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On water service ... |
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On water service ... |
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AERATED LAGOONS FOR THE
MUNICIPAL WASTE WATER TREATMENT
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General
Up to now several
new methods of treating waste water in rural districts have been applied. The
oldest and easiest way of these purification plants are sedimentation and oxidation
ponds. But they require big areas (10-15 m2/p.e.) if a rather satisfactory
purification efficiency shall be achieved, as the oxygen transfer takes place
at the water surface only.
Especially during the warm season processes of putrefaction and therefore odours can occur. Non-aerated lagoons should therefore only be build as provisional plants or in secluded areas.
Also small sewage treatment plants which were built in greater numbers in the past did often not achieve the requested results. They have shown themselves to be very maintenance-intensive and susceptible to trouble. Especially at hydraulic overload or illegal introduction of high-concentrated waste water (i.e. liquid manure or silo seepage) lead to drastic reduction of purification efficiency for a long time.
Therefore central treatment plants were preferably built in the last few years. The result is that exactly dimensioned and correctly maintained central sewage treatment plants achieve good purification efficiencies and that they are resistant to deviations of load. But the construction of such plants normally requires long intercepting sewers with respectively high investments. Additionally it has to be considered that the flow of the natural water flows can be reduced considerably in the catchment area due to the long intercepting sewers.
As the financial
means the communities dispose of decrease more and more, central sewage treatment
plants will have to be calculated more critically in the future.
Therefore it shall be endeavoured to treat the waste water of fewer populated
areas where it occurs. In this field aerated lagoons have proved themselves.
The special merits of this system are its little maintenance requirements and
the high operating reliability, also at shock (peak) loadings. The minimum requirements
in Germany (COD : < or = 110 mg/l, BOD5 : < or = 20 mg/l, suspended
solids : < or = 0.5 ml/l) can easily be achieved with such plants. To treat
also rain water the lagoons can be operated in the storage system. Due to the
mechanical aeration the area demand decreases to 2-3 m2/p.e., i.e.
in respect of non-aerated lagoons to approx 20 %. The cost for building aerated
waste water lagoons are mostly lower. Additionally waste water lagoons can be
integrated very good in the surrounding area.
Method description
The mechanic-biological
purification of the waste water takes place in one or more aerated lagoons according
to the size of the plant, which are followed by an non-aerated sedimentation
and polishing pond.
The sewage coming from the canalisation is normally led directly into the first
aerated lagoon without mechanical pre-purification. So the continuous disposal
of screenings, sand and sedimentation sludge and its maintenance efforts can
be omitted.
Coarse stuff, sand
and heavy sludge settle in the inlet zone while dissolved contaminant are distributed
in the whole first lagoon.
Liable to putrefy matter should mainly be stabilized by aerobic processes to
avoid odours and digested sludge coming up to the water surface.
According to our experience sludge at the inlet zone of the first aerated waste water lagoon has to be removed at regular intervals of several years. To exhaust and bring the sludge out liquid manure-vacuum-tankers are used.
Floating solids are retained by a scum board in the inlet area. They should be removed once or twice a week with a rake.
To design bigger plants (> 1,500 - 2,000 p.e) it has of course to be considered carefully if a mechanical pre-purification of the waste water by a fine screen or a sieve offer still more advantages.
Purification processes in an aerated waste water lagoon are best compared with those of a loaded water flow.
Unlike activated sludge plants where suspended activated sludge eliminates the dissolved contaminant out of the waste water the active biomass is essentially as a fixed biological film at the bottom of the lagoon.
Basic requirement for an extensive biological reduction of the dissolved contaminant is therefore in addition to a sufficient oxygen transfer the effective circulation and mixing of the lagoons. So stagnant zones can be avoided and an everlasting exchange of water in the area of the fixed biological film at the bottom of the lagoon is ensured.
Design fundamentals
Aerated lagoons for
municipal waste water are designed according to the volume load and the retention
time.
Referring to German regulations a volume load of < or = 25 g BOD5/(m3.d)
according to a specific volume of > or = 2.4 m3/p.e. must be chosen.
The retention time
should not be less than 5 days even at high infiltration water inflow.
An OC/load > or = 1.5 kg O2/kg BOD5 is sufficient to reduce
contaminant.
According to our experience made with more than 200 waste water lagoons equipped with FUCHS Spiral Aerator the aeration system for the first lagoon should have a specific efficiency of 4 W/p.e.
The limitating factor
in the following stages is then no longer the required oxygen transfer but the
efficiency requirement for the circulation and mixing. According to the shape
of the lagoons power densities between
1.3- 2 W/m3 are sufficient.
An aeration system designed like this ensures a sufficient oxygen transfer also at times with a higher oxygen depletion (i.e. in the early summer or at peak loading). In times of low oxygen demand the aerator can be operated intermittent to economize energy.
To grant regular favourable discharge values it is advantageous to divide the aerated lagoon volume in two stages (stage 1 approx. 60 % stage 2 approx. 40%). The result of this measure has proved itself to be more effective than for example a further subsidence of the BOD5 space loading. The reasons for this are an essentially improved retention time characteristic of the plant as well as a staggered, efficient biocenosis.
Waste water lagoons with only one aerated stage are also suitable for plants with little design capacities (< or = 500 p.e.) and for plants with combined water. The space loading should then not exceed 20 g BOD5/(m3.d). It is recommended at the same time to design the following non-aerated stage a bit larger. Normally the retention time of the waste water in the final clarification and polishing pond is approx. 1-2 days. This correspond to a specific lagoon volume of 0.3-0.5 m3/p.e.
Treatment of storm-water
Due to the long retention
times (approx. 10 days) and the low load aerated waste water lagoons dispose
of a high buffering capacity compared to the waste water load.
Unlike activated sludge plants there is no danger that the active biomass is
carried out at hydraulic overload.
Accordingly a simultaneous rainwater treatment is easily possible. There are often no special measures and the whole rainwater is led through the lagoons.
It is a good solution
to operate the first aerated lagoon in backwater.
When reaching the maximum water level a pond overflow structure will go into
operation. In the case the first aerated lagoon is designed as a storm-water
tank to retain the first amount of discharge storm-water, the storm-water overflow
in front of the pond overflow will go into operation.
Design of the lagoons
Aerated lagoons are
often designed rectangular. To enable a good circulation and mixing at low capacity
requirements, the corners of the pond should be rounded very good and the sides
should not exceed a 2/1 ratio.
In practice, oval lagoons have also shown themselves to be reliable. Round lagoons
are most efficient ones. This shape should always be used if small plants with
only one Spiral Aerator are operated.
It is also possible to adapt the shape of the lagoon to the ground. But normally,
it is then necessary to divide the aerators' capacity onto more units.
The depth of the
lagoons should be between 2.0 - 3.0 m; mostly a water depth of 2.50 m is chosen.
The angle of slope is according to the local soil conditions. Normally it is
1:1.5 to 1:2.
It is only necessary to seal the lagoons if they are in very permeable soil
layers or in water protection areas.
The inlet zone of
the first aerated lagoon should have a deepening and it should be fastened respectively
paved.
So the settled sludge and sand can later be pumped out under the water surface
without a possible damaging of the bottom and the slope of the lagoon.
To prevent damages due to muskrats, erosion and an increased vegetation near the shore the slope is fixed at the height of the water level. The shape of the final clarification and polishing pond can be chosen to a far extend. But it has to be considered that no flow circuits from the inlet to the outlet are possible. The depth of the polishing pond should be approx. 1.2 m.
Large scale emergency by-passes are not required in any case, as the ponds have to be taken out of operation very rarely. For that case it is more favourable to use flexible pipings and transportable submersible pumps for the emptying.
Aeration of waste water lagoons
FUCHS Spiral Aerators meet the requirements of an aeration system for the aeration, mixing and circulation in a special wax. For information concerning design and operation of these machines please see our prospectus "FUCHS Spiral Aerator".
If the Spiral Aerators have a suitable position in hydraulic well designed ponds an even circulation flow is formed at the lowest power requirement. It includes the whole volume of the pond and grants an even oxygen transfer and mixing.
In bigger, round or square shaped ponds also the FUCHS Circulation Aerator is in operation. According to their flow pattern they are installed in the middle of the ponds. They mainly cause a vertical shifting and mixing of the waste water and complete the flow developed by the Spiral Aerators.
Both aerator types have a robust construction and are almost maintenance-free. Due to their little weight a fast and easy installation without hoist is possible. It also causes no difficulties and no expenditure to dismount the machines for example for deslurrying the pond.
The machines are preferably installed on a floating devices. If a constant water level is granted, the aerators can also be installed on bridges.
The machines have
the following advantages : high circulation and mixing capacity minimum maintenance
requirements no danger of clogging, also at intermittent operation or power
failure no spray water no odour problems no noise problems no frost problems
no big fan station and compressed air pipes
Summary
Aerated lagoons are a complete and low-priced system to the mechanic-biological purification of waste water of small communities. They can be integrated very good into the surrounding area.
If suitable aerators are used the power consumption is not higher than those of comparable activated sludge plants. In addition to a sufficient oxygen transfer of the aerators they also effect a good circulation and mixing of the ponds' contents.
As there is no need for a continuous disposal of screenings, sand and sewage sludge, the expenditures for maintenance and operation are restricted to a minimum. These are almost the highest costs at little sewage treatment plants.
Constant good effluent
values are the last straw for such waste water treatment plants, even if peak
loads are dumped in or at increased hydraulic load.
Design examples
Design fundamentals
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| Waste water inflow |
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| Infiltration water (100%) |
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| Specific BOD5 - load |
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| BOD5 - load |
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| OC/load |
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| Required power density |
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Design of the
aerated lagoons
| Data |
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| Connected capacity |
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| Specific waste water load |
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| Inflow load |
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| Total aerated volume |
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| No. of lagoons |
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| Volume lagoon 1 |
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| Volume lagoon 2 |
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| Shape of lagoons |
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| Water depth |
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| Slope |
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| Length-width-ratio |
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Design of the
aeration equipment
| Data |
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| O2 - requirement |
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| Chosen
aeration equipment lagoon 1 lagoon 2 |
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1 WBL - 2
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2 WBL - 2
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2 WBL - 4
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| Power density |
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1.4 (pond 2) |
2.1 (pond 2) |
| WBL - 1S | Nominal power | 1.7 kW |
| WBL - 2 | Nominal power | 2.4 kW |
| WBL - 4 | Nominal power | 4.5 kW |
Aerated lagoon - capacity 2100 ep
Partial view
aerated lagoon- capacity 500 ep
Aerated lagoon
entrance
Second aerated
lagoon partial view
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ISMA
on water sevice ...
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ISMA |