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| | Nitrogen down. Phosphorus down. Satisfaction up. Upgrade sewage plant at Hunseby Strand. |
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Changing situations require flexible solutions. Nowhere is this more true than when upgrading existing arrangements to meet new, more stringent requirements. The ideal solution is the one that can satisfy the new conditions, flexibly, cost-effectively and elegantly.
The municipalities of Maribo and Sakskobing are situated on the Danish island of Lolland in the southern Baltic. Both of them are connected to the sewage treatment plant at Hunseby Strand. In 1990 the then 12-year-old treatment plant was upgraded to a population capacity of 40,000, with biological and chemical treatment.
One of the motives behind the upgrading of the plant was the recent Danish legislation on nitrogen reduction in sewage treatment; in Denmark the requirement is now BS5 < 15 mg/l, total nitrogen < 8 mg/l and total phosphorus < 1.5 mg/l for treatment plants larger than 5,000 inhabitants. at especially sensitive discharge points, the required values are somewhat lower.
Mechanical treatment
Larger solids in the incoming sewage water are removed by two "step" grids and two aerated, grid chambers remove sand and fats.
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Following this, suspended materials are separated in two pre-sedimentation basins before the flow is pumped into the new basin system.
Biological - Chamical treatment
The newly built basin block contains two parallel lines for denitrification, nitrification and sedimentation. The lines are totally separated and independent of each other, which makes for greater flexibility and simpler maintenance. The design of the layout minimized both the need for separation channels and construction costs.
"Greater flexibility and simpler maintenance"
There's an option to include an anaerobic phase before the anoxic phase during the biological phosphorus reduction process; anoxic water is then pumped back to the anaerobic stage. Nitrate-rich water from the end of the oxidation stage is pumped back to the beginning of the anoxic phase. Return sludge is collected by suction sludge scrapers in the sedimentation basins and returned to the anoxic stage via a siphon conduit.
Simultaneous treatment with iron sulfate can also be included, if required.
The treated effluent is discharged to a local bay. Sakskobing Fjord, through a thousand-meter pipe. At the discharge point, it is dispersed via diffusers at a depth of 4 m.
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Sludge treatment
Primary sludge is thickened and pumped to a digester. The biogas produced is used to generate electricity in a generator driven by three gas motors. The digested sludge is dewatered in a belt press to a TS-level up to 40%. The surplus sludge is also thickened but, to avoid the risk of phosphorus and nitrogen emissions, it is then pumped to an aerated sludge silo, which is used for both primary and surplus sludge. The dewatering to a TS-level of 20% is done in a centrifuge. Afterwards, the sludge is transported away for agricultural uses.
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PP 4451
A PP pump can be used for high flow at low head. The pump is installed through a hole in a wall. Little energy is wasted with this type of installation and power consumption is low, making it a highly cost-effective arrangement.
SR 4410
Banana blade (large diameter) mixers create high flow and velocities in the tank, at low power consumption. This is advantageous for homogenizing large volumes and keeping sediments from the bottom.
ITT Flygt Sanitaire diffusers
Fine bubble aeration with Sanitaire membrane discs. Ensures even O2 distribution over the complete aerated area. Operated at low energy consumption with proven resistance to fouling.
ITT Flygt Sanitaire diffusers
Fine bubble aeration with Sanitaire membrane discs. Ensures even O2 distribution over the complete aerated area. Operated at low energy consumption with proven resistance to fouling.
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| Treatment results: |
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| BS5 < 15 mg/l |
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| Total nitrogen < 8 mg/l |
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| Total phosphorous < 1.5 mg/l |
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| Inlet conditions: |
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| - Population equivalent, 40,000 |
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| - dry weather max./d 9,000 m3/d |
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| - rainfall max./h 1,500 m3/d |
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| BS5 2,400 kg/d |
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| Total nitrogen 470 kg/d |
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| Total phosphorous 128 kg/d |
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| Flygt equipment: |
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| Pumps: |
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| - inlet pumps, 2 x LL 3201 |
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| - lift pumps to the bio step, 4 x LL 3152 |
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| - recirculation pumps, 2 x PP 4451 |
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| - outlet pumps, 2 x 3300 |
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| Mixers: |
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| - 8 x SR 4410 |
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| - 4 x SR 4451 |
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| Aeration: |
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| 2600 Sanitaire rubber membrane discs. |
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