue “Prairie
Water Quality Problems”). Reduced DOC levels can also
improve the taste and odour of the water, and can reduce
the amount of chlorine needed for disinfection.
WHAT WERE THE
SYSTEM COMPONENTS?
The biological treatment system has three parts:
•
a slow sand filter where water flows through by
gravity; it is designed so that biological processes
establish themselves in the filter to reduce levels of
iron and arsenic;
•
a biological activated carbon (BAC) filter to reduce
manganese and DOC; and
•
a 1000 L storage tank to meet peak household
demands.
Research showed that the biological treatment system,
Prairie Farm Rehabilitation Administration
The system was designed to operate continuously,
providing high quality non-potable water to the entire
household. After the storage tank, the water was softened
to reduce hardness for washing and bathing. An
ultraviolet (UV) disinfection lamp was installed after the
softener on one ground water system to deal with
microbiological problems. For drinking and cooking
water at each site, a reverse osmosis (RO) membrane,
complete with booster pump, was installed as an
additional protective barrier at the kitchen sink. The RO
unit reduces sulphate, sodium, total dissolved solids and
hardness, which are parameters which the BAC filter will
not treat. The RO unit will also remove micro-organisms
including bacteria, viruses and parasites, providing it is
properly operated and maintained.
The biological treatment system improved the water
quality by removing:
•
85% to 90% of DOC;
•
87% to 90% of the colour;
•
93% of arsenic (in the water supply where arsenic
exceeded the regulated level of 0.025mg/L);
•
more than 99% of the turbidity;
•
34% to 94% of the manganese; and
•
more than 99% of the iron.
HOW WELL DID THE
BIOLOGICAL
TREATMENT SYSTEMS
PERFORM ON GROUND
WATER SUPPLIES?
A biological sand filter reduced high levels
of iron and arsenic, naturally-occuring
from the deep well water supply
The results of the sampling conducted on one of the two
ground water treatment systems (the water containing
higher arsenic concentrations) are shown in Table 1.
Although the sand filter was often sufficient to reduce the
concentration of target
parameters below the stated
Table 1: Ground Water Quality at Site with High Arsenic Concentrations After
goal, the BAC filter
Each Stage of the Biological Treatment System
significantly improved the
PARAMETER
BEFORE
AFTER SAND
AFTER BAC
AFTER RO
SWQI
quality of the water.
The slow sand filter
effectively reduced iron and
arsenic concentrations. The
BAC filter effectively reduced
the DOC to less than
1 mg/L. Together, the sand
and BAC filters reduced
colour and turbidity.
GOALS
SAND FILTER
FILTER
FILTER
MEMBRANE
<0.3
4.34
0.01
<0.01
0.07
<0.05
0.322
0.247
0.212
0.002
<0.025
0.029
0.002
0.002
<0.001
<5.0
4.3
4.002
0.628
0.072
Colour (TCU)
<15
33
8
4
3
Turbidity (NTU)
<1.0
39.4
0.3
0.2
0.2
Iron
(mg/L)
Manganese
Arsenic
DOC
(mg/L)
(mg/L)
NOTE:
2
(mg/L)
shaded areas indicate results met goal
The biological treatment system was inconsistent in
reducing manganese. The equipment manufacturer
continues to work on the ability of the system to reduce
manganese concentrations.
The kitchen sink RO membrane provided excellent
reductions of most parameters, including manganese and
other inorganic parameters such as hardness and total
dissolved solids.
The UV disinfection unit was originally installed before the
softener and became ineffective due to a build-up of
calcium scale. The UV lamp provided effective
disinfection after it was replaced and reinstalled after the
water softener.
The raw groundwater did not contain coliform bacteria,
which are indicators of the presence of biological
organisms. Coliforms were detected in the sand and BAC
filters, which is not surprising as these are not sealed
units. However, coliforms were generally not detected or
were found at very low levels after the water had passed
through the RO membrane.
An add-on softener will reduce hardness.
The ultraviolet light (UV) was added after
the softener to disinfect the ground water
WHAT DID IT COST AND
HOW LONG DID IT LAST?
Each treatment system was capable of supplying all
household water for one dwelling, and at one site, the
system was actually able to supply two households with
all domestic water needs, including drinking and cooking
water (a total volume of 1,700 L/day or 380 gallons
per day). The cost was about $7,000, which included
the sand and BAC filter, storage tank and kitchen sink RO
unit as well as miscellaneous components such as an air
pump. Add-on equipment such as a softener and
disinfection unit cost an additional $3,000. Both ground
water supplies required softening for washing and
bathing purposes to reduce the calcium and magnesium
which create hardness.
The systems at both sites have operated for two years.
One site required carbon replacement after about 18
months, as the filter became clogged with calcium.
Improved operating procedures such as backwashing or
acid regeneration may solve this problem.
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WHAT OPERATION &
MAINTENANCE IS
REQUIRED FOR
BIOLOGICAL SYSTEMS?
•
The biological treatment system is easy to operate
and has a long life expectancy. Good operation and
maintenance is essential to ensure it works well.
•
An appropriate air supply system is needed to supply
oxygen to the microbes in the BAC filter.
•
The slow sand filter must be backwashed manually
using air and water to remove the accumulated
material and prevent loss of productivity.
Backwashing is required about once every two to
eight weeks or after approximately 30,000 L to
40,000 L (6,000 to 9,000 gallons) of water.
•
•
Backwashing of the BAC filter with air and water is
required about every two to five months, or after
treating approximately 40,000 L to 60,000 L
(9,000 to 13,000 gallons) of water. More frequent
backwashing may be needed if the filter begins to
plug.
Acid regeneration of calcified media may be
necessary, but proper procedures must be followed.
ARE THERE ANY
LIMITATIONS TO USING
BIOLOGICAL WATER
TREATMENT?
•
•
•
•
•
Biological treatment converts ammonium to nitrate. If
the source water is high in nitrates, then biological
treatment may increase nitrates to a level that
exceeds Canadian Drinking Water Quality
Guidelines. It should be noted that an RO unit
would reduce nitrates to a safe level.
Biological treatment of water is not designed to
reduce levels of total dissolved solids, hardness,
calcium, magnesium, sodium or sulphate, all of
which are also common problems in ground water.
Iron may break through the sand filter if it is used
beyond its capabilities.
Calcium may build up in the carbon filter. On the
one ground water source where this occurred, the
carbon was replaced and the backwashing
frequency was adjusted to once every two to three
months. Acid regeneration of the carbon media may
eventually be required as a maintenance procedure.
As with other water treatment systems, the treated
water must be disinfected (through methods such as
chlorination, membrane filtration, or distillation for
example) before the water is used for drinking and
cooking.
THE BIGGER PICTURE
The biological treatment systems that were studied were
effective multiple-barrier treatment systems for ground
water supplies with iron, arsenic and dissolved organic
carbon problems. In combination with a softener and
kitchen sink reverse osmosis unit, the systems delivered
high quality domestic water and safe drinking and
cooking water for individual farms. Biological treatment
processes may prove to be valuable for treating ground
water supplies for individuals or communities. Both
producers involved in the study were satisfied with the
treatment provided by the biological system.
Further study of biological treatment systems is however
needed:
•
to determine how much treated water the system is
capable of delivering (flow rates);
•
to determine long-term performance of the systems;
and
•
to deal with iron breakthrough and calcium build-up.
The system manufacturer is continuing to research
ways to improve the system’s manganese removal
rates.
For further information on rural Prairie water quality and
treatment technology:
•
contact your local Prairie Farm Rehabilitation
Administration office (PFRA is a branch of Agriculture
and Agri-Food Canada);
•
read the other publications in PFRA’s Water
Quality Matters series;
•
get a copy of “Rural Prairie Water Quality:
Searching for Solutions for On-farm Users” available
from PFRA; or
•
read Prairie Water News, available from PFRA, or
on the Internet at www.quantumlynx.com/water
AUTHORED BY: H. Peterson, Safe Drinking Water Foundation and D. Corkal, PFRA
FUNDING: This publication was funed in part by the Canada-Saskatchewan Agriculture Green Plan Agreement and the Canada-Saskatchewan Agri-Food Innovation Fund.
ENDORSEMENT: This report should not be taken as an endorsement by PFRA or Agriculture and Agri-Food Canada of any of the products or services mentioned herein.
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