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Continuous Sand Filters
save costs & labour in water works plant
Steve Minett, PhD & Keld Fenwick
When rebuilding a water works plant supplying a suburb of Gothenburg,
the engineers decided to replace the flocculation phase followed by
sedimentation tanks and conventional sand filters, with a special
filtration system.The rebuild dates from 1993 and the system has been
operating ever since, problem free, with only two staff.
Drinking water is an emotive subject and consumers will quickly complain
if the quality deteriorates for any reason. At the same time, water
utilities are under pressure to keep costs under control and make the
most efficient use of capital
The Mölndal water works serves a community with a population of 50 000
people. The plant has a maximum capacity of 17 500 cubic metres per 24
hrs period although the average flow varies between 12 -14 000 cubic
metres. When planning the rebuild, operations manager Walter Patriksson
recalls that there were really only one alternative option to the new
filtration system they eventually choose: “This was that we ourselves
build lamella-type sedimentation systems in the existing sedimentation
tanks. The problems with this plan was that manual cleaning of the
plates would have been necessary and would have taken about half a day.
The existing tanks had to be cleaned out monthly and took about the same
time so there would have been no labour saving. In addition, the actual
building of the lamella system would have unavoidably disturbed the
operation of the plant for several months at least. Running costs would
also have been higher because the consumption of precipitation chemicals
in the new filter units is about 40% lower compared with the alternative
technique.”
With costs for both systems about the same, Nordic Water’s DynaSand
equipment won on two counts; firstly manual cleaning is not necessary
and secondly the DynaSand units could be built beside the old plant
while it continued to operate. The switch-over to the new system was
carried out within an hour or so and supply to the consumers was not
interrupted.
The old plant
Before rebuilding, the technique involved a flocculation phase
followed by sedimentation tanks and conventional sand filters. There was
a double line of process flow to allow the necessary monthly cleaning.
The flocculation chambers were 25 X 4 metres and each contained four
mixers. The flow then went on to sedimentation chambers 25 X 5m and then
finally to conventional the sand filters. The agent used for
flocculation was aluminium sulphate and the same 30 g /cubic metre
dosage was used throughout the year. Lime was also added to control the
pH.
Capacity before rebuilding was 12 000 cubic metres/24 hrs period and
part of the plan was to increase it to the present 17 500. A new pumping
station was installed to pump directly from lake Rådasjön and,
importantly, to lower the intake in the lake from 50 cm down to 15
metres. The problem with the 50 cm intake was that in the summer, the
temperature of the water could reach 23 degrees Celsius which sometimes
led to the presence of algae and other organic contaminations. At 15
metres however, the annual temperature range is 3 to 11 degrees Celsius
which discourages algae growth.
The new plant
Three pumps deliver raw water from the lake for initial screening.
Lime and carbon dioxide are added to raise hardness and alkalinity
before the water goes through a contact basin to give sufficient
reaction time. From the basin, three pumps, working in tandem with the
units in the pumping station, lift it to the DynaSand plant. The output
from one pump is varied automatically to maintain the correct water
level in the contact basin.
The feed to the 20 filters divides into five lines with a
remote-controlled shut-off valve at the inlet to each line. The number
of lines in operation is controlled automatically by the flow from the
basin to avoid excessive compressed air consumption and to reduce the
loss of drinking water as wash water. When a line is shut off, a valve
at the wash water outlet is closed automatically
New filter system
Eliminating the problem of downtime for backwashing is DynaSand’s
great operational advantage. This problem has conventionally reduced the
efficiency of sand filters. The filter has overcome it by eliminating
backwashing: fouled sand is continuously removed from the filter bed,
washed and recycled without interruption to the filtration process. The
filter is based on the counterflow principle. The water to be treated is
admitted through the inlet distributor in the lower section of the unit
and is cleaned as it flows upward through the sand bed, prior to
discharge through the filtrate outlet at the top. The sand containing
the entrapped impurities is conveyed from the tapered bottom section of
the unit, by means of an air-lift pump, to the sand washer at the top.
Cleaning of the sand commences in the pump itself, in which particles of
dirt are separated from the sand grains by the turbulent mixing action.
The contaminated sand spills from the pump outlet into the washer
labyrinth, in which it is washed by a small flow of clean water. The
impurities are discharged through the wash water outlet, while the
grains of clean sand (which are heavier) are retained to the sand bed.
As a result, the bed is in constant downward motion through the unit.
Thus, water purification and sand washing both take place continuously,
enabling the filter to remain in service without interruption.
Filtered water from the plant flows by gravity to a second contact basin
where it is disinfected with chlorine dioxide and the pH adjusted to
about 7 by dosing with sodium carbonate. It then flows to the activated
carbon filters which are backwashed once or twice a week. Walter
Patriksson explains that these are included in the process only to
insure that the water has the highest standards of taste and colour and
as a safety measure in case for any reason the DynaSand units are not
operating.
Walter points out that it is necessary to recycle the wash water from
the sand filter units because about 12 % of the raw water is used for
wash water. This is too high a percentage to remove from production;
recycling reintroduces more than 90 percent of this water so that less
than one percent of the raw water is lost in the sludge discharged from
the DynaSand process. This sludge is transported by gravity flow to a
local sewage plant.
The wash water recycling system sends the wash water to a 70 cubic
metres/hour flocculating chamber where a Magnafloc polymer agent is
added. It then flows to a Nordic Water Lamella separator, also with a 70
m³ per hour capacity. This system is necessary because the Mölndal
waterworks has 20 DynaSand units; smaller plants with only 10 units can
use one of these to recycle the wash water and thus dispense with this
extra treatment phase.
The 20 sand units have a total filtration area of 100 m² and operate in
one of three modes. The lowest level is 120 l/second using 12 units and
is the usual night-time mode. The second level is 120 l/s with 15 units
operating and is the most frequently used level. In dry periods the
system can operate at 200 l/s with all 20 DynaSand units on stream. The
operating cycles are rotated to spread the wear factors. <<
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