Full-scale deammonification system for reject-water treatment at Tartu WWTP
Tartu Waterworks Company is the second biggest municipal wastewater treatment plant in Estonia (100 000 PE) and one of the many WWTPs in the Baltic Sea Region where anaerobic digestion of sludge is applied. Nitrogen-rich reject-water produced in anaerobic digestion is commonly recirculated back to the process which increases the nitrogen load and challenging the main treatment process in keeping the discharge limits. This increases the amount of external carbon to be added to the main nitrification-denitrification process.
In the framework of constructing the anaerobic digester (2013-2015) in Tartu WWTP, the tanks were designed for nitritation of the sludge water before releasing it back to the influent of the plant. Ammonium nitrogen is oxidized to nitrite nitrogen during nitritation process, however, nitrogen removal process is still performed in main wastewater treatment process. Furthermore, University of Tartu had previously performed on-site pilot studies which showed good potential for efficient deammonification without external seed.
In order to enhance nitrogen removal with cost efficient investment, a full-scale deammonification system was implemented for reject-water treatment using existing infrastructure with regard to demonstrate in-situ start-up of deammonification.
Components installed in the solution
Since substantial infrastructure was already available in Tartu WWTP which reduced the volume of investment. Following equipment was additionally installed to the pilot:
- 200 m3 of Bioflow carriers (40-50% of tank volume) with specific surface of 800 m2/m3, special weight of 0.95. Problem with special weight being lower than water was exceeded with additional water recirculation in the process tanks until biofilm was grown on carriers.
- Current decanter (Cyklar 150, capacity of 250 m3/h) was enhanced with netting (hole size of 4.18 mm) to avoid outflow of biofilm carriers from process.
- Mixer
- Aerators near mixer were fixed with fastening strip (OTT system).
- Following sensors: redox (TriOS Mess- und Datentechnik GmbH); combined optical sensor for nitrite, nitrate and suspended solids (TriOS Mess- und Datentechnik GmbH); ammonium (NH4-N ISE TriOS Mess- und Datentechnik GmbH).
- Circulating pump to the effluent storage tank, which enables to return nitrates to the main WWT process (Grundfos SLV.65.6509.2.50B).
- 2 automatic valves (Fresto DN150)
Operational mode
There are large variety of patents available for different anammox-based processes. Therefore, first step was to exclude any possibility to have contradictions with any Estonian patents. Secondly, selection of technology and modelling were performed. Moving Bed Biofilm Reactor technology was chosen, which is based on process where nitritation and anammox are performed by microorganisms inoculated on biofilm carriers. Based on literature, nitrogen removal efficiency of 80% and removal rates up to 1.1 kgN×m-3×d-1 has been achieved with applying biofilm technology in deammonification process. However, these numbers are from ideal environment and real conditions with lower temperatures and inhibitions could substantially decrease the value. Biofilm systems are more resistant to different inhibitions than activated sludge systems. Furthermore, smaller volume of biofilm tanks allows to decrease constructional costs of water treatment systems.
Deammonification via intermittent aeration of immobilized biofilms was used for process control. Favorable environmental conditions were established for nitrifying microorganisms (AOB) by intermittent aeration and control on nitrite accumulation. Aeration capacity of 642 kgO2/d was dimensioned for nitritation process. Intermittent aeration is a tool for inhibition of nitritating microorganisms, which is contrary to activated sludge process not achievable with sludge retention time in biofilm process.
The deammonifying microbiological consortium was grown in-situ without a need of buying the sludge externally. Daily sludge flow to dewatering was 205 m3/d, which is same as hydraulic load of reject water. Loads were calculated based on average nitrogen load of 206 kg/d (max ca 280 kg/d) and COD load of 190 kg/d (max ca 260 kg/d). Average nitritation temperature was assumed to be 25°C, which requires hydraulic retention time of 1.5 days. Based on results from process modelling and previous modelling, process rates up to 0.4-0.5 kgN/(m3×d) is available in real conditions. Therefore, 2 m3 of process volume is required per 1 kg of nitrogen.
SCADA-programming was performed for enhanced process monitoring and control.