Municipal wastewater post-denitrification at Höviksnäs summer resort in western part of Sweden

Höviksnäs case study

Höviksnäs is a summer resort on the island of Tjörn, in the western part of Sweden. In 2012, it was decided to upgrade the Höviksnäs wastewater treatment plant with new equipment from Nordic Water. The existing equipment required a general upgrade, and nitrogen removal needed to be improved. Commissioning of the new plant was carried out in 2016.

The challenge

Limitation of eutrophication has long been on the agenda for authorities and municipalities on the Swedish west coast. The beautiful estuaries north of Gothenburg, the second largest city in the country, are no exception. Eutrophication along the Swedish west coast mainly favours fast-growing ephemeral macroalgae. It reduces the water quality and thus the value of recreational sea activities. Nutrient emissions from local sewage treatment plants have been identified as one source of eutrophication that needs to be reduced. In addition to meeting the new effluent constraints on nitrogen, one of the requirements at Höviksnäs WWTP was that the plant should be highly energy efficient.

 

The solution

For the above reasons, the following equipment was selected from Nordic Water:

The inlet wastewater is screened by two Meva Monoscreens with 1 mm slots, installed in parallel. The water is then further treated in an aerated grit and fat trap with Zickert bottom scrapers and a moving bed biofilm reactor (MBBR) with activated sludge process.

To improve the subsequent settling, AlCl is dosed into the flocculation tanks.

The effluent water from these stages is pumped up to an MBBR for nitrification, followed by denitrification in DynaSand Deni biofilter reactors. Prior to nitrification, caustic soda is used to correct the pH. Methanol is also added as an external carbon source in front of the DynaSand Deni bioreactors. Finally the treated water is led out into the estuary by gravity.

 

The result

The wastewater characteristics were a bit more challenging than expected and required some adjustment to perform as intended. Low inlet pH, lack of available phosphate and seasonal changes had to be compensated for. Keeping the methanol dosing proportional to the flow and using the outlet concentration of NO3-N to fine-tune the dosage were important factors in achieving stable operation and outlet results, as well as meeting the effluent constraints.

To create favourable conditions for the denitrifying bacteria, NaOH was used to raise the pH. As all phosphate is already reduced at this late stage of the treatment, some phosphoric acid had to be added. The inlet PO4-P values of about 0.3 mg/l were enough to get the reduction of nitrogen aimed for.

To improve the reactivity and keep the density of the denitrifying bacteria as high as possible, the DynaSand Deni filters are run with intermittent washing. 6 minutes of washing of the media is followed by 14 minutes of pause time. Because of the high reactivity, nitrifying bubbles can be observed on the water surface of the filters.

Post-denitrification in DynaSand Deni bioreactors is well-proven and sucessfully installed in several European countries. Now, for the fist time, it is also installed in Sweden.

 

Technical Data

Average flow                                3000 m3/day                                    Solution

Max design flow                           200 m3/h                                         Installed equipment from Nordic Water

Max. flow                                       300 m3/h                                         2 x Meva Monoscreen RSM11-70-1

Temperature range                       7-20°C                                             1 x Meva Screw Wash Press SWP20-70

1 x Meva Counter Press Screw CPS20-100

Inlet to DynaSand Deni                                                                         2 x Zickert bottom scraper Z2000

pH                                                         ~5                                                 2 x Zickert surface scraper Z3900

Oxygen                                                6-8 mg/l

Nitrate (NO3-N)                            25-35 mg/l                                       DynaSand denitrification filters

Suspended solids (SS)                  30-50 mg/l                                      Number of filters          4 psc

Model: DS5000 Deni 4.0

Outlet after DynaSand Deni                                                                   Surface per filter           5 m2

Nitrate (NO3-N)                                4-5 mg/l                                        Filter sand size              0.9-1.2 mm

Ammonia (NH4-N)                           3-5 mg/l

Suspended solids (SS)                       ~5 mg/l                                        External carbon source dosage

Methanol                                              ~4 g methanol/g NO3-N

Effluent constraints

Nitrogen (N-tot)                               <15 mg/l

BOD                                                      <8 mg/l

Phosphorus                                      <0.3 mg/l

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