The requirements for disposal of wastewater into the Atlantic Ocean in the western part of Norway are limited to screening and primary treatment. The regulation in Hommelvik is set at a removal rate of 50 % or max. 60 mg/l SS and 20 % or max. 40 mg O2/l BOD5.
The building for the new plant was engineered before the process equipment was designed. One reason for this was to improve the project timeline, another was to use as litte concrete as possible. Concrete basins have a much higher CO2 & energy lifetime footprint than stainless steel tank solutions. On the other hand, the pre-designed building posed a challenge when it came to optimising the preferred hydraulics. The engineering started in February 2018 and the plant was commissioned as early as March 2019.
The main concept of Nordic Water’s primary filtration is to adapt and integrate effective suspended solids separation to provide a high level of treatment, while reducing the energy usage and decreasing the footprint of the plant as well as the overall carbon footprint.
The system is attractive because of the low energy costs, the small footprint and because all of the equipment is above ground. The waste products are considered “clean”, since grit and inorganic screenings (such as microplastics and organic sludge) are separated individually and efficiently. The design also enables future expansion with secondary filtration, as new demands arise.
The inlet wastewater is pumped directly into a Meva Combi Unit (MCU), consisting of a Meva stepscreen with 3 mm slot width and a grit trap that removes sand and grit, > 50 µm in size. The water flows by gravity and is equally divided between two Sobye band filters with 500 µm cloth and two DynaDrum microscreens with 30 µm cloth before it enters the recipient.
Screenings, sand and sludge are separated. The sludge is pumped into a screw dewaterer, where it is thinkened before it enters into sludge storage and is finally trucked away for incineration.
The most important factors in the evaluation of the process equipment at the Hommelvik WWTP were energy efficiency, a high separation degree and the overall most economical solution. The Meva stepscreens have a very high screenings capture rate (SCR), > 80 %, but do not need any flush water at all. The energy usage is less than 0.1 kWh, which is 95 % lower than conventional perforated band screens.
The low head loss and the low height of all of the treatment equipment made it possible to gravitate the flow through the complete treatment plant.
The new treatment plant far exceeds the regulatory requirements, with an average removal rate of about 75 % of SS and 30 % of BOD, without any use of flocculation or coagulation chemicals.
Connected PE: 6000
Produced flow Qdim: 126 m3/h
Produced flow Qmax: 250 m3/h
Typical inlet values:
Suspended solids: 100-400 mg/l
BOD5: 100-250 mg O2/l
Typical outlet values:
Suspended solids: 20-60 mg/l
BOD5: 25-120 mg O2/l
Meva Combi Unit MCU 40
Step screen RS 10
Screen press CPS 15
Band screens: 2 x Sobye TD14 (500µm cloth)
Total surface: Band area 2.8 m2
Total max. surface
loading rate: 89 m/h
Microscreens: 2 x DynaDrum TDD1212 (30 µm cloth)
Total surface: Drum area 9.6 m2
Total max. surface
loading rate: 26 m/h
Chemical dosage: PAC 0 ml/m3